IGCSE Biology Section 4- Respiration and Gas Exchange



Respiration takes place in every cell in the body, and it is the process of releasing energy from glucose. This chemical energy is used to do things like create large molecules and contract muscles. It can also be released as heat energy which maintains the body temperature. 

Aerobic- Happens when there is plenty of oxygen, "with oxygen," and it is the most efficient way to release energy from glucose. 

  • Glucose + oxygen ----> carbon dioxide + water (+ energy
  • C6H12O6 + 6O2 -----> 6CO2 + 6H20 (+energy)

Anaerobic (in animals.)When there is vigorous exercise and there isn't enough oxygen, despite the heart and breathing rate increasing. Therefore, the body releases less energy through anaerobic respiration. This way the glucose is partially broken down, forming lactic acid in muscles which is painful.

Glucose ----> lactic Acid (+energy)

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Anaerobic respiration in plants- Plants can respire without oxygen, but produce ethanol and carbon dioxide, not lactic acid. Fungi like yeast also do this:

Glucose ----> ethanol + carbon dioxide (+energy)

PAPER TWO- Carbon dioxide produced from respiration can be detected by using hydrogen-carbonate solution. In the presence of carbon dioxide, the solution changes from orange to yellow. In this experiment, you could also use maggots, and the control would be glass beads.

1) soak some beans in water for a couple of days and they will begin to germinate. Therefore, they will start to respire.

2) Boil a similar number of beans, which will kill the beans and they will not be able to respire. THESE ARE THE CONTROL.

3) Put home hydrogen carbonate indicator into a test tube and place the beans on a platform made of gauze. Put in a bung, and leave for a set time. Do this for both types of beans, and the germinating beans will change the indicator, due to the carbon dioxide from respiration.

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Measuring the heat produced by respiration

PAPER TWO- You can also use dried beans to measure the heat produced from respiration. 

Prepare a batch of germinating and boiled (non-germinating) beans. Add each batch of beans to separate vacuum flasks, making sure that there is air in the flask so that the beans can respire aerobically. 

Then place a thermometer into each flask and seal the top with cotton wool. Record the temperatures of the flasks, each day, for a week. 

The germinating beans should show an increase in temperature, due to respiration, since the beans are well insulated. The control flask should show minimal or no increase in temperature, as the beans are not respiring. 

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Gas Exchange in Plants

  • Photosynthesis= exchanging carbon dioxide from the atmosphere to oxxygen. 
  • Respiration= Exchanging oxygen and producing carbon dioxide.

In photosynthesis, carbon dioxide is used, so carbon dioxide diffuses from the atmosphere (high concentration) into the plant (low concentration.) However, oxygen is being made as a waste product of photosynthesis, so it diffuses from a high concentration (in the leaf) to a low concentration (outside the leaf.) 

PAPER TWO- The net exchange of gas is dependant on light intensity. Photosynthesis happens in the day because there is light, and the plant needs to convert the energy of the light into glucose. However, respiration occurs all the time, because the plant needs to continually release energy from glucose in order to survive. 

  • During the day (light,) the plants produce more oxygen from photosynthesis than they require for respiration, so in the day, they release the excess of oxygen by diffusion. They also use up more carbon dioxide than they produce, so they take in carbon dioxide by diffusion.At night, there isn't enough light for photosynthesis, so plants take in oxygen, and produce carbon dioxide, like humans, as they just respire.
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The adaptations of a leaf- efficient gas exchange.

1) Leaves are broad and wide, so they have a large surface area for efficient diffusion.

2) Leaves are also thin, so gases have a short diffusion distance to reach and leave the cells they need to.

3) There are also spaces between the cells in a leaf, which allow the exchanging of gases to happen easily , and increases the surface area for gas exchange.


The lower surface is full of little holes called stomata. They allow water to escape (transpiration), and allow gas diffusion. 

The stoma (singular) closes at night, as there is limited light, and photosynthesis does not happen. When it is closed, the water cannot escape and the plant maintains moisture

They also close when supplies of water from the roots are limited. This stops photosynthesis, and the plant might lose too much moisture.

The opening and closing is controlled by the guard cells that surround the stomata. 

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How light affects gas exchange

PAPER TWO- Hyrdogen-carbonate indicator.

  • Orange= normal level of carbon dioxide in air.
  • Yellow= If the concentration of the carbon dioxide in the air increases.
  • Purple= If the carbon dioxide concentration decreases. 

