How do the components of a plasma membrane affect the movement of substances into and out of a cell, and why is this regulation important?

Phospholipids and proteins regulate the movement of substances through the membrane, thereby allowing the membrane to be selectively permeable. This is important since it allows the cell to, for example, maintain water potential, remove waste products, or obtain nutrients. Together, phospholipids and proteins allow for three types of transport: 1) diffusion, 2) faciliated diffusion, and 3) active transport.

Phospholipids allow for small, uncharged molecules to diffuse through the membrane. For instance, oxygen or carbon dioxide. Meanwhile, larger or charged molecules can only cross the membrane via faciliated diffusion through carrier proteins or channel proteins. These two categories of proteins are specific for particular substances, for instance, glucose or sodium ions. Carrier proteins may also carry out the third type of transport - active transport. Unlike diffusion and faciliated diffusion, active transport uses energy in the form of ATP to transport molecules across a membrane against a concentration gradient.