Biology 102

1. Biology 102 Lectures 6 & 7: Biological Membranes

2. Lecture outline • Relationship of membrane structure and function • Functions • Structure: The fluid-mosaic model of membranes • Movement of substances across membranes • Principles of Diffusion • Passive and active transport of solutes • Osmosis • Endocytosis and exocytosis • Specialization of cell surfaces

3. 1. Membrane structure and function • Biological membranes • Thin barrier separating the inside of the cell (or structure) from the outside environment • Functions (focus on plasma membrane) • Selectively isolate the cell’s contents from the external environment • Regulate the exchange of substances between the inside and outside of the cell • Communicate with other cells

4. Fluid-mosaic model of membrane structure

5. The phospholipid bilayer is the fluid portion of the membrane • Double layer • Polar head group: hydrophilic exterior • Non-polar hydrocarbon tails: hydrophobic interior • Unsaturated hydrocarbon chains: maintains fluidity

6. Phospholipid bilayer as a barrier • Hydrophilic molecules cannot pass freely through the membrane’s hydrophobic interior • Sugars, amino acids, charged ions (i.e. Na+ and Cl-) are some examples • Though polar, H20 is so small it does pass through. • Many hydrophobic molecules can pass freely through the membrane’s hydrophobic interior • Steroid hormones and other lipids are some examples

7. Cholesterol molecules are part of the lipid bilayer • Adds strength • Adds flexibility • Affects fluidity • Decreases fluidity at “moderate” temperatures • Harder for phospholipids to move • Prevents solidification at low temperatures • Keeps phospholipids from binding to each other

8. A mosaic of proteins is embeded in the membrane • Glycoproteins: proteins with attached carbohydrates

9. Types of membrane proteins • Transport proteins • For passage of materials through the plasma membrane • Channel vs. carrier proteins • Receptor proteins • Bind molecules and trigger cellular responses • Example: hormones • Recognition proteins • Self vs. non-self (glycoprotein-based) recognition • Markers during development

10. 2. Movement of substances across membranes • Definitions • Concentration • Number of molecules in a given volume • Gradient • Differences in concentration between two regions of space. • This causes molecules to move from one region to the other (if no barrier to movement) • Diffusion • Net movement of molecules from regions of high concentration to regions of low concentration • Considered as movement “down” its concentration gradient

11. Diffusion of Dye in Water RandomDispersal Dispersing Time 0 Time 1 Time 2 SteepConcentrationGradient ReducedConcentrationGradient NoConcentrationGradient

12. Passive vs. active transport • Passive transport • Movement of molecules down their concentration gradients • Requires no net energy expenditure • The gradients themselves provide energy • Active transport • Movement of molecules against their concentration gradients • Requires energy!

13. Focus: Passive transport • Simple diffusion • Facilitated diffusion • Osmosis • Remember that no energy is required, and molecules move down their concentration gradients

14. Focus: Passive transport • Simple diffusion • Molecules simply cross cell membrane on their own, down their concentration gradients • Possible only for molecules that can cross the lipid bilayer on their own • Lipid-soluble molecules • Examples: ethyl alcohol, vitamin A, steroid hormones • Very small molecules • Examples: water, carbon dioxide • Rate depends upon • Concentration gradient • Size • Lipid solubility

15. Focus: Passive transport (cont.) • Facilitated diffusion • Molecules move down their concentration gradients (as for simple diffusion), but… • Transport proteins assist these molecules in crossing the membrane • No net energy expenditure! (This is a type of diffusion…)

16. Focus: Passive transport (cont.): Facilitated diffusion via a channel

17. Focus: Passive transport (cont.): Facilitated diffusion via a carrier protein DiffusionChannelProtein (OutsideCell) Molecule inTransit DiffusionGradient Carrier proteinhas binding sitefor molecule Carrier protein changesshape, transportingmolecule across membrane Molecule entersbinding site Carrier protein resumesoriginal shape (Inside Cell)

18. Focus: Passive transport (cont.) • Osmosis • Movement of water from a high [water] to an area of low [water] concentration across a semi-permeable membrane • Note here that water can pass through, but glucose cannot Think about which way water will move (blackboard demo)

19. The effects of osmosis • Compare solute and water concentrations outside vs. inside the cell (sketches)

20. Focus: Active Transport • Movement via active transport proteins • Endocytosis • Exocytosis • Remember that energy is required, and molecules are moved against their concentration gradients

21. Focus: Active transport1.Movement via active transport proteins • ATP required (has own binding site) • Note movement of particles (Ca++) against their concentration gradient

22. Focus: Active transport2. Endocytosis • Three types of endocytosis • Pinocytosis • “cell drinking” • Extracellular fluid taken in • Receptor-mediated endocytosis • Specific for particular molecules • Molecules bind to receptors. • Receptor-molecule complex taken in • Phagocytosis • Large particles engulfed

23. Focus: Active transport3. Exocytosis

24. 3. Specialization of cell surfaces • Connections between cells • Desmosome: Membranes of adjacent cells glued together by proteins and carbohydrates • Tight junction: Cells sealed together with proteins

25. 3. Specialization of cell surfaces (cont.) • Communication between cells • Gap junctions: Channels connect adjacent cells • Plasmodesmata: Continuous cytoplasm bridges between two cells (plants) • Note also cell walls. Only certain cell types have cell walls!