• Respiration: Chemical reactions that break down nutrient molecules in living cells to release energy.
  • Uses of energy in the body of humans: 
    • Muscle contraction
    • Protein synthesis
    • Cell division
    • Active transport
    • Growth
    • Passage of nerve impulses
    • Maintenance of a constant body temperature.

Types of Respiration

Aerobic Respiration

Release of a relatively large amount of energy in cells by the breakdown of food substances in the presence of oxygen.

Glucose + Oxygen Carbon Dioxide + Water + Energy

C6H12O6 + 6O2 6CO2 + 6H2O + 36 ATP

Anaerobic Respiration

Release of a relatively small amount of energy by the breakdown of food substances in the absence of oxygen.

  • In Muscles: Glucose Lactic Acid
    • C6H12O6 2C3H6O3 + 2 ATP
  • In Yeast (Single-Cell Fungi): Glucose Ethanol + Carbon Dioxide
    •  C6H12O6 → 2C2H5OH + 2CO2 + 2 ATP 

Uses of Anaerobic Respiration


When provided a sugar source, enzymes such as yeast respire anaerobically to produce alcohol, which can later be distilled and sold.

Bread Making

  • Grapes (sugar source) are pressed pressed to allow enzymes to begin fermentation
  • Yeast converts sugar into alcohol.
  • At 8-9% the alcohol (which is toxic) kills the yeast
  • Higher concentrations are achieved by distillation
  • Flour, sugar, water and salt are mixed with yeast to make the dough.
  • The dough is kept in a warm, moist environment (28°C). Yeast ferments sugar-making carbon dioxide, which creates bubbles, so the bread rises
  • Cooking (at 180°C) – kills yeast, evaporates alcohol and hardens the outer surface.

Disadvantages of Anaerobic Respiration:

  •  Only produces 1/20 of the energy per glucose molecule that aerobic respiration would
  •  Produces poisonous lactic acid

Lactic Acid:

  • Transported in blood to heart, liver and kidneys, which oxidize it
  •  The heart, liver and kidneys need extra oxygen to do this which causes you to continue breathing heavily after exercise.
  •  The extra oxygen is called the oxygen debt.

Gas Exchange

Properties of Gas Exchange Surfaces

  • One Cell Thick
  • Short distance to diffuse
  • Large Surface Area
  • Many molecules can diffuse at once
  • Moist
  • Cells die, if not kept moist
  • Well Ventilated
  • Concentration gradients for oxygen and carbon dioxide are kept up by regular fresh supplies of air
  • Close to Blood Supply
  • Gases can be carried to/from the cells that need/produce them

 Inspired and Expired Air

  • Air taken through inspiration:
    • 21% oxygen
    • 0.04% carbon dioxide
    • 78% nitrogen
    • Water vapor varies with climate
  • Air taken through expiration:
    • 18% oxygen
    • 3% carbon dioxide
    • 78% nitrogen
    • Saturated water vapor.

Effects of Physical Activity on Breathing

  • Physical activity increases the breathing rate (more breaths per minute), and the tidal volume (more air per breath).
  • This is measured with a spirometer to produce a spirogram.
  • During exercise, tissues respire at a higher rate, the change in breathing volume and rate helps to keep CO2 concentration and pH at safe levels.

Breathing In

  • Diaphragm muscles contract – diaphragm moves downwards
  • External intercostal muscles contract – pulls rib cage upwards and outwards
  • Lung volume increases – and pressure falls
  • Air rushes in to equalize pressure

Breathing Out

  • Diaphragm muscles relax – returns to a dome shape
  • External intercostal muscles relax – rib cage falls downwards and inwards
  • Lung volume decreases – and pressure increases
  • Air is forced out

Internal Intercostal Muscles

 Are used in coughing and sneezing.

Mucus and Cilia

Goblet cells produce sticky mucus to trap and eliminate particulate matter and microorganisms.


Little hairs which sweep back and forward in a coordinated way to brush mucus up the lungs into the mouth

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