Biology Final Exam

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photosynthesis harnesses sunlight to make carbohydrate from CO2

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In photosynthesis, energy in sunlight is transformed to chemical energy by converting the C-O bonds in C02 to the C-C and C-H bonds of carbohydrate. The overall reaction-the sum of many independent reactions-can be simplified and written as

Hint 1

cellular respiration

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photosynthesis electron transfer

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photosynthesis is energy demanding

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energy demanding series of redox reactions that produce sugar and oxygen from carbon dioxide and water.

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purple sulfur bacteria

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autotrophs that manufacture their own carbohydrates from CO2, sunlight, and hydrogen sulfide.

In these organisms a simplified version of the overall reaction for photosynthesis is

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oxygen atoms released during plant photosynthesis must come from water

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In addition, the reactions responsible for producing O2 only occured in the presence of sunlight, whether or not CO2 was present.

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Two distinct sets of reactions in photosynthesis

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one that uses light to produce O2 from H20 and one that converts CO2 into sugars.

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CO2 is reduced into sugars

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CO2 is reduced into sugars

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The Calvin Cycle

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the reactions that reduce carbon dioxide and produce sugar

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The calvin cycle depends on light capturing reactions

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the calvin cycle does not directly require light, it functions if the light capturing reactions that produce O2 are also occuring.

Light capturing reactions eventually stop producing O2 if the Calvin cycle is not occuring.

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Light capturing reactions and the calvin cycle are linked

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are linked by a coordinated series of redox reactions.

The light capturing reactions provide high energy molecules that drive the Calvin cycle, which in turn regenerates that ADP, Pi, and NADP+ used by the light capturing reactions.

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Light capturing reactions

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During the light capturing reactions, electrons are promoted to a high energy state by light. This energy elevation ignites a Chain of electron transport steps that starts with the oxidation of water to form O2 and ends with the reduction of NADP+ to form NADPH.

NADP+ and NADPH are phosphorylated version of NAD+ and NADH used in cellular respiration. Some of the energy released from the redox reaction is also used to produce ATP.

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The Calvin Cycle

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During the Calvin cycle, the electrons in NADPH and the potential energy in ATP are used to reduce CO2 to carbohydrate.

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Light capturing reactions and the calvin cycle overview

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In the light capturing reactions of photosynthesis, light energy is transformed to chemical energy in the form of ATP and NADPH. The calvin cycle uses the ATP and NADPH to reduce carbon dioxide to sugar and regenerates ADP, Pi, and NADP+ for the light capturing reactions.

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Photosynthesis occurs in Chloroplasts

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Chloroplasts

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a chloroplast is enclosed by an outer and an inner membrane. The interior of the organelle is dominated buy flattened, membranous sac-like structures called thylakoids, which often occur in interconnected stacks called grana.

The space inside a thylakoid is its lumen.

The fluid filled space between the thylakoids and the inner membrane is the stroma.

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Pigments

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Pigments are molecules that absorb only certain wavelengths of light, other wavelengths are either reflected or transmitted. Pigments appear colored because people see only the wavelengths that are not absorbed.

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Chlorophyll

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The most abundant pigment in the thylakoid membranes of green plants is chlorophyll, which reflects or transmits green light. As a result, chlorophyll is responsible for the green color of plants, some algae, and many photosynthetic bacteria.

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Light capturing reactions begin with the simple act of sunlight striking chlorophyll.

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Light capturing reactions begin with the simple act of sunlight striking chlorophyll.

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The electromagnatic spectrum

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visible light

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reanges in wavelength from about 400 to about 710 nanometers.

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Shorter wavelengths of electromagnetic radiation contain more energy than longe wavelengths.

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Thus ther is more energy in blue light than in red.

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particle nature of light

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light exists in discrete packets called photons. Each photon of light has a characteristic wavelength and energy level.

Pigment molecuels absorb the energy of some of these photons. How?

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photosynthetic pigments absorb light

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When a photon strikes an object, the photon may be absorbed, transmitted, or reflected. A pigment molecule absorbs photons of particular wavelengths. If a pigment absorbs all the visible wavelengths, the pigment appears black because no Visible wavelength of light is reflected back to your eye. If a pigment absorbs many or most of the wavelengths in the blue and green parts of the spectrum but transmits or reflects longer wavelengths, it appears red.

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Different Pigments absorb different wavelengths of light

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There are two major pigment classes in plant leaves: chlorophylls and cartenoids.

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Chlorophylls

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designated chlorophyll a and chlorophyll b, absorb strongly in the blueand red regions of the visible spectrum. The presence of chlorophylls makes plants lookgreen because they mostly reflect green light, which is not absorbed well

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Carotenoids

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absorb wavelengths in the blue and green parts of the visible spectrum.Thus, carotenoids appear yellow, orange, or red. The carotenoids found in plants belongto two classes called carotenes and xanthophylls.

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Pigments that absorb violet-to-blue and red wavelengths are mosteffective at triggering photosynthesis in algae

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the action spectrum

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for photosynthesis-the wavelengths that drive the light-capturingreactions. Engelmann’s data indicate that violet-to-blue and red light are the most effective atdriving photosynthesis. Because the chlorophylls absorb these wavelengths, this earlyexperiment showed that chlorophylls are the main photosynthetic pigments.

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absorption spectrum

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An absorption spectrum measures how the wavelength of photons influences the amount of light absorbed by a pigment.

In the combined graph, peaks indicate wavelengths where absorbance or photosynthetic activity is high; troughsindicate wavelengths where absorbance or photosynthetic activity is low.

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Which part of a pigment absorbs light?

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chlorophyll a and chlorophyll b are similar in structure. Both have two fundamental parts: a long isoprenoid “tail” and a “head” consisting of a large ring structure with a magnesium atom in the middle. The tail interacts with proteins embedded in the thylakoid membrane; the head is where light is absorbed.

The structure of Beta-carotene, shown in Figure 10.8b, consists of an isoprenoid chainconnecting two rings that are responsible for absorbing light. This pigment is what givescarrots their orange color. A xanthophyll called zeaxanthin, which gives corn kernelstheir bright yellow color, is nearly identical to B-carotene, except that the ring structureson either end of the molecule contain a hydroxyl (-OH) group.

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chlorophylls are the main photosynthetic pigments, but carotenoids also absorb light.

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Carotenoids are called accessory pigments because they absorb light and pass the energy on to chlorophyll. these accessory pigments can extend the range ofwavelengths that drive photosynthesis by absorbing some of the photons not readily absorbedby chlorophyll.

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the primary function of carotenoids is to protect the plant

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To understand why carotenoids are protective, recall that photons-especially the high-energy,short-wavelength photons in the ultraviolet part of the electromagnetic spectrum-contain enoughenergy to knock electrons out of atoms and create free radicals. Free radicals, in turn, triggerreactions that can disrupt and degrade molecules.

Carotenoids “quench” free radicals by accepting or stabilizing unpaired electrons. As a result,they protect chlorophyll molecules from harm. When carotenoids are absent, chlorophyllmolecules are destroyed by free radicals and photosynthesis stops. Starvation and death follow

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When light is absorbed, Electrons Enter an Excited State

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When a chlorophyll moleculeabsorbs a photon, the photon’s energy is transferred to bonds in the chlorophyll molecule’snhead region. In response, an electron becomes “excited,” or bumped up to a higher energy state.

In chlorophyll, for example, the energy difference between the ground state and state 1 is equal to the energy in a red photon, while the energy difference between state 0 and state 2 is equalto the energy in a blue photon. Thus, chlorophyll can readily absorb red photons and blue photons.

Chlorophyll does not absorb green light well, because there is no discrete step-no difference inpossible energy states for its electrons-that corresponds to the amount of energy in a green photon.

When pigments in chloroplasts absorb photons, about 2 percent of the excited electrons produce fluorescence. The other 98 percent of the energized pigments use their excited electrons to drive photosynthesis.

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photosystems

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In the chloroplast thylakoidmembrane, 200-300 chlorophyll molecules and accessory pigments are organized byassociated proteins to form large complexes called photosystems. Most pigment molecules inphotosystems serve as light-gathering antenna pigments that guide energy toward a central reaction center.

