Interactions Between Cells and the Extracellular Environment Ch 6 Interactions Between Cells and the Extracellular Environment Part 1: Transport Mechanisms

http://img.docstoccdn.com/thumb/orig/128028380.png Cells receive nourishment from and release wastes into the extracellular environment. Cells communicate with each other by secreting chemical regulators into the extracellular environment.

3 Body Fluid Compartments Relatively free exchange Selective exchange Why ? _________ fluid plasma

?? Composition of Body Fluids ?? ICF ECF Na+ high / low K+ Cl- Protein Anions (A-) Cross out the the wrong one

Composition of Body Fluid Compartments

In summary: Barrier between .... ...plasma and interstitial fluid: ______________ Allows water, ions and other small molecules to pass freely whereas larger molecules such as _______and blood cells cannot. ... ISF and ICF: ________________ What does selectively permeable mean?

Membrane Transport Mechanisms Terminology: Permeable Impermeable Selectively permeable Passive transport Active transport

Categories of Membrane Transport Diffusion - Noncarrier-mediated Simple diffusion of lipid-soluble molecules Simple diffusion of ions through channels Simple diffusion of water = osmosis Carrier-mediated Facilitated diffusion Active transports Energy Requirements ?

Diffusion requires a ________________ Fig 6.3

Diffusion is Passive Small, nonpolar (or uncharged) lipid-soluble molecules pass through the lipid bilayer of the membrane. Examples: _______________________________________ Movement ...... Equilibrium = steady state; net movement has stopped Fig 6.2

Still passive diffusion if movement down concentration gradient What about small charged molecules, such as _____________________________ ? Still passive diffusion if movement down concentration gradient Types of Ion Channels: Non-gated channels Gated channels Mostly open Mostly closed Fig 6.2 and 6.6

Measured by the # of diffusing particles per unit of time Rate of Diffusion Measured by the # of diffusing particles per unit of time Factors influencing it Diffusion Rate Concentration Gradient Temperature Permeability Surface Area

Fick’s law of Diffusion surface conc. membrane area gradient permeability membrane thickness Diffusion rate  X

Osmosis Definition? Special channels called __________ Need solute conc. difference across membrane Non-penetrating solutes = osmotically active solutes Figs 6.7 & 6.8

Osmotic Pressure Force required to stop osmosis Can be used to describe the osmotic pull of a solution. A higher solute concentration would require a greater osmotic pressure. Pure water has an osmotic pressure of zero

Osmotic Pressure cont. Osmotic pressure Opposes movement of water across membrane Water moves freely in body until osmotic equilibrium is reached!

Molarity vs. Osmolarity In Physiology Important is not # of molecules / L but # of particles / L: osmol/L or Osm Why? In chemistry: Mole / L Avogadro’s # / L Osmolarity takes into account the dissociation of molecules in solution

Convert Molarity to Osmolarity Osmolarity = # of particles / L of solution 1 M glucose = ? Osm glucose 1 M NaCl = ? OsM NaCl 1 M MgCl2 = ? OsM MgCl2 Osmolarity of human body  300 mOsm Terminology: Isosmotic, hyperosmotic, hyposmotic

You are making up 2 solutions in 2 beakers You are making up 2 solutions in 2 beakers. Beaker 1 contains 360 g of glucose/L. You are adding 180 g of fructose and 180g of glucose to the second beaker which also contains a liter of water. The solutions in the two beakers are Iso-osmotic Hyper-osmotic Hypo-osmotic

Tonicity Physiological term describing volume change of cell if placed in a solution Always comparative. Has no units. Isotonic Hypertonic Hypotonic Depends not just on osmolarity (conc.) but also on nature of solutes: Penetrating vs. nonpenetrating solutes!

