Cellular Respiration

Enzymes lower activation energy

• Activation energy is the energy required for a chemical reaction to occur

• If a reaction has not reached the activation energy, it cannot occur

• Enzymes lower the activation energy of reactions, allowing them to occur at normal body temperature

Cellular Respiration

• Cellular respiration – the process by which organic molecules are broken down to release energy for the cell’s activities

• Cellular respiration uses glucose and oxygen, and produces carbon dioxide, water and energy

• This occurs via a series of over twenty individual reactions, each catalysed by its own specific enzyme

• Summary equation:

  • glucose + oxygen –> water + carbon dioxide

  • C6H12O6 + 6O2 –> 6H2O 6CO2

Energy from cellular respiration

• Energy created from cellular respiration must be transferred to where it is needed – but how?

• Energy cannot be directly transported, so it is used to create high-energy bonds within a recyclable energy storage molecule called adenosine triphosphate (ATP)

• ATP consists of an adenosine molecule joined to three phosphate molecules

• The bond between the second and third phosphate molecule is easily created and broken, allowing it to be used for storing and releasing energy

• When large molecules such as glucose are broken down, the energy within their bonds is used to form a bond between ADP and a phosphate molecule, creating an ATP molecule

• The ATP molecule is able to move around the cell to a location where energy is needed – when it reaches this location, the high-energy bond is broken again

• The energy released from this bond is used for the cell’s processes, and the ADP and phosphate are again available to be formed back into ATP

• ATP acts as an “energy shuttle” between reactions

Glycolysis

• First step in cellular respiration

• Occurs in the cytoplasm of the cell, and does not require oxygen

• One molecule of glucose (C6H12O6) breaks down to produce two molecules of pyruvic acid (C3H4O3)

• This produces two molecules of ATP in total

• Pyruvic acid then moves into the mitochondria to continue aerobic respiration, if oxygen is present

• If oxygen is not present, anaerobic respiration occurs in the cytoplasm

Anaerobic Respiration

• Occurs after glycolysis, if insufficient oxygen is present for aerobic respiration (eg during exercise)

• Allows glycolysis to continue producing energy

• Each pyruvic acid molecule is converted to one lactic acid molecule

• Lactic acid is taken to the liver – when oxygen is available again, lactic acid is combined with the oxygen to produce glucose

• The oxygen required for this conversion is known as an “oxygen debt” or “recovery oxygen”

• The glucose is available for cellular respiration again

Aerobic respiration

• After glycolysis, the two pyruvic acid molecules move into the mitochondria for aerobic respiration with oxygen

• Two series of reactions occur to fully break down the two pyruvic acid molecules into carbon dioxide and water

  • Kreb’s cycle (citric acid cycle) produces 2 ATP in total

  • Electron transport chain produces up to 34 ATP in total

• 38 ATP can be produced by aerobic respiration of one glucose molecule

  • 2 ATP from glycolysis

  • 2 ATP from Kreb’s cycle

  • 34 ATP from electron transport chain

Energy use by the cell

• Energy produced by cellular respiration is used for many processes within the cell

  • Movement of organelles or whole cell

  • Cell division, growth and repair

  • Synthesis of complex molecules

• 40% of energy from respiration is actually captured

  • The remaining 60% is lost as heat

• Living things must constantly take in food to continue producing energy for life processes

• ATP transfers energy from catabolic (energy-releasing) to anabolic (energy-requiring) reactions