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antenna pigments

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When antenna pigments absorb photons, the energy-but not the electron itself-is passed to anearby pigment molecule, where another electron is excited in response. This phenomenon isknown as resonance energy transfer.

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resonance energy transfer

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Resonance energy transfer is possible only between pigments that are able to absorb differentwavelengths of photons-from those absorbing higher-energy photons to those absorbinglower-energy photons. Proteins organize and tune the absorption potential of antenna pigmentsso that resonance energy is efficiently moved between pigments, as the potential energy dropsat each step.Once the energy is transferred, the original excited electron falls back to its ground state. In thisway, energy is transferred inside the photosystem in a manner that may be likened to thetransfer of sound between tuning forks, or excitement between fans at a sports event during the“wave.” But unlike the stadium wave, most of this resonance energy is directed to a particularlocation in a photosystem, called the reaction center.

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The Reaction Center

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When a photon or resonance energy from an antenna pigment reaches the reaction center, theenergy is absorbed by one of two specialized chlorophyll molecules (together called the specialpair). When this pigment is energized, an excited electron is transferred from the special pairpigment to an electron acceptor. As the acceptor becomes reduced, its potential energyincreases. This is a key step in the transformation of light energy: Electromagnetic energy fromsunlight has now been transformed to chemical energy.

Note that in the absence of light, the electron acceptor does not accept electrons. It remains inan oxidized state because the redox reaction that transfers an electron to the electron acceptoris endergonic. But when light excites electrons in chlorophyll to a high-energy state, the reactionbecomes exergonic. In this way, the energy in light transforms an endergonic reaction to an exergonic one.

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s

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two photosystem hypothesis

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According to the two-photosystem hypothesis, the enhancementeffect occurs because photosynthesis is much more efficient when both photosystems operatetogether.

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How Does Photosystem II work

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the action often begins when a mobile accessory structure called the light harvesting complex transmits resonsnace energy to an antenna pigment inside the photosystem. From there, resonsnace is relayed by other antenna pigments to transfer energy to the special pair pigments in the reaction center. At this point another type of pigment molecule called pheophytin comes into play.

Structurally pheophytin is identical to chlorophyl except that pheophytin lacks a magnesium atom in its head region.

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pheophytin

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Unlike other pigments, pheophytin does not become excited by photons or resonance energy-itaccepts excited electrons from the reaction center special pair chlorophylls. The redox reactionbetween pheophytin and special pair pigments is the key step in transforming light energy intochemical energy.Immediately after an excited electron is transferred to pheophytin, however, the oxidizedreaction center special pair pigment becomes an incredibly strong electron acceptor. Whatprevents the electron from being pulled back from pheophytin to the oxidized special pairpigment? The answer is that the electron is quickly shuttled away from the reaction center by aquinone molecule to an electron transport chain (ETC)

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When the energy from a single photon excites a special pair chlorophyll in the reaction center ofphotosystem II, the electron is accepted by pheophytin, transferred to a quinone molecule(identified as PQ), and then stepped down in energy along an electron transport chain.

In both structure and function, the thylakoid ETC is similar to components in the mitochondrialETC (Ch. 9, Section 9.5).● Structurally, the photosystem II and mitochondrial ETCs both contain quinones andcytochromes.● Functionally, the redox reactions that occur in both ETCs result in protons being activelytransported from one side of an internal membrane to the other. The resultingproton-motive force drives ATP production via ATP synthase

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The ETC

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Two photons excite two electrons to reduce one plastoquinone (PQ), which carries the electronsfrom photosystem 2 along with protons from the stroma. The cytochrome complex oxidizesplastoquinone, releasing the protons into the thylakoid lumen that drive ATP synthesis.When PQ is reduced by picking up two electrons from photosystem II, it carries them throughthe membrane to the lumen side of the thylakoid and delivers them to molecules with a higherredox potential in the cytochrome complex. In this way, PQ shuttles electrons from photosystemII to the cytochrome complex much like ubiquinone shuttled electrons between complexes I or IIand complex III in mitochondria.Similar to the mitochondrial ubiquinone, PQ also picks up protons when it becomes reduced.These protons are pulled from the chloroplast stroma. After being oxidized by the cytochromecomplex, PQ drops off the protons in the thylakoid lumen.The protons transported by PQ result in a high concentration of protons in the thylakoid lumen.The pH in the thylakoid reaches 5 while the pH of the stroma hovers around 8. Because the pHscale is logarithmic (see BioSkills 5), the difference of 3 units means that the concentration ofH+ is 10 x 10 X 10 = 1000 times higher in the lumen than in the stroma. In addition, the stromabecomes negatively charged relative to the thylakoid lumen.The net effect of electron transport, then, is a large proton electrochemical gradient. Thisgradient results in a proton-motive force that, in turn, drives H+ out of the thylakoid lumen andinto the stroma. Proton flow down the electrochemical gradient is an exergonic process that iscoupled to the endergonic synthesis of ATP from ADP and Pi. The stream of protons flowsthrough ATP synthase, causing conformational changes in the enzyme that drive production ofATP.

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photophosphorylation

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Since the synthesis of ATP in chloroplasts is initiated by the energy from light, it is calledphotophosphorylation. Although photophosphorylation is similar to the oxidativephosphorylation that occurs in plant and animal mitochondria, there is a key difference in howthis ATP is used. In mitochondria, ATP is exported and fuels many different cellular processes.In chloroplasts, however, the AT P remains within the organelle and is used for the production ofcarbohydrate.

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photosystem II obtains electrons by oxidising water

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As it turns out, the light energy harvested by photosystem II is responsible for splitting water.When excited electrons are removed from the photosystem II reaction center pigments, theredox potential of the oxidized pigments becomes so strong that enzymes can pull electronsfrom water, pass them to the pigments, and release protons and oxygen.

Photosynthetic organisms thatoxidize water will generate oxygen (02) as a by-product, and thus perform oxygenic; (“oxygenproducing”) photosynthesis.

Other organisms that have only a single photosystem do not oxidize water, and thus do notproduce 02. Instead, these organisms use different electron donors, such as H2S in the purplesulfur bacteria, to perform anoxygenic (“no oxygen-producing”) photosynthesis.

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photosystem I

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heliobacteria have only one photosystem that uses the energy insunlight to promote electrons to a high-energy state. But instead of being passed to an electrontransport chain that pumps protons across a membrane, the excited electrons in heliobacteriaare used to reduce NAD+. When NAD+ gains two electrons and a proton, NADH is produced.

In cyanobacteria and the chloroplasts of eukaryotes, a similar set of light-capturing reactionsreduces a phosphorylated version of NAD+, symbolized NADP+, to yield NADPH. Both NADHand NADPH function as strong reducing agents-that is, because they have beenreduced, they become carriers of electrons that can readily be transferred to reduce othermolecules.

When two photons excite pigments in the reaction center of photosystem I, the excited electrons leave the chlorophyll molecules and pass through a series of iron-sulfur containing proteins until they are accepted by ferredoxin. In an enzyme catalyzed reaction, the two electrons are transferred from ferredoxin to NADP+ to produce NADPH.

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photosystem I steps

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1) Antenna pigments absorb photons and pass the energy to the photosystem I reaction center

2) Two electrons ( one for each photon) are excited in reaction center chlorophyll molecules.

3) The reaction center pigments are oxidized and the excited electrons are passed through a series of carriers inside the photosystem, then to a molecule called ferredoxin, and then the enzyme called NADP+ reductase.

4) NADP+ reductase transfers the two electrons and a proton to reduce NADP+ and form NADPH.

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photosystems I & II

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Electrons from photosystem I are used to produce NADPH, which is a reducing agent similiar in function to the NADH and FADH2 produced by the citric acid cycle.

Electrons from photosystem II in contrast are used to produce a proton motive foece that drives the synthesis of ATP. The ATP and the reducing power of NADPH, will ultimately be used in the manufacture of sugar.

In combination, photosystems I & II produce chemcial energy stored in ATP and NADPH.

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Phostosystems I & II Work together

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The process starts when photons excite electrons in photosystem IIs atenna pigments. When the energy in the excited electrons is transferred to the reaction center, a special pair of chlorophyll molecules, ech called P680, passes excited electrons to pheophytin.

