Question 1

In the process of carbon fixation, RuBP attaches a CO₂ to produce a six-carbon molecule, which is then split to produce two molecules of 3-phosphoglycerate. After phosphorylation and reduction produces glyceraldehyde 3-phosphate (G3P), what more needs to happen to complete the Calvin cycle?

Accepted Answer

The regeneration of RuBP is necessary to complete the Calvin cycle, as it is required for the enzyme RuBP carboxylase to attach another CO₂ molecule and begin the process of carbon fixation again. While the other processes mentioned (addition of electrons from NADPH, inactivation of RuBP carboxylase, regeneration of ATP from ADP, and regeneration of NADP⁺) are important in the overall process of photosynthesis, they are not specifically required to complete the Calvin cycle.

Question 2

The NADPH required for the Calvin cycle comes from

Accepted Answer

The NADPH required for the Calvin cycle is generated in reactions initiated in photosystem I during the light-dependent reactions of photosynthesis.

Question 3

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 cytosol. This is because the thylakoid space is the site of proton accumulation during photosynthesis, which generates an electrochemical gradient that is used to synthesize ATP. The stroma, on the other hand, has a pH that is closer to neutral, as it is the site of the Calvin cycle reactions. The cytosol has a slightly higher pH than the stroma, as it is more exposed to the external environment and needs to maintain a more neutral pH to prevent damage to the cell.

Question 4

If plant gene alterations cause the plants to be deficient in photorespiration, what would most probably occur?

Accepted Answer

Photorespiration is a process that consumes oxygen and releases carbon dioxide, thereby reducing the efficiency of photosynthesis. However, it also plays a role in protecting plants from the harmful effects of high light intensities by consuming excess ATP and reducing power that could generate reactive oxygen species if not used. Therefore, plants deficient in photorespiration are more susceptible to light-induced damage because they lack this protective mechanism.

Question 5

Three "turns" of the Calvin cycle generate a "surplus" molecule of glyceraldehyde 3-phosphate (G3P). Which of the following is a consequence of this?

Accepted Answer

The formation of sucrose and starch in plants involves assembling G3P molecules, with or without further rearrangements. The surplus G3P generated in the Calvin cycle can be used to assemble sucrose or starch. Therefore, plants that generate surplus G3P can produce and store these complex carbohydrates.

Question 6

Reactions that require CO₂ take place in

Accepted Answer

The Calvin cycle is the metabolic pathway in which CO₂ is fixed into glucose. Therefore, reactions that require CO₂ take place in the Calvin cycle alone.

Question 7

If atmospheric CO₂ concentrations increase twofold or more, how will plants be affected, disregarding any changes in climate?

Accepted Answer

C₃ plants will benefit more from increased CO₂ levels because their photosynthesis efficiency is more CO₂ limited compared to C₄ plants, which already utilize CO₂ more efficiently.

Question 8

In an experiment studying photosynthesis performed during the day, you provide a plant with radioactive carbon (¹⁴C)dioxide as a metabolic tracer. The ¹⁴C is incorporated first into oxaloacetate. The plant is best characterized as a

Accepted Answer

C₄ plants initially fix CO₂ into a four-carbon compound, oxaloacetate, distinguishing them from C₃ plants, which fix CO₂ directly into a three-carbon compound.

Question 9

Photorespiration occurs when rubisco reacts RuBP with

Accepted Answer

Photorespiration occurs when rubisco reacts with O₂ instead of CO₂. This occurs when the concentration of O₂ increases and the concentration of CO₂ decreases, such as during hot and dry conditions. This leads to the production of toxic byproducts, which the plant must then expend energy to get rid of.

Question 10

Which of the following statements best represents the relationships between the light reactions and the Calvin cycle?

Accepted Answer

The light reactions generate ATP and NADPH, which are consumed by the Calvin cycle to produce sugars. In return, the Calvin cycle produces ADP, Pi, and NADP+ that are used in the light reactions to produce ATP and NADPH. Therefore, there is a reciprocal relationship between the light reactions and the Calvin cycle in which they exchange energy-rich molecules.

