Mitochondria
Cell Organelles | Mitochondria

Mitochondria- Structure, functions and origin.

Mitochondria also known as the powerhouse of the cell, makes up the complete energy required for the cell.

It is the double membranous organelle and is typically the most active part of the cell.

All the mitochondria present in a cell are collectively known as the Chondriome.

A mitochondrion varies in shapes and sizes form the organism to organism, but it is typically of Sausage or Cylindrical shape.

Structure of a Mitochondria

Mitochondria is not visible in the microscope until it is stained. Janus green B, a basic dye is used to stain it.

Mitochondria structure
Diagram showing different components of Mitochondria Img source

A mitochondrion is covered by the outer and inner membranes composed of phospholipid bilayers and proteins.

The membranes contain the different varieties of the proteins, because of their functions. 

A mitochondrion is make up the five distinct parts –

  1. outer mitochondrial membrane,
  2. intermembrane space (the space between the outer and inner membranes),
  3. inner mitochondrial membrane,
  4. crista (formed by infoldings of the inner membrane), 
  5. matrix (space within the inner membrane).

NoteMitochondria without their outer membrane are called mitoplasts.

Outer membrane

The outer mitochondrial membrane, which encloses the entire organelle, is 60 to 75 Å thick. 

It contains large numbers of integral membrane proteins called porins. 

The main channel protein in the outer membrane in favor to promote transport is voltage-dependent anion channel (VDAC). The VDAC is the primary transporter of nucleotides, ions, and metabolites between the cytosol and the intermembrane space.

But the Mitochondrial pro-proteins are imported through specialized translocation complexes.

The mitochondrial outer membrane can associate with the endoplasmic reticulum (ER) membrane, in a structure called MAM (mitochondria-associated ER-membrane).

This is important in the ER-mitochondria calcium signaling and is involved in the transfer of lipids between the ER and mitochondria.

Intermembrane space

Also known as peri-mitochondrial space.

The mitochondrial intermembrane space is the space between the outer membrane and the inner membrane. 

Because the outer membrane is freely permeable to small molecules, the concentrations of small molecules, such as ions and sugars, in the intermembrane space is the same as in the cytosol.

However, large proteins found difficulty to be transported across the outer membrane, so the protein composition of this space is different from the protein composition of the cytosol. 

One protein that is localized to the intermembrane space in this way is cytochrome C.

Inner membrane

The inner mitochondrial membrane contains proteins with three types of functions:

  1. Those that perform the electron transport chain redox reactions
  2. ATP synthase, which generates ATP in the matrix
  3. Specific transport proteins that regulate metabolite passage into and out of the mitochondrial matrix.

It contains more than 151 different polypeptides and has a very high protein-to-phospholipid ratio (more than 3:1 by weight).

The inner membrane is home to around 1/5 of the total protein in a mitochondrion.

Additionally, the inner membrane is rich in an unusual phospholipid, cardiolipin. 

This phospholipid is usually characteristic of mitochondrial and bacterial plasma membranes.

Cardiolipin contains four fatty acids rather than two and may help to make the inner membrane impermeable.

The inner membrane has much more restricted permeability, much like the plasma membrane of a cell.

Unlike the outer membrane, the inner membrane does not contain porins and is highly impermeable to all molecules. 

Almost all ions and molecules require special membrane transporters to enter or exit the matrix. 

Cristae

The inner mitochondrial membrane is compartmentalized into numerous cristae, which expand the surface area of the inner mitochondrial membrane, enhancing its ability to produce ATP.

In typical mitochondria, the area of the inner membrane is about five times greater than the outer membrane. 

This ratio is variable, i.e. the cells that have a greater demand for ATP, such as muscle cells, contain even more cristae, or those who have less energy requirement may contain even fewer cristae. 

These folds have the small round bodies known as F1 particles or oxysomes. These are not simple random folds but rather invaginations of the inner membrane, which can affect overall chemiosmotic function.

Oxysomes showing phosphorylation
Oxysomes showing phosphorylation

In metabolically active mitochondria number of cristae are higher.

Wondering how the oxysomes are capable to make ATP, click here to read.

