Glycolysis — Revision Notes

⚡ 30-Second Revision

Location: — Cytoplasm
Oxygen: — Anaerobic (no $O_2$ required)
Input: — 1 Glucose
Output (Net): — 2 Pyruvate, 2 ATP, 2 NADH
ATP Consumed: — 2 (Steps 1 & 3)
ATP Produced (Gross): — 4 (Steps 7 & 10, each twice)
Net ATP: — $4 - 2 = 2$
NADH Produced: — 2 (Step 6, twice)
Key Irreversible Enzymes: — Hexokinase, Phosphofructokinase-1 (PFK-1), Pyruvate Kinase
Substrate-Level Phosphorylation: — Steps 7 & 10
Redox Reaction: — Step 6 ($NAD^+ \rightarrow NADH$)

2-Minute Revision

Glycolysis is the initial, anaerobic breakdown of glucose, occurring in the cytoplasm of all cells. It's a 10-step pathway divided into two phases. The energy investment phase (Steps 1-5) consumes 2 ATP to phosphorylate glucose, converting it into two molecules of glyceraldehyde-3-phosphate.

The energy payoff phase (Steps 6-10) then generates 4 ATP via substrate-level phosphorylation (in steps 7 and 10) and 2 NADH molecules (in step 6) per glucose. The net yield is 2 ATP, 2 NADH, and 2 pyruvate molecules.

The three irreversible steps, catalyzed by Hexokinase, Phosphofructokinase-1 (PFK-1), and Pyruvate Kinase, are crucial regulatory points. Pyruvate's fate depends on oxygen: it enters the Krebs cycle aerobically or undergoes fermentation anaerobically to regenerate $NAD^+$ for glycolysis to continue.

This pathway is fundamental for immediate energy supply and serves as a metabolic hub.

5-Minute Revision

Glycolysis, meaning 'sugar splitting,' is the foundational metabolic pathway for glucose breakdown, universally present in the cytoplasm of all living cells. It is an anaerobic process, not requiring oxygen. The pathway converts one 6-carbon glucose molecule into two 3-carbon pyruvate molecules through ten enzyme-catalyzed steps.

Phase 1: Energy Investment (Steps 1-5)

1
Glucose is phosphorylated by Hexokinase (or Glucokinase) using 1 ATP to form Glucose-6-phosphate. (Irreversible)
2
Glucose-6-phosphate isomerizes to Fructose-6-phosphate by Phosphoglucose isomerase.
3
Fructose-6-phosphate is phosphorylated by Phosphofructokinase-1 (PFK-1) using 1 ATP to form Fructose-1,6-bisphosphate. (Irreversible, major regulatory step)
4
Fructose-1,6-bisphosphate is cleaved by Aldolase into Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde-3-phosphate (G3P).
5
DHAP is isomerized to G3P by Triose phosphate isomerase. (Now, two G3P molecules proceed).

Phase 2: Energy Payoff (Steps 6-10)

1
Each G3P is oxidized and phosphorylated by Glyceraldehyde-3-phosphate dehydrogenase, reducing $NAD^+$ to NADH and forming 1,3-Bisphosphoglycerate. (2 NADH produced total)
2
Each 1,3-Bisphosphoglycerate donates a phosphate to ADP, forming ATP and 3-Phosphoglycerate, catalyzed by Phosphoglycerate kinase. (2 ATP produced total via substrate-level phosphorylation)
3
Each 3-Phosphoglycerate rearranges to 2-Phosphoglycerate by Phosphoglycerate mutase.
4
Each 2-Phosphoglycerate is dehydrated by Enolase to form Phosphoenolpyruvate (PEP).
5
Each PEP donates a phosphate to ADP, forming ATP and Pyruvate, catalyzed by Pyruvate Kinase. (2 ATP produced total via substrate-level phosphorylation, Irreversible)

Net Energy Yield per Glucose:

ATP consumed: 2
ATP produced: 4
Net ATP: — 2
NADH produced: — 2

Key Takeaways for NEET:

Location: — Cytoplasm.
Oxygen: — Anaerobic.
Products: — 2 Pyruvate, 2 Net ATP, 2 NADH.
Regulatory Enzymes: — Hexokinase, PFK-1, Pyruvate Kinase (all irreversible).
Substrate-Level Phosphorylation: — Steps 7 and 10.
Fate of Pyruvate: — Aerobic (Acetyl-CoA $ightarrow$ Krebs Cycle) or Anaerobic (Fermentation $ightarrow$ Lactate/Ethanol to regenerate $NAD^+$).

