chloroplast

A green *plastid with an internal membrane system incorporating the pigment molecules that are essential to photosynthesis. Chloroplasts are present in most of the cells of autotrophic plants that are exposed to light. In algae they are frequently large and may have a complex shape, e.g. the spiral chloroplasts of Spirogyra. Many algal cells contain only one chloroplast. In bryophytes and all higher plants they are generally lens shaped and present in large numbers. They vary in size, average dimensions being 5 μm by 2.3 μm, and probably develop from the minute *proplastids of meristematic cells. The chloroplast is surrounded by a double membrane with no pores. The matrix or stroma of the chloroplast is a complex hydrophilic proteinaceous sol, containing particles that vary in size and distribution. Some are the temporarily stored products of photosynthesis, e.g. starch grains, plastoglobuli, and various crystals. Others are aggregates of ribulose bisphosphate carboxylase, ribo-somes, and polysomes, and circular fibrils of DNA. Chloroplast ribosomes are smaller than cytoplasmic ribosomes. In this respect, and in their reactions to antibiotics, chloroplast ribosomes resemble those of prokaryotes (see serial endosymbiotic theory). The internal chloroplast membranes form a complex system of granal and intergranal lamellae (see diagram).

*Grana are formed from two or three up to approximately a hundred disclike flattened vesicles (thylakoids) stacked on top of each other. They are orientated in a variety of directions relative to the long axis of the chloroplast and their number and size varies with different species. The chloroplasts in the bundle sheath cells of C4 plants do not contain grana. The intergranal lamellae are flexible interconnecting channels, continuous with and linking together the channels of individual grana. The precise arrangement of pigment molecules within the membranes is not clear but the thy-lakoid membranes have the densest concentration. Spherical structures (quanta-somes), some 17.5 nm and others 11.0 nm in diameter have been identified, which are believed to be embedded in the lipid layers with their edges protruding from the membrane surfaces. These particles have been shown to be essential to the light reactions of photosynthesis. The pigment in the smaller particles is the P700 form of chlorophyll a associated with photosystem I, and that of the larger particles is the P680 form associated with photosystem II. Superficial particles, loosely attached to the granal membranes, have also been identified. These consist of molecules of a calcium-dependent ATPase. The light reactions of photosynthesis thus occur within the granal layers. Water molecules are split, oxygen is evolved, and ATP and NADPH are formed. The dark reactions occur in the stroma, phosphoglyceric acid being reduced to glyceraldehyde 3-phosphate by NADPH. This is the basis for the biochemical reactions that ultimately produce all the organic molecules of the plant.