light reactions

Oxidation

Loss of electrons in a redox reaction.

Reduction

Gain of electrons in a redox reaction.

Photosynthesis

Process by which plants convert light energy into chemical energy (glucose).

Cellular Respiration

Process that harvests energy from glucose by oxidizing it to form ATP.

Autotrophs

Organisms that make their own food using inorganic materials and light energy.

Chloroplast

Organelle where photosynthesis occurs; contains thylakoids and stroma.

Thylakoid

Membranous sac in chloroplast where light reactions occur.

Grana

Stacks of thylakoids that increase surface area for light absorption.

Stroma

Fluid-filled space in chloroplast where the Calvin cycle takes place.

Chlorophyll

Pigment that absorbs light energy, mainly blue-violet and red-orange wavelengths.

Chlorophyll a

Main pigment that directly participates in light reactions.

Chlorophyll b

Accessory pigment that transfers energy to chlorophyll a.

Carotenoids

Pigments that absorb excess light to protect chlorophyll from damage.

Visible Light

Portion of the electromagnetic spectrum used in photosynthesis.

Photon

Packet of light energy; shorter wavelength = higher energy.

Photosystem

Light-harvesting complex in thylakoid membranes that captures solar energy.

Reaction Center

Chlorophyll a molecule and primary electron acceptor that initiate electron flow.

Photosystem II (P680)

Absorbs light at 680 nm; splits water to release O₂ and electrons.

Photosystem I (P700)

Absorbs light at 700 nm; transfers electrons to NADP⁺ to form NADPH.

Primary Electron Acceptor

Molecule that receives energized electrons from chlorophyll a.

Noncyclic Electron Flow

Linear electron flow through PS II and PS I producing ATP, NADPH, and O₂.

Water Splitting

Occurs in PS II; water molecules split into electrons, H⁺ ions, and oxygen gas.

Electron Transport Chain

Transfers electrons and pumps H⁺ into thylakoid space to form a gradient.

Proton Gradient

Build-up of H⁺ ions inside thylakoid that drives ATP production.

Chemiosmosis

Movement of H⁺ ions across thylakoid membrane that powers ATP synthesis.

ATP Synthase

Enzyme embedded in the thylakoid membrane that synthesizes ATP from ADP and Pi.

ATP Synthase Function

Uses energy from the H⁺ gradient as protons flow through it back to the stroma.

ATP Synthase Structure

Flask-shaped protein complex that provides a channel for H⁺ diffusion.

ATP Synthase Process

Converts ADP + Pi into ATP as H⁺ ions pass through by facilitated diffusion.

Photophosphorylation

ATP formation in photosynthesis driven by light energy through chemiosmosis.

Cyclic Electron Flow

Uses only PS I; electrons return to P700 via ferredoxin and cytochrome complex.

Cyclic Electron Flow Product

Produces ATP but not NADPH or O₂.

Purpose of Cyclic Flow

Balances ATP/NADPH ratio and prevents excess light damage to photosystems.

When Cyclic Flow Occurs

When NADPH levels are high or additional ATP is required by the Calvin cycle.

NADP⁺ Reductase

Enzyme that catalyzes transfer of electrons to NADP⁺, forming NADPH.

Products of Light Reactions

ATP, NADPH, and O₂.

Calvin Cycle

Light-independent process in stroma using ATP and NADPH to fix CO₂ into glucose.

Carbon Fixation

Incorporation of CO₂ into organic molecules during the Calvin cycle.

Role of ATP in Calvin Cycle

Provides energy for sugar and carbohydrate synthesis.

Role of NADPH in Calvin Cycle

Supplies high-energy electrons to reduce carbon compounds.

End Result of Photosynthesis

Formation of glucose and O₂ using light, CO₂, and water.

Overall Energy Conversion

Light energy → chemical energy (ATP and NADPH) → glucose synthesis.