Chloroplast

Chloroplasts are the energy-converting structures found in the cells of plants. As one of the many tiny organelles (structures inside a plant that have a particular function) in a plant cell, it is the site where photosynthesis (the process by which plants convert the sun's energy into food) occurs. Chloroplasts are not found in animal cells and are the most distinguishing feature of a plant cell.

Chloroplasts allow a plant to capture light energy from the sun and turn it into chemical energy. Chloroplasts accomplish this conversion because they contain chlorophyll, a bright green pigment that absorbs light energy and carries out a chain of chemical reactions. These chemical reactions result in the production of glucose, which the plant uses as food, either storing it or making cellulose to build its cell walls. Chlorophyll is stored in disk-shaped sacs or membranes called thylakoids. It is here that the light energy absorbed by the chlorophyll is directed and changed into chemical energy. This energy allows the plant to take in carbon dioxide, give off oxygen, and eventually produce the plant's food.

Some plant cells contain only one large chloroplast, but other plant cells may have hundreds of smaller ones. Those areas containing concentrations of chlorophyll are called the grana of the chloroplast, and the spaces between the grana are called the stroma. Inside a plant cell, the chloroplasts are separated from the rest of the cell by a membrane and are usually located around the edges of the cell. Although food production takes place at the cellular level, altogether those cells form an individual leaf on a plant. So it is within the cells of the leaf that photosynthesis occurs, providing the entire plant with food and energy. The energy that the plant has stored is converted back into usable food when the plant is placed in the dark and cannot photosynthesize. Organisms that eat green plants are able to obtain the light energy originally captured by photosynthesis.

[See alsoCell; Organelle; Photosynthesis; Plants ]