Respiration in Plants
Welcome to HSLC Guru! This page provides detailed Class 11 Biology Chapter 14 question answers for the chapter Respiration in Plants, prepared in line with the ASSEB (Assam State School Education Board) syllabus. Plants, like all living organisms, need a constant supply of energy to perform life activities such as growth, transport, and reproduction. This energy is obtained by breaking down food molecules — primarily glucose — through a controlled biochemical process called respiration. In this chapter, students will explore the various stages of respiration, including glycolysis, fermentation, the Krebs cycle, the electron transport chain, and oxidative phosphorylation, along with the concept of respiratory quotient and the amphibolic nature of respiratory pathways.
This page contains a complete summary of the chapter, textbook-style question answers across 1-mark, 2–3-mark, and 5–7-mark categories, MCQs, fill-in-the-blanks, true or false statements, and a glossary of important terms. All content is strictly aligned with ASSEB Class 11 Biology and crafted for English-medium students preparing for school exams, half-yearly tests, and final examinations.
Chapter Summary
Cellular respiration is the stepwise enzymatic oxidation of food molecules within living cells to release usable chemical energy in the form of ATP. In plants, the principal respiratory substrate is glucose, although fats, proteins, and organic acids may also be used under certain conditions. The complete oxidation of one glucose molecule yields carbon dioxide, water, and a large amount of energy. Respiration occurs in every living cell of the plant; there is no specialized respiratory organ. Gaseous exchange takes place through stomata in leaves, lenticels in woody stems, and the general surface of root cells. Respiration is broadly classified into aerobic respiration (in the presence of oxygen) and anaerobic respiration (in the absence of oxygen).
The first stage of respiration is glycolysis, also called the EMP pathway (Embden–Meyerhof–Parnas pathway), which occurs in the cytoplasm of the cell and is common to both aerobic and anaerobic respiration. In glycolysis, one molecule of glucose (6C) is partially oxidised into two molecules of pyruvic acid (3C). The net gain from glycolysis is 2 ATP, 2 NADH, and 2 pyruvate per molecule of glucose. In the absence of oxygen, pyruvate undergoes fermentation. In yeast, pyruvate is converted into ethanol and CO₂ — known as alcoholic fermentation. In certain bacteria and animal muscle cells, pyruvate is reduced to lactic acid — called lactic acid fermentation. Fermentation yields very little energy because glucose is only partially broken down.
In aerobic respiration, pyruvate enters the mitochondria, where it is oxidatively decarboxylated into acetyl-CoA in the link reaction, producing CO₂ and NADH. Acetyl-CoA then enters the Krebs cycle (also known as the TCA cycle or citric acid cycle), which takes place in the mitochondrial matrix. Per glucose molecule, the Krebs cycle produces 6 NADH, 2 FADH₂, 2 ATP (GTP), and 4 CO₂. The reduced coenzymes (NADH and FADH₂) carry electrons to the electron transport chain (ETS), which is located on the inner mitochondrial membrane. As electrons pass through a series of carriers, energy is released and used to pump protons across the membrane, creating a proton gradient. ATP synthase then uses this gradient to produce ATP — a process called oxidative phosphorylation. Oxygen serves as the final electron acceptor and combines with protons to form water.
The total ATP yield from complete oxidation of one glucose molecule in eukaryotes is theoretically 38 ATP, although the net usable yield is generally accepted as 36 ATP due to the energy cost of transporting NADH from the cytoplasm into the mitochondria. The respiratory quotient (RQ) is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed during respiration. RQ is 1 for carbohydrates, less than 1 (about 0.7) for fats, and about 0.9 for proteins. Respiration is also described as an amphibolic pathway because it involves both catabolism (breakdown of substrates) and anabolism (formation of intermediates that serve as precursors for the synthesis of fatty acids, amino acids, and other molecules).
1-Mark Questions
Q1. What is respiration?
Answer: Respiration is the enzymatic oxidation of food molecules in living cells to release energy in the form of ATP, with carbon dioxide and water as by-products.
Q2. Where does glycolysis take place in the cell?
Answer: Glycolysis takes place in the cytoplasm of the cell.
Q3. What is the full form of EMP pathway?
Answer: EMP stands for the Embden–Meyerhof–Parnas pathway, the metabolic route of glycolysis.
Q4. What is the end product of glycolysis?
Answer: The end product of glycolysis is pyruvic acid (pyruvate), with a net gain of 2 ATP and 2 NADH per molecule of glucose.
Q5. Name the two main types of fermentation.
Answer: The two main types of fermentation are alcoholic fermentation (carried out by yeast) and lactic acid fermentation (carried out by certain bacteria and muscle cells).
Q6. Where does the Krebs cycle occur?
