Energy Generation In Mitochondria And Chloroplasts Pt. 2

Front 1

Why can electron transfers cause the movement of entire hydrogen atoms?

Back 1

Because protons (H+) are readily accepted from or donated to water.

Front 2

How is an oxidized electron carrier molecule reduced?

Back 2

When it picks up an electron plus a hydrogen from water.

Front 3

How is a reduced electron carrier molecule oxidized?

Back 3

It loses an electron from one of its hydrogen atoms. In most instances, the electron is transferred to an electron carrier and the proton (H+) is passed to water.

Front 4

What guides electrons so that they move sequentially from one enzyme complex to the next without shortcuts?

Back 4

The protons of the respiratory chain.

Front 5

True or false: as electrons move from one molecule to the next, the molecules are oxidized.

Back 5

False. Whenever one molecule is oxidized, another is reduced. The molecule losing the electrons is oxidized while the one gaining them is reduced. These are called redox reactions.

Front 6

What are redox pairs?

Back 6

A pair of oxidizing and reducing agents that are involved in redox reactions. NADH and NAD+ would be an example because the former is converted to the latter by the loss of electrons in the reaction.

Front 7

What is redox potential?

Back 7

A measure of the tendency of redox pairs to donate or accept electrons.

Ex: Electrons will move spontaneously from a redox pair with a low redox potential (or low affinity for electrons) such as NADH/NAD+ to a redox pair with a high redox potential such as O2/H2O.

Front 8

True or false: electron transfers release large amounts of energy.

Back 8

True

Front 9

Why doesn't NADH donate electrons directly to O2?

Back 9

The huge free energy drop would cause the reaction to proceed with almost explosive force, and nearly all of the energy would be released as heat. That is why the electron transport chain exists and functions in small steps.

Front 10

How do metal atoms help each of the three respiratory enzyme complexes?

Back 10

They help the carriers be more versatile. Each complex has metal atoms tightly bound to the proteins. Electrons move within the complexes by skipping from one embedded metal ion to another with a greater affinity for electrons.

Front 11

What do electron carriers do?

Back 11

They are diffused throughout the lipid bilayer and ferry electrons from one respiratory complex to the next. In mitochondria, ubiquinone and cytochrome C are such carriers.

Front 12

What are quinones? How does ubiquinone work?

Back 12

Electron carriers diffused throughout the lipid bilayer.

It picks up one H+ from the aqueous environment for every electron it accepts, and it can carry 2 electrons as part of its hydrogen atoms. When it releases the electrons to the next carrier in the chain, protons are also released.

Front 13

What causes ubiquinone to be confined to the inner mitochondrial membrane?

Back 13

It has a long hydrophobic hydrocarbon tail that sits in the lipid bilayer.

Front 14

Where do the biggest increases in redox potential occur?

Back 14

Across each of the three respiratory complexes, which allows each of them to pump protons.

Front 15

True or false: the further down the mitochondrial electron transport chain a carrier is, the lower the redox potential.

Back 15

False. Redox potential increases the further down the chain they are located.

Front 16

What are iron-sulfur centers, and how do they work?

Back 16

They are prominent electron carriers in the early part of the electron transport chain because they have a low affinity for electrons.

Ex: one in the NADH dehydrogenase complex passes electrons to ubiquinone.

Front 17

What are iron heme groups?

Back 17

They exist within iron atoms bound to cytochrome proteins and are commonly used as electron carriers.

Front 18

What holds heme groups to the proteins? Why do different heme groups have different electron affinities?

Back 18

1. The porphyrin ring is attached covalently to the side chains of the protein

2. They differ slightly in structure and are held in different local environments within each protein.

Front 19

What catalyzes the reduction of molecular oxygen?

Back 19

Cytochrome C Oxidase.

Front 20

Describe cytochrome C oxidase.

Back 20

1. It is the last carrier in the respiratory chain and therefore has the highest redox potential.

2. It is a dimer formed from a monomer with 13 different protein subunits.

3. It is located in the inner mitochondrial membrane

4. The subunits that form the functional core of the complex are encoded by the mitochondrial genome. The rest are encoded by the nuclear genome.

5. As electrons pass through it on the way to its bound O2 molecule to produce water, it pumps protons across the membrane.

