mitochondria & peroxisomes

BYN.VIJAY

Mitochondria are membrane bound cell organelles found in

most eukaryotic cells .

It was first identified as “BIOBLASTS” by RICHARD

ALTMANN.

BENDA coined the term mitochondria.

In Greek ‘mitos’ means filament and ‘chondros’ means

granules

They are believed to be derived from endo-symbiotic

prokaryotes.

They are described as power houses of the cell

It is a semi autonomus cell organelle.

Shape :

These are filamentous or granular. They appear spherical to ellipsoid . They exhibit pleiomorphic appearance.

Size :

Approximately 1µm thick and 1-3 µm long. Width 0.5 µm ( relatively constant). Length may vary ( maximum 7µm)

Distribution : They are uniformly distributed throughout the cytoplasm

where the energy is required.

Number : A plant cell may contain hundreds or thousands of

mitochondria. Their number varies depending upon the cell type and stage

of development . Ex : The central cells of maize root cap have approximately

200 when young and 2000 – 3000 at matured stage.

Mitochondrion is composed of :

Outer mitochondrial membrane

Inter membrane space

Inner mitochondrial membrane

Cristae

Matrix

It has a protein to phospholipid ratio of 1 : 1

It contains large numbers of integral proteins called PORINS

It associates in a structure called ‘MAM’

( MITOCHONDRIA ASSOCIATED ER MEMBRANE

It is permeable to small molecules .

It is also called as PERIMITOCHONDRIAL SPACE.

It is the space between the outer and inner membranes.

The concentrations of small molecules such as ions and

sugars is same as that of cytosol.

In this space,the protein composition is different from that of

the cytosol.

It is a highly convoluted membrane.

It is much larger in area than the outer membrane.

It folds into cristae that extend deeply into the matrix.

It has a very high protein to phospholipids ratio of 3 : 1

It contains 1/5 th of total protein of mitochondrion.

It is highly impermeable to all molecules.

It contains proteins with 5 types of functions :

those perform the redox reactions of oxidative

phosphorylation

ATP-synthase

Specific transport proteins

Protein import machinery

Mitochondria fission and fusion protein

The inner membrane is compartmentalized into numerous

cristae.

They are studded with small round bodies known as ‘F1

Particles’ or ‘Oxysomes’

These particles have a spherical head about 9 nm in diameter

linked to the membrane by a short stalk.

It is the space enclosed by the inner membrane.

It contains about 2/3rd of the total protein in a mitochondrion.

It is important in the production of ATP with the aid of ATP-

synthase.

It contains a highly concentrated mixture of hundreds of

enzymes,special mt-ribosome,t-RNA and mt-DNA.

These are smaller than those in the cytoplasm.

They are 70s type and are composed of large sub units ( 50s )

and a small sub unit ( 30s ).

The large sub unit contains two r-RNA molecules, one of 23s

and other of 5s.

The small sub unit contains one r-RNA molecule of 16s.

All these r-RNA molecules are coded by mitochondrial DNA.

Mt-DNA :

Circular DNA of 16.6 kb

Double stranded DNA

Absence of histone proteins

Multiple copies are present in nucleoid

It encodes the enzymes required for oxidative

phosphorylation and mitochondrial electron transport.

The two DNA strands are different in base composition . H-strand (heavy) of high base composition L-strand (light) of low base composition D-LOOP : It is a short region ( 1121bp) on DNA. It is a DNA triple helix contains two overlapping copies of H-strand It is the site where most of the replication is controlled. No non coding DNA is present outside of the D-loop.

The origin of replication for the H-strand is in the D-loop and it is initiated by an RNA primer.

After the new H-strand is about 2/3 complete ,the L-strand origin of replication is uncovered.

The L-strand origin is on the old H-strand ; it is uncovered when the old H-strand is displaced by DNA polymerase synthesizing the new H-strand.

The L-strand origin folds into a stem loop structure which acts as a primer and replication of the L-strand begins

Replication can be said to be bi-directional.

It produces ATP through respiration and regulate cell

metabolism.

Chemical reactions of Krebs cycle takes place in

mitochondria

Bio-genesis of mitochondrial ribosomes takes place in the

matrix.

Participate in photo respiration in C3 plants .

Glyoxalate cycle occurs in the matrix.

It stores calcium a contribution process for cell

homeostasis

They are single membrane bound cell organelles .

They were first identified by CHRISTIAN DE DUVE.

They are spherical in shape

They are 0.2-1.7µm in size

It consists of finely granular matrix

Leaf peroxisomes :◦ Catalyzes oxidation of side product of CO2 fixation in

photorespiration

Glyoxysomes : ◦ Converts fatty acid in seed lipids into sugars needed for

growth in the young plant

Detoxifying cell by degradation of H2O2 produced by the perioxisomal oxidases.

Leaf peroxisomes are involved in photo respiration ( Glycolate pathway)

Breaking down fatty acids during germination of fat storing seeds ( Glyoxylate cycle)

They were involved in conversion of fixed nitrogen into ureides for N-export