Electrons are gradually stepped down in potential energy through redox reactions among a series of quinones and cytochromes. Each reduced plastoquinone (PQ) picks up protons from the stroma and transfer them to the lumen after being oxidized by the cytochrome complex. ATP synthase uses the resulting proton motive force to phosphorylate ADP, creating ATP.

When electrons reach the end of the cytochrome complex, they are passed to a small diffusable protein called plastocyanin (PC). Each reduced PC diffuses through the lumen of the thylakoid and donates one electron to an oxidized reaction center pigment in photosystem 1.

Plastocyanin is critical it forms a phsycal link between the ETC following photosytem II and photosystem I.

The flow of electrons between photosystems, by means of plastocyanin is important because it replaces electrons that are carried away from the special paiur of pigments in the photosystem I reaction center. These special pair chlorophyll molecules are called P700.

The electrons that flow into P700 are eventually excited and transferred to the protein ferredoxin, which passes electrons to the enzyme that catalyzes reduction of NADP+ to NADPH. For each O2 produced by photosystem II, four electrons have been transferred along the Z-Scheme to make two molecules of NADPH.

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oxygenic photosynthesis and the evolution of earth

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Since ozone O3 is formed from O2 gas, a protective layer of ozone could have arisen in our atmosphere only after the evolution of ogygenic photosynthesis. Without the ozone layer, Earths surface would have been bombarded continually by the searing intensity of ultraviolet radiation, making the evolution of life on land nearly impossible.

As oxygen became more abundant, bacterial cells that evolved the abilityu to use it as an elecron acceptor via cellular respiration flourished. O2 is so electronegative that it creates a huge potential energy drop for the electron transport chains involved in cellular respiration. As a result, organisms that use O2 as an electron acceptor in cellular respiration can produce ATP more efficinetly than can organisms that use other electron acceptors.

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CO2 gets into photosynthesizing tissues

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through specialized pores stoma ( stomata). Specialized pores bordered by two distinctively shaped cells called guard cells.

An open stoma allows CO2 from the atomosphere to diffuse into uncoated air filled spaced inside the leaf and excess O2 to diffuse out.

Eventually the CO2 diffuses along a concentration gradient into the chloroplasts of photosynthesizing cells. A strong concentration gradient favoring entry of CO2 is maintained by the Calvin cycle, which constantly uses up the CO2 in chloroplasts.

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carbon fixation

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carbon fixation is the addition of carbonb atoms from inorganic carbon dioxide to an organic compound. The process converts CO2 gas to a biologically useful form.

Once carbon atoms are added to an organic compound, they can be used as sources of energy and as building blocks to construct the molecules found in cells.

Carbon fixation is a redox reaction, the carbon atom in CO2 is reduced by attaching it to another carbon.

The calvin cycle fixes CO2

3PGA is the first product of carbon fixation.

ribulose bisphosphate RuBP is the initial reactant.

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All photosynetheitc organisms that use the calvin cycle to fix carbon require the CO2 fixing enzyme rubisco

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rubisco enzyme is roughly cube shaped and consists of 16 polypeptides that form eight active sites where CO2 is fixed.

Some of these polypeptide subunits are amde in the chloroplast while others are made in the cytoplasm and then imported into the organelle.

Rubisco is thought to be the most abundant enzyme on earth.

Rubisco will catalyze the addition of either O2 or CO2 to RuBP.

Oxygen and carbon dioxide compete at the enzymes active sites which slows the rate of CO2 reduction.

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photorespiration

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One of the molecules produced from the addition of oxygen to RuBP, 2-phosphoglycolate, is processed in reactions that require ATP and release CO2, regenerating 3PGA.

Part of this pathway occurs in chloroplasts, and part occurs in peroxisomes and mitochondria. The reaction sequence resembles respiration, because it consumes oxygen and produces carbon dioxide.

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Rubisco Can React with CO2 or O2

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Because photorespiration requires energy and releases fixed CO2, it "undoes" photosynthesis.

When photorespiration occurs, the overall rate of CO2 fixation declines. This does not mean that the plant does not benefit, however. Some of the products from photorespiration are known to be involved in plant signaling and development.

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The C4 Pathway

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Instead of creating a three carbon molecule as in the calvin cycle, some plant species were able to fix CO2 to produce four carbon molecules.

C4 plants actuallly fix carbon dioxide using both pathways. (to PEP carboxylase in the c4 pathway and to RuBP by rubsico.

PEP carboxylase is common in mesophyll cells near the surface of leaves, while rubisco is found in bundle sheath cells that surround the vascular tissue in the interior of the leaf. d

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The reactions that produce sugar from carbon dioxide

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depend on the ATP and NADPH produced by the light capturing reactions.

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The calvin cycle is a three step process

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All three phases of the calvin cycle take place in the stroma of chloroplasts

The number of reactants and products resulting form three turins of the cycle are shown. Of the six G3Ps that are generated during the reduction phase, one is used in the synthesis of other molecules sushc as glucose, and the other five are used to regenerate RuBP. The three RuBPs that are regenerated participate in fixation reactions for additional turns of the cycle.

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The Fixation Phase

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The Calvin cycle begins when CO2 reacts with RuBP. This phase fixes carbon and produces two molecules of 3PGA, wich is a phosphorylated three carbon organic acid.

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Reduction Phase

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the 3PGA is phosphorylated by ATP and then reduced by accepting electrons form NADPH as the phosphate is removed. The product is the phosphorylated three carbon sugar glyceraldehyde-3-phosphate G3P. Some of the G3P that is synthesized is drawn off to produce other organic molecules, like the six carbon sugar glucose.

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Regeneration Phase

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The rest of the G3P keeps the cycle going by serving as the substrate for the third phase in the cycle: reactions that use additional ATP in the regeneration of RuBP.

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One turn of the calvin cycle

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fixes one molecule of CO2.

Three turns of the cycle fix three molecules of CO2, yielding one molecule of G3P and three fully regenerated RuBP. Of the six G3Ps that are generated during the reduction phase, one is used in the synthesis of other molecules, such as glucose, and the other five are used to regenerate RuBP. The three RuBPs that are regenerated participate in fixation reaction for additional turns of the cycle.

Each mole of CO2 requires the energy from 3 moles of ATP and 2 moles of NADPH to fix it and reduce it to sugar.

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Regulation of Photosynthesis

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the presence of light triggers the production of proteins required for photosynthesis.

When sugar supplies are high, the production of proteins required for photosynthesis is inhibited, but the production of proteins required to process and store sugars is stimulated.

Rubisco is activated by regulatory molecules that are produced when light is available, but inhibited in conditions of low CO2 availibility when photorespiration is favored.

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What happens to the sugar that is produced by photosynthesis

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The products of the calvin cycle enter one of several reaction pathways that result in the production of every organic molecule in the photosynthetic organism. The most importatnt of these reaction sequences uses G3P to produce the monosaccharide glucose, a process called gluconeogenesis.

This glucose is often combined with fructose, which is also made form G3P, to form the disacchardie sucrose.

When photosynthesis is taking place slowly, almost all the G3P that is produced is used to make sucrose. Sucrose is water soluble and readily transported to other parts of the plant. If sucrose is delivered to rapidly growing parts of the plant it is broken down to fuel cellular respiration and growth.

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An alternative pathway

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An alternative pathway occurs when photosynthesis is proceeding rapidly and sucrose is abundant. Under these conditions, the glucose molecules are polymerized to form starch, which is stored in the cells of leaves and roots. Starch production occurs inside the chloroplast; sucrose synthesis takes place in the cytosol.

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Starch acts a temporary sugar storage product

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In photosynthesizing cells, starch acts as a temporary sugar storage product. At night, the starch that is stored in leaf cells is borken down to glucose molecules. The glucose is then fed into cellular respiration or used to manufacture sucrose for transport to other parts of the plant. In this way chloroplasts provide sugars for cells througout the plant by day and by night.

Virtually every carbon present in organic molecules, and the energy stored within their bonds, can be traced back to photosynthesis.

Photosynthesis is the staff of life.

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The plasma membrane

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Recall that the structure of this membrane consists of a phospholipid bilayer studded with membrane proteins. These proteins are integral, meaning embedded in the bilayer, or peripheral, meaning attached to one surface. Some membrane proteins regulate the transport of substances as part of the primary function of the plasma membrane: to create an environment insise the cell that is different from conditions outside.

