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Chloroplast

Chloroplast

Chloroplasts are the source of virtually all of the world's food and fuel and much of its oxygen supply, and as such life on Earth depends on them. They are a vital component of all photosynthetic cells in plants and algae, and are unique to them. What makes them so important is that they are the sites of photosynthesis, from the absorption of light by chlorophyll through to the production of the first simple sugars. It is chlorophyll that gives them their characteristic green color. They are present in all green-colored cells of a plant; not only in leaves, but also in green stems and green parts of a fruit (for example, in an apple peel).

Chloroplasts are approximately 4 to 6 micrometers in diameter and shaped like a satellite dish with the concave face toward the light. This shape, together with their alignment along the inner surface of the cell, maximizes their ability to capture light. Depending on the plant species there can be as many as two hundred chloroplasts in a cell.

A chloroplast is enclosed by two membranes, which together are termed the "envelope." Inside are two distinct features: a complex organization of folded and interconnecting membranes, called the thylakoids, and a protein -rich fluid region called the stroma. The proteins and pigments (chlorophyll and carotenoids) involved in the light reactions of photosynthesis are located on the thylakoid membranes. The enzymes involved in the conversion of carbon dioxide to simple sugars (the "dark reactions") are found in the stroma. Together these reactions convert carbon dioxide and water to sugars and oxygen.

As well as in making sugars, chloroplasts are important in making other essential plant products, such as fats, oils, scents, and proteins. They can even make many of the proteins needed to produce another chloroplast. They are thought to have been originally free-living, single-celled photosynthetic bacteria, which became engulfed in a nonphotosynthetic host cell. At first, the two cells lived symbiotically, where each was an individual organism that derived some benefit from the other. Eventually, through evolution, the bacteria lost more and more of their ability to live independently and became the chloroplasts we recognize today.

Many pieces of evidence support the endosymbiotic theory. Chloroplasts, for example, contain deoxyribonucleic acid (DNA), the entire sequence of which has been determined in a number of species. Chloroplast DNA codes for a number of essential chloroplast proteins. Over time, large parts of the DNA of the original bacterium have found their way into the nucleus of the host cell, giving it control over many of the functions and features of the chloroplast. Genes involved in controlling the division, and hence "reproduction," of the chloroplast are now present in the nucleus. The composition of the DNA and the way in which it is translated resembles that of bacterial cells, adding further support to the endosymbiotic origin of chloroplasts.

see also Cell Evolution; Leaves; Photosynthesis

Alyson K. Tobin

Bibliography

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.

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Chloroplast

Chloroplast

Chloroplasts are organellesspecialized 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 that 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:

6CO 2 + 6H 2O C 6H 12O 6 + 6O 2.

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, scientists have devised the serial endosymbiosis theory. It is suspected that primitive microbes were able to evolve into more complex microbes 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 Autotrophic bacteria; Blue-green algae; Evolution and evolutionary mechanisms; Evolutionary origin of bacteria and viruses

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chloroplast

chloroplast (klōr´əplăst´, klôr´–), a complex, discrete green structure, or organelle, contained in the cytoplasm of plant cells. Chloroplasts are reponsible for the green color of almost all plants and are lacking only in plants that do not make their own food, such as fungi and nongreen parasitic or saprophytic higher plants. The chloroplast is generally flattened and lens-shaped and consists of a body, or stroma, in which are embedded from a few to as many as 50 submicroscopic bodies—the grana—made up of stacked, disklike plates. The chloroplast contains chlorophyll pigments, as well as yellow and orange carotenoid pigments. Chloroplasts are thus the central site of the photosynthetic process in plants. The chloroplasts of algae are simpler than those of higher plants and may contain special, often conspicuous, starch-accumulating structures called pyrenoids.

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chloroplast

chloroplast Any of the chlorophyll-containing organelles (see plastid) that are found in large numbers in those plant and algal cells undergoing photosynthesis. Plant chloroplasts are typically lens-shaped and bounded by a double membrane. They contain membranous structures called thylakoids, which are piled up into stacks (see granum), surrounded by a gel-like matrix (stroma). The light-dependent reactions of photosynthesis occur on the thylakoid membranes while the light-independent reactions take place in the stroma.

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chloroplast

chloroplast A biconvex or planoconvex plastid, typically 5–10 μm long and 2–3 μm wide, that has a complex internal structure comprising stacks of membranaceous discs (grana) that bear photosynthetic pigments embedded in a matrix (stroma). It is a semi-autonomous organelle, which contains some genetic material and has some ability to direct the synthesis of its own proteins.

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chloroplast

chloroplast Microscopic green structure within a plant cell in which photosynthesis takes place. The chloroplast is enclosed in an envelope formed from two membranes and contains internal membranes to increase the surface area for reactions. Molecules of the light-absorbing pigment chlorophyll are embedded in these internal membranes.

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chloroplast

chlo·ro·plast / ˈklôrəˌplast/ • n. Bot. (in green plant cells) a plastid that contains chlorophyll and in which photosynthesis takes place.

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chloroplast

chloroplast •bast • cineaste • encomiast •symposiast • enthusiast • bombast •oblast • chloroplast • iconoclast •gymnast • pederast • fantast

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