Biological pigments, also known as biochromes or pigments, refer to the substances or molecules that contain color because of selective color absorption. They often operate by absorbing specific wavelengths of light. As a result, the pigment color usually differs from the structural color. Whereas the pigment color is the same from all the angles, the structural color is an outcome of selective iridescence or reflection. There are usually two types of biological pigments: flower pigments and plant pigments. The color you see in flowers is because of the flower pigment. Plant pigments are responsible for controlling growth, photosynthesis, and development.
How Do Pigments in Plants Work?
Plant Pigments work by absorption of various wavelengths of light. Light is the primary source of energy and the most important environmental factor for all plants. For a plant to survive, specifically in the photosynthesis process, it needs to be absorbent to photoreceptor-treated pigments.
The measure of the wavelength of radiation that a pigment can absorb is known as the absorption spectrum. The type of pigmentation is determined by selective absorption of different wavelengths. For example, in higher plants, chlorophyll absorbs blue and red wavelengths and not green wavelengths, and this gives their leaves the green color characteristics.
The pigment’s molecular structure determines the absorption spectrum of each pigment. In this case, a pigment molecule may absorb some wavelengths and not others. This is because the molecular structure of the pigment has limited energy states that it can absorb.
Once a pigment has absorbed some wavelengths and is rejuvenated to a higher energy state, it can utilize the energy in three of the following possible ways:
- It can discharge it as heat.
- It can discharge it as radiation of a longer wavelength (lower energy).
- The energy can be used in photochemical work, i.e., initiation of chemical change.
Betalains, carotenoids, and Flavonoids are the common plant pigments known to emit most of their absorbed light energy as heat. Other plant pigments such as chlorophyll, rhodopsin, phycobilin, and phytochrome use much of their absorbed energy to produce various chemical changes within the plant.
It is important to note that each plant pigment reacts with a narrow range of the spectrum. Thus, there is a need to produce several other pigments with different colors to capture more sunlight energy.
Principal Pigments Explained
Plants have several pigment molecules, far more than animals. It’s because plants are creatures of light. They utilize their sense of light to manage their development, growth, and rapid response to environmental changes. Importantly, plants use light as their chief source of energy.
Plant pigments have both biological and physiological functions. For example, they can control plants’ growth, development, and photosynthesis and advertise rewards for animals that disperse seeds and pollinate flowers. There are four main types of plant pigments. Their production or retention determines their molecules, the color of leaves they fall from, and the chemical formula that describes the different atoms that make up the molecule. The four types of pigments in plants include Chlorophyll, Carotenoids, Anthocyanins, Betalains.
Chlorophyll is the green photosynthetic pigment in plants and used for photosynthesis. It is produced in the chloroplasts in the photosynthetic tissues of the leaves and occurs in algae, photosynthetic bacteria, and plants.
There are ten types of chlorophyll—chlorophyll a, b, c, d, and e, bacteriochlorophyll a, b, c, d, and e, and bacterioviridin. However, only two types of chlorophyll – chlorophyll a and chlorophyll b are more diverse in higher plants and algae. The pigment drives the process of photosynthesis, which is the process by which plants make their food. Chlorophyll reflects green light and absorbs blue and red light most strongly.
Chlorophyll is an essential pigment in plants that uses light energy to synthesize carbohydrates. All living organisms depend on photosynthesis, either directly or indirectly. Other benefits of chlorophyll include:
- Conversion of sunlight energy into chemical energy.
- Giving plants a green coloration.
- Trapping sunlight, thus allowing easy and free transfer of electrons to carbon dioxide molecules.
- Absorption of energy which then used to transform carbon dioxide and water into carbohydrates and oxygen.
- Absorption of water and other soluble mineral salts from the soil.
The various types of chlorophyll absorb distinct wavelengths of light. The difference in absorption spectra allows a more significant portion of the solar spectrum during photosynthesis. Chlorophyll-a is common in higher plants, cyanobacteria, chloroxybacteria, and algae. Some groups of algae and higher plants also have chlorophyll-b. Several other algae groups contain chlorophyll-c and chlorophyll-d, and towards the end of the leaf’s lifespan, chlorophyll breaks down into nitrogen which the plant reabsorbs.
