Biol 212- Raven's Chap. 7: Photosynthesis, Light, And Life

Front 1

PHOTOSYNTHESIS: A HISTORICAL PERSPECTIVE

Photosynthesis:

Back 1

the route by which virtually all energy enters our biosphere.

Front 2

PHOTOSYNTHESIS: A HISTORICAL PERSPECTIVE

Thylakoids:

Back 2

- where the light-capturing reactions of photosynthesis occur

- chlorophyll and other pigments are embedded.

Front 3

PHOTOSYNTHESIS: A HISTORICAL PERSPECTIVE

What would happen without the flow of energy from the sun?

Back 3

whithout the flow of energy from the sun, channeled largely through the chloroplast of eukaryotic cells, the pace of life on this planet would swiftly diminish and then would virtually cease all together, as dictated by the inexorable second law of thermodynamics.

Front 4

PHOTOSYNTHESIS: A HISTORICAL PERSPECTIVE

What did Van Neil propose was the source of oxygen in photosynthesis?

Back 4

Water

Front 5

THE NATURE OF LIGHT:

What were Sir Isacc Newton's findings of light?

Back 5

was able to separate light into a spectrum of visible colors by passing it through a prism.

- showed that white light actually consists of a number of different colors, ranging from violet at one end of the spectrum to red at the other.

Front 6

THE NATURE OF LIGHT:

What were James Clark Maxwell's findings of light/ and the electromagnetic spectrum?

Back 6

- demonstrated that light is but a small part of a vast continuous spectrum of radiation (the electromagnetic spectrum).

- All the radiations in this spectrum travel in waves which have a characteristic amount of energy associated with it.

Front 7

THE NATURE OF LIGHT:

Wavelength:

Back 7

the distances form the crest of one wave to the crest of the next.

- shorter wavelength... the greater the energy.

Front 8

THE NATURE OF LIGHT [Light Has the Properties of Waves and Particles]:

What is the problem with the wave model of light?

Back 8

the wave model predicts that the brighter the light (stronger/ more intense), the beam... the greater the force with which the electrons will be dislodged from a metal.

- whether or not light can eject the electrons of a particular metal depends only on the wavelength of the light... not on its intensity.

- Increasing the intensity of light, increases the number of electrons dislodged but not the velocity at which they are ejected from the metal. (to increase velocity, must use a shorter wavelength of light).

Front 9

THE NATURE OF LIGHT [Light Has the Properties of Waves and Particles]:

Particle Model:

Back 9

light is composed of particles of energy called photons.

the energy of a photon is inversely proportional to its wavelength-- the longer the wavelength, the lower the energy.

Front 10

THE ROLE OF PIGMENTS:

Pigment:

Back 10

a substance that absorbs light

Front 11

THE ROLE OF PIGMENTS:

chlorophyll:

Back 11

the pigment that makes leaves green, absorbs light principally in the violet and blue wavelengths and also in the red.

because it reflects mainly green light... appears green.

Front 12

THE ROLE OF PIGMENTS:

Excited state:

Back 12

electrons are temporarily boosted to a higher energy level.

Front 13

THE ROLE OF PIGMENTS:

When chlorophyll molecules absorb light... electrons are temp. boosted to a higher energy level... as electrons return to their ground level what three things may occur?

Back 13

1. energy is converted to heat or partly heat and mostly released as "fluorescence" a less energetic photon.

2. energy not the electron my be transferred to a neighboring chlorophyll.

3. the high energy electron itself may be transferred to a neighboring molecule (electron acceptor) that is part of an electron transport chain.

Front 14

THE ROLE OF PIGMENTS:

Resonance energy transfer:

Back 14

energy is transferred from the excited molecule to a neighboring molecule exciting the second molecule and allowing the first nen to return to its ground state.

Front 15

THE ROLE OF PIGMENTS [The Main Photosynthetic Pigments Are the Chlorophylls, the Carotenoids, and the Phycobilins]:

Chlorophyll a:

Back 15

- occurs in all photosynthetic eukaryotes and in cyanobacteria.