Penetrating vs. Nonpenetrating Solutes Penetrating solute: can enter cell (e.g.: glucose, urea, glycerol) Nonpenetrating solutes: cannot enter or leave cell (e.g.: sucrose, NaCl*) Determine relative conc. of nonpenetrating solutes in solution and in cell to determine tonicity. Water will move to dilute nonpenetrating solutes Penetrating solutes will distribute to equilibrium

Tonicity The fate of red blood cells in isotonic, hypotonic, and hypertonic solutions Fig 6.13

Tonicity, examples A membrane only permeable to water separates a 0.3m glucose solution and a 0.15m NaCl solution. Is the 0.15 m NaCl solution hyperosmotic, hyposmotic, or isosmotic? hypertonic, hypotonic, or isotonic? RBCs are placed in a 0.3m solution of urea. (Note: urea is small and lipophilic.) Is this solution

IV Fluids – are for 2 different purposes You must decide if your patient needs IV Fluid therapy to.... ...get fluid into dehydrated cells or ...keep fluid in extra-cellular compartment

Regulation of Blood Osmolarity Osmolarity of EC fluid must be maintained, or neurons and other cells will be damaged. Hypothalamic Osmoreceptors Fig 6.14

Carrier-Mediated Transport Large or polar molecules (e.g.,___________, __________) cannot diffuse directly across the membrane Require carrier proteins Characteristics: Specificity (e.g.: GLUT transporters for hexoses) Competition (competitive inhibition applied in medicine, e.g.: gout) Saturation transport maximum (numbers of carriers can be adjusted)

saturation Fig 6.15 competition

Facilitated Diffusion Carrier mediated, _________transport Net movement from high to low conc. Transport proteins may always exist in plasma membrane or be inserted when needed Fig 6.16

Example: GLUT Transporters Four Isoforms: GLUT1 – CNS GLUT2 – pancreatic beta cells & hepatocytes GLUT3 – neurons GLUT4 – adipose tissue & skeletal muscles. Insertion regulated by exercise Cells avoid reaching glucose equilibrium Why ? How ?

Summary: Passive Transport = Diffusion (Def?) – 3 types: Simple diffusion Osmosis Facilitated diffusion (= mediated transport)

Insertion of Carrier Proteins into the Plasma Membrane Fig 6.17

Active Transport Movement from low to high conc. (move uphill) Requires ATP Creates state of ____ equilibrium Two types: Primary active transport: ATPases or “pumps” (uniport and antiport) – examples? Secondary (or coupled) active transport Symport or antiport

Primary Active Transport Hydrolysis of ATP directly fuels transport. Transport protein is also an ATPase enzyme that will hydrolyze ATP Pump activated by phosphorylation using a Pi from ATP. Fig 6.18

Na+/K+ Pump Ubiquitous uses up to 30% of cell’s ATP ATPase enzyme pumps _____out of the cell and _____ into the cell Maintains ionic imbalance of these two ions across cell membranes Sodium concentration gradient is Epot. and can be harnessed for other cell functions, e.g.: Coupled transport of other molecules Electrochemical impulses in neurons and muscles cells

Mechanism of the Na+/K+-ATPase Compare to Fig 6-19

Secondary Active Transport Indirect ATP use: uses Epot. stored in conc. gradient Also called coupled transport. Coupling of Ekin of one molecule with movement of another. Energy needed to move molecules against their concentration gradient is acquired by moving sodium back into the cell. Since the sodium was originally pumped out of the cell using ATP, this is considered active transport.

Example: SGLT Distinguish from GLUT! Sodium /Glucose transporters Fig 6.20 Sodium /Glucose transporters 1:1 ratio in kidneys 2:1 ratio in GI tract

Uniport vs. Cotransport Symport Molecules are carried in same direction Examples: Glucose and Na+ Antiport Molecules are carried in opposite direction Examples: Na+/K+ pump

Transport Across Epithelial Membranes (Trans)Epithelial Transport Uses combination of active and passive transport Maybe transcellular or paracellular Molecules have to cross two phospho- lipid bilayers Polarity of epithelial cells: Different transport proteins on Apical membrane vs. basolateral membrane

Absorption from GI tract and reabsorption from urinary filtrate Fig 6.21

Bulk Transport The end of part 1 Many molecules moved simultaneoulsy Large molecules (proteins, hormones, NTs) are secreted via exocytosis or taken up into the cells via ______________. Involves fusion of a vesicle with the plasma membrane. Requires ATP Again polarity! The end of part 1 Fig 6.23