CCEA GCSE Biology - Denmour Boyd, James Napier 2017
Unit 1
The respiratory system and cell respiration
Specification points
This chapter covers specification points 1.5.1 to 1.5.7. It covers the chemical processes of aerobic and anaerobic cell respiration and the factors affecting the respiration of yeast, the structures and adaptations of the respiratory system, the mechanism of breathing and the effects of exercise on breathing.
In Double Award Science, specification points 1.5.1 to 1.5.6 are covered, relating to the chemical processes of aerobic and anaerobic cell respiration, the factors affecting the respiration of yeast, the adaptations of the respiratory system and the effects of exercise on breathing.
Respiration
Respiration, sometimes called cell respiration, is a series of chemical reactions in every cell that continuously releases energy from food molecules. The different cells of the body can use the energy released to produce heat in the body, for movement, growth, reproduction and to carry out active transport.
Tip
Aerobic respiration is respiration which uses oxygen.
The reactions of cell respiration are described as exothermic because they release energy. They take place in special structures in the cell cytoplasm, the mitochondria, described in Chapter 1.
The cells in most living organisms use oxygen during aerobic respiration to help release energy. The word equation which summarises aerobic respiration is:
Tip
Anaerobic respiration is respiration without using oxygen.
The balanced chemical equation for aerobic respiration is:
When not enough oxygen is available, some cells, for example muscles and organisms like yeast, can still release a small amount of the energy in food molecules using a series of reactions known as anaerobic respiration.
Tip
The energy released by aerobic respiration is almost 20 times the energy released by anaerobic respiration.
This can happen during strenuous exercise in human muscles when anaerobic respiration causes lactic acid to build up, leading to muscle soreness. The word equation for anaerobic respiration in muscles is:
Anaerobic respiration by yeast produces alcohol (ethanol) and is the basis of wine, beer and bread production. The word equation is:
To demonstrate this, a solution of glucose is first boiled to remove any dissolved oxygen and to sterilise it. Yeast cells are added to this solution only after it has cooled to a temperature that will not kill the yeast. The solution of glucose with yeast is then placed in the apparatus shown in Figure 5.1. As the yeast respires anaerobically it produces carbon dioxide and alcohol, and releases energy in the form of heat.
Figure 5.1 shows how anaerobic respiration can be demonstrated using yeast.
Show you can
Compare the products of anaerobic respiration in muscles and yeast.
Test yourself
1 List the ways the different cells of the body use the energy released by respiration.
2 Which type of respiration releases less energy from a molecule of glucose?
3 Describe one situation in the human body when anaerobic respiration can take place.
4 Give the balanced chemical equation for aerobic respiration.
Prescribed practical
Biology Practical 1.5: Investigating factors affecting the respiration of yeast
Investigate the effect of temperature on the respiration of yeast
Procedure
1 Mix 3 g of yeast with 4 g of glucose with 100 cm3 of boiled and cooled water in a beaker. Remember to wear eye protection.
2 Leave at room temperature for 1 hour.
3 Fill a small test tube with this mixture.
4 Place a boiling tube upside down over the small test tube containing the yeast mixture and quickly invert both tubes.
5 Place the tubes in a water bath at a temperature of 10 °C (Figure 5.2).
6 Measure the height of the bubble at the top of the inner small tube.
7 Repeat steps 2—6 at temperatures 20 °C, 30 °C, 40 °C and 50 °C.
8 After 30 minutes measure the height of the bubble at the top of each of the inner tubes.
9 Copy Table 5.1 and use it to record the results and calculate the change in height of the bubble.
10 Draw a graph of the change in height against the temperature.
Tip
Although DAS students are expected to carry out practical work to investigate the respiration of yeast, they do not do so as a prescribed practical.
Sample results and questions
1 Explain why the glucose solution was boiled and cooled before the yeast was added.
2 Which type of respiration is the yeast mainly using in this experiment?
Table 5.2 shows data for the effect of temperature on the respiration of yeast
3 Which temperature showed the fastest respiration by yeast?
4 Explain the results at 15 °C and 55 °C.
Investigate the effect of different sugars on the respiration of yeast
Procedure
1 Mix 3 g of yeast with 4 g of glucose with 100 cm3 of water in a beaker.
Wear eye protection during this investigation.
