Glycolysis

Glycolysis is the first stage of both aerobic and anaerobic respiration. It takes place in the cytoplasm of all living cells and does not require oxygen. The word "glycolysis" literally means "sugar splitting" (from Greek: glykys = sweet, lysis = splitting). During glycolysis, one molecule of glucose (a 6-carbon hexose sugar) is partially oxidised to produce two molecules of pyruvate (a 3-carbon compound), with a small net yield of ATP and reduced NAD.

Key Definition: Glycolysis is the metabolic pathway in which one molecule of glucose (6C) is converted into two molecules of pyruvate (3C) in the cytoplasm, yielding a net gain of 2 ATP and 2 reduced NAD (NADH + H⁺).

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Why the Cytoplasm?

Glycolysis occurs in the cytoplasm because:

• Glucose enters the cell via facilitated diffusion through GLUT (glucose transporter) proteins in the cell surface membrane.
• The enzymes of glycolysis are free in the cytoplasm (not membrane-bound).
• Glycolysis evolved before mitochondria existed — it is an ancient pathway found in all domains of life (Bacteria, Archaea, Eukarya).

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Steps of Glycolysis

Glycolysis can be divided into two main phases: the energy investment phase and the energy payoff phase.

Phase 1: Energy Investment (Phosphorylation)

1. Phosphorylation of glucose — Glucose (6C) is phosphorylated by ATP to form glucose-6-phosphate (6C). This reaction is catalysed by the enzyme hexokinase. One molecule of ATP is hydrolysed to ADP.

2. Isomerisation — Glucose-6-phosphate is rearranged (isomerised) to form fructose-6-phosphate (6C) by the enzyme phosphoglucose isomerase.

3. Second phosphorylation — Fructose-6-phosphate is phosphorylated by a second ATP molecule to form fructose-1,6-bisphosphate (also called hexose bisphosphate, 6C). This is catalysed by phosphofructokinase (PFK), the key regulatory enzyme of glycolysis.

Exam Tip: Two molecules of ATP are used in the energy investment phase. This "primes" the glucose molecule, making it more reactive and easier to split.

Phase 2: Lysis (Splitting) and Energy Payoff

4. Splitting of hexose bisphosphate — Fructose-1,6-bisphosphate (6C) is split by the enzyme aldolase into two molecules of triose phosphate (glyceraldehyde-3-phosphate, also known as GALP or G3P, each 3C).

5. Oxidation of triose phosphate — Each triose phosphate is oxidised by the removal of hydrogen atoms. The hydrogen is transferred to the coenzyme NAD⁺, reducing it to NADH + H⁺ (reduced NAD). This step is catalysed by triose phosphate dehydrogenase. At the same time, an inorganic phosphate (Pi) is added to form 1,3-bisphosphoglycerate (3C).

6. Substrate-level phosphorylation (first) — A phosphate group is transferred from 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate (3C). This is catalysed by phosphoglycerate kinase. Since there are two triose phosphates, 2 ATP are produced.

7. Further reactions — 3-phosphoglycerate is converted to 2-phosphoglycerate and then to phosphoenolpyruvate (PEP) through a series of enzyme-catalysed steps including a dehydration reaction.

8. Substrate-level phosphorylation (second) — A phosphate group is transferred from PEP to ADP, forming ATP and pyruvate (3C). This is catalysed by pyruvate kinase. Again, since there are two molecules, 2 ATP are produced.

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Summary of Glycolysis Products

For every one molecule of glucose entering glycolysis: