1. Membrane Structure & Function

2. Functions of the Cell Membrane
Isolates the cytoplasm from the external environment
Regulates the flow of materials into and out of the cell
Communicates with other cells

3. The Fluid Mosaic Model Currently accepted model of the cell membrane
Proposed by Singer and Nicolson in 1972
Phospholipid bilayer
Hydrophilic “head” – exposed to the outside
Hydrophobic “tail” – hides inside
Membrane proteins are randomly dispersed in phospholipid bilayer

4. Fluid Mosaic Model

5. Fluidity of the Membrane
The lipids and proteins can drift throughout the membrane
Membrane is NOT stiff/rigid
Cholesterol makes the membrane stronger by limiting the movement of phospholipids

6. Membrane as a Mosaic
Lipid bilayer has membrane proteins embedded in it
Integral proteins
Go through the membrane (both sides)
Peripheral proteins
attached to the surface of the membrane

7. Selective Permeability of the Cell Membrane
The cell membrane can “choose” what enters and exits a cell
“Gatekeeper of the Cell”

8. Passive Transport Def’n: 3 types of passive transport:
Diffusion of a substance that does NOT require the input of energy by the cell
3 types of passive transport:
Diffusion
Osmosis
Facilitated diffusion

9. Diffusion
Movement of molecules from high to low concentration until equilibrium is reached.
Passive Transport= no energy required
What substances may diffuse across membrane? Nonpolar (non-charged) molecules; small polar molecules

10. Diffusion
Each substance diffuses down its OWN concentration gradient and is unaffected by concentration gradients of other substances

11. Diffusion Does all movement stop once equilibrium is reached?? NO!!
Equal rates in all directions

12. Osmosis
Def’n:
The passive transport of water across a selectively permeable membrane
Hyper-, hypo-, iso- tonic
RELATIVE TERMS!!
Always referring to solute concentration
Water moves from areas of lower concentration of solutes (hypotonic) to areas of higher solute concentration (hypertonic)

13. Osmosis in Plant and Animal Cells
Cell crenate/shrinks
Cytolysis
Occurs when a cell is in a hypotonic solution
Water goes from solution into cell
Plant Cells:
Turgid- vacuole swells
Flaccid-vacuole shrinks (plasmolysis)

14. Osmosis in Plant and Animal Cells

15. Facilitated Diffusion
Def’n:
The diffusion of large molecules across the cell membrane using transport proteins
Glucose; ions
Does NOT require an input of energy
Solute is still moving down its concentration gradient

16. Facilitated Diffusion
Transport proteins are specific for their solutes
Transport proteins can become saturated
Some are gated channels:
Chemical or electrical stimulus causes them to open

17. Example Which direction will sucrose move?
Which direction will glucose move?
Which direction will fructose move?

18. Active Transport Def’n: The pumping of solutes against their gradients
Requires an input of energy by the cell
Used so cells can “stockpile” extra supplies (storage of any substance in a very high concentration; i.e. Iodine stored in thyroid; glycogen stored in liver
Sodium/Potassium Pump

19. Electrogenic Pumps
Voltage across membranes = stored energy that can be used for cellular work
Sodium-Potassium Pump:
3 Na+ OUT of the cell for every 2 K+ pumped in
Net transfer of one positive charge from cytoplasm to extracellular fluid
Very important for transferring signals between nerve cells

20. Sodium-Potassium Pump

21. Cotransport
Substance that has been pumped across a membrane can do work as it “leaks” back by diffusion
Another substance “hitches a ride”

22. Endocytosis & Exocytosis
Def’n:
The movement of large molecules (polysaccharides, proteins, etc.) across the membrane using vesicles
Endocytosis = cell takes in macromolecules
Exocytosis = cell secretes macromolecules

23. Endocytosis
Cell takes in macromolecules by forming vesicles made from the plasma membrane
Phagocytosis = “cell eating”
Large molecules
Pinocytosis = “cell drinking”
Small molecules & liquids
Receptor-mediated endocytosis = seeks out specific molecules

24. Endocytosis

25. Exocytosis
The cell secretes macromolecules by the fusion of vesicles with the plasma membrane
Used to release hormones, chemical signals, etc.

26. Signal Transduction Pathway
We’ll discuss this later…
But for now:
The cell’s plasma membrane is an important player in a cell’s ability to sense and respond to environmental change