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Ch 6
Interactions
Between Cells and
the Extracellular
Environment
Part 1: Transport Mechanisms
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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
__ fluid __ fluid
_________ fluid plasma
Relatively free
exchange Selective
exchange
Why ?
?? Composition of Body Fluids ??
Cross out the the wrong one
Composition of Body Fluid Compartments
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?
In summary:
Membrane Transport Mechanisms
Terminology: • Permeable
• Impermeable
• Selectively permeable
• Passive transport
• Active transport
Categories of Membrane Transport
Diffusion - Noncarrier-mediated
1. Simple diffusion of lipid-soluble molecules
2. Simple diffusion of ions through channels
3. Simple diffusion of water = osmosis
Carrier-mediated
1. Facilitated diffusion
2. 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
• 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
Rate of Diffusion
Measured by the # of diffusing particles per unit of time
Factors influencing it Diffusion Rate
Concentration
Gradient
Temperature
Permeability
Surface Area
surface conc. membrane area gradient permeability
membrane thickness
Diffusion
rate
X X
Fick’s law of Diffusion
Osmosis Definition?
Special channels called __________
Figs 6.7 & 6.8
• Need solute conc. difference across membrane
• Non-penetrating solutes = osmotically active
solutes
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
Water moves freely in
body until osmotic
equilibrium is reached!
Osmotic Pressure cont.
Osmotic
pressure
Opposes
movement
of water
across
membrane
1. Molarity vs. Osmolarity
In chemistry:
• Mole / L
• Avogadro’s # / L
In Physiology
Important is not # of
molecules / L but
# of particles / L:
osmol/L or Osm
Why?
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.
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
1. Iso-osmotic
2. Hyper-osmotic
3. 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
1. 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?
2. RBCs are placed in a 0.3m solution of urea. (Note:
urea is small and lipophilic.) Is this solution
• hyperosmotic, hyposmotic, or isosmotic?
• hypertonic, hypotonic, or isotonic?
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)
Fig 6.15
competition
saturation
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:
1. Simple diffusion
2. Osmosis
3. 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:
1. Primary active transport: ATPases or
“pumps” (uniport and antiport) – examples?
2. 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!
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
• 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
Transport Across Epithelial Membranes
(Trans)Epithelial Transport
Absorption from GI tract and
reabsorption from urinary filtrate
Fig 6.21
Bulk Transport
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!
Fig 6.23