The plasma membrane does not exist in isolation, however. Many membrane proteins attach to cytoskeletal elements on the interior surface of the bilayer or to a complex array of extracellular structures, including those attached to the membrnaes of neighboring cells.

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The structure and function of the extracellular layer

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Most cells secrete products that are assembled into a layer or wall just beyond the membrane. The extracellular material helps define the cells shape and either attaches it to other cells or acts as first line of defense against the outside world.

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The structure of cell walls surrounding prokaryotic cells is remarkably different between bacteria and archaea.

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In bacteria, cell walls mostly consist of pilymers of the polysacharide peptidoglycan that are connected to one another by peptide bonds.

Archaea do not share any unifying characteristics in thier cell walls apart from the abscence of peptidoglycan. Often the cell walls of these organsism are formed as a dense coat of proteins on the surface of the cell called an S-layer.

Virtually all types of extracellular layers in eukaryotes from the cell walls of algae, fungi, and plants to the extracellular material that surrounds most animal cells have the same fundemental organization.

They are fiber composites: they consist of a cross linked netowkr of long filaments embedded in a stiff surrounding material called the ground substance.

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Rods and Filaments

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The rods and filaments in a fiber composite are extremely effective at withstanding stretching ans straining forces, or tension. The filaments in the extracellular material of most cells are functionally similar to the steel rodes in reinforced concrete, they resist being pulled or pushed lengthwise.

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Ground substance

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The stiff ground substance is effective at withstanding pressing forces, called compression. Concrete performs this function in highways, and a gel forming mixture of polysaccharides plays the same role in extracellular material.

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The Cell Wall in Plants

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Virtually all plant cells are surrounded by a cell wall a fiber composite that is the basis of major industries.

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Primary Cell Walls

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when plant cells first form, they secrete an initial fiber composite called a primary cell wall

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fibrous component

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the fibrous component of the primary cell wall consists of long strands of the polysaccharide cellulose. These strands are bundled into stout structures termed microfibrils, which are cross linked via hydrogen bonds to other polysaccharide filaments. The microfibrils are synthesized directly into the extracellular space by a complex of enzymes in the plasma membrane, whre they form a crisscrossed network

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polysaccharide component

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The spaces between microfibrils are filled with gelatinous polysacchardies such as pectins, the molecules that are used to thicken jams and jellies. Because these polysaccharides are hydrophillic, they attract and large amounts of water, keeping the cell wall moist. The gelatinous components of the cell wall are synthesized in the rough endoplasmic reticulum (ER) and Golgi apparatus and secreted into the extracellular space.

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primary cell wall

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The primary cell wall helps shape a plant cell. Under normal conditions, the concentrations of solutes is higher inside the cell than outside, causing water to enter the cell via osmosis. The incoing water increases the cells volume, pushing the plasma membrane up against the wall. The force exerted by the cell aginst the wall is known as turgor pressure.

Although plant cells exert turgor pressure throughout their lives, it is particularly important in young cells that are actively growing. Young plant cells secrete proteins named expansins into their cell wall. Expansins disrupt the hydrogen bonds that cross link microfibrils to other polymers in the wall, loosening the structure and allowing the microfibrils to slide past one another. Turgor pressure then forces the wall to elongate and expand, allowing for cell growth.

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Secondary Cell Walls

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As plant cells mature and stop growing, they may secrete an additional layer of material, a secondary cell wall, between the plasma membrane and the primary cell wall.

The makeup of the secondary cell wall varies from cell to cell in the plant and correlates with each cells function. Cells on the surface of a leaf have secnondary cell wall containing waxes that form a waterproof coating; cells that support a plants stem have stiff secondary cell walls the ctonatin a great deal of cellulose.

In cells that form wood, the secondary cell wall also contains lignin a complex polymer that forms an exceptionally rigid network. Thick secondary cell walls of cellulose and lignin help woody plants withstand the forces of gravity and wind.

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The Extracellular Matrix

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Most animal cells secrete a fiber composite called the extracellular matrix, or simply ECM. Like the extracellular materials found in other organisms, the ECM provides structural support.

ECM organization follows the same principles observed in the cell walls of algae, fungi, and plants. Ther is a key difference, however: The animal ECM contains much more protein relative to carbohydrate than does a cell wall.

The fibrous component of animal ECM is dominated by glycoproteins named collagen. About a quarter to a third of all the protein in your body is collagen.

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The extracellular matrix of animals is a fiber composite

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although several types of fibrous proteins are found in the ECM, the most abundant is collagen. After collagen is secreted from the cell, the triple helix proteins can assemble into fibrils and even larger cable like fibers

The spaces between collagnes are filled with a ground substance consisting of proteoglycans. Each individual proteoglycan consists of a core protein attached to many polysaccharides. In some tissues proteoglycans are assembled into even larger complexes

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ECM Proteins

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Most ECM proteins are synthesized in the rough ER, processed in the Golgi apparatus, and secreted from the cell via exocytosis. After secretion, the individual proteins may assemble into large structures. For example, groups of collagen triple helixes may coalesce to form collagen fibrils, and bundles of fibrils may link to form even larger fibrous complexes.

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Ground substance of the ECM

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The ground substance that surrounds collagen and other fibrous components of the EMC contains highly glycosylated, gel-forming proteins called proteoglycans. In addition, secreted proteoglycans may be attached to long polysaccharides synthesized by cellular enzymes in the extracellular space. The resulting huge complexes, such as the one shown in the photo are responsible for the rubber like consitency of cartilage.

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Composition of the ECM varies among tissue types

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the ECM surrounding lungs contains large amounts of a rubber like protein called elastin, which allows the ECM to expand and contract when you breathe. The structure of a tissues ECM correlates with the function of the tissue.

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ECM proteins support cell structure via their attachements to the cell surface.

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membrane proteins called integrins bind to extracellular cross linking proteins, including laminins, which in turn bind to other components of the ECM

The intracellular portions of the integrins bind to proteins that are connected tot he cytoskeleton, effectively linking the cytoskeleton and ECM. This linkage is critical. Besides keeping individual cells in place, it helps adjacent cells adhere to each other via their common connection to the ECM.

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Signalling pathways monitor the cytoskeleton-ECM linkage

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Cells monitor the cytoskeleton-ECM linkage via signaling pathways. When integrins bind to the ECM, they transmit signals that inform the cell it is in the right place and properly anchored. If this linkage breakes down, the signals are not transmitted and cells normally die as a result.

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Cell to Cell Attachment

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Materials and structures that bind cells together are particularly importnat in epithelia, tissues that form external and internal surfaces. Epithelia function as barriers between the external and internal environments of plants and animals. In animals, epithlia also serve as gatekeepers that regulate the transport of substances, such as the absorption of water and nutrients across the epithelia of the intestines.

The adhesive structures that hold cells together vary among organisms

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Indirect Cell to Cell Attachments

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The extracellular space between the walls of adjacent plant cells sandwhich a central, the middle lamella, which conisits primarily of gelatinous pectins. Because the lamella is continuous with the primary cell walls of adjacent cells, it serves to glue them together. The two cell walls are like slices of bread, andthe middle lamella is like a layer of peanut butter. If the enzymes degrade the middle lamella, as they do when flower petals and leaves detach and fall, the adjacent cells separate.

In animals integrins in the plasma membranes of cells will form connections between their cytoskeletal structures and the extracellular matrix. By interacting with the same network of ECM components, multiple cells both within and between different tissues through the ECM are particularly important in reinforcing these interactions.

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Direct Cell to Cell Attachments

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In contrast to such indirect intercellular connections, in animals, where cell walls do not exist, a varitey of membrane proteins allow for direct cell to cell attachment in epithelia and other tissues.

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Tight Junctions

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Tight junctions form waterproof seals. A tight junctions is a cell to cell attachment composed of specialized proteins in the plasma membranes of adjacent animal cells. As the drawing in FIgure 11.8b indicates, long chains of these proteins form on the surface of a cell and attach to the same proteins on adjacent cells. The tight interactions between these proteins will pull the membranes of the two cells very close together. The resulting structure resemble a quilt, wher the proteins "stitch" the membranes of two cells together. In cells, the structure forms a water tight seal that prevent solutions from flowing through the space between two cells.

