CBSE Class 11 Biology Respiration in Plants Assignments

RESPIRATION IN PLANTS

POINTS TO REMEMBER

Aerobic respiration: Complete oxidation of organic food in presence of oxygen thereby producing \(\text{CO}_2\), water and energy.

Anaerobic respiration: Incomplete breakdown of organic food to liberate energy in the absence of oxygen.

ATP Synthetase: An enzyme complex that catalyses synthesis of ATP during oxidative phosphorylation.

Biological oxidation: Oxidation in a series of reaction inside a cell.

Cytochromes: A group of iron containing compounds of electron transport system present in inner wall of mitochondria.

Dehydrogenase: Enzyme that catalyses removal of H atom from the substrate.

Electron acceptor: Organic compound which receive electrons produced during oxidation-reduction reactions.

Electron transport: Movement of electron from substrate to oxygen through respiratory chain during respiration.

Fermentation: Breakdown of organic substance that takes place in certain microbe like yeast under anaerobic condition with the production of \(\text{CO}_2\) and ethanol.

Glycolysis: Enzymatic breakdown of glucose into pyruvic acid that occurs in the cytoplasm.

Oxidative phosphorylation: Process of formation of ATP from ADP and Pi using the energy from proton gradient.

Respiration: Biochemical oxidation food to release energy.

Respiratory Quotient: The ratio of the volume of \(\text{CO}_2\) produced to the volume of oxygen consumed.

Proton gradient: Difference in proton concentration across the tissue membrane.

Pyruvic acid undergoes aerobic oxidation through Kreb's cycle.

Abbreviations

ATP - Adenosine triphosphate
ADP - Adenosine diphosphate
NAD - Nicotinamide Adenine dinucleotide
NADP - Nicotinamide Adenine dinucleotide Phosphate
NADH - Reduced Nicotinamide Adenine dinucleotide
PGA - Phosphoglyceric acid
PGAL - Phospho glyceraldehyde
FAD - Flavin adenine dinucleotide
ETS - Electron transport system
ETC - Electron transport chain
TCA - Tricarboxylic acid
OAA - Oxalo acetic acid
FMN - Flavin mono nucleotide
PPP - Pentose phosphate pathway

AEROBIC RESPIRATION

The overall mechanism of aerobic respiration can be studied under the following steps:

(A) Glycolysis (EMP pathway)
(B) Oxidative Decarboxylation
(C) Kreb's cycle (TCA-cycle)
(D) Oxidative phosphorylation

Glycolysis: The term has originated from the Greek word, glycos = glucose, lysis = splitting or breakdown means breakdown of glucose molecule.

- It is also called Embden-Meyerhof-Parnas pathway (EMP pathway).
- It is common in both aerobic and anaerobic respiration.
- It takes place outside the mitochondria, in the cytoplasm.
- One molecule of glucose (Hexose sugar) ultimately produces two molecules of pyruvic acid through glycolysis.
- During this process 4 molecules of ATP are produced while 2 molecules of ATP are utilised. Thus net gain of ATP is of 2 molecules.

Oxidative decarboxylation: Pyruvic acid is converted into Acetyle CoA in presence of pyruvate dehydrogenase complex.

\(\text{Pyruvic acid} + \text{CoA} + \text{NAD}^+ \xrightarrow[\text{Pyruvate dehydrogenase}]{\text{Mg}^{2+}} \text{Acetyle CoA} + \text{CO}_2 + \text{NADH} + \text{H}^+\)

The Acetyle CoA enters in TCA cycle.

Tri Carboxylic Acid Cycle (Kreb's cycle) or Citric acid Cycle: This cycle starts with condensation of acetyle group with oxaloacetic acid and water to yield citric acid which undergoes a series of reactions.

- It is aerobic and takes place in mitochondrial matrix.
- Each pyruvic acid molecule produces 4 \(\text{NADH} + \text{H}^+\), one \(\text{FADH}_2\), one ATP.
- One glucose molecule has been broken down to release \(\text{CO}_2\) and eight molecules of \(\text{NADH} + \text{H}^+\), two molecules of \(\text{FADH}_2\), and 2 molecules of ATP.

Electron transport system and oxidative phosphorylation: The metabolic pathway through which the electron passes from one carrier to another, is called Electron transport system and it is present in the inner mitochondrial membrane.

ETS comprises of the following:

(i) NAD and \(\text{NADH} + \text{H}^+\)
(ii) FAD and \(\text{FADH}_2\)
(iii) UQ (Ubiquinone)
(iv) Cyt b, \(\text{Cyt } c_1\), Cyt c, \(\text{Cyt } a\) and \(\text{Cyt } a_3\).

