Photosynthesis in Higher Plants explains how green plants use light, water and carbon dioxide to make glucose and release oxygen inside the chloroplast. It covers the light reaction with its two photosystems, photolysis of water and ATP synthesis by chemiosmosis, followed by the Calvin cycle, the C4 pathway, photorespiration and the factors that limit the rate. It is a high-yield NEET chapter because both the diagrams and the reaction logic are repeatedly tested.
Table of Contents
- Key Concepts - Site, pigments, light & dark reactions, C3/C4, photorespiration, limiting factors
- Weightage in Board & Entrance Exams
- Important Definitions
- Solved Examples
- Important Questions for Board Exams
- Quick Revision Points
Key Concepts
1. What is Photosynthesis?
Photosynthesis is the anabolic (building-up) process by which green plants, algae, and some bacteria synthesise organic food (glucose) from carbon dioxide and water using light energy, releasing oxygen as a by-product.
The overall balanced equation is:
6CO₂ + 12H₂O → (light, chlorophyll) → C₆H₁₂O₆ + 6O₂ + 6H₂O
- Water is both a reactant and a product; the O₂ released comes from water, not CO₂ (proven by Ruben and Kamen using the isotope ¹⁸O).
- It is the only major process that converts solar energy into chemical energy stored in food.
2. Site of Photosynthesis - The Chloroplast
Photosynthesis occurs in the chloroplast, a double-membrane organelle found mainly in mesophyll cells of the leaf.
[DIAGRAM: A chloroplast - outer and inner membrane, stroma (fluid matrix), and grana (stacks of disc-shaped thylakoids) connected by stroma lamellae.]
- Grana (thylakoids): site of the light reaction - they contain chlorophyll and trap light.
- Stroma: site of the dark reaction (Calvin cycle) - the enzymatic, light-independent carbon fixation.
Key idea: The membrane (thylakoid) handles light capture and ATP/NADPH formation; the stroma handles sugar synthesis.
3. Photosynthetic Pigments
Pigments are substances that absorb light. Leaf pigments can be separated by paper chromatography into four groups.
| Pigment | Colour | Role |
|---|---|---|
| Chlorophyll a | Bright/blue-green | Chief pigment; the reaction-centre that drives photochemistry |
| Chlorophyll b | Yellow-green | Accessory pigment; passes energy to chlorophyll a |
| Xanthophylls | Yellow | Accessory pigment |
| Carotenoids | Yellow to orange | Accessory; also protect chlorophyll from photo-oxidation |
Chlorophyll a absorbs maximally in the blue (around 430 nm) and red (around 660 nm) regions. The action spectrum of photosynthesis closely matches the absorption spectrum of chlorophyll a, proving it is the main pigment.
4. The Two Stages of Photosynthesis
Photosynthesis has two phases that occur in sequence.
- Light reaction (photochemical phase): light-dependent; occurs in the thylakoid membrane; produces ATP, NADPH, and O₂.
- Dark reaction (biosynthetic phase): light-independent; occurs in the stroma; uses ATP and NADPH to fix CO₂ into sugar.
5. The Light Reaction and Photosystems
Pigments are organised into two light-harvesting complexes called photosystems, named in the order they were discovered.
- Photosystem I (PS I): reaction-centre chlorophyll a absorbs at 700 nm (P700).
- Photosystem II (PS II): reaction-centre chlorophyll a absorbs at 680 nm (P680).
Each photosystem has a central reaction centre surrounded by a light-harvesting complex (LHC) of hundreds of accessory pigment molecules that funnel absorbed energy to the reaction centre.
6. Electron Transport - Non-Cyclic Photophosphorylation
In non-cyclic flow, electrons travel in a one-way path through both photosystems - drawn as the famous Z-scheme.
[DIAGRAM: Z-scheme - PS II (P680) → electron acceptor → plastoquinone → cytochrome b6-f → plastocyanin → PS I (P700) → ferredoxin → NADP⁺ reductase → NADPH.]
- PS II absorbs light, P680 emits an excited electron passed down the electron transport chain.
