Respiration: An Overview (Edexcel A Level Biology (A) SNAB): Revision Note

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