Major Points
1. Fluid mosaic model of membrane structure
2. Transporters allow membrane permeability
3. Vesicles carry proteins across the plasma membrane
Fluid Mosaic Model
* The membrane is fluid (movement in the plane of the membrane)
* The membrane is a mosaic (of lipids and proteins)
The Structure of the Membrane
* Membrane consists of two "leaflets"
- Lipids move easily within a leaflet
- They do not move between leaflets easily
* Specific proteins catalyze movement between leaflets
so Specificity of these enzymes makes the two leaflets different
Membrane Proteins
Membrane proteins associate in two ways:
* Integral membrane proteins
- Insert into one or both leaflets
* Peripheral membrane proteins
- Associate with the surface of the membrane
- Sometimes in association with integral proteins
Function of Membrane Proteins
* Transport proteins
- Move ions and small molecules
* Receptors
- Interact with small molecules at the cell surface
* Recognition proteins
- E.g., in the immune system: self vs. non-self
* Adhesion proteins
- E.g., form "tight junctions"
First Rules
* Ballons may be passed in any direction, but only to someone who doesn't
have a balloon.
* Ballons may not cross the aisles.
Second Rules
* People standing in the aisle may exchange balloons across the aisle.
* You must give your balloon to that person if asked.
Third Rules
* People in the aisle may only pass blue balloons to the center and red
ballons to the side.
Fourth Rules
* People in the aisle may pass balloons only in pairs: one blue and one
red, from opposite sides of the aisle.
Protein-Mediated Transport
* Passive transport
- Molecules move down a concentration gradient
* Active transport
- ATP can drive a molecule against its concentration gradient
- Pairs of dissimilar molecules can be exchanged (e.g., by the Na+/K+ transporter)
Endocytosis/Exocytosis
* Proteins in vesicles can be released at the plasma membrane
* Solvent molecules can be imported by vesicles
- This may requires first that the molecules bind to a surface
receptor (receptor mediated endocytosis)
- Molecules in solution can be imported (fluid phase endocytosis).