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Botany · Structure

Chloroplast Diagram

The parts of a chloroplast and what each does: the double envelope, the stroma, the thylakoids stacked into grana, the lumen, starch grains and its own DNA.

A chloroplast cut lengthways, not to scale. Two envelope membranes enclose the stroma, where the Calvin cycle makes sugar and starch is stored; inside it, flattened thylakoids stacked into grana and linked by stroma lamellae carry chlorophyll and the light reactions of photosynthesis. Press any part to see what it is and what it does, or take the tour in order.

Also called Chloroplast structure or Chloroplast diagram.

A labelled chloroplast in longitudinal section. Two envelope membranes, outer and inner, with a narrow intermembrane space between them, enclose the grey stroma. In the stroma, four grana, stacks of flattened thylakoid discs, are joined by thin stroma lamellae; the lumen of one disc is labelled, chlorophyll is marked as particles in another disc's membrane, and a dashed outline marks one granum. A white oval starch grain lies at the lower left, two small plastoglobules sit on the lamellae, a loop of chloroplast DNA lies at the right, and ribosomes are scattered through the stroma.Outer membraneIntermembrane spaceInner membraneStromaChloroplast DNARibosomesGranum(stack of thylakoids)PlastoglobuleChlorophyllThylakoidThylakoid lumenStarch grainStroma lamella
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Press any part of the diagram, or pick one here, to see what it is and what it does.

Download the diagramChecked against its sources on 5 November 2026.

A tour

A tour of the chloroplast

  1. 01The chloroplast at a glance

    A chloroplast cut lengthways, as in a leaf's mesophyll cell: a sunlight-driven factory that makes the sugar the rest of the plant, and most other life, runs on. Two envelope membranes enclose the stroma, and inside the stroma a third membrane system, the thylakoids, stacks into grana linked by stroma lamellae. Not to scale.

  2. 02The envelope: two membranes and a gap

    The envelope has an outer and an inner membrane with a narrow intermembrane space between them. The inner membrane controls what crosses: the triose-phosphate/phosphate translocator exports the sugar phosphate the Calvin cycle makes in exchange for phosphate, and K⁺/H⁺ antiporters set the stroma's pH. When the chloroplast divides, an FtsZ ring inside the inner membrane marks the site and proteins across the gap link it to a dynamin ring on the outer membrane.

  3. 03The thylakoid membrane holds the light reactions

    The thylakoid membrane holds the light-dependent reactions. Chlorophyll sits in the light-harvesting antennae and reaction centres of photosystems II and I; the cytochrome b₆f complex passes electrons between them, with plastocyanin carrying them through the lumen, and ATP synthase uses the proton gradient. Photosystem II and its antenna crowd the stacked grana membranes; photosystem I and ATP synthase sit mainly in the stroma lamellae and the ends of the stacks.

  4. 04Grana and stroma lamellae

    In land plants the thylakoids have two parts: grana, cylindrical stacks of discs, and stroma lamellae, unstacked thylakoids wound round the stacks in right-handed helices at 20 to 25 degrees and joined to them by slit-like openings. Light-harvesting proteins on facing membranes are thought to stick a stack together, and CURT1 proteins at its margins set the number and width of its discs. The system reshapes with light: grana change in size, unstack, and the lumen swells.

  5. 05The lumen and the proton gradient

    The light reactions pump protons into the lumen and leave the stroma alkaline: in a lit leaf the stroma sits near pH 8 and the lumen near pH 6, lower in bright light, and both drift back towards pH 7.4 in the dark. This proton-motive force, part pH gradient and part electric field, drives ATP synthase. Lumen pH is also a signal: when it falls, photoprotection switches on and the b₆f complex slows electron flow, sparing photosystem I from lasting damage.

  6. 06The stroma: the Calvin cycle and starch

    The stroma holds the Calvin cycle, shown step by step in the photosynthesis diagram: RuBisCO fixes carbon dioxide, and ATP and NADPH from the thylakoids reduce it to sugar phosphate, which leaves through the inner membrane. The alkaline stroma of a lit leaf keeps the Calvin cycle enzymes switched on. Sugar that is not exported is stored as grains of transitory starch, which enzymes such as β-amylase 3 break down at night to keep the plant supplied until morning.

  7. 07Its own DNA and ribosomes: a cyanobacterium's legacy

    Chloroplasts descend from a cyanobacterium taken in by an early eukaryote, and keep a genome of their own: in flowering plants usually a circular molecule of 120 to 160 kilobases, in many copies per cell, read by the chloroplast's own bacterial-type 70S ribosomes. Most of the ancestor's genes have moved to the nucleus, so most of its proteins are encoded there. Like a bacterium, the chloroplast divides by fission, constricted by an FtsZ ring of cyanobacterial descent.