- is essential for the oxygen generating photosynthesis carried out by organisms.

Front 16

THE ROLE OF PIGMENTS [The Main Photosynthetic Pigments Are the Chlorophylls, the Carotenoids, and the Phycobilins]:

Chlorophyll b:

Back 16

- are in plants, green algae, euglenoid algae

- is a slightly different absorption spectrum than chlorophyll a.

- is an accessory pigment. When a molecule of chlorophyll b absorbs light, the energy is ultimately transferred to a molecule of chlorophyll a which then transforms it into chemical energy during the course of photosynthesis.

Front 17

THE ROLE OF PIGMENTS [The Main Photosynthetic Pigments Are the Chlorophylls, the Carotenoids, and the Phycobilins]:

Accessory pigment:

Back 17

a pigment that is not directly involved in photosynthetic energy transduction but serves to broaden the range of light that can be used in photosynthesis.

Front 18

THE ROLE OF PIGMENTS [The Main Photosynthetic Pigments Are the Chlorophylls, the Carotenoids, and the Phycobilins]:

Chlorophyll c:

Back 18

takes the place of chlorophyll b in some groups of algae.

Front 19

THE ROLE OF PIGMENTS [The Main Photosynthetic Pigments Are the Chlorophylls, the Carotenoids, and the Phycobilins]:

Carotenoids:

Back 19

- the energy absorbed by accessory pigments must be transferred to chlorophyll a.

- are red, orange, yellow lipid-soluble pigments found in all chloroplasts and in cyanobacteria.

- are assciated with hydrophobic proteins and are embedded in the thylakoid membranes.

- are two groups:
carotenes, xanthophylls

Front 20

THE ROLE OF PIGMENTS [The Main Photosynthetic Pigments Are the Chlorophylls, the Carotenoids, and the Phycobilins]:

beta-carotene:

Back 20

- found in plants

- primary source of vit. A.

- principle function is that of anti-oxidant, preventing oxidative damage to the chlorophyll molecule by light.

Front 21

THE REACTIONS OF PHOTOSYNTHESIS:

an overview of photosynthesis:

Back 21

takes place in two stages: the energy-transduction reactions and the carbon-fixation reactions.

Front 22

THE REACTIONS OF PHOTOSYNTHESIS:

summary of the energy- transduction reaction:

Back 22

light energy absorbed by chlorophyll a molecules in the thylakoid membrane is used indirectly to power the synthesis of ATP. Simultaneously, in the interior of the tylakoid, water is split into oxygen gas and hydrogen atoms. The electrons are ultimately accepted by the NADP+ and H+ producing NADPH.

Front 23

THE REACTIONS OF PHOTOSYNTHESIS:

summary of the carbon-fixation reaction:

Back 23

occurs in the stroma of the chloroplast, sugars are synthesized from carbon dioxide and the hydrogen carried by HADPH. This process is powered by the ATP and NADPH produced in the energy-transduction reactions.

Front 24

THE REACTIONS OF PHOTOSYNTHESIS [Two Photosystems Are Involved in the Light Reactions]:

Over view of photosystem:

Back 24

- each includes an assembly of about 250-400 pigment molecules and consists of two closely linked components: the antenna complex and reaction center.

- light energy absorbed by a pigment molecule anywhere in the antenna complex is transferred from one pigment to another by resonance energy transfer until it reaches the reaction center.