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Desmosomes

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A strong cell to cell attachment particularly common in animal epithelial cells and certain muscle cells. In their structure and function, desmosomes are analogous to the rivets that hold pieces of sheet metal together.

Desmosomes comprise linking proteins and cytosolic anchoring proteins. The linking proteins span the membrane and directly connect adjacent cells and thier anchoring proteins located on the inner faces of each cell membrane. Cytoskeletal intermediate filaments help reinforce desmosomes by attaching to the intracellular anchoring proteins. In this way desmosomes help form a continuous structural support system between all the cells in the tissue.

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selective adhesion

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adhering to other cells of the same tissue type

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Cell to Cell Gaps

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In both plants and animals, direct connections between cells in the same tissue help the cells to work in a coordinated fashion. One way of accomplishing this is to have channels in the membranes of adjacent cells, allowing the cells to communicate via the diffusion of cytosolic ions and small molecules from cell to cell.

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Cell to Cell signaling

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Ions and small molecules are just two of many different forms of signals that convey information between cells. How cells respond to this exchange of information depends on the type of cell and the type of signal, but there are two general mechanisms

1) Signals may regulate gene expression, altering which proteins are produced and wich are not

2) signals may activate or inactivate particular proteins that already exist in the cell, often those involved in metabolsim, membrane transport, secretion, and the cytoskeleton.

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Gap Junctions Connect Animal Cells via Protein Channels

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In many animal tissues, structures called gap junctions connect adjacent cells. In a gap junction, specialized proteins assemble in the membrnaes of adjacent cells, creating interconnected channels that allow water, ions, and small molecules such as amino acids, sugars, and nucleotides to move between the cells.

Gap junctions are communication portals. They can help adjacent cells coordinate their activities by allowing the rapid passage of regulatory ions or small molecules. In the muscle cells of your heart, for example, a flow of ions through gap junction acts as a signal that coordinates contractions.

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Plasmodesmata Connect Plant Cells via Membrane Lined Channels

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In plants, gaps through cell walls allow direct connections between the cytoplasm of adjacent cells. At these connection, named plasmodesmata, the plasma membrane and cytoplasm of the two cells are continuous. Tubular extenstion from the smooth ER run through these membrane lined channels.

Like gap junctions, plasmodesmata are communication portals through the plasma membrnae. In plants, the plasma membrane separates most tissues into tow independent corridors

1) the symplast which is a continuous network of cytoplasm connected by plasmodesmata

2) the apoplast which is the region outside the plasma membrane. The apoplast consists of cell walls, the middle lamella, and air spaces. Small molecuels can move through plant tissues in either of these compartments without ever crossing a membrane.

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Cell to Cell Signaling in Multicellular organisms

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Biologists have classified many types of signaling molecules that keep distant tissues in touch. One, type, neurotransmitters, may open or close ion channels in the plasma membrane of distant cells, changing the electrical properties of the membrane. This type of signal is responsible for the transmission of information through the nervous system allowing you brain to control the movements of the rest of your body.

Hormones are information carrying molecules that are secreted by plant and animal cells into bodily fluids and act on distant target cells. Hormones are usually small molecules and inlcude certain peptides, steriods, and even gases. Although hormones are typically present in minute concentrations, they have a large impact on the activity of target cells.

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Signal Reception

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Hormones and other types of cell to cell signaling molecules deliver their message by binding to receptor molecules. The key characteristic of this interaction is that it changes the shape, or conformation of the receptor. A signal receptor, then, is a protein that changes its shape and activity after binding to a signaling molecule. This change in shape is how a message is passed from the signaling molecule to its receptor.

Most lipid soluble signaling molecules can diffuse across the hydrohobic region of the membrane and enter the cytosol of their target cells. The receptors for these molecules exist inside the cell.

Large or hydrophillic signaling molecules are lipid insoluble, and most cannot cross the plasma membrane. To affect a target cell, the have to be recognized at the cell surface. Their receptors are usually located in the plasma membrane.

Receptors are dynamic. The number of receptors in a particular cell may decline if hormonal stimulation occurs at high levels over a long time. The ability of a receptor to bind tightly to a signaling molecule may also decline in response to intensive stimulation. As a result , the sensitivity of a cell to a particular hormone may change over time.

Receptors can be blocked. Many drugs are used to block the interaction between hormones and their receptors. For example, certain beta blocker drugs will prevent adrenaline from binding to its receptor on heart cells.

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Signal Processing

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Once a cell receives a signal , it has to process the signal to initiate a response. This step happens in one of two ways depending on whether the receptors are located in the cytosol or at the membrane surface.

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Processing Lipid Soluble Signaling Molecules

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Steriod hormones such as estrogens and cortisol are examples of lipid soluble signaling molecules. Because they are hydrophobic, most lipid soluble signaling molecules must be carried through the bloodstream by hydrophilic proteins. After reaching their target cells, these signaling molecules are released from the carrier proteins, diffuse through the plasma membrane, and enter the cytosol. Often a hormone receptor complex is formed in the cytosol and then transported to the nucleus, where it triggers changes in gene expression. By altering the expression of genes the cell produces different proteins that will directly affect the function or shapre of the cell.

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Processing Lipid Insoluble Signaling Molecules

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Hormones that cannot diffuse across the plasma membrane and enter the cytosol do not directly participate in intracellular activities, like changin gene expression. Instead, the signal that arrives at the surface of the cell has to produce an intracellular signal, the processing step is indirect.

When a signaling molecule binds at the cell surface, it triggers signal transduction, the conversion of a signal from one form to another. A long and often complex series of events ensues, collectively called a signal transduction pathway.

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signal transduction pathways

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In a cell, signal transduction converts an extracellular signal to an intracellular signal.

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Signal amplification

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When a hormone arrives at the cell surface, the message it transmits may be amplified as the signal changes form. An increased number of intracellular signals also makes it possible for hormones to affect different molecules in the cell.

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Signal Transduction

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In cells, signal transduction begins at the plasma membrane; amplification and diversification of the signal takes place inside the cell. This may occur in a variety of ways, depending on the mechanism of signal transduction. In general, the arrival of a single signaling molecule results in a secondary signal that involves many ions or molecules that can affect several different cellular activities.

For example, when liver cells are stimulated by adrenaline to release glucose into the bloodstream, the signal is amplified by the production of numerous small molecules called second messengers. The signal then diversifies to activate enzymes that break down glycogen into glucose, inhibit enzymes that synthesize glycogen, and produce new enzymes that make glucose.

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Two major types of signal transduction systems that are distinguished based on how they are initiated.

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1) G-protein-coupled receptors initiate the production of intracellular second messengers, which then amplify and diversify the signal.

2) Enzyme-linked receptors activate a series of proteins inside the cell, through the addition of phosphate groups. The number and type of proteins activated lead to the amplification and diversification of the signal.

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Signal Transduction via G-Protein Coupled Receptors

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Many signal receptors span the plasma membrane and are closely associated with peripheral proteins inside the cell called G Proteins. When G proteins are activated by a signal receptor, they often trigger a key step in signal transduction: the production of a second messanger, a small non protein signaling molecule or ion that elicits an intracellular response to the first messenger.

G proteins link the receipt of an extracellular signal to the production of an intracellular signal.

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G proteins

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got their name because their activity is regulated by the type of nucleotide they are bound to: either

guanosine triphosphate (GTP)

or

guanosine diphosphate (GDP)

G proteins are activated when they bind GTP; they are inactivated when a phosphate group (negative charge) is removed from GTP to form GDP.

The G protein will remain inactive until the GDP is replaced with a new GTP.

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GTP

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GTP is a nucleoside triphosphate that is similar in structure to adenosine triphosphate (ATP)

Nucleoside triphosphates have high potential energy becuase their three phosphate groups have four negative charges close together.

When GTP bind to a G protein, the addition of the negative charges alters the proteins shape. Changes in shapre produce changes in activity.

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How do G protein coupled receptors work?

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STEP 1: A signaling molecule arrives and binds to a receptor in the plasma membrane. The receptor is a transmembrane protein whose intracellular portion is coupled to a G protein composed of multiple subunits. The G protein is anchored by a lipid tail to the cytosolic side of the cell membrnae. The lipid anchor permits the G protein to diffuse laterally in the membrane.

