Capture of energy from light radiation
In the membranes of the thylakoids of the chloroplasts, there is a set of photosensitive pigments (chlorophylls, carotenes, xanthophylls, ...) associated with proteins, forming the light collector complex (CCL), a system that acts as a solar antenna.
Each light-collecting complex is made up of proteins and hundreds of photosensitive pigments that, when they absorb light energy, channel it to a special molecule, the so-called chlorophyll in the reaction center. Thus, each light collector complex collects the energy of the photons of light and carries it to the photoactive components of the reaction center, which are dimers of chlorophyll a, called P700 and P680 (depending on the wavelength at which their maximum absorption occurs), and are part of photosystems I and II, respectively.
The photosynthetic pigments are located in the thylakoid membranes of the chloroplast, and are responsible for absorbing the light energy that can be transformed into chemical energy. Some examples are chlorophylls (chlorophyll a, b and bacteriochlorophyll), xanthophyll, and carotenoids. Photosynthetic pigments form the functional unit called the photosystem.
Photosynthetic organisms have several types of pigments with different molecular structures. Eukaryotes use chlorophyll a as a pigment responsible for transforming the energy of light into chemical energy. But in addition, photosynthetic cells usually have other photosynthetic pigments, such as plants and green algae, which have chlorophyll b and carotenoids, and diatoms and some protozoa, which have chlorophyll c
Each pigment absorbs light with certain wavelengths:
- The chlorophyll b absorb wavelengths corresponding to red violet light, blue, and orange.
- The carotenoids absorb the lengths of violet, blue and green wavelengths.
When these photosynthetic pigments capture the photons, they are excited, and when they return to their original state they give up an energy that excites, in turn, the neighboring molecule. Thus, the excitation passes from one molecule to another.
The concept of an excited molecule should not be confused with that of an oxidized molecule. An excited molecule has undergone a change in the distribution of its electrons after receiving energy, but when it returns to its primitive state, it gives off less energy than it absorbed to become excited.
The sunlight that reaches a photosynthetic organism is composed of many wavelengths, so the existence of different types of pigments ensures that photons can excite these pigments and begin photosynthesis.