Enhancement of Transport Selectivity through Nano-Channels ...
Transport across the cell membrane - WordPress.com...Carrier mediated transport system display three...
Transcript of Transport across the cell membrane - WordPress.com...Carrier mediated transport system display three...
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Transport across the cell membrane
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Learning objectives
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Body compartments
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ECF and ICF Constituents
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• Barrier to water and water-soluble substances
ions glucoseH2O
urea
Lipid Bilayer:
CO2
O2N2
halothane
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Cell Membrane
ions
glucose
H2O
urea
… but, other molecules still get across!
CO2
O2N2
halothane
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Molecular Gradients
Na+
K+
Mg2+
Ca2+
H+
HCO3-
Cl-
SO42-
PO3-
protein
inside
(in mM)
14
140
0.5
10-4
(pH 7.2)
10
5-15
2
75
40
outside
(in mM)
142
4
1-2
1-2
(pH 7.4)
28
110
1
4
5
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Types of Transport
• Passive and Active
• Passive
• Simple Diffusion
• Facilitated Diffusion
• Active
• Primary
• Co-Transport
• Counter Transport
• Secondary
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(a) lipid-soluble molecules move readily across the membrane
(rate depends on lipid solubility)(b) water-soluble molecules cross via channels or pores
(a) (b)
Simple Diffusion
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Fick's law of Diffusion
Several factors affect the rate of net diffusion across the membrane
• The magnitude of the concentration gradient (∆C)
• The permeability of the membrane to substance (P)
• Surface area of the membrane (A)
• Molecular weight of the substance (MW)
• Distance (thickness) ∆X
• Net rate of Diffusion Q = ∆C .P.A
MW. ∆X
Restated=
Q ∆C.A.D P =Diffusion coefficient (D)
∆X √ MW
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Factors that affect net rate of diffusion……. cont'd
1. Membrane electrical potential-
Nernst Potential
1. Pressure difference across the
membrane
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Nernst Potential:
At normal body temperature, the electrical difference
that will balance a given concentration difference of univalent ions-
such as Na ions- can be determined from the formula
called Nernst equation
EMF (in millivolts) = ±61 log C1/C2
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Diffusion Through Protein Channels and “Gating” of These Channels
• Pores Integral cell membrane proteins that form open tubes through the membrane and are always open
Characteristics of protein channels:
• Selective Permeability( due to diameter, shape & charge on channel)
• Gating
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Ungated
• determined by size, shape, distribution of charge, etc.
Characteristics:
Na+
in
outNa+ and other ions
Gated
• voltage (e.g. voltage-dependent Na+ channels)
• chemically (e.g. nicotinic ACh receptor channels)
Ion Channels
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Selective permeability of protein channels
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Gating of protein channels
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Facilitated Diffusion
• Carrier mediated diffusion• A carrier facilitates diffusionof substance to the other sidedownhill.• The rate of diffusion
approaches a maximum called Vmax, as conc of substance increases
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Carrier mediated transport system display three important characteristics
1. Specificity - Transport selectivity, Defective transport system for cysteine-
an inherited disease -cysteine urea
2. Saturation –Limited number of carrier binding sites- transport maximum (Tm)
3. Competition- Closely related substances may compete for a ride across
the membrane on the same carrier
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Osmosis
The process of net movement of water caused by a concentrationdifference of water, across a selectively permeable membrane is called osmosis
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Osmotic pressure
The amount of pressure required to stop osmosis is called osmotic pressure of any particular solution
• Osmotic pressure is determined by number of particles per unit volume of fluid, not by the mass of particles
• 1 osmole is 1 gram molecular weight of osmotically active solute ( 180 gof glucose which is 1 g molecular weight of glucose is 1 Osmole
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Osmolality
• Osmolality given in Osmoles
• One osmole is 1 gram mol. wt. of undissociated solute
• One osmole of Glucose (180 gm)
• Two osmoles NaCl (58.5 gm) (2 ions)
• Osmolality is Osmoles/1kg of solvent
• Osmolarity is Osmoles/1L of solution
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Active Transport
When a cell membrane moves molecules or ions “uphill” against the concentration gradient ( or uphill against an electrical or pressuregradient), the process is called active transport.
Examples:Sodium, potassium, Calcium, hydrogen, chloride , urate ,some amino acids etc
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• Energy is required for active transport
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Active transport is divided into two types according to the source of theenergy used
❑ Primary active transport
❑ Secondary active transport
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Primary Active Transport
Energy is derived directly from breakdown of ATP (Adenosine triphosphate)
➢ Sodium potassium pump ➢ Substances such as sodium, potassium, calcium, hydrogen, chloride
and few other ions are transported by primary active transport
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Na-K PUMP
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Functions of sodium potassium pump
Control volume of each cellWithout this function , most cells would swell until they burst.
Electrogenic nature of Na-K pump:It causes negativity inside and positivity outside- cause an electric potential Across the cell membrane
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Primary active transport of Calcium ionsTwo Calcium pumps- one in cell membrane and other in intracellular
organelle (sarcoplasmic reticulum in muscle cell) maintain very lowintracellular calcium concentration
Primary active transport of Hydrogen ions:• Gastric glands of the stomach• Late distal tubules and cortical collecting ducts of the kidney
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Secondary Active Transport
In secondary active transport, the energy stored due to
an ion gradient is used to transport another substance.
e.g
A large gradient (storehouse of energy) for sodium is created when it
moves to outside the cell membrane by primary active transport.
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Types of active transport
Co- transport
Co transport of glucose and
amino acid along with
Sodium ions in
• Epithelial cells of the
intestinal tract
• Renal tubular epithelial cells
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Counter Transport
• Sodium- Calcium counter-transport
occurs through all or almost all
Cell membranes
• Sodium- Hydrogen counter transport
in proximal tubules of kidneys
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Active transport through cellular sheets
Substances must transfer through
cellular sheets.
• Intestinal epithelium
• Epithelium of renal tubules
• Epithelium of all exocrine glands
• Epithelium of gall bladder
• Membrane of choroid plexus of
brain
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