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Knowledge check reminds you of the ideas you should have already encountered in previous work before starting the section.
Learning objectives show you what the syllabus requires in the section.
Gas exchange in humans
INTRODUCTION
Respiration uses oxygen from the air and produces carbon dioxide that is returned to the environment. These gases must get into and out of the body fast enough to support the rate at which respiration needs to work. For single-celled organisms this isn’t a problem. They have a large surface area to volume ratio, and diffusion across the cell membrane can supply and remove the Δ Fig. B11.1 The lungs are the site of gas gases at a fast enough rate. Larger organisms exchange in humans. cannot do this. Not only do they have a much smaller external surface area to volume ratio, which slows the rate of diffusion, but many of them also live on land, where the delicate surface required for gas exchange would dry out if it was directly exposed to the external environment. Different groups of organisms have different solutions to these problems but all involve structures with a large surface area. Plants exchange gases inside the leaf; insects have internal tubes (a tracheal system) inside the body where they exchange gases; fish have gills; and many vertebrates, including humans, have lungs.
KNOWLEDGE CHECK
✓Animals use oxygen from the air inspired and give out the carbon dioxide they produce in the air they expire. ✓Humans use lungs for breathing.
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LEARNING OBJECTIVES
✓Identify in diagrams and images the following parts of the breathing system: lungs, diaphragm, ribs, intercostal muscles, larynx, trachea, bronchi, bronchioles, alveoli and associated capillaries. ✓Investigate and describe the effects of physical activity on the rate and depth of breathing. ✓suPPLemenT Describe the features of gas exchange surfaces in humans, limited to: large surface area, thin surface, good blood supply, and good ventilation with air.
The human breathing system
Breathing is the way that oxygen is taken into our bodies and carbon dioxide is removed. When we breathe, air is moved into and out of our lungs. This involves different parts of the breathing system within the chest. When we breathe in, air enters though the nose and mouth. In the nose the air is moistened and warmed. The air passes over the larynx, where it may be used to make sounds, for example when we talk. The air travels down the trachea to the lungs. The air enters the lungs through the bronchi (singular: bronchus), which branch and divide to form a network of bronchioles.
Science in context boxes put the ideas you are learning into real-life context. The content in these boxes is beyond the requirements of the syllabus. However, they do provide interesting examples of scientifi c application that are designed to enhance your understanding and a challenge question to encourage you to think more deeply beyond the syllabus content.
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●SCIENCE
IN CONTEXT CONDITIONS FOR GERMINATION
Different plants are suited to different climates. Those that are adapted to colder climates will germinate at lower temperatures. They may also need a very cold period followed by an increase in temperature before they will germinate.
Other seeds will not germinate until they have been exposed to very high temperatures, such as the heat from a forest fire. The extreme heat weakens the seed coat so that water can enter the seed and germination can begin.
Germinating after a fire means that there is likely to be less competition with other species that usually cover the ground. Also, the ash left from the burning acts as a natural fertiliser for the new plants.
Challenge Question: If a winter is warmer than usual, some plant seeds may germinate and start to flower earlier than normal. Suggest why this could be a disadvantage for the plants.
Δ Fig. B14.10 Fire clears the ground of competing plants, and stimulates some seeds to germinate in ideal conditions.
Water
Water is required to swell the seed and burst the seed coat. All seeds contain some water, but during germination metabolic reactions are being carried out rapidly. More water is needed for: • activation of hormones and enzymes • breakdown of storage compounds, e.g. conversion of starch to glucose • transport of materials to be used for respiration and growth • metabolic reactions and enzyme actions that occur in solution.
Oxygen
Active living cells respire and the most useful form of respiration, aerobic respiration, requires oxygen. Seeds can use anaerobic respiration for a short while, but the rate at which energy is released is very slow (not useful in an actively growing organism) and the byproducts are toxic. That is why most seeds will only germinate successfully if there is plenty of oxygen in the soil. Δ Fig. B14.11 Waterlogged soil excludes oxygen, making it difficult for seeds to germinate and grow. At the end of the bronchioles are air sacs. The bulges on an air sac are called alveoli (singular: alveolus). The alveoli are covered in tiny blood capillaries. This is where oxygen and carbon dioxide are exchanged between the blood and the air in the lungs. This is called gas exchange. The movement of air across the alveolar surface is called ventilation.
air sacs and their blood supply
blood away from air sac bronchiole (air in and out)
blood to air sac
alveolus
blood capillary rib
intercostal muscle
bronchiole larynx
trachea
lung
bronchus
heart
air sac with alveoli
Δ Fig. B11.2 The human breathing system.
diaphragm rib
QUESTION
1. Give the structures of the human breathing system.
●SCIENCE
IN CONTEXT VENTILATION BY MACHINE
Sometimes accident or illness can damage a person’s ability to breathe. As exchange of gases is essential for respiration, and so for life, it is crucial that this process is continued artificially until the patient is sufficiently recovered to be able to do it independently again.
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Examples of investigations are included with questions matched to the practical skills you will need to learn. It is not expected you will need to perform or learn all the methods of the example investigations
Developing practical skills
We can investigate the particular conditions for germination.
Devise and plan investigations
1. Using the apparatus shown, write a plan to investigate the effect of a) light b) water c) temperature on the germination of seeds. Think carefully about what controls to use in each case.
Petri dish lined with damp paper towel
seed
Make observations and Δ Fig. B14.12 A simple set-up for investigating seed measurements germination. An investigation was carried out using two Petri dishes containing 20 seeds of the same species. Both dishes received the same amount of light and water, but they were kept at different temperatures. The table shows the number of seeds that germinated over a period of 8 days.
Day Total number germinated seedlings Cool/10 °C Warm/20 °C
1 0 0 2 0 0 3 0 5 4 1 11 5 6 15 6 16 17 7 18 17 8 18 17
2. Display the results of this investigation in a suitable way.
Analyse and interpret data
3. Describe the patterns shown by these results. 4. Give a conclusion from these results. 5. Explain the results using your scientific knowledge.
