A chloroplast is a membrane-bound organelle found in the cells of plants and many algae. It carries out photosynthesis, capturing sunlight and converting its energy into chemical forms used to build organic compounds. Chloroplasts contain chlorophyll, internal photosynthetic membranes, and their own genetic machinery, but depend extensively on the surrounding cell for their construction and operation. They also participate in biosynthetic processes beyond photosynthesis. (ncbi.nlm.nih.gov)
Structure and cellular distribution
Chloroplasts belong to the plastid family of organelles, which also includes nonphotosynthetic forms specialized for storage or pigmentation. In land plants, chloroplasts are especially abundant in the mesophyll, the photosynthetic tissue inside leaves. They also occur in other green tissues, whereas many root and storage cells contain different plastid types. A plant cell can contain numerous chloroplasts, and their abundance and development vary with tissue identity and environmental conditions. (ncbi.nlm.nih.gov)
A typical land-plant chloroplast has two envelope membranes separated by an intermembrane space. The inner membrane encloses the stroma, an aqueous compartment containing enzymes, proteins, DNA, and ribosomes. Within the stroma lies a separate membrane system composed of thylakoids, flattened sacs enclosing a space called the lumen. Thylakoids commonly form stacks known as grana, connected by unstacked membrane regions called stroma lamellae. This arrangement separates light-driven reactions in the thylakoid membrane from carbon-assimilation reactions in the stroma. (ncbi.nlm.nih.gov)
The envelope and thylakoid membranes have distinct functions. Envelope transport systems regulate exchanges between the chloroplast and the rest of the cell; thylakoid membranes organize the molecular machinery that captures light and converts it into chemical energy. Their enclosed lumen provides the compartment needed to establish a transmembrane proton gradient. (ncbi.nlm.nih.gov)
Light capture and energy conversion
Chlorophyll and accessory pigments are associated with photosystems, large protein complexes embedded in the thylakoid membrane. Antenna pigments absorb light and transfer excitation energy toward reaction centers, where an energized electron is transferred to an acceptor. Oxygenic photosynthesis uses two cooperating complexes, photosystem II and photosystem I, connected through electron-transfer reactions. Their numbering reflects the order of their discovery rather than their sequence in the principal pathway. (ncbi.nlm.nih.gov)
At photosystem II, oxidation of water replenishes electrons lost by the reaction center and releases oxygen and hydrogen ions. Electrons pass through an electron transport chain before reaching photosystem I, where light supplies another excitation step. Subsequent reactions reduce NADP⁺ to NADPH, a carrier of reducing power used in biosynthesis. The oxygen released during photosynthesis therefore comes from water rather than from carbon dioxide. (ncbi.nlm.nih.gov)
Electron transport and water oxidation help accumulate protons in the thylakoid lumen. Their return to the stroma through ATP synthase drives production of adenosine triphosphate (ATP). This coupling is an example of chemiosmosis. Cyclic electron flow around photosystem I can support additional ATP formation without net NADPH production or oxygen release, helping adjust the balance between ATP supply and reducing power. (ncbi.nlm.nih.gov)
Carbon assimilation and other metabolic functions
In the stroma, the Calvin cycle uses ATP and NADPH to incorporate carbon dioxide into organic compounds. Its initial reaction is catalyzed by Rubisco, which attaches carbon dioxide to the five-carbon compound ribulose-1,5-bisphosphate. The resulting products undergo reduction, while most of the carbon intermediates are recycled to regenerate the starting acceptor. This incorporation of inorganic carbon into organic molecules is carbon fixation. (openstax.org)
The cycle yields a net three-carbon sugar phosphate, rather than directly producing a complete glucose molecule. Fixing three carbon dioxide molecules to obtain one net glyceraldehyde-3-phosphate requires nine ATP and six NADPH. Products can contribute to carbohydrate synthesis, including starch stored within chloroplasts and sugars synthesized elsewhere in the cell. Although traditionally called “dark reactions,” these reactions are not restricted to darkness: they depend on products of the light reactions. (openstax.org)
Chloroplast functions extend beyond carbon assimilation. Their metabolic pathways contribute to the synthesis of fatty acids and certain amino acids. Photosynthesis is therefore integrated with the production of cellular building materials, rather than serving only as a mechanism for sugar formation. (ncbi.nlm.nih.gov)
Genetic system, development, and protein import
The chloroplast genome encodes only part of the machinery needed by the organelle, including components of photosynthesis and its own gene-expression system. Most chloroplast proteins are encoded in the cell nucleus, synthesized outside the organelle, and subsequently imported. Many carry an amino-terminal transit peptide that directs them to envelope transport machinery known as TOC and TIC; the targeting sequence is usually removed after import. Chloroplasts consequently have partial genetic autonomy, not independence from their host cell. (ncbi.nlm.nih.gov)
In land plants, chloroplasts develop from pre-existing plastids, including undifferentiated proplastids. Development involves coordinated production of pigments, membranes, and proteins under both nuclear and plastid control. Chloroplasts multiply by division, and their maintenance and inheritance require cellular mechanisms that regulate organelle growth, partitioning, and differentiation. (pmc.ncbi.nlm.nih.gov)
Evolutionary origin and algal diversity
The endosymbiotic theory explains chloroplast ancestry through the incorporation of a photosynthetic cyanobacterium into an ancestral eukaryotic host. Gene loss, transfer of genes to the host nucleus, and the evolution of protein-import systems progressively integrated the former organism into the cell. Comparative molecular evidence supports a shared primary plastid origin for green plants, red algae, and glaucophyte algae. (pmc.ncbi.nlm.nih.gov)
Other algal groups acquired photosynthetic plastids through secondary endosymbiosis, in which a eukaryote retained an engulfed photosynthetic alga. These complex plastids commonly possess additional surrounding membranes and more elaborate protein-targeting pathways. Chloroplast envelope architecture therefore varies among algal lineages and reflects different histories of cellular incorporation. (pmc.ncbi.nlm.nih.gov)