ALevel and AS Level Biology. Modul 3.1 Exchange and transport Summary and revision notes.
H020 and H420. OCR, Breadth and depth of biology. Featured in Paper 1,2,3.
7.1 Need for exchange surfaces:
There are two main reasons why diffusion alone is enough to supply the needs of single celled
organisms:
• Metabolic demands are usually low
• Large SA: V. ratio
Characteristics of an efficient exchange surface:
• Increases surface area-provides the area needed for exchange and overcomes the
limitations of a low SA:V ratio.
• Thin layers to give a shorter diffusion distance
• good blood supply- the steeper the concentration gradient the faster diffusion takes place
• ventilation-maintains diffusion gradient for gasses
7.2 Mammalian gaseous exchange system:
Nasal cavity:
• A large SA with a good blood supply
• a hairy lining-secrets mucus and traps dust
• moist surfaces-increases humidity reducing evaporation
Trachea: main airway from nose to chest
• Cartilage
1. incomplete C shaped
2. strong flexible rings prevents the trachea from collapsing
3. The rings are incomplete so that food can move easily down the oesophagus
• Ciliated epithelium and goblet cells
1. Goblets secrete mucus onto the lining of the trachea to trap dust or microorganisms
that escaped the nose lining.
2. The cilia beat and waft the mucus away from the lungs.
3. One of the effects of cigarette smoke is that these cilia stop beating
Bronchus: in the chest cavity:
Trachea divides into two bronchi which are similar in structure to the trachea but smaller
Bronchioles: bronchi divide to form smaller branches
• Cartilage- have none
• The walls of the bronchioles contain smooth muscle. The smooth muscle vasodilates and
vasoconstricts to change the volume of air reaching the lungs.
• Lined with flattened epithelium cells to make gas exchange possible
Alveoli: small air sacs where gas exchange takes place
• Each alveolus has a diameter of about 200-300 micrometres and consists of a layer of
flattened epithelial cells.
• Elastic fibres allow the alveoli to stretch and recoil
Adaptations:
• layer of squamous epithelial cells to give a short diffusion distance
• Large surface area to volume ratio
• good blood supply- millions of cappilaries surrounding it
• good ventilation
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