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movement across membranes - osmosis, active transport, carrier proteins…

- - - - water molecules move more easily from an area of high Ψ (more water ions molecules) to an area of low Ψ as the water molecules in the area of low Ψ are impeded (not free) from crossing the membrane by the solute molecule
      - a situation is reached where additional water molecules accumulate on the area of low Ψ until their greater concentration offsets the blocking effect of the solute.
        
        the probability of water moving in either direction is the same & equilibrium is established
      - only solvent (water) molecules can cross partially permeable membranes, solute molecules are too large
- - - - in plant cells, pressure exerted by the rigid cell wall limits further water uptake
      - Ψp will always be positive if the cell is turgid, but when the cell is flaccid the Ψ is 0kPa.
      - if no physical pressure is applied to a solution then the solute is equal to the water potential - this is the case for animal cells which don't have a cell wall
        
        for animal cells: water potential = solute potential
      - if plant cell is placed in hypotonic solution (e.g. distilled water) water will move into the cell (high Ψ to low Ψ)
        
        but when a cell's vacuole fills with water, the vacuole will push back out on the water surrounding the cell. The plant cell doesn't burst because a plant cell has a cell wall (turgor pressure)
      - when the pressure exerted outward on the water surrounding the plant is equal to the Ψs of the solution in the cell, the water potential of the cell the Ψ of the cell will be equal to zero
        
        the Ψ of the cell will be equal to the water surrounding it & there will be no net movement of water molecules
    - - a solute's concentration to the Ψ is called the solute (or osmotic) potential. It reduces the water potential.
      - the solute potential will always be negative, more negative as more solute is added to the system
- - - - across a membrane
- - - - symport carriers (e.g. glucose & oxygen)
    - - antiport carriers
        
        sodium-potassium pumps are vital in order to generate impulses in nerve cells. its an antiport as it carries two different types of ions in different directions
    - - uniport carriers
- - - - not methods of active transport (no transport proteins)