આ MCQ મોડ્યુલ આના પર આધારિત છે: Gas Exchange Glycolysis
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Gas Exchange Glycolysis
🎓 Class 11
Biology
CBSE
Theory
Ch 12 – Respiration in Plants
⏱ ~14 min
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આ મૂલ્યાંકન આના પર આધારિત હશે: Gas Exchange Glycolysis
મૂલ્યાંકન બનાવવામાં તેમની સામગ્રી સામેલ કરવા ચિત્રો, PDF અથવા Word દસ્તાવેજ અપલોડ કરો.
Do Plants Breathe? And the Glycolytic Pathway
All of us breathe to live — but why is breathing so essential to life? And do plants breathe too? Every activity of an organism, be it absorption, transport, movement, reproduction or even breathing, requires energy. We eat food for energy, but where does that energy actually come from, and how is it made available to a cell? This chapter answers that, and the answer turns out to be the mirror image of the chapter you have just finished.
12.1 Do Plants Breathe?
The answer is not quite so direct. Yes — plants require O₂ for respiration to occur and they also give out CO₂. Hence plants have systems in place that ensure the availability of O₂.
The key structural fact. Plants, unlike animals, have no specialised organs for gaseous exchange, but they have stomata and lenticels for this purpose.
Three reasons plants get along without respiratory organs
First — each plant part looks after itself. Each plant part takes care of its own gas-exchange needs. There is very little transport of gases from one plant part to another. A leaf does not breathe on behalf of a root.
Second — the demand is modest. Plants do not present great demands for gas exchange. Roots, stems and leaves respire at rates far lower than animals do. Only during photosynthesis are large volumes of gases exchanged, and each leaf is well adapted to take care of its own needs during these periods. Moreover, when cells photosynthesise, availability of O₂ is not a problem in these cells, since O₂ is released within the cell.
Third — nothing is far from the surface. The distance that gases must diffuse, even in large bulky plants, is not great. Each living cell in a plant is located quite close to the surface of the plant.
In-text question — ‘That is true for leaves, but what about thick, woody stems and roots?’ The objection is a fair one, and the chapter answers it directly. In stems, the ‘living’ cells are organised in thin layers inside and beneath the bark; these layers also have openings called lenticels. The cells in the interior are dead and provide only mechanical support — and a dead cell needs no oxygen. So a massive tree trunk is not a solid mass of respiring tissue at all: it is a thin sleeve of living cells wrapped around a dead core. Thus most cells of a plant have at least a part of their surface in contact with air. This is further facilitated by the loose packing of parenchyma cells in leaves, stems and roots, which provides an interconnected network of air spaces.
Why glucose is not simply burned
The complete combustion of glucose, which produces CO₂ and H₂O as end products, yields energy most of which is given out as heat.
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
Heat is useless to a cell. If this energy is to be useful, the cell must be able to utilise it to synthesise other molecules that the cell requires.
The cell's strategy, in one sentence. The plant cell catabolises the glucose molecule in such a way that not all the liberated energy goes out as heat. The key is to oxidise glucose not in one step but in several small steps, enabling some steps to be just large enough that the energy released can be coupled to ATP synthesis. How this is done is, essentially, the story of respiration.
During the process of respiration, oxygen is utilised, and carbon dioxide, water and energy are released as products. The combustion reaction requires oxygen — but some cells live where oxygen may or may not be available.
In-text question — can you think of situations and organisms where O₂ is not available? Several, and the chapter points to them. Waterlogged soils and marshes, where water fills the air spaces around roots; the deeper layers of soil and mud; the interior of a compact fruit or a bulky seed; stagnant water and the bottom sediments of ponds; the gut of animals; and inside a germinating seed before its coat splits. As for organisms, there are sufficient reasons to believe that the first cells on this planet lived in an atmosphere that lacked oxygen. Even among present-day living organisms we know of several adapted to anaerobic conditions: some of these are facultative anaerobes (they manage with or without O₂, like yeast and many bacteria), while in others the requirement for the anaerobic condition is obligate (they cannot tolerate O₂ at all, like the methanogens and Clostridium).
In any case, all living organisms retain the enzymatic machinery to partially oxidise glucose without the help of oxygen. This breakdown of glucose to pyruvic acid is called glycolysis.
