Final answer:
Lipid-soluble substances and small polar drugs are absorbed into cells through simple diffusion across the plasma membrane's lipid core. Larger polar substances often require transport proteins. The presence of enzymes and transporters in the GI tract can influence drug absorption.
Step-by-step explanation:
Gases, lipid-soluble, and small polar drugs are absorbed into cells using different mechanisms depending on their properties. The plasma membrane, being amphipathic with hydrophilic and hydrophobic regions, allows lipid-soluble substances to easily pass through the hydrophobic lipid core of the membrane by simple diffusion. This is why fat-soluble vitamins and drugs can easily enter cells. However, polar substances may face challenges due to the membrane's nature.
Small polar molecules may still pass through by simple diffusion, but larger polar molecules and ions often require specialized transport proteins, like channel proteins or carrier proteins, to facilitate their entry into cells. Additionally, factors such as the presence of metabolic enzymes and efflux transporters in the gastrointestinal tract can affect drug absorption.