Login Free Test Start Mock

Botany · Respiration in Plants

Glycolysis (EMP Pathway)

Glycolysis — formally the Embden-Meyerhof-Parnas (EMP) pathway — is the universal first stage of cellular respiration, converting one molecule of glucose into two molecules of pyruvate entirely within the cytosol. As Section 12.2 of NCERT Class 11 Biology explains, it is the only respiratory process in obligate anaerobes and the mandatory prelude to the Krebs cycle in aerobes. NEET regularly tests the two ATP-investment steps, the key enzymes, and the net energy yield; this subtopic has generated direct questions in 2019, 2022, and 2023.

NCERT Grounding

Section 12.2 of NCERT Class 11 Biology (Respiration in Plants) states: "The scheme of glycolysis was given by Gustav Embden, Otto Meyerhof, and J. Parnas, and is often referred to as the EMP pathway. In anaerobic organisms, it is the only process in respiration. Glycolysis occurs in the cytoplasm of the cell and is present in all living organisms." The NIOS Module 2 reinforces this by classifying glycolysis as the common opening stage of both aerobic and anaerobic respiration — a point that NEET exploits in assertion-reason questions.

"In this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid."

NCERT Class 11 Biology, Chapter 12 — Respiration in Plants

Overview and Location

Glycolysis is a cytosolic, oxygen-independent sequence of ten enzyme-controlled reactions. Its independence from mitochondria means the pathway operates in prokaryotes, all eukaryotic cells, and even highly reduced cells such as mature red blood cells. Because the reactions consume no molecular oxygen, glycolysis is classified as an anaerobic process — though its product, pyruvate, subsequently enters aerobic pathways when oxygen is available.

Location rule: Glycolysis occurs exclusively in the cytosol (cytoplasm) — never in the mitochondria. This is the most frequently tested location fact in NEET.

Universality

Present in: All living organisms (prokaryotes and eukaryotes)

Common to: Both aerobic and anaerobic respiration

Syllabus anchor — NCERT §12.2

Substrate

Starting molecule: Glucose (6-carbon)

In plants: Glucose derived from sucrose (via invertase) or storage carbohydrates

NCERT §12.2 — plant-specific detail

End Products

2 pyruvate (3-carbon each)

Net 2 ATP + 2 NADH per glucose

NEET 2022 — net ATP asked directly

Two Phases of Glycolysis

Glycolysis is conventionally divided into two sequential phases. The first phase consumes ATP to activate glucose; the second phase produces ATP and NADH as the activated intermediates are oxidised to pyruvate. Understanding this division is essential for correctly answering net-gain questions.

Phase 1 — Preparatory (Investment) Phase

The preparatory phase spans reactions 1 through 5 and is characterised by two phosphorylation events that consume 2 ATP per glucose. These reactions convert glucose into a more reactive, committed intermediate, fructose-1,6-bisphosphate, which is then cleaved into two 3-carbon units.

Preparatory Phase — 5 Key Steps

2 ATP consumed; no NADH produced
  1. Step 1

    Glucose → G6P

    Hexokinase phosphorylates glucose; 1 ATP consumed. First irreversible step.

    1 ATP used
  2. Step 2

    G6P → F6P

    Phosphoglucose isomerase converts glucose-6-phosphate to fructose-6-phosphate.

    Isomerisation
  3. Step 3

    F6P → F1,6BP

    Phosphofructokinase (PFK) phosphorylates F6P; 1 ATP consumed. Key regulatory step.

    1 ATP used
  4. Step 4

    F1,6BP → DHAP + G3P

    Aldolase cleaves fructose-1,6-bisphosphate into two 3-carbon molecules.

    C–C cleavage
  5. Step 5

    DHAP ⇌ G3P

    Triose phosphate isomerase converts DHAP to G3P; both 3-C units now enter pay-off phase.

    Isomerisation

Phase 2 — Pay-off (Yield) Phase

After aldolase cleaves the 6-carbon intermediate, two molecules of glyceraldehyde-3-phosphate (G3P) each proceed through five further reactions. Because both molecules follow the same path, all yields from this phase are doubled relative to a single G3P.

