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MEMBRANE TRANSPORT (selective permeability (achieved via selective…

- - - - ions such as Na+, K+, Cl-
        glucose
        tryptophan
        urea
        glycerol
        indole
        water
- - - - small k = hydrophilic
      - large k = hydrophobic
- - - - (pores) in bacteria
        
        usually only weakly selective of solutes
        
        act as molecular sieves (always open) or regulated by conc of solute
        
        porins abundant in out membrane of g- bacteria
      - (channel proteins) in animals
        
        can be highly selective to specific ions
        
        opens or close in response to chemical signals
      - (aquaporins) in all species
        
        occurs in cells where diffusion of water across membrane is too slow and rapid uptake is required
        
        allow passage of water and small uncharged molecules but blocks if large or charged due to funnel shape/size of pore
    - - after release conformation reverts to initial state
      - similar action to enzymes
      - some undergo a conformation change after substrate binds allowing release on the other side of membrane
      - facilitates movement down solute or charge gradient
      - no additional energy input required
      - still passive transport
- - - - finite number of pores/carriers so fixed number of sites through which substrates can be transported, so high concentrations lead to saturation (simple diffusion so not lead to saturation as it occurs across entire surface of cell)