NCERT NCERT Solutions Class 11 Biology Chapter 12 Respiration in Plants

The class 11 biology NCERT solutions chapter 12 Respiration in Plants cover every back-exercise question, according to the latest 2026-27 CBSE syllabus, and help students prepare for the CBSE Boards, NEET and CUET. Each answer follows one glucose molecule from the cytoplasm to the inner mitochondrial membrane, counting the ATP made at every step, from glycolysis through the Krebs cycle to the electron transport system.

This chapter sits in the Plant Physiology unit, and the pathway map it builds here is reused whenever respiration, photosynthesis or metabolism is tested.

• CBSE Weightage: 4 to 5 marks, part of the Plant Physiology unit that also carries photosynthesis and plant growth.
• Topics covered: cellular respiration versus combustion, glycolysis (EMP pathway), aerobic respiration, the Krebs cycle, the electron transport system, oxidative phosphorylation, the ATP balance sheet, the amphibolic pathway and the respiratory quotient.
• Exercise count: 12 back-exercise questions, mixing schematic diagrams, differentiation tables and one numerical on the respiratory quotient.

These class 11 biology NCERT solutions chapter 12 Respiration in Plants are curated by subject experts, based on the 2026-27 NCERT textbook, and checked against the last five years of CBSE Board and NEET papers.

Why Respiration in Plants Matters and What the Chapter Covers

Respiration is how a plant unlocks the energy stored in the glucose it made during photosynthesis. The chapter answers one central question: how does a cell oxidise glucose to CO₂ and water while trapping usable energy as ATP instead of losing it all as heat? Once students see the whole route, every exercise question becomes a matter of knowing which step happens where.

• Everyday role: respiration runs day and night in every living cell, powering growth, transport and every synthesis the plant carries out.
• Core skill: tracing one glucose through glycolysis, the link reaction, the Krebs cycle and the ETS, and accounting for the ATP, NADH and CO₂ at each stage.
• Why it is central: the ideas of stepwise oxidation and coupled ATP synthesis introduced here underpin the balance sheet, the amphibolic pathway and the respiratory quotient later in the same exercise.

Almost every question in this exercise hangs off knowing which of the four stages happens in the cytoplasm and which in the mitochondrion. That is why students who master the location map early find the numericals and comparisons far easier. The class 11 biology NCERT solutions chapter 12 Respiration in Plants below follow the NCERT order so students can check their working line by line.

Cellular Respiration versus Combustion and the Stepwise Release of Energy

The first and last questions of the exercise frame the whole chapter. Both respiration and combustion oxidise glucose to the same end products, CO₂ and water, by the equation C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. The difference is control. NCERT defines cellular respiration as the breaking of C-C bonds of complex organic molecules by oxidation, releasing a large amount of energy, and the whole design is to do this in small steps.

• Respiration: a stepwise, enzyme-catalysed oxidation inside living cells at body temperature, where a good part of the energy is trapped in ATP.
• Combustion: a one-step, non-enzymatic burning that needs a high ignition temperature and gives out almost all energy as heat and light.
• Why stepwise matters: a single large release cannot be captured, so it escapes as heat; small releases can each be coupled to the synthesis of one ATP from ADP and P_i.

Releasing energy in packets does three jobs at once. It lets energy be banked in a usable chemical form rather than wasted, it keeps the cell at a safe temperature so enzymes do not denature, and it gives the cell many intermediate molecules it can withdraw for other syntheses. Respiration banks its energy; combustion and fermentation do not. That single idea unlocks the differentiation questions in this exercise, so keep it in view while working through Exercise Q1 and Q7.

Glycolysis: The EMP Pathway Step by Step

Glycolysis, from the Greek for splitting sugar, is the scheme given by Embden, Meyerhof and Parnas, so it is often called the EMP pathway. It occurs in the cytoplasm, is present in all living organisms and needs no oxygen. In it one glucose (6C) is partially oxidised through ten enzyme-catalysed reactions into two molecules of pyruvic acid (3C). The stages below track what each phase costs or yields.

The two ATP-making steps and the NADH step happen once per triose, and there are two trioses per glucose, so everything after the split doubles. Four ATP are made and two are spent, so the net gain is 2 ATP and 2 NADH + H⁺ per glucose. The commonest slip is answering 4 ATP, because yield is always about the net. No CO₂ leaves and no oxygen is used, so pyruvate still holds most of the energy of the original glucose, which is exactly why the cell then turns to the mitochondrion.

Aerobic Respiration and Its Four Main Steps

Aerobic respiration is the complete oxidation of organic substances in the presence of oxygen, releasing CO₂, water and a large amount of energy. NCERT names two crucial events: the complete oxidation of pyruvate by the stepwise removal of all hydrogen atoms, and the passing of those electrons to molecular O₂ with simultaneous ATP synthesis. The first is a matrix process, the second a membrane process. The four steps map to four addresses.

A quick self-check is to trace the carbons. Glycolysis releases none, each of the two link reactions releases one, and each of the two turns of the Krebs cycle releases two, so 2 + 4 = 6, and all six carbons of glucose have left as CO₂. Aerobic respiration therefore begins in the cytoplasm and is completed in the mitochondria. Do not merge the link reaction into the Krebs cycle, since naming oxidative decarboxylation separately is often a mark of its own in Q4.

