What factors affect the transfer of drugs across cell membranes?

The transfer of drugs across cell membranes is primarily influenced by molecular size and lipid solubility. Cell membranes are composed of a phospholipid bilayer that selectively allows substances to pass through based on their chemical properties.

Molecular size plays a critical role because larger molecules may have difficulty permeating the membrane due to physical space constraints, while smaller molecules can more easily navigate through the lipid bilayer. Lipid solubility is also essential because the cell membrane is hydrophobic in nature, favoring the passage of lipophilic (fat-soluble) substances over hydrophilic (water-soluble) ones. Drugs that are more lipid-soluble can diffuse through the cell membrane more readily, as they are compatible with the membrane's lipid environment.

Together, molecular size and lipid solubility provide the necessary criteria for a drug's ability to cross biological membranes, impacting its absorption, distribution, and ultimately its therapeutic effectiveness. Understanding these factors is crucial in pharmacokinetics, as they help predict how drugs behave in the body.