Science & TechnologyNCERT Class 11 · Biology (Class 11)

Respiration in Plants

How plant and other cells break down food through glycolysis, fermentation and aerobic respiration to release energy and trap it as ATP.

⏱ 7 min readGS-III6 sections5 memory tricks

Why this matters for UPSC

Respiration in Plants is a high-yield NCERT science topic where Prelims repeatedly tests enzyme-substrate pairs, the exact location of glycolysis/Krebs/ETS, ATP yields and RQ values as single-fact or matching questions. For GS-III (Science & Technology) it underpins bioenergetics and the fermentation processes behind biotechnology and food industries. A clear grasp also sharpens the everyday-science reasoning expected across the GS papers.

Understand the chapter

The Energy Story: Respiration and ATP

Every life activity needs energy, and that energy ultimately traces back to photosynthesis, which stores light energy in the C-C bonds of carbohydrates. Respiration is the enzyme-controlled, stepwise oxidative breakdown of these C-C bonds within cells to release energy. Crucially this energy is never used directly or released in one burst; it is captured in ATP, the cell's energy currency, while the leftover carbon skeletons feed biosynthesis.

Respiratory substrate: the compound oxidised, usually carbohydrate but also fats, proteins or organic acids.
Photosynthesis occurs in chloroplasts; respiratory breakdown in cytoplasm plus mitochondria (eukaryotes).
Stepwise oxidation avoids heat loss and couples select steps to ATP synthesis.

Do Plants Breathe? Gas Exchange Without Organs

Plants do respire, taking in O2 and giving out CO2, but unlike animals they have no specialised respiratory organs. Each plant part meets its own modest needs through stomata and lenticels, aided by loosely packed parenchyma forming interconnected air spaces. Diffusion distances are short because every living cell lies near the surface, while interior woody cells are dead and only give mechanical support.

Stomata (leaves) and lenticels (stems/bark) enable gaseous exchange.
Low demand: roots, stems and leaves respire far slower than animals.
During photosynthesis O2 is released within the cell, easing supply.

Glycolysis: The Universal First Step

Glycolysis (Greek glycos = sugar, lysis = splitting), the EMP pathway named after Embden, Meyerhof and Parnas, occurs in the cytoplasm of all living organisms and is the only respiratory route in anaerobes. Through ten enzyme-controlled reactions one glucose is partially oxidised into two molecules of pyruvic acid. In plants the glucose comes from sucrose, split by invertase into glucose and fructose, or from stored carbohydrate.

ATP used at 2 steps (glucose to G6P; F6P to fructose-1,6-bisphosphate); net gain = 2 ATP.
2 NADH+H+ formed when PGAL is converted to BPGA.
Key enzymes: invertase and hexokinase.
Pyruvate is the end product and the metabolic branch point.

Fermentation: Energy Without Oxygen

Under anaerobic conditions pyruvate is only incompletely oxidised by fermentation. In yeast, pyruvic acid decarboxylase and alcohol dehydrogenase convert pyruvate to CO2 and ethanol; in some bacteria and in exercising muscle, lactate dehydrogenase reduces pyruvate to lactic acid. NADH+H+ is the reducing agent that is reoxidised to NAD+ so that glycolysis can keep running, but less than 7 per cent of glucose energy is released.

Two types: alcoholic fermentation and lactic acid fermentation.
Net ATP stays 2 (the fermentation step itself adds no ATP).
Yeast dies near 13 per cent alcohol; stronger drinks need distillation.
Facultative anaerobes can use O2 when present; obligate anaerobes cannot.

Aerobic Respiration: Link Reaction and Krebs Cycle

For complete oxidation pyruvate enters the mitochondrial matrix. In the link reaction, pyruvic dehydrogenase (needing NAD+ and Coenzyme A) decarboxylates pyruvate to acetyl CoA, releasing CO2 and NADH. Acetyl CoA then enters the tricarboxylic acid (Krebs or citric acid) cycle, elucidated by Hans Krebs, which begins when citrate synthase joins acetyl CoA with oxaloacetic acid and water to form citric acid.

