1. Chapter 7 The Cell Membrane

2. Overview
Plasma membrane separates living cell from nonliving surroundings
thin barrier = 8nm thick
Controls traffic into & out of the cell
selectively permeable
allowing some substances to cross more easily than others
Made of phospholipids, proteins & other macromolecules

3. Phospholipids Fatty acid tails are hydrophobic
Phosphate group form a hydrophilic head
interaction with H2O is complex
& very important!

4. Phospholipids in water
At surface of a cell phospholipids are arranged as a bilayer
hydrophilic heads on the outside in contact with aqueous solution & hydrophobic tails form core
phospholipid bilayer forms a barrier between cell & external environment

5. Fluid Mosaic Model
In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer

6. A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.

7. Membrane Proteins
Proteins determine most of membrane’s specific functions
Plasma membrane & organelle membranes each have unique collections of proteins
Membrane proteins:
peripheral proteins = loosely bound to surface of membrane
integral proteins = penetrate into lipid bilayer, often completely spanning the membrane (a transmembrane protein)

8. Why are proteins the perfect molecule to function embedded in the cell membrane?

9. Why proteins? Proteins are mixed molecules hydrophobic amino acids
stick in the lipid membrane
anchors the protein in membrane
hydrophilic amino acids
stick out in the watery fluid in & around cell
specialized “receptor” for specific molecules

11. Integral Proteins Within membrane Outer surfaces of membrane
hydrophobic regions with nonpolar amino acids
often coiled into alpha helices
Outer surfaces of membrane
hydrophilic regions with polar amino acids

12. Inside & outside surfaces are different
2 layers may differ in lipid composition
proteins in membrane have a direction
outer surface also has carbohydrates

13. Membranes provide a variety of cell functions
Signal transduction - transmitting a signal from outside the cell to the cell nucleus, like receiving a hormone which triggers a receptor on the inside of the cell that then signals to the nucleus that a protein must be made.

14. Fluid membranes Permeability Changing temperatures
membrane must be fluid, about as fluid as salad oil
Changing temperatures
cells alter lipid composition of membranes
cold-adapted organisms
winter wheat increases % unsaturated phospholipids in autumn
prevent membranes from solidifying during winter

15. Unsaturated fats more fluid membranes
kinks in unsaturated fatty acid tails prevent tight packing

16. Membrane carbohydrates
Play a key role in cell-cell recognition
ability of a cell to distinguish neighboring cells from another
important in organ & tissue development
basis for rejection of foreign cells by immune system
The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.

17. A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.

18. Any Questions??