Chapter 12: Respiration in Plants Long Questions | biology Class 11 CBSE

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Chapter 12: Respiration in Plants Long Questions | biology Class 11 CBSE | ByteNotes.in

Long Answer

Medium difficulty • Structured explanation

Medium

Question 1

Long Form

Compare and contrast glycolysis and the Krebs' cycle with respect to their location, substrates, products, and energy yield.

Glycolysis occurs in the cytoplasm of all living cells and involves the partial oxidation of glucose (6C) into two molecules of pyruvic acid (3C) through ten enzyme-controlled reactions; the Krebs' cycle occurs in the mitochondrial matrix and involves the complete oxidation of acetyl CoA (2C) through a cyclic series of reactions.
Glycolysis requires glucose as its primary substrate (derived from sucrose in plants via invertase), while the Krebs' cycle uses acetyl CoA, which is formed by oxidative decarboxylation of pyruvate in the mitochondrial matrix.
The direct ATP yield from glycolysis is 2 net ATP (from 4 produced minus 2 consumed) per glucose; the Krebs' cycle produces 1 GTP (equivalent to 1 ATP) per turn, totalling 2 ATP equivalents for both turns per glucose.
Glycolysis produces 2 NADH+H+ per glucose; the Krebs' cycle produces 3 NADH+H+ and 1 FADH2 per turn (6 NADH+H+ and 2 FADH2 per glucose), making it the major source of reduced coenzymes.
In glycolysis, no CO2 is released; in the Krebs' cycle, two CO2 molecules are released per turn (4 total per glucose) through successive decarboxylation steps.
Glycolysis is present in all living organisms including anaerobes; the Krebs' cycle is functional only in aerobic organisms and requires continuous regeneration of NAD+ and FAD+ through the ETS.

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Long Answer

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Hard

Question 2

Long Form

Describe the electron transport system in mitochondria and explain how it is coupled to ATP synthesis through oxidative phosphorylation.

The electron transport system (ETS) is located on the inner mitochondrial membrane and consists of four protein complexes (I–IV) and two mobile carriers — ubiquinone and cytochrome c.
NADH+H+ is oxidised by Complex I (NADH dehydrogenase), and electrons are passed to ubiquinone; FADH2 is oxidised by Complex II (succinate dehydrogenase) and also passes electrons to ubiquinone.
Reduced ubiquinol passes electrons to Complex III (cytochrome bc1), which transfers them to cytochrome c, the mobile carrier on the outer surface of the inner membrane.
Cytochrome c donates electrons to Complex IV (cytochrome c oxidase, containing cytochromes a and a3 and two copper centres), which finally reduces O2 to H2O — making oxygen the terminal electron acceptor.
As electrons pass through the complexes, the energy released is used to pump protons across the inner membrane, creating an electrochemical gradient (proton-motive force) that drives protons back through the F0-F1 ATP synthase (Complex V).
Each NADH yields 3 ATP and each FADH2 yields 2 ATP through oxidative phosphorylation; this process is distinct from photophosphorylation because it uses oxidation-reduction energy rather than light energy, explaining the name 'oxidative phosphorylation'.

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Long Answer

Hard difficulty • Structured explanation

Hard

Question 3

Long Form

Analyse the respiratory pathway as an amphibolic pathway by explaining how different substrates — fats, proteins, and carbohydrates — enter and exit the pathway.

Glucose is the preferred respiratory substrate; all carbohydrates are usually first converted to glucose before entering glycolysis as glucose-6-phosphate.
Fats are broken down into glycerol and fatty acids; glycerol enters the pathway as PGAL (a glycolytic intermediate), while fatty acids are degraded to acetyl CoA and enter at the Krebs' cycle stage.
Proteins are hydrolysed by proteases into amino acids; after deamination, depending on their structure, amino acids enter the pathway as pyruvate, acetyl CoA, or at various points within the Krebs' cycle.
Crucially, the same intermediates that are produced when substrates enter the respiratory pathway are withdrawn from it when the organism needs to synthesise the same substrates — for example, acetyl CoA is withdrawn for fatty acid synthesis.
Similarly, Krebs' cycle intermediates serve as carbon skeletons for amino acid biosynthesis, demonstrating the anabolic role of an otherwise catabolic-seeming pathway.
Because the pathway simultaneously serves catabolism (breakdown for energy) and anabolism (provision of biosynthetic precursors), it is most accurately described as amphibolic rather than purely catabolic.

