Understanding Cellular Membranes: Structure & Function

1. Chapter 5 Cellular Membranes

2. Membrane Structure • The “fluid” portion of the cell membrane is made of phospholipids • A phospholipid molecule has a hydrophilic end and a two hydrophobic ends • If surrounded by water, the hydrophobic ends face away from water, the hydrophilic ends are the outer faces • Most membranes are made of phospholipid bilayers

3. Phospholipid bilayer

4. Fluid Mosaic Model • Phospholipids are different in length of fatty acids, hydrophilic heads • Glycolipids have sugar monomers attached to the head end • Cholesterols • Phystosterols • Bilayers shift sideways and move over each other creating a fluid-like movement from an otherwise solid object.

5. Membrane proteins • Integral proteins- situated throughout the lipid bilayer with their hydrophilic ends going through both layers • Peripheral proteins- positioned at the surface of the bilayer

6. Membrane proteins • Adhesion proteins- glycoproteins that help cells stay connected to each other in a tissue • Communication proteins- form channels that allow materials to flow between two cells • Receptor proteins- binding sites for hormones that can trigger changes within a cell • Recognition proteins- identify the cell as a specific type, helps develop tissues and helps cells recognize each other • Transport proteins- passively allow water-soluble materials to flow through cell membranes

7. Membrane Proteins

8. Diffusion • All membranes show selective permeability • Gases and small neutral molecules can move across the membrane easily • Glucose and other large molecules use transport proteins • Concentration gradient refers to the differences in concentrations of a substance in a given volume(s)

9. Diffusion

10. Diffusion • The thermal energy of molecules keeps them moving • Molecules tend to move down a gradient from high concentration to low • The net movement of particles down a gradient is called diffusion • Affected by Temp, gradient, molecule size, electric gradients, pressure gradients • Diffusion is passive- does not use energy

11. Crossing mechanisms • Passive- do not use energy, move down gradients • Diffusion • Facilitated diffusion • Ion transport • osmosis • Active – use energy, move up/against gradients

13. Osmosis • Movement of water across a permeable membrane in response to solution gradients, pressure gradients or both • For example, place a cube of sugar in water, the water will move to dissolve or dilute the sugar concentration

14. Osmosis

15. Osmosis • Tonicity denotes the relative concentration of solutes in two fluids • Isotonic- the solute concentrations are equal • Hypotonic– fluid has a lower solute concentration than the cell. Water will enter the cell • Hypertonic- fluid has a higher solute concentration than the cell. Water will leave the cell

16. Tonicity

17. Osmosis • Fluid pressure-hydrostatic force is directed against a membrane, the greater the solute concentration, the greater the pressure • Countered by osmotic pressure that prevents further increase in volume • When plant cells lose water there is a shrinkage in the cytoplasm called plasmolysis- wilting

18. Active transport • Exocytosis and Endocytosis • Exocytosis- a vesicle moves large particles or bundles of molecules to the cell membrane to be release out of the cell • Endocytosis- large particles or molecules are brought into the cell using vesicles • Sodium-Potassium Pump- moves Na+ into the cell and K+ out of the cell against their concentration gradients

19. Endocytosis • Receptor mediated-specific molecules are brought to specialized regions of the membrane that form pits in the membrane • Bulk-phase- a vesicle forms around a amall volume of extracellular fluid regardless of what it contains • Phagocytocis- a cell engulfs microorganisms, debris or other particles- usu. Seen in protists

20. Endocytosis

21. Exocytosis

22. Sodium-Potassium Pump

23. Membrane cycling • During endocytosis and exocytosis- the membrane is disrupted, however it happens at a rate that the membrane can be replaced