Plasma Membrane and Mitochondria

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

general.

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 mitochondria.

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.

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