Human Physiology - Chapter 5, Membrane Transport

IV Solutions

0.9% saline Nonpenetrative Remains in ECF, increases ECF volume Increases total body solute and volume, without changing cell volume (ICF) Therefore isotonic to cells Increases ECF solute and ECF volume at same time, proportionally Therefore isosmotic to cells (same solute per volume) 5% dextrose in 0.9% saline Only saline causes water movement (nonpenetrative), increases ECF volume Therefore isotonic to cells (no ICF change) Dextrose aka glucose increases solute in ICF (penetrative, converted to G6P in cells), without volume change to ICF Therefore hyperosmotic (greater solute per volume in ICF) 5% dextrose in water Penetrative, converted to G6P in cells Increase in ICF solute Increase in volume and increase in solute proportional Isosmotic to body However, lack of nonpenetrative molecules Same osmolarity and increase in ICF solute means increase in ICF volume volume = solute / concentration Therefore hypotonic to cells (caused increase in cell volume)

Saturation of carrier-mediated transporter proteins

Rate of transport depends on: —> Substrate concentration (Molecule/ligand with higher concentration will usually bind to transporter unless transporter has higher affinity to different ligand) —> Number of carriers

Caveolae

A form of micropinocytosis mediated by lipid rafts, not receptors (not clathrin-coated pits) Concentrate & internalize small molecules Help transfer macromolecules across capillary endothelium Participate in cell signaling Forms of muscular dystrophy associated with abnormal caveolin protein Also involved in viral & parasitic infections

Cystic fibrosis transmembrane conductance regulator (CFTR)

ATP-gated anion channel that transports Cl- out of epithelia into airways Lines airways, sweat glands, and pancreas Ligand-gated aka chemically gated Defect causes airway dehydration and congestion, cystic fibrosis The only ABC transport gene superfamily member to be an ion channel

Membrane potential measurement

Absolute charge scale vs. relative charge scale

Atherosclerosis — arterial hardening due to cholesterol aggregation

Accumulation of cholesterol in blood vessels; blocks blood flow Hypercholesterolemia due to high levels of LDL (low-density lipoprotein) cholesterol predisposes people to develop atherosclerosis

Sodium-potassium pump/ATPase (primary active transport)

Antiport carrier protein Transports sodium out of cell Transports potassium into cell —>Sodium already high conc outside cell, potassium already high conc inside cell Big ATP sink, 30% of a cell's ATP can be used for just this sodium-potassium pump against gradient ATP hydrolysis directly used 3 Na+ pumped out of cell for 2 K+ pumped into cell Human body cells are 40x more permeable to K+ than to Na+

Epithelial transport

Can be either Paracellular or Transcellular transport Tight junctions separate cell membrane into two poles/regions: Apical membrane — surface that faces lumen —> Folded into microvilli to increase surface area —> Aka mucosal membrane Basolateral membrane — three other surfaces of cell that face extracellular fluid —> aka serosal membrane Absorption: Lumen -> ECF transport (example, intestines absorbing nutrients) Secretion: ECF -> Lumen (example, saliva secretion)

Carrier proteins — ACTIVE type of membrane transporter

Changes conformation when transporting/binding molecules across membrane Can transport with or against concentration gradient Can transport multiple thing at same time Exhibits: —> SPECIFICITY: Transporter binds only one molecule or family of related molecules —> COMPETITION: When binding several related molecules, they compete for binding to transporter & some ligands have greater affinity than others —> SATURATION: Rate of transport depends on substrate concentration & number of carriers SIMPLE/UNIPORTER carrier protein: transports only one thing at a time SYMPORTER carrier protein: two things transport across cell membrane to same side at same time ANTIPORTER carrier protein: two things transport in different directions across cell membrane at same time PRIMARY (DIRECT) / SECONDARY (INDIRECT) : —> Primary (direct) active transport uses ATP —> Secondary (indirect) active transport uses potential energy of another chemical gradient

Explain the difference between channel proteins and carrier proteins and why cells need both.

