Photosynthesis: The Planet’s Most Important Chemical Reaction
Every gram of food you eat, every breath of oxygen you inhale — all of it ultimately comes from photosynthesis. The process converts light energy into chemical energy stored in glucose, releasing oxygen as a by-product.
The overall equation is deceptively simple:
But the actual mechanism involves two distinct stages, dozens of enzymes, and some of the most elegant chemistry in biology. Understanding WHY each step happens is what separates a student who remembers photosynthesis from one who truly understands it.
For CBSE Class 10, the overview equation and basic inputs/outputs are sufficient. For Class 11–12 and NEET, you need the full mechanistic picture — light reactions, Calvin cycle, PS I vs PS II, electron transport, and C3/C4 pathways.
Key Terms & Definitions
Chlorophyll: The primary light-absorbing pigment in plants. Found in the thylakoid membranes of chloroplasts. Absorbs blue (430 nm) and red (680 nm) light; reflects green light (which is why plants are green).
Chloroplast: The organelle where photosynthesis occurs. Has an outer double membrane, internal thylakoid membranes (stacked into grana), and the fluid stroma.
Thylakoid: Flattened membrane sacs inside the chloroplast. The light reactions occur on the thylakoid membranes.
Stroma: The fluid-filled space surrounding the thylakoids. The Calvin cycle (dark reactions) occurs in the stroma.
Photosystem I (PS I): Absorbs light at 700 nm (P700). Produces NADPH.
Photosystem II (PS II): Absorbs light at 680 nm (P680). Splits water; produces ATP via photophosphorylation.
RuBisCO: Ribulose-1,5-bisphosphate carboxylase/oxygenase — the enzyme that fixes CO₂ in the Calvin cycle. The most abundant protein on Earth.
ATP: Adenosine triphosphate — the energy currency. Made during the light reactions.
NADPH: Reduced nicotinamide adenine dinucleotide phosphate — a reducing agent. Also made during light reactions, used to reduce CO₂ in the Calvin cycle.
The Two Stages of Photosynthesis
Stage 1: Light Reactions (Photo = Light)
Occur in: thylakoid membranes (grana).
Purpose: Convert light energy into chemical energy (ATP and NADPH).
The sequence:
1. Light absorption by PS II (P680): Chlorophyll in PS II absorbs light. This excites electrons to a higher energy level. These energised electrons are immediately passed to an electron acceptor.
2. Water splitting (photolysis):
To replace the electrons lost from PS II, water molecules are split. This is where all the oxygen we breathe comes from — not from CO₂!
3. Electron transport chain: Energised electrons pass through a series of carriers (plastoquinone, cytochrome b₆f complex, plastocyanin). As electrons move down this chain, H⁺ ions are pumped from the stroma into the thylakoid lumen, creating a proton gradient.
4. ATP synthesis (photophosphorylation): H⁺ ions flow back from lumen to stroma through ATP synthase (chemiosmosis). The energy drives ATP synthesis. This is analogous to mitochondrial ATP synthesis in respiration.
5. PS I (P700) produces NADPH: Electrons from the transport chain arrive at PS I. Re-energised by light absorption, they are passed to NADP⁺, reducing it to NADPH.
Net output of light reactions per 2 water molecules split:
- 3 ATP
- 2 NADPH
- 1 O₂ (released)
Stage 2: Calvin Cycle (Dark Reactions / Light-Independent Reactions)
Occur in: stroma.
Purpose: Use ATP and NADPH from the light reactions to fix CO₂ and produce glucose.
Despite being called “dark reactions,” they can occur in the presence of light — they just don’t directly require it. They require ATP and NADPH, which indirectly require light.
Phase 1 — Carbon Fixation: CO₂ + Ribulose-1,5-bisphosphate (RuBP, 5C) → 2 molecules of 3-phosphoglycerate (3-PGA, 3C)
This reaction is catalysed by RuBisCO. The product (3-PGA) is the first stable product of carbon fixation — this is why C3 plants are called C3 plants.