- when either of the two special chlorophyll a molecules of the reaction center absorb energy, one of its electrons is boosted to a higher energy level and transferred to an electron-acceptor molecule to initiate electron flow.
[chlorophyll molecule is therefore oxidized and positively charged]

Front 25

THE REACTIONS OF PHOTOSYNTHESIS [Two Photosystems Are Involved in the Light Reactions]:

antenna complex:

Back 25

consists of pigment molecules that gather light energy and "funnel" it to the reaction center

Front 26

THE REACTIONS OF PHOTOSYNTHESIS [Two Photosystems Are Involved in the Light Reactions]:

antenna pigments:

Back 26

pigments which are part of the light-gathering network located in the antenna complex

Front 27

THE REACTIONS OF PHOTOSYNTHESIS [Two Photosystems Are Involved in the Light Reactions]:

reaction center:

Back 27

made up of a complex of proteins and chlorophyll molecules that enable light energy to be converted into chemical energy.

Front 28

THE REACTIONS OF PHOTOSYNTHESIS [Two Photosystems Are Involved in the Light Reactions]:

Energy transfer during photosynthesis:

Back 28

light energy absorbed by a pigment molecule anywhere in the antenna complex passes by resonance energy transfer from one pigment molecule to another until it reaches one of two special chlorophyll a molecules at the reaction center. When a chlorophyll a molecule at the reaction center absorbs the energy, one of its electrons is boosted to a higher energy level and is transferred to an electron-acceptor molecule

Front 29

THE REACTIONS OF PHOTOSYNTHESIS [Two Photosystems Are Involved in the Light Reactions]:

The Z scheme: over view of the photosystems:

Back 29

shows pathway of electron transfer from water to NADP+. to raise energy of e-'s derived via photolysis form water to energy level required to reduce NADP+ to NADPH, each e- must be energized twice by photons absorbed in PSI & II. the high energy-electorns flow downhill via electron carrier chains. Protons are pumped across the thylakoid membrane into the thylakoid lumen during the water splitting reaction and during electron transfer through complexes producing proton gradient that is central to ATP formation.

Front 30

THE REACTIONS OF PHOTOSYNTHESIS [In the Light Reactions, Electorns Flow from Water to Photosystem II to Phtotsystem I to NADP+]:

Photolysis of water:

Back 30

the light-dependent oxidative splitting of water.

- contributes to the generation of a proton gradient across the thylakoid membrane [ ATP generation]

Front 31

THE REACTIONS OF PHOTOSYNTHESIS [In the Light Reactions, Electorns Flow from Water to Photosystem II to Phtotsystem I to NADP+]:

Chemiosmotic coupling mechanism of photophosphorylation:

Back 31

electrons move from water through PS II, the intermediate chain of electron carriers, PS I, and finally to NADP+. Protons are pumped from the stroma into the thylakoid lumen by the flow of electrons through the chain of carriers between PS II and PS I. Creates an electochemical proton gradient, which drives ATP synthesis as the protons flow down this gradient through an ATP synthase complex in the membrane and reenter the stroma.

Front 32

THE REACTIONS OF PHOTOSYNTHESIS [In the Light Reactions, Electorns Flow from Water to Photosystem II to Phtotsystem I to NADP+]:

Photophosphorylation:

Back 32

light provides the energy used to establish the proton gradient, thus chloroplasts and mitochondria generate ATP.

Front 33

THE REACTIONS OF PHOTOSYNTHESIS [In the Light Reactions, Electorns Flow from Water to Photosystem II to Phtotsystem I to NADP+]:

What must be absorbed to create one oxygen atom?

Back 33

two photons must be absorbed by PS II and two by PS I in order to reduce one molecule of NADP+ to NADPH and to produce one atom of oxygen.

Front 34

THE REACTIONS OF PHOTOSYNTHESIS [In the Light Reactions, Electorns Flow from Water to Photosystem II to Phtotsystem I to NADP+]:

noncylic electron flow:

Back 34

in the light, electron flow continuously from water through PS II and I to NADP+, resulting in the oxidation of water to oxygen (O2) and the reduction of NADP+ to NADPH.

Front 35

THE REACTIONS OF PHOTOSYNTHESIS [Cyclic Photophosphorylation Generates Only ATP]:

Cyclic Electron Flow in PS I:

Back 35

- energized electrons are transferred from P(700) to A(0)... the electrons are shunted to an acceptor in the electron transport chain between both photosystems.

