Photosynthesis – Light Reactions & Calvin Cycle Vocabulary

Key vocabulary terms and concise definitions covering the light-dependent reactions, electron transport, Calvin cycle, and related plant structures discussed in the lecture.

Potential Energy (in photosynthesis)

Stored energy created by pumping protons into the thylakoid lumen against their concentration gradient.

Kinetic Energy (in photosynthesis)

Energy of motion released when protons flow back through ATP synthase, driving ATP formation.

Proton Gradient

Difference in proton concentration across the thylakoid membrane that stores potential energy for ATP production.

ATP Synthase

Membrane-embedded enzyme that converts the proton motive force into ATP.

Photosystem II (PSII)

Light-harvesting complex that splits water, releases O₂, and drives ATP production via the electron transport chain.

Photosystem I (PSI)

Light-harvesting complex that receives electrons from PSII and reduces NADP⁺ to NADPH.

Electron Transport Chain (ETC) – chloroplast

Series of proteins that pass excited electrons, pump protons, and link PSII to PSI.

Plastoquinone (PQ)

Mobile electron carrier that transfers electrons from PSII to the cytochrome complex while pumping protons.

Plastocyanin (PC)

Copper-containing protein that shuttles electrons from the cytochrome complex to PSI.

Z-Scheme

Model showing the simultaneous, cooperative action of PSII and PSI as an energy “zig-zag” to make ATP and NADPH.

NADP⁺ Reductase

Enzyme on the stromal side of the thylakoid membrane that reduces NADP⁺ to NADPH using electrons from PSI.

NADPH

Reduced electron carrier produced by PSI; supplies high-energy electrons for the Calvin cycle.

Water Oxidation

Process in PSII that splits H₂O into electrons, protons, and O₂.

P680

Special chlorophyll pair in PSII reaction center that is excited by 680 nm light.

P700

Special chlorophyll pair in PSI reaction center that is excited by 700 nm light.

Light-Dependent Reactions

Exergonic stage of photosynthesis that captures sunlight to make ATP and NADPH.

Calvin Cycle

Light-independent series of reactions in the stroma that fixes CO₂ into sugars using ATP and NADPH.

RuBisCO

Abundant enzyme that catalyzes CO₂ (or O₂) addition to RuBP; key to carbon fixation.

RuBP (Ribulose-1,5-bisphosphate)

Five-carbon acceptor molecule that reacts with CO₂ in the first step of the Calvin cycle.

Carbon Fixation

Incorporation of inorganic CO₂ into an organic molecule (RuBP) to start sugar synthesis.

3-PGA (3-Phosphoglycerate)

First stable three-carbon product formed after RuBP accepts CO₂.

G3P (Glyceraldehyde-3-phosphate)

Energy-rich three-carbon sugar produced in the Calvin cycle; one out of six exits to build glucose.

Reduction Phase (Calvin cycle)

Step that uses ATP and NADPH to convert 3-PGA into high-energy G3P.

Regeneration Phase (Calvin cycle)

Step that rearranges five G3P into three RuBP, using ATP, to keep the cycle running.

Stoma (plural Stomata)

Pore on a leaf surface that regulates gas exchange of CO₂, O₂, and water vapor.

Guard Cells

Pair of cells flanking each stoma; control aperture size by swelling or shrinking.

Photorespiration

Wasteful process where RuBisCO uses O₂ instead of CO₂, releasing CO₂ and reducing photosynthetic efficiency.

Cuticle

Waxy, hydrophobic coating on plant epidermis that limits water loss.

Ferredoxin

Iron-sulfur protein that transfers electrons from PSI to NADP⁺ reductase.

Proton Motive Force

Combined effect of proton gradient and membrane potential that drives ATP synthase.

Resonance Energy Transfer

Process by which excitation energy, not electrons, is passed between pigment molecules toward the reaction center.

Cytochrome Complex

ETC component receiving electrons from PQ; pumps protons and passes electrons to PC.

Light Energy (Photon)

Discrete packet of electromagnetic energy that excites chlorophyll electrons.

Oxidation (in redox)

Loss of electrons from a molecule; e.g., water oxidation in PSII.

Reduction (in redox)

Gain of electrons by a molecule; e.g., NADP⁺ reduction to NADPH.