Pores in Plasma Membrane:
Plasma membrane is perforated by pores. These have a diameter of about 0. 35 nm (nanometer), slightly larger than the sodium ions. Less than 0.1 per cent of the plasma membrane is perforated by pores while 99.9 per cent of the cell surface is impermeable for ions. Several models of structures of pores have been proposed.
Some of them are:
1. Structural pores:
These are permanent cylindrical holes that interrupt the otherwise continuous bilayer sheet.
2. Dynamic pores:
These pores are transient cylindrical holes rather than being permanent. They appear only at the time of intake.
3. Paving channel pores:
According to this concept the pores are regarded to be the corners of the closely filled nearly hexagonal paving blocks of lipid and protein subunits.
4. Protein channel pores:
These pores are considered to be parts of the lipid-globular protein mosaic model. These form small channels of specific proteins embedded in the membrane through which ions and small molecules can diffuse.
The ionophores are small polypeptides whose one end is hydrophobic and other hydrophilic. The hydrophobic (outer) end dissolves in the membrane while the hydrophilic end (inner side) picks up ion or water-soluble materials and dumps them on the other side. The ionophores help in exchange of substances from or into the cell.