1. KEY CONCEPT The cell membrane is a barrier that separates a cell from the external environment.

2. Cell membranes are composed of two phospholipid layers.
The fluid mosaic model is used to describe the structure of the cell membrane.
Video:
Fluid Mosaic Model

3. The cell membrane has two major functions.
1. forms a boundary between inside and outside of the cell
2. controls passage of materials into and out of the cell
cell membrane
outside cell
inside cell

4. The cell membrane is made of a phospholipid bilayer.
The phosphate “head” is polar. Water is polar. Therefore, they form hydrogen bonds with each other.
The fatty acid “tails” are non-polar and therefore repelled by water (hydrophobic).

5. The cell membrane is made of a phospholipid bilayer.
There are other molecules embedded in the membrane.
Protein, cholesterol, carbohydrates
cell membrane
protein
cholesterol
carbohydrate
chain
protein channel

6. Cholesterol’s role in the cell membrane
Cholesterol is found in every cell of your body. It helps maintain the integrity of these membranes, and plays a role in facilitating cell signaling-- meaning the ability of your cells to communicate with each other. Cholesterol helps prevent extremes-- whether too fluid, or too firm-- in the consistency of the cell membrane.
Infer: Note that cholesterol is located between the fatty acid chains. Do you think cholesterol is polar or nonpolar? Explain your answer.

7. The cell membrane is selectively permeable.
- Some molecules can cross the membrane while others cannot.
Connect: Describe a semipermeable membrane with which you are already familiar.

8. Chemical signals are transmitted across the cell membrane.
Receptors bind with ligands and change shape.
Receptor is a protein that detects a signal molecule and performs and action in response. A ligand is the molecule the receptor binds to. Specific receptors bind to specific ligands.
There are two types of receptors.
1. intracellular receptor
“within, or inside, a cell”
are generally nonpolar
small in size
-- Example: hormones can interact with the nucleus to turn on genes to make needed proteins.

9. Chemical signals are transmitted across the cell membrane.
Receptors bind with ligands and change shape.
There are two types of receptors.
2. membrane receptor
Cannot cross the membrane
The receptor sends the message to the cells interior by changing its shape
Example: receptors in red blood cells trigger processes to CO2 from the body tissues to the lungs
Contrast: How do intracellular receptors differ from membrane receptors?