Photosynthesis: Chloroplast Structure, Light Reactions, and Calvin Cycle

Chloroplast

The organelle in plant cells where photosynthesis takes place. It contains thylakoids and stroma that separate the stages of photosynthesis.

Thylakoid Membrane

The inner membrane system inside the chloroplast where the light-dependent reactions occur; contains chlorophyll, photosystems, and the electron transport chain.

Thylakoid Lumen

The inner space of the thylakoid where protons (H⁺) are pumped during the light reactions to create a proton gradient.

Stroma

The fluid-filled space surrounding the thylakoids where the Calvin cycle (light-independent reactions) occurs.

Photosystem II (PSII)

The first photosystem in the light reactions; it absorbs light to split water and release electrons, oxygen, and protons.

Photosystem I (PSI)

The second photosystem; it absorbs light to re-energize electrons and helps produce NADPH.

Electron Transport Chain (ETC)

A series of proteins in the thylakoid membrane that pass electrons and use their energy to pump protons into the thylakoid lumen.

Proton Gradient

A difference in proton concentration across the thylakoid membrane that stores potential energy used to make ATP.

ATP Synthase

An enzyme that uses the energy of the proton gradient to produce ATP from ADP and phosphate.

Rubisco

The enzyme that catalyzes carbon fixation — attaching CO₂ to RuBP in the Calvin cycle.

RuBP (Ribulose Bisphosphate)

A 5-carbon sugar that binds with CO₂ to start the Calvin cycle.

G3P (Glyceraldehyde-3-Phosphate)

A 3-carbon sugar produced in the Calvin cycle; two G3P molecules combine to form glucose.

Photolysis

The process in Photosystem II where water (H₂O) is split.

3-PGA

The two 3-carbon molecules formed when a 6-carbon molecule splits during the fixation step of the Calvin cycle.

Chemiosmosis

The process by which protons flow back across the thylakoid membrane through ATP synthase.

Light-dependent reactions

The reactions in photosynthesis that use light energy to produce ATP and NADPH.

Light-independent reactions

The reactions in photosynthesis that use ATP and NADPH to fix carbon into sugars.

NADPH

A high-energy electron carrier produced during the light-dependent reactions.

ATP

The energy currency of the cell produced during the light-dependent reactions.

Glucose

A simple sugar formed from two G3P molecules during the Calvin cycle.

Carbon fixation

The process of converting inorganic CO₂ into organic compounds during the Calvin cycle.

Regeneration

The step in the Calvin cycle where some G3P molecules are used to regenerate RuBP.

Reduction

The step in the Calvin cycle where ATP and NADPH convert 3-PGA into G3P.