Pay-off Phase — 5 Key Steps (×2 per glucose)

4 ATP produced + 2 NADH per glucose
  1. Step 6

    G3P → 1,3-BPGA

    G3P dehydrogenase oxidises G3P; NAD+ reduced to NADH. Inorganic phosphate incorporated.

    2 NADH formed
  2. Step 7

    1,3-BPGA → 3-PGA

    Phosphoglycerate kinase transfers phosphate to ADP; substrate-level phosphorylation.

    2 ATP made
  3. Steps 8–9

    3-PGA → PEP

    Phosphoglycerate mutase then enolase (dehydration) convert 3-PGA to phosphoenolpyruvate.

    Rearrangement
  4. Step 10

    PEP → Pyruvate

    Pyruvate kinase transfers phosphate from PEP to ADP; final irreversible step.

    2 ATP made

Key Enzymes

Three enzymes govern the three irreversible steps of glycolysis and are preferentially tested by NEET. Knowing the substrate, product, and significance of each is mandatory.

Enzyme Reaction Catalysed Significance NEET Link
Hexokinase Glucose → Glucose-6-phosphate (G6P); 1 ATP consumed First irreversible step; traps glucose inside the cell; requires Mg²⁺ NEET 2019 Q.58 — directly asked
Phosphofructokinase (PFK) Fructose-6-phosphate → Fructose-1,6-bisphosphate; 1 ATP consumed Key regulatory (rate-limiting) enzyme of glycolysis; inhibited by ATP, citrate; stimulated by AMP NEET 2023 Q.134 — second ATP-consuming step
Aldolase Fructose-1,6-bisphosphate → DHAP + G3P (glyceraldehyde-3-phosphate) Splits the 6-carbon intermediate into two 3-carbon units; the literal "lysis" in glycolysis Often offered as a distractor for hexokinase in NEET options
Pyruvate kinase Phosphoenolpyruvate (PEP) → Pyruvate; 1 ATP made (×2 per glucose = 2 ATP) Final irreversible step; third regulatory enzyme; substrate-level phosphorylation Frequently paired with PFK in regulatory context questions
Figure 1 — Glycolysis Pathway (Simplified) Glycolysis — EMP Pathway (simplified) PREPARATORY PHASE PAY-OFF PHASE Glucose (6C) Hexokinase −1 ATP G6P (6C) Isomerase F6P (6C) Phosphofructokinase −1 ATP F-1,6-BP (6C) Aldolase (splits) DHAP (3C) G3P (3C) isomerase 2× G3P (3C each) +4 ATP, +2 NADH (Pyruvate kinase final step) 2× Pyruvate (3C) NET GAIN PER GLUCOSE 2 ATP + 2 NADH

Figure 1. Simplified EMP pathway. Red shading = preparatory (investment) phase (2 ATP consumed); green shading = pay-off (yield) phase (4 ATP + 2 NADH produced). Net yield per glucose: 2 ATP + 2 NADH.

Energy Accounting

The net energy yield of glycolysis requires careful tracking of both the investment phase (ATP consumed) and the pay-off phase (ATP and NADH produced). NEET 2022 tested this directly.

ATP Invested

2

Step 1 (hexokinase) + Step 3 (phosphofructokinase)

→

ATP Produced (substrate-level)

4

2 ATP at Step 7 (phosphoglycerate kinase) + 2 ATP at Step 10 (pyruvate kinase)

=

Net ATP Gain

2

4 produced − 2 consumed = 2 ATP net per glucose; all by substrate-level phosphorylation

Molecule Consumed Produced (gross) Net
ATP 2 (steps 1 & 3) 4 (steps 7 & 10, ×2 each) +2 ATP
NADH 0 2 (step 6, ×2) +2 NADH
Pyruvate (3C) 0 2 2 molecules

Fate of Pyruvate

NCERT identifies three principal fates for the pyruvate that exits glycolysis, each corresponding to a distinct cellular context.

