Transport Mechanism
Membrane transport refers to the transport of various substances such as ions
and small molecules across through biological membranes which are proteins
embedded lipid bilayers, membrane transport occur by several mechanisms that
regulate the passage of solutes.
Cell Membrane
Cell membranes are selectively
permeable. It allow selected, specific compound to enter or leave the cells.
It maintaining normal functioning of
cell through biological membranes.
Read here, Fluid mosaic model of Bio membrane
structure by Singer and Nicolson.
Bio membrane or Cell membrane plays
an essential role in transport of -
1. Solutes
2. Ions
3. Small molecules
4. Movement of compounds
5. Sugar
6. Steroids
7. Amino acids
8. Fatty acids
Membrane Transport Function
Membrane Transport is essential because
of key functions include -
1. It maintain PH and ionic
concentration within cell.
2. It secretes useful substance or
excretes toxic waste substances.
3. It exchange of material is
necessary to maintain cell function.
4. It obtain nutrients supply for
energy and raw material for cell.
5. It keeps balance of salts,
nutrients, and protein within range that keep cells, tissue, and organism live.
6. It generates an ionic gradient; ion movement, essential for muscular and nervous activities.
7. Bio membrane protects cell's
internal environment.
Types of Membrane Transport
There are different types of
transport mechanism across bio membrane which are-
1. Passive Transport
2. Active Transport
3. Bulk Transport
1. Passive Transport
Passive transport is a type of
membrane transport that does not require cell energy to move substances across
plasma membranes.
The four main kinds of passive
transport are diffusion and osmosis.
A. Diffusion
Diffusion is passive transport, the
type of diffusion are -
1. Passive diffusion
2. Filtration
3. Dialysis
4. Facilitated Diffusion
B. Osmosis
2. Active Transport
A. Sodium pump
3. Bulk Transport
Bulk membrane transports are -
A. Endocytosis
B. Exocytosis
1. Diffusion
Diffusion is the process where in
molecules move from a region of higher concentration to region of lower
concentration.
A. Passive Diffusion
Passive transport is type of membrane transport that doesn’t require energy to move substance across cell membranes.
Two key points -
1. Cell membrane passive role in
transfer of substance means No use of cell energy.
2. It doesn’t require semi-permeable
membrane.
“Passive diffusion is the process by
which molecule diffuse from a region of higher concentration to region of lower
concentration.”
Diffusion of substance occurs through
channels or pore. It is most important mechanism for passage of drug through
membrane.
Rate of Diffusion
Rate of Diffusion depends upon
following factor –
1. Number of particle per unit volume
2. Density of medium
3. Temperature
4. Pressure
5. Distance
6. Water soluble, solutes and ions;
K, Cl, HCO3 diffuse through tiny pore in plasma
membrane by electrochemical gradient; ion movement across membrane.
7. Molecular weight of diffusing molecules.
8. Gases always diffuse from region
of higher partial pressure to region of low partial pressure.
9. Plasma membrane is permeable to
both O2, CO2 gases due to presence of pores or channels in it.
10. Diffusion helps in respiratory
gas exchange; O2 and CO2 between tissue cells and environment.
11. Diffusion also involve in
movement of solutes and ions from one cell to another cell and also from cells
to extracellular fluids.
B. Filteration
Filteration is types of membrane transport.
Filtration is the process in which
solid particles in liquid or gaseous fluid are removed by use of filter medium;
semi permeable membrane under pressure that permits the fluid to pass through
but hold the solid particles.
Two major points -
1. It Require semi permeable membrane
2. Filteration is diffusion; movement
of molecules, substance through semi permeable membrane under pressure.
Basic Requirements for Filteration
are -
1. Filter medium
2. Fluid with suspended particle;
solids
3. Driving force; pressure
4. Mechanical device that hold
filter, medium contains fluid and permit application of force.
Example – Ultrafilteration; separation
process occurs in Malpighian tubules of kidney.
Ultrafilteration occurs when fluid
passes across a semi permeable membrane; a membrane that allow some substance
to pass through due to a driving pressure.
In glomerular capillaries due to High
pressure, cell-free and protein free-blood to flow into Bowman's capsule through
capillary endothelium and tubular endothelial. Blood is left only with corpuscles,
plasma proteins and fats.
Filtered fluid is called glomerular filterate;
it contains water urea, amino acids, sodium, and potassium ion, glucose.
Glomerular filtrate undergo tubular
reabsorption and tubular secretion to formation of urine and finally excreted
out.
