1. The Cell (Plasma) Membrane
Biology 11

2. ALL Cells of ALL Organisms have a:
CELL MEMBRANE!

3. The cell membrane separates cells from the external environment.
Analogy: a fence around a yard protects and acts as a boundary for whatever is inside.
Are fences designed to completely restrict access?

4. Function:
The cell (plasma) membrane acts as a protective barrier that regulates which particles can enter and exit the cell.
Because of this regulation it’s said to be SELECTIVELY PERMEABLE
(or semi-permeable)
Not all cells have the same permeability

5. Function cont.
The membrane separates all cells from the external environment which allows cells to be chemically different from their environment.
The cell membrane also works to keep the internal functions of the cell constant and to maintain cell “balance”.
This balance is called HOMEOSTASIS

6. Basic structural component of the plasma membrane is the phospholipid

7. Structure - The phospholipid:
Head: phosphate group (polar: hydrophilic)- likes water and is soluble in water
Two Tails: fatty acids (non-polar: hydrophobic) - hates water and is insoluble in water
The water-soluble ends face the outer environment and inner cell components

8. Cell Membrane Structure:
It is composed of two layers of lipid material with protein molecules interspersed
This is called the phospholipid bilayer

9. The Fluid Mosaic Model

10. Hydrophilic: “water loving”
Hydrophobic: “water hating”
So, how will the phospholipids behave in water?

11. Dynamic (fluid) structure NOT rigid!
Can change shape to perform different functions!
If it breaks  easily fixed!
WHY DO YOU THINK THIS IS??

13. The membrane cannot exists by phospholipids alone!!
There are several other components to the membrane that play vital parts in its function.
Cholesterol – (keep membrane fluid)
Glycoproteins - specialized sugars attached to a protein (used for cell recognition)
Proteins
Glycolipid- specialized sugar on some lipids

14. 1. Cholesterol: Stabilizes the Membrane. Steroid lipid
1. Cholesterol: Stabilizes the Membrane *Steroid lipid *Flat shape: Inserted between phospholipids *Present in animal cell membranes *Absent in bacteria; most plants--Cell walls provide stability

15. 2. Glycoproteins: sugar attached to a protein: provides cellular ID

16. 2.Glycoproteins Example:
Immune system recognizes invaders by the unique structure (the glycoprotein) on the cell membrane
Ex. Transplanted organs are often rejected by recipients because of the sugars on the proteins.
Ex. Type A red blood cell is different from a type B because of the sugar in the protein

17. Glycoproteins and blood type:

18. 3. Proteins: “gatekeepers”, receptors and transport molecules

19. Proteins in the membrane can:
1 - act as a gatekeeper, opening and closing paths through the membrane
2 - Others are receptor sites for hormones
3 - transport using cell energy to pick up needed materials