Fate of Pyruvate — Aerobic vs. Anaerobic Conditions

Aerobic Respiration

Krebs cycle

in mitochondrial matrix

  • Pyruvate enters mitochondria
  • Oxidative decarboxylation to Acetyl CoA + CO₂ + NADH
  • Requires O₂ as terminal electron acceptor
  • Yields up to 36–38 ATP per glucose overall
VS

Anaerobic (Fermentation)

Cytoplasm

no mitochondrial involvement

  • Alcoholic: pyruvate → ethanol + CO₂ (yeast)
  • Lactic: pyruvate → lactic acid (bacteria, muscles)
  • NADH reoxidised to NAD⁺ to sustain glycolysis
  • Only 2 ATP net per glucose; no further ATP from fermentation itself

Worked Examples

Worked Example 1 — ATP accounting

Starting from one molecule of glucose, trace the exact steps at which ATP is consumed and produced during glycolysis. What is the net ATP yield?

ATP consumed: Step 1 — hexokinase consumes 1 ATP (glucose → G6P). Step 3 — phosphofructokinase consumes 1 ATP (F6P → F1,6BP). Total consumed = 2 ATP.
ATP produced: Step 7 — phosphoglycerate kinase produces 1 ATP × 2 (from two G3P molecules) = 2 ATP. Step 10 — pyruvate kinase produces 1 ATP × 2 = 2 ATP. Total produced = 4 ATP.
Net gain = 4 − 2 = 2 ATP per glucose molecule, all by substrate-level phosphorylation.

Worked Example 2 — Enzyme identification

Which enzyme catalyses the first irreversible step of glycolysis? How does it differ from phosphofructokinase in terms of substrate and regulatory significance?

Hexokinase catalyses the first irreversible step: glucose + ATP → glucose-6-phosphate + ADP. Its substrate is glucose (free, 6C). Phosphofructokinase (PFK) catalyses the second irreversible step and is the primary regulatory enzyme (rate-limiting step): fructose-6-phosphate + ATP → fructose-1,6-bisphosphate + ADP. PFK activity is inhibited by high ATP and citrate (signals of energy sufficiency) and stimulated by AMP (signals of energy deficit), making it the principal control point for the entire pathway. Hexokinase is inhibited by its own product (G6P), not by downstream metabolites.

Worked Example 3 — Role of aldolase

Why is the aldolase step considered the molecular origin of the word "glycolysis"?

The term glycolysis derives from the Greek glycos (sugar) + lysis (splitting). Aldolase catalyses the cleavage of the 6-carbon fructose-1,6-bisphosphate into two 3-carbon triose phosphates: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P). Triose phosphate isomerase then converts DHAP into a second molecule of G3P, so both 3-carbon products enter the pay-off phase. Without this cleavage, no net doubling of the yield molecules would occur, and the pathway could not produce 4 ATP from a 6-carbon substrate.

Common Confusion and NEET Traps

Three Key Glycolytic Enzymes — Distinguishing Features

Hexokinase

Step 1

First irreversible step

  • Substrate: Glucose (free)
  • Product: Glucose-6-phosphate
  • Consumes 1 ATP
  • Inhibited by G6P (product inhibition)
  • Tested: NEET 2019
vs

Phosphofructokinase

Step 3

Key regulatory enzyme

  • Substrate: Fructose-6-phosphate
  • Product: Fructose-1,6-bisphosphate
  • Consumes 1 ATP
  • Inhibited by ATP, citrate; stimulated by AMP
  • Tested: NEET 2023

NEET PYQ Snapshot — Glycolysis (EMP Pathway)

Three direct questions from NEET 2019, 2022, and 2023 — all drawn from Section 12.2, NCERT Class 11 Biology.

NEET 2023 — Q.134

Assertion (A): ATP is used at two steps in glycolysis.
Reason (R): First ATP is used in converting glucose into glucose-6-phosphate and second ATP is used in conversion of fructose-6-phosphate into fructose-1,6-diphosphate.

  1. Both A and R are true and R is NOT the correct explanation of A
  2. Both A and R are true and R is the correct explanation of A
  3. A is true but R is false
  4. A is false but R is true
Answer: (2)

Why: NCERT §12.2 explicitly states: "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." Both the assertion and the reason are true, and the reason correctly identifies both ATP-consuming steps with their specific enzymes (hexokinase and phosphofructokinase respectively), making it the correct explanation.

