L8 Cellular Transport PDF

Transport across Cell Membranes Dr Pallav Sengupta February 20, Physiology 2024 Assistant Professor, College of Medicine www.gmu.ac.ae COLLEGE OF MEDICINE Learning Objectives On completion of this unit, the student will be able to: Define the terms homeostasis, internal environment, and body fluid....

Transport across Cell Membranes Dr Pallav Sengupta February 20, Physiology 2024 Assistant Professor, College of Medicine www.gmu.ac.ae COLLEGE OF MEDICINE Learning Objectives On completion of this unit, the student will be able to: Define the terms homeostasis, internal environment, and body fluid. Explain the process of diffusion and list the factors regulating it. Define the term osmosis. Describe carrier mediated transport. Describe secondary active transport with examples; and Explain exocytosis and endocytosis with examples. Clinical Case Report: Hyperkalemia Patient Presentation: You are rotating through the hemodialysis unit as part of a nephrology elective and reviewing the orders for YD, a 62year-old woman with type 2 diabetes mellitus. You are particularly interested in how her hyperkalemia is managed. The dialysis nurse hands you the pre-dialysis labs; YD’s potassium is 5.9 mEq/L (normal 3.5-5.0 mEq/L). Diagnostic Approach: You review the potassium concentration of her dialysate, the fluid used on the other side of the dialysis membrane. It is 2.0 mEq/L. At the end of her 4hour dialysis treatment, you note that YD’s potassium is 3.8 mEq/L. What is one process involved in the correction of YD’s hyperkalemia? Homeostasis The environment that surrounds the cells of a multicellular organism (their external environment) is the internal environment of the organism. Relative constancy of the conditions within the internal environment is called homeostasis (homeo- “un-changing” or “the same,” and -stasis “standing”). Homeostasis Factors homeostatically regulated include: Concentration of nutrient molecules Concentration of water, salt, and other electrolytes Concentration of waste products Concentration of O2 = 100mm Hg and CO2 = 40 mmHg pH = 7.4 Blood volume 4-6 L, osmolarity 300 mOsm and blood pressure 120/80 mm of Hg Temperature = 37℃ Body Fluid Movement Fluid compartment are separated by membranes that are freely permeable to water. Movement of fluids due to hydrostatic pressure and osmotic pressure. Membrane transport and Homeostasis To perform its normal functions, the cell uses membrane channels to establish specifically desired ICF concentrations of nearly every ion of biological importance. The differences in ionic concentration between the ICF and ECF sum to create a difference in electrical potential across the cell membrane that strongly influences the passage of ions across it. a. Simple diffusion Non-Carrier mediated transport. Involves net transport down an electrochemical gradient (from higher to lower conc). Does not need cellular metabolism energy. However, it’s powered by thermal energy of the diffusing molecules. Net diffusion stops when the conc is equal on both sides of the membrane. Simple Diffusion Requires NO energy Molecules move from area of HIGH to LOW concentration Diffusion of Liquids a. Simple diffusion (continued) Cell membrane is permeable to: Non-polar molecules (02). Lipid soluble molecules (steroids). Small polar covalent bonds (C02). H20 (small size, lack charge). Cell membrane impermeable to: Large polar molecules (glucose). Charged inorganic ions (Na+). Rate of Diffusion Speed at which diffusion occurs depends on: Magnitude of conc. gradient across the two sides of the membrane Higher gradient drives the force of diffusion. Permeability of the membrane to the diffusing substances Depending on size & shape of the molecules. Temperature of the solution Higher temperature, faster diffusion rate. Surface area of the membrane Microvilli increase surface area. Osmosis Diffusion of water across a membrane Moves from HIGH water potential (low solute) to LOW water potential (high solute) Diffusion of H2O Across A Membrane High H2O potential Low solute concentration Low H2O potential High solute concentration Aquaporins Water Channels