Carotenoids are very long-chain and water-repelling plant pigments that are synthesized by many bacteria, fungi, and plants. In plants, carotenoids are in fruits, flowers, leaves, stems, and roots. Within the plant cell, carotenoids are in the plastids membrane. Chloroplasts are the most common examples of plastids that store carotenoids as well as perform photosynthesis.
Carotenoids are often red, yellow, or orange and include a familiar compound, carotene, which gives carrots their color. Beta-carotene, produced by the ray flowers’ chromoplasts, is also used in the sunflower flower plant and gives it its bright yellow-orange colors. Beta-carotene also gives other vegetables and sweet potatoes an orange color. Lycopene is another type of carotenoid pigment that gives tomatoes their red color.
The carotenoids work by absorbing the blue wavelengths and allowing the longer wavelengths to be scattered, producing the yellow color.
There are two important benefits of carotenoids in plants. Firstly, they help in photosynthesis. Carotenoid pigment helps transfer the sunlight they receive to chlorophyll, which uses it in photosynthesis. Secondly, carotenoids help in the protection of plants that are over-exposed to sunlight. This is because they dissipate excess light energy, which is absorbed as heat.
Excess light energy can destroy the protein membrane and other molecules in the absence of carotenoids. Other important functions of carotenoids include making fruits and flowers conspicuous to animals for dispersal and pollination. In addition, Beta-carotene is a good source of vitamin A in animals.
Anthocyanins are dispersed and water-soluble pigments in vacuoles, membrane-enclosed structures inside the cells that store nutrients and water. Depending on the PH, anthocyanins may appear purple, red, blue, or black. The pigments are found in fruits, flowers, and vegetables. Plants and fruits like black soybean, blueberry, black rice, raspberry, apples, roses, and wine are some of the foods rich in anthocyanins. In addition, anthocyanins are responsible for the red leaves during autumn.
Anthocyanins are synthesized through the phenylpropanoid pathway and belong to the molecule class of flavonoids. In higher plants, they often occur in fruit where they attract animals that eat fruits and disperse seeds helping in pollination. They can also be found in flowers, where they attract insect pollinators.
Although they color beverages and foods, anthocyanins have not yet been approved to be used as food additives or supplement ingredients. Some of the major benefits of anthocyanins in plants include:
Provide colorationThe blue, red, or purple coloration provided by anthocyanins can attract various herbivorous animals and insects, which may help pollination and seed dispersal plant physiology: Anthocyanins play a good role in protecting plants against extreme temperatures. For example, the tomato plant gets protection from anthocyanins against cold stress. Anthocyanins counter the reactive oxygen species minimizing the cell death in leaves.
Light absorbanceThe light absorbance pattern associated with red color may help in the transfer of light to chlorophyll for purposes of photosynthesis. The red coloring may also protect leaves from herbivores that attract with the green color.
Betalains are nitrogen-containing and water-soluble vacuolar pigments. They are characterized by red or yellow color and contain betaxanthins and betacyanins and are used as color additives in food. The pigments are often noticeable in flower petals and can also be found in leaves, fruits, roots, and stems of plants containing them. There two types of betacyanins:
- Betacyanins: Features the reddish to violet Betalains pigments. Some of the plants that are rich in betacyanins include neobetanin, probetanin, betanin, and isobetanin.
- Betaxanthins: Features yellow to orange Betalains pigments. Some of the common plants with betaxanthins include indicaxanthin, vulgaxanthin, portulaxanthin, and miraxanthin. The key benefits of Betalains in plants include:
- The visual attraction for seed dispersers and pollinators: The red or/and yellow color of the Betalains may attract a variety of herbivorous animals and insects, which may help in pollination and seed dispersal.
- Provide anti-inflammatory, antioxidant, and detoxification support. Research has also shown that Betalains lessens the tumor cell growth and minimizes the risk of cancer.
- The red or/and yellow pigments can also protect plants from herbivores attracted by the green color.
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