Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 10.3 The Light Reactions Convert Solar Energy to the Chemical Energy of ATP and NADPH
The light reactions of photosynthesis transform solar energy into chemical energy in the form of ATP and NADPH, which power the Calvin cycle.
Light as Energy
Light is electromagnetic radiation, traveling in waves
Photons are light particles; shorter wavelengths = higher energy
Visible light (380–740 nm) drives photosynthesis
Photosynthetic Pigments
Pigments absorb specific wavelengths of light
Chlorophyll a is the main pigment; chlorophyll b and carotenoids are accessory pigments
Violet-blue and red light are most effective; green is least absorbed
Carotenoids also protect from excess light damage (photoprotection)
Absorption and Action Spectra
Absorption spectrum: shows how much light a pigment absorbs at each wavelength
Action spectrum: shows photosynthetic activity at each wavelength
Engelmann’s classic experiment confirmed that red and violet-blue light drive photosynthesis most effectively
Excitation of Chlorophyll
Absorption of light excites electrons to a higher energy state
In isolated pigments, energy is lost as heat or fluorescence
In chloroplasts, excited electrons are captured by a primary electron acceptor, enabling redox reactions
Photosystems and Linear Electron Flow
Photosystems are complexes of proteins and pigments in thylakoid membranes
Two types: Photosystem II (P680) and Photosystem I (P700)
Linear electron flow:
Water is split → electrons passed through PS II → PS I
ATP produced via chemiosmosis; NADP⁺ reduced to NADPH
O₂ is released as a by-product
Cyclic Electron Flow
Uses only Photosystem I
Electrons cycle back to the cytochrome complex
Produces ATP but no NADPH or O₂
May help balance ATP/NADPH needs or protect under intense light
Chemiosmosis and ATP Production
Electron transport chain pumps H⁺ into thylakoid space
H⁺ diffuses back into stroma through ATP synthase → ATP is produced
Chloroplast chemiosmosis is similar to that in mitochondria but powered by light rather than food-derived electrons
ATP and NADPH are released into the stroma to fuel the Calvin cycle
In a Nutshell
The light reactions use solar energy to split water, release O₂, produce ATP via chemiosmosis, and reduce NADP⁺ to NADPH. These energy carriers are essential for the sugar-making Calvin cycle that follows.