C. Dialysis
Dialysis is a
separation process procedure to removing waste
products and excess fluid from the blood when the person’s both kidneys has lost their
renal function or kidney stop working properly. It often
involves diverting blood to a machine to be cleaned.
Type of Dialysis
Kidney dialysis is two main type-
1. Haemodialysis
2. Peritoneal dialysis
D. Facilitated Diffusion
Facilitated diffusion is selective
process; membrane allows only selective molecules and ion to pass through it
without consuming any cellular energy and prevent other molecules from passing
through membrane. It requires a carrier molecule for transport across the
membrane.
Carrier protein is integral proteins
of membrane and is highly specific.
“Facilitated Diffusion refers to
transport of solute through carrier protein molecules transport membrane without
consuming any cellular energy. It is passive movement of molecule along down
concentration gradient”.
Material move across membrane from
the region of high concentration to low concentration without consuming any
cellular energy, it is downhill movement.
Examples of facilitated diffusion-
1. Glucose Transporter (GLUT)
2. Cl/HCO3 Exchangers
3. ATP/ADP Translocase
2. Osmosis
For osmosis process semi-permeable
membrane is necessary.
“Osmosis is special type of diffusion
of solvent (water) molecule through a semi permeable membrane from
region of higher solvent concentration to the region of lower solvent
concentration.”
Osmotic depends upon the number of presence
of solute particles but solute particle cannot pass through the membrane.
Note - Osmosis applicable only to solvent part of solution and it always
operates in liquid medium. For osmosis semi permeable membrane is must.
Osmotic Pressure
Osmotic pressure or osmotic potential
is minimum pressure which needs to be applied to a solution to prevent the
inward flow of its pure solvent across a semi permeable membrane.
O
Osmotic Pressure = Applied Pressure
to stop Osmosis
Types of Osmosis
The three types of osmotic conditions include-
1. Isotonic
2. Hypotonic
3. Hypertonic
1. Isotonic solution
Isotonic condition occur when
concentration of extracellular fluids is same that of intracellular fluid. In
this condition both fluids have same osmotic concentration or pressure.
Result No osmosis takes place, and
these are called isotonic solution. The cell neither shrinks nor swells.
Two solutions having same osmotic
pressure across semi permeable membrane is referred to as an isotonic solution.
Example of isotonic solution is –
RBC placed in 0.9% NaCl solution
RBC placed in 5% glucose solution
2. Hypotonic solution
If cell is placed in a hypotonic solution,
the net flow of water into the cell, cell will gains water by Endosmosis. Water move into
cell and gain volume.
When extracellular fluid has low
osmotic concentration than the intracellular fluid; solute concentration outside the cell is lower than inside
the cell, and the solutes cannot cross the membrane, then that solution is
hypotonic to the cell.
Water moves from an area of higher water concentration to one of
lower concentration and flow of water into the cell.
Hypotonic solution is used to give
fluid intravenously to patient to treat or avoid dehydration.
Example, in hypotonic solution Red
blood cell, fill up with water and swells then burst.
3. Hypertonic solution
When extracellular fluid has high
osmotic concentration than the intracellular fluid; solute concentration outside the cell is higher than inside
the cell, movement of
water outside the cell. Cell will lose water by Exosmosis.
If cell placed in hypertonic solution,
it will shrinks called plasmolysis; cell lose water.
For example - in hypertonic medium Red blood
cell become shrinken.
Hypertonic solutions are used for the soaking wounds.
Check out here the difference between Osmosis and Diffusion
2. Active Transport
1. Active Transport is process of
transporting large sized ions or molecules across a membrane, which is insoluble
in phospholipids, with the help of carrier molecules from low concentration to
high concentration using cellular ATP energy.
2. It requires semi-permeable
membrane.
3. It uses cellular Adenosine Triphosphate (ATP) energy.
4. This transport mechanism work
against concentration gradient and is more rapid than diffusion.
5. It is an uphill movement of
substance; from a region of lower concentration to higher
concentration.
6. Carrier protein binds to the
solute such as glucose, calcium, amino acid and transports it to other
side of membrane.
7. Actively transported substances
through cell membrane are ions of sodium, potassium, chloride, calcium, iron,
iodine, glucose and amino acid.
8. Active transport is known as primary active transport.
Role of Active Transport
Active membrane transport plays a
very important role in –
1. Conduction of Nerve impulse
2. Muscle contraction
3. Urine formation in kidney
Sodium-Potassium Pump
Sodium-Potassium Pump also known as Na+/K+ pump or Na+/K+-ATPase.
Active Transport occurs on the basis
of pumping action, one substance out of cell and other substance inwards cell
with the help of cellular energy. It results in the accumulation of substances
within cell in higher concentration.
