Question 1

Use the accompanying figure to answer the question(s) below. 
-As electrons are passed through the system of electron carriers associated with photosystem II, they lose energy. What happens to this energy?

Accepted Answer

The energy lost by electrons as they pass through the system of electron carriers associated with photosystem II is used to establish and maintain a proton gradient, which is essential for the production of ATP. This process is known as chemiosmosis.

Question 2

Suppose the interior of the thylakoids of isolated chloroplasts were made acidic and then transferred in the dark to a pH 8 solution. What would most likely happen?

Accepted Answer

The acidification of the thylakoid interior creates a proton gradient across the thylakoid membrane. When these chloroplasts are placed in a basic solution (pH 8), the proton gradient drives ATP synthesis through chemiosmosis, even in the dark, without the need for light-driven electron transport. This process is similar to the light-dependent reactions of photosynthesis, where the energy from the proton gradient is used by ATP synthase to generate ATP.

Question 3

The accumulation of free oxygen in Earth's atmosphere began with the origin of ________.

Accepted Answer

The accumulation of free oxygen in Earth's atmosphere began with the origin of cyanobacteria using photosystem II, which produced oxygen as a byproduct of photosynthesis.

Question 4

As a research scientist, you measure the amount of ATP and NADPH consumed by the Calvin cycle in 1 hour. You find that 30,000 molecules of ATP were consumed, but only 20,000 molecules of NADPH were consumed. Where did the extra ATP molecules come from?

Accepted Answer

Cyclic electron flow can produce additional ATP without the consumption of NADPH. This occurs when electrons cycle back from Ferredoxin to the Cytochrome complex instead of being passed to NADP+ to make NADPH. Therefore, the extra ATP consumed during the Calvin cycle may have come from cyclic electron flow.

Question 5

Use the accompanying figure to answer the question(s) below. 
-The proteins of the electron transport chain active in the light-dependent reactions ________.

Accepted Answer

The electron transport chain proteins are membrane proteins and are present in the thylakoid membrane.

Question 6

In mitochondria, chemiosmosis moves protons from the matrix into the intermembrane space, whereas in chloroplasts, chemiosmosis moves protons from the ________.

Accepted Answer

In chloroplasts, chemiosmosis moves protons from the stroma (where they accumulate following the light-dependent reactions) into the thylakoid space, creating a proton gradient that can be used to drive ATP synthesis.

Question 7

In a plant cell, where are the ATP synthase complexes located?

Accepted Answer

ATP synthase complexes are located in both the thylakoid membrane of chloroplasts (involved in photosynthesis) and the inner mitochondrial membrane (involved in cellular respiration) in plant cells. Therefore, the best choice is D, which includes both membranes.

Question 8

Which of the events listed below occurs in the light reactions of photosynthesis?

Accepted Answer

In the light reactions of photosynthesis, light energy is absorbed by pigments such as chlorophyll and funneled to reaction-center chlorophyll a. This generates excited electrons that are passed through a series of electron acceptors, ultimately resulting in the production of ATP and NADPH, which is then used in the dark reactions to fix carbon dioxide into organic molecules such as glucose. Therefore, the best choice is E.

Question 9

The pH of the inner thylakoid space has been measured, as have the pH of the stroma and of the cytosol of a particular plant cell. Which, if any, relationship would you expect to find?

Accepted Answer

The pH within the thylakoid is expected to be lower (more acidic) than that of the stroma and the cytosol. This is because during the light reactions of photosynthesis, protons (H+) are pumped from the stroma into the thylakoid space, creating an electrochemical gradient. This gradient is used to drive the production of ATP by ATP synthase as protons flow back into the stroma. Since protons are being actively transported into the thylakoid space, it is expected that the pH within the thylakoid would be more acidic than the stroma and cytosol.

Question 10

The chemiosmotic process in chloroplasts involves the ________.

Accepted Answer

The chemiosmotic process in chloroplasts involves the establishment of a proton gradient across the thylakoid membrane, which provides the energy for ATP production during the light-dependent reactions.

Question 11

Which of the following are directly associated with photosystem I?

Accepted Answer

Photosystem I is responsible for receiving electrons from the thylakoid membrane electron transport chain and using them to produce NADPH.

Question 12

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 these organisms would be to ________.

