P.04-TRANSPORT-THROUGH-CELL-MEMBRANES.pdf

PHYSIOLOGY lecture PINES CITY COLLEGES P.01 P.04Trans Title TRANSPORT THROUGH CELL MEMBRANES Dra. Anna Goyenechea Cinio | AUGUST 10, 2024 OUTLINE...

PHYSIOLOGY lecture PINES CITY COLLEGES P.01 P.04Trans Title TRANSPORT THROUGH CELL MEMBRANES Dra. Anna Goyenechea Cinio | AUGUST 10, 2024 OUTLINE I. MEMBRANE TRANSPORT........................................ 1 II. DIFFUSION....................................................... 1 III. OSMOSIS......................................................... 3 IV. ION CHANNELS................................................ 4 V. DIFFUSION AND EQUILIBRIUM POTENTIALS................. 5 VI. ACTIVE TRANSPORT............................................ 5 A. PRIMARY ACTIVE TRASNPORT............................... 6 B. SODIUM POTASSIUM PUMP................................. 6 C. CA2+ - ATPASE (CALCIUM PUMP)......................... 7 D. D. H+K+-ATPASE (OR PROTON PUMP)..................... 7 VII. SECONDARY ACTIVE TRASNPORT........................... 7 Simple diffusion can occur through the cell membrane by TWO VIII. ACTIVE TRANSPORT THROUGH CELLULAR SHEETS........ 9 pathways IX. READING ASSIGNMENTS...................................... 9 a. Through the interstices of the lipid bilayer if the diffusing X. Checkpoint..................................................... 9 substance is lipid-soluble XI. References...................................................... 9 b. Through watery channels that penetrate all the way through some of the large transport protein I. MEMBRANE TRANSPORT Lipid-Soluble Substances The lipid solubility of a substance is an important factor for determining how rapidly it diffuses through the lipid bilayer Figure 1. Transport pathways through the cell membrane and the Ø oxygen, nitrogen, carbon dioxide and alcohols basic mechanism of transport Ø rate of diffusion of each of these substances through the Ø Membrane consists of lipid bilayer with large numbers of membrane is directly proportional to its lipid solubility protein molecules in the lipid II. DIFFUSION A. SIMPLE DIFFUSION Ø is the only form of transport that is not carrier-mediated Ø occurs down an electrochemical gradient (“downhill”) Ø does not require metabolic energy (passive) Ø occurs as a result of the random thermal motion of molecules Water Ø readily passes through channels in protein molecules that penetrate all the way through the membrane Ø “pores” called aquaporins selectively permit rapid passage of water through the membrane Ø highly specialized Figure 2. Two solutions A, and B, are separated by a membrane, which is permeable to the solute. Solute A initially contains higher solute concentration than does Solution B. The rate of diffusion is determined by: Ø the amount of substance available Ø the velocity of kinetic motion Ø the number and sizes of openings in the membrane ALVIZ, TOLBE, DISCARTIN, ALICOG, SADDOY Page 1 of 9  PHYSIOLOGY lecture PINES CITY COLLEGES P.01 P.04Trans Title TRANSPORT THROUGH CELL MEMBRANES Dra. Anna Goyenechea Cinio | AUGUST 10, 2024 Other Lipid-Insoluble Molecules Thickness of the membrane (ΔX) Ø can pass through the protein pore channels in the same Ø The thicker the cell membrane, the greater the distance way as water molecules if they are water-soluble and small the solute must diffuse and the lower the rate of diffusion enough Ø However, as they become larger, their penetration falls off rapidly B. NET DIFFUSION Surface Area (A) Ø Net diffusion of the solute from A to B will continue until the Ø The greater the surface area of the membrane available, solute concentrations of the two solutions become equal the higher the rate of diffusion Ø Net diffusion of the solute is called flux or flow (J) Ø lipid-soluble gases such as oxygen and carbon dioxide have Ø Net diffusion depends on the following variables: particularly high rates of diffusion across cell membranes o Size Of The Concentration Gradient Ø high rates can be attributed to the large surface are for o Partition Coefficient diffusion provided by the lipid component of the membrane o Diffusion Coefficient o Thickness Of The Membrane Permeability o Surface Area Available For Diffusion Ø includes the partition coefficient, the diffusion coefficient, and the membrane thickness Concentration Gradient (CA-CB) Ø Driving force for net diffusion Ø The larger the difference in solute concentration between Solution A and Solution B, the greater the driving force and the greater the net diffusion Ø If the concentrations in the two solutions are equal, there is no driving force and no net diffusion Partition Coefficient (K) Ø describes the solubility of a solute in oil relative to its solubility in water Ø the higher the relative solubility in oil, the higher the partition coefficient = the more easily the solute can dissolve in the cell membrane’s lipid bilayer Ø Non-polar solutes tend to be soluble in oil and have high Sample Problem values for partition coefficient Solution A and Solution B are separated by a membrane whose Ø can be measured by adding the solute to a mixture of olive permeability to urea is 2×10−5 cm/s and whose surface area is 1cm2 oil and water The concentration of urea in A is 10 mg/mL, and the concentration Ø K: concentration in the olive oil/ concentration in the water of urea in B is 1 mg/Ml. Diffusion Coefficient (D) What is the initial rate and direction of net diffusion of urea? Ø correlates inversely with the molecular radius of the solute Ø Net flux can be calculated by substituting the following and the viscosity of the medium values in the equation for net diffusion: Assume that 1mL of Ø small solutes in non-viscous solutions have the largest water = 1 cm3 diffusion coefficients and diffuse most readily Ø Thus J = PA (CA-CB) Ø large solutes in viscous solutions have the smallest diffusion coefficients and diffuse least readily Solution Ø depends on the size of the solute molecule and the Where: J = 2x10-5 cm/s(1cm2)x(10mg/ml-1mg/ml) viscosity of the medium J = 2x10-5 cm3/s x (10mg/cm3- 1mg/cm3) = 1.8 x 10-4 mg/s Ø defined by the Stokes-Einstein equation Ø The magnitude of net flux has been calculated as 1.8 ×10−4 mg/s Ø The direction of net flux can be determined intuitively because net flux will occur from the area of high where concentration (Solution A) to the area of low concentration (Solution B) Ø Net diffusion will continue until the urea concentrations of the two solutions become equal. Diffusion of Electrolytes Ø If there is a potential difference across the membrane, that potential difference will alter the net rate of diffusion of a charged solute. Ø The diffusion of K+ ions will be slowed if K+ is diffusing into an area of positive charge. ALVIZ, TOLBE, DISCARTIN, ALICOG, SADDOY Page 2 of 9  PHYSIOLOGY lecture PINES CITY COLLEGES P.01 P.04Trans Title TRANSPORT THROUGH CELL MEMBRANES Dra. Anna Goyenechea Cinio | AUGUST 10, 2024 Ø It will be accelerated if K+ is diffusing into an area of negative charge Ø When a charged solute diffuses down a concentration gradient, diffusion can itself generate a potential difference across a membrane called a diffusion potential C. FACILITATED DIFFUSION ØOccurs down an electrochemical gradient (“downhill”) ØIt does not require metabolic energy and therefore is passive Ø More rapid than simple diffusion Ø Carrier mediated and exhibits: o Saturation o Stereospecificity o Competition Ø Requires interaction of a carrier protein o Aids passage of molecules or ions through the membrane by binding chemically with them and shuttling them through the membrane in this form o At low solute concentrations, facilitated diffusion typically proceeds faster than simple diffusion because of the function of the carrier o At higher concentrations, the carriers will become A. OSMOLARITY saturated, and facilitated diffusion will level off Ø is the concentration of osmotically active particles in a solution Ø expressed as osmoles per liter of solution Ø can be calculated using the following equation: Osmolarity = g × C where: Osmolarity = concentration of particles (Osm/L) g = number of particles in solution (Osm/mol) C = concentration (mol/L) Ø If two solutions have the same calculated osmolarity, they are called isosmotic Ø Solution with a higher osmolarity is called hyperosmotic Ø Solution with a lower osmolarity is called hypoosmotic Example: Glucose transport in muscle and adipose cells Ø “downhill” Ø carrier mediated using GLUT 4 Ø inhibited by sugars such as galactose Ø in diabetes mellitus, glucose uptake by muscle and adipose cells is impaired because the carriers for facilitated diffusion of glucose require adequate insulin III. OSMOSIS Ø The flow of water across a semipermeable membrane from Osmolarity sample problem a solution with a low solute concentration to a solution with Solution A is 2 mmol/L urea high solute concentration Solution B is 1 mmol/L NaCl Ø Concentration differences of impairment solutes establish Assume that g NaCl = 1.85 osmotic pressure Are the two solutions isosmotic? Ø Osmotic pressure difference causes water to flow by Solution osmosis Calculate the osmolarities of both solutions to compare them. Ø Osmosis occurs because of a pressure difference, whereas Solution A contains urea, which does not dissociate in solution. diffusion occurs because of a