Glossary

Transport across cell membranes

Cell membranes are selectively permeable barriers — meaning they control which substances can enter and leave the cell — and understanding how molecules cross them is central to almost every process in biology. The fluid-mosaic model describes the membrane as a flexible phospholipid bilayer (a double layer of fat-based molecules) embedded with proteins, glycoproteins, and cholesterol, each playing a specific role in regulating transport. Substances move across membranes by several distinct mechanisms — including simple diffusion, facilitated diffusion, osmosis, active transport, and co-transport — each suited to different molecules and conditions, and cells can be structurally adapted to make these processes faster or more efficient.