Principles of Biochemistry

1.1.3.6 Chloroplasts

Chloroplasts are the green plastids. They are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. They are usually disk-shaped and about 5-8 µm in diameter and 2-4 µm thick. A typical plant cell has 20-40 of them. Chloroplasts are green because they contain chlorophylls, the pigments that harvest the light used in photosynthesis. They capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.  The dark reaction of photosynthesis takes place in stroma.

The material within the chloroplast is called the stroma. Within the stroma are stacks of thylakoids, the sub-organelles, which are the site of photosynthesis. The thylakoids are arranged in stacks called grana.  A thylakoid has a flattened disk shape. Inside it is an empty area called the thylakoid space or lumen. Photosynthesis takes place on the thylakoid membrane.