PDBIO 305: Membrane Transport - Week 3
PDBIO 305: Membrane Transport - Week 3 PDBIO305
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This 4 page Class Notes was uploaded by Kirsten Notetaker on Tuesday September 20, 2016. The Class Notes belongs to PDBIO305 at Brigham Young University taught by David Thomson in Fall 2016. Since its upload, it has received 8 views. For similar materials see Human Physiology in Physiology and Developmental Biology at Brigham Young University.
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Date Created: 09/20/16
Membrane Transport Concentrations of various solutes are different on the inside vs. outside of the cell Movement of stuff across the cell membrane is regulated 2 major molecular driving forces Chemical: due to a concentration gradient Electrical: due to an electrical gradient Taken together, these are referred to as the electrochemical gradient (ECG in these notes) Diffusion: NET movement of molecules or ions from an area of high concentration to an area of low concentration Passive vs. Active membrane transport Transportation requires some sort of force (energy) to move the particles across the membrane If the process requires the cell to expend energy, then it is active transport Carried out by proteins called pumps If the process does not require the cell to expend energy, then it is called passive transport Driving force is the electrochemical gradient IMPORTANT: passive transport does still require energy, as all objects require energy in order to move (basic physics) – passive means that the cell as a whole does not need to expend energy (because it is moving with the gradient) **NOTE** There will be a question on the test about this! Be sure not to answer “true” for a true/false question that says “Ions do not use energy to move in passive transport” or similar Passive transport Simple diffusion Through lipid bi-layer or through channels Facilitated diffusion Carrier-mediated (binds to something in the membrane which helps it through) Active transport Primary active Carrier-mediated – same as with passive but in this case ATP must be used to activate the carrier Secondary active Carrier-mediated There is also tertiary and quaternary, but we won’t deal with those as much Vesicular transport (endo- or exocytosis) Fick’s Law of Diffusion Defines the effect of factors that influence the rate of diffusion across a membrane Factors that increase rate of diffusion Increased concentration gradient Increased permeability of membrane to substance Increased surface area of membrane Factors that decrease the rate of diffusion Increased molecular weight of substance Increased distance through the membrane Diffusion across a membrane Molecules/ions can diffuse through a membrane if it is permeable to that substance Permeability is determined by size of molecules/ions, presence of carriers or channels, charge of carriers/channels and molecules/ions, etc. Osmosis Special case of diffusion Net diffusion of water down its own concentration gradient through a semi-permeable membrane Osmolarity (osM) = the total solute particle concentration (as opposed to molecular (molar) concentration) To determine this, account for the fact that salts dissolve in water For the purposes of this class, if Cl is in the molecule it’s a salt Ex: 100 ml water contains 3 g KCl = 0.402 M Now in water, KCl dissociates into 2 different particles, which now creates twice as many particles (moles) in the solution Thus, in this solution KCl is 0.804 osmolar (multiply molarity by 2) Tonicity = osmolarity of a solution in relation to osmolarity of body fluids Isotonic = osM same as cells No uptake or loss of water from cells Hypertonic = osM > cells Cells lose water & become crinkled in appearance (crenation) Hypotonic = osM < cells Uptake of water by the cell causes lysis (bursting) of cells Hemolysis = lysis of red blood cells Pure water is hypotonic Channels vs. carriers Channels simply let ions through the membrane, while carriers actually change conformation to let an ion bound to it into the other side Carrier-mediated transport Specificity – most carriers will only transport 1 specific substance Competition – some substances compete for carriers Ex: glucose & galactose are close enough in structure to compete for same carrier binding Saturation – how much of the substance is present Rate of transport will increase with saturation until it is saturated enough that the carriers can’t work any faster and they are all being used Cotransport Symport (cotransport) A carrier moves multiple substances in the same direction Antiport (countertransport) A carrier moves multiple substances in opposite directions Facilitated diffusion – carrier-mediated passive transport Primary active transport Phosphate group from ATP binds to carrier & its negative charge changes structure (configuration) of protein carrier Substance bound to carrier is pushed to other side of membrane, after which it is released, the phosphate group is released & the protein changes back to original structure Sodium-Potassium ATPase Major player in many cells of the body Crucial for maintaining ion concentration in many cells Basically a pump to maintain Na and K levels in the cell 1) A pump bound to Na uses a phosphate from ATP to change shape and push the Na out of the cell 2) The pump then binds to K which is outside the cell and gives up its phosphate i+ order to change back to its original shape and release the K inside the cell Secondary active transport Ex: epithelial cells of intestine Mechanism for glucose absorption from gut to blood Vesicular transport Material is moved into or out of the cell wrapped in membrane Active method of membrane transport Two types of vesicular transport Endocytosis – moving into the cell Exocytosis – moving out of the cell Provides mechanism for secreting large polar molecules Enables cell to add specific components to membrane
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