concentration difference of Solution B contains NaCl, which dissociates partially in solution but water not completely (i.e., g < 2.0) Osmolarity A = 1 Osm/mol x 2 mmol/L = 2 mOsm/L Osmolarity B = 1.85 Osm/mol x 1 mmol/L = 1.85 mOsm/L Answer The two solutions do not have the same calculated osmolarity; therefore they are not isosmotic Ø Solution A has a higher osmolarity than Solution B and is hyperosmotic Ø Solution B is hypoosmotic ALVIZ, TOLBE, DISCARTIN, ALICOG, SADDOY Page 3 of 9  PHYSIOLOGY lecture PINES CITY COLLEGES P.01 P.04Trans Title TRANSPORT THROUGH CELL MEMBRANES Dra. Anna Goyenechea Cinio | AUGUST 10, 2024 B. OSMOLALITY Osmole Ø express the concentration of a solution in terms of numbers of particles Ø determines osmotic pressure Ø One osmole is 1 gram molecular weight of osmotically active solute Ø 1 osmole of solute dissolved in each kg = osmolality of 1 osmole/kg Ø 1/1000 osmole dissolved per kg = osmolality of 1 milliosmole/kg Reflection Coefficient (σ) Ø The normal osmolality of the extracellular and intracellular Ø a number between zero and one that describes the ease fluids is about 300 milliosmoles per kilogram of water with which a solute permeates a membrane Ø If the reflection coefficient is one, the solute is Serum Osmolality Formula impermeable therefore: o it is retained in the original solution o it creates an osmotic pressure o it causes water flow Ø Serum albumin (a large solute) has a reflection coefficient of nearly one Ø If the reflection coefficient is zero, the solute is completely permeable therefore: Osmolality o it will not exert any osmotic effect Ø Osmolality is similar to osmolarity, except that it is the o it will not cause water flow concentration of osmotically active particles expressed as Ø Urea (a small solute) usually has a reflection coefficient of osmoles (or milliosmoles) per kilogram of water close to zero and it is, therefore, an ineffective osmole Ø Because 1 kg of water is approximately equivalent to 1 L of water, osmolarity, and osmolality will have essentially the Calculating Effective Osmotic Pressure same numerical value Ø Effective osmotic pressure is the osmotic pressure (calculated by Van’t Hoff’s law) multiplied by the reflection Osmotic Pressure coefficient Ø Determined by the number of osmotic particles - Molar Ø If the reflection coefficient is one, the solute will exert Concentration maximal effective osmotic pressure. Ø each particle in a solution, regardless of its mass, experts, Ø If the reflection coefficient is zero, the solute will exert no on average, the same amount of pressure against the osmotic pressure membrane Ø The osmotic pressure of a solution can be calculated by IV. ION CHANNELS Van’t Hoff’s law o states that osmotic pressure depends on the Ø Integral proteins that span the membrane and, when concentration of osmotically active particles open, permit the passage of certain ions o the concentration of particles is converted to pressure according to the following equation: π = g × C × RT where: π = g × C × RT where: π = osmotic pressure (mm Hg or atm) g = number of particles in solution (osm/mol) C = concentration (mol/L) R = gas constant (0.082 L—atm/mol—K) T = absolute temperature (K) Ion Channels are Selective Ø The osmotic pressure increases when the solute Ø they permit the passage of some ions but not others concentration increases - The higher the osmotic pressure Ø Selectivity is based on the size of the channel and the of a solution, the greater the water flow into it distribution of charges that line it Ø Two solutions having the same effective osmotic pressure Ø a small channel lined with negatively charged groups will are isotonic because no water flows across a be selective for small cations and exclude large solutes semipermeable membrane separating them and anions Ø If two solutions separated by a semi-permeable Ø a small channel lined with positively charged groups will be membrane have different effective osmotic pressures: selective for small anions and exclude large solutes and o the solution with the higher effective osmotic pressure cations is hypertonic o the solution with the lower effective osmotic pressure Ion Channels may be Open or Close is hypotonic Ø When the channel is open, the ion(s) for which it is selective o Water flows from the hypotonic to the hypertonic can flow through solution Ø When the channel is closed, ions cannot flow through Ø Colloid osmotic pressure, or oncotic pressure, is the osmotic pressure created by proteins (e.g., plasma proteins) Ion Channels Ø The conductance of a channel depends on the probability that the channel is open ALVIZ, TOLBE, DISCARTIN, ALICOG, SADDOY