Question 11

Photorespiration lowers the efficiency of photosynthesis by

Accepted Answer

The answer of Photorespiration lowers the efficiency of photosynthesis by
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Question 12

In C₃ photosynthesis, the reactions that require ATP take place in

Accepted Answer

The answer of In C₃ photosynthesis, the reactions that require...

Question 13

Compared to C₃ plants, C₄ plants

Accepted Answer

C₄ plants can continue to fix CO₂ efficiently even when CO₂ concentrations are low and oxygen concentrations are high, due to their specialized anatomy and the enzyme PEP carboxylase, which has a higher affinity for CO₂ and does not bind oxygen. This adaptation reduces photorespiration, allowing C₄ plants to photosynthesize more efficiently under these conditions compared to C₃ plants.

Question 14

The phylogenetic distribution of the enzyme rubisco is limited to

Accepted Answer

Rubisco is a key enzyme used in the Calvin cycle of photosynthesis, and it is found in all known photoautotrophs, both prokaryotic and eukaryotic. Therefore, the phylogenetic distribution of rubisco is not limited to C3 or C4 plants, but includes all photoautotrophs.

Question 15

In a plant leaf, the reactions that produce NADH occur in

Accepted Answer

NADH is not produced in the light reactions or the Calvin cycle of photosynthesis. Instead, NADPH is produced in the light reactions and used in the Calvin cycle. NADH is typically associated with cellular respiration, not photosynthesis.

Question 16

CAM plants keep stomata closed in daytime, thus reducing loss of water. They can do this because they

Accepted Answer

CAM plants fix CO₂ into organic acids during the night and store them in vacuoles. During the day, the stomata remain closed to conserve water, and the organic acids are broken down to release CO₂ for photosynthesis in specialized mesophyll cells. This allows the plant to minimize water loss during the day while still carrying out photosynthesis.

Question 17

Plants photosynthesize only in the light. Plants respire

Accepted Answer

Plants respire both in light and dark to produce ATP. While photosynthesis only occurs in the light, respiration is a continuous metabolic process that occurs throughout the day and night. During respiration, plants use stored energy to produce ATP, which is essential for growth and maintenance.

Question 18

What is the primary function of the Calvin cycle?

Accepted Answer

The primary function of the Calvin cycle is to synthesize simple sugars (such as glucose) from carbon dioxide, using energy from ATP and reducing power from NADPH, which are produced during the light reactions of photosynthesis. The cycle also regenerates the starting molecule, ribulose bisphosphate (RuBP), which is necessary for continued carbon fixation. The cycle does not involve the release of carbon dioxide or oxygen, nor does it transport RuBP out of the chloroplast.

Question 19

The alternative pathways of photosynthesis using the C₄ or CAM systems are said to be compromises. Why?

Accepted Answer

Both C₄ and CAM photosynthesis pathways are adaptations to arid environments, designed to minimize water loss while still allowing photosynthesis to occur. However, these adaptations come at a cost of requiring more ATP to fix carbon compared to the C₃ pathway. C₄ plants achieve this by physically separating the initial CO₂ fixation and the Calvin cycle in different cell types, while CAM plants separate these processes temporally, fixing CO₂ at night and performing the Calvin cycle during the day. Both strategies reduce photorespiration but increase the energy cost of carbon fixation.

Question 20

Why are C₄ plants able to photosynthesize with no apparent photorespiration?

Accepted Answer

C₄ plants use PEP carboxylase to initially fix CO₂, thus bypassing the need for Rubisco enzyme, which is responsible for photorespiration.

Answer: E 
 C₄ plants exclude oxygen from their tissues by carrying out the initial steps of photosynthesis in the mesophyll cells and the final steps in bundle sheath cells, where carbon dioxide is concentrated and oxygen is excluded, leading to no apparent photorespiration.

Quiz 10: Photosynthesis

In the process of carbon fixation, RuBP attaches a CO₂ to produce a six-carbon molecule, which is then split to produce two molecules of 3-phosphoglycerate. After phosphorylation and reduction produces glyceraldehyde 3-phosphate (G3P) , what more needs to happen to complete the Calvin cycle?