12.2 Glycolysis
The name and the discoverers. The term glycolysis comes from the Greek glycos for sugar and lysis for splitting. The scheme was given by Gustav Embden, Otto Meyerhof and J. Parnas, and is often referred to as the EMP pathway.
Four facts frame the whole pathway:
- In anaerobic organisms, it is the only process in respiration.
- Glycolysis occurs in the cytoplasm of the cell.
- It is present in all living organisms — one of the most universal pathways in biology.
- In this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid.
Where the glucose comes from in a plant
In plants this glucose is derived from sucrose, which is the end product of photosynthesis, or from storage carbohydrates. Sucrose is converted into glucose and fructose by the enzyme invertase, and these two monosaccharides readily enter the glycolytic pathway.
The ten reactions, step by step
Glucose and fructose are phosphorylated to give rise to glucose-6-phosphate by the activity of the enzyme hexokinase. This phosphorylated form of glucose then isomerises to produce fructose-6-phosphate. Subsequent steps of the metabolism of glucose and fructose are the same. In glycolysis, a chain of ten reactions under the control of different enzymes takes place to produce pyruvate from glucose.
Where ATP is used
ATP is utilised at two steps: first in the conversion of glucose into glucose-6-phosphate, and second in the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate.
The splitting step
The fructose-1,6-bisphosphate is split into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL). This is the ‘lysis’ the name refers to — a 6-carbon sugar becoming two 3-carbon units. From here onward, everything happens twice per glucose.
Where NADH is formed
There is one step where NADH + H⁺ is formed from NAD⁺: when 3-phosphoglyceraldehyde (PGAL) is converted to 1,3-bisphosphoglycerate (BPGA). Two redox-equivalents are removed (in the form of two hydrogen atoms) from PGAL and transferred to a molecule of NAD⁺. PGAL is oxidised and, with inorganic phosphate, gets converted into BPGA.
Where ATP is made
The conversion of BPGA to 3-phosphoglyceric acid (PGA) is an energy-yielding process; this energy is trapped by the formation of ATP. Another ATP is synthesised during the conversion of PEP to pyruvic acid.
In-text question — how many ATP molecules are directly synthesised in this pathway from one glucose molecule? Count carefully, remembering that after the splitting step everything runs twice. ATP made: BPGA → PGA gives 1 ATP × 2 = 2 ATP; PEP → pyruvic acid gives 1 ATP × 2 = 2 ATP. So 4 ATP are directly synthesised. ATP spent: 1 at glucose → glucose-6-phosphate and 1 at fructose-6-phosphate → fructose-1,6-bisphosphate = 2 ATP (these two steps happen once each, before the split). Therefore the net gain is 4 − 2 = 2 ATP per glucose, along with 2 NADH + H⁺ and 2 molecules of pyruvic acid.
| Item | Where it happens | Amount |
|---|---|---|
| ATP consumed | Glucose → G-6-P (hexokinase); F-6-P → F-1,6-bisP | 2 |
| ATP produced | BPGA → PGA (×2); PEP → pyruvate (×2) | 4 (gross) |
| Net ATP | — | 2 |
| NADH + H⁺ produced | PGAL → BPGA (×2) | 2 |
| End product | PEP → pyruvic acid | 2 pyruvic acid (3C each) |
| Site | Cytoplasm — in every living organism | |
📐 Activity 12.1 — Follow the six carbons of glucose
What to do. Take a sheet of paper and write the carbon count beside every intermediate in Figure 12.1, from glucose down to pyruvic acid. Then answer three questions in your notebook: (i) At which single step does the carbon count per molecule change from 6 to 3? (ii) How many molecules of each 3-carbon intermediate exist per glucose after that step? (iii) Has any carbon been lost as CO₂ anywhere in glycolysis?
Predict: before you check, guess whether glycolysis releases any carbon dioxide at all.
(i) The count changes at the step where fructose-1,6-bisphosphate is split into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde. Before it, every intermediate is 6C; after it, every intermediate is 3C.
(ii) Two of each — which is exactly why the diagram writes “2 ×” in front of BPGA, PGA, 2-phosphoglycerate, PEP and pyruvic acid, and why the ATP yield of those steps must be doubled.
