Chloroplasts

The chloroplast is a membrane-bound organelle within a cell that conducts photosynthesis. From the molecular perspective, the chloroplast is very large and contains millions of protein molecules along with vast sheets of membranes. If we imagine an average-sized enzyme molecule to be the size of an automobile, a chloroplast in a plant leaf cell would be about 6 kilometers on its long axis and about 2 kilometers on its short axis. The approximately cube-shaped plant cell, 15 to 20 kilometers per side, would contain fifty to one hundred of these compartments.

Structure of Chloroplasts

The chloroplast is enclosed by two membranes, designated the outer and inner membranes of the chloroplast envelope. About one-half the volume within the chloroplast is occupied by stacks of fifty to one hundred flattened sacs called thylakoids, from the Greek word meaning "like an empty pouch." The thylakoid membrane surrounds the lumen or interior space and is the major membrane of the chloroplast. Groups of thylakoids adhere into stacks called grana. The remaining soluble phase of the chloroplast, outside thylakoids, is the stroma.

Function of Chloroplasts

The primary function of chloroplasts is photosynthesis, the light-driven fixation of carbon dioxide into organic compounds . The products of the photochemical reactions that occur within thylakoid membranes provide the material with which the plant cells grow and on which all forms of life on the surface of Earth depend.

Photosynthesis begins when light is absorbed by the green pigment chlorophyll, which occurs only in photosynthetic thylakoid membranes. The absorbed light energy is transferred to a reaction center called Photosystem II (PSII), where electrons are removed from water to release molecular oxygen. The electrons are carried through an electron transport chain in thylakoid membranes to Photosystem I (PSI) to eventually produce reduced compounds (for example, NADPH ) that drive carbon fixation reactions. The flow of electrons through this linked set of carriers also transfers protons (H⁺) from the stroma to the thylakoid lumen, which generates a concentration gradient. These protons can only flow back to the stroma through protein channels within the thylakoid membrane. At the stromal end of the membrane channels is adenosine triphosphate (ATP ) synthase, which uses the flow of H⁺ to drive the synthesis of H⁺ ATP. ATP is used as the primary energy source for biosynthetic reactions within the cell. The ATP and NADPH created are then used to produce sugars from carbon dioxide.

The most abundant enzyme in the biosphere, ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco, for short), catalyzes the reaction of carbon dioxide with ribulose 1,5-bisphosphate, a 5-carbon compound, to make glyceraldehyde 3-phosphate and 3-phosphoglycerate. These two 3-carbon compounds enter the reductive pentose-phosphate cycle (also called the Calvin-Benson cycle) and eventually are converted to a 6-carbon sugar, glucose 6-phosphate, the ultimate product. Glucose 6-phosphate is the precursor of many of the storage products in the plant cell, such as starch, sucrose, and lipids, and is also the starting point for biosynthesis of most of the cellular material. All fatty acids and most amino acids used by the cell are also synthesized in the chloroplast.

Rubisco is a large enzyme—containing eight large (molecular weight 52,000) and eight small (molecular weight 14,000) subunits—that is also very sluggish, catalyzing a reaction only three times per second even when saturated with carbon dioxide. The usual concentration of carbon dioxide in the watery cell interior is sufficient for only one-half this rate. Perhaps these are the reasons why plants developed mechanisms to achieve a high concentration of the enzyme in the stroma to catalyze this reaction that is essential to maintenance of life. Approximately two million molecules of rubisco are present in each chloroplast.

Development of Chloroplasts

Germination of a seed results in growth of a shoot, in which the initial plastids exist with the cells as simple, double-membrane-enclosed vesicles that contain deoxyribonucleic acid (DNA), ribosomes, and a set of enzymes needed for expression of the DNA. These structures, only about 20 percent of the size of a mature chloroplast, are called proplastids. When the shoot reaches the light, the plastid begins the synthesis of chlorophyll, which is required for nearly all remaining aspects of development. Synthesis of lipids, which form the framework of thylakoid membranes, is stimulated within the inner membrane of the envelope.

