1. Membrane Structure and Function
Selectively permeable membranes are key to the cell's ability to function
Membrane Structure and Function

2. Amphipathic Molecules
Have both hydrophilic and hydrophobic regions
Phospholipids have hydrophilic heads and hydrophobic tails

3. Phospholipid Bilayer
Phospholipids naturally arrange themselves to keep hydrophobic portions away from water

4. Fluid Mosaic Model Proteins are embedded in the membrane
These proteins have many different functions

5. Membrane Fluidity
Membranes get more rigid as the temperature gets lower
Unsaturated phospholipids and cholesterol keep the membrane from becoming completely solid

6. Proteins in the Membrane
Integral proteins– found on the inside
A transmembrane protein spans the entire membrane
Peripheral proteins – loosely bound to surface
Often bound to extracellular matrix or cytoskeleton

7. Membrane Protein Functions
Transport
Enzymatic Activity
Binding of messengers
Intercellular connections
Cell recognition
Attachment

8. Membrane Carbohydrates
Important in cell-cell recognition
Short polysaccharides
Often bonded to proteins (glycoproteins)
Distinguish cells – i.e. blood types

10. Selective Permeability
Membrane is selective in what molecules can pass through and how quickly they pass
Energy must sometimes be used to transport molecules

11. Simple Permeability
Small non-polar molecules rapidly pass through membrane
Very small polar molecules can slip through
Larger polar molecules cannot
Very impermeable to ions

12. Transport Proteins
Transmembrane proteins can provide channels for hydrophilic ions and molecules
Other proteins physically move molecules (using energy)
Proteins are specific to a particular molecule

13. Diffusion
Natural process of a substance spreading into available space
A substance diffuses from where it is more concentrated to where it is less concentrated
Down its “concentration gradient”

14. Passive Transport Diffusion of a substance across a membrane
Net flow of substance occurs until concentration is equal on both sides of membrane (equilibrium)
*Unless there is pressure

15. Hypertonic/Hypotonic/Isotonic
A solution is:
Hypertonic if it has a higher concentration of solutes
Hypotonic if it has a lower concentration of solutes
Isotonic if it has an equal concentration

16. Osmosis
If solutes cannot pass through the membrane, water will to create equilibrium
Water moves from hypotonic (High Water Potential) to hypertonic solution (Low Water Potential)
Water moves to the more “sugary” side

17. Water Follows Sugar
If you eat something very sugary, usually you will be thirsty
Water flows to the side with more sugar
This is the hypertonic side (sugar makes you hyper)
But not really, this is not actually true

18. Cells depend on water balance (osmoregulation)

19. Plant Cell Walls Hold Water In
Plant cells normally exist in hypotonic solutions
Rigid cell walls keep plant cell from bursting
Wall is instead turgid
Plants wilt in isotonic solutions

20. Facilitated Diffusion
Transport proteins allow polar molecules and ions to travel down concentration gradient
No usage of energy!
Aquaporins – special protein channels for water
Peter Agre won the 2003 Nobel Prize for the discovery of aquaporins!

21. Active Transport
Some transport proteins pump molecules against their concentration gradient
Requires energy
Necessary for cell to control cellular environment

22. Sodium-Potassium Pump
Pumps 3 molecules of sodium out of the cell
Takes ATP to move them
2 potassium molecules are transported back into the cell

23. Sodium Potassium Pump

24. Ion Pumps Generate Voltage
Voltage= the separation of charge
If one side has more + ions than – ions, it will have a + charge
- ions will flow towards sides with a net positive charge

25. Electrochemical Gradient
Two different forces create a gradient across the membrane
Electric potential (voltage)
Chemical potential (concentration gradient)
These two effects combine to form an electrochemical gradient

27. Cotransport
Cotransporter proteins use the energy of one molecule diffusing down its gradient to move another molecule against its gradient

28. Bulk Transport
Many molecules are too large to pass through the membrane
i.e. proteins
They enter the cell through endocytosis and exocytosis

29. Endocytosis The cell intakes large molecules by forming new vesicles
Small area of membrane sinks inward, forming a pocket which eventually pinches off
3 types
Phagocytosis
Pinocytosis
Receptor-mediated

31. Exocytosis
Transport vesicle fuses with plasma membrane, expelling contents from the cell
Vesicle membrane becomes part of plasma membrane