Answer: The Krebs cycle (TCA cycle) occurs in the matrix of the mitochondria.
Q7. What is the location of the electron transport chain?
Answer: The electron transport chain (ETS) is located on the inner mitochondrial membrane.
Q8. What is the net ATP yield from complete oxidation of one glucose molecule in eukaryotes?
Answer: The theoretical yield is 38 ATP, while the net usable yield in eukaryotes is generally taken as 36 ATP.
Q9. Define respiratory quotient (RQ).
Answer: Respiratory quotient is the ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed during respiration: RQ = CO₂ / O₂.
Q10. What is meant by an amphibolic pathway?
Answer: An amphibolic pathway is one that involves both catabolism (breakdown) and anabolism (synthesis). Respiration is amphibolic as its intermediates serve as precursors for biosynthesis.
2–3 Marks Questions
Q1. Differentiate between aerobic and anaerobic respiration.
Answer: Aerobic respiration takes place in the presence of oxygen, completely oxidising glucose into CO₂ and H₂O with a yield of 36–38 ATP. Anaerobic respiration occurs in the absence of oxygen and only partially breaks down glucose into ethanol or lactic acid, yielding only 2 ATP per glucose. Aerobic respiration occurs mainly in the mitochondria, while anaerobic respiration is restricted to the cytoplasm.
Q2. Write a short note on glycolysis.
Answer: Glycolysis (EMP pathway) is the stepwise breakdown of one glucose molecule (6C) into two molecules of pyruvate (3C) in the cytoplasm. It involves ten enzymatic reactions and does not require oxygen. The net gain per glucose molecule is 2 ATP, 2 NADH, and 2 pyruvate. Glycolysis is common to both aerobic and anaerobic respiration.
Q3. Explain alcoholic fermentation.
Answer: Alcoholic fermentation is an anaerobic process carried out by yeast in which pyruvate produced from glycolysis is first decarboxylated to acetaldehyde (releasing CO₂) by pyruvate decarboxylase. The acetaldehyde is then reduced to ethanol by alcohol dehydrogenase using NADH. The overall reaction is: Pyruvate → Ethanol + CO₂. This process is industrially used in baking, brewing, and wine-making.
Q4. What is the link reaction? Where does it occur?
Answer: The link reaction is the oxidative decarboxylation of pyruvate (3C) to acetyl-CoA (2C) catalysed by the pyruvate dehydrogenase complex. It occurs in the mitochondrial matrix and produces 1 CO₂ and 1 NADH per pyruvate. Two link reactions occur per glucose, yielding 2 acetyl-CoA, 2 NADH, and 2 CO₂.
Q5. What is oxidative phosphorylation?
Answer: Oxidative phosphorylation is the process by which ATP is synthesised using the energy released during the transfer of electrons from NADH and FADH₂ to oxygen via the electron transport chain. The protons pumped across the inner mitochondrial membrane create a gradient that drives ATP synthase to phosphorylate ADP into ATP.
Q6. Why is respiration considered an amphibolic pathway?
Answer: Respiration is described as an amphibolic pathway because it involves both catabolism (breakdown of glucose, fats, proteins) and anabolism (the intermediates such as acetyl-CoA, pyruvate, and α-ketoglutarate are used in the synthesis of fatty acids, amino acids, and other compounds). Hence, it is not strictly catabolic.
5–7 Marks Questions
Q1. Describe the process of glycolysis with its key steps and energy yield.
Answer: Glycolysis, or the Embden–Meyerhof–Parnas (EMP) pathway, is the partial breakdown of one molecule of glucose into two molecules of pyruvic acid in the cytoplasm of the cell. It is the universal first step of carbohydrate respiration in both aerobic and anaerobic organisms.
Glycolysis can be divided into two phases: the preparatory phase and the payoff phase. In the preparatory phase, glucose is phosphorylated to glucose-6-phosphate using one ATP, isomerised into fructose-6-phosphate, and then phosphorylated again into fructose-1,6-bisphosphate using another ATP. This 6-carbon sugar is then split into two 3-carbon molecules — glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP), which are interconvertible. In the payoff phase, each G3P is oxidised to 1,3-bisphosphoglycerate, producing one NADH per G3P. Through subsequent steps involving 3-phosphoglycerate, 2-phosphoglycerate, and phosphoenolpyruvate, four ATP molecules are produced by substrate-level phosphorylation, ultimately yielding pyruvate.
The net gain per glucose is 2 ATP (4 produced − 2 used), 2 NADH, and 2 molecules of pyruvate. Glycolysis is common to both aerobic and anaerobic respiration and is a key control point of metabolism.
Q2. Describe the Krebs cycle (TCA cycle) along with its products and significance.
