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3, Carriers and channels.pdf

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McGill University
Anatomy & Cell Biology
ANAT 262
John Presley

Carriers, Channels and Electrical Properties of the Membrane: Membrane transport is, of course, selective in nature and it has some important functions: Allows a cell to take in and release compounds in accordance with its biological function o E.g. the uptake and release of oxygen by red blood cells in response to cell needs Allows cells to regulate characteristics of this transport o E.g. the increased transport of glucose into muscle cells during activity again, depending on the need The membranes define the exterior edges of cells and divide them into compartments They are composed of amphipathic phospholipid, meaning a portion of the molecule is charged, while the rest is uncharged or hydrophobic. The lipids align to form a bilayer, with the charged (polar, hydrophilic) portions, or heads facing the aqueous environment and the uncharged (non- polar, hydrophobic) portions or tail interacting with each other within the bilayer The cell membrane is composed of various types of lipids and proteins including: Proteins for transporting substances across the membrane or linking the membrane to the cell's cytoskeleton Signalling molecules and a variety of other structures All components of the membrane vibrate randomly creating small openings in the bilayer through which water and some other small molecules can freely pass The plasma membrane acts a selective barrier in the cell, which stops some molecules from passing based on two general concepts: size (sterics) and charge Polar molecules such as ions and glucose will not pass, both because they cannot pass through the non-polar internal environment of the membrane and because they have high affinity for the water molecules in which they are dissolved The hydrophobic interior of the bilayer generates a barrier through which large molecules will not pass due to the their size o Therefore, physical constraints within the membrane generate a barrier as well Endocytosis and nuclear transport are ways of getting large molecules through membranes The barrier function is to allow the cell to maintain certain electrochemical gradients across the various membranes; each solute has its own relative permeability to a certain membane All molecules will eventually diffuse down their concentration gradient, but the rate at which they do this is defined by their permeability Permeability coefficients are therefore a reflection of how well a solute can diffuse through a membrane o The highest coefficient is at 0, and represents full permeability, while the lower values become less and less permeable There are 4 primary ways that water and other small molecules can get into or out of the cell: Simple diffusion Facilitated diffusion Primary active transport Secondary active transport Diffusion is powered by random movement of molecules in a solution in which the net movement is from regions of high concentration to low Does not saturate as the concentration or gradient changes Diffusion of different substances do not interfere with each other (i.e. no competition) Net flux (movement) is proportional to the concentration gradient and the permeability Substances can cross membranes by diffusion if they can dissolve in the hydrophobic interior of the membrane Diffusion can occur through tight junctions or within bulk solutions Diffusion of water down its concentration gradient is called osmosis Simple diffusion is passive transport and does not saturate as the concentration changes Facilitated diffusion is a process in which proteins act as carriers or pores (channels) to permit flux of substances that cannot diffuse directly through the membrane (low permeability coeff.) Movement is still passive (like diffusion), from high concentration to low, but occurs across cell membranes only The carrier protein carry solute through the membrane by a series of conformational changes; the protein itself doesnt move within the membrane Saturates when substance reaches high concentrations due to lack of available carrier proteins o Maximum rate of transport (fully saturated) is called Tm, the transport maximum (this applies to and saturable system) Related substances can compete for the same carrier or pore o e.g. Potassium, Rubidium and Cesium are all cations which use the same pore o e.g. Calcium, Strontium and barium are another set of cations which use the same pore KEY POINT: Transport is protein mediated and is therefore saturable Primary active transport is a process in which proteins in the membrane can act as pumps to move ions or small molecules from low concentration to high concentration (i.e. up their gradients or uphill) This requires cellular energy, usually as hydrolysis of ATP Saturates when substance reaches high concentrations due to lack of available protein. One example of a pump is the Na-K ATPase, which is present in nearly every cell in the body o It pumps 3 Na2+ ions out in exchange for 2 K+ ions pumped in (cost=1 ATP) Other pumps include the Ca-ATPase, which stores calcium in the cell ER, and the H- ATPase, which can acidify certain cell compartments Secondary Active Transport uses proteins similar to those for facilitated diffusion, but couples the movement of several different molecules in each cycle System saturates when substance reaches high concentrations due to lack of available protein Cotransport moves 2 or more molecules in the same direction across the membrane Counter transport moves molecules in opposite directions Uphill solute transport is coupled to "downhill transport of a different solute whose gradient was established (and maintained) by primary active transport. o Normal active transport (Na-K ATPase) makes a strong Na gradient, which in turn powers many secondary active transport mechanisms o The downhill transport acts as an energy source for the uphill, and therefore the process is ATP independent An example: Na-Glucose cotransport Membrane proteins can be specific to certain types of molecules, like ions, but can also be have higher specificity within the type of molecule, which explains their large variety Types of molecules transported: ions, sugars, amino acids, nucleotides and cell metabolites Specificity: Transport proteins exhibit specificity for the solutes (i.e. ion, sugar or amino acid) they transport In the 1950s, bacteria were mutagenized to see if they could find defects in sugar uptake Geneticists isolates the samples with different mutations and observed the resulting uptake problems These sequences were matched to the genome, and the specificities of these transporters was established Cystinuria is an example of an inherited disease with a mutation in a membrane transport protein The disease: Autosomal recessive defect in reabsorptive transport of cystine (2 cysteines joined together by a disulphide bridge) and other dibasic (2 residues) amino acids from the luminal fluid of the renal proximal tubule predominantly Diagnosis: Generation of kidney stones, enriched in cystine, which often recurs throughout a patient's lifetime Surgical intervention is necessary to remove the stones (UTIs) Defective gene: the gene products SLC3A1 SLC7A9 (SLC for solute carrier) code for the high- affinity cystine transporter present in the apical brush-border membrane of the jejunum o This shows the specificity of a transport molecule: 1 mutation can cause the whole specificity to be lost Stones arise as cystine is relatively insoluble at physiologic urine pH levels of 5-7 Multi-pass transmembrane proteins are protein in which the polypeptide traverses the membrane multiple times This continuity enables specific hydrophilic solutes to cross the membrane without coming into contact with the hydrophobic interior of the lipid bilayer. Examples are the Na/K ATP-ase, which has 2-3 subunits, each with TM domains, and the GABA A receptor in neurons, which is a pentamer, each subunit having 4 TM domains There are two classes of transport proteins: carriers and channels Carrier proteins: also known as carriers, permeases, pumps or transporters bind the specific solute to be transported o They undergo a series of conformational changes to transfer the solute across the membrane o They are involved in SLOW transport o Transmembrane carrier proteins may transport larger molecules e.g. glucose or amino acids and change their conformation as the molecules are carried through o Carriers can be involved in passive or active transport Channel proteins: form aqueous pores across the lipid bilayer, which, when open, allow the passage of specific solutes to pass through them (positive pore, negative ion and vice versa) o Interactions with the solute in channels are significantly weaker than with carriers o It is involved in FAST transport, about 10,000x faster than carriers o Transmembrane channels form hydrophilic pores which are gated
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