1. You Must Know (Chapter 3.2)
Describe the structure and function of the cell membrane.
How are cells organized into living things?

2. Cell Membrane

3. Cell Membrane The plasma membrane is selectively permeable
Allows some substances to cross more easily than others
Fluid Mosaic Model
Fluid: membrane held together by weak interactions
Mosaic: phospholipids, proteins, carbs

4. Early membrane model (1935) Davson/Danielli – Sandwich model Problems:
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Early membrane model
(1935) Davson/Danielli – Sandwich model
phospholipid bilayer between 2 protein layers
Problems:
varying chemical composition of membrane,
hydrophobic protein parts

5. The freeze-fracture method
Revealed the structure of membrane’s interior

6. Fluid Mosaic Model Hydrophobic – water-fearing
Hydrophilic – water-loving

7. Phospholipids Bilayer
hydrophilic head,
hydrophobic tail
Hydrophobic barrier keeps hydrophilic molecules out
Selective permeability

8. Selective Permeability
Small molecules (polar or nonpolar) cross easily
Fats and oils, gases, CO2, O2)
Hydrophobic core prevents passage of ions, large polar molecules

9. Membrane Proteins Integral Proteins Peripheral Proteins
Embedded in membrane
Transmembrane with hydrophilic heads/tails and hydrophobic middles
Extracellular or cytoplasmic sides of membrane
NOT embedded
Held in place by the cytoskeleton or ECM
Provides stronger framework

10. Integral & Peripheral proteins

11. Some functions of membrane proteins

12. Membrane fluidity Cholesterol hinder close packing at low temps
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Membrane fluidity
Cholesterol
limit fluidity at high temps
hinder close packing at low temps
Adaptations: bacteria in hot springs (unusual lipids); winter wheat ( unsaturated phospholipids)

13. Carbohydrates Function: cell-cell recognition; developing organisms
Glycolipids, glycoproteins
E.g. blood transfusions are type-specific

15. Cellular organization

17. Review questions 1. Describe the composition of the plasma membrane.
2. Explain why hydrophobic (“water-hating”) molecules can easily cross the plasma membrane, while hydrophilic (“water-loving”) molecules cannot.
Cells carry out all the functions of life, and they use nutrients and oxygen and produce wastes. These substances must cross the plasma membrane.
3. How do you think substances cross the plasma membrane to enter or leave the cell?
4. What if the substances are large? Protein molecules, for example, are very large. How do they enter or leave the cell?