Channel proteins allow molecules to passively diffuse along their concentration gradient —> Usually for ions and water —> Can open and close Carrier proteins actively transport molecules —> against concentration gradient —> too large to enter cell otherwise —> rapid transport of large amount of molecules —> primary active transport: uses ATP —> secondary active transport: does not use ATP, uses ion gradient to transport another molecule simultaneously

Membrane transporters

Channel proteins which create opening for facilitated diffusion Two types: Channel proteins — made from multiple copies of same protein, combine to form channel through plasma membrane, PASSIVE, selectively open and close Carrier proteins (aka carrier-mediated transport) — Exhibits —>Specificity (only one molecule or family of related molecules) —>Competition (related molecules compete for binding to transporter, some ligands have greater affinity than others) —>Saturation (rate of transport depends on substrate concentration & number of carriers) Two gene superfamilies: ABC: ATP-Binding Cassette —> uses ATP to transport small molecules & ions SLC: Solute carrier superfamily —>

Phagocytosis — vesicular transport (active)

Cytoskeleton opens up and grabs something outside of cell For very large molecules or rapid uptake of large amount of molecules —> Not performed by human cells except for immune system cells, which phagocytose bacteria & dead cells

Osmosis

DIffusion of liquids (in human body: water) Living things have osmotic equilibrium (water moves freely between cells and extracellular fluid)

Active transport — form of mediated transport by membrane proteins

Energy required to move molecule against concentration gradient across cell membrane Primary (direct) active transport: uses ATP to move molecules against concentration gradient Secondary (indirect) active transport: uses potential energy from concentration gradient of one molecule (moving downward) to move other molecule against concentration gradient

Membrane potential (Vm)

Difference in voltage across the plasma membrane; always given in terms of voltage inside the cell relative to voltage outside the cell The concentration gradient does not have to reverse to change the membrane potential Example: to change the membrane potential by 100 mV (the size of a typical electrical signal passing down a neuron), only one of every 100,000 K+ must enter or leave the cell

Facilitated diffusion — passive form of mediated transport by membrane proteins

Diffusion of molecules down concentration gradient which does not require energy, but is helped by membrane proteins

Flux

Diffusion rate per unit surface area of membrane Flux = concentration gradient x membrane permeability

Fick's law of diffusion

Diffusion rate/surface area = concentration gradient x membrane permeability ^ can describe the flux of a molecule Flux: diffusion rate per unit surface area of membrane Flux = concentration gradient x membrane permeability

Primary active transport

Directly uses ATP to provide energy allowing substance (one or more) to move against concentration gradient Important example: Sodium-Potassium pump/ATPase Carrier protein Transports sodium out of cell Transports potassium into cell —>Sodium already high conc outside cell, potassium already high conc inside cell Big ATP sink, 30% of a cell's ATP can be used for just this sodium-potassium pump against gradient ATP hydrolysis directly used

Secondary active transport

Does not use ATP directly Couples energy released from moving one molecule down concentration gradient, with moving another molecule against concentration gradient Ex: SGLT transporter

What is the difference between primary and secondary active transport?

Primary active transport: —>Energy is derived directly from breakdown of ATP Secondary active transport: —> Energy is derived secondarily from membrane ion gradient (energy that has been stored in the form of ionic concentration differences between the two sides of a membrane) —> Uses energy of ion transport along gradient to transport second molecule against gradient

Membrane equilibrium potential calculation — Nernst equation

E = (61/z) log ( [ ion out ] / [ ion in ] ) Z = ionic charge (ex: K+ z=+1) At human body temperature 37 degrees Celsius Nernst equation only works for cells freely permeable to one ion at a time (not true to life) Use Goldman-Hodgkin-Katz equation (in Chapter 8) for multi-ion permeable cell

Resting membrane potential

Electrochemical charge from concentration gradients of Na+ and K+ Na+ pumped out of cell, K+ pumped into cell via active transport Nerve cells have -70 mV voltage at rest

Bulk flow (pressure gradient)

Fluid flow caused by pressure gradient Dissolved solutes and gas also move with fluid flow Works because fluids are incompressible (unlike gas) Example: blood pumped from high pressure (heart, arteries) to low pressure (capillaries) Air flow in lungs

Chemically gated channels

Gated by intracellular messenger molecules or extracellular ligand that bind protein channel ATP-gated channels count as ligand-gated, aka chemically gated

Aquaporins — channel proteins for water

Have same protein multiple times as subunits forming water pore in cell membrane Passive like all channel proteins Can open and close; some aquaporins always closed unless specifically opened Why do cells have water channels when water can freely diffuse across membrane? —> Water diffuses slowly across membrane by entering small spaces between fatty acid tails, channels allows water to pass through more quickly Aquaporin protein AQP can also act as carrier for certain small organic molecules

Which of the following solutions has/have the most water per unit volume: 1 M glucose, 1 M NaCl, or 1 OsM NaCl?