STEP2: In response to binding of the signaling molecule, the receptor changes shape and activates its G protein. Specifically, the receptor kicks out the GDP from the inactive G protein, allowing GTP to bind to the protein. When GTP is bound, the G protein will change shape radically: The active GTP binding subunit splits off.

STEP3: The active G protein subunit interacts with a nearby enzyme that is embedded in the plasma membrane. This interaction stimulates the enzyme to catalyze production of a second messenger.

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Second messengers

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Second messengers are effective because they are small and therefore can diffuse rapidly to spread the signal throughout the cell. In addition, they can be produced quickly in large quantities. This characteristic is important. Because the arrival of a single signaling molecule can stimulate the production of many second messengers, the signal transduction event amplifies the original signal.

Several types of small molecules and ions act as second messengers in cells.

Several second messengers activate protein kinases, enzymes that activate or inactivate other proteins by adding a phosphate group to them.

Second messengers arent restricted to a single role, the same second messenger can initiate dramatically different events in the same cell or in different cell types receiving the same signaling molecule.

More than one type of second messenger may be involved in triggering a cells response to the same extracellular signaling molecule.

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Signal Transduction via Enzyme Linked Receptors via Receptor Tyrosine Kinases RTKs

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Enzyme linked receptors transduce hormonal signals by directly catalyzing a reaction inside the cell.

STEP1: A hormone binds to two subunits of an RTK and cuase them to form a dimer.

STEP2: The conformational change in the RTK turns on its catalytic activity, allowing RTK to phosphorylate itself at tyrosine residues using ATP inside the cell.

STEP3: Step 3 Proteins inside the cell bind to the phosphorylated RTK, forming a bridge between the receptor and a lipid anchored peripheral membrane protein called Ras, which is a single subunit G protein. Bridge formation activates Ras by causing it to exchange its bound GDP for a GTP.

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STEP4: When Ras is activated, it triggers the phosphorylation and activation of a protein kinase.

STEP5: The Ras-activated kinase catalyzes the phosphorylation and activation of a second kinase, which then phosphorylates and activates a third kinase. The third kinase triggers the cell response by phosphorylating additional proteins.

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phosphorylation cascade

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The sequence of protein modifications that culminates in a cell response.

Since these cascades are often initiated by mitogens signaling molecules that activate cell division, the three numbered kinases in Figure 11.16 are called mitogen activated protein kinases (MAPKs)

Although the change in MAPK conformation after it is phosphorylated is very subtle, it has a dramatic effect in MAPK catalytic activity.

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In general

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In general, intracellular signals initiated by G-Protein coupled receptors result in the production of second messengers, while enzyme linked receptors, like RTKs, drive phosphorylation cascades. Although sometimes the opposite is true.

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A signal transduction event has two results

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1) It converts an extracellular message into an intracellular message,

2) in some cases it amplifies and diversifes the original message to elicit a large and multifaceted response in the cell.

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Signal Response

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Recall that when adjacent cells share information through cell gaps two general categories of response may occur: a change in gene expression or a change in the activity of proteins that already exist in the cell. The same holds true for responses to messages carried by signaling molecuels.

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Signal Deactivation

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Cells have built in systems for turning off intracellular signals. Although many different mechanisms may be used most signal transduction systems are exquisitely sensitive to small changes in the concentration of signaling molecules or the number and activity of signal receptors. As a result, they trigger a rapid response and can be shut down quickly.

For example once an activated G protein turns on a downstream enzyme, the bound GTP is hydrolyzed by the G protein to GDP and Pi. This reaction changes the G proteins conformation and returns the protein to its inactive state. Activation of its downstream target stops, and production of the second messenger ceases. To produce a high concentration of second messengers, the pool of inactive G proteins must be continuously reactivated by the signal receptor to keep the process going. Otherwise, the signal transduction system quickly shuts down.

Phosphorylation cascades are also sensitive to the continuing presence of external signaling molecules. If stimulation of a receptor tyrosine kinase ends, enzymes called phosphatases will remove the phosphate groups from components of the phosphorylation cascade, causing the dignal transduction to cease.

The presence of second messengers in the cytosol is also short lived. For example, pumps in the membrane of the smooth ER return cytosolic calcium ions to storage in the ER lumen, and enzymes called phosphodiesterases convert active cAMP and cGMP (see Table 11.1) to inactive AMP and GMP, respectively. If the production of second messengers is halted, then they are quickly cleared from the cytosol and the signal transduction stops.

To appreciate what happens when a signal transduction system does not shut down properly, let’s return to the phosphorylation cascade illustrated in Figure 11.16. Recall that Ras is active when it is bound to GTP, but it is deactivated when it hydrolyzes GTP to GDP and Pi. If this hydrolysis activity were defective, however, Ras would remain active and continue stimulating the cascade even when the external signal is no longer present.

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Quorum Sensing

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One environmental factor that is closely monitored by populations of unicellular organisms is the density of the population. The use of signaling pathways to respond to population density in prokaryotic and eukaryotic microbes is referred to as guorum sensing. The name was inspired by the observation that cells of the same species may undergo dramatic changes in activity when their numbers reach a threshold, or quorum. Quorum sensing is based on signaling molecules that are secreted by cells and diffuse through the environment. The response to these molecules depends on the species. In bacteria, quorum sensing is often used to help glue a community of microbes to a surface in a biofilm (Ch. 2_6, Section 26.1), such as the plaque that forms on your teeth. Quorum sensing is also involved in light emission (bioluminescence) by certain bacteria. For example, bacterial species including Vibrio Fischeri are actively cultured in the light organs of the bobtail squid; after reaching a certain density, they express enzymes that catalyze a light-producing reaction (see the chapter 18 Case Study). Quorum sensing allows unicellular organisms to communicate and coordinate their activities. When it occurs, these cells take on some of the characteristics of multicellular organisms. For example, quorum sensing via a G-protein-coupled receptor causes the free-living cells (amoebae) of the slime mold Dictyostelium to aggregate into multicellular mounds (Figure 11.18). Amazingly, the slug-like body that is formed from one of these aggregates can crawl across a surface and eventually organize itself into a fruiting body that releases spores into the air

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cell division

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splitting of preexisting cells

Early studies revealed two fundamentally different ways that nuclei divide before cell division: meiosis and mitosis.

In animals meiosis leads tot he production of sperm and eggs, which ar the male and female reproductive cells termed gametes.

Mitosis leads to the production of all other cell types referred to as somatic cells.

Mitosis and meiosis are usually accompanied by cytokinesis the division of the cytoplasm into two distinct cells. When cytokinesis is complete, a so called parent cell has given rise to two daughter cells.

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The basic steps in cellular replication

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1) copying the DNA

2) separating the copies

3)dividing the cytoplasm to create two complete cells.

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chromsome

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consists of single long DNA double helix that is wrapped around proteins called histones, in a highly organized manner.

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Gene

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A region of DNA in a chromosome that codes for a particular protein or ribonucleic acid RNA

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Before Mitosis

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Each chromosome is replicated. As mitosis starts the chromsomes condense into compact structures that can be moved around the cell efficiently. Then one copy of each chromosome is distributed to each of two daughter cells.

The two chromatids are joined along their entire length by proteins called cohesins. Once mitosis begins, however these connections are removed except for those at a specialized region of the chromsome called the centromere.

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chromatid

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each of the double stranded DNA copies in a replicated chromosome

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sister chromatids

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chromatid copies that remain attached at their centromere

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Cells alternate between M phase and Interphase

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M phase- occurs when cells are in the process of separating their chromosomes.

Interphase- the rest of the time when the cell is not in M phase. The chromsomes uncoil into the extremely long, thin structures shown in Figure 12.1 and longer appear as individual threads. The cell is either grwoing and preparing to divide or fulfilling its specialized function in a multicellular individual. Cells actually spend most of their time in interphase.

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The Cell Cycle

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involves four phases

M phase

interphase consisting of G1, S, and G2 phases

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Gap phases

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In multicellular organisms, cells perform their functional roles mostly during G1 phase. G1 is also the period when the cell "decides" to begin replication and transistions to S phase.

Before mitosis can take place, a cell uses G2 phase to prepare for M phase. The time spent in both G1 and G2 allows the cell to grow and replicate organelles so it will be able to divide into two cells that can function normally.

