Chapter 3 the cellular level of organization. Intro: cells: living structural and functional units enclosed by a membrane, cell division: how all cells are formed, one cell divides into two identical cells, cell biology or cytology: the study of cellular structure and function. The plasma membrane: a flexible yet sturdy barrier that sur- rounds and contains the cytoplasm of a cell, structure: the fluid mosaic model, continuously moving and changing model. Oh group on cholesterol binds to the polar head of phospholipids and glycolipids: about 5% are glycolipids, only in extracellular facing membrane layer. Large molecules (ex. proteins) are too big and can only pass via endocytosis and exocytosis. In facilitated diffusion an integral membrane protein assists a specific substance across the membrane: channel-mediated facilitated diffusion, a solute moves down its concentration gradient through a membrane channel, most membrane channels are ion channels, channels are highly specific.