Photosynthesis in Higher Plants NEET Notes 2027: Key Points and PDF

Photosynthesis in Higher Plants is Chapter 11 of NCERT Class 11 Biology and part of Unit 4, Plant Physiology, of the NEET UG syllabus. These notes cover early experiments, the site of photosynthesis, pigments, the light reaction and Z scheme, photophosphorylation, chemiosmosis, the Calvin cycle, the C4 pathway, photorespiration and limiting factors, with a free notes PDF.

Photosynthesis in Higher Plants NEET Notes

Section-wise key points from NCERT Chapter 11, comparison tables, high-yield facts and common mistakes for NEET 2027 revision.

Photosynthesis in Higher Plants at a Glance

The topic list follows the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025, which stays the reference until the 2027 syllabus is out. The full list is on the NEET syllabus page.

What is photosynthesis and what did the early experiments show?

Photosynthesis is the physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds. It is the primary source of all food on earth and releases oxygen into the atmosphere.

• Variegated leaf or half-covered leaf experiment: starch forms only in the green parts exposed to light.
• Half-leaf experiment with KOH-soaked cotton (KOH absorbs CO2): the part inside the tube tests negative for starch, showing that CO2 is needed.

• Overall equation: 6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2 (in light). The O2 released comes from water; this was proved with radioisotope techniques.
• When H2S is the hydrogen donor (purple and green sulphur bacteria), the oxidation product is sulphur or sulphate, not O2.
• Photosynthesis is not a single reaction but a multistep process.

Where does photosynthesis take place?

Mainly in the green leaves, inside the chloroplasts of mesophyll cells, but also in other green parts of the plant.

• Chloroplasts align along the walls of mesophyll cells to get the optimum quantity of incident light.
• The chloroplast has a membrane system (grana and stroma lamellae) and a matrix, the stroma, with a clear division of labour.
• Membranes trap light energy and make ATP and NADPH: these are the light reactions (photochemical reactions).
• In the stroma, enzymatic reactions make sugar, which forms starch: these are the dark reactions (carbon reactions).
• ‘Dark reaction’ does not mean the reactions happen in darkness or are light-independent; they depend on ATP and NADPH from the light reactions.

Which pigments are involved in photosynthesis?

Paper chromatography of leaf pigments shows that leaf colour comes from four pigments, not one.

• Chlorophyll a absorbs most in the blue and red regions, and these regions also show the highest rate of photosynthesis.
• The absorption spectrum of chlorophyll a and the action spectrum of photosynthesis do not overlap one-to-one; some photosynthesis happens at other visible wavelengths.
• Accessory pigments absorb light and pass the energy to chlorophyll a. They let a wider range of wavelengths be used and protect chlorophyll a from photo-oxidation.

What happens in the light reaction?

The light reaction (photochemical phase) covers light absorption, water splitting, oxygen release and the formation of ATP and NADPH.

• Pigments are organised into two light harvesting complexes (LHC) in Photosystem I (PS I) and Photosystem II (PS II).
• The photosystems are named in the order of their discovery, not the order in which they work.
• In each photosystem all pigments except one chlorophyll a molecule form the antenna; that single chlorophyll a molecule is the reaction centre.
• PS I reaction centre absorbs at 700 nm (P700); PS II reaction centre absorbs at 680 nm (P680).

Electron Transport and the Z Scheme

• PS II absorbs 680 nm red light; excited electrons go to an electron acceptor and then down an electron transport system of cytochromes to PS I.
• PS I electrons, excited by 700 nm light, pass to another acceptor and then downhill to NADP+, reducing it to NADPH + H+.
• The whole path is called the Z scheme because of its shape when the carriers are placed on a redox potential scale.

Splitting of Water

• Water splitting is associated with PS II: 2H2O → 4H+ + O2 + 4e−. It releases oxygen, one of the net products of photosynthesis.
• Electrons from water replace those lost by PS II, while PS II supplies the electrons that PS I loses.
• The water-splitting complex lies on the inner side of the thylakoid membrane, so protons are released into the lumen.

Cyclic and Non-cyclic Photophosphorylation

Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light.

