Once the pigments in the chloroplast have captured light energy, the light energy must then be converted to chemical energy. The chemical energy is temporarily stored in ATP and NADPH. During these reactions, O2 is given off. The chlorophylls and carotenoids are grouped in clusters of a few hundred pigment molecules in the thylakoid membrane. Each cluster of pigment molecules and the proteins that the pigment molecules are embedded in are referred to collectively as a photosystem. Two types of photosystems are known: photosystem I and photosystem II. They contain similar kinds of pigments, but they have different roles in the light reactions.
The light reactions begin when accessory pigment molecules in both photosystems absorb light. By absorbing light, those molecules acquire some of the energy carried by the light. In each photosystem, the acquired energy is passed quickly to the other pigment molecules until it reaches a specific pair of chlorophyll a molecules. Chlorophyll a can also absorb light. The events that occur next can be divided into five steps, as shown in Figure 6-6.
In step Q, light energy forces electrons to enter a higher energy level in the two chlorophyll a molecules of photosystem II. These energized electrons are said to be "excited."
The light reactions, which take place in the thylakoid membrane, use energy from sunlight to produce ATP, NADPH, and oxygen.
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