Unit: Cellular Structure & Function

All cells have a cell membrane that separates the cell from its non-living surroundings. It is a thin barrier; approximately 8 nm thick!

 Problems with the cell membrane are involved in many diseases  Type 2 Diabetes, organ transplant rejection, Cholera, Cystic fibrosis, cancer

The cell membrane has several roles to play in a living cell. For example: the It helps the cell maintain homeostasis by leaves regulating what enters & leaves the cell (controls traffic in and out) It receives and produces signals to and from other cells. It identifies the cell as belonging to a particular organism and tissue It maintains connections between cells in organs and tissue

This fluidity is caused by phospholipids, which form the foundation of the plasma membrane. Remember: Lipids and water REPEL one another Cell membranes have a flexible, fluid like structure.

Phosphate group head Polar hydrophilic Fatty acid tails (2) Nonpolar hydrophobic Phosphate Head Fatty acid tails

The nonpolar “tails” cluster together trying to get as far away from the water as they can. The hydrophilic “heads” are attracted to the water The resulting formation is referred to as a lipid bilayer Outside the cell Inside the cell

polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads

The lipid bilayer is a barrier that can be described as Selectively Permeable (A membrane that allows only certain substances in the cells environment to pass through it.) Some things have no trouble crossing the Membrane (Ex. small, nonpolar things like O 2 & CO 2 ) ;others need help across (ex: glucose)

1) Proteins Some proteins stick to the surface of the lipid bilayer (peripheral proteins) and others penetrate the hydrophobic core (integral proteins). Amino acids, which make up proteins, may be polar or nonpolar. How do integral proteins stay in place?

Answer: The water found on either side of the membrane repels the non-polar part of the membrane protein. It is attracted to the interior of the lipid bilayer The polar part of a membrane protein is attracted to the water on both sides of the membrane. This duel attraction holds the protein in place within the lipid bilayer

They may be attached to some proteins (glycoproteins) or lipids (glycolipids) at the membranes surface. Function- act as a chemical identification card which allows cells to recognize & interact with one another.

Because the proteins embedded in the lipid bilayer can move around and “float” among the lipids, and because so many different kinds of molecules make up the cell membrane, scientists describe the cell membrane as a “fluid mosaic.”

Extracellular fluid Cholesterol Cytoplasm Glycolipid Transmembrane proteins Filaments of cytoskeleton Peripheral protein Glycoprotein Phospholipids