The Purpose of Chlorophyll in Plants

Chlorophyll is a plant pigment that makes plants appear green and is a molecule that harvests light energy to be converted into chemical energy. The harvesting of light energy and the subsequent steps are part of the important process known as photosynthesis.

Significance

The sun is the source of all energy on Earth. Plants are a critical part of the energy conversion system that makes life possible. By converting electromagnetic energy from the sun into chemical energy, plants are at the base of food chains, and thus provide energy to higher organisms.
Even fossil fuels owe their stored chemical energy to the sun's energy, which was initially captured by ancient plants before fossilization. The chlorophyll in plants (and some other organisms) lies at the heart of this energy conversion process.

Chlorophyll in Plants

Chlorophyll is found in plant organelles called plastids. Plastids are membrane-bounded structures exclusive to plant cells. Chromoplasts are pigment-bearing plastids, and chloroplasts are chlorophyll-containing chromoplasts.
Chromoplasts have a double membrane surrounding a liquid filling called the stroma. Within the stroma are disk-like sacs called thylakoids, and the individual thylakoids are further organized into stacks called grana.
Thylakoids contain many chlorophyll molecules. The chlorophyll molecules do not float freely in thylakoids but are bound to photoreceptor proteins. These proteins facilitate photosynthesis reactions by making the interactions between chlorophyll, carbon dioxide and other components of the pathway more efficient. Chloroplasts containing chlorophyll are generally concentrated in plant mesophyll tissues near the upper surfaces of plant leaves.

Chlorophyll Structure

Chlorophyll is similar to the oxygen- and carbon dioxide-carrying heme group found in hemoglobin, which is present in animal red blood cells. A major difference is that hemoglobin contains a central iron ion, while chlorophyll contains a central magnesium ion. Both heme and chlorophyll are porphyrins.
Several similar but distinct types of chlorophyll exist. They are differentiated by modifications to the main molecule side chains. Chlorophylls a and b are the principle green photosynthetic pigments in land plants. Plants appear green because chlorophylls absorb red and blue light, but reflect or let pass green light.

Chlorophyll and Photosynthesis

Photosynthesis is the conversion of water and carbon dioxide into sugar using light energy. The energy stored in the sugar chemical bonds is then used to synthesize adenosine triphosphate (ATP), a universal energy-transfer molecule.
When a photon of the correct wavelength (color) strikes chlorophyll, the chlorophyll absorbs the photon and ejects an electron. This energetic electron is passed through a complex chain of reactions to lower energy states, and the energy released is used to drive the chemical reactions that ultimately lead to the synthesis of sugar.

Other Plant Pigments

In addition to chlorophyll, plants contain other pigments that are red, yellow, orange or purple. These pigments absorb different wavelengths of light other than chlorophyll and are responsible for the fall colors of leaves. These pigments become apparent in tree leaves when chlorophyll disappears as the trees prepare for reduced metabolic activity before winter.
Carotenoids are red, yellow or orange. They cannot convert light energy direct into chemical energy but transfer energy to chlorophyll for further processing.
Anthocyanins occur in many colors and absorb ultraviolet (UV) light. They play a role in attracting pollinators that can see in the UV range.
Xanthophylls are yellow and, like carotenoids, are accessory pigments with a minor role in photosynthesis.

References

• Plant Physiology (4th Edition); Robert M. Devlin, Francis H. Witham; 1983

Resources

• Chlorophyll