Oxygen acts as final hydrogen acceptor. The oxidation of \(\text{NADH}_2\) or \(\text{FADH}_2\) occurs by transfer of their electrons to a series of electron acceptors and finally they combine with oxygen to form water and ATP. The oxidation of each molecule of \(\text{NADH}_2\) and \(\text{FADH}_2\) give rise to 3 ATP and 2 ATP molecules respectively.

Oxidative Phosphorylation:

The synthesis of ATP from ADP and inorganic phosphate using energy from proton gradient is called oxidative phosphorylation. This takes place in elementary particles present on the inner membrane of cristae of mitochondria. This process in mitochondria is catalysed by ATP synthetase (complex V). This complex has two major components \(\text{F}_0\) and \(\text{F}_1\). \(\text{F}_0\) acts a channel for proton and \(\text{F}_1\) acts as an ATP synthetase.

Total ATP Production

Energy production in prokaryotes during aerobic respiration = 38 ATP
Energy production in eukaryotes during aerobic respiration = \(38 - 2 = 36\) ATP

(2 ATP are used up in transporting 2 molecule of pyruvic acid in mitochondria.)

Fermentation: It is the process of anaerobic respiration which occurs in yeast and some bacteria. Fermentation involves incomplete oxidation of food into ethanol and carbon-di-oxide. It results in the production of 2 ATP molecules.

\(\text{glucose} \rightarrow \text{Pyruvic Acid} \rightarrow \text{Ethanol} + \text{CO}_2\)

Anaerobic respiration in muscles: During strenuous exercise a person feels pain and fatigue in his muscles. This is due to accumulation of lactic acid in muscles. When oxygen is inadequate pyruvic acid is reduced to lactic acid in presence of enzyme-lactic dehydrogenase

\(\text{Pyruvic acid} \xrightarrow{\text{Lactic dehydrogenase}} \text{Lactic acid}\)

During rest, lactic acid is reconverted to pyruvic acid.

Respiratory pathway as an Amphibolic pathway

During the process of cellular respiration Carbohydrates, fats and proteins are broken down to release energy and hence respiration is a catabolic process/catabolic pathway. From this pathway many compounds are withdrawn for synthesis of substrates. Here some anabolic processes are - Acetyle CoA is withdrawn for the synthesis of fat. Because respiratory pathway is involved in both catabolism and anabolism, it is better to consider the respiratory pathway as an amphibolic pathway.

QUESTIONS

Very Short Answer Questions (1 mark each)

Question 1. Name the molecule which is terminal acceptor of electron.
Answer: Oxygen (<( \text{O}_2 \)).
In simple words: Oxygen accepts electrons at the end of the electron transport chain.

Exam Tip: Always specify molecular oxygen when writing this answer.

Question 2. How many ATP molecules are produced from a molecule of glucose on its complete oxidation in eukaryotes?
Answer: 36 ATP molecules.
In simple words: Eukaryotes net 36 ATPs because 2 ATPs are used up during transport into mitochondria.

Exam Tip: Remember prokaryotes produce 38 ATPs since they lack mitochondria.

Question 3. Where does ETC found in eukaryotic cell?
Answer: Inner mitochondrial membrane.
In simple words: The electron transport chain is located on the inner membrane of mitochondria.

Exam Tip: Mention inner mitochondrial membrane or cristae for full marks.

Question 4. Name the enzyme which convert sugar into glucose and fructose.
Answer: Invertase.
In simple words: Invertase breaks down sucrose into glucose and fructose.

Exam Tip: Invertase is also known as sucrase.

Question 5. How many molecules of ATP are produced by the oxidation of one molecule of \(\text{FADH}_2\)?
Answer: 2 ATP molecules.
In simple words: Oxidation of one \(\text{FADH}_2\) yields 2 ATP molecules.

Exam Tip: Contrast this with \(\text{NADH}_2\) which yields 3 ATP molecules.

Question 6. Why do the person with sufficient white fibres get fatigued in a short period?
Answer: Due to the formation and accumulation of lactic acid.
In simple words: Anaerobic respiration in fast-twitch white muscle fibers produces lactic acid, causing fatigue.

Exam Tip: Mention anaerobic glycolysis and lactic acid accumulation.

Question 7. Write the name of end product of glycolysis.
Answer: Pyruvic acid.
In simple words: Glycolysis converts one glucose molecule into two molecules of pyruvic acid.

Question 8. Name the first product formed in Kreb's cycle.
Answer: Citric acid.
In simple words: Acetyl-CoA condenses with oxaloacetate to form citric acid, hence the name citric acid cycle.

Exam Tip: Citric acid is a 6-carbon tricarboxylic acid.

