GSEB Class 11 Biology Solutions Chapter 14 Respiration in Plants

 

Question 1. Differentiate between the following:
1. Respiration and combustion
2. Glycolysis and Krebs cycle
3. Aerobic respiration & Fermentation
Answer:
(1) Differences between respiration and combustion:
Respiration:
The breaking down of the C-C bonds in complex compounds through oxidation inside cells, which causes a significant amount of energy to be released, is termed respiration.
1. It is a regulated biochemical process.
2. Many chemical bonds break at the same time, releasing a big burst of energy all at once. Energy is not saved in this process, and no ATP is made.
3. No middle products are formed.
4. The temperature gets very high in this process.
5. No enzymes play a role.
Combustion:
The full burning of glucose, which gives \( \text{CO}_2 \) and \( \text{H}_2\text{O} \) as final products, offers energy, with most of it escaping as heat. If this energy is to benefit the cell, it needs to be used to create other molecules that the cell requires.
1. It is an unregulated physical-chemical process.
2. Here, chemical bonds break one after another, slowly releasing energy, which is then stored as ATP molecules.
3. Many intermediate substances are formed.
4. It only happens at a particular temperature.
5. Enzymes are utilized at every stage of respiration.
(2) Differences between glycolysis and Krebs cycle:
Glycolysis:
Some organisms can survive without oxygen (facultative anaerobes), while others absolutely need anaerobic conditions. Regardless, all living things possess the enzyme system to partly break down glucose without using oxygen. This breaking down of glucose into pyruvic acid is known as glycolysis.
Krebs cycle:
Pyruvic acid is a very important result of glycolysis. What happens next depends on what the cell needs. Cells manage the pyruvic acid made during glycolysis in three main ways: lactic acid fermentation, alcoholic fermentation, and aerobic respiration. Fermentation occurs without oxygen in many simple organisms and single-celled eukaryotes. For glucose to be fully broken down into \( \text{CO}_2 \) and \( \text{H}_2\text{O} \), organisms use the Krebs cycle, also called aerobic respiration, which needs oxygen.
(3) Differences between aerobic respiration and fermentation:
Aerobic respiration:
Aerobic respiration is the process where organic compounds are fully broken down when oxygen is present, releasing \( \text{CO}_2 \), water, and a lot of energy from the original substance. This kind of respiration is commonly found in more complex organisms. The key events in aerobic respiration include:
1. Pyruvate is fully oxidized by removing all hydrogen atoms step by step, which leaves three molecules of \( \text{CO}_2 \).
2. The electrons taken from hydrogen atoms are passed to molecular \( \text{O}_2 \), while ATP is created at the same time.
Fermentation:
During fermentation, for example by yeast, glucose is partially broken down without oxygen through a series of reactions where pyruvic acid becomes \( \text{CO}_2 \) and ethanol. Enzymes such as pyruvic acid decarboxylase and alcohol dehydrogenase help these reactions happen. Some other organisms, like certain bacteria, produce lactic acid from pyruvic acid. Similarly, in animal cells, like muscles during physical activity, when there isn't enough oxygen for cellular respiration, pyruvic acid is changed into lactic acid by lactate dehydrogenase. The substance that causes this reduction, \( \text{NADH} + \text{H}^+ \), gets oxidized to \( \text{NAD}^+ \) in both cases.
In simple words: Respiration is a controlled process inside cells that releases energy from complex compounds, while combustion is a faster, uncontrolled burning. Glycolysis breaks down glucose without oxygen, and the Krebs cycle is a main part of aerobic respiration that needs oxygen. Aerobic respiration fully breaks down organic substances with oxygen, producing much energy, while fermentation is an incomplete breakdown without oxygen, producing less energy and products like alcohol or lactic acid.

Exam Tip: When differentiating, ensure you list corresponding points for each category to provide a clear comparison rather than just separate descriptions.

 

Question 2. What are respiratory substrates? Name the most common respiratory substrate.
Answer: The substances that get broken down during respiration are termed respiratory substrates. Carbohydrates, especially glucose, serve as primary respiratory substrates. Fats, proteins, and organic acids can also function as respiratory substrates.
In simple words: Respiratory substrates are the materials cells break down for energy. Glucose, a type of carbohydrate, is the most common one, but fats and proteins can also be used.

