Difficulty: Intermediate | Prerequisites: Phosphagen system notes, basic understanding of enzymes and ATP.
Fast glycolysis is the anaerobic breakdown of glucose (or glycogen) into pyruvate, producing 2 net ATP and 2 NADH per glucose molecule. It kicks in when ATP demand is high and the phosphagen system can no longer keep up, typically seconds into an all-out sprint. Because oxygen is not required, it is fast but produces lactate as a by-product when NADH needs to be recycled.
Monosaccharides
The basic building blocks of carbohydrates. Single sugar molecules that cannot be broken down further. Examples: glucose, fructose, galactose. Think of these as the smallest unit of sugar your body can actually use.
Disaccharides
Two monosaccharides linked together via a glycosidic bond. Examples: sucrose (glucose + fructose), lactose (glucose + galactose), maltose (glucose + glucose). They must be enzymatically broken into monosaccharides before the body can use them.
Polysaccharides
Complex carbohydrates, meaning long chains of monosaccharides. Serve as energy storage or structural support. Key types: starch, glycogen, and cellulose.
Glycogen
The body's storage form of glucose, found in liver and muscle. Think of it as a glucose "warehouse" that can be broken down when energy demand rises.
Insulin
A hormone released by pancreatic beta cells in response to elevated blood glucose. It signals muscle and other cells to take up glucose from the blood.
AKT pathway (PI3K/AKT)
The intracellular signalling cascade triggered by insulin binding to its receptor. It activates PI3K, which makes PIP3, which recruits PDK1 and AKT. Once AKT is phosphorylated (switched on), it moves GLUT4 transporters to the cell surface so glucose can enter.
GLUT2 transporter
The glucose transporter on pancreatic beta cells. It allows glucose to enter the beta cell after eating, which ultimately triggers insulin release.
GLUT4 transporter
The glucose transporter on muscle cells (and fat cells). It is moved to the cell surface by the AKT pathway, allowing glucose uptake. Memorise GLUT4's purpose.
Glycogen phosphorylase
The enzyme that breaks glycogen into glucose for use in glycolysis. Activated by CaMK (calcium/calmodulin-dependent protein kinase) and PKA (protein kinase A).
CaMK (calcium/calmodulin-dependent protein kinase)
Activated when intracellular calcium rises. Ca2+ binds with calmodulin to activate CaMK, which in turn activates glycogen phosphorylase. Memorise: Ca2+ binds with calmodulin to activate CaMK.
PKA (protein kinase A)
Activated via the cAMP (cyclic AMP) pathway when epinephrine binds to adrenergic receptors on muscle. Increases the activity of glycogen phosphorylase. Memorise this pathway.
Phosphofructokinase (PFK)
The rate-limiting enzyme of glycolysis. It converts fructose-6-phosphate into fructose-1,6-bisphosphate. Activated by ADP and AMP when ATP demand is high. Memorise: PFK is the rate-limiting enzyme, activated by ADP.
Hexokinase
The enzyme that phosphorylates glucose as it enters the muscle cell, trapping it inside and producing glucose-6-phosphate. This is the first committed step.
Redox reaction (oxidation-reduction)
A reaction where one substance loses electrons (oxidation) and another gains electrons (reduction). Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain).
NAD+ / NADH
NAD+ is the oxidised form of nicotinamide adenine dinucleotide. It acts as an electron carrier in glycolysis. When NAD+ picks up 2 H+ ions (electrons), it is reduced to NADH. NADH must be recycled back to NAD+ to keep glycolysis running.
Lactate
The product formed when pyruvate accepts electrons from NADH (via lactate dehydrogenase). This regenerates NAD+ so glycolysis can continue under anaerobic conditions.
Pyruvate
The end product of glycolysis. Two molecules of pyruvate are produced per glucose. Under anaerobic conditions it is converted to lactate; under aerobic conditions it enters the mitochondria.
Monosaccharides: glucose, fructose, galactose. Immediate fuel, used in glycolysis or stored as glycogen.
