Respiration

1. Need for Energy and ATP

• ATP as Universal Energy Currency:

  • Adenosine triphosphate (ATP) supplies energy in small, manageable quantities for cellular processes such as active transport, muscle contraction, and anabolic reactions.

  • Hydrolysis of ATP:
    $ATP + H<em>2O \rightarrow ADP + P</em>i + 30.5\,kJ\,mol^{-1}$

• Synthesis of ATP:

  • Substrate-level phosphorylation: Direct transfer of a phosphate group from a substrate donor molecule to ADP.

  • Oxidative phosphorylation: Synthesis of ATP using energy derived from the electron transport chain and chemiosmosis in mitochondria.

2. Glycolysis

• Location: Cytoplasm of the cell.

• Process:

  1. Phosphorylation: Glucose ($6C$) is phosphorylated by $2\,ATP$ to form hexose bisphosphate ($6C$), making it more reactive and soluble.

  2. Splitting: Hexose bisphosphate is split into two molecules of triose phosphate ($3C$).

  3. Oxidation and ATP formation: Each triose phosphate is oxidized to pyruvate ($3C$).

  • Hydrogen atoms are removed and transferred to NAD, forming $2\text{ reduced NAD}$ per glucose.

  • Four molecules of ATP are produced by substrate-level phosphorylation.

• Net Yield per Glucose:

  • $2\,ATP$ (net gain)

  • $2\text{ reduced NAD}$

  • $2\text{ pyruvate}$

3. The Link Reaction

• Location: Mitochondrial matrix.

• Process:

  • Pyruvate is actively transported from the cytoplasm into the mitochondrial matrix.

  • Pyruvate ($3C$) undergoes decarboxylation (removal of $CO_2$) and dehydrogenation (removal of hydrogen to reduce NAD).

  • The resulting acetyl group ($2C$) combines with Coenzyme A (CoA) to form acetyl CoA.

• Reaction Equation:
  $\text{Pyruvate} + \text{CoA} + \text{NAD}^+ \rightarrow \text{Acetyl CoA} + \text{CO}_2 + \text{reduced NAD}$

4. The Krebs Cycle

• Location: Mitochondrial matrix.

• Process:

  1. Acetyl CoA ($2C$) combines with oxaloacetate ($4C$) to form citrate ($6C$); CoA is released and reused.

  2. Citrate ($6C$) is decarboxylated and dehydrogenated in a series of reactions to form a $5C$ intermediate, releasing $CO_2$ and reduced NAD.

  3. The $5C$ compound is further decarboxylated and dehydrogenated to regenerate oxaloacetate ($4C$).

• Yield per Turn of Cycle (per acetyl CoA):

  • $2\,CO_2$

  • $3\text{ reduced NAD}$

  • $1\text{ reduced FAD}$

  • $1\,ATP$ (via substrate-level phosphorylation)

5. Oxidative Phosphorylation

• Location: Inner mitochondrial membrane (cristae).

• Steps:

  1. Reduced NAD and reduced FAD deliver hydrogen atoms (protons and electrons) to the electron transport chain (ETC).

  2. High-energy electrons move along a series of electron carriers, releasing energy at each transfer step.

  3. This energy is used to pump protons ($H^+$) from the matrix across the inner membrane into the intermembrane space.

  4. A high concentration gradient of protons builds up in the intermembrane space (proton motive force).

  5. Protons diffuse back into the matrix down their electrochemical gradient through ATP synthase channels (chemiosmosis).

  6. The flow of protons drives ATP synthase to synthesize ATP from ADP and $P_i$

  7. Role of Oxygen: Oxygen acts as the final electron acceptor at the end of the ETC, combining with protons and electrons to form water:
     $\frac{1}{2}O<em>2 + 2H^+ + 2e^- \rightarrow H</em>2O$

6. Anaerobic Respiration

• In the absence of oxygen, oxidative phosphorylation, the Krebs cycle, and the link reaction stop because reduced NAD and reduced FAD cannot be re-oxidized.

• Glycolysis continues only if reduced NAD is re-oxidized to free up $\text{NAD}^+$.

• Lactate Pathway (Mammals):

  • Pyruvate is reduced to lactate by lactate dehydrogenase, using reduced NAD.

  • $\text{Pyruvate} + \text{reduced NAD} \rightarrow \text{Lactate} + \text{NAD}^+$

• Alcoholic Fermentation (Yeast and Plants):

  • Pyruvate is decarboxylated by pyruvate decarboxylase to form ethanal, releasing $CO_2$

  • Ethanal is then reduced to ethanol by alcohol dehydrogenase, re-oxidizing reduced NAD.

  • $\text{Pyruvate} \rightarrow \text{Ethanal} + CO_2$

  • $\text{Ethanal} + \text{reduced NAD} \rightarrow \text{Ethanol} + \text{NAD}^+$

7. Respiratory Substrates and Respiratory Quotient (RQ)

• Respiratory Substrates:

  • Lipids yield the highest amount of energy per unit mass because they contain a higher proportion of hydrogen atoms per molecule compared to carbohydrates, producing more reduced NAD and reduced FAD for oxidative phosphorylation.

• Respiratory Quotient:

  • Formula:
    $RQ = \frac{\text{volume of } CO<em>2 \text{ produced}}{\text{volume of } O</em>2 \text{ consumed}}$

  • Typical $RQ$ values:

  • Carbohydrates: $1.0$

  • Lipids: $0.7$

  • Proteins: $$