Exam Tip: Remember that while carbohydrates are the primary respiratory substrates, cells can adapt to use other molecules like fats and proteins if carbohydrates are scarce.

 

Question 3. Give the schematic representation of glycolysis?
Answer: The pathway of glycolysis was described by Gustav Embden, Otto Meyerhof, and J. Parnas, and it is frequently known as the EMP pathway. For organisms that live without oxygen, it is the sole process used in respiration. Glycolysis happens in the cell's cytoplasm and is found in all living creatures. In this procedure, glucose undergoes a partial breakdown to create two molecules of pyruvic acid. In plants, this glucose comes from sucrose, which is the final product of photosynthesis, or from stored carbohydrates. Sucrose is changed into glucose and fructose by the enzyme invertase, and both these single sugars can easily join the glycolytic pathway.
Glucose and fructose are made into glucose-6-phosphate by the enzyme hexokinase. This changed glucose then becomes fructose-6-phosphate. The steps that follow for breaking down glucose and fructose are identical. Glycolysis involves a sequence of ten reactions, managed by different enzymes, to make pyruvate from glucose. When learning about the stages of glycolysis, observe the points where ATP (energy) is used or created, or where \( \text{NADH} + \text{H}^+ \) is formed. ATP is consumed at two main points: first, when glucose turns into glucose-6-phosphate, and second, when fructose-6-phosphate changes into fructose-1,6-diphosphate.
The schematic representation of glycolysis can be summarized as follows:
Glucose (6C)
↓ (ATP is used, ADP is released)
Glucose-6-phosphate (6C)
↓
Fructose-6-phosphate (6C)
↓ (ATP is used, ADP is released)
Fructose-1,6-bisphosphate (6C)
↓
Triose phosphate (glyceraldehyde-3-phosphate) (3C) \( \rightleftharpoons \) Triose phosphate (Dihydroxy acetone phosphate) (3C)
↓ (NAD+ is used, NADH is released)
2 x Triose bisphosphate (1,3 bisphosphoglyceric acid) (3C)
↓ (ATP is released, ADP is used)
2 x Triose phosphate (3-phosphoglyceric acid) (3C)
↓
2 X 2 phosphoglycerate
↓ (\( \text{H}_2\text{O} \) is released)
2 x phosphoenolpyruvate
↓ (ATP is released, ADP is used)
2 x Pyruvic acid (3C)
Fructose-1,6-diphosphate splits into dihydroxyacetone and 3-phosphoglyceraldehyde (PGAL). We observe a step where \( \text{NADH} + \text{H}^+ \) is made from \( \text{NAD}^+ \); this happens when 3-phosphoglyceraldehyde (PGAL) changes into 1,3-bisphosphoglycerate (DPGA). The change from DPGA to 3-phosphoglyceric acid (PGA) also gives off energy; this energy is captured by making ATP. Another ATP molecule is created when PEP converts to pyruvic acid. Pyruvic acid is then the main outcome of glycolysis. What happens to it depends on the cell's requirements. There are three primary methods different cells use to process pyruvic acid produced by glycolysis, which include lactic acid.
In simple words: Glycolysis is a ten-step process that breaks down glucose into two molecules of pyruvic acid in the cytoplasm. It involves using and producing ATP and NADH, and is the only respiration process in anaerobic organisms. Sucrose in plants is converted to glucose and fructose, which then enter this pathway.

Exam Tip: Memorize the key intermediates and the points where ATP is utilized or produced, as well as the formation of NADH, to understand the energy dynamics of glycolysis.

