The leaves of deciduous trees are usually green in the summer and then turn colors and fall off in autumn. These bright fall colors come from red and orange pigments such as carotenoids. These pigments exist in the leaf all summer, but it is not until the pigment chlorophyll is depleted that the other colors become obvious.
In eukaryotes the light-dependent reactions of photosynthesis occur in the thylakoids of chloroplasts . Chloroplasts are flattened disc shaped organelles that contain stacks of thylakoids called grana . Thylakoids contain the essential components (e.g., pigments and electron transport chains) of the light reactions of photosynthesis. The thylakoid is covered with several types of pigments arranged into photosystems .
This figure shows the structure of a photosystem. In most land plants, there are two types of photosystems -photosystem I and photosystem II . Structurally, these photosystems are similar and the following description of the photosynthetic activity in a photosystem can be applied to either photosystem.
The various pigments in a photosystem (e.g., different forms of chlorophyll and carotenoids) each absorb sunlight of a different wavelength. The absorbed energy raises electrons associated with the pigment molecule to a higher energy state. This causes the electrons to become unstable and the unstable electrons are passed to a specialized chlorophyll molecule at the center of the photosystem (the reaction center). From this reaction center, the electrons will be passed to an electron transport chain.
How does this movement of electrons occur? Recall from earlier tutorials, electrons are found in association with atoms, where they can participate in bond formation. Electrons have varying amounts of energy. In a non-illuminated pigment (one in the dark), critical electrons are usually found in a low energy state. However, the energy from an incoming photon of light can bump the electron into a higher energy state. When this happens, light energy has been absorbed and energy has been transformed.