Question
Where does photosynthesis happen inside a plant cell? Describe the structure of the chloroplast and explain why the two stages of photosynthesis occur in different compartments.
Solution — Step by Step
Photosynthesis happens inside the chloroplast — a double-membrane organelle found mainly in mesophyll cells of leaves. The outer membrane is freely permeable; the inner membrane is selective, controlling what enters and exits.
Inside the inner membrane is the stroma — a fluid-filled space containing enzymes, DNA, and ribosomes. Suspended within the stroma are stacked membrane discs called thylakoids. A stack of thylakoids is called a granum (plural: grana). The space inside a thylakoid is the thylakoid lumen.
The light-dependent reactions (also called the Hill reaction or light reactions) happen on the thylakoid membranes. Why here? Because the photosystems (PS I and PS II), electron carriers, and ATP synthase are all embedded in this membrane. Light is absorbed, water is split (photolysis), and the products — ATP and NADPH — are made here. Oxygen is released as a by-product.
The Calvin cycle (light-independent reactions, or dark reactions) happens in the stroma. The enzymes for CO₂ fixation — especially RuBisCO — are dissolved in the stroma. It takes the ATP and NADPH produced in the thylakoids and uses them to fix CO₂ into G3P, which eventually becomes glucose.
| Stage | Location | Products |
|---|---|---|
| Light reactions | Thylakoid membranes (grana) | ATP, NADPH, O₂ |
| Calvin cycle | Stroma | G3P → Glucose |
Photosynthesis = Light reactions (thylakoids) + Calvin cycle (stroma).
Why This Works
The two-compartment design is not an accident — it’s engineering. The thylakoid membrane creates a proton gradient between the lumen and the stroma. When H⁺ ions flow back through ATP synthase (chemiosmosis), ATP is generated. This only works if the two spaces — lumen and stroma — are kept separate.
Think of it like a dam. The thylakoid membrane is the dam wall, the lumen is the reservoir, and the stroma is the river below. The gradient drives ATP synthesis the same way water drives a turbine. If the two sides mixed freely, the gradient would collapse and no ATP would form.
The stroma, meanwhile, is kept at the right pH and enzyme concentration for RuBisCO to work efficiently. Separating the stages means neither process interferes with the other — a beautiful case of cellular compartmentalisation.
Alternative Method
If an exam question asks you to trace the path of CO₂ to glucose, work backwards from the Calvin cycle:
- CO₂ enters via stomata → diffuses into stroma
- RuBisCO fixes CO₂ onto RuBP (5C) → 3-PGA (3C)
- 3-PGA reduced using ATP + NADPH (from light reactions) → G3P
- Some G3P exits to cytoplasm → glucose synthesis
This framing — starting from the gas entering — is how NEET MCQs often frame “trace the path” questions. Knowing the stroma is where CO₂ first gets fixed is the key anchor.
For NEET, remember: PS II comes before PS I in the electron transport chain, even though the numbering seems reversed. PS II was discovered second but acts first. This is one of the most-tested sequencing facts in plant physiology.
Common Mistake
Many students write “Calvin cycle happens in the dark” and conclude it only runs at night. This is wrong. The Calvin cycle is called “light-independent” because it doesn’t directly use light — but it runs best in the daytime because it needs the ATP and NADPH that only the light reactions can supply. It essentially shuts down at night. The old name “dark reactions” is misleading and NCERT has moved away from it for this reason.
Another frequent slip: confusing grana with stroma lamellae. Grana are the stacks; stroma lamellae (intergranal lamellae) are the unstacked thylakoid membranes connecting the grana. Light reactions happen on both, but NCERT questions usually just say “thylakoid membranes” — that’s safe enough for full marks.