આ MCQ મોડ્યુલ આના પર આધારિત છે: Balance Sheet Amphibolic Rq
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Balance Sheet Amphibolic Rq
🎓 Class 11
Biology
CBSE
Theory
Ch 12 – Respiration in Plants
⏱ ~14 min
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આ મૂલ્યાંકન આના પર આધારિત હશે: Balance Sheet Amphibolic Rq
મૂલ્યાંકન બનાવવામાં તેમની સામગ્રી સામેલ કરવા ચિત્રો, PDF અથવા Word દસ્તાવેજ અપલોડ કરો.
The Respiratory Balance Sheet, Amphibolic Pathway and Respiratory Quotient
We now have every reaction in place. This part does three things with them: it adds up the ATP and examines honestly how much that total can be trusted, it shows that the pathway is not the one-way demolition it is usually taken for, and it shows how the ratio of two gases can reveal which substrate a tissue is burning.
12.5 The Respiratory Balance Sheet
It is possible to make calculations of the net gain of ATP for every glucose molecule oxidised — but in reality this can remain only a theoretical exercise. The calculations can be made only on certain assumptions.
The four assumptions (a favourite examination question).
• There is a sequential, orderly pathway functioning, with one substrate forming the next, and with glycolysis, TCA cycle and ETS pathway following one after another.
• The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
• None of the intermediates in the pathway are utilised to synthesise any other compound.
• Only glucose is being respired — no other alternative substrates are entering the pathway at any of the intermediary stages.
• There is a sequential, orderly pathway functioning, with one substrate forming the next, and with glycolysis, TCA cycle and ETS pathway following one after another.
• The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
• None of the intermediates in the pathway are utilised to synthesise any other compound.
• Only glucose is being respired — no other alternative substrates are entering the pathway at any of the intermediary stages.
Why none of those assumptions actually holds
But this kind of assumption is not really valid in a living system. The chapter is explicit about why:
- All pathways work simultaneously and do not take place one after another.
- Substrates enter the pathways and are withdrawn from them as and when necessary.
- ATP is utilised as and when needed.
- Enzymatic rates are controlled by multiple means.
Yet it is useful to do this exercise to appreciate the beauty and efficiency of the living system in extraction and storing of energy. On those assumptions, then: there can be a net gain of 38 ATP molecules during aerobic respiration of one molecule of glucose.
📐 Activity 12.4 — Test each assumption against the living cell
What to do. Make a two-column table. In the left column write each of the four assumptions. In the right column, write one concrete biological situation from this chapter or the previous one that violates it. Then state, in one sentence, whether the real yield would be higher or lower than 38.
Predict: would the actual ATP yield in a real cell be more than 38, less than 38, or exactly 38?
| Assumption | What actually happens |
|---|---|
| Orderly, sequential pathway | All pathways work simultaneously; glycolysis, the cycle and the ETS run at once, and enzymatic rates are controlled by multiple means, so no neat queue exists. |
| Glycolytic NADH reaches the mitochondria | That NADH is made in the cytoplasm, while the ETS is on the inner mitochondrial membrane. Getting it across is not free, so its full 3-ATP value is not guaranteed. |
| No intermediate is withdrawn | The opposite is routine — and Section 12.6 is built on it. Acetyl CoA is withdrawn when fatty acids must be synthesised, and Krebs' intermediates are withdrawn for amino acids. Every carbon withdrawn is a carbon not oxidised. |
| Only glucose is respired | Fats and proteins are also respired, entering at PGAL, acetyl CoA or points within the Krebs' cycle — so the cell is never running on glucose alone. |
Answer to the prediction: less than 38. Every violation listed removes something from the tally — carbons diverted to synthesis, coenzymes not fully cashed in, ATP spent on transport. So 38 is a theoretical ceiling, not a measurement. The chapter says as much: this “can remain only a theoretical exercise”, valuable because it lets you appreciate the beauty and efficiency of the living system, not because a cell ever hits the number.
12.6 Amphibolic Pathway
Glucose is the favoured substrate for respiration. All carbohydrates are usually first converted into glucose before they are used for respiration. Other substrates can also be respired, but then they do not enter the respiratory pathway at the first step.
