Describe in detail physiochemical nature of plasma membrane and cytoplasm
Plant cell is a
unit or independent tiny or independent tiny or microscopic mass of protoplasm enclosing
in it a denser spherical or oval body called the nucleus and bounded by a
distinct wall the cell wall. Protoplasm and nucleus are living, while the cell
wall is non living; the latter has been formed by the protoplasm to maintain
its shape and firmness and to afford necessary protection. Cells vary widely in
shape and size. They may be spherical, oval, polygonal, cubical or narrow and
elongated.
PLASMA MEMBRANE
All cells are
enclosed in plasma membrane that serves as their outer boundary, separating the
cytoplasm from the external environment. This is called plasma membrane. It
allows the cell to take up and retain certain substances while excluding
others. All membrances have the same basic molecular organisation. They have double
layer 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, where as their polar
hydrophilic ends are associated with carbohydrates, protein etc.
Plasma membrane
also contains several types of lipids like cholesterol. In certain animal cells
cholesterol may constitute upto 50 percent of the lipid molecules in plasma
membrane. It is absent plasma membrane of most plant and bacteria cells.
In 1972 singer
and Nicholson proposed a working model of plasma membrane known as fluid mosaic
model. It is the lipid bilayer is retained as the core of the membrane. These
lipid 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. The proteins
occur as a mosaic of discontinuous particles that penetrate deeply into and
even completely through the lipid sheet. The components of plasma membrane are
mobile and capable of coming together to engage in various types of transient
or semi permanent interaction.
A class of
proteins that are directly incorporated within the lipid bilayer are intrinsic proteins
some of these proteins are believed to provide a channel through which water
soluble substances such as ions can pass back and forth between the extra
cellular and intra cellular compartment. Extrinsic proteins are a class of
proteins located entirely outside the lipid bilayer on extra cellular or
cytoplasmic surface. It exhibit loose association with membrane surface. Those
proteins which possess lipid or carbohydrate side chains are arranged as
mosaics within the cell membrane.
Plasma membrane
functions as protection of cell cytoplasm, and to regulate the flow of
solutions and material in and out of the cell.
CYTOPLASM
Protoplasm
outside nucleus is called cytoplasm. Cytoplasm appears as a semi fluid colloid
that fills the cell. The cytoplasm exhibits active streaming movements around
the inner surface of the cell. This movement is known as cyclosis. Cytoplasm is
composed of several types of organelles occupying half the volume of cell and a
fluid of matrix, the cytosol (cell division) in which the organelles reside.
The cytosol is a watery solution of slats, sugar, amino acids, proteins, fatty
acids, nucleotides and other material. Giving shape and organization to the
cytoplasm is a network of protein fibres, the cytoskeleton many organelles and
individual molecules of the cytoplasm are thought to be attached to the
cytoskeleton.
Observations under electron microscope reveal that the cytoplasm
is not a simple colloid since it contains many different kinds of minute
organelles and also a mesh of tiny filaments the micro fibrils that form a sort
of skeleton giving rigidity to cell and helping unicellular organisms in
movement. A variety of them are membrane bound. These organelles are grouped on
the basis of membrane. Membrane bound organelles are endoplasmic reticulum,
Mitochondria, Golgi apparatus lysogomes, plastids and microbodies. Non membrane
organlles are Ribosome, Centriole and vacuole.
Comments
Post a Comment