Disaccharides: sucrose, lactose, maltose. Must be enzymatically broken into monosaccharides first.
Polysaccharides: starch, glycogen, cellulose. Glycogen is the key storage form in muscle and liver.
Di- and polysaccharides are hydrolysed into monosaccharides in the intestines, absorbed into the blood, and brought to the liver. Some are stored as glycogen; others are released into the blood, raising blood sugar.
Glucose enters pancreatic beta cells via the GLUT2 transporter and is consumed to make ATP (glucokinase phosphorylates it). ATP binds to K+ channels and closes them, so the cell becomes more positive (depolarises). Voltage-gated Ca2+ channels open, Ca2+ enters, and this leads to exocytosis of insulin into the bloodstream.
The key trigger for insulin release from pancreatic beta cells is calcium influx through voltage-gated channels.
The AKT pathway in muscle cells:
Insulin binds its receptor, activating PI3K (phosphoinositide 3-kinase), which makes PIP3 (a signalling lipid in the membrane).
PIP3 recruits PDK1 and AKT. AKT is switched on by the addition of a phosphate.
Once active, AKT moves GLUT4 transporters to the cell surface, allowing glucose entry.
AKT also stores energy (makes glycogen, fat, and protein), blocks "self-destruct" signals, and helps grow cells.
Memorise GLUT4's purpose: it is the transporter that allows glucose into muscle cells once the AKT pathway is activated.
Glycogen phosphorylase breaks glycogen into glucose for use in glycolysis. It is activated by two pathways:
CaMK pathway: Increased intracellular Ca2+ activates CaMK. Ca2+ binds with calmodulin to activate CaMK, which activates glycogen phosphorylase.
PKA pathway: Increased epinephrine (epi) activity triggers this. Epi binds to adrenergic receptors on muscle, activating the cAMP (cyclic AMP) pathway, which increases PKA activity. PKA activates glycogen phosphorylase. Memorise this.
Oxidation: a substance loses electrons. (OIL: Oxidation Is Loss.)
Reduction: a substance gains electrons. (RIG: Reduction Is Gain.)
A redox reaction is when oxidation and reduction occur together.
Applied example: Glyceraldehyde-3-phosphate dehydrogenase is an enzyme in glycolysis that turns glyceraldehyde-3-phosphate (G3P) into 1,3-bisphosphoglycerate by oxidising it. NAD+ strips 2 H+ ions off G3P and is reduced to become NADH. This happens twice per glucose (once for each G3P molecule).
Summary of outputs per glucose:
2 NADH (from oxidation)
2 ATP (net gain: 4 produced, 2 consumed in early steps)
2 pyruvate
What triggers glycolysis during a sprint:
ADP accumulates, activating creatine kinase (phosphagen system).
Epinephrine and Ca2+ rise, glycogen is broken down into glucose. ADP also activates phosphofructokinase (PFK), which controls the rate of glycolysis.
Seconds into the sprint, ATP demand is high, PFK is being allosterically activated, driving rapid-pace glycolysis. This creates lots of NADH, ATP, and pyruvate.
Step-by-step pathway:
Glucose --> Glucose-6-phosphate (enzyme: hexokinase; costs 1 ATP). Traps glucose in the cell. After this, G6P can either enter glycolysis or be stored as glycogen.
Glucose-6-phosphate --> Fructose-6-phosphate (enzyme: isomerase). A rearrangement.
Fructose-6-phosphate --> Fructose-1,6-bisphosphate (enzyme: phosphofructokinase/PFK; costs 1 ATP). This is the rate-limiting step and the committed point.
Fructose-1,6-bisphosphate --> Dihydroxyacetone phosphate (DHAP) + Glyceraldehyde-3-phosphate (G3P) (enzyme: aldolase). After this point, everything is done twice (one for each 3-carbon molecule). Triose phosphate isomerase converts DHAP to G3P.
G3P --> 1,3-bisphosphoglycerate (enzyme: glyceraldehyde-3-phosphate dehydrogenase). NAD+ strips 2 H+ to become NADH. Each G3P also gains an additional inorganic phosphate.