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M phase

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M phase typically consists of two distinct events: the division of the nucleus and the division of the cytoplasm. Mitosis divides the replicated chromosomes to form two daughter nuclei with indentical chromsomes and genes. Cytokensis usually follows mitosis and divides the cytoplasm of the parent cell to form two daughter cells.

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chromosomes are replicated during S phase, and the cell then enters G2 phase. During M phase, the replicated chromosomes are partitioned to the two daughter cells. Each duaghter cell contain the same number of chromosomes that the parent cell had.

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Eukaryotic chromosomes

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consist of DNA wrapped around globular histone proteins. This DNA histone complex is called chromatin.

During interphase, the chromatin of each chromosome is in relaxed or less condensed state.

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G2 phase

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the cell contains replicated chromsomes before mitosis. Each chromosome now consists of two sister chromatids. Each chromatid contains one long DNA double helix and sister chromatids represent exact copies of the same genetic information.

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Mitosis

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Mitosis begins when chromatin condenses to form a much more compact structure.

During mitosis the two sister chomatids separate to form independent daughter chromosomes. One copy of each chromosome goes to each of the two daughter cells.

Biologists have identified five subphases within mitosis based on distinctive events that occur: prophase, prometaphase, metaphase, anaphase, and telophase.

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Prophase

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Mitosis begins with the events of prophase. when chromosomes condense into compact structures. Individual chromosomes first become visible in the light microscope during prophase.

Prophase is also marked by the formation of the spindle apparatus. The spindle apparatus is structure that produces mechanical forces that

- move replicated chromsomes during early mitosis

-pull chromatids apart in late mitosis

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The spindle apparatus

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consists of microtubules, components of the cytoskeleton.

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microtubules

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are composed of alpha-tubulin and beta-tubulin dimers

they have a plus end and a minus end, meaning they are asymmetric

the plus end isthe site where microtubule growth normally occurs. Microtubule disassembly is more frequent at the minus end

microtubules originate from microtubule organizing centers MTOCs. MTOCs define the two poles of the spindle apparatus and produce large numbers of microtubules, whose plus ends grow outward through the cytoplasm. Although the nature of the MTOC varies among plants, animals, fungi, and other eukaryotic groups, the spindle apparatus has the same function.

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centrosome

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a structure that contains a pair of centrioles

During S phase, the single centrosome replicates along with the DNA. At the start of prophase, the two centrosomes move to opposite sides of the nucleus to begin forming the spindle appartus. Some of these microtubules extend from each spindle pole and overlap with one another ehse are called polar microtubules.

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Prometaphase

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In many eukaryotes, once chromosomes have condensed, the nuclear envelop disintigrates. Removal of the envelope allows the cytoplasmic microtubules to attach to chromosomes at specialized structures called kinetochores. These events definet the start of prometaphase.

Each sister chromatid has its own kinetochore, which is assembled at the centromere. Because the centromere is also the attachement site for chromatids, the result is two kinetochores on opposite sides of each replicated chromsome. The microtubules attached to these structures are called kinetochore microtubules.

Early in prometaphase, kinesin and dynin motors attached to the knetochores "walk" the chromsomes up and down microtubules. This process is similar to the way the same motors transport vesicles and organelles along microtubules. When the chromosomes reach the plus ends of the microtubules, the kinetochore proteins secure their attachment.

Eventually each chromosome will have its two kinetochores attached to microtubules that originate from opposite sides of the spindle apparatus. The chromosomes are then pushed and pulled by microtubules and motor proteins until they reach the middle of the spindle.

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Metaphase

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Once all the chromosomes have migrated to the middle of the spindle, the mitotic cell enters metaphase. At this point, the chromosomes are lined up on an imaginary plane between the two spindle poles called the metaphase plate.

Formation of the spindle apparatus is now complete. The polar microtubules that extend from ech spindle pole overlap in the middle of the cell, thereby forming a pole to pole connection. Each chromosome is held by kinetochore microtubules reaching out from opposite poles and exerting the same amount of tension, or pull. The spindle poles are held in place partly because of astral microtubules that extend from the MTOCs and interact with proteins on the plasma membrane.

The polarized growth and disassembly of the kinetochore microtubules contributes to the alignment of chromosomes at the metaphase plate. The slow disassembly of the minus ends at the MTOCs is balanced by the slow growth of the plus ends at the kinetochores. Because the sister chromatids of each chromosome are connected to opposite poles, a tug of war between the poles begins during metaphase.

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Anaphase

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At the start of anaphase the cohesins that hold sister chromatids together at the centromeres are cleaved by an enzyme. Because the chromatids are under tension, each replicated chromosome is pulled apart, creating two independent daughter chromosomes. By definition, this separation of chromatids instantly double the number of chromosomes in the cell.

Two types of movement occur during anaphase. First, the daughter chromosomes move to opposite poles via the attachemnt of kinetochore proteins to the shrinking kinetochore microtubules. Second the two poles of the spindle are pushed and pulled farther apart. The push comes from motor proteins in overlapping polar microtubules, which force the poles away form each other. The pull comes from different motors on the plasma membrane, which walk along on the astral microtubules and drag the poles to opposite sides of the cell.

The separation of replicated chromosomes to opposite poles is a critical step in mitosis because it ensures that each daughter cell receives the same complement of chromosomes. When anaphase is complete, two complete sets of chromosomes are fully separated, each set identical to taht of the parent cell before chromosome replication.

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Telophase

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During telophase the nuclear envelope re-forms around each set of chromosomes, and the chromosomes begin to condense. Once two independent nuclei have formed mitosis is complete.

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How do Chromsomes move during anaphase

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The exact and equal partitioning of genetic material to the two daughter nuclei is the most fundamental aspect of mitosis. To understand how sister chromatids separate and move to opposite sides of the spindle, biologists have focused on the role of kinetochore microtubules. How do these microtubules pull chromatids apart?

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Mitotic Spindle Forces

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During mitosis, the microtubules originating from the spindle poles are highly dynamic. Rapid growth and disassembly ensures that some of the microtubules will be able to attach to kinetochores with their plus ends. Others Will be stabilized by different proteins in the cytoplasm and become polar or astral microtubules.

These observations suggest two hypotheses for the movement of chromosomes during anaphase. The simpler hypothesis is that kinetochore microtubules stop growing at their plus ends but remain attached to the kinetochores. As the minus ends disassemble at the spindle poles, the chromosomes would be reeled in like hooked fish. An alternative hypothesis is that the chromosomes move along microtubules that are being disassembled at their plus ends at the kinetochores. In this case, each chromosome would be like a yo-yo running up a string into your hand.

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Kinetochores are linked to retreating ends

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The kinetochore is a complex of many proteins that attaches the centromere region of the chromosome to one or more microtubules.

The kinetochore is a complex of many proteins that attaches the centromere region of the chromosome to one or more microtubules.

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cytokinesis Results in Two Daughter Cells

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At this point, the chromosomes have been replicated in S phase and distributed to opposite sides of the spindle via mitosis. Now it’s time to divide the cell into two daughter cells that contain identical copies of each chromosome. If these cells are to survive, however, the parent cell must also ensure that more than just chromosomes make it into each daughter cell. While the cell was in interphase, the cytoplasmic contents, including the organelles, increased in number or volume. During cytokinesis (Figure 12.5 steps 7 and 8), the cytoplasm divides to form two daughter cells, each with its own nucleus and complete set of organelles. In most types of cells, cytokinesis directly follows mitosis. In plant cells, polar microtubules left over from the spindle apparatus help define and organize the region where the new plasma membranes and cell walls will form. Vesicles from the Golgi apparatus carry components for a new cell wall to the middle of the dividing cell. These vesicles are moved along the polar microtubules via motor proteins. In the middle of what was the spindle, the vesicles start to fuse and form a flattened, sac-like structure called the cell plate (Figure 12.8a). The cell plate continues to grow as new vesicles fuse with it. Eventually, the cell plate contacts and fuses with the existing plasma membrane, dividing the cell into two daughter cells.

In animals and many other eukaryotes cytokenesis begins with the formation of a cleavage furrow. the furrow appears when a ring of overlapping actin filaments starts to contract just insidethe plasma membrnae, in the middle of what used to be the spindle. This contraction is caused by myosin motor proteins that bind to the actin filaments and use ATP to slide the filaments past one antoher.

