Question
Compare the light reactions and dark reactions (Calvin cycle) of photosynthesis in a tabular form. Where does each occur and what are the products?
Solution — Step by Step
Photosynthesis converts light energy to chemical energy stored in glucose. It occurs in two broad stages:
Light reactions (Light-dependent reactions): Capture light energy and convert it to chemical energy (ATP and NADPH). Also produce O₂ as a byproduct.
Dark reactions (Light-independent reactions / Calvin cycle): Use the ATP and NADPH from light reactions to fix CO₂ into carbohydrates (glucose). Called “dark” because they don’t directly require light — though they typically run during the day in light conditions.
Light reactions: Occur in the thylakoid membranes (stacked disc-like membranes inside the chloroplast). The light-absorbing pigments (chlorophyll, carotenoids) are embedded in these membranes.
Dark reactions: Occur in the stroma — the fluid-filled space surrounding the thylakoids inside the chloroplast.
The products of light reactions (ATP and NADPH) move from thylakoid to stroma to fuel the Calvin cycle.
| Feature | Light Reactions | Dark Reactions (Calvin Cycle) |
|---|---|---|
| Light requirement | Directly required | Not directly required (but depends on products of light reactions) |
| Location | Thylakoid membranes | Stroma of chloroplast |
| Inputs | Water (H₂O), light energy, ADP+Pi, NADP⁺ | CO₂, ATP, NADPH |
| Outputs | ATP, NADPH, O₂ | G3P (glyceraldehyde-3-phosphate → glucose) |
| Key processes | Photosystem I and II, ETC, photophosphorylation, water splitting | Carbon fixation (RuBisCO), reduction, regeneration of RuBP |
| Enzyme type | Membrane-bound complexes (PS I, PS II, ATP synthase) | Soluble enzymes (RuBisCO, others) |
| O₂ produced? | Yes (from water splitting) | No |
| CO₂ used? | No | Yes |
Photosystem II (PS II): Absorbs light at 680 nm. Water is split (photolysis):
The O₂ produced is released into the atmosphere. Electrons flow into the electron transport chain.
Electron Transport Chain (ETC): Electrons from PS II flow through plastoquinone → cytochrome b6f complex → plastocyanin → Photosystem I. Energy released pumps H⁺ ions into thylakoid lumen, creating a gradient.
ATP Synthesis: H⁺ flows back through ATP synthase (chemiosmosis) → ATP formed (photophosphorylation).
Photosystem I (PS I): Absorbs light at 700 nm. Final electron acceptor is NADP⁺, which is reduced to NADPH.
Products of light reactions: ATP + NADPH + O₂
The Calvin cycle fixes CO₂ into organic molecules in three stages:
Stage 1 — Carbon fixation: CO₂ + RuBP (5-carbon) → 2 molecules of 3-PGA (3-carbon) Enzyme: RuBisCO (ribulose bisphosphate carboxylase/oxygenase — the most abundant enzyme on Earth)
Stage 2 — Reduction: 3-PGA → G3P (glyceraldehyde-3-phosphate) Uses ATP and NADPH from light reactions
Stage 3 — Regeneration of RuBP: G3P is used to regenerate RuBP (consumes more ATP)
For every 3 CO₂ fixed → 1 G3P molecule is net output (others used for regeneration). Two G3P molecules → 1 glucose (6 carbons).
Why This Works
The two stages of photosynthesis are designed to work together:
Light reactions are limited by light intensity (they need photons). Dark reactions are limited by CO₂ concentration and temperature (enzyme kinetics). By separating the two stages — light reactions in thylakoids, dark reactions in stroma — the chloroplast can operate them semi-independently.
At high light intensity, ATP and NADPH accumulate, which drives faster Calvin cycle. At night, light reactions stop, but (for a short time) the Calvin cycle can continue using stored ATP/NADPH.
Alternative Method — Mnemonic for Products
Light reactions produce: “ANDO” = ATP, NADPH, and O₂
Calvin cycle uses ATP and NADPH, fixes CO₂, produces G3P (glyceraldehyde-3-phosphate) → glucose
NEET asks about photosynthesis every year. Key NEET facts: (1) RuBisCO is the enzyme for CO₂ fixation. (2) O₂ comes from water splitting (not from CO₂). (3) Calvin cycle occurs in stroma. (4) C4 plants (like sugarcane, maize) fix CO₂ first into oxaloacetate (using PEP carboxylase) before the Calvin cycle — a modification that reduces photorespiration. C3 vs C4 distinction is a high-frequency NEET topic.
Common Mistake
Students often say “dark reactions occur at night.” This is a significant misconception — the word “dark” means they don’t require light DIRECTLY, not that they occur in the dark. In a leaf, dark reactions run during the day alongside light reactions, fuelled by the ATP and NADPH that the light reactions produce in real time. At night, when light reactions stop producing ATP/NADPH, the Calvin cycle also stops.