Answer: The Krebs cycle, also known as the tricarboxylic acid (TCA) cycle or citric acid cycle, was discovered by Sir Hans Krebs in 1937. It takes place in the matrix of the mitochondria and is the central pathway of aerobic respiration.
The cycle begins when acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C) in the presence of citrate synthase. Citrate is isomerised to isocitrate, which is oxidatively decarboxylated to α-ketoglutarate (5C), releasing CO₂ and NADH. α-Ketoglutarate is further oxidised to succinyl-CoA (4C), again releasing CO₂ and NADH. Succinyl-CoA is converted to succinate with the formation of one GTP (equivalent to ATP) by substrate-level phosphorylation. Succinate is oxidised to fumarate, producing FADH₂. Fumarate is hydrated to malate, which is finally oxidised to oxaloacetate, regenerating NADH and completing the cycle.
Per glucose molecule (i.e., two turns of the cycle), the Krebs cycle yields 6 NADH, 2 FADH₂, 2 ATP (GTP), and 4 CO₂. Its significance is that it is the major site of CO₂ production, the chief generator of reduced coenzymes that feed the ETS, and a hub of amphibolic metabolism since several intermediates serve as precursors for the synthesis of amino acids, chlorophyll, and other biomolecules.
Q3. Explain the electron transport chain and oxidative phosphorylation.
Answer: The electron transport chain (ETS) is a series of protein complexes embedded in the inner mitochondrial membrane. It transfers electrons from NADH and FADH₂ to molecular oxygen through a sequence of redox reactions, ultimately reducing oxygen to water.
The chain consists of four major complexes (I–IV) and two mobile carriers — ubiquinone (coenzyme Q) and cytochrome c. NADH donates electrons at Complex I (NADH dehydrogenase), while FADH₂ donates electrons at Complex II (succinate dehydrogenase). Electrons pass to ubiquinone, then to Complex III (cytochrome bc₁), then to cytochrome c, and finally to Complex IV (cytochrome c oxidase), where they are transferred to O₂ to form water. As electrons move through Complexes I, III, and IV, protons (H⁺) are pumped from the matrix into the intermembrane space, generating a proton gradient.
The protons re-enter the matrix through ATP synthase (Complex V), and the energy of the gradient drives the phosphorylation of ADP into ATP. This synthesis of ATP coupled with the oxidation of NADH and FADH₂ is called oxidative phosphorylation. Each NADH yields about 3 ATP, and each FADH₂ yields about 2 ATP. Oxidative phosphorylation is the major source of ATP in aerobic respiration.
Q4. Calculate the total ATP yield from complete oxidation of one glucose molecule in a eukaryotic cell.
Answer: The complete oxidation of one molecule of glucose in a eukaryotic cell yields ATP at several stages, as summarised below:
(i) Glycolysis: 2 ATP (substrate-level phosphorylation) + 2 NADH (= 6 ATP via ETS, or 4 ATP if shuttled into the mitochondrion).
(ii) Link reaction (2 pyruvate → 2 acetyl-CoA): 2 NADH = 6 ATP.
(iii) Krebs cycle (two turns): 6 NADH (= 18 ATP), 2 FADH₂ (= 4 ATP), and 2 ATP (GTP).
Adding these: 2 + 6 + 6 + 18 + 4 + 2 = 38 ATP (theoretical maximum). However, in eukaryotes, the cytoplasmic NADH from glycolysis is transported into the mitochondria using shuttles that consume some energy, so the practical net yield is generally taken as 36 ATP per glucose. In prokaryotes, where there is no such transport cost, the full 38 ATP can be realised.
Q5. What is respiratory quotient (RQ)? Discuss its values for different respiratory substrates.
Answer: The respiratory quotient (RQ) is defined as the ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed during respiration in a given period of time. It is expressed as: RQ = Volume of CO₂ released / Volume of O₂ consumed.
The value of RQ depends on the type of respiratory substrate and the nature of respiration. For carbohydrates such as glucose, the equation C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O gives an RQ of 1.0. For fats, which are more reduced and require more oxygen for complete oxidation, RQ is less than 1, typically around 0.7 (e.g., 0.7 for tripalmitin). For proteins, the RQ is approximately 0.8 to 0.9. For organic acids, which are highly oxidised, RQ may be greater than 1 — for example, malic acid gives an RQ of about 1.33. In anaerobic respiration of carbohydrates, since CO₂ is released without oxygen consumption, RQ is theoretically infinity.
Thus, RQ values help in identifying the nature of the respiratory substrate being utilised by the organism at any given time.
