CHAPTER 5 MEMBRANE STRUCTURE AND TRANSPORT Prepared by Brenda Leady, University of Toledo

1. CHAPTER 5 MEMBRANE STRUCTURE AND TRANSPORT Prepared by Brenda Leady, University of Toledo

2. Biological Membranes • Basic framework of the membrane is the phospholipid bilayer • Phospholipids are amphipathic molecules • Hydrophobic (water-fearing) region faces in • Hydrophilic (water-loving) region faces out • Membranes also contain proteins and carbohydrates • Relative amount of each vary

3. Fluid-mosaic model • Membrane is considered a mosaic of lipid, protein, and carbohydrate molecules • Membrane exhibits properties that resemble a fluid because lipids and proteins can move relative to each other within the membrane

5. Proteins bound to membranes • Integral membrane proteins • Transmembrane proteins • One or more regions that are physically embedded in the hydrophobic region of the phospholipid bilayer • Lipid anchors • Covalent attachment of a lipid to an amino acid side chain within a protein • Peripheral membrane proteins • Noncovalently bound to regions of integral membrane proteins that project out from the membrane, or they are bound to the polar head groups of phospholipids

7. Approximately 25% of All Genes Encode Membrane Proteins • Membranes are important biologically and medically • Computer programs can be used to predict the number of membrane proteins • Estimated percentage of membrane proteins is substantial: 20–30% of all genes may encode membrane proteins • This trend is found throughout all domains of life including archaea, bacteria, and eukaryotes • Function of many genes unknown- study may provide better understanding and better treatments

9. Membranes are semifluid • Fluidity- individual molecules remain in close association yet have the ability to readily move within the membrane • Semifluid- most lipids can rotate freely around their long axes and move laterally within the membrane leaflet • “Flipflop” of lipids from one leaflet to the opposite leaflet does not occur spontaneously • Flippase requires ATP to transport lipids from one leaflet to another

11. Factors affecting fluidity • Length of fatty acyl tails • Shorter acyl tails are less likely to interact, which makes the membrane more fluid • Presence of double bonds in the acyl tails • Double bond creates a kink in the fatty acyl tail, making it more difficult for neighboring tails to interact and making the bilayer more fluid • Presence of cholesterol • Cholesterol tends to stabilize membranes • Effects depend on temperature

12. Experiments on lateral transport • Larry Frye and Michael Edidin conducted an experiment that verified the lateral movement of membrane proteins • Mouse and human cells were fused • Temperature treatment- 0°C or 37°C • Mouse membrane protein H-2 fluorescently labeled • 0°C cells- label stays on mouse side • 37°C cells- label moves over entire cell

14. FRAP • Watt Webb and colleagues used fluorescence recovery after photobleaching (FRAP) • Proteins on the surface of a cell were covalently labeled with a fluorescent chemical • Small area of cell photobleached leaving white spot • Over time, bleached molecules within the white spot spread outward, and the white region filled in with red fluorescent molecules • Indicates that proteins can laterally move in the membrane

16. Not all integral membrane proteins can move • Depending on the cell type, 10–70% of membrane proteins may be restricted in their movement • Integral membrane proteins may be bound to components of the cytoskeleton, which restricts the proteins from moving laterally • Also, membrane proteins may be attached to molecules that are outside the cell, such as the interconnected network of proteins that forms the extracellular matrix

18. Glycosylation • Process of covalently attaching a carbohydrate to a protein or lipid • Glycolipid – carbohydrate to lipid • Glycoprotein – carbohydrate to protein • Can serve as recognition signals for other cellular proteins • Often play a role in cell surface recognition • Protective effects • Cell coat or glycocalyx- carbohydrate-rich zone on the cell surface shielding cell

20. Electron microscopy • Transmission electron microscopy (TEM), uses a biological sample that is thin sectioned and stained with heavy-metal dyes • Dye binds tightly to the polar head groups of phospholipids, but it does not bind well to the fatty acyl chains

21. FFEM • Freeze fracture electron microscopy, specialized form of TEM, can be used to analyze the interiors of phospholipid bilayers • Sample is frozen in liquid nitrogen and fractured with a knife • Due to the weakness of the central membrane region, the leaflets separate into a P face (the protoplasmic face that was next to the cytosol) and the E face (the extracellular face) • Can provide significant three-dimensional detail about membrane protein form and shape

