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- is the oxidation of food substances with the release of energy
- occurs in living cells
- enzymes are involved

Respiratory system consists of
  • Lungs
  • Nose
  • Mouth
  • Trachea
  • Bronchi
  • Bronchioles
  • Alveoli

Aerobic and Anaerobic respiration

a. Aerobic respiration

- oxygen required
- large amount of energy released
- carbon dioxide and water produced

b. anaerobic respiration

- oxygen not required
- small amount of energy released
- lactic acid produced in mammals
- ethanol and carbon dioxide produced in yeast

How body takes in oxygen and removes carbon dioxide

The breathing mechanism

a. Inspiration

- external intercostal muscles contract, internal intercostal muscles relax
- ribs move upwards and outwards
- diaphragm contracts and flattens
- volume of thorax increases
- air pressure in thorax decreases
- air flows into lungs

b. Expiration

- external intercostal muscles relax
- internal intercostal muscles contract
- ribs move downwards and inwards
- diaphragm relaxes and arches upwards
- volume of thorax decreases
- air pressure in thorax increases
- air flows out of lungs







Relaxes and arches upwards

Contracts and flattens


External intercostal muscles




Internal intercostal muscles





Moves downwards and inwards
Move upwards and outwards



Moves downwards and backwards into original position

Moves upwards and forwards


Volume of thoracic cavity





Compressed by decrease in thoracic cavity volume

Expand in respond to air pressure to fill up thoracic cavity


Air pressure of lungs

Increases as lungs are compressed

Decreases due to lung expansion


Air movement

Lung pressure higher than atmospheric pressure. Air forced out of lungs

Atmospheric pressure > lung pressure. Air enters lung

Gaseous exchange

Exchange of oxygen

  • Each haemoglobin molecule present in red blood cells can combine reversibly with up to four molecules of oxygen, forming the compound oxyhaemoglobin.
  • As the red blood cells move through the blood capillaries surrounding the alveoli, the narrowness of the capillaries forces them to slow down and become distroted.
  • This increases the time available for gaseous exchange to take place, as well as exposes a larger surface area of the cell.

Exchange of carbon dioxide

  • Carbon dioxide is transported in a variety of ways by blood.
  • A very small percentage is dissolved in plasma as carbonic acid.
  • Some carbon dioxide becomes attached to haemoglobin to form the compound carbamino haemoglobin.
  • Most of the carbon dioxide is carried by the plasma as hydrogen carbonate ions.


  • The alveoli are where gaseous exchange takes place between the air in the sacs and the blood in the capillaries surrounding each alveolus.
  • The distance over which the gases must diffuse is about 0.01 mm -two cell layers thick
  • A thin film of moisture covers the surface of the alveolar wall. This has been found to contain a chemical which lowers surface tension.
  • This makes it easier for the lungs to be inflated during breathing in. Without this chemical or surfactant, the alveoli would collapse and become stuck together each time air passes out of the lungs.
Diffusion across the alveoli enhanced by:
  • Large surface area of the alveoli
  • Short distance between the air and the blood
  • Steep concentration gradient maintained by constant movement of blood through the tissues and ventilation of the lungs
  • Ability of blood to carry oxygen and carbon dioxide.

Transport of Oxygen

Transport of Carbon Dioxide

Structured Question Worked Solutions

1. Describe 3 ways in which smoking damages the lungs

  • Tar deposited in lungs contains carcinogens and increases risk of lung cancer
  • Carbon monoxide in smoke increases fat deposition, if at coronary arteries, the lumen of the arteries gets narrower and decreases blood supply, leading to heart attack
  • Irritants cause coughing. Prolonged violent coughing could lead to emphysema in which the alveolar walls break down

2. A student wanted to investigate the effect of glucose and sucrose mixtures on the rate of respiration in yeast. The experiment was set up by drawing 1cm3 of a yeast/sugar mixture into a syringe, followed by drawing 1cm3 of air (refer to fig. 1). The syringe was closed at the tip and placed in a water bath maintained at 35˚C. The volume of gas produced in the syringe was measured at 1 minute intervals. Replicates were taken and the results shown in table 1.


Volume of gas produced in syringe (cm3)

Replicate 1

Replicate 2

1 min

2 min

3 min

4 min

5 min

1 min

2 min

3 min

4 min

5 min























(a) Suggest what gas is collected in this experiment.

(b) From the results in table 1, suggest which sugar would be a better respiratory substrate for yeast.

(c) Suggest why there is a difference in the rate of respiration when yeast is given different sugars in the experiment.

(d) Highlight any anomalous data in the results.

(e) Name a possible error in this experiment and suggest an improvement or solution.


(a) carbon dioxide

(b) Glucose. It gives a higher volume of carbon dioxide gas than sucrose in the same time period, indicating that it can be utilized more quickly.

(c) The raw material used in respiration is glucose. Sucrose is a disaccharide and has to be first broken down into glucose before it can be used in respiration. Therefore, the rate of respiration using sucrose is lower than that of glucose.

(d) The reading at 5 min for replicate 1 of glucose. It is much higher than the reading of replicate 2.

(e) Some of the released carbon dioxide may not be recorded as air can be compressed; use datalogger instead. Carbon dioxide is slightly soluble in water and may dissolve in the mixture; use a hydrogencarbonate indicator instead.

3. Figure 2 shows the effect of lactic acid on the amount of oxygen released by red blood cells to actively respiring muscle cells.

(a) Explain why lactic acid is formed during vigorous muscular activity.

(b) With reference to fig. 2, explain how an increased concentration of lactic acid affects the release of oxygen to the muscle cells.

(c) How is lactic acid removed by the body? 


(a) During vigorous muscular activity, aerobic respiration alone is not enough to meet the increased energy demand. Anaerobic respiration takes place in the muscle cells to supplement the energy from aerobic respiration. Anaerobic respiration in muscle cells produces lactic acid.

(b) When there is an increased concentration of lactic acid, the percentage of oxyhaemoglobin in the blood decreases. This indicates that more oxygen is released from haemoglobin to the muscle cells.

(c) Lactic acid is removed by the body either through oxidation to produce energy, or conversion to glucose in the liver. 

4a. What are the symptoms of emphysema?

4b. Name a cause of emphysema


4a. breathing difficulty, wheezing, breathlessness

4b. Smoking

5. Explain the importane of each of the following in relation to their function in respiration

a. the exchange surface of the alveoli

b. the lining of the trachea



  • The function of the alveoli is to enable gaseous exchange by diffusion.
  • The alveoli are one cell-thick to enable rapid diffusion of gases through the wall.
  • The alveoli have a thin film of moisture on the surface to allow oxygen to dissolve in it and diffuse into the blood.
  • The exchange surface of the alveoli are richly supplied with capillaries that maintains concentration gradient of gases between the blood and alveolar air. This facilitates inward diffusion of carbon dioxide and outward diffusion of oxygen.


  • The lining of the trachea contains goblet/gland cells which secrete mucus to trap dust particles and bacteria.
  • The trachea is lined with epithelium bearing cilia which help to sweep the particles up the bronchi and trachea into the pharynx, where they can be removed by swallowing or spitting.