- The lost electron is replaced by the photolysis (splitting) of water, which releases O₂ and protons: 2H₂O → 4H⁺ + O₂ + 4e⁻.
- Electrons reach PS I, are re-energised, and finally reduce NADP⁺ to NADPH.
- Both ATP and NADPH are formed; this is called non-cyclic photophosphorylation.
Note: Photolysis of water is associated with PS II and occurs on the inner side of the thylakoid membrane.
7. Cyclic Photophosphorylation
When only PS I is functional (light beyond 680 nm, or when NADP⁺ is unavailable), electrons cycle back to P700 instead of going to NADP⁺.
- Only ATP is synthesised - no NADPH and no O₂.
- It occurs in the stroma lamellae, which lack PS II and NADP reductase.
- It helps balance the ATP : NADPH ratio needed by the Calvin cycle.
8. Chemiosmotic Hypothesis & ATP Synthesis
The chemiosmotic hypothesis (Peter Mitchell) explains how ATP is actually made - by a proton gradient across the thylakoid membrane.
- Splitting of water inside the lumen, and proton pumping by the electron transport chain, build a high H⁺ concentration in the thylakoid lumen.
- This proton gradient (high inside, low in stroma) stores potential energy.
- Protons flow back to the stroma through the F₀–F₁ ATP synthase (CF₀ channel), and this flow drives the synthesis of ATP from ADP + iP.
Key idea: ATP formation needs a membrane, a proton pump, a proton gradient, and ATP synthase - exactly the chemiosmotic requirements.
9. Biosynthetic Phase - The Calvin Cycle (C3 Pathway)
The dark reaction uses ATP and NADPH from the light reaction to fix CO₂ into sugar in the stroma. It was traced by Melvin Calvin, hence the Calvin cycle. The first stable product is a 3-carbon acid (3-PGA), so it is the C3 pathway.
The cycle has three stages:
- Carboxylation: CO₂ combines with the 5-carbon acceptor RuBP, catalysed by the enzyme RuBisCO, forming two molecules of 3-PGA.
- Reduction: 3-PGA is converted to G3P (sugar) using 2 ATP and 2 NADPH per CO₂ fixed.
- Regeneration: RuBP is regenerated using ATP so the cycle can continue.
To make one glucose, the cycle turns 6 times, using 18 ATP and 12 NADPH and fixing 6 CO₂.
10. C4 Pathway (Hatch and Slack Pathway)
C4 plants (maize, sugarcane, sorghum) have a special leaf anatomy and fix CO₂ twice to avoid photorespiration.
- Kranz anatomy: large bundle-sheath cells with many chloroplasts surround the vascular bundles.
- The first stable product is a 4-carbon acid (oxaloacetic acid, OAA) formed in mesophyll cells; the primary acceptor is PEP and the enzyme is PEP carboxylase.
- The C4 acid moves to bundle-sheath cells, releases CO₂ for the Calvin cycle there, raising CO₂ around RuBisCO.
Advantage: C4 plants show no photorespiration, higher productivity, and tolerate higher temperatures and lower CO₂ better than C3 plants.
11. Photorespiration
Photorespiration is a wasteful process in C3 plants where RuBisCO fixes O₂ instead of CO₂ when O₂ concentration is high.
- RuBisCO has a dual nature - it can act as a carboxylase or an oxygenase; high O₂ favours oxygenation.
- It produces one molecule of phosphoglycolate (2-C) and leads to release of CO₂.
- There is no synthesis of sugar, ATP, or NADPH - it lowers the net yield of C3 plants.
Note: C4 plants avoid photorespiration because their bundle-sheath cells keep CO₂ high around RuBisCO.
12. Factors Affecting Photosynthesis
The rate of photosynthesis depends on internal factors (chlorophyll, leaf area) and external factors (light, CO₂, temperature, water).
