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MembraneMembraneMembrane TransportTransportTransport By Dr. Carmen Rexach Physiology Mount San Antonio College Passive vs. Active • Passive transport – simple diffusion –Osmosis – filtration – facilitated diffusion • Active transport – primary active transport – secondary active transport Simple diffusion • Net movement of molecules from an area of high concentration to an area of low concentration by random molecular motion Diffusion through a cell membrane • Diffuse rapidly – Nonpolar molecules – small molecules with polar covalent bonds –Gases –Examples: CO2, oxygen, fatty acids, steroid hormones •usually do not diffuse – large polar molecules –Ions – Examples: phosphorylated organic molecules Rate of diffusion • Flux = amount of material crossing a surface per unit of time • Net flux = difference between one-way flux •Influenced by – Magnitude of concentration difference –Mass of the molecule –Temperature – Medium (gas, liquid, solid) –surface area of the membrane Rate of diffusion through membrane • Net flux (F) across the membrane: –F=kpA(Co-Ci) – Kp=permeability constant • Dependent on temperature and permeability of membrane – A = surface area –(Co-Ci) = concentration difference Remember: The major limiting factor in diffusion across the membrane is its hydrophobic nature! Ions diffuse through protein channels • Channels formed by integral membrane proteins • Selective – Channel diameter – Charged and polar protein surfaces • Regulation by channel gating •Ligandgated • Voltage gated • Mechanically gated Diffusion and the electrical gradient • Important consideration in ion diffusion • Membrane separates electrical charges – Same charges repel – Opposite charges attract More information on this soon! There are two driving forces in the movement of molecules across the membrane: the concentration gradient (chemical) and the electrical gradient (electrical). Together they are called the electrochemical gradient. Osmosis • The movement of water from an area of high water concentration to an area of low water concentration across a semi-permeable membrane Osmolality • Mole = molecular weight of an element or compound measured in grams • Molality = # of moles of solute per kg of solvent • Osmolality = ionic concentration of dissolved substances per kg of solvent – Osmolality of plasma = 300 mOsm = isosmotic – depends on the number of solute particles •1 m of C6H12O6 = 1 osmole (Osm) •1 m of NaCl = 2 osmoles •1m of CO2 + 1 m of KCl = 3 osmoles Osmolarity vs Osmolality • Osmolarity refers to the number of solute particles per 1 L of water • Osmolality refers to the number of solute particles per 1 kg of water • Since the mass of water is independent of temperature, osmolality is the preferred term for biological systems. Tonicity The NaCl concentration in most cells is 0.85%. This corresponds to 300mOsm of nonpenetrating solutes. 0.85% 0.85% 0.85% 0.85% 10 % 0.02% Isotonic Hypertonic =300mOsm Hypotonic = >300mOsm = <300mOsm Effect of tonicity of solutions on red blood cells hypotonic isotonic hypertonic Filtration • Movement of a fluid and solutes down the pressure gradient (from high pressure to low pressure) across a membrane or filter • What is filtered depends on the amount of pressure and the size of the pores in the filter – In kidney: • Normally, protein is not filtered into the urine because of size and charge restrictions • Glomerulonephritis: Inflammation enlarges the size of “pores” and allows protein to move out into the filtrate Filtration Filtrate forms as fluid and other Fliltrate forms as substance substances are forced through moves through filter capillary fenestrations. Force: gravity (atm pressure) Force: blood pressure Restriction: size of filter pores Restriction: size of the fenestrations Carrier mediated transport • Protein carriers transport molecules too large or polar across the plasma membrane • 4 characteristics of ligand/protein binding – chemical specificity – competition –saturation –affinity Chemical specificity • Only molecules with the requisite chemical structure are transported • Specificity is not absolute A Protein Protein carrier carrier Protein B carrier Competition • Structurally related molecules may compete for transport • Competition decreases transport rate A B Protein C carrier Saturation • Transport system is saturated when all of the binding sites are occupied • At that point, the rate of transport can not increase. Affinity • The attraction of the carrier protein for the transport substrate • Transport can be inhibited by compounds or ions which alter the shape of the binding site. +++ + --+ ---- - ----- Carrier mediated transport 1) Solute binds to transporter 2) Protein changes shape, protein (ligand/protein binding moving solute to opposite side rules) of the membrane. Facilitated diffusion • Movement of substances with the concentration gradient •Example – Glucose across plasma membrane – Aquaporins transporting water Water movement and aquaporins • Two methods for diffusion of water in and out of cells – Diffusion across lipid bilayer – Aquaporins • Usually transport solute-free water • About 10 different types of aquaporins • One will transport about 3 billion H2O molecules per second Active transport • Movement against the concentration gradient • Requires the expenditure of energy • Often called pumps •two types – primary –secondary Active transport = against the concentration gradient Primary active transport • Steps – molecule or ion binds at recognition site – carrier protein is phosphorylated – protein changes shape – transported molecule flipped to other side of membrane – phosphate removed enzymatically, releasing transported molecule Primary active transport 12 PO4 ATP ADP 3 4 PO4 PO4 Major active transport pumps •Na+K+ATPase pump •Ca++ATPase pump •H+ATPase pump •H+K+ATPase pump Na+K+ATPase pumps • 3 Na+ out & 2 K+ in •Functions: – Sets stage for secondary active transport – gradient for electrical impulses in nerve/muscles ATP ADP = Na+ ATPase = K+ Na+K+ATPase pump Secondary active transport • Energy provided by Na+ gradient – not directly by ATP •co-transport – “hitchhiking” – Uses symport •counter-transport ATP Na+ Na+ Ca++ – “revolving door” K+ – Uses anaport ADP Symport Summary of passive and active transport mechanisms Against concentration gradient With the concentration gradient Exocytosis and Endocytosis • Circumvents need to pass through plasma membrane – Allows membrane impermeable molecules to pass – Exocytosis: Adds to membrane • Also energy requiring transport mechanisms Epithelial transport • Paracellular pathway • Transcellular pathway – Luminal and basolateral membranes do not have same permeability or transport characteristics Alternative functions of endocytosis: 1. Transcellular transport 2. Endosomal processing 3. Recycling the membrane 4. Destroying engulfed materials .
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