Short Answer Questions-II (2 marks each)

Question 9. Differentiate between aerobic respiration and anaerobic respiration.
Answer:
1. Aerobic respiration takes place in the presence of oxygen, whereas anaerobic respiration occurs in the absence of oxygen.
2. Aerobic respiration leads to complete oxidation of organic substrates into \(\text{CO}_2\) and water, releasing a large amount of energy (36 or 38 ATP). Anaerobic respiration involves incomplete breakdown of substrates into ethanol and \(\text{CO}_2\) or lactic acid, releasing minimal energy (2 ATP).
In simple words: Aerobic uses oxygen and yields high energy; anaerobic does not use oxygen and yields low energy.

Exam Tip: Use a comparative table format when differentiating biological processes.

Question 10. Mention two steps of glycolysis in which ATP is utilised.
Answer:
1. Conversion of glucose to glucose-6-phosphate.
2. Conversion of fructose-6-phosphate to fructose-1,6-bisphosphate.
In simple words: ATP is consumed in the initial phosphorylation steps of glycolysis.

Exam Tip: Hexokinase and phosphofructokinase are the respective enzymes catalyzing these steps.

Question 11. Why does anaerobic respiration produces less energy than aerobic respiration?
Answer: In anaerobic respiration, there is incomplete breakdown of glucose, and organic compounds like ethanol or lactic acid still retain a large amount of potential energy. In contrast, aerobic respiration completely oxidizes glucose down to \(\text{CO}_2\) and water, releasing all stored chemical energy.
In simple words: Incomplete breakdown of food in anaerobic respiration leaves energy locked inside the end products.

Exam Tip: Focus on the concept of complete versus incomplete oxidation.

Question 12. Define Respiratory Quotient. What is its value for fat and protein?
Answer: Respiratory Quotient (RQ) is defined as the ratio of the volume of \(\text{CO}_2\) evolved to the volume of \(\text{O}_2\) consumed during respiration.
\(\text{RQ} = \frac{\text{Volume of }\text{CO}_2\text{ evolved}}{\text{Volume of }\text{O}_2\text{ consumed}}\)
- RQ for fats is less than 1 (typically around 0.7).
- RQ for proteins is approximately 0.9.
In simple words: RQ measures the ratio of carbon dioxide produced to oxygen consumed.

Exam Tip: Memorize RQ values: carbohydrates = 1.0, organic acids > 1.0, fats and proteins < 1.0.

Question 13. Distinguish between glycolysis and fermentation.
Answer:
1. Glycolysis is the first common phase of both aerobic and anaerobic respiration occurring in the cytoplasm, whereas fermentation is an anaerobic pathway that follows glycolysis in microbes like yeast.
2. Glycolysis breaks glucose down into pyruvic acid with a net gain of 2 ATP and 2 NADH, whereas fermentation further converts pyruvic acid into ethanol and \(\text{CO}_2\) or lactic acid without producing additional ATP.
In simple words: Glycolysis splits glucose into pyruvate, while fermentation converts pyruvate into alcohol or lactic acid.

Exam Tip: Clarify that fermentation acts to regenerate \(\text{NAD}^+\) for glycolysis to continue.

Question 14. What are respiratory substrates? Name the most common respiratory substrate.
Answer: Respiratory substrates are the organic compounds that are oxidized during respiration to release energy. The most common respiratory substrate is glucose (a carbohydrate).
In simple words: Respiratory substrates are energy-rich molecules broken down during respiration, with glucose being the most common.

Exam Tip: Other substrates include fats, proteins, and organic acids.

Short Answer Questions-I (3 marks each)

Question 15. Give the schematic representation of an overall view of TCA cycle.
Answer:
The Tricarboxylic Acid (TCA) cycle operates in the mitochondrial matrix as follows:
1. Acetyl-CoA (2C) condenses with Oxaloacetic acid (4C) in the presence of water and citrate synthase to form Citric acid (6C).
2. Citric acid isomerizes to isocitrate, which undergoes oxidative decarboxylation to form \(\alpha\)-ketoglutaric acid (5C), releasing \(\text{CO}_2\) and producing \(\text{NADH} + \text{H}^+\).
3. \(\alpha\)-Ketoglutaric acid undergoes another oxidative decarboxylation to form succinyl-CoA (4C), releasing \(\text{CO}_2\) and producing \(\text{NADH} + \text{H}^+\).
4. Succinyl-CoA is converted to succinic acid, coupled with substrate-level phosphorylation to form GTP (yielding ATP).
5. Succinic acid is oxidized to fumaric acid (producing \(\text{FADH}_2\)), then hydrated to malic acid, and finally oxidized back to oxaloacetic acid (producing \(\text{NADH} + \text{H}^+\)) to restart the cycle.
In simple words: The TCA cycle breaks down acetyl groups into carbon dioxide while generating high-energy electron carriers like NADH and FADH2.

Exam Tip: Practice drawing the circular pathway showing carbon numbers (6C to 4C) clearly.