23. Selectively permeable • Structure ensures … • Essential molecules enter • Metabolic intermediates remain • Waste products exit

24. Phospholipid bilayer is a barrier • Hydrophobic interior makes formidable barrier • Diffusion • Movement of solute from an area of higher concentration to an area of lower concentration • Passive diffusion- without transport protein • Solutes vary in their rates of penetration

26. Cells maintain gradients • Transmembrane gradient • Concentration of a solute is higher on one side of a membrane than the other • Ion electrochemical gradient • Both an electrical gradient and chemical gradient

27. Passive transport • Passive transport does not require an input of energy • 2 types • Passive diffusion • Diffusion of a solute through a membrane without transport protein • Facilitated diffusion • Diffusion of a solute through a membrane with the aid of a transport protein

29. Tonicity • Isotonic • Equal water and solute concentrations on either side of the membrane • Hypertonic • Solute concentration is higher (and water concentration lower) on one side of the membrane • Hypotonic • Solute concentration is lower (and water concentration higher) on one side of the membrane

30. Outside the cell Inside the cell Isotonic The solution and cell are isotonic Hypertonic The solution is hypertonic to the cell Hypotonic The solution is hypotonic to the cell

31. Osmosis • Water diffuses through a membrane from an area with more water to an area with less water • If the solutes cannot move, water movement can make the cell shrink or swell as water leaves or enters the cell • Osmotic pressure- the tendency for water to move into any cell

32. Animal cells must maintain a balance between extracellular and intracellular solute concentrations to maintain their size and shape Crenation- shrinking in a hypertonic solution

33. A cell wall prevents major changes in cell size Turgor pressure- pushes plasma membrane against cell wall Maintains shape and size Plasmolysis- plants wilt because water leaves plant cells

35. Agre Discovered That Osmosis Occurs More Quickly in Cells with Transport Proteins That Allow the Facilitated Diffusion of Water • Water passively diffuses across plasma membranes • Certain cell types allow water to move across the plasma membrane at a much faster rate than would be predicted by passive diffusion • Peter Agre and his colleagues first identified a protein that was abundant in red blood cells and kidney cells, but not found in many other cell types • CHIP28 • Striking difference was observed between frog oocytes that expressed CHIP28 versus the control • Aquaporins • Agre was awarded the Nobel Prize in 2003 for this work

37. Transport proteins • Transport proteins enable biological membranes to be selectively permeable • 2 classes • Channels • Transporters

38. Form an open passageway for the direct diffusion of ions or molecules across the membrane Aquaporins Channels

39. Most are gated- open or close Ligand-gated Intracellular regulatory proteins Phosphorylation Voltage-gated Mechanosensitive channels

40. Also known as carriers Conformational change transports solute Principal pathway for the uptake of organic molecules, such as sugars, amino acids, and nucleotides Key role in export Transporters

41. Transporter types Uniporter single molecule or ion Symporter/ cotransporter 2 or more ions or molecules transported in same direction Antiporter 2 or more ions or molecules transported in opposite directions

42. Pump Couples conformational changes to an energy source, such as ATP-driven pumps ATP hydrolysis can be uniporters, symporters, or antiporters Active transport

43. Active transport • Movement of a solute across a membrane against its gradient from a region of low concentration to higher concentration • Energetically unfavorable and requires the input of energy • Primary active transport • Directly use energy to transport solute • Secondary active transport • Use pre-existing gradient to drive transport of solute

45. ATP-Driven Ion Pumps Generate Ion Electrochemical Gradients • Na+/K+-ATPase • Actively transport Na+ and K+ against their gradients by using the energy from ATP hydrolysis • 3 Na+ exported for 2 K+ imported into cell • Antiporter • Electrogenic pump- export 1 net positive charge

47. Exocytosis/ Endocytosis • Transport larger molecules such as proteins and polysaccharides, and even very large particles • Exocytosis • Material inside the cell, which is packaged into vesicles, is excreted into the extracellular medium • Endocytosis • Plasma membrane invaginates, or folds inward, to form a vesicle that brings substances into the cell • Receptor-mediated endocytosis • Pinocytosis • Phagocytosis