Chloroplast
Chloroplasts capture light energy and use it to build sugar from carbon dioxide and water. Stacked membrane discs called thylakoids hold the chlorophyll; the fluid stroma around them runs the Calvin cycle.
- 1NucleusOrganelle · double membrane
- 2MitochondrionOrganelle · double membrane
- 3ChloroplastPlastid · double membrane
- 4RibosomeMolecular machine · no membrane
- 5Endoplasmic reticulumOrganelle · single membrane network
- 6Golgi apparatusOrganelle · single membrane stacks
- 7Cell membraneBoundary · phospholipid bilayer
- 8Cell wallCellulose layer · outside the membrane
- 9Central vacuoleWater-filled sac · up to 90% of the volume
Plant cell: what you are looking at
A boxy cell 10–100 µm across held rigid by a cellulose wall outside the membrane. A large central vacuole fills most of the interior, chloroplasts line the cytoplasm, and there are no centrioles or true lysosomes.
Cell wall. A rigid layer of cellulose fibers outside the membrane, typically 0.1–10 µm thick, that gives the cell its shape and resists turgor pressure.
Central vacuole. A water-filled sac bounded by the tonoplast that can occupy 80–90% of the cell’s volume; it stores water, ions, and pigments and keeps the cell turgid.
The chloroplast is outlined in coral; it occurs in plant cells, algae and some protists. Hover or tab through the key to light up the other structures, and click any of them to open its page.
Compare all eight organelles ↗Know its structure
| Found in | Plant cells, Algae and some protists |
| Membranes | Two envelope membranes plus a third internal system, the thylakoids, arranged in stacks called grana |
| Typical size | Lens-shaped, about 3–10 µm long and 1–3 µm thick |
| Number per cell | Around 30–100 in a leaf mesophyll cell; the alga Chlamydomonas has a single cup-shaped chloroplast |
| Contains | Chlorophyll a and b, carotenoids, grana, stroma, starch grains, circular DNA, and 70S ribosomes |
| Discovered | Chlorophyll was named in 1817 by Pelletier and Caventou; Andreas Schimper described chloroplasts in 1883 |
What it does
- Runs the light reactions. Chlorophyll in the thylakoid membranes splits water, releasing O₂ and charging up ATP and NADPH
- Fixes carbon in the Calvin cycle. In the stroma, RuBisCO attaches CO₂ to a 5-carbon sugar and the cycle turns it into 3-carbon sugars
- Stores starch. Surplus sugar is banked as starch grains by day and mobilized at night
Chloroplast: common questions
Wondering about the why? Start here.
What does the chloroplast do?
Chloroplasts run photosynthesis: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. In the thylakoid membranes, chlorophyll captures light to split water, release oxygen, and charge ATP and NADPH; in the surrounding stroma, the Calvin cycle uses that energy and the enzyme RuBisCO to fix CO₂ into 3-carbon sugars. Surplus sugar is stored as starch grains during the day and used at night.
Read: photosynthesisWhat is the difference between chloroplasts and mitochondria?
They run opposite reactions. Chloroplasts use light to build glucose from CO₂ and water and release O₂; mitochondria break glucose down with O₂ to make ATP and release CO₂ and water. Both have a double membrane, their own circular DNA, and small bacterial-style ribosomes, all relics of their bacterial ancestors, but chloroplasts add a third membrane system, the thylakoids, and occur only in plants, algae, and a few protists. Plant cells contain both organelles.
Compare with the mitochondrionDo plants respire as well as photosynthesize?
Yes, every plant cell respires in its mitochondria day and night, because photosynthesis makes sugar but respiration is what turns it into usable ATP. In bright light photosynthesis outpaces respiration, so the leaf takes in CO₂ and gives off O₂ overall; in the dark only respiration continues and the gas exchange reverses. The light level at which the two balance is called the compensation point.
Read: cellular respiration