Where each substrate joins the pathway
| Substrate | First broken down to | Enters the respiratory pathway as |
|---|---|---|
| Carbohydrates | Glucose | At the first step of glycolysis |
| Fats | Glycerol and fatty acids | See the two rows below |
| — fatty acids | Degraded to acetyl CoA | Acetyl CoA, entering Krebs' cycle |
| — glycerol | Converted to PGAL | PGAL, joining mid-glycolysis |
| Proteins | Degraded by proteases to amino acids, then deaminated | Depending on structure, at some stage within Krebs' cycle, or as pyruvate or acetyl CoA |
Is respiration really just catabolism?
Since respiration involves breakdown of substrates, the respiratory process has traditionally been considered a catabolic process and the respiratory pathway a catabolic pathway. But is this understanding correct?
The argument, in the chapter's own logic. We have just seen at which points different substrates enter the pathway if they are to be respired for energy. What is important to recognise is that it is these very compounds that would be withdrawn from the respiratory pathway for the synthesis of the said substrates. Hence fatty acids would be broken down to acetyl CoA before entering the respiratory pathway when used as a substrate; but when the organism needs to synthesise fatty acids, acetyl CoA would be withdrawn from the respiratory pathway for it. So the respiratory pathway comes into the picture both during breakdown and synthesis of fatty acids. Similarly, during breakdown and synthesis of protein too, respiratory intermediates form the link.
The conclusion. Breaking down processes within the living organism is catabolism, and synthesis is anabolism. Because the respiratory pathway is involved in both anabolism and catabolism, it would hence be better to consider the respiratory pathway as an amphibolic pathway rather than as a catabolic one.
There is a useful way to picture this. The respiratory pathway is not a one-way demolition chute but a railway junction. Molecules arrive from many directions to be broken down, and molecules are also pulled off the line half-finished to be built into something else. Acetyl CoA is the busiest platform on that junction — the same compound is the gateway for burning a fat and the starting block for making one.
12.7 Respiratory Quotient
During aerobic respiration, O₂ is consumed and CO₂ is released.
The respiratory quotient (RQ), or respiratory ratio, is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration.
RQ = volume of CO2 evolved ÷ volume of O2 consumed
The respiratory quotient depends upon the type of respiratory substrate used during respiration.
Carbohydrates — RQ = 1
When carbohydrates are used as substrate and are completely oxidised, the RQ will be 1, because equal amounts of CO₂ and O₂ are evolved and consumed respectively.
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
RQ = 6CO2 / 6O2 = 1.0
RQ = 6CO2 / 6O2 = 1.0
Fats — RQ less than 1
When fats are used in respiration, the RQ is less than 1. For the fatty acid tripalmitin:
2(C51H98O6) + 145O2 → 102CO2 + 98H2O + Energy
RQ = 102CO2 / 145O2 = 0.7
RQ = 102CO2 / 145O2 = 0.7
Proteins — RQ about 0.9
When proteins are respiratory substrates, the ratio would be about 0.9.
The honest caveat the chapter insists on. What is important to recognise is that in living organisms, respiratory substrates are often more than one; pure proteins or fats are never used as respiratory substrates. So a measured RQ in a real tissue is a mixture value, not a clean identification of one substrate.
Why fats give a lower RQ — the reasoning behind the number. Look at the molecular formulae. A carbohydrate such as glucose, C₆H₁₂O₆, is already rich in oxygen, so relatively little atmospheric O₂ is needed to oxidise it fully. A fat such as tripalmitin, C₅₁H₉₈O₆, carries an enormous number of hydrogen atoms and very little oxygen of its own — so a great deal of O₂ must be consumed just to convert all that hydrogen into water, while comparatively less CO₂ is produced per unit of O₂ used. A small numerator over a large denominator gives a ratio well below 1. The same reasoning explains why fats store so much more energy per gram than carbohydrates.