Link reaction and Krebs cycle both occur in the mitochondrial matrix.
Per Krebs turn: 3 NADH, 1 FADH2 and 1 GTP (substrate-level phosphorylation).
Oxaloacetic acid is regenerated each turn to keep the cycle running.
Two turns occur per glucose (two pyruvate and two acetyl CoA).

ETS, Oxidative Phosphorylation, Balance Sheet and RQ

The NADH and FADH2 are oxidised through the electron transport system on the inner mitochondrial membrane, passing electrons via complexes I to IV, ubiquinone and cytochrome c finally to O2 to form water, with the released energy driving ATP synthesis (oxidative phosphorylation). Summed up, NCERT's theoretical balance sheet gives a net 38 ATP per glucose. The same route is amphibolic, both breaking molecules down and supplying biosynthetic precursors, and its substrate use is read off the Respiratory Quotient.

ETS complexes: I (NADH dehydrogenase), II (FADH2 entry), III (cytochrome bc1), then cytochrome c to O2.
O2 is the final electron acceptor, combining with H+ to form water.
RQ = CO2 released / O2 consumed: carbohydrates 1, fats below 1, proteins below 1.
Amphibolic pathway: respiration links catabolism and anabolism.

Key terms

Respiration: Enzyme-controlled, stepwise oxidative breaking of C-C bonds of respiratory substrates to release energy trapped as ATP.
Respiratory substrate: The molecule oxidised in respiration, usually glucose but also fats, proteins or organic acids.
ATP: The universal energy currency of the cell, synthesised in small steps and spent wherever energy is needed.
Glycolysis (EMP pathway): Cytoplasmic splitting of one glucose into two pyruvate in all organisms, with a net gain of 2 ATP.
Fermentation: Anaerobic, incomplete oxidation of pyruvate to ethanol+CO2 or to lactic acid, releasing under 7 per cent of glucose energy.
Aerobic respiration: Complete oxidation of substrate in mitochondria using O2, yielding CO2, water and a large amount of ATP.
Krebs (TCA) cycle: Mitochondrial cyclic oxidation of acetyl CoA beginning at citric acid; per turn it yields 3 NADH, 1 FADH2 and 1 GTP.
Electron Transport System (ETS): Chain of carriers on the inner mitochondrial membrane passing electrons to O2 and driving oxidative phosphorylation.
Amphibolic pathway: A pathway that is both catabolic and anabolic; respiration both breaks down and supplies precursors for biosynthesis.
Respiratory Quotient (RQ): Ratio of CO2 evolved to O2 consumed, indicating which respiratory substrate is being used.

Must-know facts exam-ready

Respiration breaks the C-C bonds of complex compounds by stepwise oxidation, and the energy is trapped in ATP, the cell's energy currency.
Plants have no specialised respiratory organs; they exchange gases through stomata and lenticels.
Glycolysis (EMP pathway) was given by Embden, Meyerhof and Parnas, occurs in the cytoplasm of all cells, and converts glucose to two pyruvic acid in ten enzyme-controlled steps.
In glycolysis ATP is used at 2 steps; net gain is 2 ATP plus 2 NADH.
Sucrose is split into glucose and fructose by invertase; glucose is phosphorylated to glucose-6-phosphate by hexokinase.
Pyruvate has three fates: lactic acid fermentation, alcoholic fermentation and aerobic respiration.
Fermentation releases under 7 per cent of glucose energy; yeast dies near 13 per cent alcohol concentration.
The Krebs (TCA/citric acid) cycle was elucidated by Hans Krebs, occurs in the mitochondrial matrix, and starts with acetyl CoA + oxaloacetic acid + water forming citric acid via citrate synthase.
Each Krebs turn yields 3 NADH, 1 FADH2 and 1 GTP, the GTP coming from substrate-level phosphorylation.
ETS sits on the inner mitochondrial membrane; electrons pass through complexes I to IV, ubiquinone and cytochrome c to O2, the final electron acceptor, forming water.
NCERT's theoretical respiratory balance sheet gives a net 38 ATP per glucose under idealised assumptions.
RQ of carbohydrates is 1, of fats below 1 and of proteins below 1; respiration is an amphibolic pathway.

Memory tricks remember it for good

EMP = Embden, Meyerhof, Parnas

The three scientists who gave the scheme of glycolysis, hence the EMP pathway.

