The Cell (aka Plasma) Membraneintro mini-movie

Functions of the Cell Membrane

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

Selective Permeability

• 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.

Structure of the Cell Membrane

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

Phospholipids

• 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 (water-loving)

• The two fatty acid chains form the tails, which are non-polar (not charged) and hydrophobic (water-repelling)

The Phospholipid Bilayer

• 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

Proteins in the Cell Membrane

• 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)

Cholesterol

• 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

Fluid Mosaic Model

• 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