C1.3 PhotosynthesisIB Biology SL: Revision notes

In photosynthesis, light energy is transformed into chemical energy stored in the carbon compounds produced. This energy transformation supplies most of the chemical energy used for life processes in ecosystems: it passes to consumers and decomposers through food chains.

The simple word equation is:

carbon dioxide + water → glucose + oxygen

Carbon dioxide is converted to glucose using hydrogen obtained by splitting water. The oxygen from the water is released as a by-product. This happens in plants, algae and cyanobacteria.

Evidence: when the oxygen in the water supplied is labelled with ¹⁸O, the oxygen released carries the same label; labelling the carbon dioxide does not label the oxygen released.

Photosynthetic pigments can be separated by thin-layer or paper chromatography. Pigments differ in solubility in the solvent and attraction to the stationary phase, so they travel different distances.

Rf = distance moved by pigment ÷ distance moved by solvent front (both measured from the origin).

Typical order (furthest first, with a non-polar solvent): carotene (yellow-orange), xanthophyll (yellow), chlorophyll a (blue-green), chlorophyll b (yellow-green). Identify pigments by both colour and Rf value.

When a pigment molecule absorbs light, an electron is excited to a higher energy level; this is how light energy is converted to chemical energy. Only certain wavelengths are absorbed because a photon is absorbed only if its energy matches the gap between the electron's energy levels.

An absorption spectrum shows the percentage of light a pigment absorbs at each wavelength (the horizontal axis should show both wavelength in nm and colour). Chlorophyll absorbs blue-violet and red strongly and reflects green.

An action spectrum shows the rate of photosynthesis at each wavelength. Rates can be found from oxygen production or carbon dioxide consumption per unit time.

Similarities: both have peaks in blue and red and a trough in green. Differences: the action spectrum reflects all pigments together, so it is higher than chlorophyll absorption alone in the blue-green region, where carotenoids absorb.

The rate of photosynthesis may be limited by carbon dioxide concentration, light intensity or temperature. Techniques:

• CO₂: different concentrations of sodium hydrogencarbonate solution around pondweed.
• Light intensity: move a lamp to different distances, or use filters / dimmer settings.
• Temperature: water baths at a range of temperatures.

The independent variable is the factor deliberately changed; the dependent variable is the rate measured (e.g. oxygen volume per minute). Controlled variables are kept constant.

NOS: a hypothesis is a provisional explanation that needs repeated testing. It may be based on theory and tested afterwards, or suggested from evidence of an experiment already done.

Atmospheric CO₂ is rising, so experiments predict future photosynthesis and plant growth.

• Enclosed greenhouse experiments: easy to control variables, but conditions differ from natural ones.
• Free-air carbon dioxide enrichment (FACE): CO₂ released from pipes around plots in fields or natural ecosystems; more realistic but less control, and expensive.

FACE results have generally shown smaller gains than enclosed experiments, and other limiting factors (e.g. nitrogen, water) restrict the response. NOS: careful control of variables is part of experimental design; it is easier in the laboratory, but some questions can only be answered by field experiments.