Light-Independent Reactions (Calvin Cycle) Key Points

Light-Independent Reactions (Calvin Cycle)

• Converts $CO_2$ to glyceraldehyde-3-phosphate (G3P) occurs in the stroma.

• G3P is a precursor for monosaccharides, lipids, amino acids, and nucleic acids.

From Carbon Dioxide to Glucose

• $CO<em>2$ (oxidized) is converted to $C</em>6H<em>{12}O</em>6$ (glucose, reduced).

• Synthesis of glucose is endergonic, using ATP and NADPH from light-dependent reactions.

Calvin Cycle Phases

• Phase 1: Carbon Fixation

• Phase 2: Reduction

• Phase 3: Regeneration of RuBP

Phase 1: Carbon Fixation

• $CO_2$ reacts with ribulose-1,5-bisphosphate (RuBP) to produce two 3-phosphoglycerate (3PG) molecules.

• Enzyme: Rubisco

• $CO_2 + RuBP \rightarrow 2 \times 3PG$

C3 Photosynthesis

• Most plants use C3 photosynthesis: converting $CO_2$ to G3P using only the Calvin cycle.

• Example: rice, wheat, soybeans, potatoes.

• Named C3 because 3PG is the first stable product.

Phase 2: Reduction

• 3PG is converted to G3P using ATP and NADPH.

Calvin Cycle - Carbon Balance

• 3 turns of the Calvin Cycle are needed to make 1 G3P; requires 3 $CO_2$.

Phase 3: Regeneration

• G3P is used to regenerate RuBP, requiring ATP.

• 6 turns synthesize two G3P molecules which combine to form one glucose.

• For every 6 $CO_2$ fixed: 12 G3P made, 2 G3P form glucose, 10 G3P regenerate 6 RuBP.

Net Equation

• $6CO_2 + 18 ATP + 12 NADPH + water \rightarrow 2 G3P + 16 Pi + 18 ADP + 12 NADP^+$

• ATP used in reduction and RuBP regeneration; cyclic photophosphorylation needed.

Rubisco and Photorespiration

• Rubisco uses $CO<em>2$ or $O</em>2$ as substrate.

• Carboxylase activity: Calvin Cycle reduces $CO_2$ into sugars.

• Oxygenase activity: Photorespiration loses C as $CO<em>2$, occurring when $O</em>2$ is high and $CO_2$ is low.

Adaptations to Minimize Water Loss

• Small thick leaves, thick cuticle, stomata in depressions, shedding leaves.

Photorespiration Details

• $O_2$ reacts with RuBP, reversing carbon fixation, more pronounced at high temperatures.

• Products: phosphoglycolate (2C) and 3-phosphoglycerate (3C). Reduces photosynthesis efficiency (0.1% to 3%).

C4 Plants

• Examples: corn, sugarcane, sorghum.

• Use PEP carboxylase to fix $CO_2$ in mesophyll cells, then deliver it to bundle sheath cells for the Calvin cycle. Physical separation.

CAM Plants

• Examples: cacti, pineapple, aloe.

• Carbon dioxide fixation separated from the Calvin cycle by time of day (temporal separation).

• Night: stomata open, $CO_2$ fixed into organic acids.

• Day: stomata closed, Calvin cycle uses $CO_2$ released from organic acids.