B2.6 Aerobic and Anaerobic Respiration

Definition: Respiration is the process by which cells release energy from food.

• All respiration takes place inside living cells
• Living things need energy:-
  • to build up large molecules using smaller ones
  • in animals, to enable muscles to contract
  • in mammals and birds, to maintain a steady body temperature in colder surroundings
  • in plants, to build up sugars, nitrates and other nutrients into amino acids which are then built up into proteins
• There are two types of respiration:-
  • Aerobic
  • Anaerobic

The energy that is released during respiration is used:

• To build up larger molecules using smaller ones
  • In plants it is to build up sugar, nitrates and other nutrient into amino acids which are then built up into proteins
  • In animals, to build up glucose molecules into glycogen and amino acids into proteins. Fatty acids and glycerol can be built up to form lipids
• In animals, to enable muscles to contract e.g.
  • The heart muscle and muscles to allow movement
• In mammals and birds, to maintain a steady body temperature in colder surroundings
  • Mammals maintain their body at 37°C
  • Birds at 40°C

C6H1206 + 602 --> 6CO2 + 6H2O + energy

glucose + oxygen --> carbon dioxide + water + energy

Aerobic respiration takes place continuously in both plants and animals. Plants photosynthesise during daylight but respire both day and night.

Most chemical reactions in the cell take place in the cytoplasm. Different enzymes catalyse each of the chemical reaction in the cell. Respiration like many cell reactions is controlled by enzymes which are found inside the mitochondria.

Respiration is a series of separate chemical reactions each controlled by enzymes. This allows small amounts of energy to be released at each stage and the process can be controlled. Enzymes are found on the inner membrane and in the centre of the mitochondria.

Anaerobic Respiration releases energy from glucose without using oxygen. It has two main disadvantages:

• It releases less energy
• It makes poisonous waste products

Mostly our muscles use oxygen and respire aerobically. Which we do vigorous exercise oxygen cannot get to muscles quick enough and they start to respire anaerobically. In anaerobic respiration the muscles release energy(ATP) from glucose but also make a poisonous waste product called lactic acid.

Glucose --> Lactic Acid + Energy(ATP)

If the lactic acid builds up in the muscles go into cramp. Blood carries lactic acid away to the liver which uses oxygen to break it down. This extra oxygen required is called the oxygen debt. We keep breathing hard after a sprint because we are paying back the oxygen debt. Anaerobic respiration in muscles produces only 0.8KJ/g of glucose.

Aerobic Respiration

• Requires oxygen
• Occurs in mitochondria of cells
• Produces a lot of ATP per glucose molecule
• Used when heart rate and breathing rate rise

Anaerobic Respiration

• Doesn't require oxygen
• Occurs in the cytoplasm of cells
• Produces less ATP per glucose molecule
• Used during the first 1-2 minutes of exercise

When you begin exercising, glucose and oxygen move into the mitochondria of the muscle cells. As the level of activity increase, more glucose and oxygen is required. When you exercise you need more energy which is released during respiration to enable your muscle cells to contract more frequently and for longer.

To supply the extra oxygen and to remove the extra carbon dioxide produced by the cells, your blood needs to circulate faster, so your heart rate increase. Oxygen and carbon dioxide in the blood also need to be exchanged faster with the air in the lungs. As a result your breathing rate (number of breaths per minutes) increase.

It is believed that your pulse rate is a measure of your fitness. The slower the resting heart rate the fitter you are and the quicker your heart rate goes back to normal after exercise the fitter you are. On the other hand, it may not be a measure of your fitness because other factors affect heart rate e.g. stimulants.

Factors which alter pulse rate in humans

• Duration of exercise
• Intensity of exercise
• Age
• Genetics
• Stimulants e.g. drugs
• Heart rate
• Type of exercise
• High temperature
• Gender

• Muscles
  • Muscles contraction increase - Rate of respiration in muscle cells increases
    • Respiration rate increases releasing more energy to contract muscle fibres
• Heart
  • Pulse rate increases (beats per minute)
    • More blood carried to muscles
    • More oxygen carried to the muscles
  • Stroke Volume increases - volume of blood pumped per minute
    • More glucose carried to muscle cells
    • More carbon dioxide removed from muscles
• Brain
  • Blood flow constant
    • Cell activity requires a constant blood supply of oxygen and glucose

• Lungs
  • Breathing rate increases
    • More oxygen carried to the muscles
  • Depth of breathing increases
    • More carbon dioxide removed from muscles
• Skin
  • Skin becomes flushed
    • More blood carried to muscles
    • More blood carried to skin, to encourage heat loss

After exercise we experience oxygen debt.

The oxygen debt = the amount of oxygen needed to oxidise the lactic acid that has built up in muscles during anaerobic respiration.