Energy Metabolism in Biological Systems

Stage	Location	Key Inputs	Key Outputs	Key Enzymes or Complexes	ATP Yield (approx.)
Glycolysis	Cytosol	1 Glucose, 2 ATP, 2 NAD⁺, 4 ADP + Pᵢ	2 Pyruvate, 2 ATP (net), 2 NADH, 2 H⁺, 2 H₂O	Hexokinase, PFK-1, Pyruvate kinase, GAPDH	2 ATP (substrate-level) ~3–5 ATP (via NADH in ETC)*
Pyruvate Oxidation	Mitochondrial matrix	2 Pyruvate, 2 CoA, 2 NAD⁺	2 Acetyl-CoA, 2 NADH, 2 CO₂	Pyruvate dehydrogenase complex (PDC)	~5 ATP (via 2 NADH)
Krebs Cycle (Citric Acid Cycle)	Mitochondrial matrix	2 Acetyl-CoA (from glucose)	6 NADH, 2 FADH₂, 2 GTP, 4 CO₂	Citrate synthase, Isocitrate dehydrogenase, α-Ketoglutarate dehydrogenase, Succinate dehydrogenase	2 ATP (GTP) ~15 ATP (6 NADH) ~3 ATP (2 FADH₂)
β-Oxidation of Fatty Acids** (e.g., Palmitate, C₁₆)	Mitochondrial matrix	1 Palmitate, 7 CoA, 7 FAD, 7 NAD⁺, 1 ATP (for activation)	8 Acetyl-CoA, 7 FADH₂, 7 NADH, 7 H⁺	Acyl-CoA synthetase, Carnitine palmitoyltransferase I/II, Acyl-CoA dehydrogenase, Hydroxyacyl-CoA dehydrogenase	~106 ATP total: • 8 Acetyl-CoA → 80 ATP (via Krebs) • 7 NADH → ~17.5 ATP • 7 FADH₂ → ~10.5 ATP • Minus 2 ATP for activation Net: ~106 ATP
Oxidative Phosphorylation (ETC + Chemiosmosis)	Inner mitochondrial membrane	NADH, FADH₂ (from all prior stages), O₂, ADP + Pᵢ	H₂O, ATP	Complexes I–IV, ATP synthase (Complex V)	~2.5 ATP per NADH ~1.5 ATP per FADH₂
**Total per Glucose (Aerobic)**					**~30–32 ATP**
**Total per Palmitate (C₁₆)**					**~106 ATP**

*NADH from glycolysis yields ~1.5–2.5 ATP each depending on the shuttle (glycerol-phosphate vs. malate-aspartate).
**Fatty acid must be activated to palmitoyl-CoA in cytosol (costs 2 ATP equivalents) and transported into mitochondria via carnitine shuttle.

Energy generation in biological systems—primarily through cellular respiration—is a highly coordinated, multi-step process that converts energy-rich molecules like glucose and fatty acids into adenosine triphosphate (ATP), the universal cellular energy currency. This process occurs in several stages: glycolysis, the transition reaction (pyruvate oxidation), the Krebs cycle (also known as the citric acid cycle or tricarboxylic acid [TCA] cycle), and oxidative phosphorylation, which includes the electron transport chain (ETC) and chemiosmosis. Below is a detailed, stepwise explanation of each phase.

1. Substrates for Energy Production

The primary substrates for cellular respiration include:

• Glucose (C₆H₁₂O₆): Derived from dietary carbohydrates or glycogen breakdown.
• Fatty acids: From the hydrolysis of triglycerides in adipose tissue.
• Amino acids: From protein catabolism (less common under normal conditions).

While amino acids can feed into multiple points of energy metabolism, glucose and fatty acids are the principal fuels. Glucose is metabolized via glycolysis and the Krebs cycle, whereas fatty acids undergo β-oxidation to produce acetyl-CoA, which also enters the Krebs cycle.

2. Glycolysis (Cytoplasm)

Location: Cytosol of the cell
Net Input: 1 glucose, 2 ATP, 2 NAD⁺, 4 ADP + P_i
Net Output: 2 pyruvate, 2 ATP (net), 2 NADH, 2 H⁺, 2 H₂O

Glycolysis is an anaerobic (oxygen-independent) pathway that breaks down one molecule of glucose (6C) into two molecules of pyruvate (3C). It consists of two phases:

A. Energy Investment Phase

• Step 1: Glucose is phosphorylated by hexokinase (or glucokinase in liver) to glucose-6-phosphate (G6P), using 1 ATP.
• Step 3: Fructose-6-phosphate is phosphorylated by phosphofructokinase-1 (PFK-1)—a key regulatory enzyme—to fructose-1,6-bisphosphate, using another ATP.

B. Energy Payoff Phase

• Fructose-1,6-bisphosphate is split into two 3-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP), which is rapidly converted to G3P.
• Step 6: G3P is oxidized and phosphorylated by glyceraldehyde-3-phosphate dehydrogenase, producing 1,3-bisphosphoglycerate and reducing NAD⁺ to NADH.
• Substrate-level phosphorylation: High-energy phosphate groups are transferred directly to ADP:
  • Phosphoglycerate kinase generates ATP from 1,3-bisphosphoglycerate.
  • Pyruvate kinase catalyzes the final step, converting phosphoenolpyruvate (PEP) to pyruvate, yielding another ATP.

Net gain: 2 ATP and 2 NADH per glucose.

