Chapter 7: Cellular Respiration and fermentation

Free energy (ΔG)

• determines whether a reaction proceeds forward or backward.

When reactants have more free energy than products

exergonic (spontaneous, ΔG < 0).

When products have more free energy

endergonic (nonspontaneous, ΔG > 0).

Concentrations also influence direction:

• More reactants → reaction proceeds forward.
  
  

• More products → reaction proceeds backward.

Characteristics of Chemical Equilibrium

• Equilibrium: forward and reverse reactions occur at the same rate.
  
  

• No net change in concentrations of reactants or products.
  
  

• Free energy (ΔG) is at its minimum value → no more work can be done.

Living cells are open systems

• Reactants are constantly supplied (e.g., glucose, O₂).
  
  

• Products are constantly removed (e.g., CO₂, H₂O).

How Cells Avoid Equilibrium

• Living cells are open systems
  

• This keeps reactions moving forward and allows continuous work and energy flow.
  
  

Example: In cellular respiration, CO₂ diffuses out and O₂ diffuses in, preventing equilibrium.

Cellular Respiration

C6H12O6+6O2→6CO2+6H2O+Energy(ATP+Heat)C_6H_{12}O_6 + 6O_2 → 6CO_2 + 6H_2O + Energy (ATP + Heat)C6​H12​O6​+6O2​→6CO2​+6H2​O+Energy(ATP+Heat)

Oxidation

loss of electrons (or H atoms).

Reduction

gain of electrons (or H atoms).

Oxidizing agent

• accepts electrons → becomes reduced.

Reducing agent

donates electrons → becomes oxidized

 Oxidation and Reduction agents

• Cellular respiration: glucose is oxidized → CO₂; O₂ is reduced → H₂O.
  
  

• NAD⁺/NADH: NAD⁺ is reduced → NADH (electron carrier).
  
  

• Photosynthesis (reverse): CO₂ is reduced → glucose.
  
  

NAD⁺ (nicotinamide adenine dinucleotide)

accepts 2 e⁻ + 1 H⁺ → NADH.

NADH

stores high-energy electrons for use in the electron transport chain (ETC).

Each NADH oxidation releases

…free energy used to make ATP.

Role of NAD⁺ / NADH Diagrammatically

• NAD⁺ + 2e⁻ + H⁺ → NADH (reduction, energy stored)

• NADH → NAD⁺ + 2e⁻ + H⁺ (oxidation, energy released)

Outer membrane

permeable to small molecules.

Inner membrane

folded into cristae → increases surface area for ETC.

Intermembrane space

site of proton (H⁺) buildup.

Matrix

• contains enzymes for Krebs cycle and pyruvate oxidation.

Structure supports function

 the inner membrane’s large surface area allows efficient ATP production through electron transport and proton gradient formation.

Substrate-Level Phosphorylation

• Direct transfer of phosphate from a substrate to ADP.
  
  

• Occurs in glycolysis and Krebs cycle.
  
  

• Makes small amounts of ATP.

Oxidative Phosphorylation

• Uses energy from electrons moving through ETC to power ATP synthase.
  
  

• Occurs in mitochondrial inner membrane.
  
  

• Makes majority of ATP (~90%).

Pyruvate oxidation:

Pyruvate → Acetyl-CoA + CO₂ + NADH

Krebs cycle

• Acetyl-CoA → 2 CO₂

• Produces: 3 NADH, 1 FADH₂, 1 ATP per turn

Free energy: Stored in NADH/FADH₂ for next stage.

Electron Transport Chain

• NADH/FADH₂ donate electrons → O₂ (final acceptor → H₂O)
  
  

• Energy released as electrons move → pumps H⁺ into intermembrane space.

Chemiosmosis

H⁺ diffuses back through ATP synthase, driving ATP formation.

ATP Production Without Oxygen

Fermentation and Anaerobic Respiration. When O₂ is limited, ETC can’t operate (no final electron acceptor).

Fermentation

regenerates NAD⁺ so glycolysis can continue.

Lactic acid fermentation

Pyruvate → lactate (in animals).

Alcohol fermentation

 Pyruvate → ethanol + CO₂ (yeast, some bacteria).

Pathway Disruptions

• If one step is blocked, reactions upstream accumulate and downstream stop.

  • Example: ETC inhibition → NADH builds up → NAD⁺ runs out → glycolysis stops.

• Feedback regulation adjusts pathway activity.

  • Example: High ATP inhibits phosphofructokinase in glycolysis.

Connections to Other Pathways (Light Focus)

• Carbohydrates, fats, and proteins can all enter cellular respiration at different points.

  • Fats → glycerol and fatty acids → glycolysis or acetyl-CoA.

  • Proteins → amino acids → intermediates of glycolysis or Krebs cycle.

These connections maintain metabolic flexibility for energy balance.

Energy flow

Glucose → NADH/FADH₂ → ETC → Proton gradient → ATP

Matter flow

Glucose → CO₂ + H₂O