bio 178 lecture 11
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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Bio 178 Lecture 11Biological Membranes (Cntd.)
http://www.cellsalive.com/channels.htm
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Reading
• Chapter 6
Quiz Material
• Questions on P 124
• Chapter 6 Quiz on Text Website (www.mhhe.com/raven7)
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Outline• Biological Membranes Proteins (Cntd.)
Membrane Transport
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Transmembrane Proteins1. Single-Pass Transmembrane ProteinsProtein passes through membrane once - one non-polar region.
2. Multiple-Pass Transmembrane ProteinsProtein passes through membrane several times using helices Channel
Example
• Bacteriorhodopsin - 7 pass protein Channel for protons to pass during photosynthesis.
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Multiple-Pass Transmembrane Protein
http://www.enzim.hu/hmmtop1.1/doc/model.html
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Channel Protein in Halobacterium halobium
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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.
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Structure of a Pore Protein
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Porin - Barrel
http://fig.cox.miami.edu/~cmallery/255/255etc/porin.htm
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Membrane Transport1. Passive ProcessesDo not require an input of energy. Include:• Diffusion
• Osmosis
2. Active ProcessesRequire an input of energy. Include:
• Endocytosis
• Exocytosis
• Active Transport
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Diffusion
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Diffusion
Movement of molecules and ions down a concentration gradient until they are evenly distributed, eg. O2.
Description
Selective Diffusion in CellsSubstances that do not cross the membrane by simple diffusion (polar substances) can cross via specific transporters.
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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
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Selective Diffusion (Cntd.)Facilitated Diffusion - CarriersTransport 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 MethodTransmembrane protein transports glucose by conformational change.
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Facilitated Diffusion
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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
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OsmosisDescriptionDiffusion of water across a selectively permeable membrane.AquaporinsMembrane 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].
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Osmosis
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Osmotic ConcentrationDetermined 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 TransportHypoosmotic Hyperosmotic
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Osmotic PressurePressure 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).
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Osmotic Pressure (What is the Ideal Extracellular Concentration for Cells?)
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Methods Used to Maintain Osmotic Balance• Isosmotic CytoplasmIsosmotic with environment, eg. certain marine organisms.
• TurgorCytoplasm is hyperosmotic to environment, eg. Plant cells.
Plants - hydrostatic pressure (turgor pressure) pushes plasma membrane against cell wall.
• ExtrusionWater is pumped out of the organism to the hypoosmotic environment, eg. Contractile vacuole of Paramecium.
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Bulk Transport
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Bulk Transport
2 types:• Endocytosis
• ExocytosisEndocytosis
Utilization of a membrane to take material into a cell.• PhagocytosisUptake of solid material, eg. Neutrophil uptake of bacteria.
• PinocytosisUptake of liquid material.
Use of a membrane to envelope material to be transported.
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Phagocytosis
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Pinocytosis
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Endocytosis (Cntd.)• Receptor Mediated Endocytosis (RME)1. Specific molecules bind to specific receptors in the PM.
2. These accumulate in coated pits (clathrin).
3. 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.