Chemiosmotic Hypothesis

• ATP synthesis is linked to a proton gradient across the thylakoid membrane. Protons collect inside, in the lumen (in respiration they collect in the mitochondrial intermembrane space).
• Three causes of the gradient: protons from water splitting are released into the lumen; the primary electron acceptor passes its electron to an H carrier, moving a proton from the stroma to the lumen; NADP reductase on the stroma side uses protons from the stroma.
• Result: fewer protons in the stroma, more in the lumen, and a measurable drop in lumen pH.
• ATP synthase has two parts: CF0, embedded in the membrane, forms the channel for facilitated diffusion of protons; CF1 protrudes on the stroma side.
• Proton flow through CF0 causes a conformational change in CF1, which then makes ATP.
• Chemiosmosis needs a membrane, a proton pump, a proton gradient and ATP synthase.

Where Are ATP and NADPH Used? the Calvin Cycle

The light reaction gives ATP, NADPH and O2. O2 diffuses out of the chloroplast, while ATP and NADPH drive the biosynthetic phase that makes sugars.

• The biosynthetic phase continues for a while after the light goes off and then stops, so calling it the ‘dark reaction’ is a misnomer.
• C3 plants: first CO2 fixation product is 3-phosphoglyceric acid (PGA). C4 plants: first product is oxaloacetic acid (OAA), a 4-carbon acid.
• The primary CO2 acceptor in the Calvin cycle is ribulose bisphosphate (RuBP), a 5-carbon ketose sugar. Scientists first searched for a 2-carbon acceptor for years.
• The Calvin pathway occurs in all photosynthetic plants, whether C3 or C4.

• Per CO2 fixed: 3 ATP and 2 NADPH.
• One glucose needs 6 CO2 and 6 turns of the cycle: 18 ATP and 12 NADPH in, one glucose, 18 ADP and 12 NADP out.
• Cyclic photophosphorylation probably makes up the extra ATP needed compared with NADPH.

What is the C4 pathway?

C4 plants, adapted to dry tropical regions, form the 4-carbon OAA first but still use the Calvin cycle as their main biosynthetic pathway.

• Special features: Kranz anatomy, tolerance of higher temperatures, response to high light intensity, no photorespiration and greater biomass productivity.
• Kranz means ‘wreath’. The large bundle sheath cells around the vascular bundles may form several layers, have many chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces.
• Examples to observe Kranz anatomy: maize and sorghum.
• Hatch and Slack pathway: PEP (3-carbon) in mesophyll cells accepts CO2 through PEP carboxylase (PEPcase) to form OAA. OAA becomes malic acid or aspartic acid, which moves to the bundle sheath cells.
• In the bundle sheath, the C4 acid releases CO2 and a 3-carbon molecule. The 3-carbon molecule returns to the mesophyll and is converted back to PEP; the CO2 enters the Calvin cycle.

What is photorespiration?

Photorespiration is the wasteful oxygenation reaction of RuBisCO in C3 plants.

• RuBisCO, the most abundant enzyme in the world, can bind both CO2 and O2 at its active site. The binding is competitive and depends on the relative concentration of the two gases.
• RuBisCO has a much greater affinity for CO2 when CO2 and O2 are nearly equal.
• When O2 binds, RuBP forms one molecule of phosphoglycerate and one of phosphoglycolate (2-carbon) instead of two PGA.
• No sugar, ATP or NADPH is made; CO2 is released and ATP is used. Its biological function is not yet known.
• C4 plants avoid it: breakdown of the C4 acid in the bundle sheath raises the CO2 concentration at the enzyme site, so RuBisCO works as a carboxylase.

Which factors affect photosynthesis?

Internal factors are the number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal CO2 concentration and the amount of chlorophyll. External factors are sunlight, temperature, CO2 concentration and water.

• Blackman’s (1905) Law of Limiting Factors: if a chemical process is affected by more than one factor, its rate is determined by the factor nearest to its minimal value.

Most-asked Facts for NEET: Photosynthesis in Higher Plants

Revise these one-line facts from the NCERT chapter just before the exam.

