Photosynthesis: Light Reactions and the Calvin Cycle

Vocabulary flashcards covering the components, pathways, and mechanisms of the light reactions and the Calvin cycle in photosynthesis based on lecture notes.

Reaction Center Complex

A complex within a photosystem that contains a special pair of chlorophyll molecules and a primary electron acceptor.

Light-Harvesting Complexes

Pigment-protein complexes surrounding the reaction center that absorb photons and transfer excitation energy efficiently between molecules until reaching the reaction center.

Primary Electron Acceptor

A molecule in the reaction center that captures excited electrons from the special chlorophyll $a$ pair, becoming reduced while the chlorophyll is oxidized.

Thylakoid Lumen

The internal space enclosed by the thylakoid membrane where protons accumulate from water splitting and electron transport to establish a proton gradient.

Stroma

The fluid compartment outside the thylakoid membrane where ATP and NADPH are synthesized and where the reactions of the Calvin cycle take place.

P680

The reaction center chlorophyll $a$ of Photosystem II, named because it is most efficient at absorbing light at a wavelength of $680\,\text{nm}$.

$$P_{680}^+$$

The oxidized form of the P680 reaction center chlorophyll, recognized as one of the strongest biological oxidizing agents capable of extracting electrons from water.

Photosystem II (PSII)

The photosystem that functions first in the light reactions, absorbing light to excite electrons from P680 and using water-derived electrons to replenish them.

Photosystem I (PSI)

The photosystem discovered first but functioning second in the light reactions, which contains P700 and transfers excited electrons downstream to ferredoxin.

P700

The reaction center chlorophyll of Photosystem I, which has an absorption peak that is best at a wavelength of $700\,\text{nm}$.

Water Splitting (Photolysis)

The reaction that extracts electrons to reduce oxidized $P_{680}^+$, releasing oxygen as a byproduct and releasing protons into the thylakoid space.

Plastoquinone

An intermediate electron carrier molecule that transfers electrons from the primary electron acceptor of Photosystem II toward the cytochrome complex.

Cytochrome Complex

An electron transport complex that uses the energy released as electrons fall in energy to pump protons from the stroma into the thylakoid space.

Plastocyanin

An electron carrier protein that accepts electrons from the cytochrome complex and transfers them to reduce oxidized P700 in Photosystem I.

Ferredoxin

An iron-containing protein that accepts electrons from the primary electron acceptor of Photosystem I and passes them to NADP+ reductase.

NADP+ Reductase

An enzyme that transfers electrons from ferredoxin along with a stromal proton to $\text{NADP}^+$, reducing it to form NADPH.

NADPH

A reduced coenzyme produced on the stromal side of the thylakoid membrane that serves as a general reducing equivalent and store of chemical energy for the Calvin cycle.

Calvin Cycle

The anabolic, cyclical pathway in the stroma that uses the chemical energy of ATP and the reducing power of NADPH to convert carbon dioxide into sugar.

Rubisco

An abundant plant enzyme that catalyzes the initial carbon fixation step of the Calvin cycle by attaching carbon dioxide to ribulose bisphosphate.

Ribulose Bisphosphate (RuBP)

A five-carbon sugar containing two phosphate groups that acts as the carbon dioxide acceptor in carbon fixation and is regenerated at the end of the Calvin cycle.

3-Phosphoglycerate

The three-carbon intermediate produced when the unstable six-carbon compound formed from RuBP and carbon dioxide splits in the Calvin cycle.

1,3-Bisphosphoglycerate

A phosphorylated intermediate produced in the Calvin cycle when six molecules of 3-phosphoglycerate each receive a phosphate group from ATP.

Glyceraldehyde 3-Phosphate (G3P)

A three-carbon sugar produced during the reduction phase of the Calvin cycle using NADPH; one molecule is set aside as net output for every three fixed molecules of $\text{CO}_2$.

Carbon Fixation Phase

The first phase of the Calvin cycle where inorganic carbon from carbon dioxide is incorporated into an organic molecule (RuBP) via Rubisco.

Reduction Phase

The second phase of the Calvin cycle where 1,3-bisphosphoglycerate accepts electrons and hydrogens from NADPH to form glyceraldehyde 3-phosphate.

Regeneration Phase

The final phase of the Calvin cycle where five remaining molecules of G3P ($15$ carbons total) are rearranged using three ATP to regenerate three molecules of RuBP.