1,3-bisphosphoglycerate --> 3-phosphoglycerate (enzyme: phosphoglycerate kinase). ADP is phosphorylated back to ATP. Makes 2 ATP total (one per G3P), which is the payback for the 2 ATP invested.
3-phosphoglycerate --> 2-phosphoglycerate (enzyme: phosphoglycerate mutase).
2-phosphoglycerate --> Phosphoenolpyruvate (PEP) (enzyme: enolase; releases H2O in a condensation reaction). PEP still has one high-energy phosphate attached.
PEP --> Pyruvate (enzyme: pyruvate kinase). Takes the remaining phosphate group and makes ATP. This produces 2 more ATP (the net gain).
Net gain = +2 ATP (4 total produced, 2 used in the early steps).
Phosphofructokinase (PFK) is the rate-limiting enzyme. Activated by ADP. Memorise this.
When ATP demand is high and consistently being consumed, lots of ADP and Pi are produced, activating PFK and making glycolysis faster.
When lots of ATP and pyruvate are being made, glycolysis will slow down (feedback inhibition).
When ATP demand is high and no oxygen is available, pyruvate is reduced to lactate by lactate dehydrogenase. At the same time, NADH is oxidised back to NAD+. This is a redox reaction.
Why does this matter? NADH must be regenerated to NAD+ to keep glycolysis running. Without this conversion, NADH would over-accumulate and glycolysis would stop. The transfer of electrons from NADH to pyruvate converts pyruvate to lactate while regenerating NAD+ that can be used again in glycolysis.
Glycolysis net yield (per glucose)
Glucose --> 2 Pyruvate + 2 ATP (net) + 2 NADH
Hexokinase reaction
Glucose + ATP --> Glucose-6-phosphate + ADP
PFK reaction (rate-limiting step)
Fructose-6-phosphate + ATP --> Fructose-1,6-bisphosphate + ADP
G3P dehydrogenase (redox step)
G3P + NAD+ + Pi --> 1,3-bisphosphoglycerate + NADH + H+ (x2 per glucose)
Lactate formation (NAD+ recycling)
Pyruvate + NADH --> Lactate + NAD+ (catalysed by lactate dehydrogenase)
Redox example
A(2e-) + B --> A + B(2e-). A is oxidised (lost electrons), B is reduced (gained electrons).
Fast glycolysis is the dominant energy pathway during activities like a 200-400 metre sprint, high-rep weightlifting sets, or the final push up a steep hill on a bike. Any time you feel that deep muscular burn and sudden fatigue, that is lactate accumulating as a by-product of NADH recycling during anaerobic glycolysis. Understanding this system also explains why athletes train with interval work: short bursts push into the glycolytic zone, improving the body's tolerance and clearance of lactate.
Students often think lactate itself causes fatigue. The relationship is more complex: lactate accumulation is a marker of high anaerobic demand, and the associated hydrogen ion build-up (acidosis) contributes to the sensation of fatigue.
Students confuse GLUT2 and GLUT4. GLUT2 is on pancreatic beta cells (senses blood glucose). GLUT4 is on muscle cells (allows glucose uptake after insulin signalling). Different cells, different jobs.
Students sometimes believe glycolysis produces 4 ATP net. It does not: 4 ATP are produced, but 2 ATP are consumed in the early investment phase. Net gain is 2 ATP.
Students often think PFK is activated by ATP. PFK is inhibited by high ATP. It is activated by ADP and AMP, which signal that the cell needs more energy.
⚠️ Know the full 10-step glycolysis pathway, including enzyme names at each step. Especially hexokinase, PFK, and pyruvate kinase.
⚠️ PFK is the rate-limiting enzyme of glycolysis, activated by ADP. This is a very common exam question.
⚠️ Memorise GLUT4's purpose and how the AKT pathway moves it to the cell surface.
⚠️ Memorise the two pathways that activate glycogen phosphorylase: CaMK (calcium-driven) and PKA (epinephrine/cAMP-driven).
⚠️ Understand the redox step at G3P dehydrogenase: NAD+ --> NADH, happens twice per glucose.
