Glycolysis

Glycolysis is the anaerobic breakdown of glucose (a 6-carbon sugar) into two 3-carbon pyruvate molecules. This process occurs in the cytoplasm of cells and yields a small amount of energy: a net gain of 2 ATP molecules and 2 NADH molecules per glucose.

• Nearly all living organisms perform glycolysis as the first stage of extracting energy from glucose. It does not require oxygen (glycolysis itself is anaerobic).
• The pathway uses 2 ATP in its initial 'investment' phase and produces 4 ATP in the 'payoff' phase, for a net gain of 2 ATP per glucose. It also produces 2 NADH by reducing NAD+ when glyceraldehyde-3-phosphate is oxidized.
• If oxygen is available, the pyruvate produced will be shuttled into mitochondria for further oxidation (pyruvate oxidation and Krebs cycle). If oxygen is absent, cells can divert pyruvate to fermentation pathways (like lactic acid fermentation) to regenerate NAD+ and allow glycolysis to continue.

• Location clue: A question might ask 'Where in the cell does glucose get converted to pyruvate-' The answer is the cytoplasm (glycolysis occurs in the cytosol, not in mitochondria).
• Energy accounting trap: Exams often test the net vs gross ATP yield of glycolysis. Remember: 4 ATP are produced in total, but 2 are used, so the net gain is 2 ATP per glucose (plus 2 NADH).
• Oxygen scenario: If a question describes a cell producing ATP from glucose without mitochondria or without oxygen, it-s pointing to glycolysis (and likely fermentation to recycle NAD+). Glycolysis is the universal anaerobic pathway for ATP production.