During the process of cellular respiration, plants, algae and cyanobacteria absorb light energy and convert it to chemical energy through photosynthesis. However, the colour seen by the naked eyes is the light that's reflected, not absorbed, which affects the efficiency of photosynthesis.
Complex photosynthetic organisms like plants and algae have organelles inside their cells called chloroplasts. These organelles have two outer membranes and a semi-fluid interior called a stroma. Inside are a series of tiny compartments stacked on top of one another called thylakoid membranes, which are linked by channels. Here is where pigment molecules harvest light energy. At least one region of atoms in the pigment acts as an antenna to collect the light. Cyanobacteria have a similar system, but they lack some of the organelles of eukaryotic life.
Light is essentially a wave on the electromagnetic spectrum. A wavelength measures the distance between each wave. Visible light, for instance, has a wavelength of 380 to 750 nanometres (a billionth of a meter). Purple is on the shorter end, and red is on the longer end, and on either side of them are ultraviolet and infrared. Shorter wavelengths have higher energy. White light is composed of a combination of the visible spectrum, but different wavelengths may not reach all areas equally.
Pigments can only absorb certain wavelengths of light. The rest of the visible light is reflected back, which gives photosynthetic organisms their colour. Chlorophyll A, for instance, absorbs red and blue-violet but reflects mostly green, giving plants with chlorophyll a an intense green colour. Chlorophyll B varies slightly, reflecting green and yellow light. The reason some deciduous trees and shrubs begin to change colour in the autumn is because they stop making food during that time, and the chlorophyll slowly disappears.
Types of Pigment
Carotenoids absorb blue light and reflect red, orange and yellow. They are often found in fruits and vegetables, but algae and some bacteria use them too. There are over 600 types of carotenoids. Xanthophylls (which use oxygen) and carotenes (which don't use oxygen) are the major groups. Another kind of pigment is anthocyanin, which absorbs blue and green light and gives some flowers and fruits, like blueberries and cherries, a purple or red colour. Phycobilin absorbs orange, yellow, and green light and is often found in algae and cynobacteria.
Since plants, algae, and cyanobacteria are dependent upon the efficient absorption of light so that they can drive cellular metabolism or release hormones at specific times, they are adapted to use a specific pigment for their unique situations and environments. Algae in shallow water, for instance, may use phycobilin to easily capture red-yellow light, but algae deep in water tend to capture blue-green light where it is most prevalent. However, colours in the green wavelength range only need to be absorbed at low levels or not at all, so green is the most abundant colour reflected by photosynthetic organisms, which gives the Earth a familiar green colour. Since blue has a shorter wavelength and provides more energy, it is the colour most abundantly absorbed. Red can provide the remainder of the necessary energy. An organism may grow or work at different variables of efficiency depending on the wavelength of light, but they also can have many different kinds of pigments within each light harvesting complex, preventing stunted or lethargic functionality.
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