Exchange - alex

Sourced from the AQA textbook. Includes plants, insects, diffusion, adaptations and fish.

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  • Created by: 2ralewis
  • Created on: 14-01-18 14:28
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  • Exchange
    • Diffusion
      • High Surface Area : Volume Ratio = gases can diffuse easily across into cells (e.g. leaf of plant)
    • Fish
      • Small Surface Area : Volume Ratio. Therefore requires a specialised gas exchange system.
        • The gills are made up of stacks of gill filaments which have gill lamellae. The gill lamellae greatly increase the surface area.
      • The counter-current system maintains a favourable concentration gradient across the entire gill.
        • If the counter-current system did not exist, then the concentrations of oxygen in the blood and water would reach equilibrium and diffusion would not occur.
      • Water enters the fish through the mouth and is then pushed over the gill filaments and then exits via the gills.
    • Single-celled organisms
      • They're small and have a high SA:Vol ratio.
      • Oxygen is absorbed by diffusion across their body surface. In the same way carbon dioxide diffuses out.
      • An example would be an Amoeba
    • Insects
      • Insects have evolved an internal network of tubes called tracheae. They're supported by strengthened rings to prevent them collapsing.
      • The tracheoles extend throughout all the body tissues of the insect and so atmospheric air is delivered right to the respiring tissues.
      • Respiratory gases move in and out of the tracheal system in three ways:
        • Along a diffusion gradient - when cells are respiring, O2 is used up so the concentration at the ends of the tracheoles falls and creates a diffusion gradient. A carbon dioxide diffusion gradient is created in the opposite direction.
        • Mass transport - The contraction of muscles in insects squeeze the trachea enabling mass movements of air in and out.
        • The ends of the tracheoles are filled with water - during major activity periods, the muscle cells around the ends of the tracheoles respire anaerobically. This produces lactate, which lowers the water potential of the muscle cells and so water moves into the cells by osmosis.
      • A limitation of the tracheal system is that it relies on diffusion in order to exchange gases. In order for diffusion to be effective there must be a short diffusion pathway. This means that insects cannot grow to a large size.
    • Plants (Leaves)
      • Photo-synthesis - carbon dioxide + water -> oxygen + glucose
      • Respiration - oxygen + glucose -> carbon dioxide + glucose + ATP
      • Respiration occurs 24/7. Photo-synthesis only happens in daylight
      • Although some carbon dioxide produced from respiration could be used for photo-synthesis, most is obtained is from external air. The same applies with oxygen from photo-synthesis.
        • Spongy mesophyll layer in the leaf. The numerous air-spaces means that gases readily come into contact with the cells. Air spaces provide a large surface area.
        • Thin so that no living cell is far from the external air, and therefore has a source of oxygen and carbon dioxide.
        • Many stomata found mainly on the underside of leaves. Controlled by guard cells to monitor the rate of gaseous exchange. Stomata close in order to prevent excessive water loss.
    • Adaptations to limit water loss
      • In Insects
        • Small SA:VOL ratio to minimise area in which water is lost
        • Waterproof coverings
        • Spiracles. They're mostly closed when the insect is at rest as closing the spiracles conflicts with the need for oxygen
      • In Plants (Leaves)
        • Xerophytes are plants that have adapted to living in extreme areas
        • A thick cuticle. The thicker = less water evaporation
        • rolled leaves to protect the stomata on the underside of the leaves and create a region of trapped still, moist air.
        • Hairy leaves to trap a layer of still air and reduce the water potential gradient between the inside and outside of the leaf. evaporation is therefore reduced
        • Sunken stomata to trap still, moist air and reduce the water potential gradient.
        • Reducing the SA:Vol ratio e.g. pine needles. The reduction in SA is balanced against the need for photo-synthesis




This is amazing! It really helped me with my revision.

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