2 Leave at room temperature for 1 hour.
3 Fill a small test tube with this mixture.
4 Place a boiling tube upside down over the small test tube containing the yeast mixture and quickly invert both tubes.
5 Place the tubes in a water bath at a temperature of 30 °C (Figure 5.2).
6 Measure the height of the bubble at the top of the inner small tube.
7 Repeat steps 1—5 with yeast mixtures containing sucrose, fructose, lactose and maltose.
8 After 30 minutes measure the height of the bubble at the top of each of the inner tubes.
9 Copy Table 5.3 and use it to record the results and calculate the change in height of the bubble.
Sample results and questions
Table 5.4 shows data for the effect of different sugars on the respiration of yeast.
1 Draw a bar graph of the change in height produced by the different sugars using this data.
The respiratory system
As we learned in Chapter 1 (page 8), multi-celled organisms have developed special respiratory surfaces inside their bodies (lungs) to efficiently exchange gases with their environment.
In humans the respiratory system is inside the thorax, a space surrounded by ribs of bone with intercostal muscles between them and a muscular sheet, the diaphragm below, as shown in Figure 5.6. Air enters through the nasal cavity, where it is warmed and filtered before continuing into the lungs and the other parts of the respiratory system. Figure 5.3 shows the lungs and other parts of the respiratory system.
Respiratory surfaces
In humans gas exchange takes place in the alveoli. Oxygen diffuses into the blood and carbon dioxide diffuses from the blood into the alveoli, where it is then breathed out.
Respiratory surfaces are adapted in a number of ways. For example, in humans they have:
• a large surface area — there are many alveoli in each lung, and each alveolus has a large surface area; together these give a gas exchange surface (where the alveolar walls are in contact with blood capillaries) in humans of many square metres
• thin walls with short diffusion distances — Figure 5.3 shows that there are only two layers of cells separating the oxygen in the alveolus from the red blood cells; this means that there is only a short diffusion distance for the gases involved
• moist walls — these help the gases to pass through the respiratory surfaces because the gases dissolve in the moisture
• permeable surfaces — the moist, thin walls make the respiratory surfaces permeable
• a good blood supply — alveoli are surrounded by capillaries to ensure that any oxygen diffusing through is carried around the body; this also ensures that carbon dioxide is continually taken back to the lungs
• a diffusion gradient — the process of breathing ensures that there is a large diffusion gradient that encourages oxygen to diffuse into the blood and carbon dioxide to diffuse from the blood into the alveoli; when fresh air rich in oxygen is breathed in, it makes the concentration of oxygen in the alveoli higher than that in the capillaries and therefore oxygen diffuses from the alveoli into the capillaries.
Tip
A permeable surface allows substances to pass through it.
Respiratory surfaces in plants
The same principles apply to the respiratory surfaces in plants. The main respiratory surfaces in plants are the spongy mesophyll cells surrounding the air spaces in the leaves. Because there are a lot of cells in contact with the air spaces, there is a large surface area and the cell membranes (where gas exchange takes place) are thin, moist and permeable. Figure 5.4 shows how the spongy mesophyll of a leaf is adapted for gas exchange.
The process of breathing
The process of breathing can be examined using a lung model. Many lung models resemble the bell jar apparatus shown in Figure 5.5. Lungs are represented by balloons and the diaphragm is represented by a thin sheet of rubber.
When the rubber representing the diaphragm is pulled down the lungs inflate and they deflate as the rubber sheet is released.
This model demonstrates the following key features of the breathing process:
• As the diaphragm (rubber sheet) moves down, the volume inside the glass jar increases.
• This causes the pressure inside the glass jar to decrease.
• This causes air to enter the lungs until the pressures inside and outside the bell jar become equal.
In reality the process of breathing is more complicated than is shown by the model.
The diaphragm and intercostal muscles, together with the other parts of the respiratory system shown in Figure 5.3, are involved in the processes of inhaling and exhaling (breathing). Breathing ensures that fresh air rich in oxygen is brought into the lungs and air rich in carbon dioxide is expelled from the lungs. Figure 5.6 shows the process of breathing in humans.