The Krebs Cycle and Its Energy Yield

The Krebs cycle, also called the tricarboxylic acid (TCA) or citric acid cycle, operates in the mitochondrial matrix. It opens with acetyl CoA (2C) condensing with oxaloacetic acid (4C) and water to give citric acid (6C), catalysed by citrate synthase. Two decarboxylations then strip two carbons, and a series of oxidations regenerates oxaloacetic acid so the cycle can turn again.

• Entry and descent: citric acid (6C) → isocitrate → α-ketoglutaric acid (5C) → succinyl-CoA (4C), releasing one CO₂ at each decarboxylation.
• Substrate-level phosphorylation: succinyl-CoA to succinic acid makes one GTP, which is converted to one ATP.
• Closing the loop: succinic acid → fumaric acid (making FADH₂) → malic acid → oxaloacetic acid (making the third NADH).

There are three points where NAD⁺ is reduced and one where FAD⁺ is reduced. Per turn the cycle yields 3 NADH + H⁺, 1 FADH₂, 1 ATP and 2 CO₂; per glucose, with two turns, that doubles to 6 NADH, 2 FADH₂, 2 ATP and 4 CO₂. Almost every mark lost here comes from quoting the per-turn figures as if they were per-glucose. The continued oxidation of acetyl CoA also requires the continued replenishment of oxaloacetic acid and the regeneration of NAD⁺ and FAD⁺, both of which depend on oxygen downstream.

The Electron Transport System and Oxidative Phosphorylation

The electron transport system (ETS) is the chain of carriers on the inner mitochondrial membrane through which electrons pass from one carrier to the next. Its job is to oxidise the NADH + H⁺ and FADH₂ made earlier, hand their electrons to O₂ and form water. Answer it as a relay of five stations, each named for what it receives and what it passes on.

Because the phosphorylation is driven by the energy of oxidation-reduction, the process is called oxidative phosphorylation, in contrast with photophosphorylation, which uses light energy for the same proton gradient. The number of ATP made depends on the electron donor: oxidation of 1 NADH yields 3 ATP, while oxidation of 1 FADH₂ yields only 2 ATP. FADH₂ is worth less because it joins at complex II, downstream of complex I, so fewer proton-pumping complexes are used. Oxygen acts only at the terminal stage as the final hydrogen acceptor, yet its presence is vital because it keeps the far end of the chain empty so electrons can keep flowing.

The ATP Balance Sheet and the Assumptions Behind 38 ATP

NCERT is careful to say that the net gain of ATP per glucose can be calculated, but in reality this remains a theoretical exercise. The figure of 38 ATP is not a measurement; it is the answer you get only if four simplifying assumptions hold. Q8 asks for exactly those four, and stating them shows students understand why real yields differ.

• Sequential pathway: glycolysis, the TCA cycle and the ETS run in a strict, orderly sequence, one substrate forming the next.
• Glycolytic NADH is used fully: the NADH made in the cytoplasm is transferred into the mitochondria and undergoes oxidative phosphorylation.
• No intermediates are withdrawn: nothing is siphoned off for synthesis of other compounds along the way.
• Only glucose is respired: no other substrate enters the pathway at an intermediate step.

In a living cell none of these assumptions holds strictly. The pathways work simultaneously rather than one after another, substrates enter and are withdrawn as the cell needs them, and ATP is used up as soon as it is made. That is why real cells never bank a clean 38 ATP, and why some textbooks quote 36 instead. The number is a teaching figure that shows the ceiling of the pathway, not a value you would measure in a mitochondrion.

Amphibolic Pathway and the Respiratory Quotient

Respiration has traditionally been called a catabolic, energy-releasing process, but NCERT shows it is really an amphibolic pathway, one that both breaks down and builds up. Fatty acids, amino acids and other building blocks are withdrawn from respiratory intermediates such as acetyl CoA and the Krebs-cycle acids for synthesis. The same intermediates that substrates break into on the way in are drawn out on the way out, so respiration serves both catabolism and anabolism.

The respiratory quotient (RQ) is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed, and it is a fingerprint of the substrate being respired. For fats the RQ is 0.7, because fats carry a great deal of hydrogen and little oxygen, so more O₂ is needed to oxidise them. Working the tripalmitin case, RQ = 102/145 = 0.7. When students answer Q10 they should give the definition, name the value for fats explicitly, and show the calculation, since a bare 0.7 without the ratio loses the reasoning mark.

Respiration in Plants Exercise-wise Breakdown

The NCERT back exercise has 12 questions, mixing schematic diagrams, differentiation tables and one numerical. The table below maps the question blocks to their topics so students can revise by theme and know where the diagrams are expected.

The diagram questions carry the most marks, so students should practise drawing the glycolysis flow chart, the Krebs circle and the ETS relay by hand. Mark ATP in, ATP out and NADH on the correct arrows, and always show the doubling after the split. Every question in the class 11 biology NCERT solutions chapter 12 Respiration in Plants PDF is solved with each step shown, so students can compare their working against the model answer.

Common Mistakes Students Make in Respiration in Plants

Most marks in this chapter are lost on counting slips and misplaced labels, not on hard ideas. Each mistake below costs 1 to 2 marks, so watch for it at the exact step.

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