life11e_ch10

Light absorption results in photochemical change

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When a pigment molecule absorbs light, it enters an excited state. This is an unstable situation, and the molecule rapidly returns to its ground state, releasing most of the absorbed energy. So rapid is this process that it is measured in picoseconds (trillionths of a second). Within the antennae system of a photosystem (Figure 10.6A), the energy released by a pigment molecule (for example, chlorophyll b) is absorbed by other, adjacent pigment molecules. The energy (not as electrons but in the form of chemical energy called resonance) is passed from molecule to molecule until it reaches a chlorophyll a molecule at the reaction center of the photosystem (Figure 10.6B).

Figure 10.6 Energy Transfer and Electron Transport (A) The molecular structure of a single light-harvesting complex shows the polypeptide in brown with three helices that span the thylakoid membrane. Pigment molecules (carotenoids and chlorophylls a and b) are bound to the polypeptide. (B) This simplified illustration of the entire photosystem uses chlorophyll molecules to represent the light-harvesting complexes. Energy from a photon is transferred from one pigment molecule to another, until it reaches a chlorophyll a molecule in the reaction center. The chlorophyll a molecule can give up its excited electron to an electron acceptor.

A ground-state chlorophyll a molecule at the reaction center (symbolized by Chl) absorbs the energy from the adjacent chlorophylls and becomes excited (Chl*), but to return to the ground state this chlorophyll does not pass the energy to another pigment molecule—something very different occurs. The reaction center converts the absorbed light energy into chemical energy (Figure 10.7). The chlorophyll molecule in the reaction center absorbs sufficient energy that it actually gives up its excited electron to a chemical acceptor:

Chl* + acceptor → Chl⁺ + acceptor^– (10.6)

This, then, is the first consequence of light absorption by chlorophyll: the reaction center chlorophyll (Chl*) loses its excited electron in a redox reaction and becomes Chl⁺. As a result of this transfer of an electron, the chlorophyll gets oxidized, while the acceptor molecule is reduced.

Figure 10.7 Noncyclic Electron Transport Uses Two Photosystems Absorption of light energy by chlorophyll molecules in the reaction centers of photosystems I and II allows them to pass electrons into a series of redox reactions.

Question

Q: An herbicide can act as an electron acceptor, becoming reduced by ferredoxin (Fd). What effect would this have on a plant?

The herbicide rather than NADP reductase would accept electrons from noncyclic photosystem I. NADPH would not be formed. This would severely reduce the transfer of solar energy to chemical energy in the light-requiring reaction system.