| Respiratory substrate | RQ | Reason |
|---|---|---|
| Carbohydrates (completely oxidised) | 1.0 | Equal volumes of CO₂ evolved and O₂ consumed |
| Fats (e.g. tripalmitin) | 0.7 — less than 1 | Hydrogen-rich, oxygen-poor substrate needs much more O₂ per CO₂ released |
| Proteins | about 0.9 | Intermediate composition; enter after deamination at various points |
| Organic acids | Greater than 1 | Already highly oxidised, so little O₂ is needed (not required by NCERT, given for context) |
🎯 Interactive: Substrate, entry point and RQ
Enters the pathway as: Glucose, at the first step of glycolysis
Glucose is the favoured substrate, and all carbohydrates are usually first converted into glucose before being respired. Completely oxidised, it gives RQ = 6CO2/6O2 = 1.0, because equal volumes of CO2 are evolved and O2 consumed.
🎯 Competency-Based Questions
Scenario: Three germinating seeds are placed in separate respirometers and their gas exchange measured. Seed P (a cereal grain) gives RQ = 1.0. Seed Q (a castor or groundnut seed) gives RQ = 0.7. Seed R gives RQ = 0.9. All three are at the same temperature and all are respiring aerobically.
Q1. Identify the main respiratory substrate in each seed. L3 Apply
P — carbohydrate, since when carbohydrates are completely oxidised the RQ is 1, equal volumes of CO₂ and O₂ being evolved and consumed. Q — fat, since when fats are used the RQ is less than 1, and 0.7 is the value calculated for tripalmitin. R — protein, for which the ratio would be about 0.9. The identification is consistent with what these seeds store: cereals store starch, castor and groundnut store oil.
Q2. How confident can you be in those identifications? L5 Evaluate
Reasonably, but not fully. The chapter's own caveat applies: in living organisms, respiratory substrates are often more than one; pure proteins or fats are never used as respiratory substrates. A measured RQ is therefore a weighted average. An RQ of 0.9 could mean protein, but equally a mixture of carbohydrate and fat. So an RQ narrows the possibilities and tells you the direction — below 1 means fat is contributing — but confirming the substrate needs independent evidence, such as a biochemical assay of what the seed actually stores.
Q3. Fill in the blanks: The theoretical net gain from one glucose is ______ ATP. This rests on the assumptions of a ______ pathway, that glycolytic ______ reaches the mitochondria, that no ______ is withdrawn for synthesis, and that only ______ is being respired. L1 Remember
38; sequential, orderly; NADH; intermediate; glucose.
Q4. A cell is actively synthesising fatty acids while also respiring. Explain, using one named intermediate, why this makes the 38 ATP figure unattainable. L4 Analyse
Because it breaks the third assumption directly — that none of the intermediates in the pathway are utilised to synthesise any other compound. The named intermediate is acetyl CoA: when the organism needs to synthesise fatty acids, acetyl CoA is withdrawn from the respiratory pathway for that purpose. Every acetyl CoA diverted is two carbons that never enter the Krebs' cycle, so its 3 NADH, 1 FADH₂ and 1 GTP are never generated — a loss of about 12 ATP per acetyl CoA withdrawn. This is exactly why the pathway is called amphibolic: the same junction serves breakdown and synthesis, and the two compete for the same molecules.
Q5. “Respiration is a catabolic process.” Discuss whether this traditional description should be retained. L5 Evaluate
It should be replaced, or at least heavily qualified. The traditional label is defensible as far as it goes: respiration does involve breakdown of substrates, and breaking down processes within the living organism is catabolism. But it describes only one direction of traffic. The same compounds at which substrates enter the pathway are the very compounds withdrawn from the respiratory pathway for the synthesis of those substrates. Fatty acids are broken to acetyl CoA to be respired; acetyl CoA is withdrawn from the pathway when fatty acids must be made. The same holds for protein, where respiratory intermediates form the link in both breakdown and synthesis. Since synthesis is anabolism and the pathway serves both, it is better to consider the respiratory pathway as an amphibolic pathway rather than as a catabolic one. The wider lesson is methodological: a pathway should be classified by all the traffic it carries, not by the direction in which it was first studied.
