Question
Draw a labelled diagram of a chloroplast and describe the structure and function of its main components: outer membrane, inner membrane, thylakoids, grana, stroma, and stroma lamellae. Where do the light reactions and dark reactions of photosynthesis occur?
(NCERT Class 11, Cell: The Unit of Life)
Solution — Step by Step
The chloroplast is bounded by a double membrane — an outer membrane and an inner membrane separated by a narrow intermembrane space. The outer membrane is permeable to small molecules. The inner membrane is selective and controls what enters the stroma.
The inner membrane does NOT fold into cristae (unlike mitochondria). Instead, the internal membrane system is a separate third compartment — the thylakoid system.
Thylakoids are flattened, disc-shaped membrane sacs inside the chloroplast. They form the site of light reactions (photophosphorylation and photolysis of water).
Thylakoids are stacked in piles called grana (singular: granum). Each granum has 10-20 thylakoids stacked like a pile of coins. The stacking increases the surface area for light-absorbing pigments (chlorophyll, carotenoids).
Stroma lamellae (or frets) are unstacked thylakoid membranes that connect adjacent grana, allowing the transport of molecules between grana.
The stroma is the gel-like matrix surrounding the thylakoids, inside the inner membrane. It contains:
- Enzymes for the Calvin cycle (dark reactions / carbon fixation)
- Chloroplast DNA (circular, ~120 kb)
- 70S ribosomes (prokaryotic type)
- Starch granules (stored photosynthetic product)
The stroma is where CO₂ is fixed into glucose through the Calvin cycle.
| Stage | Location | What happens |
|---|---|---|
| Light reactions | Thylakoid membrane | Photolysis, electron transport, ATP + NADPH production |
| Calvin cycle (dark reactions) | Stroma | CO₂ fixation, G3P synthesis, glucose production |
The thylakoid membrane houses Photosystem I, Photosystem II, cytochrome b6f complex, and ATP synthase. The stroma has RuBisCO and other Calvin cycle enzymes.
Why This Works
The chloroplast’s compartmentalised structure is essential for efficient photosynthesis. The thylakoid membrane creates a sealed compartment where protons (H⁺) accumulate during the light reactions, building up a proton gradient. This gradient drives ATP synthase — exactly the same principle as oxidative phosphorylation in mitochondria, just in a different organelle.
The stroma provides the right environment (alkaline pH, high NADPH concentration) for the Calvin cycle enzymes. Separating the two stages into different compartments allows them to be regulated independently.
Chloroplasts are believed to have evolved from ancient cyanobacteria through endosymbiosis — evidence includes the double membrane, 70S ribosomes, circular DNA, and the ability to self-replicate.
Alternative Method
For quick diagram recall in exams, remember the “three compartment” model: (1) intermembrane space, (2) stroma, (3) thylakoid lumen. Protons are pumped into the thylakoid lumen during light reactions, creating the gradient for ATP synthesis.
NEET regularly asks diagram-based questions on chloroplast structure. The most common MCQ pattern: “Calvin cycle occurs in ___” (stroma), “Photolysis of water occurs in ___” (thylakoid lumen), “ATP synthase is located on ___” (thylakoid membrane, facing stroma). Know the exact locations — this is guaranteed marks.
Common Mistake
Students confuse stroma lamellae with grana. Grana are stacks of thylakoids (like coin stacks). Stroma lamellae are the connecting tubes between grana (like bridges between stacks). Another frequent error: writing that “dark reactions don’t need light.” The Calvin cycle doesn’t directly use light energy, but it depends on ATP and NADPH produced by light reactions. So it indirectly requires light and actually stops in prolonged darkness.