Lesson 3: Light-dependent Reaction

photosynthetically active radiation

For the same reasons, plants pigment molecules absorb only light in the wavelength range of 700 nm to 400 nm; plant physiologists refer to this range for plants as _______.

violet

400 nm

red

700 nm

700 nm red

lower energy

400 nm violet

higher energy

Chlorophyll

Absorb blue/red light; reflect green

Carotenoids

Absorb blue; reflect yellow, orange, red. Found in fruits (e.g., lycopene, β-carotene) and act as energy disposal systems in leaves.

Light-dependent Reactions

Purpose: Convert solar energy into chemical energy (NADPH, ATP).

Photosystem II

Reaction center: P680 (named for its absorption peak at 680 nm). First step in the light-dependent reactions.

Photosystem II

Photon absorption, splits water, transfers electrons to PSI.

Photosystem I (PSI)

Reaction center: P700 (named for its absorption peak at 700 nm). Second step in the light-dependent reactions.

Photosystem I (PSI)

Electrons arriving to this (via plastocyanin) are low-energy. It re-energizes these electrons using light energy. Re-energizes electrons and ensures their delivery to NADP⁺, forming NADPH.