d. substances diffuse because of their inherent random molecular movement (ie, following the principle of Brownian motion) e. Diffusion across membranes occurs if the membrane is permeable to the f. The net rate of diffusion ) is proportional to the membrane area(A)and lute concentration difference(C1-C2) and the permeability(p)of the membrane g. Diffusion is measured using the formula]=PA(C1-C2) 2. Facilitated diffusion is the transport of a substrate by a carrier protein down a. Facilitated diffusion is required for substrates that are not permeable to the lipid bilayer and is faster than simple diffusion b. Facilitated diffusion is used cellular survival, including glucose and amino acids. 3. Osmosis is the movement of water across a semipermeable membrane due to a water concentration difference. Osmosis follows the same principles as diffu of any solute For example, if two solutions, A and B, are separated by a membrane im- permeable to solute but permeable to water and A contains a higher solute concentration than B, a driving force exists for water movement from B to A to equilibrate water concentration differences. Thus, water moves toward a solution with a higher osmolal b. Osmolality is a measure of the total concentration of discrete solute parti les in solution and is measured in osmoles per kilogram of water c. Because it is much more practical to measure the volume than the weigh of physiological solution, the concentration of solute particles is typicall expressed as osmolarity, which is defined as osmoles per liter: Osmolarity =gx C here number of particles in solution(Osm/mol) concentration(mol/L) d. Consider the following example: What is the osmolarity of a 0. 1 mol/L NaCl solution(for NaCl, g= 2)? Osmolarity =2 Osm/mol x 0. 1 mol/L=0.2 Osm/L or 200 mOsm/L e. Two solutions that have the same osmolarity are described as isosmotic. 4. An isotonic solution is one in which the volume of cells incubated in it does not change, implying that there is no movement of water in or out of the cell a. Under normal conditions. an isotonic solution is isosmotic with intracell lar fluid, which is isosmotic with plasma(290 mOsm/L) b. Not all isosmotic solutions are isotonic. A 290 mM(millimolar)solution of urea will be isosmotic(290 mOsm/L) but not isotonic because urea is meable to the cell membrane and will diffuse inside the cell. this cause increased concentration of urea inside the cell, which induces water influx and an increase in cell volumeChapter 1: Cell Physiology 3 N d. Substances diffuse because of their inherent random molecular movement (ie, following the principle of Brownian motion). e. Diffusion across membranes occurs if the membrane is permeable to the solute. f. The net rate of diffusion (J) is proportional to the membrane area (A) and solute concentration difference (C1−C2) and the permeability (P) of the membrane. g. Diffusion is measured using the formula J = PA (C1−C2). 2. Facilitated diffusion is the transport of a substrate by a carrier protein down its concentration gradient. a. Facilitated diffusion is required for substrates that are not permeable to the lipid bilayer and is faster than simple diffusion. b. Facilitated diffusion is used to transport a variety of substances required for cellular survival, including glucose and amino acids. 3. Osmosis is the movement of water across a semipermeable membrane due to a water concentration difference. Osmosis follows the same principles as diffu￾sion of any solute. a. For example, if two solutions, A and B, are separated by a membrane im￾permeable to solute but permeable to water and A contains a higher solute concentration than B, a driving force exists for water movement from B to A to equilibrate water concentration differences. Thus, water moves toward a solution with a higher osmolality. b. Osmolality is a measure of the total concentration of discrete solute parti￾cles in solution and is measured in osmoles per kilogram of water. c. Because it is much more practical to measure the volume than the weight of physiological solution, the concentration of solute particles is typically expressed as osmolarity, which is defined as osmoles per liter: where g = number of particles in solution (Osm/mol) C = concentration (mol/L) d. Consider the following example: What is the osmolarity of a 0.1 mol/L NaCl solution (for NaCl, g = 2)? e. Two solutions that have the same osmolarity are described as isosmotic. 4. An isotonic solution is one in which the volume of cells incubated in it does not change, implying that there is no movement of water in or out of the cell. a. Under normal conditions, an isotonic solution is isosmotic with intracellu￾lar fluid, which is isosmotic with plasma (290 mOsm/L). b. Not all isosmotic solutions are isotonic. A 290 mM (millimolar) solution of urea will be isosmotic (290 mOsm/L) but not isotonic because urea is per￾meable to the cell membrane and will diffuse inside the cell. This causes an increased concentration of urea inside the cell, which induces water influx and an increase in cell volume. Osmolarity = 2 Osm/ mol 0.1 mol/ L = 0.2 Osm/ L or 200 mOsm/ L × Osmolarity g = × C 5506ch01.qxd_ccII 2/17/03 2:08 PM Page 3