Substance Exchange

    Cards (187)

    • 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.
    • Ringing - removal of the bark and phloem, theoretically prevents translocation to the sinks below the ring.
    • To track the movement of sugars through the phloem, scientists use radioactive isotopes in tracer experiments with radioactive isotopes.
    • At the sink, solutes are removed by osmosis.
    • Gas exchange in fish occurs 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, 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 by controlling the open and closing of their spiracles, hair around spiracles and a waterproof, waxy cuticle.
    • In humans, gas exchange occurs via the lungs.
    • The alveolar epithelium is adapted for gas exchange by having a large surface area, good blood supply, thin walls and 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, pressure in the thorax decreases so the atmospheric pressure is greater than the 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.
    • 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 surfaces have a large surface area, thin barriers and associated transport systems to maintain a steep diffusion gradient.
    • Organisms with a higher metabolic rate require more nutrients and produce more waste, therefore require a specialised exchange surface.
    • Red blood cells transport oxygen using the protein haemoglobin.
    • Haemoglobin is made up of four polypeptide chains, each containing a prosthetic haem group.
    • Each haem group binds one oxygen molecule.
    • Binding of the first O2 molecule causes a conformational change in the haemoglobin, making the haem groups more accessible to oxygen.
    • Amino acids and carbohydrates are absorbed via co-transportation with sodium.
    • Water evaporates from the leaves creating tension (transpiration), and the cohesive nature of water moves the whole column of water up the xylem (cohesion-tension theory).
    • Animals living in a low partial pressure of oxygen are shifted to the left promoting oxygen association due to a high oxygen affinity.
    • Tissue fluid.
    • Capillaries - area of metabolic substance exchange.
    • This releases the non-polar monoglyceride and fatty acids, which diffuse straight into the epithelial cell.
    • During digestion, large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes.
    • Remaining fluid returns to circulation via the lymphatics system.
    • Sucrose is actively transported into the companion cells and moves via diffusion into the sieve tube followed by the osmosis of water.
    • Assimilates move from area of high to low pressure (mass flow).
    • Chylomicrons leave epithelial cells by exocytosis and move into lacteals.
    • Bile salts made by the liver, emulsify lipids in order to increase the surface area of the lipids, for greater access to lipases.
    • Venule: Hydrostatic pressure < water potential.
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