BIOL005A Lecture 14: Photosynthesis II (The Calvin Cycle)

Vocabulary practice flashcards covering the Calvin cycle, the three reaction phases, RuBisCO functionality and limitations, and metabolic differences between photosynthetic stages from Lecture 14.

Calvin Cycle

The light-independent pathway of photosynthesis that takes place in the chloroplast stroma, using $CO_2$, ATP, and NADPH to synthesize organic sugar molecules such as G3P.

Stroma

The dense fluid within the chloroplast surrounding the thylakoid membranes, serving as the site of the Calvin cycle (dark reactions).

RuBisCO (RuBP Carboxylase Oxygenase)

The enzyme that catalyzes the covalent addition of $CO_2$ to RuBP during carbon fixation; it is by far the most abundant protein on the planet.

RuBP (Ribulose-1,5-bisphosphate)

A $5$-carbon sugar with two phosphate groups that acts as the initial carbon dioxide acceptor in the Calvin cycle.

Carbon Fixation (Phase 1)

The first phase of the Calvin cycle in which RuBisCO covalently joins $CO_2$ to RuBP to create an unstable $6\text{C}$ intermediate that immediately splits into two $3\text{C}$ molecules of $3$-phosphoglycerate.

3-Phosphoglycerate (PGA)

The $3$-carbon molecule formed when the unstable $6\text{C}$ product of carbon fixation splits in two during the Calvin cycle.

Reduction (Phase 2)

The phase of the Calvin cycle in which electrons from NADPH and energy from ATP are added to convert intermediates into six molecules of G3P, one of which exits the cycle as net output.

Glyceraldehyde-3-Phosphate (G3P)

The $3$-carbon sugar output produced by the Calvin cycle that can be utilized to make glucose, fructose, sucrose, starch, and cellulose, or broken down for energy in cellular respiration.

Regeneration of RuBP (Phase 3)

The final phase of the Calvin cycle in which the five remaining $3\text{C}$ G3P molecules are rearranged using $3$ ATP to regenerate three $5\text{C}$ RuBP molecules.

RuBisCO Catalytic Speed

A measure of the catalytic efficiency of RuBisCO, which functions notably slowly by fixing only $3\text{ to }10$ molecules of $CO_2$ per second.

RuBisCO Oxygenase Flaw

The inability of RuBisCO to completely discriminate between $CO_2$ and $O_2$, causing it to fix $O_2$ instead of $CO_2$ in approximately $25\%$ of reactions to produce a wasteful byproduct.

Origin of Plant Dry Mass

Atmospheric $CO_2$ absorbed from the air, which plants reduce using energy from sunlight and electrons and protons from $H_2O$ to synthesize organic polymers.

Light Reactions vs. Dark Reactions

Light reactions occur in the grana/thylakoids, depend on light and photosystems, split $H_2O$, release $O_2$, and produce ATP and NADPH; dark reactions occur in the stroma, require no light or photosystems, absorb $CO_2$, and consume ATP and NADPH to yield sugar.

Overall Chemical Equation of Photosynthesis

The balanced chemical formula summarizing photosynthetic production of hexose: $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$.