Photosynthesis: Key Concepts, Pigments, and Calvin Cycle

Light dependent reaction

Uses light energy in the thylakoid membranes to make ATP, NADPH, and oxygen.

Light independent reaction (Calvin Cycle)

Uses ATP and NADPH in the stroma to make sugars from CO₂.

Photosynthetic pigments

Absorb light energy in the visible spectrum, mainly blue and red light.

Chlorophyll a

The main pigment that directly drives photosynthesis.

Chlorophyll b

An accessory pigment that passes energy to chlorophyll a.

Carotenoids

Accessory pigments that absorb blue-green light and protect chlorophyll.

Xanthophyll

A yellow pigment that helps protect the photosystems.

Phycobilins

Pigments in cyanobacteria and red algae that absorb green and yellow light.

Antenna pigments

Capture light and transfer energy to the reaction center.

Action spectrum

Shows how effective different wavelengths of light are for photosynthesis.

Chloroplast

The organelle where photosynthesis occurs.

Grana

Stacks of thylakoids that increase surface area for light reactions.

Stroma

The fluid where the Calvin Cycle occurs.

Thylakoids

Membrane sacs where light reactions take place.

Photosystem

A pigment-protein complex that absorbs light energy.

Reaction center

The chlorophyll a molecule that releases excited electrons.

Photosystem I (PS I)

Produces NADPH using light energy.

Photosystem II (PS II)

Splits water to replace lost electrons.

P700

The reaction center of PS I that reduces NADP⁺ to NADPH.

P680

The reaction center of PS II that drives water splitting.

Noncyclic flow

Moves electrons from water to NADPH, producing ATP, NADPH, and oxygen.

Cyclic photophosphorylation

Produces ATP only by cycling electrons in PS I.

Noncyclic photophosphorylation

Produces ATP using a linear flow of electrons.

Primary electron acceptor

The first molecule to receive electrons from chlorophyll.

Photolysis

The splitting of water into oxygen, protons, and electrons.

Electron transport chain

Transfers electrons and pumps protons across the thylakoid membrane.

Cytochromes

Iron-containing proteins that carry electrons.

Chemiosmosis

Makes ATP using a proton gradient.

ATP synthase channel

Produces ATP as protons flow through it.

Thylakoid space (lumen)

Where protons build up during light reactions.

NADP⁺ and NADPH

Electron carriers where NADP⁺ gains electrons to become NADPH.

Phosphoglyceraldehyde (PGAL)

A three-carbon sugar made in the Calvin Cycle.

Carbon fixation

Attaches CO₂ to RuBP.

Reduction reaction

Adds electrons to form higher-energy molecules.

Ribulose biphosphate (RuBP)

The five-carbon molecule that accepts CO₂.

3-phosphoglycerate (3-PGA)

The first stable product of carbon fixation.

C-3 plants

Have the foundational photosynthetic system, use only the Calvin Cycle, and are inefficient in hot climates.

Rubisco

The enzyme that fixes CO₂ but can also bind oxygen.

C-4 plants

Evolved to reduce photorespiration in hot, dry environments.

Photorespiration

Occurs when rubisco binds oxygen, wasting energy.