Page 4 of 9  PHYSIOLOGY lecture PINES CITY COLLEGES P.01 P.04Trans Title TRANSPORT THROUGH CELL MEMBRANES Dra. Anna Goyenechea Cinio | AUGUST 10, 2024 o The higher the probability that a channel is open, the higher the conductance, or permeability o Opening and closing of channels are controlled by gates o Voltage-gated channels are opened or closed by changes in membrane potential o Ligand-gated channels are opened or closed by hormones, second messengers, or neurotransmitters VI. ACTIVE TRANSPORT A. PRIMARY ACTIVE TRANSPORT V. DIFFUSION AND EQUILIBRIUM POTENTIALS Ø Substances move against the concentration gradient, from Ø A diffusion potential is the potential difference generated an area of low concentration to an area of high across a membrane because of a concentration concentration difference of an ion Ø Occurs against an electrochemical gradient (“uphill”) Ø A diffusion potential can be generated only if the Ø Requires direct input of metabolic energy in the form ATP membrane is permeable to the ion Ø Energy is derived directly from the breakdown of adenosine Ø The size of the diffusion potential depends on the size of the triphosphate (ATP) or some other high-energy phosphate concentration gradient compound Ø The sign of the diffusion potential depends on whether the Ø Carrier mediated diffusing ion is positively or negatively charged Ø Exhibits stereospecificity, saturation, and competition Ø Diffusion potentials are created by the diffusion of very few because of the use of carrier protein ions and, therefore, do not result in changes in Carrier protein- capable of imparting energy to concentration of the diffusing ions move it against the electrochemical gradient Nernst Equation Ø helps explain and predict the behavior of charged species in solutions, especially in relation to concentration gradients and their effects on electrochemical processes * The equilibrium potential is the potential difference that would exactly balance (oppose) the tendency for diffusion down a concentration difference B. NA+ K+ - ATPASE (or NA+ - K+ Pump) In Cell *At electrochemical equilibrium, the chemical and electrical Membranes driving forces that act on an ion are equal and opposite, and no Ø Transports Na+ from intracellular to extracellular fluid and K+ further net diffusion of the ion from o extracellular to intracellular fluid Ø Maintains low intracellular Na+ and high intracellular K+ ALVIZ, TOLBE, DISCARTIN, ALICOG, SADDOY Page 5 of 9  PHYSIOLOGY lecture PINES CITY COLLEGES P.01 P.04Trans Title TRANSPORT THROUGH CELL MEMBRANES Dra. Anna Goyenechea Cinio | AUGUST 10, 2024 Ø Both Na+ and K+ are transported against their of energy. This liberated energy is believed to cause electrochemical gradients chemical a conformational change in the protein carrier o Note: This is ACTIVE TRANSPORT molecule extruding 3 sodium ions to the outside and 2 Ø Energy is provided from the terminal phosphate bond of ATP potassium ions to the inside Ø The usual stoichiometry is 3 Na+ /2 K+ 3. 3 Na+ outside, 2 K+ inside means 1+ (positive charge) is Ø Establishes negative electrical voltage inside the cells moved from the interior to the exterior of the cell for each pump cycle 4. This action creates positivity outside the cells that results to a deficit of positive ions inside the cells, this, causes negativity on the inside 5. Therefore, Na-K pump is said to be electrogenic because it creates electrical potential to the cell membrane 6. Electrical potential is a basic requirement to nerve and muscle fibers for transmitting signals C. SODIUM POTASSIUM PUMP Ø Controls the cell volume Ø Without this, most cell would swell then burst D. CA2+ - ATPASE (CALCIUM PUMP) Ø Once swelling of cell is detected, this pump is activated: moving more ions to the exterior and carrying water with Ø in the sarcoplasmic reticulum (SR) or cell membranes them Ø Transports Ca2+ against an electrochemical gradient Ø Carrier protein is a complex of 2 separate globular proteins Ø Sarcoplasmic and endoplasmic reticulum Ca2+-ATPase is larger - mw of 100,000 (alpha subunit) called SERCA ▪ has 3 binding sites for sodium ions at the portion of the protein that protrudes to the inside of the cell ▪ has 2 binding sites for potassium ions on the outside Ø smaller- mw of 55, 000 (beta subunit) function not known anchors the lipid membrane The inside portion of this protein near the sodium binding site has ATP activity 1. When 2 potassium ions bind on the outside of the carrier Ø Calcium ions are maintained at extremely low protein, and 3 sodium ions bind on the inside, the ATPase concentrations in the intracellular cytosol of virtually all cells