Higher osmolarity = lower water per volume (due to higher # particles per volume) 1 M NaCl = 2 OsM NaCl (actually 1.8 due to NaCl disassociation factor) The 1 M (= 1 OsM) glucose and 1 OsM NaCl have the most water 1.8 osmol NaCl has less water per volume due to more solute particles

Law of conservation of electrical charge

Human body is overall electrically neutral If one ion exists of one charge, another ion of the opposite charge must also exist

Absorption

Molecules pass from organ's lumen to extracellular fluid Ex: Nutrients pass from intestinal lumen to blood

Tonicity

If the cell has a higher concentration of nonpenetrating solutes than the solution, there will be net movement of water into the cell. The cell swells, and the solution is hypotonic If the cell has a lower concentration of nonpenetrating solutes than the solution, there will be net movement of water out of the cell. The cell shrinks, and the solution is hypertonic If the concentrations of nonpenetrating solutes are the same in the cell and the solution, there will be no net movement of water at equilibrium. The solution is isotonic to the cell *Tonicity depends on nature of solutes in solution (penetrating or no penetrating solutes) in addition to concentration (osmolarity) * Figure out Tonicity by relative concentrations of non penetrating solutes in cell and in solution

Channel proteins — PASSIVE type of membrane transporter

Made of multiple of same type of protein subunit Combine to form channel (hole) through membrane Only passive transport Can selectively open and close (some aquaporins closed all the time unless they are opened) Have specificity by charge of amino acids lining pore and pore diameter: —Some channels only transport one molecule i.e. potassium channels, no other ions can enter —Monovalent channels only allow chemically similar molecules (same-charge ions); positively-charged channel allows negative ions to pass, blocks positively charged ions; negatively-charged channel allows positive ions to pass, blocks negatively charge ions Ions and water commonly allowed through channels (water channel proteins called aquaporins)

Endocytosis — vesicular transport (active)

Membrane surface indentation takes in molecules, forms vesicle around it For very large molecules or rapid intake of large amount of molecules Receptor-mediated endocytosis —> Membrane surface indentation (called coated pits, where cytoplasmic side of membrane has high protein concentration) takes in thing (invaginates), forms vesicle around it —> Most common common pit protein: clathrin —> Caveolae: flask-shaped indentations without clathrin, are membrane regions with lipid rafts (lipid-anchored proteins); function to concentrate/internalize small molecules, transfer macromolecules across capillary endothelium, & participate in cell signaling; abnormalities in caveolin protein associated w muscular dystrophy All human cells capable of this Cells take up extracellular fluid constantly via endocytosis (called pinocytosis, which is nonselective) Opposite of Exocytosis Membrane-bound receptors can be reused via membrane recycling

Voltage gated channels

Open and close when electrical state of the cell changes

NaCl — the (functionally) non penetrating solute

Most important non penetrating solute in physiology If a cell is placed in a solution of NaCl, the Na+ and Cl- ions do not enter the cell This makes NaCl a nonpenetrating solute. In reality, a few Na+ ions may leak across, but they are immediately transported back to the extracellular fluid by the Na+ - K+ - ATPase For this reason, NaCl is considered a functionally nonpenetrating solute *Osmolarity calculation: NaCl's disassociation factor is actually 1.8, not 2 1 mole NaCl/L = 1.8 osmol/L *Note: Higher osmolarity = lower water per volume 1 mol NaCl has higher osmolarity, lower water per volume than 1 osmol NaCl (approx. 0.5 mol NaCl) or 1 mol glucose (1 osmol glucose)

Mediated transport by membrane proteins (Facilitated diffusion & active transport)

Most molecules require MEMBRANE PROTEINS to cross cell membrane Two forms: Facilitated diffusion — no energy required, but membrane proteins still help diffusion Active transport — requires energy and membrane protein —> can use ATP (primary, direct) or potential energy of another chemical gradient (secondary, indirect)

Secretion

Movement from extracellular fluid into organ's lumen

Diffusion

Movement from high concentration to low concentration (down concentration gradient) Properties cause molecules to diffuse across membranes at different RATES —>Higher temperatures make diffusion faster (cooler temperatures make diffusion slower) —>Short distances make diffusion faster (long distances make diffusion slower) —>Thinner membrane makes diffusion faster (thicker membrane makes diffusion slower) —> Higher concentration gradient makes diffusion faster (Lower concentration gradient makes diffusion slower) —> Smaller molecules make diffusion faster (larger molecules make diffusion slower) —> More permeable membrane (lipid composition) makes diffusion faster (less permeable membrane makes diffusion slower) —>Net movement of molecules occur until osmolarity made equal

Cystic fibrosis

Movement of NaCl into lumen of airways impaired Water moves into hyperosmotic regions Lung mucus thick and dehydrated NaCl normally moves water into lung lumen, inability to move NaCl means inability to move water into lungs Defect in cystic fibrosis transmembrane conductance regulator (CFTR) (ATP-gated anion channel protein) — located in airway, sweat gland, & pancreas epithelia —> Gate opens when ATP binds to protein, transports Cl- out of epithelial cells into airways —> Ligand-gated, aka chemically gated

