Pigments A pigment is a molecule which absorbs some wavelengths (colours) of light but not others. The wavelengths it does not absorb are either reflected, or transmitted through the substance. These unabsorbed wavelengths enter our eyes, so we see the pigment in these colours. Chloroplasts of higher plants contain two different groups of pigments: chlorophylls and carotenoids.

Chlorophylls Chlorophylls absorb mainly in the red and blue violet regions of the light spectrum. They reflect green light, which is why plants look green. Carotenoids absorb mainly in the blue- violet region of the spectrum so look yellow/orange.

Structure of chlorophylls Chlorophylls consist of a head and a tail. The head contains a porphyrin ring centered on magnesium (in many ways the porphyrin structure of chlorophyll resembles the structure of the haem group in animal blood). The porphyrin ring is attached to a long hydrocarbon tail. The different chlorophylls differ in the side chains on the head of the molecule (chlorophyll a has –CH3 as a side chain whereas in chlorophyll b this side chain is –CHO). The different side chains affect the absorption of the chlorophylls.

The structure of chlorophyll a

Structure of Chlorophyll b

Primary pigments and accessory pigments The photosynthetic pigments fall into two groups: primary pigments and accessory pigments. The primary pigments are two forms of chlorophyll a with slightly different absorption peaks. The accessory pigments include other forms of chlorophyll a and b and the carotenoids.

Accessory Pigments Accessory pigments help in absorbing wavelengths of light which otherwise could not be used by the plant, and passing on the energy from this light to chlorophyll. So with different types of pigments in a chloroplast, a wider range of wavelengths of light can be used in photosynthesis than with any one pigment alone.

Absorption spectrum A graph of the absorbance of different wavelengths of light by a pigment.

Action spectrum If you supply a plant with light of a single wavelength and measure the rate of photosynthesis, you find that the rate is greatest with wavelengths around 680 to 700 nm, with another, lower, peak at around 460 nm. A graph of rate of photosynthesis against wavelength is called an action spectrum.

The shape of the action spectrum is very similar to the combined shapes of the absorption spectra of the individual pigments. This similarity can be taken as evidence that the light energy absorbed by the pigments is used in photosynthesis.

Redox reactions Oxidation is the loss of electrons by a substance. When oxygen combines with hydrogen to form water (hydrogen oxide), oxygen takes electrons from hydrogen. The hydrogen is oxidised as it loses electrons. Oxygen is called an oxidising agent because it has a strong tendency to take electrons from other substances. Other oxidising agents are NAD and NADP.

Reduction is the gain of electrons by a substance. In the above example, oxygen has gained electrons from hydrogen and the oxygen becomes reduced as a result. If a substance has a strong tendency to lose electrons and donate them to other substances, it is called a reducing agent. Reduced NADP (NADPH) and reduced NAD (NADH) are reducing agents

Oxidation and reduction always takes place simultaneously, and a reaction in which they occur is called a redox reaction. In biological redox reactions, hydrogen atoms are often involved. A hydrogen atom is, in effect, a hydrogen ion and an electron. Gaining a hydrogen atom means gaining a hydrogen ion and an electron, so this is an example of reduction. During some of the stages of respiration, the coenzyme NAD accepts hydrogen ions and electrons, and becomes reduced: