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Revised Tested Examready


Unit 1 Biology and disease

1 The causes of disease XX Pathogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX Lifestyle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■2 The digestive system XX The digestive system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX How enzymes work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX Enzyme properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ xx Carbohydrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX Carbohydrate digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■3 Cells, diffusion and absorption XX Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX Lipids and plasma membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX Osmosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX Active transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX Absorption of glucose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX Cholera — a bacterial disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■4 The lungs and lung disease XX Lung function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX The biological basis of lung disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■5 The heart and heart disease XX Heart structure and function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX The biological basis of heart disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■6 The immune system XX Principles of immunology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■

My revision planner

Unit 2 The variety of living organisms

7 The nature of variation XX Investigating variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX Causes of variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■8 DNA and cell division XX Structure of DNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX Genes and polypeptides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ XX DNA and chromosomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■

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1 The causes of disease

There are three basic causes of disease:● pathogens● lifestyle● genes

The actual cause of any one disease is often a complex mixture of two or more of the above factors. For example, some people inherit genes that make them more likely to develop heart disease or a particular type of cancer, but their lifestyle also has a lot to do with it.

PathogensTypes of pathogen

Pathogens are disease-causing organisms. Most pathogens are microorganisms such as such as bacteria, viruses and fungi, but there are also larger pathogens such as worms and flukes.

The terms transmissible, infectious and communicable all refer to the same thing: diseases you can catch. Sneezing, kissing, drinking dirty water, insect bites — there are lots of ways to transfer pathogens from one person to another.

BacteriaBacterial diseases include tuberculosis (TB), cholera and typhoid. Bacteria often cause disease because their waste products are toxic. For example, the bacteria that cause botulism (severe food poisoning) produce a toxin that paralyses muscles. This is the ‘botox’ (botulinum toxin) that is commonly used in cosmetic surgery.

VirusesViruses are tiny, infectious particles. They consist simply of some DNA or RNA (containing a few genes) surrounded by an outer protein coat. They are not classed as living organisms because they do not perform the classic signs of life such as feeding, growth and excretion. All they do is reproduce, and they cannot do that on their own. They need a host cell. Viruses get into cells and use them to make thousands of identical copies of the virus, which have the potential to burst out and infect new cells.

FungiCompared to bacteria and viruses, fungi do not pose much of a threat to someone with a healthy immune system. Athlete’s foot and thrush are about as bad as fungal diseases get.

Revised

Pathogens are microorganisms that can cause disease.

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Unit 1 Biology and disease

1 The causes of disease 8 Pathogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 10 Lifestyle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■2 The digestive system 13 The digestive system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 14 Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 16 How enzymes work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 17 Enzyme properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 20 Carbohydrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 23 Carbohydrate digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■3 Cells, diffusion and absorption 26 Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 30 Lipids and plasma membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 33 Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 35 Osmosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 37 Active transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 37 Absorption of glucose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 38 Cholera — a bacterial disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■4 The lungs and lung disease 41 Lung function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 44 The biological basis of lung disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■5 The heart and heart disease 47 Heart structure and function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 51 The biological basis of heart disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■6 The immune system 55 Principles of immunology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■

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5My revision planner

Unit 2 The variety of living organisms

Revised Tested Examready7 The nature of variation

60 Investigating variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 62 Causes of variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■8 DNA and cell division 64 Structure of DNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 66 Genes and polypeptides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 68 DNA and chromosomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 69 Meiosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 70 Replication of DNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 72 Mitosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 73 Cell cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■

9 Genetic diversity and selection

77 Genetic diversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 79 Cell differentiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 80 Antibiotics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 80 Genetic variation in bacteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■

10 Surface area, volume, gas exchange and transport

84 Size and surface area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 85 Gas exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 87 Mass transport systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 94 Haemoglobin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 97 Carbohydrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 98 Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■

11 Biodiversity and classification

101 Principles of taxonomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 103 Genetic comparisons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 105 Behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■ 105 Biodiversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . . .■ . . . . . . . . . . . . . . . . .■

109 Now test yourself answers

Exam practice answers and quick quizzes at www.therevisionbutton.co.uk/myrevisionnotes

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6–8 weeks to go

l Start by looking at the specification — make sure you know exactly what material you need to revise and the style of the examination. Use the revision planner on pages 4 and 5 to familiarise yourself with the topics.

l Organise your notes, making sure you have covered everything on the specification. The revision planner will help you to group your notes into topics.

l Work out a realistic revision plan that will allow you time for relaxation. Set aside days and times for all the subjects that you need to study, and stick to your timetable.

l Set yourself sensible targets. Break your revision down into focused sessions of around 40 minutes, divided by breaks. These Revision Notes organise the basic facts into short, memorable sections to make revising easier.

