Calvin Cycle & Photorespiration Vocabulary

Vocabulary flashcards summarizing key terms, molecules, phases, organelles, and energetic costs related to the Calvin cycle and photorespiration as discussed in the lecture.

Calvin Cycle

Light-independent series of reactions in the chloroplast stroma that fixes CO₂ and ultimately produces sugars.

Stroma

Fluid interior of the chloroplast where the Calvin cycle (dark reactions) occurs.

Chloroplast

Plant organelle responsible for photosynthesis; houses thylakoids (light reactions) and stroma (Calvin cycle).

Carbon Fixation

First step of the Calvin cycle in which inorganic CO₂ is attached to RuBP to form an unstable 6-carbon intermediate.

Rubisco

Ribulose-1,5-bisphosphate carboxylase/oxygenase; enzyme that catalyzes CO₂ (or O₂) addition to RuBP.

RuBP (Ribulose-1,5-bisphosphate)

5-carbon sugar that accepts CO₂ in the Calvin cycle; must be regenerated each turn.

3-PGA (3-Phosphoglycerate)

Stable 3-carbon molecule produced after the 6-carbon intermediate splits during carbon fixation.

G3P (Glyceraldehyde-3-phosphate)

3-carbon carbohydrate produced in the Calvin cycle; exported to form glucose and other sugars.

ATP

Energy currency; 3 ATP are used per CO₂ fixed in the Calvin cycle (18 ATP for one glucose).

NADPH

Reduced electron carrier; 2 NADPH are consumed per CO₂ fixed (12 NADPH for one glucose).

Fixation Phase

Calvin cycle stage where CO₂ combines with RuBP via Rubisco to form 3-PGA.

Reduction Phase

Stage where 3-PGA is phosphorylated and reduced to G3P using ATP and NADPH.

Regeneration Phase

Stage that rearranges some G3P to reform RuBP, enabling the cycle to continue.

C₃ Cycle

Another term for the Calvin cycle, named for its 3-carbon first stable product (3-PGA).

C₂ Cycle

Alternative pathway (photorespiration) triggered when Rubisco fixes O₂ instead of CO₂.

Photorespiration

Process where Rubisco incorporates O₂, producing toxic 2-PG, wasting ATP/NADPH and releasing CO₂.

Cyclic Photophosphorylation

Light reaction that produces ATP but not NADPH; can still power the Calvin cycle’s ATP demands.

Non-cyclic Photophosphorylation

Standard light reaction producing both ATP and NADPH for the Calvin cycle.

2-Phosphoglycolate (2-PG)

Toxic 2-carbon compound formed when Rubisco fixes O₂; initiates the photorespiratory pathway.

Peroxisome

Organelle where glycolate is converted during photorespiration; involved in detoxification.

Serine

Amino acid intermediate generated in the mitochondrion during photorespiration before returning carbon to the Calvin cycle.

Energetic Cost of Calvin Cycle

Each CO₂ fixed requires 3 ATP and 2 NADPH; synthesizing one glucose (6 CO₂) uses 18 ATP and 12 NADPH.