Photosynthesis: Calvin Cycle, Light Reactions, and Adaptations

Vocabulary flashcards covering the mechanisms of the Calvin cycle, photophosphorylation, photorespiration, and C4 and CAM plant adaptations.

Photosynthesis

An anabolic process that utilizes light energy to reduce atmospheric $CO_2$ using electrons from water, synthesizing sugars and releasing $O_2$.

Photophosphorylation

The production of ATP during the light reactions, driven by ATP synthase as protons move down their concentration gradient out of the thylakoid space into the stroma.

Cyclic Flow of Electrons

A pathway in the light reactions that uses only photosystem I to pump protons and generate ATP without producing NADPH or releasing oxygen.

RuBP (Ribulose Bisphosphate)

A five-carbon sugar that accepts atmospheric $CO_2$ during the initial carbon fixation step of the Calvin cycle.

Rubisco

The enzyme that catalyzes the attachment of one molecule of atmospheric $CO_2$ to ribulose bisphosphate (RuBP) to form an unstable six-carbon intermediate.

Three-Phosphoglycerate (3-Phosphoglycerate)

The stable three-carbon molecule formed when the unstable six-carbon intermediate splits, giving the $C_3$ cycle its name.

Carbon Fixation (Phase 1)

The initial phase of the Calvin cycle where atmospheric inorganic carbon ($CO_2$) is incorporated into the organic compound RuBP by the enzyme Rubisco.

Reduction (Phase 2)

The Calvin cycle phase where six molecules of 3-phosphoglycerate receive phosphate groups from $6$ ATP and electrons from $6$ NADPH to form six molecules of G3P.

Regeneration of RuBP (Phase 3)

The phase of the Calvin cycle where five remaining molecules of G3P are rearranged using ATP to regenerate three molecules of RuBP.

G3P (Glyceraldehyde 3-Phosphate)

The three-carbon carbohydrate output of the Calvin cycle, of which one molecule leaves the cycle for every three molecules of $CO_2$ fixed.

Photorespiration

An energetically wasteful evolutionary relic occurring when stomata partially close in hot, dry conditions, causing Rubisco to bind $O_2$ instead of $CO_2$ and release $CO_2$ without producing sugar.

PEP Carboxylase

An enzyme in the mesophyll cells of $C_4$ plants that possesses high affinity for $CO_2$ over $O_2$ and fixes low levels of $CO_2$ into the four-carbon compound oxaloacetate.

Bundle Sheath Cells

Specialized cells in $C_4$ leaves connected to mesophyll cells via plasmodesmata, where malate releases concentrated $CO_2$ to Rubisco to run the Calvin cycle.

$C_4$ Pathway

An adaptation to prevent photorespiration that employs spatial separation, fixing atmospheric carbon in mesophyll cells and conducting the Calvin cycle in bundle sheath cells.

CAM (Crassulacean Acid Metabolism)

An adaptation to arid environments utilizing temporal separation, opening stomata at night to store $CO_2$ in organic acids and closing them by day to run the Calvin cycle using light-reaction products.