Describe methods of solute transport across a cell membrane.

For non-polar small molecules simple diffusion down a concentration gradient through the phospholipid bilayer is possible as it is soluble in the hydrophobic fatty acid tails of the membrane. An example of this is oxygen diffusion into the cell from the surrounding tissue fluid. For polar molecules down a concentration gradient facilitated transport is the method of transport with the use of proteins embedded in the membrane such as channel proteins (consisting of a hydrophilic pore which the solute associates with) and carrier proteins (consisting of a hydrophilic solute binding domain and involves changes in tertiary structure to transfer the bound solute across the membrane). This enables the hydrophilic solute to associate with hydrophilic parts of the protein so makes the transport energetically favourable. An example is the K+ channel. For solutes against their concentration gradient the carrier proteins, now referred to as pumps, are coupled to an energy source such as ATP hydrolysis or an ion gradient and this drives the conformational changes needed for transport of the bound solute. The Na+/K+ pump is an example where three Na+ are transported out of the cell per two K+ into the cell and is needed in neurons for maintenance of the resting potential.