TRANSCYTOSIS
Transcytosis is a transport mechanism
in which an extracellular macromolecule enters through one side of a cell,
migrates across cytoplasm of the cell and exits through the other side.
Mechanism of
Transcytosis
Cell encloses the extracellular
substance by invagination of the cell membrane to form a vesicle. Vesicle then moves
across the cell and thrown out through opposite cell membrane by means of
exocytosis. Transcytosis involves the receptor-coated pits as in
receptor-mediated
endocytosis. Receptor protein coating the pits in this process is caveolin
and not clathrin. Transcytosis is also called, vesicle trafficking
or cytopempsis.
Transcytosis plays an important role in selectively transporting the substances
between two environments across the cells without any distinct change in the composition
of these environments. Example of this type of transport is the movement of proteins
from capillary blood into interstitial fluid across the
endothelial cells of
the capillary. Many pathogens like human immuno deficiency virus
(HIV) are also transported by this mechanism.
MOLECULAR MOTORS
Molecular motors are
the protein-based molecular machines that perform intracellular movements in
response to specific
stimuli.
FUNCTIONS OF
MOLECULAR MOTORS
1. Transport of
synaptic vesicles containing neurotransmitters from the nerve cell body to
synaptic
terminal
2. Role in cell
division (mitosis and meiosis) by pulling the chromosomes
3. Transport of viruses and toxins to
the interior of the cell for its own detriment.
TYPES OF MOLECULAR
MOTORS
Molecular motors are
classified into three super families:
1. Kinesin
2. Dynein
3. Myosin.
1. Kinesin
Kinesin transports
substances by moving over the microtubules. Each kinesin molecule has two heads
and a tail portion. One of the heads hydrolyses ATP to obtain energy. By
utilizing this energy, the other head swings continuously causing movement of the
whole kinesin molecule . End portion of the tail carries the
cargo (substances to be transported). Kinesin is responsible for anterograde transport (transport
of substances towards the positive end of microtubule).
2. Dynein
Dynein is almost similar to kinesin
and transports substances by moving over the microtubules. But
it
is responsible for retrograde transport (transport
of substances towards the negative end of microtubule).
3. Myosin
Myosin transports substances
by moving over micro filaments. Myosins are classified into 18 types according to
the amino acid sequence. However, myosin II and V are functionally significant.
Myosin II is involved in muscle contraction . Myosin V is involved in transport of
vesicles.
ABNORMALITIES OF
SODIUMPOTASSIUM PUMP
Abnormalities in the number or
function of Na+-K+ pump are associated
with several pathological conditions. Important examples are:
1. Reduction in either
the number or concentration of Na+-K+ pump
in myocardium is associated with
cardiac failure
2. Excess reabsorption
of sodium in renal tubules is associated with hypertension.
CHANNELOPATHIES OR
ION CHANNEL DISEASES
Channelopathies or ion channel
diseases are caused by mutations in genes that encode the ion channels.
1. Sodium Channel
Diseases
Dysfunction of sodium channels leads
to muscle spasm and Liddle’s
syndrome (dysfunction of sodium channels in kidney resulting
in increased osmotic pressure in the blood and hypertension).
2. Potassium
Channel Diseases
Potassium channel dysfunction
causes disorders of heart, inherited deafness and epileptic seizures in
newborn.
3. Chloride
Channel Diseases
Dysfunction of chloride channels
results in formation of renal stones and cystic fibrosis. Cystic
fibrosis is
a generalized disorder affecting the functions of many organs
such as lungs (due to excessive mucus),
exocrine glands like pancreas, biliary
system and immune system.
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