C1.3 PhotosynthesisIB Biology HL: Revision notes

Photosynthesis transforms light energy into chemical energy in carbon compounds, supplying most of the energy for life in ecosystems.

carbon dioxide + water → glucose + oxygen

Hydrogen from splitting water is used to reduce carbon dioxide to glucose; oxygen from the water is released as a by-product by plants, algae and cyanobacteria.

Pigments are separated by thin-layer or paper chromatography; Rf = distance moved by pigment ÷ distance moved by solvent front. Identify pigments by colour and Rf: carotene (yellow-orange), xanthophyll (yellow), chlorophyll a (blue-green), chlorophyll b (yellow-green).

Absorbing a photon excites an electron. Only wavelengths whose photon energy matches the energy gap are absorbed. An absorption spectrum shows light absorbed at each wavelength; an action spectrum shows the rate of photosynthesis at each wavelength. Both peak in blue and red; green is poorly absorbed.

CO₂ concentration (sodium hydrogencarbonate), light intensity (lamp distance) and temperature (water baths) can be varied to test hypotheses. Identify the independent, dependent and controlled variables. A hypothesis is a provisional explanation that needs repeated testing.

Enclosed greenhouse experiments give good control; FACE (free-air CO₂ enrichment) experiments in fields and natural ecosystems are more realistic but less controlled.

A photosystem is an array of chlorophyll and accessory pigments in a membrane, with a special chlorophyll at the reaction centre that emits an excited electron. Photosystems occur in cyanobacteria and in chloroplasts of photosynthetic eukaryotes. The array absorbs a wide range of wavelengths and funnels energy to the reaction centre; a single pigment molecule could not perform any part of photosynthesis.

• Photosystem II: photolysis of water replaces lost electrons, giving protons, electrons and oxygen (a waste product).
• Electron carriers use energy from electrons to pump protons into the thylakoid space.
• Chemiosmosis: protons diffuse back through ATP synthase, making ATP. In non-cyclic photophosphorylation electrons come from PSII; in cyclic photophosphorylation electrons from PSI return to the carriers.
• Photosystem I: NADP accepts two electrons from PSI and a hydrogen ion from the stroma to form reduced NADP.

The thylakoid membrane holds the photosystems, electron carriers and ATP synthase. Photolysis releases protons into the thylakoid space (lumen), and proton pumping adds more, so the small lumen quickly builds a steep proton gradient. ATP synthase and the reduction of NADP face the stroma, where the ATP and reduced NADP are used in the Calvin cycle.

The advent of oxygen generation by photolysis had immense consequences: oxidation of dissolved iron (banded iron formations), toxicity to anaerobes, aerobic respiration, and the ozone layer.

• Carbon fixation: Rubisco combines CO₂ with RuBP, producing glycerate 3-phosphate (GP). Rubisco is the most abundant enzyme on Earth; high concentrations are needed because it works slowly and is ineffective at low CO₂.
• Reduction: GP is converted to triose phosphate (TP) using reduced NADP and ATP.
• Regeneration: five TP make three RuBP, using ATP. If glucose is the product, five-sixths of the TP must be recycled.

All carbon in the organism is fixed in the Calvin cycle; other carbon compounds (amino acids, lipids and so on) are made by pathways that start from cycle intermediates, using mineral nutrients such as nitrate.

The light-independent reactions need ATP and reduced NADP from the light-dependent reactions: in the dark, GP accumulates and RuBP falls. The light-dependent reactions need NADP and ADP returned from the Calvin cycle: without CO₂, NADP stays reduced, electrons have no acceptor and photosystem II stops.