Structures
Responsible for Membrane Transport
We have discussed how the
lipid bilayer acts as an efficient barrier by only allowing a very small number
of non-polar molecules to freely enter or exit a cell. While for the most part
this selectivity is a valuable function and allows the cell to maintain its
integrity, cells do need to move certain large, polar
molecules such as amino acids, sugars, and nucleotides across their membranes.
As a result, cell membranes require specific structures that allow for the
transport of certain molecules.
Membrane
Transport
There are a number of
different ways that molecules can pass from one side of a cell membrane to the
other. Some such means, like diffusion and osmosis, are natural processes that
require no expenditure of energy from the cell and are called passive
transport. Other methods of transport do require cellular energy and are called
active transport. In addition to these two forms of transport, there exist
other forms of transport such as endocytosis and exocytosis, which will be
discuss later and do not require the same set of membrane proteins for their
function.
Passive
Transport
Diffusion is the natural
phenomenon in which nonpolar molecules naturally flow from an area of higher
concentration to an area of lower concentration. Osmosis is a similar process,
but refers specifically to water molecules. Both of these classes of molecules
we have already discussed as capable of crossing the lipid bilayer. As seen in
, neither diffusion nor osmosis require the expenditure of energy.
Active
Transport
Active transport occurs
when a cell actively pumps a molecule across its membrane, against the natural
direction dictated by diffusion, osmosis, or polarity. As seen in , such
transport requires energy.
Transport
Proteins
Both of passive and active transport
are mediated with the help of transmembrane proteins that act as transporters.
shows the two main classes of transport proteins: carrier proteins and channel
proteins. For the most part, carrier proteins mediate active transport while
channel proteins mediate passive transport. Carrier proteins create an opening
in the lipid bilayer by undergoing a conformational change upon the binding of
the molecule. Channel proteins form hydrophilic pores across the lipid bilayer.
When open, these pores allow specific molecules to pass through. There is one
other class of transport proteins called ionophores. These are small,
hydrophobic proteins that increase bilayer permeability for specific ions.
Transport proteins are
critical to cell life and cell interactions. They allow for the proper
distribution of ions and molecules in multicellular organisms. Additionally,
they can help to maintain proper intra- and extra-cellular pH levels,
facilitate communication between cells, and are involved numerous other essential
functions including protein sythesis.
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