 

Question 4. What are the main steps in aerobic respiration? Where does it take place?
Answer: For aerobic respiration to happen inside the mitochondria, pyruvate, which is the last product of glycolysis, moves from the cytoplasm into the mitochondria. The key events in aerobic respiration are:
1. The full breakdown of pyruvate by removing all hydrogen atoms step by step, leaving three molecules of \( \text{CO}_2 \).
2. The transfer of electrons, taken from hydrogen atoms, to molecular \( \text{O}_2 \), with ATP being created at the same time.
The first of these procedures occurs in the mitochondria's matrix, while the second procedure is found on the inner membrane of the mitochondria. Pyruvate, formed from the breakdown of carbohydrates in the cytosol during glycolysis, enters the mitochondrial matrix and then undergoes oxidative decarboxylation through a complex series of reactions helped by pyruvic dehydrogenase. The reactions catalyzed by pyruvic dehydrogenase require \( \text{Mg}^{2+} \).
\( \text{Pyruvic acid} + \text{CoA} + \text{NAD}^+ \xrightarrow{\text{Pyruvate dehydrogenase}}^{\text{Mg}^{2+}} \text{Acetyl CoA} + \text{CO}_2 + \text{NADH} + \text{H}^+ \)
During this particular process, two molecules of \( \text{NADH} \) are formed from the breakdown of two molecules of pyruvic acid (which comes from one glucose molecule during glycolysis).
In simple words: Aerobic respiration mainly happens in mitochondria after glycolysis. Pyruvate moves into mitochondria, where it's fully broken down, releasing carbon dioxide and producing ATP when electrons are passed to oxygen. The first part is in the mitochondrial matrix, and the second part is on the inner membrane.

Exam Tip: Differentiate clearly between the location of glycolysis (cytoplasm) and the subsequent stages of aerobic respiration (mitochondria), and remember the main products at each stage.

 

Question 5. Give the schematic representation of an overall view of Kreb's cycle.
Answer: The tricarboxylic acid cycle, also widely known as the Krebs cycle, is named after scientist Hans Krebs, who first explained it. The TCA cycle begins when the acetyl group combines with oxaloacetic acid (OAA) and water to produce citric acid. The enzyme citrate synthase help this reaction, and a molecule of CoA is released. Citrate then changes into isocitrate. This is followed by two steps of decarboxylation, which create an acid and then succinyl-CoA. In the last steps of the citric acid cycle, succinyl-CoA is oxidized to OAA, allowing the cycle to keep going. During the change of succinyl-CoA to succinic acid, a molecule of GTP is formed.
This is called substrate-level phosphorylation. In a linked reaction, GTP changes to GDP along with the creation of ATP from ADP. Also, there are three stages in the cycle where \( \text{NAD}^+ \) is converted to \( \text{NADH} + \text{H}^+ \) and one stage where \( \text{FAD} \) is changed to \( \text{FADH}_2 \). The ongoing breakdown of acetic acid through the TCA cycle needs a steady supply of oxaloacetic acid, which is the cycle's initial component. Furthermore, it also requires \( \text{NAD}^+ \) and \( \text{FAD} \) to be re-formed from \( \text{NADH} \) and \( \text{FADH}_2 \) respectively. The overall equation for this part of respiration can be expressed as:
\( \text{Pyruvic acid} + 4\text{NAD}^+ + \text{FAD} + 2\text{H}_2\text{O} + \text{ADP} + \text{Pi} \xrightarrow{\text{Mitochondrial Matrix}} 3\text{CO}_2 + 4\text{NADH} + 4\text{H}^+ + \text{FADH}_2 + \text{ATP} \)
The schematic representation of the Citric Acid Cycle (Krebs Cycle) can be outlined as follows:
Pyruvate (3C)
↓
Acetyl Coenzyme A (2C)
↓
**Citric Acid Cycle:**
Oxaloacetic acid (4C)
↓ + Acetyl Coenzyme A (2C)
Citric acid (6C)
↓
α-ketoglutaric acid (5C) (releases \( \text{CO}_2 \), \( \text{NADH} + \text{H}^+ \) from \( \text{NAD}^+ \))
↓
Succinic acid (4C) (releases \( \text{CO}_2 \), \( \text{NADH} + \text{H}^+ \) from \( \text{NAD}^+ \))
↓
Malic acid (4C) (releases \( \text{FADH}_2 \) from \( \text{FAD} \), produces ATP from ADP + Pi)
↓
Oxaloacetic acid (4C) (releases \( \text{NADH} + \text{H}^+ \) from \( \text{NAD}^+ \))
↑ (Cycle continues)
In simple words: The Krebs cycle is a main part of aerobic respiration that breaks down acetyl-CoA. It starts with acetyl-CoA joining oxaloacetic acid to form citric acid. Through several steps, it releases carbon dioxide, generates electron carriers like NADH and FADH2, and produces some ATP, eventually regenerating oxaloacetic acid to continue the cycle.

