1. BIOL 2402 Chapter 3 Online Tutorial

2. Topics: I. Review of membrane structure II. Membrane transport A. Permeability B. Diffusion

3. I. Review of membrane structure A. the main structure of the plasma membrane is a fluid lipid bilayer: made of phospholipids and cholesterol forms the foundation of the membrane allows the membrane to be flexible prevents passage of most polar molecules

5. 1. characteristics of phospholipids polar head is hydrophilic and lines up with water in ICF and ECF nonpolar tails are hydrophobic and face the inside of the membrane these factors cause a bilayer to form automatically whenever phospholipids come into contact with water the molecules are not bonded to each other, so they can move around laterally in the membrane

7. 2. characteristics of cholesterol individual molecules are positioned between the tails of the phospholipid molecules they prevent the fatty acids from crystallizing they stabilize the membrane and promote flexibility

8. B. proteins are embedded in the lipid bilayer 1. location of proteins proteins “float” in the lipid bilayer they are not bonded to the phospholipids they may extend through the membrane or they may be anchored to the inner or outer surface of the membrane

9. 2. each protein performs a specific function: channel carrier receptor enzyme structural adhesion self-recognition

11. C. carbohydrates are located on the outer surface (see previous slide) 1. carbohydrates are attached to lipids and proteins in the main membrane layer 2. functions of carbohydrates: a. responsible for recognition of self b. allow cells to recognize cells of the same type c. help control tissue growth

12. D. cell to cell adhesions 1. cells are held together by a. cell adhesion molecules b. the extracellular matrix c. cell junctions

13. 2. there are three types of cell junctions a. desmosomes hold adjacent cells together at discrete spots b. tight junctions form impermeable bonds between adjacent cells and prevent the passage of molecules c. gap junctions form “tunnels” between adjacent cells and allow the passage of ions from the cytosol of on cell to the cytosol of adjacent cells

14. II. Membrane Transport A. whether or not a molecule can pass through a membrane depends on: the permeability of the membrane the penetration of the molecule the presence of energy (kinetic or ATP)

15. 1. membrane permeability biological membranes are selectively or semipermeable factors affecting permeability are: a. the presence of a hydrophobic lipid bilayer prevents the passage of molecules that are not lipid soluble b. the presence of channels allows specific ions to go through the membrane c. the presence of carrier molecules allows specific molecules to be transported through the membrane

16. 2. molecule penetration molecules that can go through a membrane are called “penetrating” factors affecting a molecule’s penetration are: a. solubility in lipids: high solubility increases penetration b. electrical charge: charge decreases penetration c. polarity: polarity decreases penetration d. size: large size decreases penetration

17. self-test: which molecule is most likely to pass through a cell membrane? molecular weight polarity X56nonpolar Y42polar Z187nonpolar

18. B. Diffusion 1. the physical basis for diffusion: molecules have kinetic energy that causes them to move constantly and in random directions as the molecules move around they collide and bounce off of each other given enough time they reach an even distribution the NET result is that molecules move from an area of higher to an area of lower concentration (down their concentration gradient)

21. self-test: will net diffusion occur if there is no concentration gradient?

22. 2. factors that influence diffusion rate a. higher temperature increases diffusion rate b. higher concentation gradient increases diffusion rate c. higher membrane permeability increases diffusion rate d. higher surface area of membrane increases diffusion rate e. larger molecule decreases diffusion rate f. longer distance molecule has to diffuse decreases diffusion rate

23. 3. electrical gradients affect diffusion of ions like charges repel each other opposite charges attract each other the combination of the concentration gradient and electrical gradient is called the “electrochemical” gradient

24. 4. osmosis is the diffusion of water through a membrane water moves down its concentration gradient water moves away from the higher water concentration water moves towards the higher solute concentration think of the solute as “pulling” water to its side of the membrane

26. a. penetrating vs nonpenetrating solutes if a solute can move through a membrane it will eventually become evenly distributed, osmosis will stop, the volumes on both sides of the membrane will be equal

28. if a solute cannot move through a membrane it will continue to “pull” water towards it until water movement is stopped by hydrostatic pressure, and the volume on one side will be larger than the other side

30. b. osmotic pressure think of osmotic pressure as a force that “pulls” water to the side of the membrane with the highest solute concentration the side of the membrane with the higher solute concentration has the higher osmotic pressure

31. 300 mOsm c. tonicity describes the effect of the extracellular fluid (ECF) on the volume of a cell in the situation diagrammed here the intracellular fluid (ICF) and extracellular fluid (ECF) both have solute concentrations of 300 milli osmoles ICF ECF 300 mOsm

32. a. isotonic solutions have the same concentration of solutes as the ICF do not cause osmosis do not cause changes in cell volume300 mOsm

33. 300 mOsm b. hypotonic solutions have lower solute concentrations than the ICF cause osmosis into the cell cause the cell volume to increase 200 mOsm water

34. c. hypertonic solutions have higher solute concentrations than the ICF cause osmosis out of the cell cause the cell volume to decrease 300 mOsm 400 mOsm water