SGLT Transporter (secondary active transport) — Glucose & Na+ Symporter

Moves glucose against concentration gradient by cotransport with Na+ movement In intestinal mucosa (enterocytes) (SGLT1) and kidney nephron's proximal tube (SGLT1 and SGLT2) to reabsorb glucose from getting excreted Symporter, Na+ and glucose together ONE-DIRECTIONAL unlike glut transporters —>for example when reabsorbing glucose from urine (to not excrete it) —> requires energy to move glucose from low to high conc —> glucose normally does not require energy to enter cell (since it is converted to G6P within the cell, creating constant concentration gradient), but activity in intestinal mucosa and kidneys is reabsorbing glucose into body, against gradient

Simple Diffusion (Passive unmediated transport)

No energy required for molecule to move across cell membrane down its concentration gradient Rate of diffusion directly proportional to surface area of membrane; Fick's Law of Diffusion — Rate of diffusion proportional to surface area, concentration gradient, & membrane permeability (rate of diffusion : surface area x conc gradient x membrane permeability) Lipid-soluble (liphophilic) molecules can passively diffuse across phospholipid bilayer (lipids, steroids, small lipophilic molecules) Small size of water molecules does allow it to slip between lipid tails of some phospholipid membrane, allowing passive diffusion of water —> High cholesterol content in membrane occludes water, fills spaces between fatty acid tails (ex: sections of the kidney impermeable to water) —> Most water movement still takes place through protein channels

Exocytosis — vesicular transport (active)

Opposite of endocytosis Requires ATP for energy Membrane surface releases molecules from vesicles Used to export large lipophobic molecules Constitutive (continous) in some cells, like intestinal goblet cells (constantly release mucus via Exocytosis) and connective tissue fibroblasts (release collagen) Endocrine cells store hormones in secretory vesicles in cytoplasm, release in response to extracellular signal Two families of proteins: Rabs — help vesicles dock onto membrane SNAREs — facilitate membrane fusion Regulated Exocytosis uses increase of intracellular Ca2+ to signal, interacts w/ calcium-sensing protein that initiates secretory vesicle docking & fusion

Conductor

Oppositely charged (positive and negative) ions can move freely toward each other Water is a good conductor

Insulator

Oppositely charged (positive and negative) ions cannot move freely toward each other The phospholipid bilayer is a good insulator

Osmolarity and Tonicity Rules

Osmolarity describes the number of solute particles dissolved in a volume of solution. It has units, such as osmoles/liter. The osmolarity of a solution can be measured by a machine called an osmometer. Tonicity has no units; it is only a comparative term Osmolarity can be used to compare any two solutions, and the relationship is reciprocal (solution A is hyperosmotic to solution B; therefore, solution B is hyposmotic to solution A). Tonicity always compares a solution and a cell, and by convention, tonicity is used to describe only the solution—for example, "Solution A is hypotonic to red blood cells." Osmolarity alone does not tell you what happens to a cell placed in a solution. Tonicity by definition tells you what happens to cell volume at equilibrium when the cell is placed in the solution.

Osmotic pressure

Pressure that must be applied to oppose osmotic movement of water Proportional to concentration gradient that causes osmosis

Organic anion transporter (OAT) in kidneys

Reclaims urate (anion form of uric acid) From urine, returns acid to plasma Gout caused by excess uric acid in plasma Curable by competitive inhibition of OAT by probenecid administration (binds probenecid instead of urate) More urate leaves body in urine, reducing plasma uric acid

Mechanically gated channels

Respond to physical forces that put tension on the membrane, such as increased temperature or pressure

How does molecular weight affect the rate of diffusion?

Smaller molecular weight -> faster rate of diffusion Larger molecular weight -> slower rate of diffusion

Semipermeable cell membranes

Some substances can freely cross through: Water Lipids Other small, uncharged molecules: —>Ethanol, all lipids, gases like carbon dioxide ^Because most of cell membrane is hydrophobic Membrane permeability (proportional to) lipid solubility / / molecular size Rate of diffusion (proportional to) Surface area x concentration gradient x membrane permeability

Two compartments are separated by a membrane that is permeable to water and urea but not to NaCl. Which way will water move when the following solutions are placed in the two compartments?