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Long Answer

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Medium

Question 4

Long Form

Describe the process of alcoholic fermentation in yeast, including the enzymes involved, energy yield, and significance. Compare it with lactic acid fermentation.

Alcoholic fermentation occurs in yeast under anaerobic conditions; pyruvic acid produced by glycolysis is first decarboxylated by pyruvic acid decarboxylase to produce acetaldehyde and CO2.
Acetaldehyde is then reduced to ethanol by alcohol dehydrogenase, using NADH+H+ as the reducing agent; this reoxidises NADH to NAD+, allowing glycolysis to continue.
In lactic acid fermentation (in bacteria and animal muscle cells under low O2), pyruvic acid is directly reduced to lactic acid by lactate dehydrogenase, also using NADH+H+ as the reducing agent.
Both processes yield only 2 net ATP per glucose (from glycolysis), representing less than 7% efficiency in energy extraction; neither additional decarboxylation nor ETS is involved.
The key difference is the end product: ethanol + CO2 in alcoholic fermentation vs. lactic acid in lactic acid fermentation; the reoxidation of NADH to NAD+ occurs in both.
Fermentation is hazardous — ethanol poisons yeast at concentrations above ~13%, and lactic acid accumulation causes muscle fatigue in animals — yet it was critical for the earliest life forms on Earth before oxygen became available in the atmosphere.

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Long Answer

Hard difficulty • Structured explanation

Hard

Question 5

Long Form

Describe the structure and function of ATP synthase (Complex V) in the mitochondria, explaining the role of the proton gradient in driving ATP synthesis.

ATP synthase, also called Complex V, is located on the inner mitochondrial membrane and is responsible for synthesising ATP using the proton gradient generated by the ETS.
It consists of two major components: F1, a peripheral membrane protein complex forming the headpiece, and F0, an integral membrane protein forming the channel for proton transport.
F1 contains the catalytic sites for ATP synthesis from ADP and inorganic phosphate; its activity is driven by the passage of protons through the F0 channel.
F0 is embedded in the inner membrane and forms a proton channel; protons flow through F0 from the intermembrane space (high concentration) to the matrix (low concentration) down the electrochemical gradient.
For each ATP produced, 4H+ pass through F0 from the intermembrane space to the matrix; the flow of protons is tightly coupled to the rotation of F0 and the conformational change in F1 that catalyses ATP synthesis.
This mechanism, described by the chemiosmotic hypothesis, is called oxidative phosphorylation because the energy driving the proton gradient comes from oxidation-reduction reactions in the ETS, unlike photophosphorylation where light energy is used.

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Long Answer

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Medium

Question 6

Long Form

Explain the concept of the Respiratory Quotient (RQ) and analyse how the type of respiratory substrate determines the value of RQ with specific examples.

The Respiratory Quotient (RQ) is the ratio of the volume of CO2 evolved to the volume of O2 consumed during aerobic respiration; it reflects the chemical composition of the substrate being oxidised.
When carbohydrates (e.g., glucose) are the substrate, the RQ = 1.0 because the number of oxygen atoms relative to carbon and hydrogen is such that equal volumes of CO2 and O2 are produced and consumed: C6H12O6 + 6O2 → 6CO2 + 6H2O.
When fats are the substrate (e.g., tripalmitin), the RQ is less than 1 — approximately 0.7 — because fats have a lower proportion of oxygen relative to carbon and hydrogen, requiring more O2 for oxidation than CO2 produced: 2(C51H98O6) + 145O2 → 102CO2 + 98H2O.
When proteins are the respiratory substrate, the RQ is approximately 0.9; the intermediate value reflects the partial oxidation of nitrogen-containing amino acids and the variable elemental composition of proteins.
In living organisms, pure proteins or pure fats are never used as the sole respiratory substrate; the measured RQ reflects the simultaneous oxidation of a mixture of carbohydrates, fats, and proteins.
Measuring RQ in an organism thus provides useful physiological information about metabolic activity, substrate preference, and the relative proportions of different substrates being oxidised at any given time.

