General Physiology: Transport Across Cell Membranes - 1 PDF

GENERAL PHYSIOLOGY 4. TRANSPORT ACROSS CELL MEMBRANES-1  LEARNING OBJECTIVES 1. Understanding of body water and body fluids. 2. Definition of mole, equivalent and osmoles. 3. Types of cell transport 4. Simple diffusion and facilitated diffusion with examples. 5. Osmosis, os...

GENERAL PHYSIOLOGY 4. TRANSPORT ACROSS CELL MEMBRANES-1  LEARNING OBJECTIVES 1. Understanding of body water and body fluids. 2. Definition of mole, equivalent and osmoles. 3. Types of cell transport 4. Simple diffusion and facilitated diffusion with examples. 5. Osmosis, osmotic pressure,  BODY WATER & BODY FLUIDS - In an average young adult male: (i) 7% is mineral (ii) 15% is fat (iii) 18% is protein & related substances & (iv) 60% is water, called total body water (TBW).  - TBW is about 10% lower in young females due to relatively greater amount of adipose tissue (subcutaneous fat). - In infants TBW is 65- 75% of BW.  DISTRIBUTION OF TBW IN A NORMAL ADULT 70 KG PERSON: COMPARTME VOLUM BODY BODY NT E (L) WEIGHT WATER (%) (%) TBW 42 60 100 ECF 14 20 33 (1/3rd TBW) ICF 28 40 67 (2/3rd TBW)  DISTRIBUTION OF ECF IN A NORMAL ADULT 70 KG PERSON: COMPARTMEN VOLU BODY BODY T ME (L) WEIGHT WATER (%) (%) Plasma (25% of 3 4-5 8 ECF) Interstitial plus 11 15 25 trans cellular fluid (75% of ECF)  TRANSCELLULAR FLUID: - It represents fluid in the lumen of the structures lined by epithelium. - It includes: digestive secretions, sweat, CSF, pleural, peritoneal, synovial, intraocular & pericardial fluids, bile & luminal fluids of the gut, thyroid & cochlea.  - Trans cellular fluid volume is relatively small, about 1L = 15 ml/kg of body weight (1.5% of the body weight).  IONIC COMPOSITION OF BODY FLUIDS - Ions constitute approximately 95% of the solutes in the body fluids. - Na⁺, Ca²⁺, Cl⁻ & HCO₃⁻ are largely extracellular; whereas, K⁺, Mg²⁺, PO₄³⁻ & proteins are  IONIC COMPOSITION OF BODY FLUIDS - Essentially all of the body K⁺ is in the exchangeable pool, whereas only 65% - 70% of the body Na⁺ is exchangeable.  IONIC COMPOSITION OF BODY FLUIDS - Only the exchangeable solutes are osmotically active. - Almost all of the body Ca²⁺ (in bone) & most of the body Mg²⁺ (in bone & cells) are non  UNITS FOR MEASURING CONCENTRATION OF SOLUTES MOLES: - A mole is the gram molecular weight of a substance (the molecular weight of the substance in grams).  - Each mole (mol) consists of approximately 6 x 10²³ molecules. - Thus, 1 mole of KCl = 39 + 35.5 gm = 74.5 gm. - The millimole (mmol) is 1/1000 of a mole, therefore, 1 mmol of KCl = 74.5 mg.  UNITS FOR MEASURING CONCENTRATION OF SOLUTES EQUIVALENTS: - The equivalent is the standard unit for expressing the solutes in the body which are in the form of charged particles. - One equivalent (Eq) is 1 mole of an ionized substance divided by its valency.  - One mole of KCl dissociates into 1 Eq of K⁺ & 1 Eq of Cl⁻. - One Eq of K⁺ = 39 gm/1 = 39 gm but 1 Eq of Ca²⁺ = 40 gm/2 = 20 gm. - The miliequivalent (mEq) is 1/1000 of 1 Eq.  UNITS FOR MEASURING CONCENTRATION OF SOLUTES OSMOLES: - The amount of concentrations of osmotically active particles are usually expressed in osmoles (osm).  - The milliosmole (mosm) is 1/1000 of 1 osm. As 1 osmole = Gram molecular weight of a substance Number of freely movable particles, each molecule liberates in solution.  - Therefore, if solute is a non ionizing compound (e.g. glucose), then one osmole is equal to 1 mole of solute particle. - Thus, 1 molar solution of glucose has a concentration of 1 osm (1  - If the solute ionizes & forms an ideal solution, each ion is an osmotically active particles thus in a 1 molar solution of NaCl, NaCl would dissociate  - So that each mole in solution would supply 2 osmoles of solute per liter of solution. - Similarly, one mole of CaCl₂ would dissociate into Ca²⁺, Cl⁻ & Cl⁻ supplying 3 osmoles.  - The number of osmoles per liter of solution is called osmolarity. - Whereas, the number of osmoles per kilogram of solvent is osmolality. - Therefore, osmolarity is affected by the volume of the various solutes in the solution & the temperature while the  CELL TRANSPORT  PASSIVE TRANSPORT PROCESSES - Here substances move across the cell membrane without any energy expenditure by the cell. - It includes (a) Diffusion  DIFFUSION - Diffusion is a passive process (no external source of energy is required) by which molecules move from areas of high concentration to areas of low concentration (down their ‘chemical gradient’).  