Exam Tip: Focus on the cyclic nature of the Krebs cycle, the inputs (acetyl-CoA, water), outputs (\( \text{CO}_2 \), ATP, NADH, FADH2), and the regeneration of oxaloacetic acid, as these are critical for understanding its role in energy production.

 

Question 6. Explain ETS.
Answer: The metabolic route where electrons move from one carrier to another is known as the electron transport system (ETS), and it is found in the inner mitochondrial membrane. Electrons from \( \text{NADH} \) formed in the mitochondrial matrix during the citric acid cycle are oxidized by \( \text{NADH} \) dehydrogenase (Complex I), and these electrons are then moved to ubiquinone, which is located inside the inner membrane. Ubiquinone also gets reducing equivalents through \( \text{FADH}_2 \) (Complex II) that is created during the oxidation of succinate in the citric acid cycle.
The reduced ubiquinol is then oxidized when it transfers electrons to cytochrome C via the cytochrome \( \text{bc}_1 \) complex (Complex III). Cytochrome c is a tiny protein connected to the outer part of the inner membrane and serves as a mobile carrier for moving electrons between complex III and IV. Complex IV includes cytochromes a and \( \text{a}_3 \), along with two copper centers.
When electrons travel from one carrier to another through complexes I to IV in the electron transport chain, they are linked to ATP synthase (Complex V) to produce ATP from ADP and inorganic phosphate. The quantity of ATP molecules made depends on the type of electron donor. Breaking down one molecule of \( \text{NADH} \) results in 3 molecules of ATP, while breaking down one molecule of \( \text{FADH}_2 \) generates 2 molecules of ATP. Although the aerobic respiration process only occurs when oxygen is present, oxygen's function is restricted to the final stage of the process.
The general flow of the Electron Transport System (ETS) in the mitochondrion:
**Mitochondrial Matrix / Inner mitochondrial membrane / Inter-membrane space**
\( 2\text{H}^+ \) and \( \text{NADH} + \text{H}^+ \) (from \( \text{NAD}^+ \) + FMN)
↓
**Complex I (NADH dehydrogenase):** \( 2\text{e}^- \), \( \text{FeS} \)
↓
Ubiquinone (OH\( _2 \))
↓
**Complex II:** \( 2\text{e}^- \), \( \text{FeS} \) (receives \( \text{FADH}_2 \) from \( \text{FAD} \))
↓
**Complex III (Cytochrome \( \text{bc}_1 \)):** \( \text{Cy b} \), \( \text{FeS} \), \( \text{Cy c} \)
↓
**Complex IV (Cytochrome \( \text{a} \), \( \text{a}_3 \)):** \( 2\text{e}^- \), Copper centers
↓
\( \text{O}_2 \) + \( 2\text{H}^+ \) → \( \text{H}_2\text{O} \)
(Throughout this flow, \( 2\text{H}^+ \) are pumped from the matrix to the inter-membrane space, creating a proton gradient used by ATP synthase (Complex V) to make ATP.)
In simple words: The Electron Transport System (ETS) is how electrons move through a series of carriers in the inner mitochondrial membrane, releasing energy that is used to make ATP. NADH and FADH2 deliver these electrons, and oxygen is the final electron acceptor, forming water.

Exam Tip: Understand the role of each complex in the electron transport chain, specifically how electrons are passed, the pumping of protons, and the final role of oxygen in forming water.