Phase 2 — Reduction: 3-PGA is reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.
Phase 3 — Regeneration of RuBP: Most G3P molecules (5 out of every 6) are used to regenerate RuBP (using more ATP), keeping the cycle going.
The remaining 1 G3P molecule (net output) is used to synthesise glucose and other organic compounds.
To make 1 glucose molecule (), the cycle runs 6 times:
ATP:NADPH ratio required = 3:2. The light reactions produce this ratio.
Photosystems — PS I vs PS II
This distinction is heavily tested:
| Feature | Photosystem II (PS II) | Photosystem I (PS I) |
|---|---|---|
| Peak absorption | 680 nm (P680) | 700 nm (P700) |
| Reaction centre pigment | P680 | P700 |
| Product | ATP (via ETC + ATP synthase) | NADPH |
| Electron donor | Water (H₂O) | Electron from PS II via ETC |
| Also called | Photolytic (splits water) | Reductive (makes NADPH) |
NEET frequently asks which photosystem is responsible for O₂ evolution — it’s PS II (it splits water). Students often incorrectly say PS I. The memory trick: PS II comes first in the Z-scheme (electrons go PS II → ETC → PS I), and the first step is water splitting.
Solved Examples
Easy — Class 10 Level
Q: What are the raw materials and products of photosynthesis?
Solution:
- Raw materials: CO₂ (from air), H₂O (from soil), light energy (from sun)
- Products: Glucose (C₆H₁₂O₆) and oxygen (O₂)
- Site: Chloroplasts (chlorophyll-containing cells in leaves)
Medium — CBSE Class 12 Level
Q: Where in the chloroplast does the Calvin cycle occur? Name the enzyme responsible for CO₂ fixation.
Solution: The Calvin cycle occurs in the stroma (fluid-filled space inside the chloroplast). The enzyme is RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) — the most abundant enzyme on Earth.
Hard — NEET Level
Q: In the Z-scheme of photosynthesis, explain how the electrons from PS II ultimately lead to NADPH formation.
Solution:
- PS II (P680) absorbs light → electrons excited to high energy state
- Water is split to replace lost electrons:
- Electrons pass through plastoquinone (PQ) → cytochrome b₆f complex (pumps H⁺) → plastocyanin → PS I (P700)
- PS I absorbs light → further excites electrons to even higher energy
- Electrons pass to ferredoxin (Fd) → NADP⁺ reductase
- Final step:
The entire electron pathway from water to NADPH forms a Z-shape when plotted on an energy level diagram — hence “Z-scheme.”
Exam-Specific Tips
CBSE Class 10: Know the overall equation, the raw materials, products, and the role of chlorophyll. Draw a labelled diagram of a chloroplast.
CBSE Class 11–12: The complete mechanism — light reactions, Calvin cycle, PS I vs PS II, Z-scheme, chemiosmosis — is required. Know the products of each stage and where each stage occurs in the chloroplast.
NEET: 3–5 questions from this chapter most years. High-frequency MCQ topics: site of O₂ evolution (PS II), first stable product in C3 plants (3-PGA), enzyme for CO₂ fixation (RuBisCO), number of turns of Calvin cycle to make 1 glucose (6 turns), ATP:NADPH ratio needed (3:2).
NEET 2022 asked: “Which of the following is NOT produced during the light reactions?” Options included O₂, ATP, NADPH, and glucose. Answer: glucose (made in Calvin cycle, not light reactions). This tests whether you know which stage produces which product.
Common Mistakes to Avoid
Mistake 1: Saying oxygen comes from CO₂. The O₂ released during photosynthesis comes from water splitting (photolysis in PS II), NOT from CO₂. This was proved by isotopic labelling experiments using ¹⁸O-labelled water.
