Cellular Respiration

Cellular Respiration

• The central purpose of cellular respiration is to produce ATP (adenosine triphosphate), the energy currency of the cell.

ATP Production Methods

• There are two primary methods of ATP production:

  1. Substrate-Level Phosphorylation: ATP is produced directly in a reaction without the need for an intermediate electron transport chain.

  2. Electron Transport Phosphorylation (Chemiosmosis): ATP synthesis driven by a chemiosmotic gradient. This process involves the flow of protons (H+) through a specialized channel protein called ATP Synthase.

ATP Synthase Structure and Function

• ATP Synthase facilitates the conversion of ADP (adenosine diphosphate) and inorganic phosphate (P) into ATP using energy stored in a proton gradient.

  • The equation can be represented as: ADP + ATP

• The influx of H+ ions provides the energy necessary for this phosphorylation reaction.

Stage 1: Glycolysis

• Overview: It occurs in the cytoplasm and involves the breakdown of glucose into two molecules of pyruvate with a net yield of 2 ATP.

Glycolysis Steps

• Energy Investment Phase:

  • 2 ATP molecules are used to phosphorylate glucose and other intermediates.

  • Initial glucose undergoes transformation into glucose-6-phosphate.

  • Two conversions occur to yield 2 molecules of phosphoglyceraldehyde (PGAL).

• Energy Return Phase:

  • The subsequent steps generate ATP and NADH.

  • Key reactions convert PGAL into biphosphoglycerate (BPGA).

  • The net yield at the end of glycolysis includes:

  • 2 ATP (from substrate-level phosphorylation)

  • 2 NADH (electron carriers)

  • 2 Pyruvate (end product transitioning to Krebs Cycle).

• Glycolysis Summary:

  • Anaerobic process occurring in the cytoplasm.

  • Energy investment: -2 ATP.

  • Net yield: +2 ATP.

  • Produces 2 NADH which will feed into the Electron Transport Chain (ETC).

  • Produces 2 Pyruvate that will enter the Krebs Cycle.

Stage 2: Krebs Cycle (Citric Acid Cycle)

• Overview: This stage includes the conversion of pyruvate to acetyl-CoA, which enters the Krebs cycle occurring in the mitochondria.

Prep Step of Krebs Cycle

• Process: During conversion:

  • 2 Pyruvate yield:

  • 2 Acetyl-CoA

  • 2 CO₂ as waste

  • 2 NADH to enter the ETC.

Krebs Cycle Steps

1. Acetyl-CoA combines with oxaloacetate to form citrate.

2. Citrate undergoes transformations leading to:

   • 4 CO₂ released (as waste).

   • Generation of 6 NADH, 2 FADH2, and 2 ATP through substrate-level phosphorylation.

• Overall Reaction:

  • 2 Acetyl-CoA} + 2 Oxaloacetate} Citrate + 4 CO²+ 6 NADH + 2 FADH² + 2 ATP

  • Enzymes involved in the reactions include:

    • Pyruvate dehydrogenase (converts pyruvate to acetyl-CoA)

    • Citrate Synthase (combines Acetyl-CoA with oxaloacetate)

    • Isocitrate dehydrogenase and others for further conversions.

Stage 3: Electron Transport Phosphorylation

• Overview: This stage occurs across the inner mitochondrial membrane. It involves the transfer of electrons from NADH and FADH2 through a series of protein complexes known as the electron transport chain.

Mechanism

1. Electron Transfer: High energy electrons from NADH and FADH2 are transferred through protein complexes, facilitating the pumping of H+ ions into the intermembrane space, creating a proton gradient.

2. Chemiosmosis: H+ ions flow back through ATP Synthase due to the gradient, leading to ATP synthesis.

3. Oxygen is the final electron acceptor in the chain, combining with electrons and H+ ions to form water as a byproduct:

   • O2 + 4 e + 4 H + 2 H2O

4. The approximate yield of ATP from this stage is about 32 ATP.

Summary of Cellular Respiration Phases

• Overall Reaction for Cellular Respiration:

  • {C}6 {H}{12} {O}6 + 6 {O}2 to 6 {CO}2 + 6 H2O + (36 ATP)

• Yield from Each Stage:

  • Glycolysis: 2 ATP, 2 NADH, 2 Pyruvate.

  • Prep Step: 2 CO₂, 2 NADH, 2 Acetyl-CoA.

  • Krebs Cycle: 4 CO₂, 6 NADH, 2 FADH2, 2 ATP.

  • Total: 36-38 ATP produced through cellular respiration.