Enhancement of Transport Selectivity through Nano-Channels ...
Transport across the cell membrane - WordPress.com...Carrier mediated transport system display three...
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Transport across the cell membrane
Learning objectives
Body compartments
ECF and ICF Constituents
• Barrier to water and water-soluble substances
ions glucoseH2O
urea
Lipid Bilayer:
CO2
O2N2
halothane
Cell Membrane
ions
glucose
H2O
urea
… but, other molecules still get across!
CO2
O2N2
halothane
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
Types of Transport
• Passive and Active
• Passive
• Simple Diffusion
• Facilitated Diffusion
• Active
• Primary
• Co-Transport
• Counter Transport
• Secondary
(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
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
Factors that affect net rate of diffusion……. cont'd
1. Membrane electrical potential-
Nernst Potential
1. Pressure difference across the
membrane
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
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
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
Selective permeability of protein channels
Gating of protein channels
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
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
Osmosis
The process of net movement of water caused by a concentrationdifference of water, across a selectively permeable membrane is called osmosis
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
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
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
• Energy is required for active transport
Active transport is divided into two types according to the source of theenergy used
❑ Primary active transport
❑ Secondary active transport
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
Na-K PUMP
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
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
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.
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
Counter Transport
• Sodium- Calcium counter-transport
occurs through all or almost all
Cell membranes
• Sodium- Hydrogen counter transport
in proximal tubules of kidneys
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