Cell Biology & Energy (BIO 101) · Module 3 of 8
Glycolysis
The universal first stage of sugar breakdown: investment, payoff, net yield, and regulation.
The Investment Phase
- Glycolysis begins with two ATP investments that phosphorylate glucose and fructose-6-phosphate, trapping the sugar in the cell and destabilizing it for splitting.
- The six-carbon sugar is then split into two three-carbon molecules of glyceraldehyde-3-phosphate (G3P).
Why does the cell "invest" ATP at the start of glycolysis, seemingly working against its own energy goal?
Solution. Phosphorylating glucose traps it inside the cell (phosphorylated sugars can't cross the membrane) and raises its energy state, making the molecule unstable enough to be split apart in later steps.
The Payoff Phase
- Each G3P is oxidized and phosphorylated, producing NADH, and then substrate-level phosphorylation generates ATP directly (not via the electron transport chain) at two steps.
- Because glycolysis splits glucose into two G3P molecules, the payoff phase steps happen twice per glucose.
Why is the payoff phase's ATP and NADH yield doubled compared to what a single G3P step would produce?
Solution. Glycolysis splits one glucose molecule into two G3P molecules, so every step of the payoff phase occurs twice per original glucose molecule.
Net Yield & Substrate-Level Phosphorylation
- Per glucose, glycolysis produces a net of 2 ATP (4 made in the payoff phase minus 2 invested), 2 NADH, and 2 pyruvate molecules.
- Substrate-level phosphorylation directly transfers a phosphate group from a high-energy intermediate to ADP, distinct from the electron-transport-chain-driven phosphorylation used later.
What is the net ATP yield of glycolysis, and how is it calculated?
Solution. Net 2 ATP per glucose: 4 ATP are produced in the payoff phase (2 per G3P × 2), minus the 2 ATP invested in the investment phase.
Regulation of Glycolysis
- Phosphofructokinase (PFK), which catalyzes the first committed step, is the main regulatory enzyme of glycolysis — inhibited by high ATP and citrate levels (signs of abundant energy) and activated by AMP (a sign of low energy).
Why would high levels of ATP inhibit phosphofructokinase?
Solution. High ATP signals the cell already has plenty of energy, so inhibiting PFK slows glycolysis to avoid wastefully breaking down more glucose than needed.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What is the purpose of phosphorylating glucose at the start of glycolysis?
2. How many net ATP does glycolysis produce per glucose molecule?