The Cell (aka Plasma) Membrane intro mini-movie intro 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 ExplanationExplanation / 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

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

Passive Transport Particles are constantly in motion, colliding and scattering. –This motion is random. Does not require the cell to use ATP / energy The membrane is still semi-permeable, so only certain substances can cross (in or out) by passive transport.

Passive Transport A concentration gradient is the difference in the concentration of a substance from one location to another When there is a concentration gradient, the NET movement is DOWN their concentration gradient - from areas of higher concentration to areas of lower concentration ANIMATION

Diffusion Movement of molecules in a fluid or gas from an area of higher concentration to an area of lower concentration ANIMATION

Equilibrium Diffusion continues until the solution is at equilibrium (no concentration gradient). Molecules still move, but there is no NET movement.