NEET 2022 — Q.127

What is the net gain of ATP when each molecule of glucose is converted to two molecules of pyruvic acid?

  1. Six
  2. Two
  3. Eight
  4. Four
Answer: (2) Two

Why: Glycolysis produces 4 ATP (substrate-level phosphorylation at steps 7 and 10, 2 per G3P × 2 G3P molecules) but consumes 2 ATP in the preparatory phase (hexokinase + phosphofructokinase). Net gain = 4 − 2 = 2 ATP. The distractor "Four" represents gross production; "Six" and "Eight" are fabricated values with no biochemical basis.

NEET 2019 — Q.58

Conversion of glucose to glucose-6-phosphate, the first irreversible reaction of glycolysis, is catalyzed by:

  1. Aldolase
  2. Hexokinase
  3. Enolase
  4. Phosphofructokinase
Answer: (2) Hexokinase

Why: Hexokinase phosphorylates glucose to glucose-6-phosphate using 1 ATP — the first committed, irreversible reaction of glycolysis. Aldolase (Step 4) cleaves F1,6BP; Enolase (Step 9) catalyses dehydration of 2-phosphoglycerate to PEP; Phosphofructokinase (Step 3) catalyses the second irreversible phosphorylation. All three are offered as distractors because they are all glycolytic enzymes.

FAQs — Glycolysis (EMP Pathway)

High-frequency conceptual questions based on NEET PYQ patterns and NCERT §12.2.

Where does glycolysis take place in the cell?

Glycolysis occurs in the cytoplasm (cytosol) of the cell. It does not require mitochondria and takes place in all living organisms, making it a universal pathway common to both aerobic and anaerobic respiration.

How many ATP molecules are consumed and produced during glycolysis?

During glycolysis, 2 ATP molecules are consumed (invested) in the preparatory phase and 4 ATP molecules are produced in the pay-off phase. The net gain is therefore 2 ATP per glucose molecule (4 produced minus 2 invested).

At which two steps is ATP consumed during glycolysis?

ATP is consumed at exactly two steps: (1) conversion of glucose to glucose-6-phosphate, catalysed by hexokinase, and (2) conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, catalysed by phosphofructokinase. Both are irreversible reactions.

What is the end product of glycolysis?

The end product of glycolysis is 2 molecules of pyruvate (pyruvic acid), each a 3-carbon compound. From one 6-carbon glucose molecule, the aldolase step splits the intermediate fructose-1,6-bisphosphate into two 3-carbon units that both ultimately yield pyruvate.

Which enzyme catalyses the first irreversible step of glycolysis?

Hexokinase catalyses the first irreversible step of glycolysis: the phosphorylation of glucose to glucose-6-phosphate using one ATP. This was the subject of NEET 2019 Q.58, where the correct answer was hexokinase (option 2).

What is the role of aldolase in glycolysis?

Aldolase cleaves fructose-1,6-bisphosphate (a 6-carbon compound) into two 3-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P or PGAL). DHAP is subsequently isomerised to G3P, so both 3-carbon units feed the pay-off phase.

Is glycolysis aerobic or anaerobic?

Glycolysis itself is an anaerobic process — it does not require oxygen. It is the common first stage of both aerobic and anaerobic respiration. In aerobic organisms, the pyruvate produced then enters mitochondria for the Krebs cycle; in anaerobic organisms, pyruvate undergoes fermentation.

Related Topics
Respiration in Plants Back to the full chapter notes — all subtopics, overview, and PYQ summary. Fermentation Anaerobic fate of pyruvate — alcoholic and lactic acid pathways, NAD⁺ regeneration. Krebs Cycle (TCA Cycle) Aerobic oxidation of acetyl CoA; NADH, FADH₂, and ATP production in the mitochondrial matrix. Electron Transport System Inner mitochondrial membrane complexes; oxidative phosphorylation and the role of O₂. Respiratory Balance Sheet Complete ATP accounting across glycolysis, Krebs cycle, and ETS for one glucose molecule. Amphibolic Pathway Why the respiratory pathway serves both anabolic and catabolic functions. Do Plants Breathe? Gas exchange in plants — stomata, lenticels, and the absence of specialised respiratory organs.