Multiple Choice Questions (MCQs)
Q1. Glycolysis takes place in the:
(a) Mitochondria
(b) Cytoplasm
(c) Chloroplast
(d) Nucleus
Answer: (b) Cytoplasm
Q2. The end product of glycolysis is:
(a) Lactic acid
(b) Acetyl-CoA
(c) Pyruvic acid
(d) Ethanol
Answer: (c) Pyruvic acid
Q3. The Krebs cycle occurs in:
(a) Cytoplasm
(b) Inner mitochondrial membrane
(c) Mitochondrial matrix
(d) Outer mitochondrial membrane
Answer: (c) Mitochondrial matrix
Q4. Net ATP yield in glycolysis is:
(a) 2 ATP
(b) 4 ATP
(c) 36 ATP
(d) 38 ATP
Answer: (a) 2 ATP
Q5. Alcoholic fermentation is carried out by:
(a) Muscle cells
(b) Yeast
(c) Lactobacillus
(d) E. coli
Answer: (b) Yeast
Q6. The electron transport chain is located in the:
(a) Cytoplasm
(b) Outer mitochondrial membrane
(c) Inner mitochondrial membrane
(d) Mitochondrial matrix
Answer: (c) Inner mitochondrial membrane
Q7. The respiratory quotient (RQ) of carbohydrates is:
(a) 0.7
(b) 0.9
(c) 1.0
(d) Infinity
Answer: (c) 1.0
Q8. The total ATP yield from complete oxidation of one glucose molecule (theoretical) is:
(a) 30 ATP
(b) 36 ATP
(c) 38 ATP
(d) 40 ATP
Answer: (c) 38 ATP
Q9. The final electron acceptor in aerobic respiration is:
(a) NAD⁺
(b) FAD
(c) Oxygen
(d) Water
Answer: (c) Oxygen
Q10. Pyruvate is converted into acetyl-CoA in the:
(a) Cytoplasm
(b) Mitochondrial matrix
(c) Inner membrane
(d) Nucleus
Answer: (b) Mitochondrial matrix
Fill in the Blanks
Q1. The full form of EMP pathway is __________.
Answer: Embden–Meyerhof–Parnas pathway.
Q2. The Krebs cycle yields __________ NADH per glucose molecule.
Answer: 6.
Q3. The respiratory quotient of fats is approximately __________.
Answer: 0.7.
Q4. The synthesis of ATP using energy from the proton gradient is called __________.
Answer: Oxidative phosphorylation.
Q5. Respiration is described as an __________ pathway because it involves both catabolism and anabolism.
Answer: Amphibolic.
True or False
Q1. Glycolysis takes place in the mitochondria.
Answer: False — Glycolysis takes place in the cytoplasm.
Q2. Yeast performs alcoholic fermentation producing ethanol and CO₂.
Answer: True.
Q3. The electron transport chain is present on the outer mitochondrial membrane.
Answer: False — It is present on the inner mitochondrial membrane.
Q4. Each NADH yields about 3 ATP through the ETS.
Answer: True.
Q5. The respiratory quotient of an organic acid like malic acid is less than 1.
Answer: False — RQ of organic acids like malic acid is greater than 1 (about 1.33).
Glossary
| Term | Meaning |
|---|---|
| Respiration | Enzymatic oxidation of food in cells to release ATP energy. |
| Glycolysis (EMP) | Breakdown of glucose to pyruvate in cytoplasm; net 2 ATP, 2 NADH, 2 pyruvate. |
| Fermentation | Anaerobic breakdown of pyruvate to ethanol or lactic acid. |
| Alcoholic Fermentation | Conversion of pyruvate to ethanol and CO₂ by yeast. |
| Lactic Acid Fermentation | Conversion of pyruvate to lactic acid in muscle/bacteria. |
| Link Reaction | Oxidative decarboxylation of pyruvate to acetyl-CoA in mitochondrial matrix. |
| Krebs Cycle (TCA) | Cyclic oxidation of acetyl-CoA in matrix; per glucose: 6 NADH, 2 FADH₂, 2 ATP, 4 CO₂. |
| Acetyl-CoA | 2C compound entering Krebs cycle. |
| ETS | Electron transport chain on inner mitochondrial membrane. |
| Oxidative Phosphorylation | ATP synthesis driven by proton gradient across inner membrane. |
| ATP Yield | 38 ATP (theoretical) / 36 ATP (net) per glucose in eukaryotes. |
| Respiratory Quotient (RQ) | Ratio of CO₂ released to O₂ consumed during respiration. |
| Amphibolic Pathway | Pathway involving both catabolism and anabolism. |
| NADH / FADH₂ | Reduced coenzymes carrying electrons to ETS. |
| ATP Synthase | Enzyme complex that synthesises ATP using proton gradient. |