PHOTOSYNTHETIC PIGMENTS: CHARACTERISTICS AND MAIN TYPES - BIOLOGY - 2025

The photosynthetic pigments are chemical compounds that absorb and reflect certain wavelengths of visible light, which makes them appear "colorful". Different types of plants, algae and cyanobacteria have photosynthetic pigments, which absorb at different wavelengths and generate different colors, mainly green, yellow and red.

These pigments are necessary for some autotrophic organisms, such as plants, because they help them take advantage of a wide range of wavelengths to produce their food in photosynthesis. As each pigment reacts only with some wavelengths, there are different pigments that allow more light to be captured (photons).

characteristics

As previously mentioned, photosynthetic pigments are chemical elements that are responsible for absorbing the light necessary for the photosynthesis process to take place. Through photosynthesis, the energy from the Sun is converted into chemical energy and sugars.

Sunlight is made up of different wavelengths, which have different colors and energy levels. Not all wavelengths are used equally in photosynthesis, which is why there are different types of photosynthetic pigments.

Photosynthetic organisms contain pigments that absorb only the wavelengths of visible light and reflect others. The set of wavelengths absorbed by a pigment is its absorption spectrum.

A pigment absorbs certain wavelengths, and those that it does not absorb are reflected; the color is simply the light reflected by the pigments. For example, plants appear green because they contain many chlorophyll molecules a and b, which reflect green light.

Types of photosynthetic pigments

Photosynthetic pigments can be divided into three types: chlorophylls, carotenoids, and phycobilins.

Chlorophylls

Chlorophylls are green photosynthetic pigments that contain a porphyrin ring in their structure. They are stable ring-shaped molecules around which electrons are free to migrate.

Because electrons move freely, the ring has the potential to gain or lose electrons easily and therefore has the potential to provide energized electrons to other molecules. This is the fundamental process by which chlorophyll "captures" energy from sunlight.

Types of chlorophylls

There are several types of chlorophyll: a, b, c, d, and e. Of these, only two are found in the chloroplasts of higher plants: chlorophyll a and chlorophyll b. The most important is chlorophyll "a", as it is present in plants, algae and photosynthetic cyanobacteria.

Chlorophyll "a" makes photosynthesis possible by transferring its activated electrons to other molecules that will make sugars.

A second type of chlorophyll is chlorophyll "b", which is found only in so-called green algae and plants. For its part, chlorophyll "c" is found only in the photosynthetic members of the chromista group, such as dinoflagellates.

The differences between the chlorophylls in these major groups was one of the first signs that they were not as closely related as previously thought.

The amount of chlorophyll "b" is about a quarter of the total chlorophyll content. For its part, chlorophyll "a" is found in all photosynthetic plants, which is why it is called universal photosynthetic pigment. It is also called a primary photosynthetic pigment because it performs the primary reaction of photosynthesis.

Of all the pigments that participate in photosynthesis, chlorophyll plays a fundamental role. For this reason, the rest of the photosynthetic pigments are known as accessory pigments.

The use of accessory pigments allows it to absorb a wider range of wavelengths and therefore capture more energy from sunlight.

Carotenoids

Carotenoids are another important group of photosynthetic pigments. These absorb violet and blue-green light.

Carotenoids provide the bright colors that fruits present; For example, the red in tomatoes is due to the presence of lycopene, the yellow in corn seeds is caused by zeaxanthin, and the orange in orange peels is due to β-carotene.

All of these carotenoids are important in attracting animals and promoting the dispersal of the plant's seeds.

Like all photosynthetic pigments, carotenoids help capture light but they also serve another important function: eliminating excess energy from the Sun.

Thus, if a leaf receives a large amount of energy and this energy is not being used, this excess can damage the molecules of the photosynthetic complex. Carotenoids are involved in absorbing excess energy and helping to dissipate it as heat.

Carotenoids are generally red, orange, or yellow pigments, and include the well-known compound carotene, which gives carrots their color. These compounds are made up of two small six-carbon rings connected by a "chain" of carbon atoms.

As a result of their molecular structure, they do not dissolve in water but instead bind to membranes within the cell.

Carotenoids cannot directly use the energy of light for photosynthesis, but must transfer the absorbed energy to the chlorophyll. For this reason, they are considered accessory pigments. Another example of a highly visible accessory pigment is fucoxanthin, which gives marine algae and diatoms their brown color.

Carotenoids can be classified into two groups: carotenes and xanthophylls.

Carotenes

Carotenes are organic compounds widely distributed as pigments in plants and animals. Their general formula is C40H56 and they do not contain oxygen. These pigments are unsaturated hydrocarbons; that is, they have many double bonds and belong to the isoprenoid series.

In plants, carotenes impart yellow, orange, or red colors to flowers (calendula), fruits (pumpkin), and roots (carrot). In animals they are visible in fats (butter), egg yolks, feathers (canary) and shells (lobster).

The most common carotene is β-carotene, which is the precursor to vitamin A and is considered very important for animals.

Xanthophylls

Xanthophylls are yellow pigments whose molecular structure is similar to that of carotenes, but with the difference that they contain oxygen atoms. Some examples are: C40H56O (cryptoxanthin), C40H56O2 (lutein, zeaxanthin) and C40H56O6, which is the characteristic fucoxanthin of brown algae mentioned above.

Carotenes are generally more orange in color than xanthophylls. Both carotenes and xanthophylls are soluble in organic solvents such as chloroform, ethyl ether, among others. Carotenes are more soluble in carbon disulfide compared to xanthophylls.

Functions of carotenoids

- Carotenoids function as accessory pigments. They absorb radiant energy in the middle region of the visible spectrum and transfer it to chlorophyll.

- They protect the chloroplast components from the oxygen generated and released during the photolysis of water. Carotenoids pick up this oxygen through their double bonds and change their molecular structure to a lower energy (harmless) state.

- The excited state of chlorophyll reacts with molecular oxygen to form a highly damaging oxygen state called singlet oxygen. Carotenoids prevent this by turning off the excited state of chlorophyll.

- Three xanthophylls (violoxanthin, antheroxanthin and zeaxanthin) participate in the dissipation of excess energy by converting it into heat.

- Due to their color, carotenoids make flowers and fruits visible for pollination and dispersal by animals.

Phycobilins

Phycobilins are water-soluble pigments and are therefore found in the cytoplasm or stroma of the chloroplast. They occur only in cyanobacteria and red algae (Rhodophyta).

Phycobilins are not only important to organisms that use them to absorb energy from light, but are also used as research tools.

When compounds such as pycocyanin and phycoerythrin are exposed to strong light, they absorb the energy of the light and release it by fluorescing in a very narrow range of wavelengths.

The light produced by this fluorescence is so distinctive and reliable that the phycobilins can be used as chemical "tags." These techniques are widely used in cancer research to "tag" tumor cells.

References

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