Question

Question 4 (4 points) Provide the number of net ATP molecules produced in each of the following scenarios: A glucose molecule that does not proceed beyond glycolysis In the electron transport chain (only, do not count prior processes) after 6 NADH molecules (only) drop off their electrons For each FADH2 that brings a pair of electrons to the ETC Per glucose molecule in aerobic respiration (in the presence of oxygen)

Answer 1

A glucose molecule that does not proceed beyond glycolysis- 2 ATP molecules. (Glycolysis is divided into two phases- preparatory and pay-off. During preparatory phase, 2 ATP molecules are consumed to produce glyceraldehyde 3-phosphate. Pay-off phase is the stage, where energy is generated in the form of 4 ATP molecules. Thus, a total of 2 ATP molecules are produced in glycolysis).

In the electron transport chain after 6 NADH molecules (only) drop off their electrons- 15 ATP molecules. (NADH in the electron transport chain, donates 2 electrons to Complex-I, the electrons are then transferred to Complex-III and Complex-IV, and then to oxygen. In the process, protons are pumped across the inter-membrane space. Complex-I and Complex-III pumps 4 protons each, and Complex-IV pumps 2 protons. Thus, a total of 10 protons are pumped, for one molecule of NADH. These protons create an electro-chemical gradient, which is used to drive ATP synthesis. The 10 protons flow back into the matrix through ATP synthase, which then makes ATP. As 4 protons are moved into the matrix, 1 ATP is made. Therefore, from 10 protons, 2.5 ATP will be made. Thus, for 1 NADH, 2.5 ATP are made. Hence, for 6 NADH, total number of ATP synthesized will be 15).

For each FADH2 that brings a pair of electrons to the ETC- 1.5 ATP molecules. (FADH2, in the electron transport chain, donates electrons to Complex-II. Electrons are then transferred to Complex-III and Complex-IV, and finally to oxygen. The transfer of electrons leads to proton pumping across the inter-membrane space. 4 protons are pumped through complex-III and 2 protons through complex-IV, thus, a total of 6 protons are pumped. These 6 protons will then flow through ATP synthase into the matrix, and ATP will be made. Flowing back of 4 protons makes 1 ATP molecule, therefore, 6 protons will make 1.5 ATP molecules. Hence, from one FADH2 molecule, 1.5 ATP molecules are made).

Per glucose molecule in aerobic respiration- 32 ATP molecules. (Aerobic respiration consists of Glycolysis, Pyruvate Dehydrogenase Complex (PDC) and Krebs cycle. At the end of aerobic respiration, energy is produced in the form of ATP, NADH and FADH2. NADH and FADH2 donate electrons to the ETC, from which, more ATP molecules are made. The number of ATP molecules made during different steps of aerobic respiration, has been summarised in the table given below-

STEPS	NUMBER OF ATP	NUMBER OF NADH	NUMBER OF FADH2
GLYCOLYSIS	2	2	-
PDC	-	2	-
KREB'S CYCLE	2	6	2
TOTAL	4	10	2

In the electron transport chain, 10 NADH will produce (10 x 2.5)= 25 ATP molecules and 2 FADH2 will produce (2 x 1.5)= 3 ATP molecules.

Thus, total number of ATP produced at the end of aerobic respiration is = 4+25+3= 32).