Cells & molecules

Chloroplast

Cell biology · Organelle

How does a leaf turn sunlight, water and air into sugar — and where does the oxygen come from?

Chloroplasts are the tiny green parts of plant cells that run photosynthesis. Inside, stacks of green membrane discs catch sunlight and use its energy to split water, which releases the oxygen we breathe. The captured energy is then used in the surrounding fluid to build sugar from carbon dioxide taken from the air.

Chloroplasts capture light energy in the thylakoid membranes and store it as ATP and NADPH, releasing O₂ from water. In the surrounding stroma, the Calvin cycle spends that ATP and NADPH to fix CO₂ into sugar.

Key facts

Size

≈ 5–10 µm long, 2–4 µm thick (lens-shaped)

Per leaf mesophyll cell

≈ 20–100 chloroplasts

Grana

≈ 10–20 thylakoids per stack, ≈ 300–600 nm across

Chloroplast genome

≈ 120–160 kb circular; ≈ 100–130 genes

Lumen pH in light

≈ 5.8–6.5 (stroma ≈ 7.8–8)

ATP synthase c-ring

14 subunits in spinach ⇒ ≈ 4.7 H⁺ per ATP

Parts you can explore (13)

• Outer membrane
• Inner membrane
• Stroma
• Grana
• Thylakoid lumen
• Stroma lamellae
• Photosystems
• ATP synthase
• Starch grains
• Plastoglobuli
• cpDNA
• Ribosomes
• Sunlight

In the 3D model

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