Both yeast and mammalian muscle cells utilize anaerobic respiration to generate ATP when oxygen is absent. However, the specific processes and end products differ.

Yeast cells undergo alcoholic fermentation. Pyruvate is converted to ethanol and carbon dioxide. This process is carried out by yeast cells, such as Saccharomyces cerevisiae, and is used in the production of alcoholic beverages and bread. The end products are ethanol (CH₃CH₂OH) and carbon dioxide (CO₂).

Mammalian muscle cells undergo lactate fermentation. Pyruvate is converted to lactate. This process occurs in muscle cells when oxygen supply is limited. The end product is lactate (CH₃COO^-). This process is essential for providing a quick source of ATP during intense exercise, although it is not sustainable for prolonged periods.

Here's a comparison table:

Glycolysis is the breakdown of glucose into two molecules of pyruvate. It can be divided into two main phases: the energy-investment phase and the energy-payoff phase.

Energy-Investment Phase:

• Glucose is phosphorylated to glucose-6-phosphate (G6P) by hexokinase (or glucokinase in the liver). This requires one ATP molecule.
• G6P is isomerized to fructose-6-phosphate (F6P) by phosphoglucose isomerase.
• F6P is phosphorylated to fructose-1,6-bisphosphate (F1,6BP) by phosphofructokinase. This is a key regulatory step and requires another ATP molecule.

Energy-Payoff Phase:

• F1,6BP is cleaved into two 3-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP) by aldolase. DHAP is isomerized to G3P by triose phosphate isomerase.
• G3P is oxidised and phosphorylated to 1,3-bisphosphoglycerate by glyceraldehyde-3-phosphate dehydrogenase. This generates one NADH per G3P molecule.
• 1,3-bisphosphoglycerate transfers a phosphate group to ADP, producing ATP and 3-phosphoglycerate by phosphoglycerate kinase.
• 3-phosphoglycerate is converted to 2-phosphoglycerate by phosphoglycerate mutase.
• 2-phosphoglycerate is dehydrated to phosphoenolpyruvate (PEP) by enolase.
• PEP transfers a phosphate group to ADP, producing ATP and pyruvate by pyruvate kinase.

Net Gain: The net gain of ATP is 2 molecules (4 ATP produced - 2 ATP invested). Two molecules of NADH are also produced per glucose molecule.

Importance: Glycolysis is the first stage of cellular respiration and occurs in the cytoplasm. It provides pyruvate, which can then enter the Krebs cycle (citric acid cycle) for further oxidation and ATP production. It also generates reducing power (NADH) which is used in the electron transport chain.