Q.Differentiate between: Glycolysis and Krebs cycle
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Glycolysis: The First Step in Energy Extraction
Imagine you have a glucose molecule — a six-carbon sugar packed with energy. Your cells need that energy, but they can't burn glucose directly like fuel in a furnace. They need to break it down step by step, capturing the released energy in usable forms. Glycolysis is that first, universal breakdown pathway.
It happens in the cytoplasm of every living cell — bacteria, yeast, plant, or human. No oxygen required. That's why it's ancient: it worked before Earth's atmosphere had oxygen.
The Big Picture
Glycolysis takes one molecule of glucose (6 carbons) and splits it into two molecules of pyruvate (3 carbons each). Along the way, it produces a small net gain of ATP (the cell's energy currency) and NADH (an electron carrier that stores energy for later use).
The full name is the Embden-Meyerhof-Parnas (EMP) pathway, after the scientists who worked it out.
Glucose (6C)+2 NAD++2 ADP+2 Pi→2 Pyruvate (3C)+2 NADH+2 H++2 ATP+2 H2O
The Two Phases
Glycolysis has ten enzyme-catalysed steps, but they fall into two clear phases.
Phase 1: Energy Investment (Steps 1–5)
You spend 2 ATP molecules to phosphorylate glucose, making it reactive. The 6-carbon sugar is then split into two 3-carbon molecules called glyceraldehyde-3-phosphate (G3P). No energy has been earned yet — you're in the red by 2 ATP.
Phase 2: Energy Payoff (Steps 6–10)
Each G3P is oxidised and converted to pyruvate. For each G3P, you gain 2 ATP and 1 NADH. Since you started with two G3P molecules, the total gain is 4 ATP and 2 NADH.
Net result: 4 ATP earned − 2 ATP spent = 2 ATP net gain, plus 2 NADH.
Remember the net ATP as 2 — one from each of the two 3-carbon fragments after the investment phase.
Key Details for Exams
The substrate-level phosphorylation happens in two steps: one produces ATP from 1,3-bisphosphoglycerate (via phosphoglycerate kinase), the other from phosphoenolpyruvate (via pyruvate kinase). Both are direct transfers of a phosphate group to ADP — no electron transport chain involved.
The oxidation step is step 6: G3P dehydrogenase removes two hydrogens (as a hydride ion and a proton) and passes them to NAD⁺, forming NADH. This is the only oxidation step in glycolysis. …
Glycolysis occurs in the cytoplasm, needs no oxygen, breaks glucose into 2 pyruvate (net 2 ATP, 2 NADH+H+), and needs no preceding step; the Krebs cycle occurs in the mitochondrial matrix, depends on continuous ETC/oxygen activity to keep running, oxidises acetyl-CoA (from pyruvate) all the way to CO2 (per turn: 3 NADH+H+, 1 FADH2, 1 ATP), and needs the link reaction to feed it. …
Step 1. Location. Glycolysis takes place in the cytoplasm of the cell. The Krebs cycle takes place in the matrix of the mitochondrion.
Step 2. Substrate and product. Glycolysis starts with one glucose molecule (6C) and produces two molecules of pyruvic acid (3C each). The Krebs cycle starts with acetyl-CoA (2C, derived from pyruvate via the separate link reaction) and, combining it with oxaloacetate, ultimately oxidises it completely to CO2, regenerating oxaloacetate to close the cycle.
Step 3. Number of steps. Glycolysis proceeds through ten steps (in a preparatory and a pay-off phase). The Krebs cycle proceeds through nine steps per turn, and turns twice for every glucose molecule (since each glucose gives two pyruvate/acetyl-CoA).
Step 4. Oxygen dependence. Glycolysis itself needs no oxygen directly and can run under fully anaerobic conditions (as in fermentation). The Krebs cycle can only keep running as long as its NAD+/FAD+ supply is continuously regenerated by the electron transport chain, which in turn needs oxygen as its final electron acceptor - so the Krebs cycle is indirectly, but firmly, oxygen-dependent. …
Differentiating glycolysis from the Krebs cycle across location, substrate/product, number of st …
- Saying the Krebs cycle 'directly uses oxygen', when in fact none of its own nine steps consumes O2 directly - its oxygen dependence is indirect, through its reliance on the electron transport chain to keep regenerating NAD+/FAD+. …
Showing the 12 most recent of 14 on this concept.
- CBSE 2026Set ANNUAL1 markMCQQ.Glycolysis takes place in:(a) Chloroplast(b) Cytoplasm(c) Ribosome(d) Mitochondria
›Reveal solutionSolution
Glycolysis occurs in the cytoplasm (cytosol) of the cell.
