Chloroplast

A chloroplast is a membrane-bound organelle in plants and algae where photosynthesis converts light energy into chemical energy. Its internal thylakoid membranes contain chlorophyll and other light-absorbing pigments.

Thylakoid: A flattened membrane sac inside a chloroplast where light-dependent reactions of photosynthesis occur. Thylakoid membranes hold the photosystems that capture light within the chloroplast.
Stroma: The fluid-filled space inside a chloroplast’s inner membrane, surrounding the thylakoids. The Calvin cycle takes place in this compartment outside the thylakoid lumen.
Endosymbiotic theory: The theory that some eukaryotic organelles originated as free-living organisms incorporated into ancestral cells. It explains how chloroplasts arose from bacteria living inside an ancestral eukaryotic cell.
C3 carbon fixation: A photosynthetic pathway in which carbon dioxide is first fixed into a three-carbon compound. Most plant chloroplasts use this pathway, whose efficiency falls when photorespiration increases.
Great Oxidation Event: A major rise in atmospheric oxygen caused by oxygen-producing photosynthesis roughly 2.4 billion years ago. Chloroplast ancestry links plant photosynthesis to the long evolutionary history of atmospheric oxygen.
Photosystem II: A protein–pigment complex that uses light energy to extract electrons from water during photosynthesis. This complex begins linear electron flow by splitting water in chloroplast thylakoid membranes.
Thylakoid lumen: The internal space enclosed by a thylakoid membrane. Protons accumulate here during light reactions, creating a gradient used to make ATP.
Cyanobacteria: A group of bacteria that perform oxygen-producing photosynthesis. Chloroplasts descend from an endosymbiotic lineage related to these photosynthetic bacteria.
C4 carbon fixation: A photosynthetic pathway that concentrates carbon dioxide around Rubisco using a four-carbon intermediate. C4 plants reorganize carbon delivery to chloroplasts, reducing photorespiration in hot conditions.
Oxygenic photosynthesis: Photosynthesis that uses water as an electron donor and releases oxygen. Chloroplasts perform this process, connecting their activity to global oxygen and carbon cycles.

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