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Glycolysis Summary

• Glycolysis is the anaerobic oxidation of glucose, occurring in all tissues when oxygen is limited (anaerobic conditions).
• It is the sole energy source for cells without mitochondria.
• The process takes place in the cytoplasm.

Aerobic Conditions

• The reaction involves 10 steps.
• Results in 2 pyruvate molecules, 4 ATP molecules, and 2 NADH molecules (if sufficient oxygen for the Krebs cycle).

Anaerobic Conditions

• The reaction involves 11 steps.
• Results in 2 lactate (lactic acid) molecules and 2 ATP molecules (when oxygen is limited).
• If oxygen supply is restored, lactate is oxidized into pyruvate and enters the Krebs cycle.

Glycolysis Steps (Phase 1 - Preparatory Phase)

• Steps 1 through 5 utilize 2 ATP, and produce 2 identical triose phosphate molecules.

Glycolysis Steps (Phase 2 - Energy Payoff Phase)

• Steps 6 through 10 produce 4 ATP and 2 NADH from 2 triose phosphates, resulting in 2 pyruvate molecules.

Fermentation (Anaerobic)

• Yeast and some microorganisms can convert glucose to ethanol and CO2.
• Humans and other higher organisms do not use fermentation to process glucose.

Phosphorylation - Step 1

• Hexokinase (or glucokinase) phosphorylates glucose using ATP, creating glucose-6-phosphate (glucose-6-P).
• This reaction is irreversible.

Isomerization - Step 2

• Phosphoglucoisomerase converts glucose-6-P to fructose-6-P.

Second Phosphorylation - Step 3

• Phosphofructokinase-1 phosphorylates fructose-6-P to fructose-1,6-bisphosphate, consuming another ATP.
• This reaction is irreversible and a major regulatory step in glycolysis.

Cleavage - Step 4

• Aldolase cleaves fructose-1,6-bisphosphate into two 3-carbon molecules: glyceraldehyde-3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP).

Isomerization - Step 5

• Triose phosphate isomerase converts DHAP into GAP.
• Now, there are two molecules of glyceraldehyde-3-phosphate, which can proceed to phase two of glycolysis.

Oxidation - Step 6

• Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) oxidizes glyceraldehyde-3-phosphate; NAD+ is reduced to NADH and a high-energy intermediate is formed (1,3-bisphosphoglycerate).

Dephosphorylation - Step 7

• Phosphoglycerate kinase transfers a phosphate group from 1,3-bisphosphoglycerate to ADP, forming ATP.
• This is substrate-level phosphorylation.

Transfer - Step 8

• Phosphoglycerate mutase converts 3-phosphoglycerate into 2-phosphoglycerate.

Dehydration - Step 9

• Enolase removes water from 2-phosphoglycerate, forming phosphoenolpyruvate (PEP).

Dephosphorylation - Step 10

• Pyruvate kinase transfers a phosphate group from PEP to ADP, forming a second ATP.
• This is another substrate-level phosphorylation reaction, and the final step of glycolysis.

Lactate Formation (Anaerobic)

• In the absence of oxygen, pyruvate is reduced to lactate by lactate dehydrogenase, regenerating NAD+.
• This allows glycolysis to continue.

Glycolysis Summary (Aerobic)

• Net yield of energy per glucose molecule: 2 ATP, 2 NADH
• Location: Cytoplasm

Glycolysis Summary (Anaerobic)

• Net yield of energy per glucose molecule: 2 ATP
• Location: Cytoplasm