Mistake 2: Calling the Calvin cycle “dark reactions” and thinking they happen only at night. They happen continuously whenever light reactions provide ATP and NADPH. The term “dark reactions” just means “light-independent” — they can occur in the dark if ATP and NADPH are supplied externally.
Mistake 3: Confusing where each stage occurs. Light reactions → thylakoid membranes (grana). Calvin cycle → stroma. Students often reverse these or say “both in chloroplast” (too vague).
Mistake 4: Saying RuBisCO is in the thylakoid membrane. It’s a soluble enzyme in the stroma, where the Calvin cycle occurs.
Mistake 5: Stating that PS I comes before PS II in the electron transport chain. Electrons flow PS II → ETC → PS I. PS II is numbered “II” because it was discovered second, not because it acts second.
Practice Questions
1. How many turns of the Calvin cycle are needed to produce one molecule of glucose?
6 turns. Each turn of the Calvin cycle fixes one CO₂ molecule (forming 3-PGA). Since glucose has 6 carbons, 6 CO₂ molecules must be fixed, requiring 6 turns. Each turn requires 3 ATP and 2 NADPH, so 6 turns require 18 ATP and 12 NADPH.
2. What is the role of NADPH in the Calvin cycle?
NADPH acts as a reducing agent. In Phase 2 of the Calvin cycle, it reduces 3-phosphoglycerate (3-PGA) to glyceraldehyde-3-phosphate (G3P). This reduction step incorporates hydrogen into the organic molecule, converting a partially oxidised compound (3-PGA) into a reduced, energy-rich compound (G3P) that can be used to make glucose.
3. Why is RuBisCO considered an inefficient enzyme?
RuBisCO can react with both CO₂ (carboxylation, useful) and O₂ (oxygenation, wasteful — this is photorespiration). At high temperatures and low CO₂ concentrations (like in hot, dry conditions), the oxygenase activity increases, reducing the net yield of photosynthesis. C4 plants evolved to concentrate CO₂ around RuBisCO, minimising this wasteful oxygenation.
4. What pigments are found in a photosystem, and what is the difference between antenna pigments and the reaction centre?
A photosystem has: (a) antenna pigments — many chlorophyll a, chlorophyll b, and carotenoid molecules that absorb light and pass the energy (as excitons, not electrons) to the reaction centre; (b) the reaction centre — a special chlorophyll a molecule (P680 in PS II, P700 in PS I) that actually donates electrons when excited. The antenna pigments funnel light energy to the reaction centre, increasing the efficiency of light capture.
FAQs
Q: Why do leaves appear green? Chlorophyll absorbs red light (~680 nm) and blue/violet light (~430 nm) most effectively. Green light (~550 nm) is reflected — our eyes see this reflected green light. In autumn, chlorophyll breaks down, revealing other pigments (carotenoids — yellow and orange; anthocyanins — red), which is why leaves turn colourful.
Q: Can photosynthesis occur in non-green plants? Yes. Many plants have non-green coloration but still photosynthesise — they contain other pigments in addition to chlorophyll. Purple-leafed plants have anthocyanins that mask green chlorophyll. Red algae use phycoerythrin. The key requirement is chlorophyll, which is present even when hidden by other pigments.
Q: What is the quantum yield of photosynthesis? The quantum yield is the ratio of moles of CO₂ fixed to the number of photons absorbed. The theoretical minimum is 8 photons per CO₂ (4 for PS I and 4 for PS II). Experimentally, the quantum yield is lower due to energy losses. This measurement helped establish the two-photosystem model.
Q: Why does the rate of photosynthesis plateau at high light intensities? At high light intensities, the light reactions produce ATP and NADPH faster than the Calvin cycle can use them. The Calvin cycle becomes the limiting step — it saturates because RuBisCO can only process CO₂ at a certain rate. Adding more light beyond this point doesn’t increase photosynthesis. CO₂ concentration or temperature then becomes the limiting factor.