A-Level Biology AQA Notes

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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.
Gas exchange in fish occur via gills. 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.

Gas exchange in dicotyledonous plants occurs in the leaves. The stomata can open to allow gases diffuse in and out of the leaf. The mesophyll cells have a large surface area for rapid diffusion.

Gas exchange can lead to water loss. Plants can control the opening of their stomata to limit this, and xerophytes may have additional adaptations such as: hairs, waxy cuticle, small leaves, sunken stomata, rolled leaves. Insects can also control water loss but controlling open and closing of their spiracles, hair around spiracles and a waterproof, waxy cuticle.

Human Gas Exchange System

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
Pulmonary ventilation rate is the total volume of air moved into the lungs during a minute.
Tidal volume is the volume of air moved in and out of the lungs with a normal breath.
Breathing rate is the number of breaths per minute.
Pulmonary Ventilation Rate (dm^3min^−1) = Tidal Volume (dm^3) × Breathing Rate (min^−1)

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