DIFFUSION - Cations (positively charged molecules)move to negatively charged areas whereas anions move to the positively charged areas (down their ‘electrical gradient’). - It is of 2 types: (1) Simple diffusion (2) Facilitated diffusion. SIMPLE DIFFUSION CHARACTERISTICS FEATURES: - It occurs because the heat content of the solution keeps the solvent & the solute particles of the solution in constant motion. - It is the only form of SIMPLE DIFFUSION - Net movement stops when the concentration of the molecules is equal everywhere within the solution (diffusional equilibrium). SIMPLE DIFFUSION - Although random movements of the molecules continue after diffusional equilibrium is achieved, the concentration of the molecules throughout the solution remains the same.  FACTORS AFFECTING DIFFUSION: - The rate at which a material diffuses through a membrane (flux) is given by Fick’s law of diffusion. - Net rate of diffusion (flux) = Diffusion coefficient (D) * Area of the membrane (A) * (Cin – Cout) Thickness of membrane (T) (or diffusion distance) - Cin & Cout = Concentration of the material inside & outside of the membrane. (1) Distance: - The greater the distance, the longer the time required. - In human body, diffusion distances are usually small as diffusion of substances occurs across the cell membranes of uniform thickness (10 nm). (2) Size of the gradient: - The larger the concentration gradient, faster the diffusion proceeds. (3) Temperature: - The higher the temperature, faster the diffusion rate. - At normal body temperature of 37⁰C diffusion is optimal (maximum). (4) Molecular size: - The permeability of cell membrane to a substance falls rapidly with increase in MW in the range b/w 10,000 to 60,000. - This is why glucose diffuses faster than large proteins. (5) Lipid solubility: (i) Lipid soluble molecules : - O₂, CO₂, N₂ & alcohols diffuses rapidly with ease through the lipid layer of the membrane. (ii) Water soluble molecules: - Ions, glucose, urea can cross the cell membrane slowly as they diffuse through the aqueous channels formed by trans membrane proteins.  FACILITATED DIFFUSION - It is a carrier mediated process that enables molecules that are too large to flow through membranes channels by simple diffusion. - It is more rapid than  FACILITATED DIFFUSION - For example, (1) Glucose transport by the glucose transporter (GLUT) across intestinal epithelium. (2) The transport of glucose into RBCs, muscles & adipose tissues in the presence of  FACILITATED DIFFUSION - Its rate of diffusion increases with increase in concentration gradient to reach a plateau when all the binding sites on the carrier proteins are filled. - This is called  OSMOSIS - Osmosis is the passive flow of the solvent (water across a selectively permeable membrane) (membrane permeable to solvent but not to the solute), into a region where there is a higher concentration of a solute to which the  THE OSMOTIC PRESSURE - The amount of pressure exactly required to prevent solvent migration (osmosis) is called the osmotic pressure of the solution.  THE OSMOTIC PRESSURE - Osmotic pressure of a solution is related to the: (1) Number of particles (molecules or ions) dissolved in the solution rather than their size, type, MW or chemical composition.  THE OSMOTIC PRESSURE (2) Temperature & volume in the same way as the pressure (P) of a gas P = nRT V - Where n = Number of particles R = Gas constant T = Absolute temperature  THE OSMOTIC PRESSURE - If ‘T’ is kept constant then osmotic pressure is proportionate to the number of particles in a solution per unit volume of solution. OSMOLAL CONCENTRATION OF PLASMA: TONICITY - The osmolality of normal human plasma is 290 mosm/L. - The osmolality of a solution relative to plasma is called tonicity. OSMOLAL CONCENTRATION OF PLASMA: TONICITY - Solutions that have the same osmolality as plasma are said to be isotonic (e.g. 0.9% sodium chloride solution or 5% glucose solution). - Those with greater osmolality are hypertonic & those with lesser osmolality are - Clinically, a rough estimation of plasma osmolality can be made by using the formula: Osmolality (mosm/L) = 2[Na⁺] + 0.055[glucose] + 0.36[blood urea] (mEq/L) (mg/dl) (mg/dl) CLINICAL SIGNIFICANCE OF OSMOSIS - The total plasma osmolality is important in assessing: (i) Dehydration (ii) Overhydration (iii) Other fluid & electrolyte SPECIAL TYPES OF PASSIVE TRANSPORT 1. Bulk flow 2. Filtration 3. Osmosis.  BULK FLOW - Bulk flow is the diffusion of large quantity of substances from a region of high pressure to the region of low pressure. - Best example for bulk flow is the exchange of gases across the respiratory membrane in lungs.  FILTRATION - Movement of water and solutes from an area of high hydrostatic pressure to an area of low hydrostatic pressure is called filtration. THANK YOU