Cellular Respiration

What is respiration?

Cellular respiration is the process by which cells break down glucose to release energy in the form of ATP.

Respiration can occur:

• Aerobically - in the presence of oxygen.

• Anaerobically - in the absence of oxygen.

The overall aerobic equation is: Glucose + Oxygen → Carbon dioxide + Water + ATP (energy)

1. Glycolysis (Occurs in the Cytoplasm)

   
   

   Purpose: Split one glucose molecule (6 carbons) into two pyruvate molecules (3 carbons each).

   
   

   Step 1: Glucose enters the cell.

   
   

   Step 2: 2 ATP molecules are used to phosphorylate glucose.

   
   

   Step 3: The 6-carbon molecule becomes unstable and splits into two 3-carbon molecules.

   
   

   Step 4: These molecules are oxidized, reducing NAD to NADH.

   
   

   Step 5: ATP is produced by substrate-level phosphorylation.

   
   

   Products per glucose: 2 Pyruvate, 2 NADH, net gain of 2 ATP

   
   

2. Link Reaction (Occurs in the Mitochondrial Matrix)

   
   

   Purpose: Convert pyruvate into acetyl-CoA before entering the Krebs cycle.

   
   

   For each pyruvate: Pyruvate (3C) is oxidized. NAD is reduced to NADH. One carbon is removed

   and released as CO₂ (decarboxylation). The remaining 2-carbon molecule combines with

   Coenzyme A.

   
   

   Products per glucose: 2 Acetyl-CoA, 2 NADH, 2 CO₂

   
   

3. Krebs Cycle / Citric Acid Cycle (Occurs in the Mitochondrial Matrix)

   
   

   Purpose: Complete the breakdown of glucose and generate reduced coenzymes for ATP

   production.

   
   

   Step 1: Acetyl-CoA (2C) combines with a 4-carbon compound to form citrate (6C).

   
   

   Step 2: Citrate is gradually broken down through a series of reactions.

   
   

   Step 3: Carbon dioxide is released.

   
   

   Step 4: NAD and FAD are reduced to NADH and FADH₂.

   
   

   Step 5: ATP is produced by substrate-level phosphorylation.

   
   

   Step 6: The original 4-carbon compound is regenerated.

   
   

   Products per glucose: 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP (substrate-level)

   
   

4. Oxidative Phosphorylation (Electron Transport Chain)

   
   

   Location: Inner mitochondrial membrane.

   
   

   This stage produces most of the ATP.

   
   

   Step 1: NADH and FADH₂ donate electrons to the electron transport chain.

   
   

   Step 2: Electrons pass through protein complexes embedded in the membrane.

   
   

   Step 3: Energy released pumps H+ ions into the intermembrane space.

   
   

   Step 4: A proton gradient forms.

   
   

   Step 5: H+ ions flow back through ATP synthase.

   
   

   Step 6: ATP synthase produces ATP from ADP + Pi.

   
   

   Step 7: Oxygen acts as the final electron acceptor and combines with electrons and H+ to form

   water.

   
   

   Products: Approximately 28 - 34 ATP, 6 H₂O

Substrate-Level vs Oxidative Phosphorylation

• Substrate-Level Phosphorylation: ATP produced directly from a phosphorylated substrate. Occurs in glycolysis and the Krebs cycle. Direct transfer of phosphate to ADP.

  
  

• Oxidative Phosphorylation ATP produced using energy from electron transport chain. Occurs in the inner mitochondrial membrane. Indirect ATP production through ATP synthase.

Anaerobic Respiration

When oxygen is absent: Only glycolysis occurs. Pyruvate is converted into lactic acid (animals).

Lactic acid dissociates into lactate and H+ ions.

Only 2 ATP are produced per glucose molecule.

Respiratory Quotient (RQ)

RQ = Volume of CO₂ Produced ÷ Volume of O₂ Consumed

RQ helps determine which respiratory substrate (carbohydrate, fat, or protein) is being used.

Summary of Aerobic Respiration

Per glucose molecule:

Glycolysis: 2 ATP + 2 NADH + 2 Pyruvate

Link Reaction: 2 NADH + 2 CO₂ + 2 Acetyl-CoA

Krebs Cycle: 2 ATP + 6 NADH + 2 FADH₂ + 4 CO₂

Oxidative Phosphorylation: ~28-34 ATP + 6  H₂O

Total ATP yield: Approximately 30-32 ATP per glucose molecule.

Figure 1: Cellular respiration pathway and output per glucose molecule.