Differentiating mediated and non-mediated transport:
Glucose and many other compounds can enter cells by a non-mediated pathway; that is, they slowly diffuse into cells at a rate proportional to their membrane solubility and their concentrations on either side of the membrane. The flux (rate of transport per unit area) of a substance across the membrane increases with the magnitude of its concentration gradient. If glucose moves across a membrane by means of a transport protein, its flux is no longer linear.

This is one of four characteristics that distinguish mediated from non-mediated transport:

1. Speed and specificity-The solubilities of the chemically similar sugars D-glucose and D-mannitol in a synthetic lipid bilayer are similar. However, the rate at which glucose moves through the erythrocyte membrane is four orders of magnitude faster than that of D-mannitol. The erythrocyte membrane therefore contains a system that transports glucose and that can distinguish D-glucose from D-mannitol.
   
2. Saturation-The rate of glucose transport into an erythrocyte does not increase infinitely as the external glucose concentration increases. Such an observation is evidence that a specific number of sites on the membrane are involved in the transport of glucose; which becomes saturated at high [glucose] and the plot of glucose flux versus [glucose] is hyperbolic. The non-mediated glucose flux increases linearly with [glucose].
   
3. Competition-The curve is shifted to the right in the presence of a substance that competes with glucose for binding to the transporter; for example, 6-Obenzyl-D- galactose.Competition is not a feature of non-mediated transport, since no transport protein is involved.
   
   
4. Inactivation-Reagents that chemically modify proteins and hence may affect their functions may inhibit the rapid, saturatable flux of glucose into the erythrocyte.
   
   
   

Interesting facts:

• The binding of the neurotransmitter acetylcholine at certain synapses opens channels that admit Na⁺ and initiate a nerve impulse or muscle contraction.
  
• Sound waves bending the cilia-like projections on the hair cells of the inner ear open up ion channels leading to the creation of nerve impulses that the brain interprets as sound.
  
• Mechanical deformation of the cells of stretch receptors opens ion channels leading to the creation of nerve impulses.
  
• The crucial roles of the Na⁺/K⁺ ATPase are reflected in the fact that almost one-third of all the energy generated by the mitochondria in animal cells is used just to run this pump.
  
• ABC transporters must have evolved early in the history of life. The ATP-binding domains in archaea, eubacteria, and eukaryotes all share a homologous structure, the ATP-binding "cassette".