Biology: Unit 3 Gas and Solute Exchange

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  • Created by: Nat Meier
  • Created on: 05-05-13 19:06
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  • Biology: Unit 3 Gas and Solute Exchange
    • Substances move by diffusion, osmosis and active transport
    • Life processes need gases or other dissolved substances before they can happen
      • For example, for photosynthesis to happen, CO2 and water need to get into plant cells.
      • And for respiration to take place, glucose and oxygen both have to get inside cells.
        • For example, for photosynthesis to happen, CO2 and water need to get into plant cells.
    • Waste substances also need to move out of the cells so that the organism can get rid of them.
    • These substances move to where they need to be by diffusion, osmosis and active transport...
      • Diffusion: where particles move from an area of high concentration to low. Eg: gases can diffuse through one another, like smell spreading through a room
        • Alternatively, dissolved particles can diffuse in and out of cells through cell membranes
      • Osmosis: movement of water molecules across a partially permeable membrane  from a region of high water concentration to a region of low water concentration.
      • Active Transport: This is when substances need to move in the other direction from what diffusion and osmosis normally do (high concentration to low concentration)
    • In life processes, the gases and dissolved substances have to move through some sort of sxchange surface.
      • The exchange surface structures have to allow enough of the necessary substances to pass through
        • Exchange surfaces are adapted to maximise effectiveness...
          • They are thin, so substances only have a short distance to diffuse
          • They have a large surface area so lots of a substance can diffuse at once
          • Exchange surfaces in animals have lots of blood vessels, to get stuff into and out of the blood quickly
          • Gas exchange surfaces in animals (like alveoli) are often ventilated too-air moves in and out
    • Exchanging substances gets more difficult in bigger and more complex organisms
      • the place where the substances are needed (or the waste is made) ends up being a long way away from exchange surfaces
    • The structure of leaves lets gases diffuse in and out of cells
      • 1. CO2 diffuses into the air spaces within the leaf, then it diffuses into the cells where photosynthesis happens. The leaf's structure is adapted so that this can happen easily.
      • 2. The underneath of the leaf is an exchange surface. It's covered in small little holes called stomata which the CO2 diffuses in through
      • 3. Oxygen produced in photosynthesis and water vapour also diffuse out through the stomata. Water vapour is actually lost from all over the leaf surface, but most is lost through the stomata.
      • 4. The size of the stomata are controlled by guard cells. These close the stomata if the plant i slosing water faster than it is being replaced by the roots. Without these guard cells the plant would soon wilt
      • 5. The flattened shape of the leaf increases the area of this exchange surface so that it is more effective
      • 6. The walla of the cells inside the leaf form another exchange surface. The air spaces inside the leaf increase the area of this surface so there is more chance for CO2 to get into the cells
      • The water vapour evaporates from the cells inside the leaf. Then it escapes by diffusion because there is a lot of it inside the leaf and less of it in the air outside. Evaporation is quickest in hot, dry and windy conditions.



A good use of colour to separate the main ideas and a lot of key phrases and definitions  have been incorporated. Mind maps are often seen to be better if fewer words are used. Single strong words and short, meaningful phrases can  sometimes convey the same concepts.  Adding images can also help recall of the map.

This key topic is found in many GCSE science specifications.

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