Add the same volume of the indicator to four boiling tubes. Put three similar sized leaves into three of the tubes, and seal them all with a bung. Wrap one of the leaf tubes with foil, an the other with gauze. Then, place all the tubes in bright light. Leave the tubes for an hour, then check the colour of the indicator.

There should be no change in the control tube. In the tube wrapped in foil, the indicator should turn yellow. This is because no light is entering, so the leaf will only respire, not photosynthesise. Therefore the carbon dioxide concentration would increase. In the tube wrapped in gauze, you would expect no change, as there are both processes happening. This means that similar volumes of carbon dioxide are being produced, as they are being used up. Finally, the tube exposed to strong lighting should turn the indicator purple. This is because there would be lots of photosynthesis, and the leaf would take in more carbon dioxide than it produced. Thus the concentration of carbon dioxide would be lower.

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The respiratory system and Ventillation

The thorax is the top part of the body, and is separated from the lower part by the diaphragm. The lungs are surrounded by pleural membranes and are protected by the ribcage. The air that is breathed in travels down the trachea, and splits down the two bronchi (bronchus.) This air goes into each lung, and into the bronchioles. At the end of the bronchioles are alveoli where GAS EXCHANGE takes place. 

  • The intercostal muscles (between the ribs) contract, pulling the ribcage up and out. The diaphragm contracts which pulls it downwards. Therefore there is a decrease in pressure, and the air is drawn in.
  • The intercostal muscles relax, lowering the ribcage. The diaphragm relaxes and pushes upwards. This increases the pressure in the lungs, and air rushes out.

Test the effect of exercise: Sit still for 5 minutes and then count the number of breath intakes in one minute. Then exercise for 4 minutes, and count your breaths in one minute. Get 3 people to do this. Exercise increases breathing rate, since your muscles are respiring, and there is a higher demand of oxygen. 

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The Alveoli

There are millions of pulmonary alveoli (alveolus singular) where gas exchange happens. They are like air sacs, and their walls are one cell thick.  The blood flowing through the capillary surrounding the alveolus is de-oxygenated from the body, so there is a high concentration of carbon dioxide, and very little oxygen. Therefore, carbon dioxide diffuses from the blood, into the alveolus where there is little carbon dioxide from the air breathed in. This waste is then breathed out. Oxygen diffuses into the blood (low concentration) from the air in the alveolus where there is a higher concentration of oxygen. 

When the blood arrives at the body cells, oxygen (carried by the red blood cells where there is a high concentration) diffuses into the cell where there is a low concentration of oxygen. Also in the cell, there is a high concentration of carbon dioxide due to respiration, so it diffuses back into the blood where there is very little. The blood is then de-oxygenated again, and pumped back to the lungs. 

The alveoli are adapted for their job. Firstly, the huge number of the gives the lungs an enormous surface area. They also have permeable, one cell thick walls, so that gases don't have far to diffuse/ can easily diffuse in and out of the blood. There is also a moist lining to the alveolus so that the gases can easily dissolve. Finally, the alveolus has good blood supply, and is in close contact with the blood vessels so there remains a high concentration gradient, and the gases diffuse quickly. 

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The alveolus


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The risks of smoking tobacco

Smoking can damage the walls of the alveoli, reducing the surface area available for gas exchange and diffusion of gases. This therefore leads to diseases such as emphysema, where the sufferer finds they are often breathless, and that they have breathing difficulties. 

The tar in cigarettes also damages the cilia in the lungs and trachea, stopping them from doing the job (working with mucus to catch dust and bacteria before it enters the lungs.) Due to the cilia being damaged, chest infections would be more likely. 

In addition, tar irritates the bronchi and bronchioles, as a result more mucus is formed, however this cannot be cleared by the damaged cilia. This results in smoker's cough, and chronic bronchitis. 

Carbon monoxide found in cigarettes also reduces the volume of oxygen the red blood cells can carry, since CARBOXYHEMOGLOBIN is formed on the red blood cell. Because of this, the heart rate increases, which increases blood pressure. Furthermore, this damages the artery walls and makes the occurance of blood clots more likely. This increases the risk of coronary heart disease,

Tobacco contains carcinogens, which are chemicals that can cause mutations/damage to the DNA of a cell. This then multiplies causing cancerous tissues. 

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