Active channel sodium pump is primary
active transport system; It is sodium-potassium exchange pump that works in
plasma membrane of cells to adjust the concentration of sodium (Na+) and potassium (K+) ion inside and
outside the cells.
Active Transport is energy- requiring
process of pumping molecules and ions across membrane against a concentration
gradient. It transfers Na+ and K+ ion against their concentration gradient.
Active Transport process help
maintain Homeostasis.
How does the Sodium-Potassium Exchange Pump Works?
1. Sodium-potassium exchange pump
operates with carrier protein transport membrane with the help of enzyme
ATPase. Carrier protein is needed for Sodium-Potassium Exchange function.
2. 3 sodium (Na+) ions bind with
protein pump inside cell, and 2 potassium (K+) ions are bind with protein pump
outside cell. Carrier protein then get energy from ATP and change shape.
3.
It pumps the 3 Na+ ions out of cell and 2 K+ ions pumped into the cell.
4. Cell maintains high concentration
of Na+ ions outside plasma membrane than inside it.
5. Electrical events in nerve fibres
are controlled by differential permeability of plasma membrane to Na+ and K+
ions.
6. Cell membrane is more permeable to
K+ ion than to Na+ ions entering it.
7. This leads to increase positive (+) charge on
outside cell membrane and negative (-) charge on inside of cell membrane.
8. This condition makes resting nerve
fibre "polarize"; neuron is not stimulated.
9. Entry of Na+ leads to
depolarization of nerve membrane; Na+ ions flow
into the cell.
Whole process of pumping out Na+ ion
and pumping in K+ ion into nerve fibre called Na+ / K+ exchange pump.
Na+/K+ exchange pump also help to
control the water contents. It involves counter current mechanisms to excrete
hypertonic urine for conserving body water. Used to concentrate urine in kidney
by nephrons.
Factor Affective Active Transport
1. Carrier protein
2. Formation of ATP
3. Temperature
4. Inhibitors and toxic substance
Significance of Active Transport
1. Maintain ionic concentration and
osmotic pressure.
2. Enabling
cells to accumulate molecules or ions against the concentration gradient.
3. Enable flow of stimulus, maintains
differential potential inside and outside nerve membrane and muscle cells.
4. Controlling cell osmo-regulation,
maintaining osmotic balance; salt and water balance.
5. Maintain high K+ salt
concentration required for protein synthesis, glycolysis and physiological
process.
6. Helps cells
maintain cell homeostasis.
What are the differences between Active and Passive transport?
3. Bulk Transport
Bulk transport is a type of membrane transport, involves the
transport of macromolecules and large amount of substance such as solid food particles, lipid
droplets, across plasma membrane by utilising cell energy.
1. It transport macromolecules by
vesicles formation.
2. Material is transported in
vesicles of the membrane, called carrier vesicles.
3. It requires energy from cell ATP,
Ca+ ions and cytoskeleton system support.
4. Macromolecules are transported like
proteins, fat droplet, polysaccharides, and polynucleotide.
Types of Bulk Transport
Bulk transport involve two Process-
1. Endocytosis
2. Exocytosis
1. Endocytosis
Endocytosis inside cell, protein from
RER to Golgi complex, substance brought into cell.
Three methods of endocytosis are -
A. Pinocytosis
B. Phagocytosis
C. Receptor-mediated endocytosis
A. Pinocytosis
1.
Liquid droplets are ingested into cell, also called
drinking of cell.
2. Bulk transport of fluid matter and
substances dissolved in it (sugars, ions, amino acids) across the cell membrane
by forming detachable vesicles called pinosome.
3. In pinocytosis is a cellular process in which substances are brought inside a cell. Cell
membrane folds and creates small pockets and captures the cellular fluid and
dissolved substances like ions sugars, amino acids.
B. Phagocytosis
1. Also called eating of cells.
2. Phagocytosis
is a cellular process for ingesting and eliminating solid matter, larger particles, including microorganisms, foreign substances, and
apoptotic cells.
3. It transport of solid matter like food,
foreign particles, pathogens (bacteria, viruses) across membrane by forming
detachable vesicles called phagosome.
2. Exocytosis
1. Exocytosis process is reverse of
endocytosis and called celvomiting or ephagy.
2. In Exocytosis process, excretion of the substances with the help of the cell membrane. Substances are transported outside of cell.
3. It is process of exudating (emitted through pores) secretary or waste products outside the cell through plasma membrane. Exudated products include proteins, hormones, enzymes, undigested matter.
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