 

Question 7. Distinguish between the following:
1. Aerobic respiration and Anaerobic respiration
2. Glycolysis and Fermentation
3. Glycolysis and Citric acid Cycle.
Answer:
(1) Differences between Aerobic respiration and anaerobic respiration:
Aerobic respiration:
1. It happens when molecular oxygen is present.
2. It consistently makes \( \text{CO}_2 \).
3. It forms water.
4. It takes place in the cytoplasm and mitochondria of cells.
5. It creates a large quantity of energy.
6. Glucose is entirely broken down into water.
7. It occurs in most plant and animal cells, but not in yeasts, specific bacteria, internal parasitic worms, skeletal muscles, or mammalian red blood cells.
Anaerobic respiration:
1. It occurs without oxygen.
2. It might or might not produce \( \text{CO}_2 \).
3. It does not produce water.
4. It happens only in the cytoplasm and outside the cell's mitochondria.
5. It creates a very small amount of energy.
6. Glucose is partially broken down into \( \text{CO}_2 \), ethyl alcohol, or lactic acid.
7. It occurs only in yeasts, certain types of bacteria, endoparasitic worms, skeletal muscles, and mammalian red blood cells.
(2) Differences between glycolysis and fermentation:
Glycolysis:
1. Glycolysis occurs in the cell's cytoplasm and is found in all living organisms.
2. In this process, glucose undergoes a partial breakdown to form two molecules of pyruvic acid.
3. In plants, this glucose comes from sucrose, which is the final product of photosynthesis, or from stored carbohydrates.
4. Sucrose is changed into glucose and fructose by the enzyme invertase, and both these single sugars can easily join the glycolytic pathway.
5. Glucose and Fructose are made into glucose-6-phosphate by the enzyme hexokinase.
Citric acid cycle (as counterpoint to Fermentation in source):
1. The reaction is helped by the enzyme citrate synthase, and a CoA molecule is released.
2. Citrate then changes into isocitrate. This is followed by two successive decarboxylation steps, leading to the creation of alpha-ketoglutaric acid and then succinyl-CoA.
3. In both lactic acid and alcohol fermentation, not much energy is released, less than seven percent of the energy in glucose is set free, and not all of it is captured as high-energy ATP bonds. Also, the processes are dangerous—either acid or alcohol is created.
4. Yeasts harm themselves to death when the alcohol level reaches about 13 percent.
5. In eukaryotes, these steps happen inside the mitochondria and require \( \text{O}_2 \).
(3) Differences between glycolysis and citric acid cycle:
**Glycolysis:**
1. Glycolysis takes place in the cell's cytoplasm and is found in all living organisms.
2. In this process, glucose undergoes a partial breakdown to form two molecules of pyruvic acid.
3. In plants, this glucose comes from sucrose, which is the final product of photosynthesis, or from stored carbohydrates.
4. Sucrose is changed into glucose and fructose by the enzyme invertase, and both these single sugars can easily join the glycolytic pathway.
5. Glucose and Fructose are made into glucose-6-phosphate by the enzyme hexokinase.
6. This modified glucose then changes to produce fructose-6-phosphate.
7. Subsequent steps for breaking down glucose and fructose are identical.
8. In glycolysis, a series of ten reactions, managed by various enzymes, occur to create pyruvate from glucose.
9. ATP is used at two stages: first, in converting glucose into glucose-6-phosphate, and second, in changing fructose-6-phosphate into fructose-1,6-diphosphate.
10. The fructose-1,6-diphosphate splits into dihydroxyacetone phosphate and 3-phosphoryl acetaldehyde (PGAL). PGAL is oxidized and with inorganic phosphate, it gets converted into DPGA.
11. The conversion of DPGA to 3-phosphoglyceric acid (PGA) is also an energy-giving process; this energy is captured by making ATP. Another ATP is created during the conversion of PEP to pyruvic acid.
12. Pyruvic acid is then the key outcome of glycolysis.
**Citric acid cycle:**
1. The reaction is helped by the enzyme citrate synthase, and a molecule of CoA is oxidized to OAA, allowing the cycle to continue.
2. Citrate then changes into isocitrate. This is followed by two successive decarboxylation steps, leading to the creation of alpha-ketoglutaric acid and then succinyl-CoA.
3. In the remaining steps of the citric acid cycle, succinyl-CoA is oxidized to OAA, enabling the cycle to continue.
4. During the conversion of succinyl-CoA to succinic acid, a molecule of GTP is formed. This is called substrate-level phosphorylation.
5. In a linked reaction, GTP changes to GDP along with the creation of ATP from ADP.
6. Also, there are three stages in the cycle where \( \text{NAD}^+ \) is converted to \( \text{NADH} + \text{H}^+ \) and one stage where \( \text{FAD}^+ \) is changed to \( \text{FADH}_2 \).
7. The ongoing breakdown of acetic acid through the TCA cycle needs a steady supply of oxaloacetic acid, which is the cycle's initial component.
8. In addition, it also requires \( \text{NAD}^+ \) and \( \text{FAD} \) to be re-formed from \( \text{NADH} \) and \( \text{FADH}_2 \) respectively.
In simple words: Aerobic respiration needs oxygen and produces much energy, while anaerobic respiration occurs without oxygen and yields less. Glycolysis is the first step, breaking glucose into pyruvic acid. Fermentation is an anaerobic pathway to process pyruvic acid, making products like alcohol. The Citric Acid Cycle is a core aerobic pathway that completely breaks down molecules, producing more energy carriers.

