Membrane Structure and Function

1. Membrane Structure and Function Chapter 7

2. Objectives • Describe the fluid mosaic model of cell membranes and the roles of proteins in the membranes • Define the terms diffusion, osmosis, facilitated diffusion, active transport, exocytosis, endocytosis, isotonic, hypotonic and hypertonic

3. Membrane Function • The plasma membrane organizes chemical activities of cell • separates cells from outside environment • controls passage of molecules across membranes • Selectively permeable • partitions organelle function in eukaryotes • provides reaction surfaces and organizes enzymes and their substrates

4. Membrane Structure • Phospholipids of membranes form bilayers • phospholipids have polar “head” and nonpolar “tail” • form stable bilayer in water with heads out and tails in • hydrophobic interior forms barrier to hydrophilic molecules

5. Membrane Structure • Membrane is fluid mosaic of lipids and proteins • proteins embedded in phospholipid bilayer • individual molecules free to move laterally • some proteins extend through both sides of bilayer • cholesterol helps stabilize animal cell membranes at different temperatures

7. Proteins in the membrane • lipids can move in membrane - semi-fluid nature of membrane • Two major classes of proteins in membrane • integral – transmembrane • peripheral - loosely associated with membrane surface • membrane also shows “sidedness” • interior - attachment to cytoskeleton • exterior - carbohydrates, extracellular matrix

10. An integral protein could be a transporter • True • False

11. Membrane Function • Proteins make membrane a mosaic of function • identification tags-glycoproteins • enzymes • receptors-trigger cell activity when molecular messenger binds • cell junctions • transporters

12. The role of membranes in cell-cell recognition • Cells recognize each other by binding to surface molecules, often carbohydrates, on the surface of the cell membrane • Membrane carbohydrates may be covalently bonded to lipids (glycoproteins) • Or more commonly to proteins (glycoproteins) • Improper molecule means no bonding

13. Synthesis and sidedness of membranes • Membranes have a distinct inside and outside faces • This affects movement of proteins synthesized in the endomembrane system

14. Permeability of the Lipid Bilayer • A cell must exchange materials with its surroundings, a process controlled by the plasma membrane • Hydrophobic molecules are lipid soluble and can pass through the membrane rapidly • Polar molecules do not cross the membrane rapidly • Transport proteins allow passage of hydrophilic substances across the membrane

15. How does water move? • Is water polar or unpolar? • So how does water move? • Aquaporins!!

16. Membrane Function • Diffusion across a membrane • diffusion is tendency of molecules to spread out spontaneously from area of high concentration to area of low concentration • passive diffusion across membranes occurs when molecules diffuse down concentration gradient • at equilibrium molecules diffuse back and forth-no net gain or loss • Different molecules diffuse independently of each other • Ex. Water, Sodium, Chloride

18. Membrane Function • Osmosis is diffusion of water (passive) • if cell membrane permeable to water but not solute separates area of high solute concentration (hypertonic) from area of low concentration (hypotonic), water diffuses from hypotonic area to hypertonic area until concentrations are equal • direction of osmosis is determined by differences in relative concentrations • Gradient = concentration difference

19. Water balance of cells without walls • Tonicity is the ability of a solution to cause a cell to gain or lose water • Has a great impact on cells without walls

20. Tonicity • Isotonic = solutions of equal solute concentration • Hypotonic = less solute relative to the solution is it being compared • Hypertonic = more solute relative the solution it is being compared

22. Water movement • Water will always move from the solution with a lower solute concentration to the high solute solution.

23. Membrane Function • Water balance between cells and surroundings critical • cell membranes semi-permeable • cells in isotonic solution do not change size -no osmosis • cells in hypotonic solution gain water • cells in hypertonic solutions lose water

24. The cell contains 20% solute, the beaker 10% salt. What will happen to the cell? 20 10 H2O

25. Water balance with cell walls • Cell walls help maintain water balance • It a plant cell is turgid it is in a hypotonic environment • It’s very firm, a healthy state in most plants • If a plant cell is flaccid it is in an isotonic or hypertonic environment

27. A cell that contains 22% solute is in a beaker that contains 22% solvent. Which of the following is correct? • The cell is hypotonic to the solution in the beaker • The cell is hypertonic to the cell in the beaker • The cell is isotonic to the solution in the beaker

28. Membrane Function • Specific proteins facilitate diffusion across membranes • facilitated diffusion occurs when protein pore in membrane allows solute to diffuse down concentration gradient • no energy required • rate depends on number of transport proteins and strength of gradient

30. Membrane Function • Cells expend energy for active transport • transport protein involved in moving solute against concentration gradient • energy from ATP-mediated phosphorylation changes protein shape and moves solute molecule across membrane • active transport of two solutes in opposite directions often coupled, but not always

31. Electrogenic Pumps • Sometimes the transporters pump ions that cause an electrical gradient to from across the membrane • An example: This occurs in your nerve cells • Electrogenic pumps, like the Na+-K+ pump, generate voltage across membranes. The resulting voltage, or membrane potential, is energy that can be used to drive the transport of ions against a chemical gradient

34. Transporters have complex subcategories • Uniports- One Substance in One Direction • Symport- Two Substances in Same Direction • Antiports- Two Substances in Opposite Directions • Aquaporins – Rapid water transport channels

35. Co-transport • Membrane proteins co-transport two solutes by coupling the downhill diffusion of one solute with the uphill diffusion of the other

37. Membrane Function • Exocytosis and endocytosis transport large molecules • exocytosis: membrane-bound vesicles containing large molecules fuse with plasma membrane and release contents outside cell • endocytosis: plasma membrane surrounds materials outside cell, closes around materials, and forms membrane-bound vesicles

38. Types of Endocytosis • Three important types of endocytosis are: • phagocytosis • pinocytosis • receptor-mediated endocytosis

40. Signal Transduction Across Membranes • Proteins embedded in the cell membrane can carry messages and cause a chain reaction of signals that “tell” you cells its time to do an activity or make a chemical in short supply • You will learn more about this in other classes

42. Checklist • What forms the membrane bilayer? • What are the two major components of membranes? • What are the 2 classes of proteins? • What are the functions of proteins in the membrane? • Do membranes show sidedness? • What types of molecules more through a membrane rapidly? • How can water move rapidly?

43. Checklist • What are the types of movement? • What are the types of tonicity? • What are the three types of endocytosis? • What is signal transduction?