Cell Membrane Transport PDF

In The Name of God Cell Physiology Prepared by: Dr. Abdollahzade –Fard A, Ph.D. Assistant professor of physiology Cell Membrane Transport Passive Transport - requires no energy input Diffusion Simple Diffusion Facilitated Diffusion Osm...

In The Name of God Cell Physiology Prepared by: Dr. Abdollahzade –Fard A, Ph.D. Assistant professor of physiology Cell Membrane Transport Passive Transport - requires no energy input Diffusion Simple Diffusion Facilitated Diffusion Osmosis Filtration Active Transport - metabolic energy ATP required Primary Active Transport Secondary Active Transport Vesicular Transport Exocytosis Endocytosis Passive Membrane Transport: Diffusion Diffusion is the tendency of molecules or ions to scatter evenly throughout their environment. Molecules are in constant motion (kinetic energy) and move around in a random fashion, colliding with other molecules and/or walls of the container. Molecules move from areas where they are in higher concentration to areas where their concentration is lower Molecules diffuse down their Concentration Gradient Passive Membrane Transport: Diffusion Diffusion is the tendency of molecules or ions to scatter evenly throughout their environment. Passive Membrane Transport: Diffusion Molecules move from areas where they are in higher concentration to areas where their concentration is lower Molecules diffuse down their Concentration Gradient Passive Membrane Transport: Diffusion Molecules diffuse down their Concentration Gradient Gradient – rate of change of some variable (temperature, pressure, density, concentration) as a function of distance A change in concentration from one place to another No Concentration Gradient High Low Low High Fick‟s law J= -DA(∆C/∆X) J: net rate of diffusion in moles or grams per unit time D: diffusion coefficient of the diffusing solute in the membrane A: area of the membrane ∆C: concentration difference across the membrane ∆X: thickness of the membrane Cell Membrane Properties – Semi-Permeable The plasma membrane is a selectively permeable barrier. It only allows “selected” substances to pass through. Passive Membrane Transport: Diffusion The driving force for diffusion is the kinetic energy of the particles The speed or rate of diffusion is influenced by: Molecular size (the smaller, the faster) Temperature (the warmer, the faster) In a closed system, diffusion eventually results in a uniform distribution of particles and the system reaches equilibrium with no net movement Passive Membrane Transport: Simple Diffusion – Simple diffusion – some nonpolar and lipid soluble substances can diffuse directly through the lipid bilayer Ex. Oxygen, carbon dioxide, fat-soluble vitamins Passive Membrane Transport: Facilitated Diffusion – Facilitated Diffusion – some molecules Combine with protein carriers Move through transmembrane protein channels Passive Membrane Transport: Facilitated Diffusion – Facilitated Diffusion – some molecules Combine with protein carriers Move through transmembrane protein channels  Osmolarity Definition: The total solute concentration of a solution is known as its osmolarity 1 osmol is equal to 1 mol of solute particle 1 Osm = 1 osmol per liter example: 1M glucose  1 Osm 1M NaCl 2 Osm refer to the concentration of the solute particles, also determine the water concentration, higher the osmolarity, lower the water concentration Passive Membrane Transport: Osmosis Passive Membrane Transport: Osmosis Osmosis is the net movement (net diffusion) of water across a semipermeable membrane. It is driven by a difference in solute concentrations on the two sides of the membrane. Occurs when the concentration of solvent is different on opposite sides of a membrane (when the concentration of water differs on the two sides of the membrane). Semipermeable Selectively Permeable Differentially Permeable Osmolarity- total concentration of solute particles in a solution Passive Membrane Transport: Osmosis Different concentrations of solute molecules mean that the concentrations of water molecules are different. Low solute concentration - High water concentration High solute concentration - Low water concentration If one side of a membrane has a higher water concentration, chances are that more water molecules will contact that side of the membrane in a given time interval. More contacts mean a greater chance for diffusion and more molecules passing through the membrane. This leads to the net diffusion of water from the side with a higher concentration of water to the side with a lower concentration of water. Effect of Membrane Permeability on Diffusion and Osmosis – Membrane Permeable to Solute and Water Effect of Membrane Permeability on Diffusion and Osmosis – Membrane Permeable to Water Only Osmotic Pressure Osmotic pressure is the pressure that must be applied to a solvent to stop osmosis Pressure Tonicity (tono = Tension) The ability of a solution to change the shape (size) or „tone‟ of a cell by altering the internal water volume Many molecules, especially intracellular proteins and selected ions cannot diffuse through the cell membrane. A change in their concentration changes the water concentration and can result in the cell having a net loss or gain of water. Tonicity Isotonic – solutions with the same solute concentration as the cytosol (.9% saline or 5% glucose) Hypertonic – solutions having greater solute concentration than the cytosol Hypotonic – solutions having lesser solute concentration than the cytosol Hypotonic – Isotonic - Hypertonic Cell Membrane Transport Passive Transport - requires no energy input Diffusion Simple Diffusion Facilitated Diffusion Osmosis Filtration Active Transport - metabolic energy (ATP) required Primary Active Transport Secondary Active Transport Vesicular Transport Exocytosis