What is the net gain of ATP molecules produced directly by glycolysis?
A. 2
B. 4
C. 6
The correct answer and explanation is :
The correct answer is A. 2.
Explanation:
Glycolysis is the first step of cellular respiration, taking place in the cytoplasm of the cell. It involves the breakdown of one molecule of glucose (a 6-carbon sugar) into two molecules of pyruvate (3 carbons each). In the process, a small amount of energy is produced, which is stored in the form of ATP and NADH.
Here’s a step-by-step breakdown of ATP production in glycolysis:
- Energy Investment Phase:
The process starts by “investing” 2 ATP molecules to activate glucose. These ATPs are used to phosphorylate glucose and its intermediate product, fructose-1,6-bisphosphate, which helps in splitting the molecule later in the pathway. - Energy Payoff Phase:
After the initial investment, the pathway leads to the formation of two molecules of glyceraldehyde-3-phosphate (G3P), which is then converted into pyruvate. In this phase, 4 ATP molecules are produced through substrate-level phosphorylation (direct transfer of a phosphate group from a substrate to ADP, forming ATP).
- Step 7: 1 ATP is produced per G3P molecule, and since there are two molecules of G3P, this results in 2 ATP.
- Step 10: Similarly, 1 ATP is produced per G3P molecule, contributing 2 more ATP.
- Net Gain of ATP:
Glycolysis results in a total of 4 ATP molecules produced, but since 2 ATP molecules were consumed in the investment phase, the net gain of ATP is 4 – 2 = 2 ATP molecules.
Additionally, glycolysis also produces 2 NADH molecules, which can later be used in the electron transport chain to generate more ATP in the presence of oxygen, but these do not contribute directly to the ATP yield from glycolysis itself.
Therefore, the net gain of ATP molecules produced directly by glycolysis is 2.