Chlorophyll

Chlorophyll is a type of molecule (a chemical unit made up of bonded atoms) found in plants, algae, and light-using bacteria. It is the substance that gives leaves their green colour, and it captures sunlight energy for use in photosynthesis—the process by which plants convert carbon dioxide and water into sugar and oxygen.

Chlorophyll is contained in small structures within plant cells called chloroplasts. There it is bound within membranes called thylakoids, which are stacked on top of one another. The arrangement of chlorophyll and proteins makes up a light-capturing system called a photosystem.

Chemical Structure

The chlorophyll molecule has two main parts. The first is a ring made up of four nitrogen-containing rings, with a magnesium atom at its centre. The second is a long fatty tail that anchors the molecule to the thylakoid membrane. The magnesium atom at the centre is essential to how chlorophyll absorbs light.

Why It Is Green

The white light from the sun is made up of different colours. Chlorophyll strongly absorbs blue and red light, but reflects and sends back green light. For this reason, the human eye sees leaves as green.

Types

There are several types of chlorophyll distinguished by letter. Chlorophyll a is the most abundant and is found in all organisms that produce oxygen through photosynthesis. Chlorophyll b is found in land plants and green algae, and helps capture light wavelengths that chlorophyll a does not absorb well. Other types, such as chlorophyll c and d, are found in algae and certain other organisms.

Changes in Leaf Colour

In regions where seasons change, some plants stop making new chlorophyll at certain times of year. When existing chlorophyll breaks down, other pigments that were always present in the leaves—such as red and yellow—become visible. Similarly, the flesh of ripening fruit usually loses its chlorophyll.