The part of the plant cell that gives the plant its green color is an organelle called the chloroplast. Inside the chloroplast there are pigment molecules called chlorophyll, which is the substance that gives plants their green color.
Within the chloroplasts, the chlorophyll molecules lie on membranes of stacked disks called thylakoids. Chlorophyll captures light energy from the sun which plant cells use to power the process of photosynthesis, which is the making of glucose from carbon dioxide and water.
Although cells in the stem and immature fruit also contain chloroplasts, the majority of chloroplasts are housed in the cells on the surface of the leaves where much of the photosynthesis takes place. The cells on the surface of leaves contain about half a million chloroplasts per square millimeter; therefore, leaves tend to appear greener than the rest of the plant.