Exam Tip: Be mindful of the conditions (presence/absence of oxygen) and locations (cytoplasm vs. mitochondria) for each pathway, as these are crucial distinguishing features.

 

Question 8. What are the assumptions made during the calculation of the net gain of ATP?
Answer: It is possible to perform calculations for the net ATP gained from each glucose molecule oxidized, but in reality, this remains a theoretical exercise. These calculations can only be done based on specific assumptions:

• It is assumed that different parts of aerobic respiration, like glycolysis, the TCA cycle, and ETS, happen in a specific and organized order.


• \( \text{NADH} \) formed during glycolysis is presumed to enter the mitochondria to undergo oxidative phosphorylation.


• The glucose molecule is taken as the only substance being broken down, and it is assumed that no other molecule joins the pathway at intermediate points.


• The intermediate products made during respiration are not used in any other process.

However, these types of assumptions are not truly correct in a living system; all pathways work at the same time and do not happen one after another; substances enter and leave the pathways as needed; ATP is used when required; and enzyme speeds are controlled in various ways. Still, it is helpful to do this exercise to appreciate the beauty and efficiency of the living system in getting and storing energy. Consequently, there can be a net gain of 36 ATP molecules during aerobic respiration from one molecule of glucose.
In simple words: When calculating ATP gain, we assume that respiration pathways happen in a perfect, step-by-step order, that NADH always goes to make ATP, and that glucose is the only fuel used, with no other molecules interfering. These assumptions simplify a complex biological process.

Exam Tip: Understand that these ATP yield calculations are theoretical models; in reality, biological systems are dynamic and more complex, making exact figures hard to pinpoint.

 

Question 9. Discuss "The respiratory pathway is an amphibolic pathway".
Answer: Glucose is the preferred substance for respiration. All carbohydrates are usually changed into glucose first before being used for respiration. Other substances can also be broken down, as mentioned earlier, but they do not join the respiratory pathway at its initial step. Fats would first need to be broken into glycerol and fatty acids. If fatty acids were to be respired, they would first be reduced to acetyl CoA and then join the pathway. Glycerol would join the pathway after being changed into PGAL. Proteins would be broken down by proteases, and the individual amino acids (after deamination), depending on other structures, would enter the pathway at some point within the Krebs cycle, or even as pyruvate or acetyl CoA.
The entry points of different substrates into the respiratory pathway can be represented as follows:
**Fats** → Fatty acids and glycerol → Glucose 6-phosphate ← Simple sugars (e.g., Glucose) ← **Carbohydrates**
↓
Fructose 1,6 bisphosphate
↓
Dihydroxy Acetone Phosphate \( \rightleftharpoons \) Glyceraldehyde 3-phosphate
↓
Pyruvic acid
↓
Acetyl CoA
↓
Krebs Cycle → \( \text{CO}_2 \)
**Proteins** → Amino acids
Since respiration involves breaking down substrates, the respiratory process has traditionally been seen as a catabolic process, and the respiratory pathway is indeed catabolic. Thus, fatty acids would be broken down to acetyl CoA before joining the respiratory pathway when used as a fuel. However, when an organism needs to create fatty acids, acetyl CoA would be taken from the respiratory pathway for this purpose. Therefore, the respiratory pathway plays a role in both the breaking down and creation of fatty acids. Similarly, during the breakdown and creation of proteins, respiratory intermediate compounds act as the connection. Breaking down processes within a living organism is catabolism, and building up is anabolism. Because the respiratory pathway participates in both anabolism and catabolism, it is more fitting to view it as an amphibolic pathway rather than solely a catabolic one.
In simple words: The respiratory pathway is amphibolic because it's involved in both breaking down molecules (catabolism) for energy and building new molecules (anabolism). It can use different fuels like fats, carbohydrates, and proteins, and its intermediates can be diverted to make new substances when needed.

