Chloroplast: the photosynthetic organelle in plants and algae

Structure and components

Typical chloroplasts are enclosed by a double membrane and contain an internal fluid called the stroma. Within the stroma are stacks of flattened thylakoid membranes that form grana. The thylakoid membranes embed pigment–protein complexes that harvest light. The best-known pigment is the molecule chlorophyll, which absorbs mainly blue and red wavelengths and gives plants their green color. Chloroplasts also contain accessory pigments such as carotenoids that extend the range of light that can be used and protect the photosynthetic apparatus.

• Envelope: outer and inner membranes delimiting the organelle
• Stroma: aqueous matrix with enzymes and its own DNA
• Thylakoids and grana: membrane system where light-dependent reactions occur
• Pigments: chlorophyll and accessory pigments for light capture
• Plastid genome: small circular DNA inherited often maternally

How photosynthesis works (overview)

Photosynthesis in chloroplasts couples two major sets of reactions. In the light-dependent reactions, absorbed photons drive electron transport in the thylakoid membranes, creating energy-rich molecules and a proton gradient that powers synthesis of ATP and reducing power. Water (H2O) is split during these reactions, releasing O2 as a by-product. In the subsequent carbon fixation reactions, ATP and reducing power are used to incorporate carbon dioxide (CO2) into organic molecules, ultimately producing sugars and other compounds.

Functions beyond light capture

Chloroplasts host biosynthetic pathways for amino acids, fatty acids and pigments, and contribute to cellular signaling and stress responses. Their metabolic output supports not only the cell that contains them but also herbivores and decomposer communities. The number and form of chloroplasts vary by species, tissue type and environmental conditions; some cells contain many small chloroplasts while others have fewer, larger organelles.

Origin, diversity and significance

Chloroplasts are thought to have arisen by an ancient endosymbiotic event in which a eukaryotic ancestor acquired a photosynthetic cyanobacterium; over evolutionary time the endosymbiont became an integrated organelle. Different algal lineages and plant groups show diversity in chloroplast membrane architecture and pigment composition, reflecting complex evolutionary histories. Chloroplasts are central to Earth's biosphere because they produce organic matter and oxygen, and they drive major flows in global carbon and energy cycles.

Research and applications

Chloroplast biology is a focus of research in plant physiology, ecology and biotechnology. Understanding chloroplast function informs crop improvement, efforts to enhance photosynthetic efficiency, and studies of environmental responses. For further background and technical resources on organelles, algae, light capture and photosynthetic mechanisms, see entries on organelles, algal biology, light-driven energy conversion, sugar metabolism, photosynthesis, starch storage, the molecule chlorophyll, CO2 and H2O dynamics, O2 production, the role of carotenoids, and the behavior of photons in light capture.