⚠️ Understand why pyruvate is converted to lactate: to regenerate NAD+ so glycolysis can continue anaerobically.
⚠️ What triggers insulin release from pancreatic beta cells? Calcium influx through voltage-gated channels. Common exam question.
True or False: The net ATP gain from glycolysis is 4 ATP per glucose. (False. Net gain is 2 ATP.)
Fill in the blank: The rate-limiting enzyme of glycolysis is ______. (Phosphofructokinase / PFK.)
True or False: GLUT4 is the glucose transporter found on pancreatic beta cells. (False. GLUT4 is on muscle cells. GLUT2 is on pancreatic beta cells.)
Fill in the blank: Pyruvate is converted to ______ to regenerate NAD+ under anaerobic conditions. (Lactate.)
True or False: PKA activates glycogen phosphorylase via the cAMP pathway when epinephrine levels rise. (True.)
Q: Describe the sequence of events that leads to insulin release from pancreatic beta cells after a meal.
A: Glucose enters beta cells via GLUT2. Glucokinase phosphorylates it, and it is consumed to make ATP. ATP binds to K+ channels and closes them, depolarising the cell. Voltage-gated Ca2+ channels open, Ca2+ enters, and exocytosis of insulin occurs.
Q: What is the rate-limiting enzyme of glycolysis and what activates it?
A: Phosphofructokinase (PFK). It is activated by ADP (and AMP), which signal that ATP demand is high.
Q: Explain why pyruvate is converted to lactate during fast glycolysis.
A: NADH must be recycled back to NAD+ to keep glycolysis running. Lactate dehydrogenase transfers electrons from NADH to pyruvate, producing lactate and regenerating NAD+. Without this step, NAD+ would be depleted and glycolysis would halt.
Q: Describe the role of the AKT pathway in glucose uptake by muscle cells.
A: Insulin binds to the muscle cell receptor, activating PI3K, which produces PIP3. PIP3 recruits PDK1 and AKT. Once AKT is phosphorylated, it moves GLUT4 transporters to the cell surface, allowing glucose to enter. AKT also promotes glycogen, fat, and protein synthesis.
Q: Name the two pathways that activate glycogen phosphorylase and describe what triggers each.
A: (1) CaMK pathway: rising intracellular Ca2+ binds calmodulin, activating CaMK, which activates glycogen phosphorylase. (2) PKA pathway: epinephrine binds adrenergic receptors, activating the cAMP pathway, which activates PKA, which activates glycogen phosphorylase.
Q: In the glycolysis redox step, what happens to NAD+ and to glyceraldehyde-3-phosphate?
A: NAD+ is reduced to NADH (it gains 2 H+ ions/electrons). Glyceraldehyde-3-phosphate is oxidised to 1,3-bisphosphoglycerate. This is catalysed by glyceraldehyde-3-phosphate dehydrogenase and occurs twice per glucose.
This connects to the phosphagen system because glycolysis takes over as the primary ATP source once phosphocreatine stores are depleted. The two systems overlap during a sprint: the phosphagen system handles the first few seconds, and glycolysis ramps up as ADP accumulates and activates PFK.
It also connects to slow (aerobic) glycolysis, because the same glycolytic pathway produces pyruvate and NADH. The difference is what happens next: under aerobic conditions, pyruvate enters the mitochondria for oxidative phosphorylation instead of being converted to lactate.
Fast glycolysis, anaerobic glycolysis, glucose metabolism, glycolysis steps, phosphofructokinase, PFK, hexokinase, pyruvate kinase, GLUT4, GLUT2, insulin signalling, AKT pathway, PI3K, glycogen phosphorylase, CaMK, PKA, cAMP, epinephrine, redox reactions, OIL RIG, NAD+ NADH, lactate, lactate dehydrogenase, pyruvate, monosaccharides, disaccharides, polysaccharides, glycogen, glucose-6-phosphate, fructose-1,6-bisphosphate, substrate-level phosphorylation, allosteric regulation, rate-limiting enzyme