One week to go

l Try to fit in at least one more timed practice of an entire past paper and seek feedback from your teacher, comparing your work closely with the mark scheme.

l Check the revision planner to make sure you haven’t missed out any topics. Brush up on any areas of difficulty by talking them over with a friend or getting help from your teacher.

l Attend any revision classes put on by your teacher. Remember, he or she is an expert at preparing people for examinations.

4–6 weeks to go

l Read through the relevant sections of this book and refer to the examiner’s tips, examiner’s summaries, typical mistakes and key terms. Tick off the topics as you feel confident about them. Highlight those topics you find difficult and look at them again in detail.

l Test your understanding of each topic by working through the ‘Now test yourself’ questions in the book. Look up the answers at the back of the book.

l Make a note of any problem areas as you revise, and ask your teacher to go over these in class.

l Look at past papers. They are one of the best ways to revise and practise your exam skills. Write or prepare planned answers to the exam practice questions provided in this book. Check your answers online and try out the extra quick quizzes at www.therevisionbutton.co.uk/myrevisionnotes

l Use the revision activities to try different revision methods. For example, you can make notes using mind maps, spider diagrams or flash cards.

l Track your progress using the revision planner and give yourself a reward when you have achieved your target.

My exams

AS Biology Unit 1

Date: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Time: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Location: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

AS Biology Unit 2

Date: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Time: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Location: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Revised

Revised

The day before the examination

l Flick through these Revision Notes for useful reminders, for example the examiner’s tips, examiner’s summaries, typical mistakes and key terms.

l Check the time and place of your examination.

l Make sure you have everything you need — extra pens and pencils, tissues, a watch, bottled water, sweets.

l Allow some time to relax and have an early night to ensure you are fresh and alert for the examination.

Revised

Revised

Countdown to my exams

179705_01_Biology_1-111.indd 6 31/10/12 12:16 PM

41Unit 1 Biology and disease

4 The lungs and lung disease

Lung functionWhy exchange gas?

It’s all about respiration — the process that releases the energy locked in organic molecules such as glucose and lipids. All cells, in all living things, respire all the time. The process requires a constant supply of oxygen and produces carbon dioxide that needs to be eliminated.

The lungs are organs that are adapted for gas exchange in air. Their function is simple: to get as much fresh air as possible in contact with blood. As you can see in Figure 4.1, the bronchial tree is a branching system of tubes that takes air to the alveoli deep in the lungs. All of the tubes are held open by rings of cartilage except the terminal bronchioles.

Rib section

Right bronchus

Pleural fluid(acts as a lubricant)

Intercostalmuscles

Muscularpart of thediaphragm

Two pleural layersLeft lung

Terminalbronchiole

Alveolus

Spaceoccupiedby heart

Trachea

Figure 4.1 The structure of the human gas exchange system

The alveolar epithelium

The alveolar epithelium is a surface over which gas exchange takes place. Its structure is related to function:● Millions of alveoli provide a large surface area.

Revised

Examiner’s tipVentilation and breathing refer to the same process, but respiration is different. Make sure you use the correct terms.

Revised

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4 T

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ease ● The alveolar walls consist of flat cells called squamous epithelium

that are as thin as possible. This makes the diffusing pathway as small as possible.

● The dense network of blood capillaries around the alveoli means that a lot of blood is in close contact with the air.

● The beating of the heart, along with constant breathing, makes sure that deoxygenated blood meets fresh air. This maintains a diffusion gradient.

All these features serve to make diffusion as fast and efficient as possible. The rate of diffusion of a substance across an exchange surface is inversely proportional to the thickness of the exchange surface.

