From one glucose, during the prep reaction,
a. how many ATPs are formed during substrate-level ATP synthesis?
b. how many ATPs are formed during oxidative phosphorylation?
The correct answer and explanation is:
a. Substrate-Level ATP Synthesis
During the preparatory reaction of cellular respiration, which includes glycolysis and the conversion of pyruvate to Acetyl-CoA, 2 ATPs are produced through substrate-level phosphorylation.
Glycolysis occurs in the cytoplasm, where glucose (a 6-carbon molecule) is broken down into two molecules of pyruvate (3 carbons each). During this process, two ATP molecules are invested at the beginning of the pathway, but four ATP molecules are generated later through substrate-level phosphorylation, resulting in a net gain of 2 ATPs.
However, in the preparatory phase (including the conversion of pyruvate to Acetyl-CoA), no further ATP is generated directly through substrate-level phosphorylation. The key role here is the decarboxylation of pyruvate, which forms Acetyl-CoA while generating 1 NADH per pyruvate (so, 2 NADH total per glucose). The production of ATP directly in the preparation reaction does not occur.
b. ATPs Formed During Oxidative Phosphorylation
Oxidative phosphorylation occurs in the mitochondria, specifically in the electron transport chain (ETC) and ATP synthase. The electron transport chain uses electrons from NADH and FADH2 (produced earlier in glycolysis, the preparatory reaction, and the Krebs cycle) to create a proton gradient across the mitochondrial membrane, which drives ATP synthesis via chemiosmosis.
For each glucose molecule (which generates 2 NADH during glycolysis and 2 NADH and 2 FADH2 during the Krebs cycle), a total of about 32 to 34 ATPs are produced during oxidative phosphorylation.
Here’s a breakdown:
- NADH generates 2.5 ATP each through the electron transport chain, contributing about 20 ATPs from NADH.
- FADH2 generates 1.5 ATP each, contributing about 4 ATPs from FADH2.
Thus, the total ATP generated during oxidative phosphorylation from one glucose molecule is 32 to 34 ATPs.
In summary:
- Substrate-level ATP synthesis: 2 ATPs (via glycolysis).
- Oxidative phosphorylation: 32-34 ATPs (via the electron transport chain and ATP synthase).