Biology 107 Topic 7
How does ferredoxin participate in linear electron flow?
Ferredoxin acts as an electron transporter, picking excited electrons up from the primary electron acceptor of PSI and delivering them to NAHP+ reductase.
All of the following are directly associated with photosystem I except: 1. harvesting of light energy by chlorophyll 2. receiving electrons from plastocyanin (Pc) 3. P700 chlorophyll 4. splitting water 5. passing electrons to ferredoxfin (Fd)
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What are the three phases of the Calvin cycle? What happens in each phase?
1. Carbon Fixation: 3 CO2 is combined with a sugar (3 ribulose bisphosphate; RuBP) by the enzyme RUBISCO to incorporate C into sugar (6 3-phosphoglycerate) molecules 2. Reduction: energy from 6 ATP and 6 NADPH is used to cover C-containing sugar (6 -phosphoglycerate) into 6 glyceraldehyde-3-phosphate molecules. One of these G3Ps is released from the cycle as a net gain, and the other 5 G3Ps carry on to the next phase of the cycle. 3. Regeneration: 3 ATP are used to provide the energy to convert 5 G3P into 3 RuBP, thereby regenerating the RuBP molecules needed to start the cycle again.
Which of the following is not true of the cyclic electron flow pathway? 1. it is needed to operate the Calvin cycle 2. it generates ATP, but not NADPH 3. it generates oxygen 4. it requires light to operate 5. it incorporates part of the electron transport chain
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Which of the following statements regarding events in the functioning of photosystem II is false? 1. light energy excites electrons in a pigment molecule in a light-harvesting complex 2. the excitation is passed along to a molecule of P680 chlorophyll in the reaction centre 3. the P680 chlorophyll donates a pair of protons to NADPH 4. the electron vacancies in P680 are filled by electrons derived from water 5. the splitting of water yields molecular oxygen as a by-product
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Which of the following statements about photosynthesis is true 1. NADP+ is reduced to NADPH in the thylakoid space 2. the ATP synthase allows protons to leave the stroma 3. the prof the thylakoid space is about 8 4. H2O is split at photosystem I 5. photosystem II is not involved in cyclic electron flow
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What are the products of the light-independent reactions of photosynthesis?
ADP, P, NADP+, and CnH2nOn
What are the products of the light reactions that are subsequently used by the Calvin cycle?
ATP and NADPH
What molecules does the Calvin cycle receive from light reactions?
ATP and NADPH
What does the cytochrome complex do and how does it work?
As electrons move down the electron transport chain, they are transferred from plastoquinone to the cytochrome complex, and then from the cytochrome complex to plastocyanin. As these electrons are transferred energy is released, which causes a conformational change in the cytochrome complex. This shape change causes the cytochrome complex to act as an active transport, and pump protons from the stroma into the thylakoid space against their concentration gradient.
Why do cells have cyclic electron flow?
Because sometimes cells do not have enough ATP to carry out the Calvin cycle (i.e. too much NADPH for the amount. of ATP that is present). The process of cyclic electron flow allows the cell to generate extra ATP without generating more NADPH, thereby providing enough ATP to carry out the Calvin cycle in the stroma of the chloroplasts.
What is an electron transport chain? What happens to the electrons at the end of an electron transport chain?
Electron transport chain: a series of membrane proteins that act as electron carriers Each protein in the chain is reduced then oxidized as electrons are transferred from one to the next. The molecule at the end is the most electronegative, acting to pull the electrons down the chain. As electrons are transferred, they release energy in a series of small steps, which the cell uses for other processes (i.e. chemiosmosis to produce ATP).
How does ferredoxin participate in cyclic electron flow?
Ferredoxin acts as an electron transporter. It picks up excited electrons from the primary electron acceptor of PSI and delivers them to the cytochrome complex of the electron transport chain.
What is the source of electrons for the ETCs in photosynthesis?
High energy electrons from excited reaction center chlorophyll a molecules (P680 and P700) are transferred to the primary electron acceptors of PSII and PSI, and then are transferred to the ETCs. In P680 of PSII, these electrons are replaced by the electrons produced from the splitting water. When the high energy electrons from PSII get to the bottom of the PSII ETC, they fill the electron hole in the reaction centre chlorophyll a molecules (P700) of PSI.
When an electron in a chlorophyll molecule gets excited, what are the three possible fates of the released energy as the electron returns to its ground state from the excited state? Where does each process occur?
If the chlorophyll molecule is isolated (i.e. in solution in a beaker), the excited electron falls back to ground state and releases its energy as heat and light (fluorescence). In the light-harvesting complexes of the photosystems, electrons may undergo resonance transfer. In this process, the energy released as the electron falls back to the ground state is transferred to a neighbouring chlorophyll molecule, where it excites another electron from the ground state to the excited state. Lastly, the high energy electron may be transferred to an electron acceptor (occurs in the reaction centre complex of the photosystems). In this process, the electron retains its energy during transfer to the electron acceptor. The electron lost from the chlorophyll molecule is then replaced by a low-energy electron from an electron donor (H2O), in order to fill the electron hole
What is the fate of light energy when it hits chlorophyll a in a test tube compared to chlorophyll a in the thylakoid membrane? What accounts for the difference?
