physio membrane transp (4) - mt. sac...diffusion through a cell membrane • diffuse rapidly –...
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Membrane TransportMembrane TransportMembrane Transport
By Dr. Carmen Rexach
PhysiologyMount San Antonio College
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Passive vs. Active• Passive transport
– simple diffusion– Osmosis– filtration– facilitated diffusion
• Active transport– primary active transport– secondary active transport
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Simple diffusion• Net movement of molecules from
an area of high concentration to an area of low concentration by random molecular motion
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Diffusion through a cell membrane
• Diffuse rapidly– Nonpolar molecules– small molecules with polar covalent bonds– Gases– Examples: CO2, oxygen, fatty acids, steroid
hormones
• usually do not diffuse– large polar molecules– Ions– Examples: phosphorylated organic molecules
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Rate of diffusion• Flux = amount of material crossing a surface per
unit of time• Net flux = difference between one-way flux
• Influenced by– Magnitude of concentration difference– Mass of the molecule– Temperature– Medium (gas, liquid, solid)– surface area of the membrane
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Rate of diffusion through membrane
• Net flux (F) across the membrane:– F=kpA(Co-Ci)– Kp=permeability constant
• Dependent on temperature and permeability of membrane
– A = surface area– (Co-Ci) = concentration difference
Remember: The major limiting factor in diffusion across the membrane is its hydrophobic nature!
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Ions diffuse through protein channels
• Channels formed by integral membrane proteins
• Selective– Channel diameter– Charged and polar protein surfaces
• Regulation by channel gating• Ligand gated• Voltage gated• Mechanically gated
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Diffusion and the electrical gradient• Important consideration in ion diffusion• Membrane separates electrical charges
– Same charges repel– Opposite charges attract
More information on this soon!
There are two driving forces in the movement of molecules acrossthe membrane: the concentration gradient (chemical) and the electrical gradient (electrical). Together they are called the electrochemical gradient.
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Osmosis
• The movement of water from an area of high water concentration to an area of low water concentration across a semi-permeable membrane
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Osmolality• Mole = molecular weight of an element or compound
measured in grams• Molality = # of moles of solute per kg of solvent• Osmolality = ionic concentration of dissolved
substances per kg of solvent– Osmolality of plasma = 300 mOsm = isosmotic– depends on the number of solute particles
• 1 m of C6H12O6 = 1 osmole (Osm)• 1 m of NaCl = 2 osmoles• 1 m of CO2 + 1 m of KCl = 3 osmoles
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Osmolarity vs Osmolality
• Osmolarity refers to the number of solute particles per 1 L of water
• Osmolality refers to the number of solute particles per 1 kg of water
• Since the mass of water is independent of temperature, osmolality is the preferred term for biological systems.
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Tonicity
0.85% 0.85% 0.85%
0.85% 10 % 0.02%
Isotonic=300mOsm
Hypertonic = >300mOsm
Hypotonic = <300mOsm
The NaCl concentration in most cells is 0.85%. This corresponds to 300mOsm of nonpenetrating solutes.
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Effect of tonicity of solutions on red blood cells
hypotonic isotonic hypertonic
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Filtration• Movement of a fluid and solutes down the
pressure gradient (from high pressure to low pressure) across a membrane or filter
• What is filtered depends on the amount of pressure and the size of the pores in the filter– In kidney:
• Normally, protein is not filtered into the urine because of size and charge restrictions
• Glomerulonephritis: Inflammation enlarges the size of “pores” and allows protein to move out into the filtrate
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Filtration
Fliltrate forms as substance moves through filterForce: gravity (atm pressure)Restriction: size of filter pores
Filtrate forms as fluid and other substances are forced through capillary fenestrations.Force: blood pressureRestriction: size of the fenestrations
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Carrier mediated transport• Protein carriers transport molecules
too large or polar across the plasma membrane
• 4 characteristics of ligand/protein binding– chemical specificity– competition– saturation– affinity
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Chemical specificity
• Only molecules with the requisite chemical structure are transported
• Specificity is not absolute
Proteincarrier
A
B Protein carrier
Proteincarrier
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Competition
• Structurally related molecules may compete for transport
• Competition decreases transport rate
Proteincarrier
A
B
C
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Saturation• Transport system is saturated when
all of the binding sites are occupied • At that point, the rate of transport
can not increase.
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Affinity
• The attraction of the carrier protein for the transport substrate
• Transport can be inhibited by compounds or ions which alter the shape of the binding site.
+++ + - - +- - - - - - - - - -
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Carrier mediated transport
1) Solute binds to transporter protein (ligand/protein bindingrules)
2) Protein changes shape, moving solute to opposite side of the membrane.
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Facilitated diffusion
• Movement of substances with the concentration gradient
• Example– Glucose across plasma membrane– Aquaporins transporting water
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Water movement and aquaporins
• Two methods for diffusion of water in and out of cells– Diffusion across lipid bilayer– Aquaporins
• Usually transport solute-free water• About 10 different types of aquaporins• One will transport about 3 billion H2O
molecules per second
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Active transport
• Movement against the concentration gradient
• Requires the expenditure of energy• Often called pumps• two types
– primary– secondary
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Active transport = against the concentration gradient
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Primary active transport
• Steps– molecule or ion binds at recognition site– carrier protein is phosphorylated– protein changes shape– transported molecule flipped to other
side of membrane– phosphate removed enzymatically,
releasing transported molecule
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Primary active transport
PO4
PO4
ATP ADP
PO4
1 2
3 4
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Major active transport pumps
• Na+K+ATPase pump• Ca++ATPase pump• H+ATPase pump• H+K+ATPase pump
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Na+K+ATPase pumps• 3 Na+ out & 2 K+ in• Functions:
– Sets stage for secondary active transport– gradient for electrical impulses in
nerve/muscles
= Na+
= K+
ATPase
ATP ADP
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Na+K+ATPase pump
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Secondary active transport
• Energy provided by Na+ gradient – not directly by ATP
• co-transport– “hitchhiking”– Uses symport
• counter-transport– “revolving door”– Uses anaport
Na+
Na+
Ca++
K+
ATP
ADP
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Symport
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Summary of passive and active transport mechanisms
Against concentration gradient
With the concentration gradient
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Exocytosis and Endocytosis• Circumvents need to pass through
plasma membrane– Allows membrane impermeable molecules
to pass– Exocytosis: Adds to membrane
• Also energy requiring transport mechanisms
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Epithelial transport
• Paracellular pathway• Transcellular pathway
– Luminal and basolateral membranes do not have same permeability or transport characteristics
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Alternative functions of endocytosis:
1. Transcellular transport
2. Endosomal processing
3. Recycling the membrane
4. Destroying engulfed materials
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