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Chloroplasts are double-membrane organelles found in plant cells and algae that convert light energy into chemical energy via photosynthesis. Inside the inner membrane lies the stroma, an aqueous matrix containing enzymes for the Calvin cycle. Embedded within the stroma are stacked thylakoid membranes (grana), which harbor the photosynthetic pigments and protein complexes of the light reactions. Like mitochondria, chloroplasts contain their own DNA and ribosomes, supporting their endosymbiotic origin.
Map each stage of photosynthesis onto a chloroplast diagram: light reactions occur in the thylakoid membranes; Calvin cycle occurs in the stroma. Contrast with mitochondria structure to reinforce both.
From your study of organelles, you know that eukaryotic cells compartmentalize their functions into membrane-bound structures, each specialized for particular tasks. Chloroplasts are the organelles responsible for photosynthesis — the conversion of light energy into chemical energy — and they are found exclusively in plant cells and algae. If you have already studied mitochondria, chloroplasts will feel familiar in many ways: both are double-membrane organelles with their own DNA, both have a soluble matrix where key metabolic cycles run, and both use internal membrane systems to generate energy-storing molecules. The key difference is the direction of energy flow — mitochondria break down organic fuel to release energy, while chloroplasts capture light to build organic fuel.
A chloroplast is enclosed by an outer membrane (freely permeable to small molecules) and an inner membrane (selectively permeable, with specific transporters). Inside the inner membrane lies the stroma, an enzyme-rich aqueous space analogous to the mitochondrial matrix. The stroma contains all the enzymes of the Calvin cycle, the chloroplast's own circular DNA, and 70S ribosomes — evidence of the organelle's ancient bacterial ancestor. Suspended within the stroma is a third membrane system found nowhere else in the cell: the thylakoid membranes. These form flattened, fluid-filled sacs that stack into columns called grana (singular: granum), connected by unstacked regions called stroma lamellae. This extensive internal membrane provides an enormous surface area for the photosynthetic machinery.
The spatial organization of the chloroplast maps directly onto the two stages of photosynthesis. The light reactions occur in the thylakoid membranes, where chlorophyll and associated pigment-protein complexes (photosystems I and II) absorb photons and use that energy to split water, generate a proton gradient across the thylakoid membrane, and produce ATP and NADPH. The Calvin cycle runs in the stroma, using that ATP and NADPH to fix CO₂ into organic sugars. The thylakoid interior (lumen) is where protons accumulate — analogous to the mitochondrial intermembrane space — so ATP synthase in the thylakoid membrane faces its catalytic head into the stroma, where ATP is needed for the Calvin cycle. This tight spatial coupling means the products of the light reactions are delivered directly to where the Calvin cycle enzymes are working.
Chloroplasts also have a remarkable evolutionary origin that explains many of their features. The endosymbiotic theory holds that an ancient eukaryotic cell engulfed a photosynthetic cyanobacterium, and over billions of years the bacterium became the chloroplast. The evidence is compelling: chloroplasts have double membranes (the inner one from the original bacterium, the outer one from the host's engulfing vesicle), their own circular DNA resembling bacterial genomes, 70S ribosomes matching bacterial size, and they divide by binary fission independently of the host cell's division cycle. Most of the original bacterial genes have migrated to the host nucleus over evolutionary time, so chloroplast proteins are largely encoded in the nucleus, synthesized in the cytoplasm, and imported back into the chloroplast via transit peptides — a process requiring the TOC and TIC translocon complexes in the outer and inner membranes, respectively.