GCSE AQA Biology 3 Movement in & out of Cells

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• Type of diffusion

1. Definition: the movement of water from an area of high concentration to an area of low concentration through a partially permeable membrane
   1. Partially permeable membrane has very small holes in it - only tiny molecules like water can pass through
2. Water molecules pass both ways through the membrane during osmosis - molecules move about randomly
   1. More on one side - net flow of water to area of low concentration
      1. Stronger sugar solution gets more dilute - water acts like it's trying to even up the concentration on either side
3. Tissue fluid surrounds cells in body - water with oxygen, glucose etc
   1. Will usually have different concentration to fluid inside cell - water will move in or out of the cell
      1. If cell is short of water it will be concentrated - solution outside is dilute, water will move in by osmosis
         1. If cell has lots of water, will be more dilute - water will be drawn out of cell by osmosis

1. Substances move by diffusion, osmosis & active transport
   1. Life processes need gases/other dissolved substances & waste substances also need to move out of cells
      1. Have to move through exchange surfaces which are adapted by:
         1. Being thin - shorter diffusion distance
         2. Having a large SA - lots of substance can diffuse at once
         3. Having lots of blood vessels (animals) - to get stuff in/out of blood
         4. Being ventilated (animals) - air moves in/out
2. Structure of leaves lets gases diffuse in/out
   1. Carbon dioxide diffuses into air spaces in leaf, then into cells
      1. Underneath of leaf is an exchange surface - covered in stomata which carbon dioxide diffuses in through
         1. Oxygen & water vapour diffuse out through stomata
            1. Size of stomata controlled by guard cells - close if plant is losing water faster than getting from roots
   2. Flattened shape of leaf increases area of exchange surface
      1. Walls of cells inside leaf = another exchange surface - air spaces inside leaf increase area of surface
         1. Water vapour evaporates from cells inside leaf, escapes by diffusion (lots inside, less outside) = evaporation quickest in hot, dry, windy conditions

1. Gas exchange happens in the lungs - lungs transfer oxygen to RBC & remove waste carbon dioxide from plasma
   1. Lungs contain aveoli where gas exchange takes place
      1. Adapted to maximise diffusion of oxygen & carbon dioxide by having:
         1. Large SA
         2. Moist lining - for dissolving gases
         3. Thin walls
         4. Good blood supply
2. Small intestine is covered in villi - they increase the surface area so digested food is absorbed quickly into the blood
   1. Adapted by having:
      1. Single layer of surface cells
      2. Good blood supply to assist quick absoption

1. Definition: The process that allows substances to be absorbed against a concentration gradient (from low to high concentration) which requires energy (from respiration)
2. Root hairs - specialised to absorb water & minerals
   1. Cells on surface of plant roots grow into hairs which increase SA
      1. The concentration of minerals is usually higher in the root hair cell - active transport has to happen
3. Used in gut - when there's low concentration of nutrients in gut but high in blood
   1. Has to absorb against concentration gradient