The lipoid bilayer serves as barrier to any molecules that are not small and hydrophobic. Even single atoms that are tiny will not penetrate the lipid bilayer if they are changed (ions)
Co2, 02, n2, steroid hormone and really among the only molecules that can freely move thus the bilayer.
q) how can a cell exchange polar larger molecules with its environment?
a) By placing channels, pores, carriers in the lipid bilayer.
Most channels and carriers etc are made of proteins that are insisted into the bilayer are called integral proteins. If the integral protiens spans the bilayer, its called a transmembrane protein.
A protein that associates with the memebrane but doesn’t get inserted is called a peripheral membrane protein.
Many of the transmembrane proteins are carriers or pores or channels but not all (at all). Some are receptor for signaling molecules, some serve as tethers for the cell to connect to the extracellular matrix
Basic structures and general functions of the barrier of the cell membrane is provided by the phospholipids that make up the bilayer but membrane proteins carry out most of the specific functions of the membrane.
A transmembrane protein needs about 20 hydrophobic amino acids in a stretch to cross the lipid bilayer.
Conformational change: Something binds something else => shape change.
q) can integral including transmembrane proteins diffuse laterally in its membrane?
A) if they are not connected to the filament of the cytoskeleton or the extracellular matrix (ECM) or connected to proteins of another cell.
If a molecule is small and non-polar. It may be able to pass directly through the membrane (no assistance by proteins needed). The concentration of the molecule will dictate its net movement.
Something that moves from a higher concentration ro a lower concentration