Prelims Revision Notes

Glycolysis: NEET Quick Facts

1. Definition & Overview:

'Glykys' (sweet) + 'lysis' (splitting) = splitting of sugar.
Breaks down 1 glucose (6C) into 2 pyruvate (3C) molecules.
Universal pathway, occurs in virtually all living cells.

2. Location & Oxygen Requirement:

Site: — Cytoplasm (cytosol).
Oxygen: — Anaerobic process; does NOT require $O_2$.

3. Phases & Steps:

Energy Investment Phase (Steps 1-5): — Consumes ATP to phosphorylate glucose.

* Step 1: Glucose $ightarrow$ Glucose-6-P (Enzyme: Hexokinase/Glucokinase; consumes 1 ATP; irreversible) * Step 3: Fructose-6-P $ightarrow$ Fructose-1,6-bisP (Enzyme: Phosphofructokinase-1 (PFK-1); consumes 1 ATP; irreversible, rate-limiting) * Step 4: Fructose-1,6-bisP splits into DHAP + G3P (Enzyme: Aldolase) * Step 5: DHAP $ightarrow$ G3P (Enzyme: Triose phosphate isomerase; ensures 2 G3P molecules proceed)

Energy Payoff Phase (Steps 6-10): — Produces ATP and NADH.

* Step 6: G3P oxidation & phosphorylation (Enzyme: Glyceraldehyde-3-phosphate dehydrogenase; produces 1 NADH per G3P, so 2 NADH total) * Step 7: 1,3-Bisphosphoglycerate $ightarrow$ 3-Phosphoglycerate (Enzyme: Phosphoglycerate kinase; produces 1 ATP per molecule via substrate-level phosphorylation, so 2 ATP total) * Step 10: Phosphoenolpyruvate (PEP) $ightarrow$ Pyruvate (Enzyme: Pyruvate kinase; produces 1 ATP per molecule via substrate-level phosphorylation, so 2 ATP total; irreversible)

4. Net Energy Yield (per glucose molecule):

ATP Consumed: — 2
ATP Produced (Gross): — 4
Net ATP Gain: — $4 - 2 = 2$ ATP
NADH Produced: — 2
Pyruvate Produced: — 2

5. Key Enzymes & Regulation:

Irreversible Steps (Major Regulatory Points):

* Hexokinase (Step 1) * Phosphofructokinase-1 (PFK-1) (Step 3) - Most important regulatory enzyme. * Pyruvate Kinase (Step 10)

These enzymes are often allosterically regulated by ATP, ADP, AMP, citrate, etc.

6. Fate of Pyruvate:

Aerobic Conditions (presence of $O_2$): — Pyruvate $ightarrow$ Acetyl-CoA $ightarrow$ Krebs Cycle (in mitochondria).
Anaerobic Conditions (absence of $O_2$): — Fermentation.

* Lactic Acid Fermentation: Pyruvate $ightarrow$ Lactic Acid (e.g., muscle cells, some bacteria); regenerates $NAD^+$. * Alcoholic Fermentation: Pyruvate $ightarrow$ Ethanol + $CO_2$ (e.g., yeast); regenerates $NAD^+$.

7. Significance:

Provides rapid energy.
Essential for cells lacking mitochondria (e.g., RBCs).
Metabolic hub for other pathways.

Vyyuha Quick Recall

For the 10 steps of glycolysis, remember: Good Girls Find Fresh Glucose By Picking Peaches Every Peak.

Glucose
Glucose-6-phosphate
Fructose-6-phosphate
Fructose-1,6-bisphosphate
Glyceraldehyde-3-phosphate (and DHAP)
Bisphosphoglycerate (1,3-)
Phosphoglycerate (3-)
Phosphoglycerate (2-)
Enolpyruvate (Phosphoenolpyruvate, PEP)
Pyruvate