Chloroplasts are discoid cytoplasmic organules that resemble a biconvex lens about 10 micrometers in diameter.
They have two enveloping membranes and numerous inner membranes, which form small flattened discoid pockets, the tilacoides (from Greek thylakos, handbag).
The tilacoids are arranged on top of each other, forming cylindrical structures that resemble piles of coins. Each stack is a granumwhich means grain in Latin (plural, money).
The internal space of the chloroplast is filled with a viscous fluid called stroma, which corresponds to the mitochondrial matrix, and contains, like these, DNA, enzymes and ribosomes.
Chlorophyll molecules are arranged neatly on the tilacoid membranes to capture sunlight with maximum efficiency.
If mitochondria are the energy centers of cells, chloroplasts are the energy centers of life itself. They produce organic molecules, especially glucose, that fuel the mitochondria of all organisms that feed directly or indirectly on plants.
Chloroplasts produce organic substances through the photosynthesis process.. In this process, the light energy is transformed into chemical energy, which is stored in the molecules of the manufactured organic substances. The raw materials used in the production of these substances are simply carbon dioxide and water.
During photosynthesis, chloroplasts also produce and release oxygen gas (O2), necessary for the breathing of both animals and plants. Scientists believe that virtually all of the oxygen gas that exists today in the earth's atmosphere originated through photosynthesis.