KEY CONCEPT The cell membrane is a barrier that separates a cell from the external environment.

Cell membranes are composed of two phospholipid layers. The fluid mosaic model is used to describe the structure of the cell membrane. Meaning the membrane is flexible, not rigid and composed of a variety of molecules. Video: Fluid Mosaic Model

The cell membrane has two major functions. 1. forms a boundary between inside and outside of the cell 2. controls passage of materials into and out of the cell cell membrane outside cell inside cell

The cell membrane is made of a phospholipid bilayer. The phosphate “head” is polar. Water is polar. Therefore, they form hydrogen bonds with each other. (hydrophilic) The fatty acid “tails” are non-polar and therefore repelled by water (hydrophobic).

There are other molecules embedded in the membrane. Protein, cholesterol, carbohydrates Proteins can help materials cross the membrane or can be a part of the cytoskeleton. Cholesterol strengthens the membrane. Carbohydrates serve as identification tags enabling cells to distinguish one type of cell from another. cell membrane protein cholesterol carbohydrate chain protein channel

Cholesterol’s role in the cell membrane Cholesterol is found in every cell of your body. It helps maintain the integrity of these membranes, and plays a role in facilitating cell signaling-- meaning the ability of your cells to communicate with each other. Cholesterol helps prevent extremes-- whether too fluid, or too firm-- in the consistency of the cell membrane. Infer: Note that cholesterol is located between the fatty acid chains. Do you think cholesterol is polar or nonpolar? Explain your answer.

The cell membrane is selectively permeable or sometimes called semipermeable. - This means some molecules can cross the membrane while others cannot. This helps the cell to maintain homeostasis. Connect: Describe a semipermeable membrane with which you are already familiar.