GCSE AQA Biology 3 Movement in & out of Cells

GCSE AQA Biology 3 Movement in & out of Cells

1. Osmosis

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

2. Gas & solute exchange

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

3. Diffusion through cell membranes

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

4. Active transport

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