Exam Tip: To explain an amphibolic pathway, always highlight examples of both catabolic (energy generation) and anabolic (biosynthesis) roles, showing how intermediates are utilized in both directions.

 

Question 10. Define RQ. What is its value for fats?
Answer: As we know, during aerobic respiration, oxygen \( (\text{O}_2) \) is used and carbon dioxide \( (\text{CO}_2) \) is given off. The ratio of the volume of \( \text{CO}_2 \) produced to the volume of \( \text{O}_2 \) consumed during respiration is termed the respiratory quotient (RQ) or respiratory ratio.
\( \text{RQ} = \frac{\text{Volume of CO}_2 \text{ evolved}}{\text{Volume of O}_2 \text{ consumed}} \)
The respiratory quotient changes based on the type of substance used during respiration. When carbohydrates are used as the substrate and are fully broken down, the RQ will be 1, because equal amounts of \( \text{CO}_2 \) and \( \text{O}_2 \) are produced and consumed, as seen in the equation below:
\( \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \longrightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy} \)
\( \text{RQ} = \frac{6\text{CO}_2}{6\text{O}_2} = 1.0 \)
When fats are used in respiration, the RQ is less than 1. Calculations for the fatty acid tripalmitin, if used as a substrate, are shown:
\( 2(\text{C}_{51}\text{H}_{98}\text{O}_6) + 145\text{O}_2 \longrightarrow 102\text{CO}_2 + 98\text{H}_2\text{O} + \text{Energy} \)
\( \text{RQ} = \frac{102\text{CO}_2}{145\text{O}_2} = 0.7 \)
When proteins are the respiratory substrates, the ratio would be about 0.9.
In simple words: Respiratory Quotient (RQ) is the ratio of carbon dioxide produced to oxygen consumed during respiration. For fats, the RQ value is less than 1, typically around 0.7, because fats require more oxygen for their complete breakdown compared to the carbon dioxide they release.

Exam Tip: Remember the formula for RQ and how it varies for different substrates: 1 for carbohydrates, less than 1 for fats and proteins, and often greater than 1 for organic acids, reflecting the level of oxygen required for their oxidation.

 

Question 11. What is oxidative phosphorylation?
Answer: Oxidative phosphorylation is a process where electrons are moved from electron donors to oxygen, which takes on the electrons. These oxidation-reduction reactions help create a proton gradient. The primary job in oxidative phosphorylation belongs to the enzyme ATP synthase. This enzyme group includes F₀ and F₁ components. For every two protons that move through the F₀-F₁ complex, one ATP molecule is made.
In simple words: It's how cells make a lot of energy (ATP) using oxygen. Electrons move in a special way, creating a difference in charge that helps an enzyme make ATP.

Exam Tip: Key terms to remember are 'electron transport system', 'proton gradient', and 'ATP synthase' as these are central to explaining the process.

 

Question 12. What is the significance of the step-wise release of energy in respiration?
Answer: The process of aerobic respiration is broken into four stages: glycolysis, TCA cycle, ETS, and oxidative phosphorylation. It is commonly believed that respiration and ATP production happen in each stage in a step-by-step way. The result of one pathway becomes the starting material for the next. Many different molecules created during respiration are used in other biochemical processes. The respiratory substrates go into and come out of the pathway as needed. So, releasing energy step by step helps the system work better at getting and keeping energy.
In simple words: Breaking down energy in small steps helps the body get and store more energy from food efficiently. It prevents wasting energy as too much heat and allows the use of different starting materials.

Exam Tip: Emphasize 'efficiency' and 'control' when discussing the significance of stepwise energy release, and mention that it minimizes energy loss as heat.

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