interactions between cells and the extracellular environment
TRANSCRIPT
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Ch 6
Interactions
Between Cells and
the Extracellular
Environment
Part 1: Transport Mechanisms
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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.
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3 Body Fluid Compartments
__ fluid __ fluid
_________ fluid plasma
Relatively free
exchange Selective
exchange
Why ?
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?? Composition of Body Fluids ??
Cross out the the wrong one
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Composition of Body Fluid Compartments
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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:
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Membrane Transport Mechanisms
Terminology: • Permeable
• Impermeable
• Selectively permeable
• Passive transport
• Active transport
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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 ?
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Diffusion requires a ________________
Fig 6.3
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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
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• 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
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Rate of Diffusion
Measured by the # of diffusing particles per unit of time
Factors influencing it Diffusion Rate
Concentration
Gradient
Temperature
Permeability
Surface Area
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surface conc. membrane area gradient permeability
membrane thickness
Diffusion
rate
X X
Fick’s law of Diffusion
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Osmosis Definition?
Special channels called __________
Figs 6.7 & 6.8
• Need solute conc. difference across membrane
• Non-penetrating solutes = osmotically active
solutes
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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
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Water moves freely in
body until osmotic
equilibrium is reached!
Osmotic Pressure cont.
Osmotic
pressure
Opposes
movement
of water
across
membrane
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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
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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
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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
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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!
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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
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Tonicity
The fate of red blood cells in
isotonic, hypotonic, and hypertonic solutions
Fig 6.13
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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?
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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
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Regulation of Blood Osmolarity
• Osmolarity of EC
fluid must be
maintained, or
neurons and other
cells will be
damaged.
• Hypothalamic
Osmoreceptors
Fig 6.14
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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)
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Fig 6.15
competition
saturation
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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
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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 ?
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Summary: Passive Transport
= Diffusion (Def?) – 3 types:
1. Simple diffusion
2. Osmosis
3. Facilitated diffusion (= mediated
transport)
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Insertion of Carrier Proteins into the
Plasma Membrane
Fig 6.17
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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
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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
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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
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Mechanism of the Na+/K+-ATPase
Compare
to Fig 6-19
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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.
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Example: SGLT
Distinguish from GLUT!
Fig 6.20
Sodium /Glucose transporters
1:1 ratio in kidneys
2:1 ratio in GI tract
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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
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• 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
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Absorption from GI tract and
reabsorption from urinary filtrate
Fig 6.21
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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