Physio T1 Membrane Transport 1 (passive) PDF

Physiology PPP-112 Module Faculty of medicine Tutorial 1 Membrane Transport (1) Passive Transport Staff Members of Medical Physiology  Objectives After studying this section, you should be able to:  En...

Physiology PPP-112 Module Faculty of medicine Tutorial 1 Membrane Transport (1) Passive Transport Staff Members of Medical Physiology  Objectives After studying this section, you should be able to:  Enlist types of membrane transport.  Describe with the help of an illustration: passive transport, diffusion, osmosis and filtration.  Explain factors affecting the rate of diffusion.  Distinguish between simple and facilitated diffusion.  Discuss special types of passive transport. There are two main types of transport through cell membrane: 1- Passive transport (Diffusion):  Simple & facilitated. 2- Active transport:  Primary & secondary & Endocytosis & Exocytosis.  It is transport of a substance across a semipermeable Definition membrane down its electrochemical gradient.  Criteria of Passive  Occurs down concentration (chemical), electrical or pressure gradient. Transport:  Does not need energy.  Directly proportional with Inversely proportional with 1) The gradient for diffusion whether 1) Molecular weight chemical, electrical or pressure. (molecular size) of the  Factors 2) Temperature: increases the random substance. affecting motion of the molecules. rate of diffusion: 3) Surface area of the membrane 2) Thickness of the available for diffusion. membrane. 4) Permeability of the membrane. 5) Lipid solubility of the substance or number of protein channels according to the type of diffusion. Types  Simple Diffusion  Facilitated Diffusion Definition Diffusion without a need for carrier. It is concerned A- lipid soluble substances. with: B- Small water-soluble substances, e.g. ions. lipid bilayer: This is for lipid soluble substances, e.g. O2, CO2, fatty acids, glycerol Protein channels (permeation): This is for small water-soluble substance e.g. It occurs ions (Na+, K+, Cl- etc). The types of channels in the cell membrane include: through:  Leak channels: has no gate (continuously opened), e.g. Na+ - K+ leak channels.  Gated channels: have gates (open and close) and are of 2 types:  Voltage – gated channels: their gates open as a result of change in the electric potential across the cell membrane, e.g., voltage gated Na+ channel, K+ channel and Ca2+ channel.  Ligand – gated channels: their gates open as a result of binding (ligation) of a chemical substance with receptor on the channel protein, e.g., acetylcholine – gated ion channel at motor end plate (neuromuscular junction). Simple Diffusion Definition diffusion that needs carrier protein. It is concerned with: lipid insoluble large molecules, e.g. glucose and most amino acids.  The carrier protein has a channel large enough to transport the specific molecule. Mechanism  Binding of the specific molecule, e.g., glucose with a receptor on the carrier protein (a type of integral proteins) → widening of the inner part of the channel of the carrier protein → facilitation of transport of the molecule across the membrane.  Simple diffusion Facilitated diffusion 1. For lipid soluble and small lipid 1- For lipid insoluble large molecule. insoluble 2. Does not need carrier. 2- Needs carrier. 3. No structural specificity because there 3. High structural specificity: - each carrier is specific is no carrier. for one or very few substances. 4. No competitive inhibition because 4. Competitive inhibition: similar molecules there is no carrier compete with each other for the same carrier and ↓ the transport of each other. 5. No saturation, i.e. it ↑ with increasing 5. Saturation: has a maximum limit (v. max) or the concentration gradient without transport maximum (Tm) i.e. the rate of diffusion limit (no carrier). is directly proportional to the concentration gradient till a certain limit Tm. 6. Less sensitive to temperature changes 6. More sensitive to temperature changes (3 times (no carrier). that of simple diffusion) because binding of the substance with the carrier is through an enzyme.  It is the diffusion of water (the solvent) from area of less concentrated solution (more solvent) to the area of more concentrated solution (less solvent).  Osmotic pressure is the pressure needed to stop osmosis. Osmotic pressure depends on number rather than the size of particles in a solution.  Diffusion of water (osmosis) takes place through both lipid bilayer and protein channels (specific protein channels called aquaporin). This is because water molecules have very small size and high kinetic energy; so that the molecules penetrate the lipid bilayer like bullets before hydrophobic characters of lipid layer stop them.  Means diffusion of fluid through a membrane that is caused by difference in hydrostatic pressure. Examples: o Filtration at arterial end of systemic capillaries to form interstitial fluid. o Filtration through glomerular capillaries in the kidney to form glomerular filtrate.  REFERENCES Oxford Handbook of Medical Sciences ,third edition 2021, Robert Wilkins, Simon Cross, Ian Megson, David Meredith, OXFORD university press. (Handbook) Lippincott Illustrated Reviews: Integrated Systems. Sandra K. Leeper-Woodford, MS, Macon, Georgia Linda R. Adkison, MS Copyright © 2015 Wolters Kluwer. (Student book) Guyton and Hall Textbook of Medical Physiology 14th edition John E. Hall, PhD and Michael E. Hall, MD, MS. Copyright © 2021 by Elsevier, Inc. All rights reserved http://evolve.elsevier.com/Hall/physiology/ (Text book)