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Hydrocarbon tails secure chlorophyll molecules to hydrophobic proteins inside the thylakoid membrane.

Chlorophyll molecules

Stroma

Hydrocarbon tails secure chlorophyll molecules to hydrophobic proteins inside the thylakoid membrane.

Chlorophyll Membrane

Thylakoid membrane

Thylakoid interior charged. In plants, the pigment molecule in the reaction center is always a molecule of chlorophyll a. There are many other chlorophyll a molecules in the antenna system, but all of them absorb light at shorter wavelengths than does the molecule in the reaction center.

Thylakoid membrane

Thylakoid interior ergy is given off as heat and the rest is given off as light energy, or fluorescence. Because some of the absorbed light energy is lost as heat, the fluorescence has less energy and longer wavelengths than the absorbed light. When there is fluorescence, there are no permanent chemical changes or biological functions—no chemical work is done.

Any pigment molecule can become excited when its absorption spectrum matches the energies of incoming photons. If fluorescence does not occur, that pigment molecule may pass the absorbed energy along to another molecule, provided that the target molecule is very near, has the right orientation, and has the appropriate structure to receive the energy.

The pigments in photosynthetic organisms are arranged into energy-absorbing antenna systems. In these systems, the pigments are packed together and attached to thylakoid membrane proteins in such a way that the excitation energy from an absorbed photon can be passed along from one pigment molecule in the system to another (see Figures 8.7 and 8.8). Excitation energy moves from pigments that absorb shorter wavelengths (higher energy) to pigments that absorb longer wavelengths (lower energy). Thus the excitation ends up in the one pigment molecule in the antenna system that absorbs the longest wavelengths; this molecule is in the reaction center of the antenna system.

It is the reaction center that converts the light absorbed into chemical energy. It is in the reaction center that a molecule absorbs sufficient energy that it actually gives up its excited electron (is chemically oxidized) and becomes positively

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