Glycolysis (also called the EMP pathway, after Embden, Meyerhof, and Parnas) is a sequence of ten enzyme-catalysed reactions that convert one 6-carbon glucose molecule into two 3-carbon pyruvate molecules, with a net gain of 2 ATP and 2 NADH. Because all the enzymes of this pathway are dissolved, soluble enzymes present freely in the cytosol — and no membrane-bound organelle is needed for this stage — glycolysis takes place in the cytoplasm. This is true whether the cell subsequently respires aerobically (pyruvate then enters the mitochondria for the Krebs cycle and ETC) or ferments anaerobically (pyruvate is converted to ethanol/lactic acid, s …
- CBSE 2026Set ANNUAL1 markMCQQ.What is the site of glycolysis in a cell ?(a) Mitochondria(b) Cytoplasm(c) Nucleus(d) Chloroplast
›Reveal solutionSolution
Glycolysis occurs in the cytoplasm, so the answer is (B).
Glycolysis (the Embden-Meyerhof-Parnas or EMP pathway) is the partial oxidation of one glucose molecule into two molecules of pyruvic acid. It occurs in the cytoplasm (cytosol) of the cell and does not require oxygen. It yields a net gain of 2 ATP and 2 NADH.
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- CBSE 2026Set ANNUAL1 markMCQQ.Which of the following processes does not occur in mitochondria ?(a) Krebs cycle(b) Glycolysis(c) Electron transport chain(d) Oxidative phosphorylation
›Reveal solutionSolution
Glycolysis does not occur in mitochondria (it occurs in the cytoplasm), so the answer is (B).
Locations of the stages of aerobic respiration:
- Glycolysis — cytoplasm (does NOT occur in mitochondria).
- Krebs cycle (citric acid cycle) — mitochondrial matrix. …
- CBSE 2025Set ANNUAL1 markQ.Assertion (A) : Glycolysis take place in Cytoplasm and produce 2 ATP and 2 NADH. Reason (R) : Glycolysis is aerobic process and oxidise substrate fully. Choose the correct option using the codes given below.
›Reveal solutionSolution
Assertion (A) is a correct description of glycolysis; Reason (R) is factually wrong, since glycolysis is anaerobic and only partially oxidises the substrate.
Assertion (A): Glycolysis (the Embden-Meyerhof-Parnas pathway) does take place in the cytoplasm, converting one molecule of glucose into two molecules of pyruvic acid, with a net gain of 2 ATP and 2 NADH. This is correct.
Reason (R): Glycolysis is described here as "aerobic" and as fully oxidising the substrate. Both parts of this are wrong:
- Glycolysis occurs whether or not oxygen is present — it is essentially an anaerobic process; the fate of pyruvate afterwards (aerobic respiration in mitochondria, or anaerobic fermentation) depends on O2 availability, but glycolysis itself does not require O2. …
- CBSE 2025Set ANNUAL1 markMCQQ.Which of the following statements describe the characteristics of glycolysis? (I) In this process, three molecules of pyruvic acid are formed from one molecule of glucose. (II) CO2 is not generated in this process. (III) Two molecules of NADH2 are formed in this process.(a) I and II(b) II and III(c) I and III(d) I, II and III
›Reveal solutionSolution
Glycolysis yields 2 pyruvic acid + 2 ATP (net) + 2 NADH per glucose, with no CO2 released — so only statements II and III are correct.
Glycolysis is the sequence of ten enzyme-catalysed reactions (occurring in the cytoplasm) that breaks down one molecule of glucose (a 6-carbon sugar) into two molecules of pyruvic acid (a 3-carbon compound) — not three molecules, so statement I is incorrect.
During this pathway, no CO2 is released at any step — CO2 is only produced later, during the oxidative decarboxylation of pyruvate (link reaction) and in the Krebs cycle. So statement II, 'CO2 is not generated in this process,' is correct.
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- CBSE 2023Set ANNUAL1 markQ.Name the end product of Glycolysis.
›Reveal solutionSolution
Glycolysis converts one glucose molecule into two molecules of pyruvic acid (pyruvate), the pathway's end product, along with net 2 ATP and 2 NADH.
Glycolysis (also called the EMP pathway, after Embden, Meyerhof and Parnas) is the partial oxidation of glucose that takes place in the cytoplasm of the cell and is common to both aerobic and anaerobic respiration.
The process: glucose, a six-carbon molecule, is broken down through a series of ten enzyme-catalysed reactions. It is first phosphorylated (using ATP) and eventually split into two three-carbon molecules, which are further oxidised and dephosphorylated. The overall result is that one 6-carbon glucose molecule is converted into two molecules of a 3-carbon compound.