6. It receives 4 electrons to produce H2O.

Front 21

Describe the conformation changes of proton pumping coupled to electron transport

Back 21

Thought to be used by NADH dehydrogenase and cytochrome C oxidase as well as many other proton pumps.

1. The protein is driven through three conformational changes.

2. In one conformation, it has a high affinity for H+, causing it to pick one up in the matrix side of the membrane.

3. In another conformation, it has a low affinity for H+, causing it to release one into the intermembrane space.

4. The cycle only goes in this direction because one of the steps is driven by an allosteric change in conformation driven by energetically favorable transport of electrons.

Front 22

What are uncoupling agents?

Back 22

H+ carriers that can insert into the inner mitochondrial membrane. They render the membrane permeable to protons, allowing H+ to flow into the matrix without passing through ATP synthase. This short circuit uncouples electron transport from ATP synthase.

Front 23

Describe the experiment that provided direct evidence that proton gradients can power ATP production

Back 23

Experiments used bacteriorhodopsin and bovine mitochondrial ATP synthase introduced into liposomes.

1. When bacteriorhodopsin is added to liposomes, the protein generates a proton gradient in response to light.

2. When both bacteriorhodopsin and an ATP synthase are present, the light generated proton gradient drives the formation of ATP from ADP and a phosphate.

3. Liposomes only containing ATP synthase don't produce ATP in response to light.

4. In liposomes with both features, uncoupling agents that abolish the proton gradient eliminate light-induced ATP synthesis.

Front 24

Why does cytochrome C oxidase hold its oxygen molecule until it receives the 4 electrons needed to convert it into two molecules of H2O?

Back 24

O2 has a high affinity for electrons. Once it picks up one, it forms the superoxide radical O2-. It is dangerously reactive and will pick up 3 more electrons wherever it can find them, which can damage nearby DNA, proteins, and lipid membranes.

Front 25

What is photosynthesis?

Back 25

A series of light-driven reactions that creates organic molecules from atmospheric carbon dioxide.

Front 26

What is a chloroplast?

Back 26

A specialized intracellular organelle in plants which contains light capturing pigments such as the green pigment chlorophyll.

Front 27

What is carbon fixation?

Back 27

Photosynthesis produces ATP and NADPH. Once activated, these carriers can be used to convert CO2 into sugar in the chloroplast. This process is called carbon fixation.

Front 28

How do we know microorganisms that carry out oxygen-producing photosynthesis changed Earth's atmosphere?

Back 28

Fossilized stromatolites contain the remnants of the photosynthetic cyanobacteria, some of which are 3.5 billion years old. Stromatolites still exist living today.

Front 29

What are thylakoids?

Back 29

Chloroplasts are similar to mitochondria, but have this extra component. They are flattened disc-like sacs that are arranged in stacks called grana. They are contained in a third membrane called the thylakoid membrane.

Front 30

What are the three sets of membranes in chloroplasts?

Back 30

Outer membrane, inner membrane, and thykaloid membrane.

Front 31

What does the thylakoid membrane contain?

Back 31

The light-capturing and ATP-generating systems of plant chloroplasts.

Front 32

What is stage 1 of photosynthesis?

Back 32

A series of photosynthetic electron transfer reactions produce ATP and NADPH. In the process, electrons are extracted from water and oxygen is released as a byproduct. Occurs in the thylakoid membrane.

Front 33

What is stage 2 of photosynthesis?

Back 33

When carbon dioxide is assimilated (fixed) to produce sugars and a variety of other organic molecules. Begins in the chloroplast stroma (the equivalent of the mitochondrial matrix) and continues in the cytosol.

Front 34

What are the steps of stage 1 of photosynthesis?

Back 34

1. Light is absorbed and used to drive electrons derived from water to create NADPH with oxygen as a byproduct.

2. An electron transport chain (Starting with NADPH) in the thylakoid membrane harnesses energy of electron transport to pump protons into the thylakoid space.

3. The resulting proton gradient drives the synthesis of ATP by ATP synthase.

Front 35

What is the difference between photosynthesis and oxidative phosphorylation?

