7.2.1 Respiration: An Overview

• Glucose is the main respiratory substrate used by cells
• Aerobic respiration is the process of breaking down a respiratory substrate in order to produce ATP using oxygen
• The equation for aerobic respiration:

glucose + oxygen →  carbon dioxide + water + energy

C₆H₁₂0₆ + 6 O₂ →  6 CO₂ + 6 H₂0 + 2870kJ

• The energy that is released during the process is used to phosphorylate (add a phosphate) ADP to form ATP
• The ATP provides energy for other biological processes in cells
• The process of aerobic respiration using glucose can be split into four stages:
  • Glycolysis
  • The Link reaction
  • The Krebs cycle
  • Oxidative phosphorylation
• Glycolysis occurs in the cytoplasm of the cell, while the other three stages all occur within different parts of the mitochondria
• The reactions in each stage of respiration are controlled by enzymes found inside the cell
• The enzyme that catalyses these reactions the slowest will determine the overall rate of aerobic respiration
• Several coenzymes are required during respiration to transfer various molecules involved in the process
  • NAD and FAD are the coenzymes responsible for transferring hydrogen between molecules
  • Depending on whether they give or take hydrogen, they are able to reduce or oxidise a molecule
  • Coenzyme A is responsible for the transfer of acetate (also known as acetic acid) from one molecule to another
• Although glucose is the main fuel for respiration, organisms can also break down other molecules (such as fatty acids or amino acids) to be respired

Structure of mitochondria

• Mitochondria have two phospholipid membranes
• The outer membrane is:
  • Smooth
  • Permeable to several small molecules
• The inner membrane is:
  • Folded (cristae)
  • Less permeable
  • The site of the electron transport chain (used in oxidative phosphorylation)
  • Location of ATP synthase enzymes (used in oxidative phosphorylation)
• The intermembrane space:
  • Has a low pH due to the high concentration of protons
  • The concentration gradient across the inner membrane is formed during oxidative phosphorylation and is essential for ATP synthesis
• The matrix:
  • Is an aqueous solution within the inner membranes of the mitochondrion
  • Contains ribosomes, enzymes and circular mitochondrial DNA necessary for mitochondria to function

The structure of a mitochondrion