Chloroplasts are organelles found in the cells of green plants and algae. They are unique in that they have the ability to change light into chemical energy that can be used by plants to create glucose from carbon dioxide and water – a process known as photosynthesis. Chloroplasts are believed to have arisen early in the evolutionary history of life, when an ancient bacterium was engulfed by a larger cell. The smaller bacterium continued to live inside the larger cell, and over time, the two cells began to work cooperatively. The chloroplast is thought to be descended from this ancient bacterium.
While all chloroplasts share a common ancestor, there is considerable diversity among them today. This is due in part to their location within the plant cell. Plants are not stationary organisms – they grow and change shape as they mature. As a result, chloroplasts must be able to move around within the plant cell so that they can be exposed to sunlight (which they need for photosynthesis). Some plant cells have just one or two large chloroplasts, while others may have hundreds or even thousands of small chloroplasts scattered throughout the cell.
The structure of a chloroplast also varies depending on its location within the plant cell. For example, those chloroplasts near the surface of the leaf (where they will receive more light) tend to be flattened so that they can absorb as much light as possible. Those located further away from the surface may be more spherical in shape. However, all chloroplasts share some common features:
• A double membrane – The outer membrane is similar to that of other organelles in plant cells (such as mitochondria), while the inner membrane is unique to chloroplasts and has many folds (known as thylakoids) which increase its surface area . . .