  8. 08Where chloroplasts are, and how many

    Chloroplasts fill a leaf's mesophyll cells: an Arabidopsis mesophyll cell holds about 80 to 120, and as the cell grows its chloroplasts divide to keep pace, so their total area stays a steady share of the cell's. Leaves grown in strong light make more chloroplasts per cell and more photosystem II, cytochrome b₆f and ATP synthase per chlorophyll, but the same photosystem I. Plastoglobules swell under stresses such as high light or a shortage of nitrogen.

  9. 09Chloroplast and mitochondrion

    Chloroplast and mitochondrion are cousins: both have two membranes, a circular genome and ribosomes of their own, and both began as bacteria inside another cell, a cyanobacterium and an alphaproteobacterium. Both make ATP from a proton gradient. The chloroplast has a third membrane system, the thylakoids; the mitochondrion folds its inner membrane into cristae instead. One captures light to build sugar from carbon dioxide; the other burns sugar for ATP.

Parts

What each part of the chloroplast does

Outer membrane
The outer of the two envelope membranes, the chloroplast's boundary with the cytosol. When the chloroplast divides, a ring of dynamin-related protein forms on its outer face and PDV proteins in it link the outer and inner division machinery across the intermembrane space.
Intermembrane space
The narrow gap between the two envelope membranes. Proteins reach across it: when the chloroplast divides, PDV2 in the outer membrane meets ARC6 in the inner membrane here, so the two membranes pinch in together.
Inner membrane
The inner envelope membrane, which encloses the stroma. Its transporters control traffic with the cytosol: the triose-phosphate/phosphate translocator exports the sugar phosphate the Calvin cycle makes in exchange for phosphate, and K⁺/H⁺ antiporters in it help set the pH of the stroma.
Stroma
The fluid inside the inner membrane, around the thylakoids. It holds the Calvin cycle enzymes, including RuBisCO, the chloroplast's DNA and ribosomes, and its starch. In the light it becomes alkaline, near pH 8, which with magnesium and thioredoxin switches the Calvin cycle enzymes on.
Thylakoid
A flattened membrane sac in the stroma. Its membrane holds chlorophyll, the two photosystems, the cytochrome b₆f complex and ATP synthase, so the light reactions run here. Thylakoids stack into grana and run between the stacks as stroma lamellae.
Thylakoid lumen
The space inside a thylakoid. In the light, protons pumped into it bring it to about pH 6, lower in bright light, against about pH 8 in the stroma; the gradient drives ATP synthase, and the acidity switches on photoprotection. Plastocyanin diffuses through it, and in the light it swells, easing that diffusion.
Granum
A stack of thylakoid discs, piled like coins. The stacked membranes hold photosystem II with its light-harvesting antenna; adhesion between those antenna proteins on facing membranes is thought to hold the stack together, and CURT1 proteins at the margins set the number and size of its discs.
Stroma lamella
An unstacked thylakoid that runs through the stroma from one granum to the next, wound round the stacks in a helix and joined to them by slit-like openings. Photosystem I and ATP synthase sit mainly here and in the end membranes of the grana.
Chlorophyll
The green pigment, held in the light-harvesting antennae and reaction centres of the two photosystems in the thylakoid membrane, drawn here as particles in one membrane. Leaves grown in stronger light carry more chlorophyll per unit area and a higher ratio of chlorophyll a to b.
Starch grain
A grain of transitory starch in the stroma, a store of the sugar made in photosynthesis. At night, enzymes such as β-amylase 3 break it down in the mesophyll cells to keep the plant supplied until morning.
Plastoglobule
A lipid droplet attached to a thylakoid, bounded by a single layer of lipid, with about 30 core proteins. It stores and processes prenyl lipids such as tocopherols and plastoquinone, helps recycle thylakoid lipids as leaves age, and grows under stresses such as high light.
Chloroplast DNA
The chloroplast's own genome: in flowering plants usually a circular molecule of 120 to 160 kilobases holding 110 to 130 genes, in which two inverted repeats separate a large and a small single-copy region. It descends from a cyanobacterium's genome, and most of the ancestor's genes have since moved to the nucleus.
Ribosomes
The chloroplast's own ribosomes, which translate its genes. They are bacterial-type 70S ribosomes, and its protein synthesis still works much as a bacterium's does, though both have gained features of their own: one more sign of where the organelle came from.

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