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Long Answer

Hard difficulty • Structured explanation

Hard

Question 7

Long Form

Trace the complete journey of pyruvic acid from glycolysis to its final oxidation products in aerobic conditions, naming all intermediates, enzymes, and products at each stage.

Pyruvic acid (3C), produced by glycolysis in the cytoplasm, is transported into the mitochondrial matrix where it undergoes oxidative decarboxylation catalysed by pyruvate dehydrogenase (requiring NAD+ and CoA) to produce acetyl CoA (2C), CO2, and NADH+H+.
Acetyl CoA enters the Krebs' cycle by condensing with oxaloacetic acid (OAA, 4C) and water via citrate synthase to form citric acid (6C), releasing CoA.
Citric acid is isomerised to isocitrate; two successive decarboxylations then form alpha-ketoglutaric acid (5C) and then succinyl-CoA (4C), releasing 2 CO2 and 2 NADH+H+.
Succinyl-CoA is converted to succinic acid (4C) with substrate-level synthesis of GTP; succinic acid is oxidised to fumarate with FAD+ → FADH2; fumarate is hydrated to malate, then oxidised to OAA with NAD+ → NADH+H+.
OAA is regenerated and ready for another turn; per turn the cycle produces 3 NADH+H+, 1 FADH2, 1 GTP, and 2 CO2 — so per glucose (two turns), the Krebs' cycle yields 6 NADH+H+, 2 FADH2, 2 ATP, and 4 CO2.
The NADH+H+ and FADH2 from the cycle, combined with those from glycolysis and pyruvate oxidation, feed into the ETS where electrons ultimately reduce O2 to H2O and drive oxidative phosphorylation, producing the bulk of the 38 theoretical ATP per glucose.

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Long Answer

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Medium

Question 8

Long Form

Discuss how plants manage gaseous exchange without specialised respiratory organs, with reference to their structural adaptations.

Unlike animals, plants have no specialised lungs or gills for gaseous exchange; instead, each plant part independently manages its own O2 and CO2 exchange using stomata (in leaves) and lenticels (in woody stems).
Plants respire at rates far lower than animals, so the metabolic demand for gas exchange is minimal; only during active photosynthesis are large gas volumes exchanged, and leaves are structurally adapted to handle their own needs during these periods.
Photosynthesising cells release O2 internally, so O2 availability is not a concern during photosynthesis; non-photosynthesising cells rely on diffusion from nearby air spaces.
Parenchyma cells in leaves, stems, and roots are loosely packed, creating an interconnected network of air spaces that allows rapid diffusion of gases to virtually every living cell.
In woody stems, living cells are arranged in thin layers just beneath the bark; lenticels in the bark provide direct diffusion pathways to these cells; interior cells are dead (providing mechanical support) and do not have respiratory needs.
Because each living cell is located close to an air-exposed surface and diffusion distances are short, plants can sustain all respiratory gas exchange without active ventilation or specialised transport organs.

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Long Answer

Hard difficulty • Structured explanation

Hard

Question 9

Long Form

Critically examine the assumptions and limitations of the respiratory balance sheet, and explain why 38 ATP is a theoretical maximum and not the actual yield in a living cell.

