Instant Notes: Plant Biology

ring structure, with a magnesium atomat its center, and a tail of hydrocarbons,

which anchors the molecule to a membrane. Chlorophyll ais the main pigment

of photosynthesis. Most chloroplasts also contain accessory pigments, pigments

that broaden the range of wavelengths at which light can be absorbed and

that pass the energy obtained to chlorophyll aor protect against damage.

Chlorophyllb,carotenoidsandxanthophyllsare examples of these pigments.

To harvest the energy of light, the pigments of photosynthesis must be arranged

so that the energy is focused to a point from which it can be used. Energy

absorbed by the pigment molecules is transferred to one of a central pair of

chlorophyllamolecules (termed the reaction center chlorophylls) by resonance

energy transfer(Fig. 3). From here, the high energy electron is passed on to an

electron acceptor. The pigments are arrayed in flat sheets in the thylakoid

membrane, orientated to capture the incoming radiation. Each reaction center is

surrounded by 200–400 pigment molecules, the antenna complex, and the

whole structure constitutes a photosystem. The thylakoids are stacked in grana

held within the stromaof the chloroplast (Topic B3).

There are two types of photosystem, known as photosystem I(PS-I) and

photosystem II(PS-II). PS-I was the first to be discovered, and the reaction

center chlorophylls in it are known as P 700 because they absorb maximally at

700 nm. The reaction center chlorophylls of PS-II absorb maximally at 680 nm

(P 680 ). The functions of PS-I and PS-II are explained in Topic J2.

The reaction
center

136 Section J – Metabolism

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400 500 600 700

Wavelength (nm)

Percentage absorbance

Rate of photosynthesis(% of rate at 670 nm)

Action

spectrum

Chlorophyll b

Carotenoids

Chlorophyll a

Fig. 1. Absorption spectra of three major pigments of photosynthesis: chlorophyll a,

chlorophyllband carotenoids, together with the action spectrum of photosynthesis. The

absorption spectrum represents the degree to which the pigment is able to absorb energy at

each wavelength of light; the action spectrum represents rate of photosynthesis at each

wavelength (Govindjee, unpublished data, 1961; redrawn with permission of Govindjee,

University of Illinois at Urbana, Illinois, USA).