There are a number of important differences between the lung model in Figure 5.5 and the process of breathing in humans.
• In humans, the ribs move out and in and work together with the diaphragm in changing the volume of the thorax. In the lung model, only the diaphragm (rubber sheet) is involved.
• In humans, the diaphragm is normally a domed shape — it flattens when breathing in (in the bell jar it starts as a flat shape and is pulled down).
• The space between the lungs (balloons) and the chest wall (glass jar) is much greater in the model than between the lungs and ribcage in reality.
Tip
If an injury causes a hole in the chest wall, inhaling becomes very difficult. As the volume of the chest increases and the pressure decreases, air will be drawn into the chest cavity through the hole and the lungs will only partially inflate.
Pleural membranes line the inside of the chest wall (ribs) and the outside of the lungs. Their role is to reduce friction during breathing. The space between the membranes (the pleural cavity) usually contains a small amount of pleural fluid, which further helps to reduce friction during breathing.
Test yourself
5 Name the part of the lungs where gas exchange takes place.
6 Explain why moist surfaces are beneficial for gas exchange.
7 Which part of the respiratory system is represented by a rubber sheet in a lung model?
8 What is the function of the pleural membranes during breathing?
Show you can
Use Figure 5.3 to help describe the pathway taken by a molecule of oxygen from the air into the blood.
The effect of exercise on breathing
To investigate the effect of exercise on breathing rate and recovery rate you first need to calculate your breathing rate at rest. Then carry out vigorous exercise for a short period of time. Measure your breathing rate immediately after exercising and then at intervals, for example every minute, until it returns to normal. The time taken for the breathing rate to return to normal can be referred to as the recovery time. Compare your results with other members of your class.
Tip
Breathing rate is the number of breaths taken each minute.
You should be able to use the results you obtain to discuss the following points.
• Is there a link between levels of fitness and breathing rate?
• Is there a link between levels of fitness and recovery time?
• If a student became fitter over time, how would this affect his or her recovery time?
Example
Figure 5.7 shows the results of an investigation into how exercise affected the breathing rate of two boys.
The results show Rory is fitter because he shows:
• a lower resting breathing rate (between 0—5 minutes)
• a slower rate of increase during exercise (between 5—10 minutes)
• a lower maximum breathing rate (between 10—15 minutes)
• a faster recovery to his normal resting breathing rate after exercise (between 15—20 minutes).
Tip
Depth of breathing is the volume of air breathed in during each breath.
As well as increasing the rate of breathing, exercise can increase the depth of breathing.
Explaining the effect of exercise on breathing
Exercise involves the movement of muscle cells which use energy released from food (glucose) molecules by cell respiration. Cell respiration, as we have learned, requires oxygen and produces carbon dioxide. Increased exercise therefore means that the muscle cells will use more oxygen from the blood and pass more carbon dioxide into the blood. When that blood reaches the lungs, a larger volume of air is needed to replace the oxygen and remove the carbon dioxide. The increased depth and rate of breathing is how the body brings in a larger volume of air and increases the rate of gas exchange.
Practice questions
1 Figure 5.8 shows a model of the respiratory system.
a) Name the parts of the respiratory system represented by the glass tube and the bell jar.
(2 marks)
b) Describe and explain what would happen to the balloons if the rubber sheet was pushed up.
(3 marks)
2 Figure 5.9 shows an alveolus.
a) Name parts A and B.
(2 marks)
b) Make a copy of the alveolus and draw an arrow to show the pathway of a molecule of carbon dioxide from the blood.
(1 mark)
c) Describe one feature of the alveolus and explain how it increases the rate at which oxygen is absorbed.
(2 marks)
3 a) Copy and complete the balanced chemical equation for aerobic respiration.
(3 marks)
Aerobic respiration uses oxygen; anaerobic respiration does not.
b) Describe other ways aerobic respiration differs from anaerobic respiration in yeast.
(3 marks)
c) i) Give the word equation for anaerobic respiration in human muscles.
(2 marks)
ii) Describe one difference between the products of anaerobic respiration in human muscles and yeast.
(1 mark)