The respiratory balance sheet calculates a theoretical maximum of 38 ATP per glucose under four key assumptions: sequential operation of pathways, transfer of cytoplasmic NADH to mitochondria, no diversion of intermediates, and glucose as the sole substrate.
In reality, all metabolic pathways work simultaneously rather than sequentially; the TCA cycle, glycolysis, and ETS operate concurrently, and intermediates are constantly being withdrawn for other biosynthetic reactions.
Cytoplasmic NADH produced during glycolysis may not always be efficiently transferred into the mitochondrial matrix; this transfer involves a shuttle mechanism with an energy cost, reducing the effective ATP yield from glycolytic NADH.
In living cells, substrates other than glucose — fatty acids, amino acids — continuously enter the respiratory pathway at intermediate steps, altering the actual ratio of ATP, NADH, and FADH2 produced.
ATP is continuously consumed by the cell as it is synthesised, and enzyme activity is regulated by feedback mechanisms (e.g., high ATP inhibits key glycolytic enzymes), so the pathway never runs at theoretical maximum efficiency.
Despite these limitations, the balance sheet exercise remains valuable as it demonstrates the inherent efficiency of aerobic over anaerobic respiration (38 vs. 2 ATP) and illustrates the elegance of biological energy extraction; it provides a useful framework for understanding respiratory metabolism even if the numbers are idealised.

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Long Answer

Hard difficulty • Structured explanation

Hard

Question 10

Long Form

Explain the ten steps of glycolysis, highlighting the key reactions involving ATP utilisation, ATP synthesis, and NADH production.

Glycolysis begins when glucose (6C) is phosphorylated to glucose-6-phosphate by hexokinase using one ATP; in plants, sucrose is first hydrolysed by invertase to glucose and fructose, both of which can enter the pathway.
Glucose-6-phosphate isomerises to fructose-6-phosphate; this is then phosphorylated to fructose-1,6-bisphosphate (6C) by phosphofructokinase, consuming a second ATP — these are the only two ATP-consuming steps.
Fructose-1,6-bisphosphate is split into dihydroxyacetone phosphate (DHAP) and 3-phosphoglyceraldehyde (PGAL), both 3C triose phosphates; DHAP is interconverted to PGAL, so effectively two molecules of PGAL proceed.
PGAL (2 molecules) is oxidised to 1,3-bisphosphoglycerate (BPGA) with reduction of NAD+ to NADH+H+; this is the sole NADH-producing step in glycolysis, yielding 2 NADH+H+ per glucose.
BPGA is converted to 3-phosphoglyceric acid (PGA) by substrate-level phosphorylation, producing 2 ATP; PGA proceeds through 2-phosphoglycerate and phosphoenolpyruvate (PEP), with H2O removed in the latter conversion.
PEP is converted to pyruvic acid by pyruvate kinase via substrate-level phosphorylation, producing 2 more ATP; the net ATP yield from glycolysis is 2 (4 produced minus 2 consumed), and the overall product is 2 pyruvic acid, 2 ATP, and 2 NADH+H+ per glucose.

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Chapter 12: Respiration in Plants | Long Questions

Compare and contrast glycolysis and the Krebs' cycle with respect to their location, substrates, products, and energy yield.

Describe the electron transport system in mitochondria and explain how it is coupled to ATP synthesis through oxidative phosphorylation.

Analyse the respiratory pathway as an amphibolic pathway by explaining how different substrates — fats, proteins, and carbohydrates — enter and exit the pathway.

Describe the process of alcoholic fermentation in yeast, including the enzymes involved, energy yield, and significance. Compare it with lactic acid fermentation.

Describe the structure and function of ATP synthase (Complex V) in the mitochondria, explaining the role of the proton gradient in driving ATP synthesis.

Explain the concept of the Respiratory Quotient (RQ) and analyse how the type of respiratory substrate determines the value of RQ with specific examples.

Trace the complete journey of pyruvic acid from glycolysis to its final oxidation products in aerobic conditions, naming all intermediates, enzymes, and products at each stage.

Discuss how plants manage gaseous exchange without specialised respiratory organs, with reference to their structural adaptations.

Critically examine the assumptions and limitations of the respiratory balance sheet, and explain why 38 ATP is a theoretical maximum and not the actual yield in a living cell.

Explain the ten steps of glycolysis, highlighting the key reactions involving ATP utilisation, ATP synthesis, and NADH production.

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