The
cell membrane also plays a role in anchoring the cytoskeleton to
provide shape to the cell, and in attaching to the extracellular
matrix and other cells to hold them together to form tissues. The
cell membrane is selectively permeable and able to regulate what
enters and exits the cell, thus facilitating the transport of
materials needed for survival.
INTRACELLULAR
TRANSIT: The
movement of substances across the membrane can be either "passive",
occurring without the input of cellular energy, or "active",
requiring the cell to expend energy in transporting it. The membrane
also maintains the cell potential. The cell membrane thus works as a
selective filter that allows only certain things to come inside or go
outside the cell. The cell employs a number of transport mechanisms
that involve biological membranes: Passive osmosis and diffusion:
Some substances (small molecules, ions) such as carbon dioxide (CO2)
and oxygen (O2),
can move across the plasma membrane by diffusion, which is a passive
transport process. Because the membrane acts as a barrier for
certain molecules and ions, they can occur in different
concentrations on the two sides of the membrane. Such a concentration
gradient across a semipermeable membrane sets up an osmotic flow for
the water. Transmembrane protein channels and transporters:
Nutrients, such as sugars or amino acids, must enter the cell, and
certain products of metabolism must leave the cell. Such molecules
diffuse passively through protein channels such as aquaporins (in the
case of water (H2O))
in facilitated diffusion or are pumped across the membrane by
transmembrane transporters.
ENDOCYTOSIS:
Endocytosis is the process in which cells absorb molecules by
engulfing them. The plasma membrane creates a small deformation
inward, called an invagination, in which the substance to be
transported is captured. The deformation then pinches off from the
membrane on the inside of the cell, creating a vesicle containing the
captured substance. Endocytosis is a pathway for internalizing solid
particles ("cell eating" or phagocytosis), small molecules
and ions ("cell drinking" or pinocytosis), and
macromolecules. Endocytosis requires energy and is thus a form of
active transport.
EXOCYTOSIS:
Just as material can be brought into the cell by invagination and
formation of a vesicle, the membrane of a vesicle can be fused with
the plasma membrane, extruding its contents to the surrounding
medium. This is the process of exocytosis. Exocytosis occurs in
various cells to remove undigested residues of substances brought in
by endocytosis, to secrete substances such as hormones and enzymes,
and to transport a substance completely across a cellular barrier.
In the process of exocytosis, the undigested waste-containing food
vacuole or the secretory vesicle budded from Golgi apparatus, is
first moved by cytoskeleton from the interior of the cell to the
surface. The vesicle membrane comes in contact with the plasma
membrane. The lipid molecules of the two bilayers rearrange
themselves and the two membranes are, thus, fused. A passage is formed
in the fused membrane and the vesicles discharges its contents
outside the cell.
REFERENCES;
1. TISSUES
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