Definition · Plain-language
Chloroplast
A chloroplast is the organelle in plant cells where photosynthesis takes place, containing the green pigment chlorophyll.
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Where photosynthesis happens
A chloroplast is the organelle in which a plant makes its own food by photosynthesis. Inside the chloroplast, light energy is captured and used to combine carbon dioxide and water into glucose, releasing oxygen. Because this is where plants build the sugars that feed not only themselves but, through food chains, almost all other life, chloroplasts are among the most important structures in the living world. They are found in plant cells and in algae, but never in animal cells, since animals cannot photosynthesise.
Chlorophyll, the green pigment
Chloroplasts contain chlorophyll, a green pigment that absorbs light energy to power photosynthesis. Chlorophyll is what gives leaves and other plant parts their green colour: it absorbs red and blue light and reflects green, which is the green we see. The amount of light a chloroplast can capture helps set how fast photosynthesis runs. In autumn, when many plants break down their chlorophyll, the green fades and other pigments show through, which is why leaves change colour.
Where chloroplasts are found
Chloroplasts are not spread evenly through a plant. They are concentrated in the cells that are exposed to light, mainly in the leaves and the green parts of stems, where photosynthesis can happen. Cells in the roots, which are underground and get no light, have few or no chloroplasts. This is a good example of how cells are specialised for their jobs: a leaf cell is packed with chloroplasts to make food, while a root cell, which absorbs water instead, has little use for them.
Key facts
At a glance
- Definition: the organelle in plant cells where photosynthesis happens
- Contains: chlorophyll, the green pigment that absorbs light
- Job: uses light to make glucose from carbon dioxide and water
- Colour: gives plants their green colour
- Found in: plant cells and algae (not animal cells)
- Located: mainly in leaves and green stems, not roots
Common misconceptions
What people often get wrong
Often heard: Both plant and animal cells contain chloroplasts.
Actually: Only plant cells and algae have chloroplasts. Animals cannot photosynthesise, so animal cells have no chloroplasts. They still have mitochondria for respiration, but not chloroplasts for making food.
Often heard: Every cell in a plant is full of chloroplasts.
Actually: Chloroplasts are concentrated in cells exposed to light, mainly in leaves and green stems. Root cells, which are underground, have few or none. Not all plant cells are green or photosynthetic.
Often heard: The chloroplast and the mitochondrion do the same job.
Actually: They do opposite jobs. The chloroplast captures light to make glucose by photosynthesis; the mitochondrion releases energy from glucose by respiration. Plant cells have both, because they make food and also need to release its energy.
Going deeper