The exchange of gases at the alveoli

The exchange of gases at the alveoli (Figure 4.2) occurs by simple diffusion. Oxygen and carbon dioxide are small, simple molecules that pass easily through cell walls. There is more oxygen in the air than in the blood, so oxygen diffuses into the blood. There is more carbon dioxide in the blood than in the air, so this gas diffuses in the opposite direction.

Capillaries

Alveoli

Alveolarduct

Alveolarsac

Pulmonary arteriole(deoxygenated blood)

Pulmonary venule(oxygenated blood)

Terminalbronchiole

Figure 4.2 Alveoli are found at the end of terminal bronchioles

Pulmonary ventilation

Pulmonary ventilation is the amount of air we inhale in 1 minute. It is calculated as:

pulmonary ventilation = tidal volume × ventilation rate per minute

Typical values would be 0.5 litres for tidal volume and 14 breaths per minute, giving a pulmonary ventilation of 7 litres per minute.

Tidal volume is the volume of air breathed in and out with each breath.

Ventilation rate is the number of breaths per minute.

Revised

Examiner’s tipGas exchange always occurs by diffusion. There is no active transport, facilitated diffusion or osmosis.

Revised

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4 T

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easeLung volumes are measured using a spirometer, which produces a trace

like the one in Figure 4.3. When we exercise, our breathing undergoes a series of changes. Key points on the trace are labelled with the letters A to E:● A to B shows normal, tidal breathing while at rest.● B to C shows the effect of exercise — breathing is deeper and more

frequent.● C to D shows a return to normal after exercise.● D shows the subject breathing out as far as possible.● E shows the subject inhaling as much as possible.● So the difference in volume between D and E shows the vital

capacity — the total usable lung volume.● Residual volume is the air that must remain in the lungs.

5

4

3

2

1

0

Volumeof air in

lungs/dm3

0Time/s

30 60 90 120 150

tidalvolume

A B C D E

vitalcapacity

residualvolume

totallungcapacity

Figure 4.3 Changes in breathing during exercise

The mechanism of breathing

The lungs are spongy organs that simply fill with air and then empty again. They cannot do this on their own because they have no muscle. To inflate, they need the intercostal muscles and the diaphragm.

The mechanism of breathing is shown in Table 4.1. Inspiration is an active process — it requires energy. Expiration is largely a passive process. The lungs, diaphragm and intercostal muscles are all elastic. When stretched, they tend to go back to their original shape.

Table 4.1 The mechanism of breathing

Stage Breathing in (inspiration) Breathing out (expiration)

1 The external intercostal muscles contract, pulling the ribs up and out

The external intercostal muscles relax

2 The diaphragm muscles contract, flattening the diaphragm

The diaphragm relaxes. The abdominal organs (liver and intestines) push upwards

3 Their combined effect is to increase the volume of the thorax

The volume of the thorax decreases

4 This lowers the pressure in the thorax to below atmospheric pressure

The pressure inside the thorax increases

5 Atmospheric pressure forces air into the lungs Air is forced out

Examiner’s tipYou may have been taught terms such as inspiratory reserve volume and expiratory reserve volume, but they are not in the specification and so cannot be tested in the exam.

Revised

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4 T

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ease The biological basis of lung disease

Pulmonary tuberculosis

Pulmonary tuberculosis (TB) is an infection that usually affects the lungs and is caused by the bacterium Mycobacterium tuberculosis. Transmission is by means of inhaling droplets from an infected person if they cough or sneeze over a period of time. You won’t get TB if an infected person coughs or sneezes on you just once.

The bacteria set up small pockets of infection in the lungs. The immune system surrounds the infection and creates nodules called tubercles. These reduce the surface area available for gas exchange. Symptoms of the disease include:● constant coughing that produces sputum/phlegm● loss of appetite● night sweats● weight loss

When antibiotics became available in the early 1940s, they were very effective against TB and so the disease became rarer. However, resistant strains of bacteria have since developed and TB now has to be treated with a combination of antibiotics over a number of months.

TB is prevented by the BCG vaccine, which used to be recommended for all UK children but now it is restricted to high-risk groups such as health workers or people who have travelled from a country where the incidence of TB is high.