- electrons then pass downhill through that chain back into the reaction center of PS I.
---- driving the transport of protons across the thylakoid membrane and powering the generation of ATP.
[no O2, no NADPH, only ATP]

Front 36

THE CARBON-FIXATION REACTIONS:

overview of carbon-fixation reactions:

Back 36

the ATP and NADPH generated by the light reactions are used to fix and reduce carbon and to synthesize simple sugars.

Front 37

THE CARBON-FIXATION REACTIONS:

How does carbon dioxide reach the photosynthetic cells in plants?

Back 37

through special openings in the leaves and green stems called the stromata.

Front 38

THE CARBON-FIXATION REACTIONS:

Where does the calvin cycle occur?

Back 38

in the stroma of the chloroplast

Front 39

THE CARBON-FIXATION REACTIONS [in the Calvin Cycle, carbon dioxide Is Fixed via a Three-Carbon Pathway]:

What is the starting (ending) compound in the calvin cycle?

Back 39

a 5-carbon sugar with 2 phosphate groups called RuBP.

Front 40

THE CARBON-FIXATION REACTIONS [in the Calvin Cycle, carbon dioxide Is Fixed via a Three-Carbon Pathway]:

What is another name for the calvin cycle?

Back 40

C3 pathway

Front 41

THE CARBON-FIXATION REACTIONS [in the Calvin Cycle, carbon dioxide Is Fixed via a Three-Carbon Pathway]:

What is RuBP carboxylase/oxygenase:

Back 41

- often called rubisco for short

- enzyme that catalyzes the crucial initial reaction of the cavin cycle.

Front 42

THE CARBON-FIXATION REACTIONS [in the Calvin Cycle, carbon dioxide Is Fixed via a Three-Carbon Pathway]:

What are the 3 stages of the calvin cycle:

Back 42

1: Fixation:
- begins when carbon dioxide enters the cycle and is enzymatically combined with RuBP.
- results: 6-carbon compound (unstable enzyme-bound intermediate) which is hydrolyzed to generate two PGA (PGA contains 3-carbon atoms)

2. Reduction:
PGA is reduced to PGAL
[occurs in 2 steps]
- PGA kinase catalyzes the transfer of phosphate from ATP to PGA
- NADPH donates electrons into a reduction catalyzed PGAL denhydrogenase producing PGAL.

3. Regeneration of acceptor:
5 of the 6 molecules of PGAL are used to generate 3 molecules of RuBP.

Front 43

THE CARBON-FIXATION REACTIONS [in the Calvin Cycle, carbon dioxide Is Fixed via a Three-Carbon Pathway]:

What is the overall equation of the calvin cycle:

Back 43

Front 44

THE CARBON-FIXATION REACTIONS [in the Calvin Cycle, carbon dioxide Is Fixed via a Three-Carbon Pathway]:

What is the immediate product of the cavlin cycle?

Back 44

Clyceraldehyde 3-Phosphate (PGAL)

[the primary molecule transported form the chlroplast to the cytosol of the cell]

Front 45

THE CARBON-FIXATION REACTIONS [in the Calvin Cycle, carbon dioxide Is Fixed via a Three-Carbon Pathway]:

Summary of the Calvin cycle:

Back 45

at each full "turn of the cycle, one molecule of carbon dioxide enters the cycle. Three turns are the number required to make one molecule of PGAL, the equivalent of one molecule of a 3-carbon sugar. The energy that drives the Calvin cycle is provided in the form of ATP and NADPH, produced by the light reaction of photosynthesis.

Front 46

THE CARBON-FIXATION REACTIONS [Most fixed carbon is converted to sucrose or starch]:

What happens to PGAL produced by the calvin cycle?

Back 46

- much is exported to the cytosol which is converted to sucrose.

- PGAL which remains in chloroplast is converted to strach, which is stored temp. during the light period as starch grains in the stroma. at night sucrose is then produced from the starch.