The non penetrating solute is NaCl Compare the NaCl concentration to the cell's osmolarity Assume osmolarity is due only to non penetrating solutes Ignore urea since it is penetrating solute Compartment A has approximately 1.5 M NaCl Compartment B has approximately 0.5 M NaCl Water will move toward Compartment A

Paracellular transport

Transport of materials through the interstitial space without interactions with the cytoplasm or cell membrane through junctions between adjacent cells

Specificity of carrier-mediated transporter proteins

Transporter can only bind one molecule or family of related molecules

Uniporter (simple) carrier protein

Transports molecules one at a time across cell membrane Examples: Ca2+ — ATPase H+ — ATPase (proton pump)

Symporter carrier protein

Two different things can pass in same direction across cell membrane (at same time) Example: Na+ — K+ — 2Cl- (NKcc) Na+ — Glucose (SGLT) Na+ — Cl- Na+ — HCO3- Na+ — Amino acids (several types) Na+ — Bile salts (small intestine) Na+ — choline uptake (nerve cells) Na+ — Neurotransmitter uptake (nerve cells) H+ — peptide symporter (pepT)

Antiporter carrier protein

Two different things pass in different directions across cell membrane (at same time) Example: Na+ — K+ — ATPase (sodium-potassium pump) H+ — K+ — ATPase Na+—H+ (NHE) Na+ — CA2+ (NCX) HCO3- — Cl-

Secondary (indirect) active transport

Use kinetic energy (from potential energy chemical gradient) of molecule moving down gradient to move other molecules against concentration gradient Many are Na+-dependent, with exception of H+—peptide symporter (pepT)

Primary (direct) active transport (ATPase)

Uses ATP as energy source Hydrolysis ATP to ADP + Pi Ex: Na+ — K+ — ATPase (sodium-potassium pump) -> ANTIPORT CA2+ — ATPase -> UNIPORT H+ — ATPase (proton pump) -> UNIPORT H+ — K+ — ATPase -> ANTIPORT

Aquaporins are cell membrane water channels. Why does the cell need water channels when the plasma membrane is permeable to water anyway?

Water can passively diffuse across cell membrane since the small molecules can enter between spaces in the fatty acid tails, but process is slow Aquaporin channels allow water to enter cell more quickly Important to maintaining osmotic equilibrium

Vesicular Transport — ACTIVE, large molecules or rapid transport of large amount of molecules

Way for very large molecules or rapid rate of large amount of molecules to enter or leave cell REQUIRE ATP Phagocytosis —> Cell engulfs something by "opening up and grabbing" with cytoskeleton —> Most human cells cannot, only immune system to engulf bacteria or dead cells Receptor-mediated endocytosis —> Membrane surface indentation (called coated pits, where cytoplasmic side of membrane has high protein concentration) takes in thing (invaginates), forms vesicle around it —> Most common common pit protein: clathrin —> All human cells capable of this —> Take up extracellular fluid constantly via endocytosis (called pinocytosis, which is nonselective) Exocytosis —> Movement of something out of cell —> Opposite process of endocytosis

Competition for carrier-mediated transporter proteins

When transporter can bind several related molecules, they compete for binding to it Some ligands have greater affinity than others Molecule with greater concentration will usually bind unless other one has higher affinity

GLUT Transporter — Facilitated diffusion (passive) of glucose into cell

family of facilitated diffusion carriers for glucose and other hexose sugars (monosaccharides) Maltose (disaccharide) acts as a competitive inhibitor, binds to GLUT transporter but is not carried across membrane Works in both directions —Glucose is always passively diffused into cell because cell immediately converts to Glucose-6-phosphate, creating concentration gradient (G-6-P is not glucose) —> Ensures cells always have low glucose concentration GLUT1: in most cells of the body GLUT2: liver, kidneys, intestinal epithelia GLUT3: neurons GLUT4: insulin-regulated skeletal muscle GLUT5: intestinal fructose transporter 14 GLUT genes have been found

Membrane proteins

integral proteins and peripheral proteins Functions can be membrane transport, structural (cell junctions, cytoskeletal), membrane enzymes, membrane receptors Ex: Transporting epithelia microvilli are cytoskeletal structural membrane proteins Tight & gap junctions hold tissues together via

Osmolarity

number of osmotically active particles (ions or intact molecules) per liter of solution determines the movement of water across a selectively permeable membrane Higher osmolarity = lower water per volume (due to higher particles per volume) Equation: Solute/volume = concentration Convert between molarity and osmolarity equation: Molarity (mol/L) x particles/molecule (osmolarity/mol) = osmolarity (osmolarity/mol)

Transcellular transport

transport of materials through the cell (in one side of cell & out the other) requires interaction w/ the cytoplasm may require transport proteins

Reasons to compartmentalize cells

—Can prevent interactions between things we don't want to interact —When we want two things to interact, can place them together in small compartment to encourage reaction Downside: —Compartmentalizing things requires work