Q.Oxygen is an essential requirement for aerobic respiration but it enters the respiratory process at the end? Discuss.

The question asks only for the energy from mitochondrial NADH$_2$, i.e. the NADH$_2$ generated inside the mitochondrion (the $2$ NADH$_2$ made in cytoplasmic glycolysis are excluded).

Count the mitochondrial NADH$_2$ from one glucose:

• Pyruvate $\to$ acetyl-CoA (link reaction): $2$ pyruvate $\times\,1 = 2$ NADH$_2$.
• Krebs cycle: $2$ turns $\times\,3 = 6$ NADH$_2$.

$\text{Total mitochondrial NADH}_2 = 2 + 6 = 8.$ …

Glucose is split in glycolysis into two molecules of glyceraldehyde-3-phosphate (G-3-P). Every downstream product — pyruvate, acetyl-CoA, the reducing equivalents fed to the electron transport chain — comes in a set of two per glucose. So the ATP tally per G-3-P is exactly half the per-glucose yield.

Per-glucose count (standard convention: 1 NADH = 3 ATP, 1 FADH₂ = 2 ATP):

Concept & Intuition

Each of these three molecules enters cellular respiration at a different point, so the total ATP obtainable from each differs by how many extra energy-yielding steps precede the Krebs cycle:

• Acetyl-CoA enters the Krebs cycle directly.
• Pyruvic acid must first be oxidised to Acetyl-CoA (via pyruvate dehydrogenase), generating one extra NADH, before entering the cycle.
• PEP is converted to pyruvate first — a substrate-level phosphorylation step that directly yields 1 ATP — and then follows the same path as pyruvic acid.

Using the standard textbook ATP-equivalents (1 NADH = 3 ATP, 1 FADH$_2$ = 2 ATP, 1 GTP = 1 ATP):

Step-by-step reasoning

1. ATP from one Acetyl-CoA

   One turn of the Krebs cycle from Acetyl-CoA produces 3 NADH, 1 FADH$_2$, and 1 GTP:

   $3 \times 3 + 1 \times 2 + 1 = 9 + 2 + 1 = 12$ ATP.

2. ATP from one Pyruvic acid

   Pyruvate → Acetyl-CoA (via pyruvate dehydrogenase) produces 1 NADH = 3 ATP, and the resulting Acetyl-CoA then yields 12 ATP as above.

   Total = $3 + 12 = 15$ ATP.

3. ATP from one PEP

   PEP → Pyruvate (via pyruvate kinase) is a substrate-level phosphorylation that directly yields 1 ATP. The resulting pyruvate then yields 15 ATP as above. …

Concept. The Electron Transport System (ETS) itself doesn't produce ATP directly — it uses the high-energy electrons carried by NADH and FADH₂ to build a proton gradient that drives ATP synthase. In the standard convention used in NCERT/Indian exam biology: cytoplasmic NADH (from glycolysis) yields 2 ATP each on entering the ETS (via the glycerol phosphate shuttle), while mitochondrial NADH yields 3 ATP each and FADH₂ yields 2 ATP each.

1. Glycolysis (A). Produces 2 NADH in the cytoplasm (along with 2 ATP by substrate-level phosphorylation, which is not counted here since it doesn't come from the ETS).

2. Oxidative decarboxylation of 2 pyruvate (B). Each pyruvate → acetyl CoA + NADH (mitochondrial), giving 2 NADH total for 2 pyruvate molecules.

3. 2 Acetyl CoA through the Krebs cycle (C). Each turn of the Krebs cycle produces 3 NADH, 1 FADH₂, and 1 ATP (substrate-level, excluded here). For 2 acetyl CoA (2 turns):

Concept. One turn of the Krebs (citric acid) cycle produces the reduced coenzymes 3 NADH and 1 $\mathrm{FADH_2}$ (plus 1 GTP by substrate-level phosphorylation, which is not ETS-derived).

ETS yield (classical convention).

• Each NADH → 3 ATP ⇒ $3 \times 3 = 9$ ATP.
• Each $\mathrm{FADH_2}$ → 2 ATP ⇒ $1 \times 2 = 2$ ATP. …

Aerobic respiration extracts energy from glucose through a series of stages: glycolysis, the link reaction (pyruvate oxidation), the Krebs cycle, and the electron transport chain. To count FADH₂ molecules, we need to identify exactly where this coenzyme is reduced.