Blackman’s Law of Limiting Factors
Blackman’s law (1905): when a process is governed by several factors, the rate is limited by the factor that is in shortest supply (the limiting factor). Changing only the limiting factor will change the rate.
| Factor | Effect on Photosynthesis |
|---|---|
| Light | Rate rises with intensity, then plateaus; very high light causes photo-oxidation (breakdown) |
| CO₂ concentration | Major limiting factor; raising CO₂ increases rate up to a saturation point |
| Temperature | Dark reaction is enzyme-controlled; C4 plants have a higher optimum than C3 |
| Water | Stress closes stomata, reduces CO₂ intake and leaf area |
Read the rest of the chapter →Hide the rest ↑
Weightage in Board & Entrance Exams
| Exam | Typical Weightage | Most-Tested Areas |
|---|---|---|
| CBSE Board (Class 11) | 5–7 marks | Light vs dark reaction, Calvin cycle, C3 vs C4, photorespiration |
| NEET | 2–4 questions (high yield) | Photosystems, Z-scheme, photophosphorylation, RuBisCO, C4 anatomy |
| CUET / State Boards | 2–3 questions | Pigments, site of photosynthesis, limiting factors |
[TABLE: Question-type split - VSA (1 mark): pigments, P700/P680, first product; SA (2–3 marks): cyclic vs non-cyclic, C3 vs C4, photorespiration; LA (5 marks): Calvin cycle, chemiosmotic ATP synthesis.]
Important Definitions
| Term | Definition |
|---|---|
| Photosynthesis | Synthesis of food from CO₂ and water using light energy, releasing O₂ |
| Photolysis | Light-driven splitting of water in PS II releasing O₂, H⁺, and electrons |
| Photosystem | Light-harvesting complex with a reaction centre (PS I = P700, PS II = P680) |
| Non-cyclic photophosphorylation | One-way electron flow through PS II and PS I making ATP + NADPH + O₂ |
| Cyclic photophosphorylation | Electron flow only in PS I making ATP alone (no NADPH, no O₂) |
| Chemiosmosis | ATP synthesis driven by a proton gradient across the thylakoid membrane |
| Calvin cycle (C3) | Dark-reaction pathway fixing CO₂ via RuBP; first product is 3-PGA |
| RuBisCO | Enzyme of carboxylation; acts as both carboxylase and oxygenase |
| Kranz anatomy | Special leaf structure of C4 plants with bundle-sheath cells |
| Photorespiration | Wasteful O₂ fixation by RuBisCO in C3 plants; no ATP/NADPH/sugar made |
Solved Examples
Example 1
How many ATP and NADPH molecules are needed to make one glucose molecule in the Calvin cycle?
Answer: The cycle turns 6 times for one glucose. Per CO₂: 3 ATP + 2 NADPH. So total = 18 ATP and 12 NADPH.
Example 2
Name the first stable products of the C3 and C4 pathways and the number of carbons in each.
Answer: C3 - 3-PGA (3 carbons); C4 - oxaloacetic acid, OAA (4 carbons).
Example 3
Which products of the light reaction are used in the dark reaction, and which is released into the air?
Answer: ATP and NADPH are used in the Calvin cycle; O₂ is released into the atmosphere.
Example 4
Why is non-cyclic photophosphorylation called “non-cyclic”?
Answer: Electrons travel a one-way path from water through PS II and PS I to NADP⁺ and do not return to their source - hence non-cyclic. The lost electrons are replaced by photolysis of water.
Example 5
A C3 and a C4 plant are kept in identical bright light, high temperature, and low CO₂. Which fixes carbon more efficiently and why?
Answer: The C4 plant, because Kranz anatomy concentrates CO₂ around RuBisCO in bundle-sheath cells, preventing photorespiration even at high temperature and low CO₂.
Example 6
In an experiment, light intensity is increased but the photosynthesis rate stops rising. According to Blackman’s law, what is happening?
Answer: Light is no longer the limiting factor; another factor (usually CO₂ concentration) has become the limiting factor, so increasing light alone cannot raise the rate.
Important Questions for Board Exams
1-Mark Questions (VSA)
- Name the pigment that forms the reaction centre of PS I.