Question 16. Where does electron transport system operative in mitochondria? Explain the system giving the role of oxygen.
Answer: The Electron Transport System (ETS) operates on the inner mitochondrial membrane (cristae).
- Mechanism: Reduced coenzymes (\(\text{NADH}_2\) and \(\text{FADH}_2\)) produced in glycolysis, link reaction, and the Krebs cycle are oxidized by passing their electrons through a series of electron carriers (Complexes I to IV, including ubiquinone and cytochromes). As electrons move down the gradient, protons are pumped into the intermembrane space, creating a proton gradient that drives ATP synthesis via ATP synthase.
- Role of Oxygen: Oxygen acts as the ultimate and final hydrogen/electron acceptor at the end of Complex IV. It combines with electrons and protons to form metabolic water (\(\text{H}_2\text{O}\)), ensuring the continuous flow of electrons through the transport chain.
In simple words: ETS passes electrons along membrane carriers to generate a proton gradient, with oxygen acting as the final trash collector for electrons and protons to form water.

Exam Tip: Emphasize that without oxygen, the entire electron transport chain backs up and stops.

Question 17. Give a brief account of ATP molecules produced in aerobic respiration in eukaryotes.
Answer:
In eukaryotes, the complete aerobic oxidation of one glucose molecule yields a net total of 36 ATP molecules, accounted for as follows:
1. Glycolysis (Cytoplasm): Produces 2 ATP directly and 2 \(\text{NADH}\) molecules. In eukaryotes, transporting these cytoplasmic \(\text{NADH}\) molecules into the mitochondria consumes 2 ATP (via the malate-aspartate shuttle), yielding a net of 4 ATP.
2. Oxidative Decarboxylation (Pyruvate to Acetyl-CoA): Produces 2 \(\text{NADH}\), which yield \(2 \times 3 = 6\) ATP.
3. Kreb's Cycle (Mitochondrial Matrix): Produces 2 GTP (equivalent to 2 ATP), 6 \(\text{NADH}\) (\(6 \times 3 = 18\) ATP), and 2 \(\text{FADH}_2\) (\(2 \times 2 = 4\) ATP), totaling 24 ATP.
Total Net ATP = \(4 + 6 + 24 = 36\) ATP.
In simple words: Eukaryotes get 36 ATPs total after accounting for the 2 ATPs spent transporting pyruvic acid into the mitochondria.

Exam Tip: Clearly state the difference between prokaryotes (38 ATP) and eukaryotes (36 ATP).

Question 18. Discuss the respiratory pathway is an amphibolic pathway.
Answer:
- Respiration is traditionally viewed as a catabolic process because complex organic molecules (carbohydrates, fats, and proteins) are broken down into simpler molecules like \(\text{CO}_2\) and water to release energy.
- However, intermediates of the respiratory pathway are frequently withdrawn to synthesize various cellular components (anabolic function). For example, acetyl-CoA is withdrawn for fatty acid and lipid synthesis, and \(\alpha\)-ketoglutaric acid is used to synthesize amino acids.
- Because the respiratory pathway serves both catabolic (breakdown for energy) and anabolic (biosynthesis) roles, it is aptly termed an amphibolic pathway.
In simple words: Respiration is amphibolic because it both breaks down food for energy (catabolism) and provides building blocks for synthesis (anabolism).

Question 19. What is glycolysis? Where does glycolysis takes place in a cell? Give schematic representation of glycolysis.
Answer:
- Definition: Glycolysis (EMP pathway) is the enzymatic breakdown of a glucose molecule into two molecules of pyruvic acid with the net production of ATP, occurring in the absence or presence of oxygen.
- Location: It takes place in the cytoplasm of the cell.
- Schematic Representation:
Glucose (6C)
\(\downarrow\) (ATP \(\rightarrow\) ADP)
Glucose-6-phosphate
\(\downarrow\)
Fructose-6-phosphate
\(\downarrow\) (ATP \(\rightarrow\) ADP)
Fructose-1,6-bisphosphate (6C)
\(\downarrow\)
Dihydroxyacetone phosphate + Glyceraldehyde-3-phosphate (3C)
\(\downarrow\) (\(2\text{NAD}^+ \rightarrow 2\text{NADH}\), \(2\text{Pi}\))
1,3-Bisphosphoglycerate
\(\downarrow\) (\(2\text{ADP} \rightarrow 2\text{ATP}\))
3-Phosphoglycerate
\(\downarrow\)
2-Phosphoglycerate
\(\downarrow\) (\(-\text{H}_2\text{O}\))
Phosphoenolpyruvate (PEP)
\(\downarrow\) (\(2\text{ADP} \rightarrow 2\text{ATP}\))
Pyruvic acid (3C) (\(\times 2\))
In simple words: Glycolysis is the cytoplasm-based splitting of glucose into two pyruvate molecules, yielding a net of 2 ATPs and 2 NADHs.

Exam Tip: Outline the preparatory phase (energy investment) and the pay-off phase (energy yield) when describing glycolysis.