function of the protein becomes activated of the body 2. ACTIVATION leads to cleavage of one molecule of ATP, Ø At a concentration of about 10, 000x less than that of the splitting it to ADP and liberating a high phosphate bond extracellular fluid ALVIZ, TOLBE, DISCARTIN, ALICOG, SADDOY Page 6 of 9  PHYSIOLOGY lecture PINES CITY COLLEGES P.01 P.04Trans Title TRANSPORT THROUGH CELL MEMBRANES Dra. Anna Goyenechea Cinio | AUGUST 10, 2024 Ø This level of maintenance is achieved by 2 primary active 2. As the secretory ends at the gastric gland parietal cells, the transport pumps hydrogen ion concentration is increased as much as a Cell membrane- pumps calcium to outside of the cell million-fold, then released to the stomach along with The other pumps calcium to one or more of the chloride ions to form hydrochloric acid intracellular vesicular organelles of the cell such as 3. In the renal tubules, special intercalated cells found in the sarcoplasmic reticulum of muscle cells and late distal tubules and cortical collecting ducts also transport mitochondria in all cells hydrogen ions with primary active transport Ø Carrier proteins have high specific binding site for calcium Large amounts of H+ ions are secreted from Penetrates the membrane and functions as enzyme blood into renal tubular fluid to eliminate excess ATPase with the same capability to cleave ATP, as hydrogen ions from body fluids the ATPase of Na-K protein 4. The hydrogen ions can be secreted into the renal tubular fluid against a concentration gradient of 900-fold NOTE: Concentration gradient is formed by primary active transport (Na-K pump) SECONDARY ACTIVE TRANSPORT- Na+ is diffused once again B. SECONDARY ACTIVE TRANSPORT Ø The transport of two or more solutes is coupled Ø One of the solutes (usually Na+) is transported “downhill” and provides energy for the “uphill” transport of the other solute Ø Metabolic energy is not provided directly but indirectly from the Na+ gradient that is maintained across cell membranes Ø If the solutes move in the same direction across the cell membrane, it is called cotransport or symport Ø The energy in this transport is derived secondarily from energy stored in the form of ionic concentration differences Note: Calcium is maintained at low levels in the cytosol of secondary molecular or ionic substances between the Calcium moves into extracellular space and into the sarcoplasmic two sides of cell membrane created originally by primary reticulum active transports Ø When Na+ ions are transported outside, a large E. H+K+-ATPASE (OR PROTON PUMP) concentration gradient of Na+ ions usually develop with a Occurs in the: high concentration of sodium outside cells and low Ø gastric parietal cells concentration inside Ø renal α-intercalated cells in the late distal tubules and Ø This gradient is a storehouse of energy because excess cortical collecting ducts of the kidneys sodium outside is always attempting to diffuse to the interior Ø Transports hydrogen (H+) ions into the lumen (of the stomach Ø This diffusion can pool other along with it thru cell or renal tubule) against its electrochemical gradient membrane Ø Inhibited by proton pump inhibitors, such as omeprazole. Phenomenon: POOL TRANSPORT o For sodium to pool other substances, coupling mechanism is required o Achieved by other carrier protein - serves as attachment for Na+ and other substances to be cotransported o Once both are attached, energy gradient of Na+ ion causes them to be transported together to the interior of the cell 1. In the gastric gland, the deep lying parietal cells have the Sodium Counter Transport most potent primary active mechanism for hydrogen ion Ø Sodium ions again attempt to diffuse to the interior of the transport cell because of their large concentration gradient This mechanism is the basis for secreting Ø Substance to be transported is on the inside of the cell and hydrochloric acid in stomach digestive is transported to the outside secretion ALVIZ, TOLBE, DISCARTIN, ALICOG, SADDOY Page 7 of 9  PHYSIOLOGY lecture PINES CITY COLLEGES P.01 P.04Trans Title TRANSPORT THROUGH CELL MEMBRANES Dra. Anna Goyenechea Cinio | AUGUST 10, 2024 Ø The sodium ion binds to the carrier protein, where it projects to the exterior surface of the membrane, and the substance to be counter transported binds to the interior projection of the carrier protein. Ø Once both have become bound, a conformational change occurs, and energy released by the action of the sodium ion moving to the interior causes the other substance to move to the exterior Sodium-Glucose Cotransport Ø carrier is located in the luminal membrane of intestinal mucosal and renal