(iii) No carbon is lost as CO₂ in glycolysis. Add up the end products: 2 pyruvic acid × 3 C = 6 C, the same six carbons the glucose began with. This is precisely why glycolysis is described as a partial oxidation of glucose: the carbon skeleton is only split and rearranged, never fully broken down. Complete oxidation to CO₂ waits for the Krebs cycle, and it needs oxygen.
Why this matters for the next part. Since no CO₂ leaves and only 2 net ATP are made, most of the energy of the glucose is still locked up in the two pyruvate molecules. What happens to them next — fermentation or aerobic respiration — decides whether the cell extracts 2 ATP or nearly forty.
Pyruvic acid is then the key product of glycolysis. What is its metabolic fate? This depends on the cellular need. There are three major ways in which different cells handle pyruvic acid produced by glycolysis: lactic acid fermentation, alcoholic fermentation, and aerobic respiration. Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes. For the complete oxidation of glucose to CO₂ and H₂O, however, organisms adopt Krebs' cycle, which is also called aerobic respiration. This requires an O₂ supply.
🎯 Interactive: Trace any glycolytic step
Energy book-keeping: 1 ATP is used up
Glucose is phosphorylated by the enzyme hexokinase to give glucose-6-phosphate. This is the first of the two steps at which ATP is invested, and it happens only once per glucose.
🎯 Competency-Based Questions
Scenario: A biology class sets up three flasks of a yeast suspension in glucose solution. Flask 1 is bubbled continuously with air. Flask 2 is sealed under nitrogen. Flask 3 is sealed under nitrogen and, in addition, the glucose is replaced with an equal weight of sucrose. Gas production and final ATP yield per glucose are compared.
Q1. In which flask is glycolysis occurring? Justify your answer. L3 Apply
In all three. Glycolysis occurs in the cytoplasm and is present in all living organisms, and it does not require oxygen — it is the enzymatic machinery that all living organisms retain to partially oxidise glucose without the help of oxygen. Oxygen decides only what happens to the pyruvate afterwards, not whether glycolysis runs. In the anaerobic flasks glycolysis is in fact the only process in respiration.
Q2. Flask 3 was given sucrose instead of glucose, yet glycolysis proceeds normally. Explain how. L3 Apply
Sucrose is converted into glucose and fructose by the enzyme invertase, and these two monosaccharides readily enter the glycolytic pathway. Fructose, like glucose, is phosphorylated towards fructose-6-phosphate, and from that point the subsequent steps of metabolism of glucose and fructose are the same. In plants this is the normal route anyway, since the glucose respired is derived from sucrose, the end product of photosynthesis, or from storage carbohydrates.
Q3. Fill in the blanks: Glycolysis is also called the ______ pathway after ______, ______ and ______. It occurs in the ______ of the cell and converts one glucose into ______ molecules of ______ acid, with a net gain of ______ ATP and ______ NADH. L1 Remember
EMP; Gustav Embden, Otto Meyerhof, J. Parnas; cytoplasm; two; pyruvic; 2; 2.
Q4. A student argues that since combustion of glucose and respiration have the same equation, a cell might as well simply burn its glucose. Analyse the error. L4 Analyse
The overall equation is indeed the same, but an equation says nothing about the path, and the path is the entire point. In complete combustion, most of the energy is given out as heat, and heat is a form the cell cannot use to build molecules. So the cell adopts a different strategy: it catabolises glucose in such a way that not all the liberated energy goes out as heat. The key is to oxidise glucose not in one step but in several small steps, each small enough that the energy released can be coupled to ATP synthesis. The student has confused thermodynamics (how much energy) with mechanism (in what usable form). A second error follows: burning would also destroy the cell, since the reaction is uncontrolled.
Q5. “Plants do not need respiratory organs because they photosynthesise and make their own oxygen.” Evaluate this explanation. L5 Evaluate
The explanation is partly right but badly incomplete, and as a general claim it is wrong. What is right: the chapter does note that when cells photosynthesise, availability of O₂ is not a problem in these cells, since O₂ is released within the cell. What is wrong: this applies only to green, illuminated cells during photosynthesis. Roots do not photosynthesise; nor do stems' inner living layers; nor does any part of the plant at night. Yet none of these has a respiratory organ either. The real explanation rests on three separate reasons: each plant part takes care of its own gas-exchange needs with very little transport between parts; plants do not present great demands for gas exchange, respiring at rates far lower than animals; and each living cell is quite close to the surface, helped by stomata, lenticels and the interconnected air spaces between loosely packed parenchyma. A good answer credits the photosynthesis point as one convenience among several, not as the reason.
