Describe physiochemical nature of plasma membrane in the cell
All cells are
enclosed in a membrane that serves as their outer boundary, separating the
cytoplasm from the external environment; this membrane is known as plasma
membrane. It allows the cell to take up and retain certain substances while
excluding others.
All biological
membranes have the same basic molecular organisation. They consist of a double
layer of (bilayer) of phospholipids interspersed with proteins. The
phospholipids molecules in the plasma membrane are arranged in two parallel
layers. Their non polar hydrophobic ends face each other, whereas their polar
hydrophilic ends are associated with carbohydrate protein etc. Plasma membrane
also contains several types of lipids like cholesterol. In certain animal cells
cholesterol may constitute upto 50% of the lipid molecules in plasma membrane.
It is absent from plasma membrane of most plat and bacteria cells.
In 1972 Singer
and Nicolson proposed working model of plasma membrane known as fluid mosaic
model. In this model the lipid bilayer is retained as the core of the membrane.
These fluid molecules are present in a fluid state capable of rotating and
moving laterally within the membrane. The structure and arrangement of
membrane.
Proteins in the
fluid mosaic model are like ice bergs in the sea. Proteins occur as a mosaic of
discontinuous particles that penetrate deeply into and even completely through
the lipid sheet. The components of the plasma membrane are mobile and capable
of coming together to engage in various types of semi permanent interactions.
Proteins
associated with lipid bi-layer can be divided into two large groups,
(a) Integral
Proteins (Intrinsic Proteins):
A class of
proteins that are directly incorporated within the lipid bi-layer. Some of
these proteins are believed to provide a channel through which water soluble
substances such as lions, can pass back and forth between the extra cellular
and intracellular compartment.
(b) Peripheral
Proteins (Extrinsic Proteins):
A class of
protein located entirely outside of the lipid bi-layer or either the extra
cellular or cytoplasmic surface, exhibit a loose association eith membrane
surface. These proteins which may possess lipid (lipprotein) or carbohydrates
(Glycoproteins and Protoglycon) side chains are arranged as mosics within cell
membrane.
The plasma
membrane performs several function but the main functions are Protection of
cell cytoplasm and to regulate the flow of solution and material in and out of
the cell with certain limitation. The limitations or check flow across the
membrane is called differential or selective permeability. Transport across
membranes is necessary to maintain suitable pH ionic concentration for enzyme
activity and excrete toxic substances. Foe entry or exit there are two main
processes, Passive transport i.e. diffusion and osmosis and active transport.
The passive process do not require energy while active require energy with
these, there are two other phenomenon i.e. endocytosis and exocytosis.
(i) DIFFUSION OR PASSIVE TRANSPORT
It occurs
spontaneously and no extra energy is required to bring it about. A few
substances freely diffuse across plasma membrane e.g. respiratory gases (O and
CO2) diffuse in and out of the cells.
(ii) OSMOSIS
It maintains a
balance between the osmotic pressure of the intracellular fluid and that of
interstitial fluid known as osmoregulation.
(iii) ACTIVE TRANSPORT
Movement of
molecules from lower concentration ot the higher concentration by consuming
energy called active transport.
(iv) ENDOCYTOSIS
It is the process
in which the cell membrane helps to take in material by infolding in the form
of vacuole. Endocytosis may be (a) Phagocytosis in which solid particles are
picked up and ingested by the cell e.g. W. B. C. picked up foreign bodies blood
stream. In this way they destroy the harmful bacteria which enter into our
body. It is also called cell eating process (b) Pinocytosis when liquid
material in bulk in the form of vesicles is taken in by endocytosis, the
process is called Pinocytosis which is also called drinking process.
(v) EXOCYTOSIS
The process of
membrane fusion and the movement of material out of a cell is called
endocytosis.
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