Chloroplasts are the organelles inside plant and algae cells that carry out photosynthesis, converting sunlight, water, and carbon dioxide into sugar and oxygen. They’re relatively large as cell components go, typically 5 to 10 micrometers long, and a single leaf cell can contain over 200 of them. Everything green about a plant traces back to these structures.
Basic Structure
A chloroplast is wrapped in a double membrane called the chloroplast envelope. Inside that envelope sits a third membrane system, the thylakoid membrane, which forms a network of flattened discs called thylakoids. These discs frequently stack on top of one another like coins in a roll, and each stack is called a granum (plural: grana). The thylakoid membranes are where the actual light-capturing chemistry happens.
The fluid that fills the space between the envelope and the thylakoid membranes is called the stroma. Think of it as the chloroplast’s workshop floor. It contains the enzymes that take CO₂ from the air and build it into sugar, along with the chloroplast’s own small set of DNA and the machinery to read it. This three-layer design, with the envelope, stroma, and thylakoid system, creates distinct compartments that each play a role in photosynthesis.
How Chloroplasts Capture Light
The pigments inside chloroplasts, primarily chlorophyll, absorb light most strongly in two bands: blue wavelengths (400 to 500 nanometers) and red wavelengths (650 to 680 nanometers). Green light, around 530 nanometers, is barely absorbed at all. It bounces off and scatters, which is why leaves look green. Plants also contain carotenoids, the yellow-orange pigments that become visible in autumn when chlorophyll breaks down, and these help capture some additional light in the blue range.
There’s actually a large chunk of the solar spectrum beyond 700 nanometers (infrared) that plant chloroplasts can’t use at all. From a pure energy standpoint, a lot of available sunlight goes unharvested.
The Light Reactions
Photosynthesis happens in two linked stages, and the first takes place in the thylakoid membranes. Here, chlorophyll absorbs photons and uses that energy to split water molecules into oxygen, protons, and electrons. The oxygen is released as a waste product (the very oxygen we breathe). The electrons get passed along a chain of proteins embedded in the thylakoid membrane, somewhat like a bucket brigade, generating two key energy carriers: ATP and NADPH.
This electron chain runs through two major protein complexes called Photosystem II and Photosystem I, each powered by its own light-absorbing event. Because splitting a single water molecule requires removing four electrons, and each light-driven reaction only moves one electron at a time, the system needs four separate cycles of light absorption to produce one molecule of O₂ from two water molecules. As electrons flow through the chain, protons accumulate on one side of the thylakoid membrane, creating a concentration gradient. That gradient drives an enzyme called ATP synthase, which works like a tiny turbine to produce ATP.
Building Sugar in the Stroma
The second stage doesn’t need light directly. It uses the ATP and NADPH produced by the light reactions as fuel to convert CO₂ into sugar, and it takes place in the stroma through a cycle of chemical reactions called the Calvin cycle.
The central player here is an enzyme called RuBisCO, which grabs CO₂ molecules and attaches them to an existing carbon-based molecule. RuBisCO is the most abundant protein on Earth, which sounds impressive until you learn why: it’s extremely slow. It processes only about three CO₂ molecules per second, so plants compensate by producing enormous quantities of it. RuBisCO is also imprecise. It sometimes grabs oxygen instead of CO₂, triggering a wasteful side process called photorespiration. When CO₂ levels around the leaf are low, this oxygen-grabbing tendency increases, reducing the plant’s efficiency.
Their Own DNA
Unlike most organelles, chloroplasts carry their own small genome. In flowering plants, this genome typically ranges from 120,000 to 180,000 base pairs and contains around 110 to 130 genes, of which roughly 80 code for proteins. The rest encode the RNA molecules needed to read those genes. This is a tiny fraction of what a free-living organism needs, so chloroplasts depend heavily on the cell’s nucleus for the vast majority of their proteins, which are made in the cytoplasm and imported.
This small, semi-independent genome is one of the strongest clues to where chloroplasts came from.
Evolved From Ancient Bacteria
Chloroplasts didn’t arise from scratch inside plant cells. The prevailing evidence points to an ancient event in which an early single-celled organism engulfed a photosynthetic cyanobacterium and, instead of digesting it, kept it alive. Over hundreds of millions of years, the captured bacterium lost most of its genes (transferring many to the host’s nucleus) and became the chloroplast we see today.
The evidence for this is layered. Chloroplast genes are overwhelmingly cyanobacterial in origin, confirmed by decades of phylogenetic analysis comparing DNA sequences. The ribosomal RNA of chloroplasts hybridizes with cyanobacterial DNA. Even the order in which certain genes are arranged along the chloroplast genome mirrors gene arrangement in modern cyanobacteria. The double membrane fits the story as well: one membrane likely came from the cyanobacterium itself, the other from the host cell’s engulfing process. This is the same type of origin story that explains mitochondria, which descended from a different group of bacteria.
How Chloroplasts Protect Themselves
Too much light is genuinely dangerous to chloroplasts. Excess photons can damage the photosynthetic machinery and generate harmful reactive molecules. To cope, chloroplasts physically move within the cell. Under low light, they spread out along the cell walls facing the light source, positioning themselves to absorb as much as possible. Under intense light, they migrate to the edges of the cell that run parallel to the incoming rays, minimizing their exposure. This avoidance response is triggered by a blue-light receptor called phototropin and is essential for plant survival in bright conditions.
This movement is visible under a microscope: in dim light, a leaf cell’s chloroplasts carpet the top and bottom surfaces, and in strong light, they retreat to the sides like beachgoers moving into the shade.
Where Chloroplasts Are Found
Chloroplasts exist in plants and algae but not in animals or fungi. Within a plant, they’re concentrated in the green parts, especially the mesophyll cells of leaves. Older counting methods that looked at thin slices of cells estimated only 8 to 20 chloroplasts per cell, but more accurate three-dimensional techniques have revealed the true number is far higher. In one study using advanced 3D counting, the average mesophyll cell contained roughly 210 chloroplasts, about ten times what traditional flat-section counting suggested.
Chloroplasts are also part of a larger family of plant organelles called plastids. In non-green parts of the plant, the same basic organelle takes on different forms: chromoplasts store the pigments that color fruits and flowers, and amyloplasts store starch in roots and tubers. All of these plastid types share a common origin and can, under certain conditions, convert from one form to another.