Asthma, fibrosis and emphysema

Asthma is a condition in which the muscles lining the terminal bronchioles constrict and over-produce mucus. Both of these responses narrow the airway, making it particularly difficult to breathe out. The narrow airway produces a wheezing sound during exhaling. In asthmatics, an attack is brought on by an environmental trigger, which can be stress or cold air, but is commonly an allergen such as house dust or animal fur. Asthma affects lung function because it reduces air flow. It does not damage alveoli and so there is no reduction in surface area.

Fibrosis is a response to tissue damage. When normal body tissue such as lung, liver, bone or heart becomes damaged, the body responds by producing connective tissue, commonly called scar tissue.

Emphysema is fibrosis of the alveoli. Lung damage results from long-term exposure to irritants such as cigarette smoke and air pollution. When the delicate alveolar walls become damaged and replaced by connective tissue, the effects are:● reduced surface area● increased diffusing pathway due to the thicker walls

Revised

Now test yourself

1 Suggest why hostels for the homeless can be a common source of new TB infections.

2 If you were to look down a microscope at the organisms that cause TB, would they have a nucleus? Explain your answer.

Answers on p. 109Tested

Revised

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4 T

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easeBoth of these combine to make gas exchange less efficient. Emphysema is

long-term, irreversible damage to the lungs.

(a) (b)

Figure 4.4 (a) Healthy alveoli (b) Alveoli in a person with emphysema

There will always be questions about data in the exam. Tables and graphs are nothing to be afraid of. In fact, they often bring easy marks as the information is there for you.

Example question 1Study the graph below, which shows the change in the number of asthma patients and four common air pollutants over a 20-year period. The air pollutants are particle matter, sulphur dioxide, nitrogen oxide and carbon monoxide.

Describe the pattern of incidence of asthma. [3]

200%1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996

Year

100%

0%

100%

200%

300%

400% all 4 main pollutantsasthma incidence

AnswerYou have to match up your answer to the number of marks. If the graph is one straight line, there will usually be 1 mark for writing something like ‘as X increases, Y increases’. However, for a more complex graph like the one here, there will often be 2 or 3 marks available. A good answer is:● There is a slow increase up to 1986.● There is a rapid increase from 1986 to 1993.● There is a fall after the peak in 1993.

Note that the question says ‘describe’. There is no need to explain anything here.

Example question 2Evaluate the claim that air pollution causes asthma. [3]

AnswerEvaluate means to look at both sides. A good answer is:● There is a correlation between the number of cars and

the incidence of asthma.● However, a correlation does not mean a cause.● Something else could be causing the increase, such as

more allergens.

Graphs and patterns Exam technique

Now test yourself

3 Explain the difference between the terms respiration and breathing.4 List three ways in which the lungs are adapted to speed up the process of diffusion.

Answers on p. 109–110

Tested

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4 T

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1 The graph below shows the pressure and volume changes in the lungs during one breathing cycle.

+0.15

+0.10

+0.05

0.0

–0.05

–0.10

–0.15

Pressurein

alveoli/kPa

0.50

0.40

0.30

0.20

0.10

0.0

Volumeof air in

lungs/dm3

0 1 2Time/s

3 4

(a) During which time is the person:

(i) inhaling [1](ii) exhaling [1]

(b) Use the scale to work out the ventilation rate. [1](c) What is value of the tidal volume? [1](d) Work out the pulmonary ventilation in litres. [1]

2 An individual who suffers from emphysema will find exercise difficult, often getting out of breath simply by walking upstairs. Explain why. [4]

Answers and quick quizzes onlineOnline

Examiner’s summaryBy the end of this chapter you should be able to understand:

4 The basic structure of the human gas exchange system including the alveoli and bronchial tree.

4 The features of the alveolar epithelium as a surface over which gas exchange occurs.

4 How the lungs are adapted for gas exchange.

4 Pulmonary ventilation as the product of tidal volume and ventilation rate.

4 The mechanism of breathing.

4 The course of infection, symptoms and transmission of pulmonary tuberculosis.

4 The effects of fibrosis, asthma and emphysema on lung function.

179705_01_Biology_1-111.indd 46 31/10/12 12:16 PM

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