Calvin Cycle
What cycle uses ATP and NADPH to convert CO₂ into carbohydrate?
The Calvin Cycle.
What is the first phase of the Calvin Cycle?
Carbon fixation
What gas competes with CO₂ for Rubisco’s active site?
O₂
What group of plants fixes carbon twice to avoid photorespiration?
C4 Plants
What molecule provides reducing power for the Calvin Cycle?
NADPH
What 3‑carbon molecule is the actual product of the Calvin Cycle?
G3P (Glyceraldehyde‑3‑phosphate)
What enzyme catalyzes carbon fixation?
Rubisco
What wasteful process occurs when Rubisco binds O₂?
Photorespiration
In C₄ plants, where does the first carbon fixation step occur?
Mesophyll cells
What molecule provides energy for the anabolic reactions of the Calvin Cycle?
ATP
In what part of the chloroplast does the Calvin Cycle take place, and why there?
The stroma, because that’s where ATP and NADPH accumulate.
During the reduction phase, 3‑PG is converted into what molecule?
G3P.
Why is photorespiration considered wasteful?
It consumes ATP and NADPH but produces no sugar.
When do CAM plants fix carbon?
At night
How many ATP are used per CO₂ fixed?
Three
Why does the Calvin Cycle depend on the light reactions even though it does not require light?
It needs ATP and NADPH, which are only produced during the light reactions.
How many ATP and NADPH are required to produce one G3P?
9 ATP and 6 NADPH.
When Rubisco binds O₂, what two molecules are produced?
One 3‑carbon molecule (3‑PG) and one 2‑carbon molecule (phosphoglycolate).
How do CAM plants reduce water loss while still fixing carbon?
They open stomata at night, store carbon as organic acids, and release CO₂ internally during the day.
What is one way a plant can use the G3P produced by the Calvin Cycle?
G3P can be used to make glucose/starch, to make sucrose for transport, or be oxidized to produce ATP for the plant.
Why does producing one molecule of G3P require three turns of the Calvin Cycle?
Each turn fixes one CO₂, and three CO₂ are needed to build the 3‑carbon backbone of G3P.
Why must five G3P molecules be used to regenerate RuBP instead of being used to make sugar?
Because the cycle must regenerate the CO₂ acceptor (RuBP) to continue; only one G3P can exit without collapsing the cycle.
What group of plants fixes carbon twice to avoid photorespiration?
C₄ plants.
What is the key difference between C₄ and CAM strategies in how they avoid photorespiration?
C₄ separates carbon fixation by space (different cells), while CAM separates it by time (night vs. day).
Why is oxygen released during photosynthesis even though the Calvin Cycle does not use oxygen?
Because O₂ comes from water splitting in the light reactions, not from carbon fixation.