• Priestley (1770) used a mint plant in a bell jar; he discovered oxygen in 1774.
• Engelmann used Cladophora and aerobic bacteria to describe the first action spectrum.
• van Niel showed that the O2 released in photosynthesis comes from water.
• Four leaf pigments: chlorophyll a (blue green), chlorophyll b (yellow green), xanthophylls (yellow), carotenoids (yellow to yellow-orange).
• PS I reaction centre is P700; PS II reaction centre is P680.
• Water splitting is linked to PS II: 2H2O → 4H+ + O2 + 4e−.
• Cyclic photophosphorylation uses only PS I, occurs in stroma lamellae and makes only ATP.
• Protons accumulate in the thylakoid lumen; CF0 is the proton channel, CF1 faces the stroma.
• Calvin cycle stages: carboxylation, reduction, regeneration; RuBP is the 5-carbon acceptor.
• One glucose needs 6 turns, 18 ATP and 12 NADPH.
• C4 primary acceptor PEP, enzyme PEPcase, first product OAA; Hatch and Slack pathway.
• Kranz anatomy: bundle sheath cells with many chloroplasts and no intercellular spaces (maize, sorghum).
• Photorespiration forms phosphoglycolate (2-carbon); no ATP or NADPH is made.
• Light saturation occurs at 10% of full sunlight; CO2 is the major limiting factor.
• C4 plants saturate at about 360 µL/L CO2; C3 plants only beyond 450 µL/L.
• Blackman’s Law of Limiting Factors dates from 1905.

Common Mistakes to Avoid

Watch for these mix-ups when you answer questions on this chapter.

• Saying the O2 released comes from CO2. It comes from water.
• Thinking PS I works before PS II. They are numbered by discovery; in the Z scheme electrons flow from PS II to PS I.
• Writing that dark reactions take place only at night. They do not need light directly but depend on ATP and NADPH from the light reaction.
• Saying cyclic photophosphorylation makes NADPH. It makes only ATP.
• Placing the proton build-up in the stroma. In chloroplasts protons collect in the thylakoid lumen.
• Swapping CF0 and CF1. CF0 is the membrane channel; CF1 protrudes into the stroma and makes ATP.
• Calling OAA the primary acceptor in C4 plants. PEP is the acceptor; OAA is the first product.
• Saying C4 plants have no Calvin cycle. They run it in the bundle sheath cells.
• Saying photorespiration produces ATP. It uses ATP and releases CO2.
• Writing NADH for the light-reaction product. The text gives NADPH + H+ (the NCERT summary line saying ‘NAD forming NADH’ is a slip).

Photosynthesis in Higher Plants Notes PDF and Related Pages

Download the AglaSem notes for Photosynthesis in Higher Plants (key notes and important questions with answers) free: View PDF

• All chapters: NEET Biology notes
• Full topic list: NEET syllabus
• Practice: NEET question papers
• Source text: NCERT Chapter 11 (NCERT, Class 11 Biology)

NEET Biology Notes: All Chapters

Botany

• The Living World
• Biological Classification
• Plant Kingdom
• Cell: The Unit of Life
• Plant Growth and Development
• Anatomy of Flowering Plants
• Cell Cycle and Cell Division
• Respiration in Plants
• Photosynthesis in Higher Plants
• Morphology of Flowering Plants
• Principles of Inheritance and Variation
• Microbes in Human Welfare
• Sexual Reproduction in Flowering Plants
• Molecular Basis of Inheritance
• Organisms and Populations
• Ecosystem
• Biodiversity and Conservation

Zoology

• Animal Kingdom
• Biomolecules
• Breathing and Exchange of Gases
• Excretory Products and their Elimination
• Body Fluids and Circulation
• Structural Organisation in Animals
• Locomotion and Movement
• Reproductive Health
• Chemical Coordination and Integration
• Neural Control and Coordination
• Evolution
• Human Health and Disease
• Human Reproduction
• Biotechnology and its Applications
• Biotechnology: Principles and Processes

All chapters with their notes PDFs are on the NEET Biology notes page.

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