Diffusion through lipid membrane

Basic terms to get things into head:-

Diffusion:-                   

Typically a random motion of molecules the net movement takes place from a region of high concentration to a region of low concentration.

Permeability coefficient:- 

Number of molecules crossing the membrane per unit area in unit time.

Partition coefficient :-

The measure of lipid solubility i.e. (hydrophobicity/lipophilicity) in a drug, a high partition coefficient substance states that it is more lipophilic.

Non-polar molecules which freely occurs in the lipid membrane continuously diffuse across the cell-membrane ,the number of molecules diffusing through the membrane depends on permeability coefficient(P) and also on concentration gradient. Permeant molecules must be present within the membrane in sufficient numbers and must be mobile within the membrane if rapid permeation is to occur. Two physicochemical factors contribute to P, namely solubility in the membrane  and diffusivity, which is a measure of the mobility of molecules within the lipid and is expressed as a diffusion coefficient (diffusion coefficient varies only modestly between conventional drugs).

In case of drug A the transmembrane concentration gradient is more than drug B(lipid insoluble)   the small molecules or drug molecules diffuses rapidly.