1. Chapter 3 Cells
3.3 Cell Membrane

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

3. The cell membrane has two major functions
Aka Plasma Membrane
Forms a boundary between inside and outside of the cell
Controls import and export of materials
cell membrane
outside cell
inside cell

4. Cell membranes are made of Phospholipids
Molecule with a phosphate head region and two fatty acid tails
Head is polar = charge
(Hydrophilic)
Tails are non-polar = no charge
(Hydrophobic)

5. Phospholipid Bilayer
Properties of the polar heads and nonpolar tails cause the phospholipids to arrange themselves in layers
Like a sandwich
Heads like bread
Interact with watery environment inside and outside the cell
Tails
Like filling protected from water

6. Other Membrane Molecules
Cholesterol
Strengthens the cell membrane
Protein Channels
Allow specific molecules into the cell
Cytoskeletal Protein
Help cell maintain its shape
Carbohydrates (Glycoproteins)
Carb chain serves as an ID badge for the cell

7. Fluid Mosaic Model
Describes the arrangement of molecules that make up a cell membrane

8. Fluid Mosaic Model Takes it’s name from 2 characteristics
Fluid = Membrane is flexible not ridged
Allows phospholipids and proteins to move side to side and slide past each other
Proteins cannot flip end to end though
Mosaic = because of the variety of scattered molecules

9. Selectively Permeable
Some molecules can cross over, while others cannot
Helps to maintain homeostasis – equilibrium – by monitoring what enters and exits cells. For Example:
Small molecules and larger hydrophobic molecules move through easily. Examples: O2, CO2, H2O

10. Selectively Permeable
Small non-polar molecule
Pass right through the membrane
Small polar molecules
Transported by channel proteins
Large molecules (polar and non-polar)
Moved in vesicles by Endocytosis/Exocytosis

11. Chemical signals are transmitted across the cell membrane
Receptors bind with ligands and change shape.
Two types
of receptors

12. Intracellular Receptor
Found totally within cell
Ligand moves across the membrane, binds with receptor inside cell, and receptor-ligand complex cause something to happen
Ex. Many Hormones work this way

13. Membrane receptor Ligand cannot enter cell
Ligand bind to external receptor
Receptor changes shape
Shape change causes molecules in cell to respond