Selective Permeability and Diffusion: How Molecules Cross the Plasma Membrane

1. 5.2-5.4 PDQ AP Biology

2. 1. Why is the plasma membrane described as selectively permeable? • Because it allows certain molecules through

3. 2. Which types of molecules can passively cross a membrane? • Some small molecules • Hydrophobic molecules • Molecules that are soluble in lipids • The more lipid soluble the molecule is, the faster it can diffuse through the phospholipid bilayer

4. 2. Which types of molecules can NOT cross a membrane freely? • Electrically charged molecules • Polar molecules • Examples: amino acids, sugars, ions, water • Two reasons why they can’t cross: • Not very soluble in the hydrophobic interior of the bilayer • They form hydrogen bonds with water and ions in the aqueous solutions on either side of the membrane

5. 3. Diffusion • Process of random movement toward a state of equilibrium • Net movement of molecules from a high concentration to a lower concentration • So concentration drives diffusion to take place

6. 4. affects on rate of diffusion Molecular Size – smaller molecules diffuse faster Temperature – higher temperatures lead to faster diffusion (heat provides more energy for movement of molecules) Concentration Gradient – The greater the concentration gradient, the more rapidly a substance will diffuse

7. 5. Osmosis and Osmotic Pressure Osmosis Osmotic Pressure Also called turgor pressure Pressure on a cell due to the build up of water Keeps plants upright If enough water leaves the cells, turgor pressure drops and the plant will wilt. • Diffusion process of water molecules passing through specialized channels in membranes • Depends on the concentration of water on both sides of the membrane

8. 6. Let’s look at A • Top picture is animal, bottom picture is plant cell • Arrows are showing water movement • What type of environment is this cell in? • Isotonic, arrows are going in both directions, meaning there is no net change in water movement

9. 6. Let’s look at B • Top picture is animal, bottom picture is plant cell • Water is moving INTO the cell • What type of environment is this cell in? • Hypotonic. There must be more water outside the cell than inside the cell. • In terms of solute, there must be more solute inside the cell and a LOW amount of solute outside the cell • Causes animal cells to explode and plant cells will get a lot of turgor pressure

10. 6. Let’s look at C • Top picture is animal, bottom picture is plant cell • Water is moving OUT of the cell • What type of environment is this cell in? • Hypertonic. There must be more water inside the cell than outside the cell. • In terms of solute, there must be less solute inside the cell and a HIGH amount of solute outside the cell • Causes animal cells to shrivel and plant cells will have less turgor pressure and wilt (vacuole gets small and cell membrane moves away from cell wall)

11. 6. Answers. • C • B (you have to examine what is being referenced in the question) • A • Animal cells shrunk and are going through crenation. Plant cells are losing water and are going through plasmolysis. • Outside the cell – HIGH solute, low water Inside the cell – LOW solute, high water Water moves from high to low, therefore it moves out of the cell. F. Animal cells are exploding and going through cytolysis. Plant cells are getting more water, and experiencing turgor pressure.

12. 7. How do substances that cannot readily diffuse through the cell membrane get transported across? • Through integral proteins!

13. 8. Facilitated diffusion • Substances diffuse the plasma membrane using channel or carrier proteins. • They are diffusing based on concentration gradients.

14. 9. Channel vs. Carrier proteins Channel Carrier Integral proteins Substances bind to them to allow them to diffuse through the phospholipid bilayer Example – this is done with sugars and amino acids • Integral proteins • Form channels across the membrane through which certain substances can pass • Example is an ion channel • They are either open or closed and most of them open when there is a stimulus or chemical signal (ligand)

15. Channel proteins Facilitated diffusion

16. Facilitated Diffusion Carrier Proteins

17. Saturation of a carrier protein • There are so many carrier proteins per unit of plasma membrane • So there is a maximum rate that facilitated diffusion can occur

18. 10. Gated Channel • Channel protein that opens when a stimulus is involved • Ligand- gated channels – channels that are controlled by a chemical signal, called a ligand. • Voltage – gated channel – channels that are stimulated to open or close by a change in the voltage (electrical charge) across the membrane

19. Ligand Gated Channel

20. Voltage Gated Channel

21. 11. Water Channel Protein • Aquaporin • Water moves through these proteins and is still PASSIVE transport, no energy is required.

22. 12. Facilitated Diffusion vs Active Transport Facilitated Diffusion Active transport Active Transport ATP is required Uses proteins or vesicles to move molecules across the membrane Moves molecules from low to high concentrations • Passive Transport • No ATP needed • Uses proteins to move molecules across membrane • Moves molecules from high to low concentrations

24. 13. Why does the Na-K pump result in the formation of an electric gradient? • The transfer of Na and K is not even • 3 Na+ go out • Only 2 K+ go in. • There will more of a positive charge on the outside of the cell

25. Primary Active Transport Vs Secondary Primary Secondary Does not use ATP directly Its energy is supplied by an ion concentration gradient or an electrical gradient, established by primary active transport Example – Glucose uptake when Na passively diffuses back into cell . • Involves direct hydrolysis of ATP • Example - Na-K Pump

27. 25. Three methods of Endocytosis

31. 17. Transport Processes