- From which molecule is the oxygen released during photosynthesis derived?
- What is the primary CO₂ acceptor in the C4 pathway?
- Name the enzyme responsible for photorespiration.
- In which part of the chloroplast does the Calvin cycle occur?
2–3-Mark Questions (SA)
- Differentiate between cyclic and non-cyclic photophosphorylation.
- Compare C3 and C4 plants with respect to first product, CO₂ acceptor, and photorespiration.
- Explain why RuBisCO is described as having a dual nature.
- State Blackman’s law of limiting factors with one example.
5-Mark Questions (LA)
- Describe the three stages of the Calvin cycle and state how many ATP and NADPH are needed per glucose.
- Explain the chemiosmotic hypothesis of ATP synthesis in the chloroplast.
- Draw and explain the Z-scheme of non-cyclic electron transport, naming the products formed.
Quick Revision Points
- Overall: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O; O₂ comes from water
- Site: chloroplast - grana (light reaction), stroma (dark reaction)
- Pigments: chlorophyll a (chief), chlorophyll b, xanthophylls, carotenoids
- PS I = P700, PS II = P680; LHC funnels energy to the reaction centre
- Non-cyclic: PS II + PS I → ATP + NADPH + O₂ (Z-scheme; water split in PS II)
- Cyclic: only PS I → ATP alone; occurs in stroma lamellae
- Chemiosmosis: H⁺ gradient in thylakoid lumen drives ATP synthase
- Calvin cycle (C3): RuBP + CO₂ → 2 × 3-PGA via RuBisCO; 18 ATP + 12 NADPH per glucose
- C4: Kranz anatomy; first product OAA (4-C); PEP carboxylase; no photorespiration
- Photorespiration: RuBisCO fixes O₂; no sugar/ATP/NADPH; only in C3
- Blackman’s law: rate set by the factor in shortest supply (often CO₂)
Next Chapter: Chapter 12 - Respiration in Plants
Class 11 Botany · Chapter 13 – swipe through all 10 cards to understand the whole chapter.
The Big Picture
Photosynthesis traps light energy to turn CO2 and water into glucose, releasing O2.
The O2 released comes from water, not from CO2.
- Two stages: light reaction (on thylakoids) + Calvin cycle (in stroma)
- Light reaction makes ATP + NADPH; Calvin cycle uses them to fix CO2
- 12 H2O are split so all 6 released O2 atoms trace back to water
Site & Pigments
Everything happens inside the chloroplast, split between thylakoids and stroma.
Action spectrum overlaps chl a absorption spectrum → chl a is the main pigment.
- Thylakoids stack into grana, linked by stroma lamellae; light reactions sit on thylakoid membranes
- Calvin cycle (dark reaction) runs in the stroma
- Max photosynthesis in the blue and red regions of light
Two Photosystems & the Z-Scheme
Light excites electrons that travel a downhill electron-transport chain between two photosystems.
PS II acts FIRST even though its number is higher.
- PS I reaction centre = P700; PS II = P680 (named by absorption peak in nm)
- Electrons end on NADP⁺, reducing it to NADPH
- Whole flow looks like a ‘Z’ when drawn against energy
Photolysis of Water
PS II splits water on the inner (lumen) side of the thylakoid to replace its lost electrons.
Happens in the thylakoid lumen and is associated with PS II.
- Electrons released replace those lost by P680
- O2 of photosynthesis is released here — from water
- Protons (H⁺) pile up in the lumen, building the gradient for ATP
Cyclic vs Non-cyclic
Photophosphorylation makes ATP using light; the path taken decides the products.
Cyclic flow runs in stroma lamellae, which lack PS II.
- Non-cyclic: one-way flow, electrons never return to the donor
- Cyclic: electrons loop PS I → cyt complex → PS I; no NADPH, no O2
- Cell uses cyclic flow when it needs extra ATP
How ATP Is Actually Made
A proton gradient across the thylakoid drives ATP synthase, just like in mitochondria.
ATP is released into the stroma to power the Calvin cycle.