Back 35

The difference from oxidative phosphorylation is the high energy electrons come from a molecule of chlorophyll that has absorbed energy from sunlight. These electrons ultimately end up donated to NADP+ to create NADPH rather than to O2 to create water.

Front 36

What are the steps of stage 2 of photosynthesis?

Back 36

1. The ATP and NADPH produced by the photosynthetic electron transfer reactions of stage 1 are used to drive the manufacture of sugars from CO2.

2. These carbon fixation reactions occur in the absence of sunlight and are called dark reactions.

3. Beginning in the stroma, they generate a 3 carbon sugar called glyceraldehyde-3 phosphate, which is transported to the cytosol.

4. It is then used to produce sucrose and a large number of other organic molecules in plants.

Front 37

True or false: chlorophylls only absorb wavelengths of green light.

Back 37

False. Chlorophylls absorb light of blue and red wavelengths.

Front 38

Describe the structure of a chlorophyll molecule.

Back 38

Each contains a porphyrin ring with a magnesium atom at its center. It is structurally similar to the one that binds iron in heme in mitochondria. Light is absorbed within the bond network while the long hydrophobic tail helps hold the chlorophyll in the thylakoid membrane.

Front 39

Describe a photosystem and how it works.

Back 39

1. A photosystem consists of a reaction center surrounded by chlorophyll-containing antenna complexes.

2. Once light has been captured in an antenna complex, it will pass randomly from one chlorophyll molecule to the next until it gets trapped by a chlorophyll dimer called a special pair located in the reaction center.

3. The special pair holds its electrons at a somewhat lower energy than the antenna chlorophylls, which is how it traps the energy from the antenna.

4. Note: in the antenna complex, energy is moved from one chlorophyll molecule to the next, not electrons.

Front 40

What provides a rapid path from the special pair to a mobile electron carrier?

Back 40

Intermediary carriers embedded within the reaction center.

Front 41

What converts light into chemical energy?

Back 41

The transfer of high energy electrons from the excited chlorophyll special pair, which leaves behind a positive charge that creates a charge separated state.

Front 42

What happens once the electron in the special pair has been replaced?

Back 42

The mobile carrier diffuses away from the reaction center, transferring the high-energy electron to the transport chain.

Front 43

True or false: a pair of photosystems cooperate to generate both ATP and NADPH.

Back 43

True.

Front 44

Describe part one of the process by which the two photosystems cooperate to generate ATP and NADPH.

Back 44

1. Photosystem II captures light energy.

2. Once this happens, a high energy electron is transferred to the mobile carrier plastoquinone (similar to ubiquinone of mitochondria).

3. This carrier transfers its electrons to a proton pump called the cytochrome b6-f complex (resembles cytochrome c reductase complex).

4. Cytochrome b6-f is the sole site of active proton pumping in the chloroplast electron transport chain.

5. An ATP synthase embedded in the membrane then uses the energy of the electrochemical proton gradient to produce ATP.

Front 45

Describe part two of the process by which the two photosystems cooperate to generate ATP and NADPH.

Back 45

The special pair of photosystem 1 receives its electrons from photosystem 2.

1. Photosystem 1 captures light energy, which causes a high energy electron to pass to the mobile electron carrier ferredoxin.

2. Ferredoxin is a small protein that contains an iron-sulfer center.

3. Ferredoxin transfers its electrons to ferredoxin-NADP+ reductase, which is the final protein in the electron transport chain.

4. The reductase catalyzes the production of NADPH.

Front 46

What is the water-splitting enzyme, and where is it located?

Back 46

It's located in the reaction center of photosystem 2. It catalyzes the extraction of electrons from water.

When light energy excites the chlorophyll special pair, an electron is passed to plastoquinone. An electron is then returned to the special pair by then water-splitting enzyme.

Front 47

True or false: photosystem 2 exists as a monomer in the membrane and thus contains one copy of the water-splitting enzyme.

Back 47

False. It exists as a dimer and thus contains two copies of the water-splitting enzyme.

Front 48

How many electrons must be withdrawn from two water molecules to release O2 into the atmosphere?

Back 48

Four

Front 49

How do plants protect themselves from free radicals (charged versions of water)?

