Respiration in Plants NEET Notes 2027: Key Points and PDF

Respiration in Plants is Chapter 12 of NCERT Class 11 Biology and part of Unit 4, Plant Physiology, of the NEET UG syllabus. These notes cover gas exchange in plants, glycolysis, fermentation, the link reaction, Krebs’ cycle, the electron transport system, the respiratory balance sheet, the amphibolic pathway and respiratory quotient, with a free notes PDF.

Respiration in Plants NEET Notes

Section-wise key points from NCERT Chapter 12, comparison tables, high-yield facts and common mistakes for NEET 2027 revision.

Respiration in 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 cellular respiration?

Respiration is the breaking of C-C bonds of complex compounds by oxidation within cells, releasing a considerable amount of energy.

• The compounds oxidised are respiratory substrates. Usually carbohydrates are used, but proteins, fats and even organic acids can be respired in some plants under certain conditions.
• Energy is released in slow, stepwise, enzyme-controlled reactions and trapped as ATP, the energy currency of the cell.
• The carbon skeletons produced are used as precursors for the biosynthesis of other molecules.
• All the food respired for life processes ultimately comes from photosynthesis.
• In eukaryotes, breakdown of food takes place in the cytoplasm and the mitochondria.
• Overall: C6H12O6 + 6O2 gives 6CO2 + 6H2O + energy. Cells oxidise glucose in small steps so that some energy can be coupled to ATP synthesis instead of being lost as heat.

Do plants breathe?

Plants need O2 and give out CO2, but they have no specialised respiratory organs; stomata and lenticels allow gas exchange.

• Each plant part takes care of its own gas-exchange needs, with very little transport of gases between parts.
• Plants have low demands: roots, stems and leaves respire at rates far lower than animals; only photosynthesis exchanges large volumes of gas.
• Diffusion distances are short: living cells lie close to the surface, woody stems have living cells in thin layers beneath the bark and lenticels, and loosely packed parenchyma gives a network of air spaces.

Glycolysis (EMP Pathway)

Glycolysis is the partial oxidation of one glucose into two molecules of pyruvic acid in the cytoplasm; it occurs in all living organisms and is the only respiratory process in anaerobic organisms.

• Name: Greek glycos (sugar) and lysis (splitting). The scheme was given by Gustav Embden, Otto Meyerhof and J. Parnas, hence the EMP pathway.
• It is a chain of ten enzyme-controlled reactions.
• In plants, glucose comes from sucrose (the end product of photosynthesis) or storage carbohydrates. Invertase converts sucrose into glucose and fructose.
• Hexokinase phosphorylates glucose and fructose to glucose-6-phosphate, which isomerises to fructose-6-phosphate; after this, glucose and fructose follow the same steps.

• Glycolysis gives a net gain of 2 ATP per glucose degraded to pyruvic acid.
• Pyruvic acid has three fates: lactic acid fermentation, alcoholic fermentation or aerobic respiration (Krebs’ cycle, which needs O2).

Fermentation: Anaerobic Respiration

Fermentation is the incomplete oxidation of glucose under anaerobic conditions, in many prokaryotes, unicellular eukaryotes and germinating seeds.

• In both, NADH + H+ is the reducing agent and is reoxidised to NAD+.
• Less than 7% of the energy in glucose is released, and not all of it is trapped as ATP.
• The processes are hazardous because acid or alcohol is produced; yeasts poison themselves to death when alcohol reaches about 13%.

Aerobic Respiration: Link Reaction and Krebs’ Cycle

Aerobic respiration completely oxidises organic substances in the presence of O2 to CO2 and water, releasing a large amount of energy; in eukaryotes it takes place in the mitochondria.

• Pyruvate is transported from the cytoplasm into the mitochondria.
• Two crucial events: complete oxidation of pyruvate by stepwise removal of all hydrogen atoms, leaving three CO2 molecules (in the matrix), and passing the electrons to O2 with ATP synthesis (on the inner membrane).
• Link reaction: pyruvic acid + CoA + NAD+ gives acetyl CoA + CO2 + NADH + H+, by oxidative decarboxylation catalysed by pyruvic dehydrogenase (needs Mg2+, NAD+ and Coenzyme A).
• Two NADH are produced from the two pyruvic acid molecules of one glucose.

Tricarboxylic Acid (TCA) Cycle

• Also called Krebs’ cycle after Hans Krebs, who first elucidated it; it runs in the mitochondrial matrix.
• Acetyl group + oxaloacetic acid (OAA) + water gives citric acid, catalysed by citrate synthase, releasing CoA.
• Citrate isomerises to isocitrate, followed by two successive decarboxylations to alpha-ketoglutaric acid and then succinyl-CoA.
• Succinyl-CoA to succinic acid forms one GTP (substrate level phosphorylation); in a coupled reaction GTP gives GDP and ATP is made from ADP.
• Per turn, NAD+ is reduced to NADH + H+ at three points and FAD+ to FADH2 at one point.
• The cycle needs continued replenishment of OAA and regeneration of NAD+ and FAD+.
• Summary per pyruvic acid: pyruvic acid + 4NAD+ + FAD+ + 2H2O + ADP + Pi gives 3CO2 + 4NADH + 4H+ + FADH2 + ATP.
• Up to the end of the TCA cycle, one glucose gives eight NADH + H+ and two FADH2 besides just two ATP in the TCA cycle.