- Gradient built by: water splitting in lumen, H⁺ pumped during transport, NADP⁺ reduction removing stromal H⁺
- Protons flow back through the F0 channel of ATP synthase
- This is the chemiosmotic hypothesis
Calvin Cycle (C3 Pathway)
In the stroma, CO2 is fixed onto RuBP and reduced to sugar over three phases.
Per 1 CO2 fixed: 2 NADPH + 3 ATP. Per glucose (6 CO2): 18 ATP + 12 NADPH.
- RuBisCO adds CO2 to 5-C RuBP → two molecules of 3-C 3-PGA (most crucial step)
- 3-PGA → G3P using ATP & NADPH; RuBP regenerated using ATP
- 6 turns of the cycle make one glucose; ATP:NADPH demand ratio is 3:2
C4 Pathway & Kranz Anatomy
C4 plants (maize, sugarcane, sorghum) concentrate CO2 around RuBisCO to boost efficiency.
Bundle-sheath chloroplasts lack grana; primary acceptor is PEP, not RuBP.
- Kranz anatomy: bundle-sheath cells ring the vascular bundles
- First product is 4-C OAA → hence ‘C4’
- No photorespiration → higher productivity, higher optimum temperature
Photorespiration
In C3 plants, when O2 is high and CO2 low, RuBisCO works backwards and wastes energy.
No sugar, no ATP, no NADPH produced — purely wasteful.
- RuBisCO has dual nature: carboxylase AND oxygenase
- C4 plants avoid it: PEPcase ignores O2 and CO2 is concentrated for RuBisCO
- Favoured by high O2 / low CO2 conditions
Factors Affecting Rate
Blackman’s Law: the factor nearest its minimum sets the overall rate.
Light saturation at ~10% of full sunlight; CO2 helps up to ~0.05%.
- Light: rate rises then plateaus at light saturation
- Temperature: dark (enzymatic) reaction is most temperature-sensitive; C4 optimum > C3
- Water acts indirectly — stomatal closure cuts CO2 uptake
📝 Practice Photosynthesis in Higher Plants — 10 NEET PYQs
Real previous-year questions · with answers & solutions
Start →Close ✕
Chapter Navigation
Previous: Transport in Plants Class 11 Notes
Next: Respiration in Plants Class 11 Notes
Related Chapters in Class 11 Biology
- Cell The Unit of Life Class 11 Notes
- Biomolecules Class 11 Notes
- Respiration in Plants Class 11 Notes
Practice What You Learned
Take your biology further with our Class 12 Biology notes once you are board-ready.
Explore More
Frequently Asked Questions
Photosynthesis is the process by which green plants use light energy trapped by chlorophyll to convert carbon dioxide and water into glucose, releasing oxygen as a by-product. It happens inside the chloroplast in two stages, the light reaction on the thylakoids and the Calvin cycle in the stroma.
The overall reaction is 6 CO2 plus 12 H2O giving C6H12O6 plus 6 O2 plus 6 H2O in the presence of light and chlorophyll. Fixing one CO2 in the Calvin cycle needs 3 ATP and 2 NADPH, so making one glucose from 6 CO2 needs 18 ATP and 12 NADPH.
Yes, it is one of the most scoring Class 11 Botany chapters and questions appear almost every year in NEET. The Z-scheme, photolysis of water, the Calvin cycle steps, C4 and Kranz anatomy, and the limiting-factor concept are the most frequently asked areas.
In C3 plants the first stable product is the 3-carbon 3-PGA and CO2 is fixed directly by RuBisCO, while in C4 plants the first product is the 4-carbon OAA formed by PEPcase. C4 plants like maize and sugarcane use Kranz anatomy to concentrate CO2 around RuBisCO, so they avoid photorespiration and are more productive.
The oxygen comes from water, not from carbon dioxide. During photolysis, water is split on the inner lumen side of the thylakoid by photosystem II, releasing O2, protons and electrons, which is why 12 water molecules are shown so that all 6 oxygen molecules trace back to water.