Accepted Answer

The liberation of O2 in the light is a direct result of photosynthesis and is produced through the light-dependent reactions of photosystem II. If an organism lacks photosystem II, it will not produce O2 in the light, making this the best way to detect its absence. While thylakoids are necessary for photosynthesis, their presence alone does not indicate the presence of photosystem II. Testing for CO2 fixation in the dark or for the production of sucrose or starch would detect the overall process of photosynthesis, but not specifically the absence of photosystem II. Experimenting to generate an action spectrum would also not specifically detect the absence of photosystem II.

Question 13

The electrons of photosystem II are excited and transferred to electron carriers. From which molecule or structure do the photosystem II replacement electrons come?

Accepted Answer

The replacement electrons for photosystem II come from the splitting of water molecules during the light-dependent reactions of photosynthesis. This process, known as photolysis, releases oxygen as a byproduct and transfers electrons to the oxidized form of the artificial electron acceptor, P680+.

Question 14

Assume a thylakoid is somehow punctured so that the interior of the thylakoid is no longer separated from the stroma. This damage will most directly affect the ________.

Accepted Answer

The puncturing of the thylakoid will lead to a loss of the proton gradient required for the synthesis of ATP through chemiosmosis. In normal conditions, proton pumps move H+ ions from the stroma to the thylakoid lumen, creating a gradient that drives ATP synthesis. However, puncturing the thylakoid would eliminate this gradient, hampering ATP synthesis.

Question 15

What is the main purpose of light-dependent reactions of photosynthesis?

Accepted Answer

The main purpose of light-dependent reactions of photosynthesis is to produce NADPH and ATP, which are used as energy sources in the light-independent reactions to make glucose. The splitting of H2O also occurs in the light-dependent reactions, but its purpose is to generate electrons for the production of NADPH, not to generate oxygen. The production of NADPH is not for use in respiration, but rather for use in photosynthesis.

Question 16

Which statement describes the functioning of photosystem II?

Accepted Answer

Photosystem II functions by using light energy to excite electrons in the P680 pigment molecules. These excited electrons are then transferred to a nearby electron acceptor molecule in the thylakoid membrane, creating a flow of electrons that generates ATP. The electron vacancies in P680+ are filled by electrons derived from the splitting of water into oxygen and protons, which replenishes the electron supply. This process is known as photolysis and is critical for oxygen evolution during photosynthesis.

Question 17

Plastoquinone (PQ), an electron carrier of small molecular weight, is found in the electron transport chain associated with photosystem II. If PQ is not directly anchored to other membrane or cytoplasmic structures, it is probably ________.

Accepted Answer

PQ is lipid soluble and serves as a shuttle between the electron transport chain and ATP synthase. Its small molecular weight allows it to move freely within the thylakoid membrane.

Question 18

Use the accompanying figure to answer the question(s) below. 
-What is the difference between NAD+ and NADP?

Accepted Answer

Both NAD+ and NADP function as electron carriers in cells, but the difference between them is that NADP has a phosphate group attached to its ribose ring, whereas NAD+ does not. This phosphate group allows NADP to participate in different metabolic reactions that require the transfer of high-energy electrons, such as photosynthesis. NAD+ is primarily involved in catabolic reactions, where it accepts electrons and becomes reduced to NADH, which is then used to produce ATP through the electron transport chain.

Question 19

In its mechanism, photophosphorylation is most similar to ________.

Accepted Answer

Photophosphorylation and oxidative phosphorylation both involve the generation of ATP through a proton gradient. In photophosphorylation, light energy is used to pump protons across a membrane, generating a gradient that powers the production of ATP. In oxidative phosphorylation, the protons are pumped across the inner mitochondrial membrane by the electron transport chain, generating a gradient that powers the production of ATP. Substrate-level phosphorylation in glycolysis involves the transfer of a phosphate group from a substrate to ADP to generate ATP, which is a different mechanism than photophosphorylation. The Calvin cycle and carbon fixation are not mechanisms for the generation of ATP. Reduction of NADP+ is important in photosynthesis, but it is not directly related to photophosphorylation.

Question 20

P680+ is said to be the strongest biological oxidizing agent. Given its function, why is this necessary?

Accepted Answer

P680+ needs to be the strongest biological oxidizing agent because it must extract electrons from water molecules, which requires a stronger attraction for electrons than oxygen has. This is essential for the process of photolysis in photosynthesis, where water is split to release oxygen, protons, and electrons.

Quiz 10: Photosynthesis

Use the accompanying figure to answer the question(s) below. -As electrons are passed through the system of electron carriers associated with photosystem II, they lose energy. What happens to this energy?