In the test tube, the absorbed energy is lost as light (fluorescence) and heat. In the thylakoid membrane, energy is transferred to reaction centre chlorophyll a molecules, thereby electrons get excited and are transferred to the primary electron acceptor. The reason for this difference is due to the special structure of the photosystems, in which chlorophyll molecules are arranged together in the light harvesting complexes so that they are able to transfer energy from molecule to molecule.
Where do the light reactions and Calvin cycle take place and what is produced during each?
Light reactions take place in the thylakoid space and thylakoid membrane, and produces ATP, NADPH (both produced by enzymes in the thylakoid membrane and released into the stroma), and O2 (released in thylakoid space due to splitting of water). Calvin cycle takes place in the stroma, and produces carbohydrates (G3P)
What is the purpose of an electron shuttle molecule? Which shuttle molecule is used in photosynthesis?
Shuttle molecules store and transfer high energy electrons in cells. When electrons are added to the molecule, it is reduced, and is high in potential energy. When the electrons are removed, the molecule is oxidized, and its potential energy is released. The shuttle molecule used in photosynthesis is NADP+/NADPH
What components of the electron transport chain participate in cyclic electron flow? What roles do they play?
The cytochrome complex receives electrons from ferredoxin, and passes them to plastocyanin. The plastocyanin then transfers the electrons to the P700 of photosystem I to fill the electron hole. As the electrons pass through the cytochrome complex, the energy released is used to power the pumping of protons from the stroma into the thylakoid space. This proton gradient can then be used to power row production of ATP by ATP synthase.
What happens to chlorophyll molecules when they absorb a photon of light in the light-harvesting complex of PSII?
The electrons in the chlorophyll molecules become excited by the light energy. This energy is transferred from one chlorophyll molecule to the next via resonance transfer, and eventually this energy reaches the P680 chlorophyll a molecules of the PSII reaction centre complex, where it excites electrons in P680.
What happens when a photon of light hits the light-harvesting complex of PSI?
The electrons in the chlorophyll molecules become excited by the light energy. This energy is transferred from one chlorophyll molecule to the next via resonance transfer, and eventually this energy reaches the P700 chlorophyll a molecules of the PSI reaction centre complex, where it excites electrons in P700.
What is H2O used for in the light reactions?
The electrons released from H2O are used to fill the electron hole in P680 of photosystem II. The protons released from H2O contribute to the proton gradient in the thylakoid space.
Where do the activated electrons from P700 go? Where do the replacement electrons for P700 come from?
The excited electrons from P70 are transferred to the primary electron acceptor of the PSI reaction centre, then to ferredoxin, and finally to NADP+ reductase, where they are added to NADP+ to reduce it to NADPH. The replacement electrons come from plastocyanin at the bottom of the preceding electron transport chain.
What happens to the electrons, oxygen, and protons (H+) when water is split during the light reactions of photosynthesis?
The oxygen is released from the cell of O2. The protons become part of the proton gradient in the thylakoid space and diffuse out through ATP synthesis. The electrons fill the electron hole in the reaction centre chlorophyll (P680) of photosystem II, then become excited and pass through the electron transport chains to eventually end up in NADPH.
Show and explain the relationship between the P680 electron transport chain and the protein gradient. In what direction are the protons pumped and in which direction do they diffuse? What process is being depicted?
The process is chemiosmosis. As electrons move down the electron transport chain, energy is released. The energy released by these redox reactions changes the conformation of the cytochrome complex of the ETC, allowing it to pump protons from the stroma into the thylakoid space. Protons diffuse back into the stroma by moving through ATP synthase.
Suppose a mutation occurred that resulted in an altered cytochrome complex that was still able to transfer electrons, but was unable to pump protons. Do you think this mutation would have an effect on photosynthesis?
Yes, it would have an effect. The pumping of protons is necessary because it produces the proton gradient, which is used by ATP synthase to make ATP. ATP is needed in the Calvin cycle, so without proton pumping there would be no ATP and the Calvin cycle would not occur. Photosynthesis would therefore not be able to produce complex carbohydrate products.
The colour of light least effective in driving photosynthesis is:
green
In leaves, photosynthesis occurs:
in mesophyll cells
a photosystem consists of:
light harvesting complexes plus one reaction centre
The reaction centre chlorophyll of photosystem I is known as P700 because:
this pigment is best at absorbing light with a wavelength of 700 nm
What is the primary function of the light reactions of photosynthesis?
the produce ATP and NADPH
Where does the Calvin cycle occur?
the stroma
Some photosynthetic organisms contain chloroplasts that lack photosystem II, yet are able to survive. The best way to detect the lack of photosystem II in this organism would be:
to test for liberation of O2 in the light