BIOLOGY Volume 1 - A Guide to General Biology - 2004

7. AUTOTROPHIC NUTRITION

7.5. Photosynthetic Pigments

7.5.4. Excitation of Chlorophyll by Light

When a molecule of chlorophyll or another photosynthetic pigment absorbs light, it is said to enter an excited state. The light energy is used to promote electrons to a higher energy level. Light energy is captured by chlorophyll and converted into chemical energy. The excited state of chlorophyll is unstable, and its molecules tend to return to their normal (ground) state. For example, if light is passed through a chlorophyll solution and it is then observed in the dark, the solution can be seen to fluoresce. This occurs because the excess excitation energy is converted into light of a longer wavelength (and lower energy), while the remaining energy is lost as heat. The excited electrons return to their normal low-energy state. In a living plant, the released energy may be transferred to another chlorophyll molecule (see below). Alternatively, the excited electron may be transferred from the chlorophyll molecule to another molecule called an electron acceptor. Since the electron is negatively charged, its departure leaves a positively charged 'electron hole' in the chlorophyll molecule.

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The process of losing electrons is called oxidation, while the process of gaining them is called reduction. Consequently, chlorophyll is oxidized, and the electron acceptor is reduced. Chlorophyll replaces its lost electrons using low-energy electrons from other molecules known as electron Donors.

The initial Stages of Photosynthesis involve The transfer of both energy and excited electrons between molecules within the Photosystems described below.