Biological membranes are composed of phospholipid molecules, cholesterol, and proteins. They have a fluid mosaic structure with phospholipid bilayers and embedded proteins that allows for selective transport. Membranes demonstrate lateral and transverse asymmetry, with unique compositions in each layer and set of proteins. Cholesterol regulates fluidity. Passive transport relies on diffusion and requires small, polar molecules, while active transport uses membrane proteins and ATP to move ions and large molecules against gradients.