Fate of pyruvate:

• Under aerobic conditions: transported into mitochondria for oxidation.
• Under anaerobic conditions (e.g., in muscle during intense exercise): reduced to lactate by lactate dehydrogenase, regenerating NAD⁺ for glycolysis to continue.

3. Pyruvate Oxidation (Mitochondrial Matrix)

Before entering the Krebs cycle, pyruvate is converted to acetyl-CoA:

• Enzyme complex: Pyruvate dehydrogenase complex (PDC)—a multi-enzyme complex including E1 (pyruvate decarboxylase), E2 (dihydrolipoyl transacetylase), and E3 (dihydrolipoyl dehydrogenase).
• Reaction:
  Pyruvate + CoA + NAD⁺ → Acetyl-CoA + CO₂ + NADH + H⁺
• Key features: Irreversible; regulated by product inhibition (acetyl-CoA, NADH) and covalent modification (phosphorylation inactivates PDC).

Each glucose yields 2 acetyl-CoA, 2 NADH, and 2 CO₂.

4. Krebs Cycle (Citric Acid Cycle; Mitochondrial Matrix)

Input per acetyl-CoA: 1 acetyl-CoA
Output per acetyl-CoA:

• 3 NADH
• 1 FADH₂
• 1 GTP (≈ATP)
• 2 CO₂

Since one glucose yields two acetyl-CoA molecules, all outputs are doubled per glucose.

Key Steps:

1. Citrate formation: Acetyl-CoA condenses with oxaloacetate (4C) via citrate synthase to form citrate (6C).
2. Isomerization: Citrate → isocitrate (via aconitase).
3. First oxidative decarboxylation: Isocitrate → α-ketoglutarate (5C) by isocitrate dehydrogenase, producing NADH + CO₂.
4. Second oxidative decarboxylation: α-Ketoglutarate → succinyl-CoA (4C) by the α-ketoglutarate dehydrogenase complex (similar to PDC), yielding another NADH + CO₂.
5. Substrate-level phosphorylation: Succinyl-CoA → succinate via succinyl-CoA synthetase, producing GTP (or ATP in some cells).
6. FAD reduction: Succinate → fumarate via succinate dehydrogenase (embedded in inner mitochondrial membrane), reducing FAD to FADH₂.
7. Hydration: Fumarate → malate (fumarase).
8. Regeneration of oxaloacetate: Malate → oxaloacetate via malate dehydrogenase, producing NADH.

Regulation: Controlled by substrate availability and key enzymes (citrate synthase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase), which are inhibited by ATP, NADH, and succinyl-CoA.

5. Oxidative Phosphorylation (Inner Mitochondrial Membrane)

This final stage produces the majority (~90%) of ATP and consists of two linked processes:

A. Electron Transport Chain (ETC)

• Location: Inner mitochondrial membrane (cristae).
• Function: Re-oxidizes NADH and FADH₂, using O₂ as the final electron acceptor.
• Complexes:
  • Complex I (NADH dehydrogenase): Accepts electrons from NADH, pumps 4 H⁺.
  • Complex II (succinate dehydrogenase): Accepts electrons from FADH₂ (no proton pumping).
  • Complex III (cytochrome bc₁ complex): Transfers electrons via ubiquinone (Q) and cytochrome c; pumps 4 H⁺.
  • Complex IV (cytochrome c oxidase): Transfers electrons to O₂, forming H₂O; pumps 2 H⁺.

Overall reaction:
½O₂ + 2H⁺ + 2e⁻ → H₂O (final step at Complex IV)

As electrons move down the energy gradient through the complexes, protons (H⁺) are pumped from the matrix into the intermembrane space, creating an electrochemical proton gradient.

B. Chemiosmosis & ATP Synthesis

• Proton-motive force: The gradient (ΔpH and ΔΨ) drives H⁺ back into the matrix through ATP synthase (Complex V).
• ATP synthase: A rotary motor enzyme that uses proton flow to catalyze:
  ADP + P_i → ATP
• P/O ratios: ~2.5 ATP per NADH, ~1.5 ATP per FADH₂ (due to entry points into ETC).

Total ATP yield per glucose (approximate):

• Glycolysis: 2 ATP + 2 NADH → ~5 ATP (cytosolic NADH requires shuttles; yield varies)
• Pyruvate oxidation: 2 NADH → ~5 ATP
• Krebs cycle: 2 GTP + 6 NADH + 2 FADH₂ → 2 + 15 + 3 = ~20 ATP

Total: ~30–32 ATP per glucose molecule in eukaryotes.

6. End Products of Cellular Respiration

The complete aerobic oxidation of one glucose molecule yields:

• ~30–32 ATP (usable energy)
• 6 CO₂ (2 from pyruvate oxidation, 4 from Krebs cycle)
• 6 H₂O (formed when O₂ accepts electrons and H⁺ at Complex IV)
• Heat (as a byproduct of exergonic reactions)

For fatty acids (e.g., palmitate, C₁₆):

• Undergo β-oxidation to yield 8 acetyl-CoA, 7 NADH, and 7 FADH₂.
• Each acetyl-CoA enters Krebs cycle → net ~106 ATP per palmitate (after subtracting 2 ATP for activation).
• CO₂, H₂O
• Heat

Starting from diverse substrates like glucose and fatty acids, cells channel carbon through glycolysis, the Krebs cycle, and ultimately the electron transport chain. Energy is captured in the form of ATP primarily via oxidative phosphorylation, driven by redox reactions and a proton gradient. The process is highly efficient, tightly regulated, and essential for sustaining life—converting fuel molecules into usable energy while producing CO₂ and H₂O as harmless waste products.