Endocytosis Passive Membrane Transport: Filtration The passage of water and solutes through a membrane due to hydrostatic pressure Pressure gradient pushes solute-containing fluid from a higher-pressure area to a lower-pressure area Hydrostatic pressure exerted by the blood forces fluid out of the capillaries. Filtration also occurs in the Kidney (glomerulus). Cell Membrane Transport Passive Transport - requires no energy input Diffusion Simple Diffusion Facilitated Diffusion Osmosis Filtration Active Transport - metabolic energy (ATP) required Primary Active Transport Secondary Active Transport Vesicular Transport Exocytosis Endocytosis Active Transport Active Transport - metabolic energy (ATP) required Primary Active Transport Secondary Active Transport Utilize carrier proteins that can bind specifically and reversibly with the transported atoms/molecules Primary Active Transport – energy comes directly from ATP hydrolysis Secondary Active Transport – energy comes from the ionic gradients created by 1° Active Transport Symporter vs. Antiporter  Active transport solute moves against its concentration gradient primary active transport: ATP directly consumed (e.g., Na+ K+ATPase) secondary active transport: energy of ion gradient (usually Na+) used to move second solute (e.g., nutrient absorption in gut)  Two types of active transporters Primary active transporters use ATP as energy transporter is an ATPase e.g. Na-K ATPase pump, Ca 2+ -ATPase, H+-ATPase, H+/K+ATPase Secondary active transporters use electrochemical gradient Other major primary active-transporters Ca 2+ -ATPase, keep low[Ca2+ ]i Pump direction: plasma membrane: Cytosol extracellular organelle membrane: cytosol organelle H+-ATPase, move H+ out of cell to maintain cellular pH H+/K+-ATPase, one H+ out of and one K+ into the cell in the plasma membrane of acid secreting cells in the stomach & kidney Sodium-Potassium Pump (Na+- K+ ATPase) Types of Active Transport Secondary active transport – indirect use of an exchange pump (such as the Na+-K+ pump) to drive the transport of other solutes Secondary active transport Co- transport- Na-glucose transporter Counter transport- Na- Ca exchanger;3Na-Ca Structure of the GLUT family of glucose transporter  SGLT Na+/glucose co transporter SGLT1: kidney, intestinal. SGLT2: kidney.  SGLT SGLT1 is in the later part of the proximal tubule a high-affinity/low-capacity cotransporter, that responsible for apical glucose uptake (2:1). SGLT1 carry glucose and galactose and cannot carry fructose. SGLT2 is in the early part of the proximal tubule a high-capacity/low-affinity cotransporter that mediates apical glucose uptake (1:1).  Epithelial cells luminal membrane: one surface of an epithelial cellfaces a hollow or fluid filled chamber and the plasmamembrane on this side is referred to basaolateral membrane: the opposite side of luminal membran 2 Pathways for Epithelial Transport diffusion(paracellular pathway) --between adjacent cells --require tight junction --small area available for diffusion transcellular pathway --move into the cell, through cytosol, exit across the whole cell --luminal and basolateral membranes contain different transporters & ion channels Cell Membrane Transport Passive Transport - requires no energy input Diffusion Simple Diffusion Facilitated Diffusion Osmosis Filtration Active Transport - metabolic energy (ATP) required Primary Active Transport Secondary Active Transport Vesicular Transport Exocytosis Endocytosis Vesicular Transport Transport of large particles, macromolecules, and fluid across plasma and intracellular membranes Exocytosis – out of the cell – moves substance from the cell interior to the extracellular space Endocytosis – into the cell – moves substance from the outside into the intracellular space Phagocytosis – pseudopods engulf solids and bring them into the cell Pinocytosis – fluid-phase endocytosis – cell membrane invaginates (infolds) and brings extracellular fluid and solutes into the cell  Endocytosis and Exocytosis endocytosis: plasma membrane fold into the cell, forming small pockets that pinch off to produce intracellular, membrane-bound vesicles that enclosed a small volume of extracellular fluid exocytosis: membrane bound vesicles in the cytoplasm fuse with the plasma membrane and release their contents to the outside of the cell Exocytosis Hormone secretion, neurotransmitter release, mucus secretion, ejection (excretion) of wastes  Three types of endocytosis fluid endocytosis (pinocytosis, cell drinking) non-specific, water, ion, nutrients, small molecules phagocytosis (cell eating), eg. Immune system bacteria, large molecules, cell debris internalized phagosomes migrate to and fuse with lysosomes  destroyed  Three types of endocytosis receptor mediated endocytosis specific, molecules bind to membrane bound receptors, leads to a concentrated specific ligand in the endocytotic vesicles e.g. cholesterol bind lipoprotein then bind lipoprotein _receptorendocytosis cholesterol delivered into the cell_ Endocytosis - Phagocytosis bacteria, cell debris X Phagocytosis – pseudopods extend, engulf solids, and bring them into the cell (phagosome may fuse with a lysosome → phagolysosome) Endocytosis - Pinocytosis Pinocytosis – fluid-phase endocytosis – cell membrane invaginates (infolds) and brings extracellular fluid and solutes into the cell Receptor-mediated Endocytosis Receptor-mediated transport – uses Clathrin- coated pits (and also Caveolae) as part of the mechanism for uptake of specific substances (enzymes, Fe, hormones, some viruses) Other Terminology Transcytosis - vesicular transport from one side of the cell to other side Vesicular Trafficking – refers to intracellular traffic – vesicles „pinch‟ off from organelles and travel to other organelles to deliver their contents Any questions?