Back 49

The water molecules are held by the water-splitting enzyme until it is safe to release. Much like cytochrome c oxidase holds the O2 molecule until it is safe to release.

Front 50

In short, describe the serial movement of electrons through the two photosystems.

Back 50

1. Water-splitting enzyme supplies electrons to photosystem 2.

2. Electron energy is raised by absorption of light.

3. This powers the pumping of protons by the cytochrome b6-f complex.

4. Electrons are then donated to a copper-containing protein (electron carrier, plastoquinone).

5. Plastoquinone ferries electrons to reaction center of photosystem 1.

6. A second energy boost from light allows these electrons to be used to produce NADPH.

Front 51

Which photosystem has a lower redox potential? What does this result in?

Back 51

Photosystem 2. That means it is more likely to donate electrons to photosystem 1, which has a higher affinity for them.

Front 52

Describe how Ribulose bisphosphate carboxylase (Rubisco) catalyzes the carbon fixation process of photosynthesis.

Back 52

1. Reaction takes place in the chloroplast stroma.

2. CO2 from the atmosphere is covalently bonded to the energy-rich molecule ribulose 1,5-bisphosphate.

3. This union generates a chemical intermediate that reacts with water

4. This reaction generates two molecules of 3-phosphoglycerate.

Front 53

What occurs in the three stages of the carbon fixation cycle?

Back 53

1. Carbon fixation: CO2 is added to ribulose 1,5-bisphosphate. This results in production of 3-phosphoglycerate.

2. Sugar formation: ATP and NADPH are consumed to convert 3-phosphoglycerate into glyceraldehyde 3-phosphate.

3. Regeneration of ribulose 1,5-bisphosphate: Most of the glyceraldehyde 3-phosphate produced is transported out of the chloroplast stroma into the cytosol.

This is the highlight reel. There are many intermediates throughout the cycle.

Front 54

For every 3 molecules of CO2 that enter the cycle, how many molecules of glyceraldehyde 3-phosphate are produced? How many molecules of ATP are consumed? How many molecules of NADPH are consumed?

Back 54

1. 1 molecule of glyceraldehyde 3-phosphate

2. 9 molecules of ATP

3. 6 molecules of NADPH

Front 55

What happens to sugars generated by carbon fixation?

Back 55

They can be stored as starch or consumed to produce ATP. Chloroplasts often contain large stores of carbohydrates and fatty acids.

Front 56

True or false: the chloroplast inner membrane is permeable to ATP and NADPH produced in the stroma during the light reactions of photosynthesis.

Back 56

False. It is impermeable to them. These molecules are then funneled into the carbon fixation cycle, where they are used to make sugars.

Front 57

If sugars are not stored in the chloroplast as starch or fats, what happens to them?

Back 57

They are exported to the rest of the plant cell. There, they can enter the energy-generating pathway that ends in ATP synthesis in the mitochondria.

Front 58

How did oxidative phosphorylation probably evolve?

Back 58

In stages. Stage 1 may have involved evolution of an ATPase that pumped protons out of the cell using energy from ATP hydrolysis.

Stage 2 could have involved the evolution of a different proton pump, driven by an electron transport chain.

Stage 3 could then link these two systems together to generate an ATP synthase that uses the protons pumped by the electron transport chain to synthesize ATP.

An early cell with this final system would have had a large selective advantage over cells with neither or only one.

Front 59

What does green sulfur bacteria use in photosynthesis? Why?

Back 59

Hydrogen sulfide (H2S). This is because electrons are easier to extract from H2S than H2O because H2S has a much higher redox potential. Therefore, only one photosystem is needed to produce NADPH and elemental sulfur instead if O2 as a byproduct.

Front 60

What do green sulfur bacteria use as electron carriers in their photosystem?

Back 60

Iron-sulfur centers that eventually donate their high-energy electrons to ferredoxin.

Front 61

What delayed the accumulation of O2 in the atmosphere?

Back 61

The initial reaction of O2 with abundant ferrous iron (Fe2+) dissolved in the early oceans. Once saturated, O2 was able to accumulate in the atmosphere.

Front 62

How do Methabococcus bacteria get energy in the hot, dark oceans?

Back 62

They use hydrogen gas bubbling from deep sea vents for chemiosmotic coupling.