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

2.
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. 3 Body Fluid Compartments
Relatively free exchange
Selective exchange
Why ?
_________ fluid
plasma

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

5. Composition of Body Fluid Compartments

6. 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?

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

8. 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 ?

9. Diffusion requires a ________________
Fig 6.3

10. 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

11. 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

12. 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

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

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

15. 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

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

17. 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

18. 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

19. 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

20. 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!

21. 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

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

23. 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

24. 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

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

26. 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)

27. saturation
Fig 6.15
competition

28. 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

29. 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 ?

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

31. Insertion of Carrier Proteins into the
Plasma Membrane
Fig 6.17

32. 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

33. 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

34. 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

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

36. 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.

37. 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

38. 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

39. 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

40. Absorption from GI tract and reabsorption from urinary filtrate
Fig 6.21

41. 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