🧠 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): A thick woody tree trunk needs no elaborate system to deliver oxygen to its interior.
Reason (R): The cells in the interior of the stem are dead and provide only mechanical support, while the living cells lie in thin layers beneath the bark, which has lenticels.
Answer: A. Both are true and the reason is exactly the explanation. A dead cell has no oxygen demand, so the trunk's bulk is not respiring tissue at all.
Assertion (A): Glycolysis is a partial oxidation of glucose.
Reason (R): All six carbon atoms of glucose are retained in the two molecules of pyruvic acid, and no CO₂ is released during the pathway.
Answer: A. Two pyruvic acid molecules of three carbons each account for all six carbons, so the carbon skeleton has been split but not oxidised away — hence “partial”.
Assertion (A): Four molecules of ATP are the net gain from the glycolysis of one glucose molecule.
Reason (R): ATP is synthesised at the conversion of BPGA to PGA and again at the conversion of PEP to pyruvic acid, each occurring twice per glucose.
Answer: D. The assertion is false — four is the gross yield; 2 ATP are utilised in the two phosphorylation steps, so the net gain is 2 ATP. The reason is true and correctly accounts for the gross figure.
Coming next. Part 2 takes up Section 12.3 and the opening of 12.4 — the two kinds of fermentation and why they are both wasteful and hazardous, the yeast alcohol limit, and then the entry of pyruvate into the mitochondrion through oxidative decarboxylation to acetyl CoA.
Frequently Asked Questions - Do Plants Breathe and Glycolysis
Do plants breathe, and do they have respiratory organs?
Plants do require O2 for respiration and give out CO2, but unlike animals they have no specialised organs for gaseous exchange. They use stomata and lenticels instead, because each plant part meets its own gas needs, the demand is low, and every living cell lies close to the surface.
Why can a thick woody stem respire without any transport system for gases?
Because the living cells of a stem are organised in thin layers inside and beneath the bark, which also carries openings called lenticels, while the cells in the interior are dead and provide only mechanical support. A dead cell has no oxygen demand.
Why does a cell oxidise glucose in many small steps instead of burning it?
Complete combustion releases most of the energy as heat, which the cell cannot use to synthesise molecules. By oxidising glucose in several small steps, some steps release just enough energy to be coupled to ATP synthesis, so the energy is captured in a usable form.
What is glycolysis and who discovered it?
Glycolysis, from the Greek glycos for sugar and lysis for splitting, is the partial oxidation of glucose to two molecules of pyruvic acid. The scheme was given by Gustav Embden, Otto Meyerhof and J. Parnas, and is often called the EMP pathway.
Where does glycolysis occur and in which organisms?
It occurs in the cytoplasm of the cell and is present in all living organisms. In anaerobic organisms it is the only process in respiration, because all living organisms retain the enzymatic machinery to partially oxidise glucose without oxygen.
How many ATP are produced in glycolysis from one glucose molecule?
Four ATP are directly synthesised, two at the conversion of BPGA to PGA and two at the conversion of PEP to pyruvic acid. Two ATP are used up at the hexokinase step and at the formation of fructose-1,6-bisphosphate, so the net gain is 2 ATP, along with 2 NADH and 2 pyruvic acid.
At which step of glycolysis is NADH formed?
At a single step: the conversion of 3-phosphoglyceraldehyde, PGAL, to 1,3-bisphosphoglycerate, BPGA. Two hydrogen atoms are removed from PGAL and transferred to NAD+. Since two PGAL exist per glucose, 2 NADH + H+ are formed.
How does a plant get glucose for respiration?
From sucrose, the end product of photosynthesis, or from storage carbohydrates. Invertase converts sucrose into glucose and fructose, and both monosaccharides readily enter the glycolytic pathway, meeting at fructose-6-phosphate.
What are the three fates of pyruvic acid?
Lactic acid fermentation, alcoholic fermentation and aerobic respiration. Which one occurs depends on the cellular need and on whether oxygen is available. Only aerobic respiration, through Krebs cycle, oxidises glucose completely to CO2 and H2O.
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