A-Level Biology OCR Notes
3.1.1 Exchange surfaces
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Surface Area to Volume Ratio
- The greater the size of an organism, the smaller its surface area: volume ratio
- Larger organisms therefore require specialised exchange surfaces and transport mechanisms to meet their metabolic requirements
- Specalised exchange surface have: a large surface area, thin barriers and associated transport systems to maintain a steep diffusion gradient.
- Also, organisms with a higher metabolic rate require more nutrients and produce more waste, therefore require a specialised exchange surface
- Larger organisms therefore require specialised exchange surfaces and transport mechanisms to meet their metabolic requirements
- Specalised exchange surface have: a large surface area, thin barriers and associated transport systems to maintain a steep diffusion gradient.
- Also, organisms with a higher metabolic rate require more nutrients and produce more waste, therefore require a specialised exchange surface
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- The alveolar epithelium is adapted for gas exchange by having a large surface area, good blood supply, thin walls & elastic fibres which help recoil
- Ventilation is the process of breathing in (inspiration) and out (expiration).
- Inspiration: external intercostal muscles contract, rib cage moves up & out, diaphragm contracts, volume of the thorax is increased, atmospheric pressure is greater than pulmonary pressure and air is forced into the lungs.
- Expiration: internal intercostal muscles contract, ribs move down and inwards, diaphragm relaxes, volume of the thorax is decreased, pulmonary pressure is greater than atmospheric pressure, air is forced out of the lungs
- Spirometers measure the volume and function of the lungs. Inhalation and expiration moves the lid of the spirometer which is recorded by the datalogger. Carbon dioxide is absorbed by soda lime in the chamber.
- Vital capacity is the maximum amount of air that can be moved by the lungs in one breath
- Residual volume is the volume of air left in the lungs after a forced expiration
- Tidal volume is the volume of air moved in and out of the lungs with a normal breath.
Gas Exchange
- Single celled organisms can exchange oxygen and carbon dioxide directly through their plasma membrane via diffusion.
- Insects exchange gas in their tracheal system. Air enters via spiracles, travels through trachea and tracheoles, delivering oxygen directly to every tissue. Tracheal fluid limits diffusion to cells. Larger insects can also ventilate by movement of the abdomen, flight muscles changing the volume of the thorax and the presence of air sacs.
- Gas exchange in fish occur via gills.
- They ventilate their gills by a buccal-opercular pump
The orientation of the gill filaments and lamellae ensures that the water flowing over them moves in the opposite direction to the flow of blood through the capillaries (countercurrent flow), maintaining a diffusion gradient.
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