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Bio 178 Lecture 11

Bio 178 Lecture 11. Biological Membranes (Cntd.). http://www.cellsalive.com/channels.htm. Reading. Chapter 6. ?. Quiz Material. Questions on P 124 Chapter 6 Quiz on Text Website (www.mhhe.com/raven7). Outline. Biological Membranes Proteins (Cntd.) Membrane Transport.

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Bio 178 Lecture 11

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  1. Bio 178 Lecture 11 Biological Membranes (Cntd.) http://www.cellsalive.com/channels.htm

  2. Reading • Chapter 6 ? Quiz Material • Questions on P 124 • Chapter 6 Quiz on Text Website (www.mhhe.com/raven7)

  3. Outline • Biological Membranes • Proteins (Cntd.) • Membrane Transport

  4. Transmembrane Proteins 1. Single-Pass Transmembrane Proteins Protein passes through membrane once - one non-polar region. 2. Multiple-Pass Transmembrane Proteins Protein passes through membrane several times using  helices Channel Example • Bacteriorhodopsin - 7 pass protein  Channel for protons to pass during photosynthesis.

  5. Multiple-Pass Transmembrane Protein http://www.enzim.hu/hmmtop1.1/doc/model.html

  6. Channel Protein in Halobacterium halobium

  7. Transmembrane Proteins 3. Pores Large non-polar regions that form ß-pleated sheets, which form a barrel structure within the membrane. Example Porin proteins of bacteria.

  8. Structure of a Pore Protein

  9. Porin -  Barrel http://fig.cox.miami.edu/~cmallery/255/255etc/porin.htm

  10. Membrane Transport

  11. Membrane Transport 1. Passive Processes Do not require an input of energy. Include: • Diffusion • Osmosis 2. Active Processes Require an input of energy. Include: • Endocytosis • Exocytosis • Active Transport

  12. Passive Processes

  13. Diffusion

  14. Diffusion Description Movement of molecules and ions down a concentration gradient until they are evenly distributed, eg. O2. Selective Diffusion in Cells Substances that do not cross the membrane by simple diffusion (polar substances) can cross via specific transporters.

  15. Selective Diffusion Ion Channels Polar groups line the channel, allowing ions to pass through the membrane down their concentration gradient. Each ion channel is specific. Direction of Transport • Dependent on: • Ion Concentration • Voltage across the membrane

  16. Selective Diffusion (Cntd.) Facilitated Diffusion - Carriers Transport of molecules and ions down their concentration gradient that is achieved by a protein that physically binds them. Example: Glucose transporter in RBCs • Concentration Gradient Maintained by addition of phosphate to glucose - prevents it from crossing back through membrane. • Transportation Method Transmembrane protein transports glucose by conformational change.

  17. Facilitated Diffusion

  18. Facilitated Diffusion (Cntd.) Carrier Saturation Occurs when all the carriers are saturated - increased concentration gradient does not increase transport rate. Key Features of Facilitated diffusion • Passive • Specific • Saturates

  19. Osmosis Description Diffusion of water across a selectively permeable membrane. Aquaporins Membrane channels for water. Mechanism • Different concentrations of solutes on the 2 sides of the membrane  Different concentrations of free water. • Free water moves down its concentration gradient ( higher [solute].

  20. Osmosis

  21. Osmotic Concentration Determined by the concentration of all solutes in a solution. • Hyperosmotic Solution Solution with the higher concentration (of solute). • Hypoosmotic Solution Solution with the lower concentration (of solute). • Isosmotic solution Concentration of solutes in the 2 solution is equal. Direction of Transport Hypoosmotic  Hyperosmotic

  22. Osmotic Pressure Pressure that must be applied across a membrane to stop the osmotic movement of water across a membrane. Hydrostatic Pressure Pressure exerted by the cytoplasm pushing against the plasma membrane (increases as water flows in). Counteracts osmotic pressure (water flowing into the cell).

  23. Osmotic Pressure (What is the Ideal Extracellular Concentration for Cells?)

  24. Methods Used to Maintain Osmotic Balance • Isosmotic Cytoplasm Isosmotic with environment, eg. certain marine organisms. • Turgor Cytoplasm is hyperosmotic to environment, eg. Plant cells. Plants - hydrostatic pressure (turgor pressure) pushes plasma membrane against cell wall. • Extrusion Water is pumped out of the organism to the hypoosmotic environment, eg. Contractile vacuole of Paramecium.

  25. Active Processes

  26. Bulk Transport

  27. Bulk Transport Use of a membrane to envelope material to be transported. 2 types: • Endocytosis • Exocytosis Endocytosis Utilization of a membrane to take material into a cell. • Phagocytosis Uptake of solid material, eg. Neutrophil uptake of bacteria. • Pinocytosis Uptake of liquid material.

  28. Phagocytosis

  29. Pinocytosis

  30. Endocytosis (Cntd.) • Receptor Mediated Endocytosis (RME) Specific molecules bind to specific receptors in the PM. These accumulate in coated pits (clathrin). The clathrin then causes a vesicle to form (only when the target molecule binds to the receptor)  endocytosis. Example - LDL (low density lipoprotein) • Means of transportation of cholesterol. When cholesterol is required for membranes the LDL is taken up by RME. • Hypercholesterolemia - LDL receptors lack tails  LDL not taken up by RME  cholesterol remains in blood  atherosclerosis.

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