Plasma Membrane and Mitochondria.
It is the outer limiting membrane of both prokaryotic and eukaryotic
cells. It is an ultra thin, elastic, living membrane. Plasma membrane is a dynamic
and selective transport barrier.
Since the plasma membrane is ultra thin, it could be observed only
under electron microscope. Structure of the membrane is studied by isolating
the same from the cell and conducting biochemical investigations.
In 1895 Overton suggested that the membrane is made of fatty substances.
Other workers later concluded that two layers of lipid were present
in the cell membrane. According to a model proposed by Danielli and Davson
in 1935, the lipid bilayer of the membrane was coated on either side with
protein. In 1960, Robertson using electronmicrographs proposed a unit
membrane hypothesis. According to this hypothesis the two outer layers of
protein are about 2 nm thick and appear densely granular. They enclose a
clear central area of about 3.5 nm wide consisting of lipids. The lipids are
mainly phospholipid molecules.
Singer and Nicholson (1972) have proposed a fluid mosaic model
for the plasma membrane. The fluid mosaic membrane is a dynamic
structure. In this structure much of the protein molecules float about. Some of
them are anchored to the organelles within the cell. Lipid molecules also move
about. ‘Fluid mosaic model’ is applied to all biological membranes in
The cell membrane controls the passage of materials both into and out
of the cell. It regulates the passage of water and dissolved substances. Water
passes through the membrane by Osmosis. Water soluble substances cross
the membrane by diffusion or by active transport. Many water soluble solutes
are transported by carrier proteins. Lipid soluble compounds pass more
quickly by dissolving in the phospholipid layer.
The mitochondria are filamentous or granular cytoplasmic organelles
of all aerobic cells of higher animals and plants. They are also found in micro
organisms including Algae, Protozoa and Fungi.
They were first observed by Kolliker in 1850 as granular structures
in the striated muscles. The name ‘mitochondria’ was given to them by Benda
(1897-98). Various steps of glycolysis in mitochondria was discovered by two
German biochemists Embden and Meyerhof. Embden got the Nobel Prize in
1922. Sir Hans Adolph Krebs, in 1937 found out various reactions of citric
acid cycle. Kennedy and Lehninger (1948-50) showed that Citric acid
cycle, oxidative phosphorylation and fatty acid oxidation took place in
The number of mitochondria in a cell depends on the type and functional
state of the cell. Certain cells contain large number of mitochondria
e.g., eggs of sea urchin contain 140,000-150,000 mitochondria. Oocytes of
amphibians contain 300,000 mitochondria. Liver cells of rat contain only 500-
1600 mitochondria. Some algal cells may contain only one mitochondrion.
The mitochondria may be filamentous or granular in shape. They vary
in size from 0.5 μm to 2.0 μm. Due to their minute nature they can not be seen
under light microscope.
Each mitochondrion is bound by two highly specialized membranes.
The outer membrane is smooth. It is separated from the inner membrane by a
6-8 nm wide space. The inner membrane is highly convoluted, forming a
series of inflodings known as cristae.
Thus mitochondria are double membrane envelopes. The inner membrane
divides the mitochondrial space into two distinct chambers. The outer
compartment is the peri-mitochondrial space. It is found between outer and
inner membranes. The inner compartment is the matrix space. It is filled
with a dense gel like substance called mitochondrial matrix. The matrix
contains lipids, proteins and circular DNA molecules.
The outer and inner membranes, intermembrane space and mitochondrial
matrix contain several enzymes. Hence the mitochondria perform several
important functions such as oxidation, dehydrogenation, oxidative
phosphorylation and respiratory chain of the cell.
Since mitochondaria play a key role in the oxidation of carbohydrates
and fats, they are considered as the actual respiratory organs of the cells.
During such biological oxidations large amount of energy is released. The
energy is utilized by the mitochondria for synthesis of the energy rich compound
known as adenosine tri phosphate or ATP. Due to this function, the
mitochondria are also known as “power houses” of the cell. In animal cells
mitochondria produce 95 % of ATP molecules.