Photosynthesis review

In plant cells, where does photosynthesis take place?

When water is split in photosynthesis, what products are formed? You may choose more than one response.

• Water

• Oxygen

• Hydrogen gas

• Electrons

• Protons

What molecule serves as the initial electron donor for photosynthesis?

• Water

• Carbon Dioxide

• NADH

• Oxygen

• Glucose

Electrons transferred on the specialized chlorophyll molecule of photosystem ___ come from ___. 

• I, water

• I, NADH

• I, pigments of the antenna

• II, water

• II, plastocyanin

What are the main product(s) of the light-dependent reactions of photosynthesis?

• Water

• Oxygen

• Glucose and oxygen

• ATP and NADPH

• G3P

Answer explanation

The light-dependent reactions of photosynthesis are cyclic phosphorylation and noncyclic photophosphorylation. They produce ATP and NADPH for the Calvin cycle. 

In terms of carbon atoms, how can the Calvin cycle afford to lose 1 G3P molecule for every 6 G3P molecules synthesized?

• By phosphorylating 6 1,3-Bisphosphoglycerate molecules to 6 G3P molecules

• By regenerating 5 G3P molecules into 3 RuBP molecules

• By converting 3 RuBP molecules into six 3-phosphoglycerate molecules.

• By using 9 ATPs and 6 NADPH from the light-dependent reactions

• By phosphorylating six 3-phosphoglycerate into six 1,3-bisphosphoglycerate molecules.

Answer explanation

The only way that the Calvin cycle is able to lose 1 G3P molecule for 6 that get generated is by gaining more carbon in one of the steps. This is done by converting RuBp to 3-phosphoglycerate molecules, fixing 3 CO2 molecules in the process. 

Which of the following is the final electron acceptor in cyclic photophosphorylation?

• NADP⁺

• The plastoquinone pool

• P700

• Ferredoxin

• There is no final electron acceptor in cyclic photophosphorylation

Considering the factors that contribute to building the concentration gradient in the thylakoid lumen, would the concentration gradient be stronger, weaker, or the same if only cyclic photophosphorylation was used instead of a combination of both light-dependent reactions?

• Stronger

• Weaker

• The same

Answer explanation

It would be weaker since in cyclic photophosphorylation, you don't have the splitting of water releasing H⁺ in the thylakoid lumen and you don't have the reduction of NADP⁺ to NADPH removing a proton from the stroma. Since both of these processes contribute to building the proton gradient, the gradient would be weaker if these processes are not there.

How many carbon dioxide molecules must be fixed to produce one molecule of sucrose (C₁₂H₂₄O₁₂)?

• 1

• 3

• 5

• 6

• 12

Answer explanation

Sucrose is a disaccharide composed of 1 glucose and 1 fructose, both of which are 6-carbon molecules.

Why do CAM plants perform the C4 pathway only at night?

• Because their stomata are closed at night.

• To prevent water loss during the day.

• Because the CO₂ concentrations in the atmosphere are higher at night.

• Because cellular respiration occurs mostly at night in plants.

• There is no real advantage to performing the C4 pathway at night, they just evolved like that. 

Answer explanation

CAM plants are found mainly in hot and dry environments like a desert. If the plants were to open their stomata during the day, they would lose a tremendous amount of water. To avoid this problem, they open only their stomata at night. Because gas exchanges can mainly occur at night when stomata are open, the CAM plants will fix large amounts of CO₂ during the night for RuBisCO to have enough CO₂ to fix and avoid photorespiration during the day.