Glycolysis (EMP Pathway)
Glycolysis is the first and universal pathway of cellular respiration. It operates in all living organisms—plants, animals, fungi, bacteria—and functions under both aerobic and anaerobic conditions.
Definition: Glycolysis (Greek: glykos = sugar, lysis = splitting) is the partial oxidation of one glucose molecule (6C) into two molecules of pyruvic acid (3C each).
Location: Occurs in the cytoplasm (cytosol) and does not require oxygen.
EMP Pathway: Named after Embden, Meyerhof, and Parnas, who elucidated its steps.
Phases of Glycolysis
Glycolysis consists of 10 enzyme-controlled steps, divided into two functional phases:
Preparatory Phase (Energy Investment Phase)
In this phase, glucose is activated by phosphorylation.
- ATP Consumed: 2 ATP
- Purpose: Make glucose more reactive and prepare it for breakdown
Key Steps:
- Step 1: Glucose → Glucose-6-phosphate (enzyme: Hexokinase, 1 ATP used)
- Step 3: Fructose-6-phosphate → Fructose-1,6-bisphosphate (enzyme: Phosphofructokinase, 1 ATP used)
- Step 4: Fructose-1,6-bisphosphate (6C) splits into:
- PGAL (Glyceraldehyde-3-phosphate)
- DHAP (Dihydroxyacetone phosphate)
- DHAP is converted into PGAL → 2 molecules of PGAL formed
Key Point: This phase uses energy but is essential for later ATP generation.
Payoff Phase (Energy Generation Phase)
This phase converts the two PGAL molecules into two molecules of pyruvic acid.
- ATP Produced: 4 ATP
- Reducing Power Produced: 2 NADH + H⁺
Key Steps:
- Step 6: PGAL → 1,3-bisphosphoglycerate
- NAD⁺ is reduced to NADH (oxidation step)
- Step 7: 1,3-bisphosphoglycerate → 3-phosphoglyceric acid
- ATP produced (substrate-level phosphorylation)
- Step 10: PEP → Pyruvic acid
- ATP produced (substrate-level phosphorylation)
Key Point: This phase releases energy in the form of ATP and NADH.
Net Gain from Glycolysis (Per Glucose Molecule)
- ATP used: 2
- ATP produced: 4
- Net ATP gain: 2 ATP
- NADH produced: 2 NADH + H⁺
- End product: 2 Pyruvic acid molecules
Fate of Pyruvic Acid
The fate of pyruvate depends on oxygen availability:
- Aerobic conditions: Converted to Acetyl-CoA → enters Krebs cycle
- Anaerobic (plants/yeast): Ethanol + CO₂
- Anaerobic (animals): Lactic acid
Significance of Glycolysis
- Common pathway for aerobic and anaerobic respiration
- Occurs even in cells lacking mitochondria (e.g., RBCs)
- Supplies metabolic intermediates for amino acids, fats, etc.
- Produces ATP via substrate-level phosphorylation
Important: Glycolysis is a partially anaerobic pathway occurring in the cytoplasm of all living organisms.
Memory Capsules – Glycolysis at a Glance
- Universal pathway → occurs in all cells
- Location: Cytoplasm
- No O₂ needed
- Glucose (6C) → 2 Pyruvate (3C)
- ATP used: 2 | ATP made: 4 | Net: 2 ATP
- NADH formed: 2
- Key enzymes: Hexokinase, Phosphofructokinase (PFK)
- ATP formed by: Substrate-level phosphorylation
Exam Hack: PFK is the rate-limiting and regulatory enzyme of glycolysis.
💡 Questions and Answers
Q1. What is substrate-level phosphorylation? Give examples from glycolysis.
A1: Substrate-level phosphorylation is the direct formation of ATP by transfer of a phosphate group from a high-energy intermediate to ADP. It does not involve the electron transport chain.
Examples from glycolysis:
- 1,3-bisphosphoglycerate → 3-phosphoglyceric acid
- Phosphoenolpyruvate (PEP) → Pyruvic acid
Q2. Where does glycolysis occur and does it require oxygen?
A2: Glycolysis occurs in the cytoplasm of the cell and is independent of oxygen, so it functions under both aerobic and anaerobic conditions.
Q3. What is the net gain of ATP and NADH in glycolysis?
A3: From one glucose molecule, glycolysis gives a net gain of 2 ATP and 2 NADH after accounting for ATP used in the preparatory phase.
Q4. Why is glycolysis called a universal pathway?
A4: Because it occurs in all living organisms and is the first step of both aerobic and anaerobic respiration.
Q5. Mention any two significances of glycolysis.
A5:
- It provides ATP even in absence of oxygen.
- It supplies intermediates for other metabolic pathways.