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Respiration Notes for IGCSE cambridge


Respiration is a chemical reaction which happens in almost all cells in the body to produce energy from nutrient molecules. This energy can be used in a variety of processes including:

● Muscle contraction

● Protein synthesis

 ● Cell division

● Active transport

● Growth

● Nerve impulses

● Maintaining body temperature

 Respiration usually occurs with the presence of oxygen (aerobic respiration), although it can occur in the absence of oxygen (anaerobic respiration).

Anaerobic respiration is less efficient and leads to fatigue in humans. Both types of respiration are catalysed by enzymes. This means that the rate of respiration can be influenced by factors such as temperature and pH.

Aerobic Respiration:

 Aerobic respiration occurs in the presence of oxygen.

Glucose is broken down into carbon dioxide, water and energy with the help of oxygen. This occurs in the cell mitochondria. Cells which require lots of energy, such as muscle cells, therefore have high amounts of mitochondria.

 Equations for aerobic respiration:

 ● glucose + oxygen → carbon dioxide + water

● C6H12O6 + 6O2 → 6CO2 + 6H2O

Anaerobic respiration:

 Anaerobic respiration occurs when oxygen is not present. It is less efficient than aerobic respiration and produces less energy per glucose molecule.

It occurs in the cell cytoplasm and thus does not require mitochondria.

 Animal cells undergo anaerobic respiration during vigorous exercise as not enough oxygen is delivered to muscles.

 In this reaction, glucose is broken down to produce lactic acid, as well as releasing energy.

This lactic acid builds up in muscles and causes muscle fatigue. Anaerobic respiration also produces an ‘oxygen debt’. To repay this, the lactic acid must be transported to the liver where it is broken down into carbon dioxide and water using oxygen. This is the reason why the breathing and heart rates remain high after exercise

Microorganisms, such as yeast, also undergo anaerobic respiration. Yeast breaks down anaerobically to form alcohol and carbon dioxide instead of lactic acid. Equations for anaerobic respiration in yeast:

 ● glucose → alcohol + carbon dioxide

● C6H12O6 → 2C2H5OH + 2CO2

Equation for anaerobic respiration in animal cells: ● glucose → lactic acid

Difference between aerobic and anaerobic respiration

Aerobic Anaerobic
1. uses oxygen 2.No alcohol or lactic acid is formed 3.Large amount of energy is released per a molecule of glucose 4. Carbon dioxide is made 5. most steps occurs inside a mitochondria 1. does not use oxygen 2. alcohol in microorganisms and lactic acid in animals is produced 3. much less energy is released 4. carbon dioxide is given out by plants and microorganisms but not by animals 5. All steps Occurs in the cytoplasm of the cell.

Gas exchange in Humans

It is done via breathing. Breathing is a process through which chest muscular movements keep the respiratory surfaces supplied with oxygen.

Human respiratory system Key structures:

● Lungs – The lungs are the main organs in the respiratory system, containing the surfaces where gas exchange takes place.

● Ribs and intercostal muscles – Intercostal muscles are found between the ribs. Internal and external intercostal muscles work antagonistically in pairs to expand and contract the rib cage during breathing. The ribs also protect the lungs and heart from physical damage.

● Larynx – contains the vocal cords.

● Trachea – connects the throat to the bronchi. C-shaped cartilage rings are present to provide structural strength, keeping the trachea open so that air can pass through it.

● Bronchi – hollow tubes composed of cartilage rings that carry air from the trachea to the lungs. The bronchi splits into two tubes to enter the left and right lung, before branching further inside the lungs.

● Bronchioles -Smaller tubes which branch off from the bronchi in the lungs, leading to the alveoli.

● Alveoli – Where gas exchange occurs; comprised of tiny air sacs with a capillary network. Oxygen from the air diffuses into the capillaries, whilst waste carbon dioxide diffuses out. Waste gases are then breathed out. Ventilation: Ventilation is the act of moving air into and out of the lungs to allow gas exchange to occur.

Breathing in

The internal intercostal muscles relax whilst the external intercostal muscles contract, pulling the ribs up and out while the diaphragm flattens, pushing the abdominal muscles downwards. The volume in the thorax (chest cavity) increases, so air enters the lungs. Air diffuses into the lungs, rather than being ‘sucked’ in. This is because when the volume of the chest increases, there is a lower concentration of air inside the lungs compared to outside, thus air diffuses in.

Breathing out

 volume of thorax decreases, increasing pressure so that air is forced out. This is passive (does not require muscle contraction) except when forcibly breathing out like when coughing, where the internal intercostal muscles contract forcefully.

The majority of air in the atmosphere is composed of nitrogen, oxygen and carbon dioxide. Inhaled air is made up of more oxygen than exhaled air, as oxygen is absorbed into the blood in the alveoli instead of being exhaled.

Oxygen is used in cells for respiration, and carbon dioxide is produced as a waste product. This carbon dioxide is released from the blood at the alveoli and diffuses out into the lungs, before being exhaled, thus there is more carbon dioxide in exhaled air.

Exhaled air also contains more water vapour than inhaled air. This is because, as you breath out, some water vapour is carried from the surface of the lungs by the expired gas.

When exercise is carried out, muscles increase the rate of respiration to produce energy for muscle contraction. Aerobic respiration requires oxygen; thus, a greater amount of oxygen is demanded.

In addition, a greater amount of carbon dioxide is produced as a waste substance, which diffuses into the blood. This increase in carbon dioxide in the blood is detected by the brain, which causes the rate of breathing to speed up, allowing gas exchange to happen more rapidly, expelling the carbon dioxide whilst taking in more oxygen. The heart rate is also increased to pump substances around the body more quickly in the blood.

Adaptations of exchange surfaces:

● Large surface area – allows more efficient diffusion. The alveoli allow the lungs to have a huge surface area of 80-100 square meters.

● Thin surface – this means that there is a short diffusion distance, thus exchange can occur more rapidly.

● Good blood supply – Maintains concentration gradient by carrying away substances which have diffused across already.

● Good ventilation with air – this means that waste gases can diffuse out of the blood into the air in the lungs whilst oxygen diffuses into the blood.

● Moist – Allows gases to dissolve before diffusing across the membrane.

 The lungs are also adapted to protect from foreign pathogens and particles. Goblet cells, found in the trachea and bronchi, are adapted to secrete mucus into the respiratory tract. Foreign pathogens and particles stick to this mucus, which is then moved upwards towards the throat by cilia (hair-like projections from some cells). Mucus is then swallowed, and pathogens are destroyed in the acidic conditions in the stomach.

Exercise and breathing rate.

When exercising, the breathing rate increases in order to increase oxygen supply in the muscles. When doing thorough exercises such as a splint, the amount of oxygen required can not be provided by the lungs. The body starts to respire anaerobically. This leads to accumulation of lactic acid in the body. The body will therefore need to break this lactic acid using oxygen at the end of the exercise. So, the body is said to be in an Oxygen debt. [will need oxygen after exercise]

After exercise, the high rate of breathing and fast heartbeat continues until all lactic acid is broken down.


  Inspired Expired Air Reason of the difference
Oxygen 21% 16% Oxygen is absorbed the gas surface then used by the cells in respiration
Carbon Dioxide 0.04% 4% As cells respire, they form more carbon dioxide which diffuses out across the gas exchange surface.
Argon and other noble gases 1% 1% Not absorbed and produced by the body cells
Water content[humidity] variable Always high The gas exchange surface is moist. Some of this water vapour evaporates and gets out with the expired air.
Temperature variable Always warm Air is warmed by the respiratory surface as it passes.