FADH₂ forms when FAD accepts two hydrogen atoms (two electrons and two protons). Unlike NADH, which appears in multiple stages, FADH₂ is produced exclusively in the Krebs cycle during a single specific reaction: the oxidation of succinate to fumarate, catalyzed by succinate dehydrogenase.

Let's trace the path from one glucose molecule:

1. Glycolysis (cytoplasm): One glucose splits into two pyruvate molecules. This stage produces 2 NADH but no FADH₂.

2. Link reaction (mitochondrial matrix): Each pyruvate is oxidized to acetyl-CoA, releasing CO₂ and producing NADH. Since we have two pyruvates, this gives 2 NADH but again no FADH₂.

3. Krebs cycle (mitochondrial matrix): Each acetyl-CoA enters the cycle. During one complete turn:

   • Isocitrate → α-ketoglutarate: produces 1 NADH
   • α-ketoglutarate → succinyl-CoA: produces 1 NADH …

Aerobic respiration is the process by which cells break down glucose in the presence of oxygen to produce a large amount of ATP. This process occurs in several stages: glycolysis, pyruvate oxidation (link reaction), the Krebs cycle (citric acid cycle), and oxidative phosphorylation (electron transport chain and chemiosmosis). ATP is generated in two main ways:

1. Substrate-level phosphorylation (SLP): A phosphate group is directly transferred from a high-energy substrate molecule to ADP to form ATP. This occurs during glycolysis and the Krebs cycle.
2. Oxidative phosphorylation: This is the major ATP-generating mechanism. Electron carriers, NADH and FADH$_2$, produced during glycolysis, pyruvate oxidation, and the Krebs cycle, donate their electrons to the electron transport chain. This creates a proton gradient across the inner mitochondrial membrane, which is then used by ATP synthase to produce ATP. The question specifically asks for ATP formed by the oxidation of NADH and FADH$_2$, which refers to this process.

To solve this problem, we will first determine the net ATP from substrate-level phosphorylation, and the total NADH and FADH$_2$ produced, for one glucose molecule. Then, we will calculate the ATP yield from the oxidation of these electron carriers, using the standard convention for eukaryotic cells (where glycolytic NADH yields 2 ATP and mitochondrial NADH yields 3 ATP, while FADH$_2$ yields 2 ATP). Finally, we will multiply these values by five for five glucose molecules.

Step-by-step Derivation:

1. ATP, NADH, and FADH$_2$ production per glucose molecule:

   Let's break down the products of each stage of aerobic respiration for one molecule of glucose:

   • Glycolysis (in cytoplasm):

     • Net ATP (by SLP): $2$ ATP
     • NADH produced: $2$ NADH

   • Pyruvate Oxidation (Link Reaction, in mitochondrial matrix): (Since one glucose yields two pyruvate molecules, this stage occurs twice per glucose.)

     • ATP (by SLP): $0$ ATP
     • NADH produced: $2 \times 1 = 2$ NADH

   • Krebs Cycle (in mitochondrial matrix): (Since one glucose yields two acetyl-CoA molecules, the cycle runs twice per glucose.)

     • ATP (by SLP, via GTP): $2 \times 1 = 2$ ATP
     • NADH produced: $2 \times 3 = 6$ NADH
     • FADH$_2$ produced: $2 \times 1 = 2$ FADH$_2$

   Now, let's sum these up for one glucose molecule:

   • Total ATP from Substrate-Level Phosphorylation (SLP):

     $2 \text{ ATP (glycolysis)} + 2 \text{ ATP (Krebs cycle)} = 4 \text{ ATP}$

   • Total NADH produced:

     $2 \text{ NADH (glycolysis)} + 2 \text{ NADH (pyruvate oxidation)} + 6 \text{ NADH (Krebs cycle)} = 10 \text{ NADH}$

   • Total FADH$_2$ produced:

     $2 \text{ FADH}_2 \text{ (Krebs cycle)} = 2 \text{ FADH}_2$

2. ATP yield from oxidation of NADH and FADH$_2$ per glucose molecule:

   The ATP yield from the oxidation of NADH and FADH$_2$ in the electron transport chain depends on the shuttle system used to transport glycolytic NADH into the mitochondria. For eukaryotic cells, the glycerol phosphate shuttle is often assumed, which results in a lower ATP yield for glycolytic NADH.

   Important

   The standard ATP yield conventions used for aerobic respiration in eukaryotes are:

   • $1$ NADH (from glycolysis, via glycerol phosphate shuttle) $\rightarrow 2$ ATP
   • $1$ NADH (from mitochondrial processes) $\rightarrow 3$ ATP …