The Cell (aka Plasma) Membrane intro mini-movie

 Found in all cell types
 Is more flexible than a cell wall
 Controls what enters and exits the cell
 Forms a boundary between the cell and its environment

• Allows the cell to maintain homeostasis
(internal balance) despite changes in its environment
• Allows some, but not all, materials to cross
– Small, nonpolar (uncharged) molecules pass through the membrane easily
– Small, polar (charged) molecules pass through the membrane with the aid of proteins
– Large molecules require vesicles to get in / out of the cell.

The cell membrane consists of a double layer of phospholipids interspersed with a variety of other molecules, including proteins, cholesterol, and carbohydrates.
Quick-time movie

• A phospholipid has three major parts
:
–Phosphate group (which is charged)
–Glycerol
–Two fatty acid chains
Explanation / animation

Phospholipids – Heads and Tails
• The phosphate and glycerol form the head , which is polar (charged) and hydrophilic (waterloving)
• The two fatty acid chains form the tails , which are non-polar
(not charged) and hydrophobic (waterrepelling)

• Cells are filled with and surrounded by water
• So, phospholipids line up in a “bilayer”
– Water-loving heads form the outer layer (like bread on a sandwich)
– Water-repelling tails are protected inside (like the filling of a sandwich)
Click for Animation

• Some proteins extend all the way through the cell membrane
– These are integral proteins
• Some proteins are only on the surface of the cell membrane
– These are peripheral proteins

Functions of Membrane Proteins
• Receive information = receptor
– Respond only to specific molecules
– Can bind to a molecule outside of the cell and cause changes inside the cell
• Help move substances = transport
– This may or may not require energy
– Move only specific molecules
• Identify the cell = marker
– Often these are glycoproteins (have a carbohydrate attached)

• Strengthens the cell membrane
• Without cholesterol, the cell membrane would be too fluid / not firm enough / too permeable

• The illustration above show the variety of molecules embedded within the cell membrane.
• Animation

• Describes the arrangement of the molecules that make up a cell membrane
• The cell membrane is flexible, thus “fluid”
– The phospholipids can move from side to side and slide past each other
• Molecules are embedded within the phospholipid bilayer, thus it is a “mosaic”
• Practice labeling