Calvin Cycle and Photosynthesis: Key Concepts and Enzymes

Where does the Calvin cycle occur within the chloroplast?

In the stroma.

What are the primary inputs for the Calvin cycle derived from light reactions?

ATP and NADPH.

What is the primary enzyme responsible for carbon fixation in the Calvin cycle?

Rubisco.

What is the first stable product formed during the carbon fixation phase of the Calvin cycle?

PGA (3-phosphoglycerate).

What is the high-energy sugar product (triose phosphate) produced during the reduction phase?

PGAL (G3P).

Which phase of the Calvin cycle requires ATP but not NADPH?

Regeneration (rearranging PGAL into RuBP).

Why does the Calvin cycle require extra ATP beyond what is produced in linear electron flow?

Because the cycle uses more ATP than NADPH, requiring cyclic electron flow to generate the surplus.

What is the difference in energy levels between PGA and PGAL?

PGA is a lower-energy first product, while PGAL is a higher-energy sugar product.

What is the consequence of Rubisco binding to O2 instead of CO2?

Photorespiration occurs, which consumes energy and produces no sugar.

What is the primary trade-off regarding the enzyme Rubisco?

Higher specificity for CO2 results in slower catalytic speed.

What is the key difference between C3, C4, and CAM plants regarding their carbon fixation strategies?

C3 uses standard fixation, C4 uses spatial separation (different cells), and CAM uses temporal separation (different times).

In C4 plants, which enzyme is used to fix CO2 into oxaloacetate?

PEP carboxylase (PEPCase).

How do C4 plants achieve spatial separation of carbon fixation?

CO2 is fixed into malic acid in mesophyll cells, then transported to bundle sheath cells to release CO2 for the Calvin cycle.

When do CAM plants fix CO2, and where is it stored?

They fix CO2 at night and store it as malic acid in the vacuole.

Under what environmental conditions is the C4 pathway most favored?

High temperatures, high light, and low CO2 concentrations.

How does increased temperature affect the efficiency of photosynthesis?

It decreases CO2 solubility, which increases the rate of photorespiration.

What is the function of the Bowen Ratio in plant physiology?

It represents the ratio of sensible heat to evaporative heat, indicating how stomatal opening affects leaf temperature.

What is the purpose of Non-Photochemical Quenching (NPQ)?

It protects the plant from excess light energy by converting it into heat using the xanthophyll cycle.

How do chloroplasts adapt to varying light intensities?

They move to avoid damage under high light and maximize absorption under low light.

What are the observed effects of increased CO2 levels on plants in FACE experiments?

Increased photosynthesis, decreased stomatal opening, and increased plant temperature.

What is the difference between starch and sucrose in terms of plant sugar usage?

Starch is used for storage within the chloroplast, while sucrose is used for transport in the cytosol.