🧠 Assertion–Reason Questions
For each pair choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
Assertion (A): The respiratory pathway is best described as amphibolic.
Reason (R): Intermediates such as acetyl CoA are withdrawn from the pathway for synthesis, as well as entering it for breakdown.
Answer: A. Both are true and the reason is exactly the chapter's argument: because the pathway is involved in both anabolism and catabolism, “amphibolic” is the accurate term.
Assertion (A): The RQ of fats is less than 1.
Reason (R): Fats are hydrogen-rich and oxygen-poor, so a large volume of O₂ must be consumed relative to the volume of CO₂ evolved.
Answer: A. Both are true and the reason explains the assertion. For tripalmitin the calculation gives 102CO₂ / 145O₂ = 0.7.
Assertion (A): A net gain of 38 ATP per glucose is what a plant cell actually obtains from aerobic respiration.
Reason (R): All pathways work simultaneously, substrates are withdrawn as needed, and enzymatic rates are controlled by multiple means.
Answer: D. The assertion is false — 38 is a theoretical figure resting on assumptions that are not really valid in a living system. The reason states precisely why those assumptions fail, and so is true.
Coming next. Part 5 is the exercise part: the chapter summary followed by worked solutions to all twelve NCERT exercise questions of Chapter 12, including the three “differentiate” questions, the two schematic representations, and the significance of step-wise release of energy.
Frequently Asked Questions - Respiratory Balance Sheet, Amphibolic Pathway and RQ
What is the net gain of ATP from one molecule of glucose in aerobic respiration?
There can be a net gain of 38 ATP molecules during aerobic respiration of one molecule of glucose. This is a theoretical figure: 8 ATP from glycolysis, 6 from the two link reactions and 24 from the two turns of the Krebs cycle with their ETS yield.
What are the assumptions made in calculating the net gain of ATP?
Four. That there is a sequential, orderly pathway with glycolysis, TCA cycle and ETS following one after another; that the NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation; that none of the intermediates are used to synthesise any other compound; and that only glucose is being respired.
Why are those assumptions not valid in a living system?
Because all pathways work simultaneously rather than one after another, substrates enter and are withdrawn as and when necessary, ATP is utilised as and when needed, and enzymatic rates are controlled by multiple means. The 38 ATP figure is therefore a theoretical ceiling.
How do fats and proteins enter the respiratory pathway?
Fats are first broken into glycerol and fatty acids. Fatty acids are degraded to acetyl CoA and enter there, while glycerol enters after being converted to PGAL. Proteins are degraded by proteases and the amino acids, after deamination, enter at some stage within Krebs cycle or as pyruvate or acetyl CoA, depending on their structure.
Why is the respiratory pathway called amphibolic rather than catabolic?
Because the compounds at which substrates enter the pathway for breakdown are the very compounds withdrawn from it for synthesis. Fatty acids are broken to acetyl CoA to be respired, but acetyl CoA is withdrawn from the pathway when fatty acids must be made. Since the pathway serves both catabolism and anabolism, amphibolic is the accurate term.
What is the respiratory quotient?
The respiratory quotient, or respiratory ratio, is the ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration. It depends on the type of respiratory substrate used.
What is the RQ value for carbohydrates, fats and proteins?
For carbohydrates completely oxidised it is 1, because equal amounts of CO2 and O2 are evolved and consumed. For fats it is less than 1, calculated as 0.7 for tripalmitin. For proteins the ratio would be about 0.9.
Why is the RQ of fat less than that of carbohydrate?
Because a fat molecule is rich in hydrogen and poor in oxygen, so a large volume of O2 must be consumed to convert all that hydrogen into water while comparatively less CO2 is released. For tripalmitin the figures are 102 CO2 against 145 O2, giving 0.7.
Can RQ alone identify the respiratory substrate of a tissue?
Not reliably. In living organisms respiratory substrates are often more than one, and pure proteins or fats are never used as respiratory substrates, so any measured RQ is a mixture value. It narrows the possibilities but needs independent biochemical evidence to confirm.
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