The Two Pathways of Photosynthesis: An Overview

In plants, photosynthesis takes place in chloroplasts.

In the light reactions of photosynthesis, electron transport and photophosphorylation produce ATP and reduce NADP+ to NADPH + H+.

ATP and NADPH + H+ are needed for the reactions that fix and reduce CO₂ in the Calvin–Benson cycle, forming carbohydrates.

The Interactions of Light and Pigments

Light energy comes in packets called photons, but it also has wavelike properties.

Absorption of a photon puts a pigment molecule in an excited state that has more energy than its ground state.

Pigments absorb light in the visible spectrum

Each compound has a characteristic absorption spectrum. An action spectrum reveals the biological effectiveness of different wavelengths of light. The absorption spectrum of the plant pigment chlorophyll a correlates well with the action spectrum for photosynthesis.

Chlorophylls and accessory pigments form antenna systems for absorption of light energy.

An excited pigment molecule may lose its energy by fluorescence or by transferring it to another pigment molecule.

Electron Transport, Reductions, and Photophosphorylation

Noncyclic electron transport uses two photosystems (I and II) and produces ATP, NADPH + H+, and O₂. Photosystem II uses P680 chlorophyll, from which light-excited electrons are passed to a redox chain that drives chemiosmotic ATP production.

Light-driven oxidation of water releases O₂ and passes electrons from water to the P680 chlorophyll. Photosystem I passes electrons from P700 chlorophyll to another redox chain and then to NADP+, forming NADPH + H+.

Cyclic electron transport uses P700 chlorophyll and produces only ATP. Its operation maintains the proper balance of ATP and NADPH + H+ in the chloroplast.

Chemiosmosis is the mechanism of ATP production in photophosphorylation. Electron transport pumps protons from the stroma into the thylakoids. Diffusion of the protons back to the stroma via ATP synthase channels drives ATP formation.

Making Carbohydrate from CO₂: The Calvin–Benson Cycle

The Calvin–Benson cycle makes sugar from CO₂. This pathway was elucidated through the use of radioactive tracers.

The Calvin–Benson cycle consists of three phases: fixation of CO₂, reduction and carbohydrate production, and regeneration of RuBP. RuBP is the initial CO₂ acceptor, and 3PG is the first stable product of CO₂ fixation. The enzyme rubisco catalyzes the reaction of CO₂ and RuBP to form 3PG.