Fundamentals of Molecular Biology. Part 1: Molecular Biology of the Cell - A.N. Ogurtsov 2011

Photosynthetic Systems
Light-Harvesting Complexes

Although the chlorophyll a molecule of the reaction center is capable of directly absorbing light and initiating Photosynthesis, it is excited primarily indirectly via energy received from light-harvesting complexes (LHCs) associated with the reaction center of the light-harvesting antenna. An example of an LHC is the light-harvesting antenna of a cyanobacterium (Figure 179).

Indeed, even under maximum illumination conditions, such as those at noon in the tropics, each reaction center chlorophyll a absorbs only one photon per second, which is insufficient to maintain an adequate rate of photosynthesis in plants.

The Use of light-harvesting antenna complexes significantly enhances photosynthetic efficiency both by increasing absorption at a wavelength of 680 nm and by utilizing the absorption properties of other pigment molecules across different spectral ranges.

Energy absorbed by pigment molecules is rapidly transferred (in under one nanosecond) to one of the two Components of the dimer, formed by two chlorophyll a molecules in the reaction center, where it triggers The process of photosynthetic charge Separation.

The Proteins that make up the light-harvesting complexes spatially organize the pigment molecules in such a way that Light absorption and energy transfer to the reaction centers occur optimally. This maximizes rapid and efficient Resonance energy transfer from the antenna pigment molecules to the reaction centers.

In cyanobacteria and higher plants, As a result of photon absorption by the pigment molecule system, energy is resonantly transferred first to the "bridging" chlorophyll molecule in each of the light-harvesting complexes, and from them to the chlorophylls of the reaction centers (Figure 182).

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Figure 182 - Resonance energy transfer from light-harvesting complexes (LHCs) to the reaction center in Photosystem I of a cyanobacterium. Squares indicate chlorophyll molecules

Notably, the molecular Structure of light-harvesting complexes in cyanobacteria and plants is entirely different from that of analogous complexes in green and purple Bacteria, even though both types of complexes contain similar clusters of carotenoid and chlorophyll molecules.

Unlike reaction centers, light-harvesting complexes can only absorb photons, but they cannot release (generate) photoelectrons.

Self-Control Questions

1. Name the main products of photosynthesis.

2. What is the function of the third, thylakoid, membrane of METABOLISM/14.html">Chloroplasts?

3. Name the Stages of Photosynthesis. What processes occur at each stage?

4. What are the Similarities and differences between heme molecules and the chlorophyll a molecule?

5. What are the light-dependent reactions of photosynthesis, and how do they differ from the dark reactions?

6. What components make up Photosystems?

7. What are light-harvesting complexes? Which pigment molecules are constituents of light-harvesting complexes?

8. What is photoelectron transport?

9. What is the function of plastoquinone Q?

10. How is photosynthetic charge separation achieved in the reaction center of a photosystem?

11. Why is the quantum yield of photosystems independent of the incident light wavelength?

12. What is the purpose of resonance energy transfer in light-harvesting antennas?