Glycolysis

Core Concepts

Location and the big picture

Glycolysis happens in the cytosol, not in the mitochondria. This matters for two reasons: it means the pathway is physically available to every cell, and it explains why red blood cells (no mitochondria) can still make ATP. The overall reaction can be summarized as:

Glucose (C₆H₁₂O₆) + 2 NAD⁺ + 2 ADP + 2 Pᵢ → 2 pyruvate (C₃H₄O₃) + 2 NADH + 2 ATP + 2 H₂O + 2 H⁺

(Textbook formulations of pyruvate's formula vary slightly — some show the ionized form — but the stoichiometry of 2 pyruvate, 2 NADH, and a net 2 ATP is standard.)

Phase 1: Energy investment (steps 1–5)

Glycolysis starts by spending energy to get the glucose ready. Two ATP molecules are hydrolyzed to add phosphate groups to the sugar:

1. Hexokinase adds the first phosphate to glucose, forming glucose-6-phosphate (this traps glucose inside the cell — the charged phosphate can't cross the membrane).
2. After a rearrangement to fructose-6-phosphate, phosphofructokinase (PFK) adds a second phosphate, forming fructose-1,6-bisphosphate. PFK is the most important regulatory step of the whole pathway — more on this below.
3. The six-carbon sugar is then split into two three-carbon molecules (glyceraldehyde-3-phosphate, G3P). From here on, everything happens twice — once for each three-carbon half.

Net effect of Phase 1: 2 ATP spent.

Phase 2: Energy payoff (steps 6–10)

Now the pathway pays dividends. For each three-carbon molecule:

• G3P is oxidized: NAD⁺ picks up electrons and a proton, becoming NADH (2 NADH total per glucose).
• A series of reactions generates ATP directly from ADP + Pᵢ through substrate-level phosphorylation — an enzyme transfers a phosphate group from a reaction intermediate onto ADP. This happens twice per three-carbon molecule, giving 4 ATP total.

Net effect of Phase 2: 4 ATP made. Overall net: 4 − 2 = 2 ATP, plus 2 NADH, per glucose.

Regulation: the cell's throttle

Glycolysis is regulated mainly at phosphofructokinase (PFK):

• High ATP and high citrate (signals of plenty) inhibit PFK — the cell slows glycolysis when it already has energy.
• High ADP and AMP (signals of need) activate PFK — the cell speeds up glycolysis when energy is low.

This is feedback inhibition: the end product of the pathway's purpose (ATP) turns the pathway off. It prevents cells from burning glucose wastefully.

The fork in the road: what happens to pyruvate

Pyruvate is a crossroads molecule:

• Aerobic conditions (O₂ present): pyruvate enters mitochondria, becomes acetyl-CoA, and feeds the citric acid cycle and oxidative phosphorylation, harvesting much more ATP (commonly taught total of ~36–38 ATP per glucose across all stages, though figures vary — verify against your text).
• Anaerobic conditions (no O₂): pyruvate is reduced in fermentation, which regenerates NAD⁺ so glycolysis can continue producing its 2 ATP.

Worked Example: Tracing One Glucose Through Glycolysis

Start with one glucose in the cytosol of a muscle cell:

1. Investment: hexokinase adds a phosphate (1 ATP spent). The sugar rearranges, and PFK adds a second phosphate (2nd ATP spent). The six-carbon compound splits into two G3P molecules.
2. Payoff, round one: each G3P is oxidized, reducing NAD⁺ to NADH (2 NADH total). Two enzymes then perform substrate-level phosphorylation, producing 2 ATP.
3. Payoff, round two: the same two enzymes process the second half, producing 2 more ATP and converting the intermediates to pyruvate.
4. Tally: 2 ATP spent, 4 ATP made → net 2 ATP; plus 2 NADH; plus 2 pyruvate.
5. Decision point: the muscle cell is sprinting and low on O₂, so pyruvate moves into lactic acid fermentation (regenerating NAD⁺) rather than the mitochondria. Glycolysis keeps running, supplying the 2 ATP per glucose that keeps the muscle contracting.

Common Confusions