proximal tubule cells Ø Glucose is transported “uphill” Ø Na+ is transported “downhill” Ø Energy is derived from the “downhill” movement of Na+ Ø The inwardly directed Na+ gradient is maintained by the Ø SGLT2: transporter; a basis for a type of medicine for Na+K+ pump on the basolateral (blood side) membrane diabetes mellitus Ø SGLT2 inhibitors prevent reabsorption of glucose into kidneys and bloodstream Ø Sodium-Glucose Cotransport Sodium - Amino Acid Cotransport Ø Uses a different set of transport proteins Ø Five amino acid transport proteins have been identified, each of which is responsible for transporting one subset of amino acids with specific molecular characteristics Ø Epithelial cells of the intestinal tract and the renal tubules of the kidneys Ø Other important co-transport mechanisms in at least Ø Some cells include co-transport of potassium, chloride, bicarbonate, phosphate, iodine, iron, and urate ions. Ø Glucose and amino acids are transported against high concentration gradient with cotransport mechanism, Ø Transport carrier protein has 2 binding sites on exterior- 1 for sodium, 1 for glucose Ø Na+ ions concentration is higher on the outside and lower on the inside Ø Transport protein has conformational change to allow Na+ movement to interior will not occur until glucose attaches Ø CONFORMATIONAL CHANGE Ø When both attached Na+ & glucose are transported inside of cell at the same time Ø Important to transport glucose across renal and interstitial epithelial cells ALVIZ, TOLBE, DISCARTIN, ALICOG, SADDOY Page 8 of 9  PHYSIOLOGY lecture PINES CITY COLLEGES P.01 P.04Trans Title TRANSPORT THROUGH CELL MEMBRANES Dra. Anna Goyenechea Cinio | AUGUST 10, 2024 Na+ - Ca2+ Countertransport or Exchange D. CHECKPOINT Ø transports calcium (Ca2+) “uphill” from low intracellular 1. Only form of transport that is not carrier-mediated Ca2+ to high extracellular Ca2+ a. Active transport Ø Ca2+ and Na+ move in opposite directions across the cell b. Simple diffusion membrane Ø The energy is derived from the “downhill” movement of Na+ c. Facilitated Diffusion Ø As with cotransport, the inwardly directed Na+ gradient is 2. Net diffusion depends on the following variables, maintained by the Na+ K+ pump except: a. Size of the diffusion gradient Sodium Hydrogen Countertransport b. Partition Coefficient Ø occurs in several tissues. c. Diffusion Coefficient Ø important example is in the proximal tubules of the kidneys Ø Sodium ions move from the lumen of the tubule to the interior d. Velocity of kinetic motion of the tubular cell and hydrogen ions are counter- 3. True or False. Facilitated diffusion requires energy, transported into the tubule lumen therefore is an active transport. Ø not nearly as powerful as the primary active transport of 4. At high solute concentrations, facilitated diffusion hydrogen ions that occurs in the more distal renal tubules, typically proceeds faster than simple diffusion because but it can transport extremely large numbers of hydrogen of the function of the carrier. At higher concentrations, ions, thus making it a key to hydrogen ion control in the body the carriers will become saturated, and facilitated fluids diffusion will level off a. Only the first statement is correct C. ACTIVE TRANSPORT THROUGH CELLULAR SHEETS b. Only the second statement is correct Occurs through the following: c. Both statements are correct Ø intestinal epithelium d. Both statements are wrong Ø epithelium of the renal tubules 5. The flow of water across a semipermeable membrane Ø epithelium of all exocrine glands from a solution with a low solute concentration to a Ø epithelium of the gallbladder solution with high solute concentration Ø membrane of the choroid plexus of the brain, along with 6. T or F. Ions channels are non-selective other membranes The basic mechanism is as follows: Ø active transport through the cell membrane on one side of the transporting cells in the sheet False – Ion channels are selective. 6. Osmosis 5. Ø either simple diffusion or facilitated diffusion through the facilitated diffusion will level off membrane on the opposite side of the cell the carrier. At higher concentrations, the carriers will become saturated, and diffusion typically proceeds faster than simple diffusion because of the function of B. Only the second statement is correct - At low solute concentrations, facilitated 4. False – Facilitated diffusion does not require energy; therefore, it is passive. 3. D. Velocity of kinetic motion – determines rate of diffusion 2. B. Simple Diffusion 1. E. REFERENCES Ø Guyton and Hall Textbook of Medical Physiology, 14th Ed ALVIZ, TOLBE, DISCARTIN, ALICOG, SADDOY Page 9 of 9