As myosin moves the actin filaments, the ring shrinks and tightens. Because the ring is attached to the inside of the plasma membrane, the contracting ring pulls the membrane with it. As a result, the plasma membrane is drawn inward. Myosin continues to slide the actin filaments past each other, tightening the ring further until the plasma membrane fuses and cell division is complete. Chromosome separation and cytoplasmic division are common requirements for all organisms, not just eukaryotes. What is known about cell division in prokaryotes? Is the process of cell division in your cells similar to that in bacteria?

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Bacterial Cell Replication

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Many bacteria divide using a process called binary fission. Although binary fission does not involve mitosis, recent research has shown that chromosome segregation and cytokinesis in bacteria are stikingly similar to what occurs in the eukaryotic M phase. Protein filments attach to the replicating bacterial chromosomes, then the filaments pull the chromosomes apart.

Once the chromosome copies have been moved to opposite sides of the cell, other filaments attach to the plasma membrane and form a ring between the chromosome copies. A signal from the cell causes the filaments to draw in the membrane, eventually cleaving the parent cell into two genetically identical cells. Having explored What occurs during cell division, let’s focus on how it is controlled in eukaryotes. When does a eukaryotic cell divide, and when does it stop dividing?

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M-phase promoting factor

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the substance that initiates M-phase in oocytes.

Virtually all eukaryotes use a similar MPF to induce M phase. As a result, G2 arrested frog ocytes have been used extensively to measure MPF activity from a variety of sources. For example, when frog oocytes are injected with M-phase cytoplasm from human cells, the oocytes immediatley entered M phase.

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Proteins responsible for MPF activity

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After discovering that MPF caused cells to enter M phase, researchers began looking for proteins that might be responsible for this activity. ONe candidate belonged to a family of proteins called cyclins, based on the cylcic nature of their expression during the cell cycle. The pattern of cyclin expression was similar to MPF its concentration increased just before M phase, and then plummeted after cells divided and reentered interphase. The activity of MPF is dependent on the presence of cyclin protein, suggeseting that cyclin is a required part of MPF.

The activity of MPF is dependent on the presence of cyclin protein, suggeseting that cyclin is a required part of MPF.

After further research, it was determined that MPF is made up of two distinct polypeptide subunits. One subunit is the cyclin protein, and the other is a protein kinase that is more or less constant in its concentration throughout the cell cycle. Protein kinases are enzymes that catalyze the transfer of a phosphate group from ATP to a target protein. Recall that phosphorylation may activate or inactivate the function of proteins by changing their shape . This observation suggested that MPF phosphorylates proteins that trigger the onset of M phase.

As Figure 12.11 shows the concentration of the cyclin associated with MPF builds during interphase and peaks in M phase. The timing of this increase is important because the protein kinase subunit in MPF is functional only when it is bound to the cyclin subunit. As a result the protein kinase subunit of MPF is called cyclin dependent kinase of CdK.

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MPF Summary

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To summarize, MPF is a dimer consisting of cyclin and a cyclin depedendent kinase. THe cyclin subunite regulates the formation of the MPF dimer; the kinase subunit catalyzes the phosphorylation of other proteins to start M phase.

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How is MPF turned on?

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According to Figure 12.11 the concentration of cyclin builds up steadily during interphase. Why doesnt the resulting increase in the concentration of MPF trigger the onset of M phase earlier in the cell cycle?

The answer is that the activity of MPF's CdK subunit is further regulated by two phosphorylation sites on the subunit. Phosphorylation of one site activates the kinase, but phosphorylation of the second site ihibits the kinase. Both sites are phosphorylated after cyclin binds to the CdK subunit. This allows the concentration of the dimer to increase without prematurely starting M phase. Late in G2 phase, however, a phosphatase removes the inhibitory phosphate. This dephosphorylation reaction, coupled with the addition of the activating phosphate, changes the CdKs shape in a way that turns on its kinase activity.

Once MPF is active it trigger a chain of events. Although the exact mechanisms involved are still under investigation, the result is that chromsomes begin to condense and the spindle apparatus starts to form. In this way, MPF triggers the onset of M phase.

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How is MPF Turned Off

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During anaphase, an enzyme complex begins degrading MPF's cyclin subunit, triggering a chain of events that leads to the deactivation of MPF.

MPF deactivation illustrates two key concepts about regulatory systems in cells.

1) Negative Feedback occurs when a process is slowed or shut down by one of its products. Thermostats shut down furnacses when temperature are high; enzymes in glycolysis are inihibited by ATP; MPF is turned off by an enzyme complex that is activated by events in mitosis.

2) Destroying specific protiens is a common way to control cell processes. In the case of MPF, the enzyme complex that is activated in anaphase attaches small proteins called ubiquitins to MPF's cyclin subunit. This marks the subunit for destruction by a protein complex knwon as proteasome.

In response to MPF activity, then, the concentration of cyclin declines rapidly. It slowly builds up gain during interphase.

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Cell cycle checkpoints can arrest the cell cycle

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MPF is only one of many protein complexes involved in regulating the cell cycle, however. A different cyclin-CdK complex triggers the passage from G1 phase into S phase, and several regulatory molecules hold cells in particular stages.

A cell cycle checkpoint is a critical point in the cell cycle that is regulated.

There are distinct checkpoints in three of the four phases of the cell cycle. In effect, interactions among regulatory molecules at each checkpoint allow a cell to "decide" whether to proceed with division or not. If these regulatory molecules are defective, the checkpoint may fail and cells may start dividing in an uncontrolled fashion.

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G1 checkpoint

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The first cell cycle checkpoint occurs late in G1 phase. For most cells, this checkpoint is the most important in establishing whether the cell will continue through the cycle and divide, or exit the cycle and enter G0. What factors are important in determining whether a cell passes the G1 checkpoint?

Size Becuase a cell must reach a certain size before its daughter cells will be large enough to function normally, biologists hypothesize that some mechanism exists to arrest the cell cycle if the cell is too small.

Availability of nutrients unicellular organisms arrest at the G1 checkpoint if nutrient conditions are poor.

Social signals cells in multicellular organisms pass ( or do not pass) the G1 checkpoint in response to signaling molecules form other cells, which are termed social signals.

Damage to DNA if DNA is physically damaged, the p53 protein activates genes that either stop the cell cycle until the damge can be repaired or cause the cells programmed, controlled destruction, a phenomenon known as apoptosis. In this way, p53 acts as a brake on the cell cycle.

If brake molecules such as p53 are defective, damaged DNA remain unrepaired. Damage in genes that regulate cell growth can lead to uncontrolled cell division. Conseqeuntly regulatory proteins such as p53 are called tumor suppressors.

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G2 checkpoint

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The second checkpoint occurs after S phase, at the boundary between the G2 and M phases. Because MPF is the key signal triggering the onset of M phase, investigators were not surprised to find that it is involved in the G2 checkpoint.

Data suggest that if DNA is damaged or if chromosomes are not replicated correctly, the phophatase that removes the inhibitory phosphate on MPF's CdK subunit is not active. As a result, MPF is not turned on, and cells remain in G2 phase. Cells at the G2 checkpoint may also respond to signals from other cells and to internal signals realtin to cell size.

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M phase Checkpoints

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The final two checkpoints occur during mitosis. The first regulates the transition from metaphase to anaphase. This checkpoint ensures that the sister chromatids do not split until all kinetochores are attached properly to the spindle apparatus. If the metaphase checkpoint does not function properly, chromosomes may not separate correctly, and daughter cells could receive either too many or too few chromosomes. The second Checkpoint regulates the transition from anaphase to telophase. To exit M phase and progress into G1 phase, cells must degrade all of their cyclins and thus turn off MPF activity. The enzymes responsible for degrading cyclins are activated only when all the chromosomes have been properly separated. If chromosomes do not fully separate during anaphase, the remaining MPF activity will prevent the cell from entering telophase and undergoing cytokinesis. If cells are arrested by either of these two checkpoints, they will remain in M phase.

To summarize the four cell cycle checkpoints have the same purpose: They prevent the division of cells that are damaged or that have other problems. The G11 checkpoint also prevents mature cells that are in the G0 state from dividing.

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