Electron Transport System and Oxidative Phosphorylation

The electron transport system (ETS) in the inner mitochondrial membrane releases and uses the energy stored in NADH + H+ and FADH2 by passing electrons to O2, forming H2O.

• Oxidation of one NADH gives 3 ATP; one FADH2 gives 2 ATP.
• Oxygen is the final hydrogen acceptor. Its role is limited to the terminal stage, but it is vital because it drives the whole process by removing hydrogen.
• The energy of oxidation-reduction builds the proton gradient, so the process is called oxidative phosphorylation (in photophosphorylation, light energy builds the gradient).
• F1 headpiece: peripheral membrane protein complex with the site of ATP synthesis. F0: integral membrane protein complex forming the proton channel.
• For each ATP made, 4H+ pass through F0 from the intermembrane space to the matrix, down the electrochemical proton gradient.

Respiratory Balance Sheet

NCERT calculates a theoretical net gain of 38 ATP for the aerobic respiration of one glucose molecule, based on assumptions that do not fully hold in a living system.

• Assumption 1: glycolysis, TCA cycle and ETS work one after another in a sequential, orderly pathway.
• Assumption 2: NADH made in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
• Assumption 3: no intermediates are withdrawn to make other compounds.
• Assumption 4: only glucose is respired; no alternative substrates enter at intermediate stages.
• In reality all pathways work simultaneously, substrates enter and leave as needed and enzyme rates are controlled in many ways.

Why is respiration an amphibolic pathway?

The respiratory pathway is used both to break down substrates (catabolism) and to supply intermediates for synthesis (anabolism), so it is amphibolic rather than purely catabolic.

• Glucose is the favoured substrate; other carbohydrates are usually converted to glucose first.
• Fats are broken into glycerol and fatty acids. Fatty acids are degraded to acetyl CoA; glycerol enters after conversion to PGAL.
• Proteins are degraded by proteases, and the amino acids (after deamination) enter within the Krebs’ cycle or as pyruvate or acetyl CoA.
• The same compounds are withdrawn for synthesis: for example, acetyl CoA is withdrawn when the organism needs to make fatty acids.

Respiratory Quotient (RQ)

RQ, or respiratory ratio, is the volume of CO2 evolved divided by the volume of O2 consumed, and it depends on the respiratory substrate.

• In living organisms, respiratory substrates are often more than one; pure proteins or fats are never used as respiratory substrates.

Most-asked Facts for NEET: Respiration in Plants

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

• Plants exchange gases through stomata and lenticels; they have no specialised respiratory organs.
• Glycolysis (EMP pathway: Embden, Meyerhof, Parnas) occurs in the cytoplasm of all living organisms.
• Invertase converts sucrose to glucose and fructose; hexokinase phosphorylates them.
• ATP is used at two steps of glycolysis: glucose to glucose-6-phosphate and fructose-6-phosphate to fructose 1,6-bisphosphate.
• NADH + H+ is formed in glycolysis when PGAL becomes BPGA.
• Net gain of glycolysis: 2 ATP per glucose.
• Alcoholic fermentation: pyruvic acid decarboxylase and alcohol dehydrogenase; lactic acid: lactate dehydrogenase.
• Fermentation releases less than 7% of the energy in glucose; yeast dies at about 13% alcohol.
• Pyruvic dehydrogenase needs Mg2+, NAD+ and Coenzyme A.
• Citrate synthase joins acetyl CoA with oxaloacetic acid to form citric acid.
• GTP is formed at succinyl-CoA to succinic acid (substrate level phosphorylation).
• Per TCA turn: 3 NADH + H+, 1 FADH2 and 1 GTP/ATP.
• Cytochrome c is a mobile carrier between complex III and IV; complex IV has cytochromes a, a3 and two copper centres.
• 1 NADH gives 3 ATP; 1 FADH2 gives 2 ATP; 4H+ pass through F0 per ATP.
• Oxygen is the final hydrogen acceptor and is reduced to water.
• Theoretical net gain: 38 ATP per glucose in aerobic respiration.
• RQ: carbohydrate 1.0, tripalmitin (fat) 0.7, protein about 0.9.

Common Mistakes to Avoid

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

• Saying glycolysis happens in the mitochondria. It occurs in the cytoplasm; the link reaction and TCA cycle are in the matrix and ETS on the inner membrane.
• Writing a gross gain of 4 ATP as the net gain of glycolysis. Two ATP are used, so the net gain is 2 ATP.
• Thinking oxygen is used throughout aerobic respiration. Its role is limited to the terminal stage as the final hydrogen acceptor, though it is vital.
• Calling the respiratory pathway only catabolic. It is amphibolic because intermediates are also withdrawn for synthesis.
• Giving an RQ of more than 1 for fats. NCERT gives less than 1 (0.7 for tripalmitin).
• Treating 38 ATP as an exact, real yield. NCERT says it is a theoretical figure based on assumptions.
• Mixing up substrate level and oxidative phosphorylation: the GTP/ATP from succinyl-CoA is substrate level; ATP via ETS and ATP synthase is oxidative.
• Placing the ATP synthesis site in F0. F1 has the catalytic site; F0 is the proton channel.
• Thinking only animals make lactic acid. Some bacteria also produce lactic acid from pyruvic acid.

Respiration in Plants Notes PDF and Related Pages

Download the AglaSem notes for Respiration in 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 12 (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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