End product: this 3-carbon end product is pyruvic acid (pyruvate). In this process, there is a net gain of 2 ATP molecules and 2 molecules of NADH (from NAD+) per glucose molecule.
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- CBSE 2023Set ANNUAL1 markMCQQ.How many carbon atoms are found in pyruvic acid?(a) 3(b) 2(c) 4(d) 5
›Reveal solutionSolution
Pyruvic acid has the molecular formula CH3-CO-COOH, which contains 3 carbon atoms.
Glycolysis breaks down one molecule of glucose (a 6-carbon sugar) into two molecules of pyruvic acid (3-carbon each), along with a net gain of 2 ATP and 2 NADH:
Glucose (C6H12O6) → 2 Pyruvic acid (CH3-CO-COOH)
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- CBSE 2023Set ANNUAL1 markMCQQ.Match the column. Item: Cytoplasm(a) Stigma(b) Cell division(c) Platyhelminthes(d) Cochlea(e) Earthworm(f) Lactation(g) Glycolysis
›Reveal solutionSolution
Glycolysis, the universal first step of cellular respiration, occurs in the cytoplasm, so Cytoplasm is matched with Glycolysis.
Unlike the later stages of aerobic respiration (Krebs cycle and the electron transport chain, both of which occur inside the mitochondria), glycolysis is common to both aerobic and anaerobic respiration and takes place entirely in the cytosol/cytoplasm of the cell, not requiring any membrane-bound organelle. In this pathway, one glucose molecule (6C) is broken down through a series of ten enzyme-catalysed steps …
- CBSE 2022Set ANN1 markQ.Name the process which is common for both aerobic and anaerobic respiration.
›Reveal solutionSolution
Glycolysis, the breakdown of glucose to pyruvic acid in the cytoplasm, is the one process common to both aerobic and anaerobic respiration.
Respiration begins in the cytoplasm with glycolysis, in which one molecule of glucose (6C) is partially oxidised to two molecules of pyruvic acid (3C), with a net gain of 2 ATP and 2 NADH. This pathway does not require oxygen. After glycolysis, the pathway diverges: in the presence of O2 (aerobic respiration), pyruvic acid enters the mitochondrion for the TCA cycle and electron transport chain; in the abs …
- CBSE 2021Set ANNUAL1 markMCQQ.Glycolysis takes place in(a) mitochondria(b) grana(c) endoplasmic reticulum(d) cytoplasm
›Reveal solutionSolution
Glycolysis is a cytoplasmic (cytosolic) process, common to both aerobic and anaerobic respiration.
Glycolysis is the first stage of cellular respiration and takes place in the cytoplasm of the cell, independent of the presence of oxygen. During glycolysis, one molecule of glucose (6-carbon) is partially oxidised and split into two molecules of pyruvic acid (3-carbon each), with a net gain of 2 ATP (via substrate-level phosphorylation) and 2 NADH.
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- CBSE 2020Set ANNUAL1 markMCQQ._______ is the common pathway for aerobic and anaerobic respiration.(a) Krebs' cycle(b) ETS(c) Calvin cycle(d) Glycolysis
›Reveal solutionSolution
(d) Glycolysis — the common cytoplasmic pathway shared by aerobic and anaerobic respiration.
Glycolysis is the universal first stage of respiration common to both its aerobic and anaerobic forms.
Glycolysis (the Embden-Meyerhof-Parnas pathway) occurs in the cytoplasm and breaks down one molecule of glucose into two molecules of pyruvic acid, yielding a small net gain of ATP and NADH, without requiring oxygen. This pathway is common to every respiring cell: under aerobic conditions the pyruvate is subsequently oxidised via the Krebs' cycle and the electron transport system (ETS) inside mitochondria, while under anaerobic conditions the same …
- CBSE 2020Set ANNUAL1 markMCQQ.How many pyruvic acids are formed after glycolysis of one molecule of glucose ?(a) 2(b) 4(c) 6(d) 8
›Reveal solutionSolution
Glycolysis converts one molecule of glucose (6C) into two molecules of pyruvic acid (3C each), with a net gain of 2 ATP and 2 NADH.
Glycolysis (the Embden-Meyerhof-Parnas pathway) occurs in the cytoplasm and is common to both aerobic and anaerobic respiration. Glucose (a 6-carbon sugar) is phosphorylated and eventually cleaved into two 3-carbon sugar-phosphate units, which are each oxidised through a series of steps to yield pyruvic acid. Since …
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