Transmembrane
Transportation
[A]
(1)
Passive method: Energy independent.
Simple diffusion:
It is the random movement of particles along their concentration gradient. It is governed by:
(i)
Particle size
(ii)
Concentration gradient
(iii)
whether the particles are ions or nonpolar molecules
(a)
Ions: Diffusion rate of ions depends on the potential gradient across the membrane.
(b)
Nonpolar molecules: Diffusion rate depends on its solubility in the bimolecular
phospho-
lipids layers of the plasma membrane.
[Osmosis is a specialized form of diffusion]
(2)
Facilitated diffusion:
Carrier molecules (made of proteins) are located in plasma membrane and transport specific particles
across the plasma membrane from high to low concentration. It is faster than simple diffusion.
Facilitated diffusion has selective manner.
Example: Transport of glucose from plasma into erythrocytes is by facilitated diffusion.
[B]
Active Process: Energy dependent ; requires consumption of ATP.
Active transport is faster in aerobic condition; slower in anaerobic condition or when respiratory inhibitor
is present.
(1)
Active Transport: This is the transmembrane transport of a substance against concentration gradient. A specific carrier molecule( made of proteins) take up and combine temporarily with the
substance to be transported to form a complex. The carrier then release the substance on the other
side of the plasma membrane. Since specific carrier is involved, active transport has a selective
manner.
e.g.
absorption of ions by root hairs from soil water.
absorption of nutrients by ileum.
reabsorption of glucose in the proximal tubule of nephron.
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(2)
ENDOCYTOSIS
This involves the bulk transport of materials through membrane into a cell.
(a)
Phagocytosis ("Cell eating")
It is bulk transport of solid material into a cell.
e.g. Amoeba ingests food particles
Mammalian phagocytes ingest pathogens
Kupfer cell of mammalian liver ingests aged erythrocytes.
When a phagocytic cell approaches a solid particle, its plasma membrane pushes out to form
pseudopodia to enclose the solid. The tips of the pseudopodia then fuse, forming a phagocytic
vesicle enclosing the solid food. Lysozomes then fuse with it to form a vesicle in which intracellular digestion of the solid is carried out.
The end product of digestion is absorbed into cytoplasm by active transport and diffusion.
(b)
Pinocytosis ("cell drinking")
It is bulk transport of liquid into a cell.
e.g.
Epithelial cell of ileum villus incorporate the digestive end products in the ileum lumen.
Epithelial cell of nephron proximal tubule reabsorb the fluid in its lumen.
Plasma membrane of such cell invaginates forming a pinocytic channel. The channel is then cut
off from the plasma membrane to give a pinocytic vesicle. Lysozome approaches and carries out
intracellular digestion as mentioned in phagocytosis.
(3)
EXOCYTOSIS
This is the bulk transport of materials out of a cell.
(a)
Exophagocytosis
e.g. Undigested remains from phagocytic cell is egested.
(b)
Exopinocytosis
e.g. secretion from Golgi body.
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Transmembrane Transport
1996 Edition
OSMOSIS
Osmosis is a passive process by which water molecules move from a region of higher water potential through a
selectively permeable membrane to a region of lower water potential.
THIS CONCEPT IS ONLY ACCEPTED BY BOTANISTS
Chemical Potential of a substance is the measure of free energy of the substance. This is related to the linear,
circular and vibrational motion of their ions/ molecules.
Chemical potential of pure water at 1 atmsphere and room temperature is arbitarily set as zero. This is referred
to water potential of pure water.
Water Potential of a solution is the difference in free energy between the water molecules of the solution and
those of pure water.
Water potential of a solution is made of two components namely solute potential and pressure potential.
Solute Potential is a component of water potential due to the presence of dissolved substances. Formally,
solute potential is called osmotic potential. Usually dissolved substances attract water molecules and
weaken their motion. Therefore dissolved substances lows down water potential. Solute potential is always negative value.
Pressure Potential is a component of water potential due to the hydrostatic pressure.
High hydrostatic pressure renders the water molecules move more vigorously and hence increases the
water potential of a solution. Pressure potential is usually positive value except under highly reduced
pressure.
Plasmolysis is a phenomenon that the protoplast of a plant cell shrinks and detaches from its cell wall due to loss
of water to its environment which has a much lower water potential than that of the plant cell sap.
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Transmembrane Transport
1996 Edition
Incipient plasmolysis is the phenomenon that the protoplast of a plant cell is just out of contact with the cell wall.
Protoplast is the living parts of a plant cell. It consists of plasma membrane, cytoplasm, nucleus and vacuoles.
plant cell = cellulose cell wall + protoplast
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