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Chloroplasts are organelles—specialized parts of a cell that function in an organ—like fashion. They are found in vascular plants, mosses, liverworts, and algae. Chloroplast organelles are responsible for photosynthesis, the process by which sunlight is absorbed and converted into fixed chemical energy in the form of simple sugars synthesized from carbon dioxide and water.

Chloroplasts are located in the mesophyll, a green tissue area in plant leaves. Four layers or zones define the structure of a chloroplast. The chloroplast is a small lens-shaped organelle which is enclosed by two membranes with a narrow intermembrane space, known as the chloroplast envelope. Raw material and products for photosynthesis enter in and pass out through this double membrane, the first layer of the structure.

Inside the chloroplast envelope is the second layer, which is an area filled with a fluid called stroma. A series of chemical reactions involving enzymes and the incorporation of carbon dioxide into organic compounds occur in this region.

The third layer is a membrane-like structure of thylakoid sacs. Stacked like poker chips, the thylakoid sacs form a grana. These grana stacks are connected by membranous structures. Thylakoid sacs contain a green pigment called chlorophyll. In this region the thylakoid sacs, or grana, absorb light energy using this pigment. Chlorophyll absorbs light between the red and blue spectrums and reflects green light, making leaves appear green. Once the light energy is absorbed into the final layer, the intrathylakoid sac, the important process of photosynthesis can begin.

Scientists have attempted to discover how chloroplasts convert light energy to the chemical energy stored in organic molecules for a long time. It has only been since the beginning of this century that scientists have begun to understand this process. The following equation is a simple formula for photosynthesis:

A transmission electron micrograph (TEM) of a chloroplast from a tobacco leaf (Nicotiana tabacum.). The stacks of flattened membranes that can be seen within the chloroplast are grana. The membranes that run between the stacks are stroma. The faint white patches within the chloroplast are nucleoids, where chloroplast DNA is stored. © Dr. Jeremy Burgess/Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc. Reproduced with permission.

Carbon dioxide plus water produce a carbohydrate plus oxygen. Simply, this means that the chloroplast is able to split water into hydrogen and oxygen.

Many questions still remain unanswered about the complete process and role of the chloroplast. Researchers continue to study the chloroplast and its evolution. Based on studies of the evolution of early complex cells, scientist have devised the serial endosymbiosis theory. It is suspected that primitive microbes were able to evolve into more complex ones by incorporating other photosynthetic microbes into their cellular structures and allowing them to continue functioning as organelles. As molecular biology becomes more sophisticated, the origin and genetic makeup of the chloroplast will be more clearly understood.

See also Leaf.

Additional topics

Science EncyclopediaScience & Philosophy: Chimaeras to Cluster