Chlorophyll Definition and Role in Photosynthesis

Understand the importance of chlorophyll in photosynthesis

This is the chlorophyll B molecule. Chlorophyll is used for photosynthesis. The molecule features a magnesium atom at the center of the chlorin pigment.
This is the chlorophyll B molecule. Chlorophyll is used for photosynthesis. The molecule features a magnesium atom at the center of the chlorin pigment. LAGUNA DESIGN / Getty Images

Chlorophyll Definition

Chlorophyll is the name given to a group of green pigment molecules found in plants, algae, and cyanobacteria. The two most common types of chlorophyll are chlorophyll a, which is a blue-black ester with the chemical formula C55H72MgN4O5, and chlorophyll b, which is a dark green ester with the formula C55H70MgN4O6. Other forms of chlorophyll include chlorophyll c1, c2, d, and f.

The forms of chlorophyll have different side chains and chemical bonds, but all are characterized by a chlorin pigment ring containing a magnesium ion at its center.

The word "chlorophyll" comes from the Greek words chloros, which means "green", and phyllon, which means "leaf". Joseph Bienaimé Caventou and Pierre Joseph Pelletier first isolated and named the molecule in 1817.

Chlorophyll is an essential pigment molecule for photosynthesis, the chemical process plants use to absorb and use energy from light. It's also used as a food coloring (E140) and as a deodorizing agent. As a food coloring, chlorophyll is used to add a green color to pasta, the spirit absinthe, and other foods and beverages. As a waxy organic compound, chlorophyll is not soluble in water. It is mixed with a small amount of oil when it's used in food.

Also Known As: The alternate spelling for chlorophyll is chlorophyl.

Role of Chlorophyll in Photosynthesis

The overall balanced equation for photosynthesis is:

6 CO2 + 6 H2O → C6H12O6 + 6 O2

where carbon dioxide and water react to produce glucose and oxygen. However, the overall reaction doesn't indicate the complexity of the chemical reactions or the molecules that are involved.

Plants and other photosynthetic organisms use chlorophyll to absorb light (usually solar energy) and convert it into chemical energy.

Chlorophyll strongly absorbs blue light and also some red light. It poorly absorbs green (reflects it), which is why chlorophyll-rich leaves and algae appear green.

In plants, chlorophyll surrounds photosystems in the thylakoid membrane of organelles called chloroplasts, which are concentrated in the leaves of plants. Chlorophyll absorbs light and uses resonance energy transfer to energize reaction centers in photosystem I and photosystem II. This happens when energy from a photon (light) removes an electron from chlorophyll in reaction center P680 of photosystem II. The high energy electron enters an electron transport chain. P700 of photosystem I works with photosystem II, although the source of electrons in this chlorophyll molecule can vary.

Electrons that enter the electron transport chain are used to pump hydrogen ions (H+) across the thylakoid membrane of the chloroplast. The chemiosmotic potential is used to produce the energy molecule ATP and to reduce NADP+ to NADPH. NADPH, in turn, is used to reduce carbon dioxide (CO2) into sugars, such as glucose.

Other Pigments and Photosynthesis

Chlorophyll is the most widely recognized molecule used to collect light for photosynthesis, but it's not the only pigment that serves this function.

Chlorophyll belongs to a larger class of molecules called anthocyanins. Some anthocyanins function in conjunction with chlorophyll, while others absorb light independently or at a different point of an organism's life cycle. These molecules may protect plants by changing their coloring to make them less attractive as food and less visible to pests. Other anthocyanins absorb light in the green portion of the spectrum, extending the range of light a plant can use.

Chlorophyll Biosynthesis

Plants make chlorophyll from the molecules glycine and succinyl-CoA. There is an intermediate molecule called protochlorophyllide, which is converted into chlorophyll. In angiosperms, this chemical reaction is light-dependent. These plants are pale if they are grown in darkness because they can't complete the reaction to produce chlorophyll.

Algae and non-vascular plants don't require light to synthesize chlorophyll.

Protochlorophyllide forms toxic free radicals in plants, so chlorophyll biosynthesis is tightly regulated. If iron, magnesium, or iron are deficient, plants may be unable to synthesize enough chlorophyll, appearing pale or chlorotic. Chlorosis may also be caused by improper pH (acidity or alkalinity) or pathogens or insect attack.

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Helmenstine, Anne Marie, Ph.D. "Chlorophyll Definition and Role in Photosynthesis." ThoughtCo, Dec. 1, 2016, thoughtco.com/chlorophyll-definition-role-in-photosynthesis-4117432. Helmenstine, Anne Marie, Ph.D. (2016, December 1). Chlorophyll Definition and Role in Photosynthesis. Retrieved from https://www.thoughtco.com/chlorophyll-definition-role-in-photosynthesis-4117432 Helmenstine, Anne Marie, Ph.D. "Chlorophyll Definition and Role in Photosynthesis." ThoughtCo. https://www.thoughtco.com/chlorophyll-definition-role-in-photosynthesis-4117432 (accessed November 18, 2017).