1. Membrane Structure and Transport Section 3-2 Chapter 7

2. Phospholipid Bilayer The membrane is composed of two layers of phospholipids Phospholipids are ampipathic Hydrophobic tail Hydrophyllic head

3. Selectively Permeable  Selectively= only allows specific substances  Permeable= to cross the membrane

4. Fluid Mosaic

5. Fluidity Phospholipids can change places rapidly Proteins can also move fluidly throughout the membrane Slower rate because they are larger More structured movement, likely guided by the cytoskeleton in some way

6. Maintaining Fluidity Decrease in temperature can cause the phospholipid bilayer to solidify Unsaturated hydrocarbons help maintain fluidity because they cause kinks in the tails and keep the phospholipids from packing too close

7. Maintaining Fluidity- Cholesterol At normal temperatures, animal membranes are less fluid because of the inability of the phospholipids to move Advantage as the temperature drops however, because the cholesterol helps the membrane keep from solidifying

8. Maintaining Fluidity

9. Mosaic Refers to the various proteins that are found in/on the membrane Integral Proteins/Transmembrane Proteins- span the entire membrane Peripheral Proteins- occur on one side or another of the membrane

10. General Function

12. Carbohydrates and Cell to Cell Recognition Membrane carbohydrates are usually short, branched chains Function in cell-to-cell recognition Distinguish one cell type from another Help sort cells during development Help the organism to recognize foreign cells

13. Carbohydrates and Cell to Cell Recognition Glycolipid- lipid with an attached carbohydrate Glycoprotein- protein with an attached carbohydrate

14. The Rules of Membrane Passage The plasma membrane is hydrophobic and phospholipids are tightly packed- only small nonpolar molecules can travel across the membrane quickly Oxygen Carbon dioxide Hydrocarbons

15. The Rules of Membrane Passage If a chemical is “too charged or too large” it must have another way to get across the membrane

16. Channel Proteins Provide a hydrophyllic channel for polar molecules and ions to pass through Aquaporins- specific channel proteins allow the passage of water

17. Carrier Proteins Capable of physically changing their shape to carry large molecules across the membrane

18. Passive Transport movement of a substance across a membrane without the cell expending energy to make it happen

19. Diffusion The tendency for molecules to spread out evenly into the available space Diffusion is a spontaneous process It can occur for small, nonpolar substances directly across a membrane

20. Diffusion

21. Facilitated Diffusion  The diffusion of a substance with the assistance of a transport protein

22. Channel Proteins provided channels through which charged or polar substances may pass Aquaporins Ion Channels Gated Channels

23. Carrier Proteins  Carrier proteins- undergo subtle changes that help larger solutes move across the membrane

24. Osmosis Diffusion of water across a selectively permeable membrane

25. Tonicity The ability of a solution to cause a cell to gain or lose water Dependent on the concentration of solutes that cannot cross the membrane relative to the cell itself

26. Hypotonic Solution Contains less dissolved solute than the cell itself Will cause water to rush into the cell to equalize the solute concentration Animal cell will burst Plant cell will turgid or firm as the cell will push out against the cell wall- this is the favorable condition for a plant

27. Hypotonic Solution

28. Isotonic Solution Contains the same amount of dissolved solute as the cell Will cause water to move in and out of the cell at equal rates Animal cell will stay the same- this is the favorable condition for an animal cell A plant cell will become saggy and the plant will be droopy

29. Isotonic Solution

30. Hypertonic solution Contains more dissolved solute than the cell Will cause water to rush out of the cell to equalize the solute concentrations Animal cell will shrivel Plant cell will undergo plasmolysis- the process of the plasma membrane pulling inward away from the cell wall

31. Hypertonic Solution

32. Active Transport movement of a substance against a concentration gradient that requires the cell to expend energy

33. Electrogenic Pumps Membrane protein that generates voltage across a membranes by actively pumping ions across a membrane to contribute to membrane potential Membrane Potential Voltage (potential energy caused by the difference in charges across a membrane) Cytoplasm is in general more negatively charged

34. Sodium-Potassium Pump Found in animal cells Pumps out 3 sodium ions for every two potassium ions that enter the cell Helps to keep the inside of the cell at a net negative charge

36. Proton Pump Found in plants Pumps hydrogen ions (protons) out of the cell

37. Cotransport Coupling the “downhill” diffusion of a substance with the “uphill” diffusion of another substance against its concentration gradient

38. Exocytosis

39. Endocytosis- Phagocytosis

40. Endocytosis- Pinocytosis

41. Receptor-Mediated Endocytosis

42. Water Potential

43. The Problem with Cell Walls Osmosis can easily be predicted when there is not a physical barrier to stand in the way of the movement of water into our out of a cell Plant cells have cell walls so osmosis into our out of a plant cell depends on two factors: The concentration of solutes Physical pressure on both sides of the cell wall

44. Water Potential Water potential (Ψ) is the measurement of the combined effects of solute concentration and physical pressure on osmosis into our out of a plant cell Measured in units of megapascals (MPa) When water is bound to a solute it does not have as much freedom to move; this can also be viewed as the inability of water to perform work (little potential energy) Water will ALWAYS move from a region of high water potential to regions of lower water potential

45. The Formula Ψ= Ψ s + Ψ p Ψ s = solute potential or osmotic potential Solute potential of pure water is 0MPa Adding a solute ALWAYS lowers solute potential so it can be negative, but never positive Ψ p = pressure potential Can be positive or negative relative to atmospheric potential Turgor Pressure results when the cell is under positive pressure relative to the atmosphere

48. Another Wacky Formula Solute Potential Ψ s = -iCRT i= ionization constant always 1 for a molecular compound because they do not ionize unique to ionic compounds in chemistry represented by K eq C= molar concentration of the solute (molarity) R= Pressure constant 0.831 L*atm/mol*K T= Temperature in Kelvin (°C+ 273)