💡 Recall who described glycolysis and its alias.

Cyto-Matrix-Inner

Glycolysis in the CYTOplasm, link reaction and Krebs in the mitochondrial MATRIX, ETS on the INNER mitochondrial membrane.

💡 Pin each stage of respiration to its exact location.

LAA = the 3 fates of pyruvate

Lactic-acid fermentation, Alcoholic fermentation, Aerobic respiration.

💡 Remember the three destinies of pyruvic acid.

Krebs 3-1-1

Each Krebs turn yields 3 NADH, 1 FADH2 and 1 GTP.

💡 Recall the per-cycle energy carriers of the citric acid cycle.

RQ ladder: 1 then below

Carbohydrates sit on top with RQ = 1; fats and proteins are lower rungs with RQ below 1.

💡 Recall respiratory quotient values by substrate.

Traps to avoid

Glycolysis makes 4 ATP gross but only a NET 2 ATP because two are consumed; UPSC tests the net figure.
Glycolysis happens in the cytoplasm, not the mitochondria; only the link reaction, Krebs and ETS are mitochondrial.
Fermentation is incomplete oxidation releasing under 7 per cent energy; it is not zero energy and adds no ATP beyond glycolysis's net 2.
The Krebs cycle directly makes GTP by substrate-level phosphorylation, which is then converted to ATP, not ATP directly.
Plants use stomata and lenticels and have no specialised respiratory organs; do not equate them with animal lungs or gills.
NCERT's eight NADH refers to the mitochondrial phase (link reaction plus Krebs); glycolysis's two NADH are counted separately.

Exam focus

🧠 Prelims angles

Enzyme-reaction matching: invertase, hexokinase, citrate synthase, pyruvic dehydrogenase, lactate dehydrogenase, alcohol dehydrogenase, pyruvic acid decarboxylase.
Stage-location matching: glycolysis (cytoplasm), Krebs (matrix), ETS (inner mitochondrial membrane).
ATP accounting: net 2 from glycolysis, per-Krebs-turn carriers, and the theoretical 38 ATP per glucose.
Scientists and terms: EMP pathway (Embden-Meyerhof-Parnas), Hans Krebs, and the literal meaning of glycolysis.
RQ values for carbohydrates, fats and proteins; facultative versus obligate anaerobes.
Gas-exchange structures stomata and lenticels, and the amphibolic nature of respiration.

✍️ Mains angles GS-III

Fermentation as the basis of biotechnology and food industriesLink microbial anaerobic respiration to alcohol, dairy/lactic acid and baking, citing the roughly 13 per cent alcohol limit and the need for distillation.
Why life oxidises glucose stepwise rather than by direct combustionExplain energy-coupling to ATP, avoidance of heat loss and enzyme control as a model of bioenergetic efficiency.
Respiration as the amphibolic hub of cellular metabolismShow how intermediates feed both breakdown and biosynthesis of fats and proteins, integrating the cell's metabolism.

Practice Science & Technology questions from this syllabus →

Last-minute revision tick as you recall

Respiration = stepwise oxidation of C-C bonds; energy trapped as ATP, the cell's currency.
Plants breathe via stomata and lenticels; no special organs and low gas demand.
Glycolysis: cytoplasm, all cells, glucose to 2 pyruvate, net 2 ATP + 2 NADH; EMP pathway.
Pyruvate's three fates: lactic-acid fermentation, alcoholic fermentation, aerobic respiration.
Fermentation: anaerobic, under 7 per cent energy, yeast dies near 13 per cent alcohol.
Link reaction and Krebs in the matrix (Hans Krebs); per turn 3 NADH + 1 FADH2 + 1 GTP.
ETS on the inner membrane; O2 is the final electron acceptor forming water; oxidative phosphorylation.
Net about 38 ATP per glucose (NCERT theoretical); respiration is amphibolic.
RQ = CO2/O2: carbohydrates 1, fats below 1, proteins below 1.

← PreviousPhotosynthesis in Higher Plants Next →Plant Growth and Development

Distilled from NCERT Class 11 · Biology (Class 11) for UPSC. Always cross-check facts with the original NCERT.