Protein pores used during OSMOSIS Cells in Solutions WATER MOLECULES Isotonic Solution Hypotonic Solution NO NET MOVEMENT OF H2O (equal amounts entering & leaving) CYTOLYSIS Hypertonic Solution PLASMOLYSIS Facilitated diffusion Doesn’t require energy Uses transport proteins to move high to low concentration Examples: Glucose or amino acids moving from blood into a cell. Facilitated Diffusion Molecules will randomly move through the pores in Channel Proteins. Facilitated Diffusion Some Carrier proteins do not extend through the membrane. They bond and drag molecules through the lipid bilayer and release them on the opposite side. Carrier Proteins Other carrier proteins change shape to move materials across the cell membrane Active Transport Requires energy or ATP Moves materials from LOW to HIGH concentration AGAINST concentration gradient I. Primary Active Transport Energy is supplied directly from hydrolysis of ATP for the functions of the protein carriers. Molecule or ion binds to “recognition site” on one side of carrier protein. Binding stimulates phosphorylation (breakdown of ATP) of carrier protein. Carrier protein undergoes conformational change. Hinge-like motion releases transported molecules to opposite side of membrane. Some of these carriers transport only one molecule or ion for another. Sodium-Potassium Pump 3 Na+ pumped in for every 2 K+ pumped out creates a membrane potential Primary active transport (continued) Examples: a. Sodium-Potassium pump (Na+/K+ pump). b. Primary active transport of calcium (Ca2+ ATPase). c. Primary active transport of hydrogen ions (H+/K+ ATPase) Secondary Active Transport (continued) If the other molecule or ion is moved in the same direction as Na+ (into the cell), the coupled transport is called either: ‘cotransport’ or ‘symport’. If the other molecule or ion is moved in the opposite direction as Na+ (out of the cell), the process is called either: ‘countertransport’ or ‘antiport’. Secondary Active Transport a. Co-transport (Symport) All solutes move in the same direction “to the inside of the cell” e.g. - Na+– glucose Co transport - Na+– amino acid Co transport In the intestinal tract, & kidney’s brush borders. Moving the “Big Stuff” Exocytosis- moving things out. Molecules are moved out of the cell by vesicles that fuse with the plasma membrane. This is how many hormones are secreted and how nerve cells communicate with one another. Exocytosis Exocytic vesicle immediately after fusion with plasma membrane. Moving the “Big Stuff” Large molecules move materials into the cell by one of three forms of endocytosis. Pinocytosis Most common form of endocytosis. Takes in dissolved molecules as a vesicle. Pinocytosis Cell forms an invagination Materials dissolve in water to be brought into cell Called “Cell Drinking” Receptor-Mediated Endocytosis Some integral proteins have receptors on their surface to recognize & take in hormones, cholesterol, etc. Endocytosis – Phagocytosis Used to engulf large particles such as food, bacteria, etc. into vesicles Called “Cell Eating” Clinical Case Report: Hyperkalemia Patient Presentation: You are rotating through the hemodialysis unit as part of a nephrology elective and reviewing the orders for YD, a 62year-old woman with type 2 diabetes mellitus. You are particularly interested in how her hyperkalemia is managed. The dialysis nurse hands you the pre-dialysis labs; YD’s potassium is 5.9 mEq/L (normal 3.5-5.0 mEq/L). Diagnostic Approach: You review the potassium concentration of her dialysate, the fluid used on the other side of the dialysis membrane. It is 2.0 mEq/L. At the end of her 4hour dialysis treatment, you note that YD’s potassium is 3.8 mEq/L. What is one process involved in the correction of YD’s hyperkalemia? Findings on Case Report Thinking back to YD, what is one process involved in the correction of her hyperkalemia? Discussion You postulate that the semipermeable membrane used in the hemodialysis filter allows for passive diffusion. Potassium shifted from the blood into the dialysate via a concentration gradient, going from a higher-concentration compartment to a lower-concentration compartment.