6 min readPathway sheetOften a 10-mark answer

Glycolysis

Glucose is split into 2 pyruvate in the cytoplasm, giving a small, fast burst of energy, with or without oxygen.

WhereCytoplasm, all cells

Start → EndGlucose → 2 pyruvate

Net gain2 ATP + 2 NADH

Control pointPFK-1 (step 3)

The 10 steps

Steps 1–5 spend energy. Steps 6–10 earn it, and run twice per glucose. Starred steps are one-way and set the speed.

1★
Glucose → Glucose-6-P
Hexokinase · uses 1 ATP
2
Glucose-6-P → Fructose-6-P
Phosphohexose isomerase
3★
Fructose-6-P → Fructose-1,6-bisP
Phosphofructokinase-1 (PFK-1) · uses 1 ATP · rate-limiting
4
Fructose-1,6-bisP → DHAP + G-3-P
Aldolase · splits 6C into two 3C
5
DHAP ⇌ G-3-P
Triose phosphate isomerase
6
G-3-P → 1,3-Bisphosphoglycerate
Glyceraldehyde-3-P dehydrogenase · makes NADH
7
1,3-BPG → 3-Phosphoglycerate
Phosphoglycerate kinase · makes ATP
8
3-PG → 2-Phosphoglycerate
Phosphoglycerate mutase
9
2-PG → Phosphoenolpyruvate
Enolase · blocked by fluoride
10★
Phosphoenolpyruvate → Pyruvate
Pyruvate kinase · makes ATP

★ one-way, controls the pace

Used (steps 1, 3)− 2 ATP

Made (steps 7, 10, twice each)+ 4 ATP

Net per glucose2 ATP + 2 NADH

With oxygenEnters mitochondria and goes on to the TCA cycle.

Without oxygenBecomes lactate (lactate dehydrogenase), freeing NAD+ so glycolysis keeps going.

Red blood cells have no mitochondria, so they rely on glycolysis alone.

Nutrition link

NAD+ (step 6) is made from niacin, vitamin B3. The kinases need magnesium to work with ATP.

Exam angle

10 marks: “Describe glycolysis with its energetics.” Draw the pathway, name the 10 enzymes, add the energy count and one line on significance.

2 marks: “Why is fluoride added to blood glucose tubes?” It blocks enolase, so the cells in the sample stop using up glucose.

Remember it

"Goodness Gracious, Father Franklin Did Go By Picking Pumpkins (to) Prepare Pies": the 11 molecules from glucose to pyruvate, in order.