Proteins are also imported into the chloroplasts after synthesis on cytosolic ribosomes as precursor molecules. Such proteins contain an extension at their amino-terminal end, designated the transit sequence, that serves as a targeting signal for import into the chloroplast. As soon as the protein reaches the stroma, the transit sequence is removed by a specific protease. The chloroplast envelope contains an elaborate apparatus made of numerous protein molecules that function to guide proteins through the membranes into the interior. While some proteins remain embedded in the membrane, others pass through the envelope into the stroma. Of these, a relative few are also transported across thylakoid membranes into the thylakoid lumen. The two major proteins that are imported are the precursor of the small subunit of rubisco, which is released into the stroma, and the chlorophyll-binding proteins, which are integrated into large light-harvesting antenna complexes within the envelope inner membrane. These complexes absorb and funnel light energy to reaction centers to drive the light reactions of photosynthesis. The addition of lipids, pigments, and proteins causes expansion of this membrane, which pinches off into vesicles that subsequently fuse to construct the large thylakoid structure in the interior of the organelle.

Chloroplasts grow and divide along with the cell they reside in as the plant grows. Nearly one hundred copies of the chloroplast genome , a circular, rather small molecule of DNA, are present in each chloroplast. The genes are expressed by transcription to make messenger ribonucleic acid (mRNA), which is translated on chloroplast ribosomes. These ribosomes, about one million in total number per chloroplast, are synthesized inside the chloroplast and are slightly smaller than the cytosolic ribosomes that are encoded by nuclear DNA. Therefore, chloroplasts are able to synthesize their own proteins, but in fact make only about 10 percent of the proteins they contain. Although a chloroplast may contain 500 to 1,000 different proteins, the chloroplast genome contains only 70 to 80 genes for proteins among its total of about 150 genes. The remainder of the proteins are encoded in nuclear DNA and imported.

Evolution of Chloroplasts

The presence of a separate genome, along with similarities between the structures of the chloroplast and photosynthetic cyanobacteria , prompted scientists to propose that chloroplasts originated as the result of an early eukaryotic cell engulfing a prokaryotic cyanobacterium. This proposal has recently received nearly unequivocal support in view of the remarkable similarities in sequences in genes that occur within chloroplasts and cyanobacteria. The evidence suggests that this event, called endosymbiosis, happened once, or a few times, about one billion years ago and that chloroplasts in all photosynthetic eukaryotic organisms are descendants of this event. Shortly after the cyanobacterium became resident within the host cell, much of the genetic information in the bacterium was transferred to the nucleus of the host. Following this endosymbiosis event, as photosynthetic organisms evolved, their chloroplasts diverged as well.

The divergent evolutionary heritages of chloroplasts in various organisms has led to a collection of unique properties. Most of the variety occurs among the algae. Light-harvesting complexes in green algae (Chlorophyceae) contain chlorophylls a and b bound to proteins within the membrane that are very similar to higher plants. The red algae (Rhodophyceae) are similar to green algae except that they contain phycobilisomes as major light-harvesting complexes attached to the surface of thylakoid membranes and do not contain chlorophyll b, which is similar to cyanobacteria. Brown algae (Phaeophyceae), yellow-green algae (Chrysophyceae), diatoms (Bacillariophyceae), and dinoflagellates (Dinophyceae) contain proteins similar to those in light-harvesting complexes in green algae; they differ in that they contain chlorophyll c instead of chlorophyll b. These latter algal families contain an additional pair of membranes surrounding the chloroplast. These extra membranes are thought to have originated when a second eukaryotic cell engulfed an entire chloroplast-containing eukaryotic alga.

see also Cells; Chlorophyll; Endosymbiosis; Photosynthesis, Carbon Fixation and; Photosynthesis, Light Reactions and; Plastids.

J. Kenneth Hoober

Bibliography

Hoober, J. Kenneth, and Laura L. Eggink. "Assembly of Light-Harvesting ComplexII and Biogenesis of Thylakoid Membranes in Chloroplasts." Photosynthesis Research 61 (1999): 197-215.

Raghavendra, A. S., ed. Photosynthesis. A Comprehensive Treatise. Cambridge, UK: Cambridge University Press, 1998.

Tomitani, Akiko, Kiyotaka Okada, Hideaki Miyashita, Hans C. P. Matthijs, Terufumi Ohno, and Ayumi Tanaka. "Chlorophyll b and Phycobilins in the Common Ancestor of Cyanobacteria and Chloroplasts." Nature 400 (1999): 159-62.