Matrix

The matrix is the space enclosed by the inner membrane. It contains about 2/3 of the total proteins in a mitochondrion.

The matrix is important in the production of ATP with the help of the ATP synthase in the inner membrane. 

The matrix contains a highly concentrated mixture of hundreds of enzymes, special mitochondrial ribosomes, tRNA, and several copies of the mitochondrial DNA genome. 

The majority of the enzyme present is associated with the oxidation of pyruvate and fatty acids and to the citric acid cycle.

The matrix contains the DNA of the mitochondria-

It is double-stranded, circular, and without the histones.

The DNA molecules are packaged into nucleoids by proteins, one of which is TFAM.

Mitochondria have their own genetic material and the machinery to manufacture their own RNAs and proteins.

A published human mitochondrial DNA sequence revealed 16,569 base pairs encoding 37 genes: 22 tRNA, 2 rRNA, and 13 peptide genes.

The 13 mitochondrial peptides in humans are integrated into the inner mitochondrial membrane, along with proteins encoded by genes that reside in the host cell’s nucleus.

Function of Mitochondria

The main function of the mitochondria is cellular respiration, to form the ATP for the cellular needs.

Where the different structures are involved in the different steps of the respiration.

Mitochondria Membranes

The inner membrane is loaded with proteins involved in electron transport and ATP synthesis. 

The outer membrane contains enzymes involved in such diverse activities as the elongation of fatty acids, oxidation of epinephrine, and the degradation of tryptophan. 

These enzymes include monoamine oxidase, rotenone-insensitive NADH-cytochrome c-reductase, kynurenine hydroxylase, and fatty acid Co-A ligase. 

Matrix

The citric acid cycle (Kreb’s cycle) occurs here which in result produces the NADH2.

Cristae 

The hydrogen from the NADH2 is passed on to a protein complex and from it to the other complex.

At the end of this electron transport chain, the final electron is accepted by the oxygen, and this ultimately forms water (H20). 

Notethis water is called the metabolic water, which is even the major source of the water in some animals like kangaroo rat.

At the same time, the electron transport chain produces ATP. (This is why the process is called oxidative phosphorylation.)

During electron transport, the participating protein complexes push protons from the matrix out to the intermembrane space. 

This creates a concentration gradient of protons that another protein complex, called ATP synthase (oxysomes), uses to power synthesis of the energy carrier molecule ATP.

Mitochondrial Genome

Mitochondrial genomes are very small, circular and show a great variety of variation as a result of divergent evolution. 

Mitochondrial genes that have been conserved across evolution include rRNA genes, tRNA genes, and a small number of genes that encode proteins involved in electron transport and ATP synthesis. 

The mitochondrial genome retains a similarity to its prokaryotic ancestor. 

Mitochondrial rRNAs show close resemblance with the bacterial rRNAs than the eukaryotic rRNAs found in the cell cytoplasm. 

Besides, some of the codons that mitochondria use to specify amino acids differ from the standard eukaryotic codons.

Still, the vast majority of mitochondrial proteins are synthesized from nuclear genes and transported into the mitochondria. 

These include the enzymes required for the citric acid cycle, the proteins involved in DNA replication and transcription, and ribosomal proteins. 

The protein complexes of the respiratory chain are a mixture of proteins encoded by mitochondrial genes and proteins encoded by nuclear genes. 

Due to this mitochondria is also known as the semi-autonomous organelle, same as the chloroplast.

Proteins in both the outer and inner mitochondrial membranes help transport newly synthesized, unfolded proteins from the cytoplasm into the matrix.

What Is the Origin of Mitochondria?

There are two hypotheses about the origin of mitochondria: endosymbiotic and autogenous. 

The endosymbiotic hypothesis suggests that mitochondria were originally prokaryotic cells, capable of implementing oxidative mechanisms that were not possible for eukaryotic cells; they became endosymbionts living inside the eukaryote.

In the autogenous hypothesis, mitochondria were born by splitting off a portion of DNA from the nucleus of the eukaryotic cell at the time of divergence with the prokaryotes; this DNA portion would have been enclosed by membranes, which could not be crossed by proteins.

Since mitochondria have many features in common with bacteria, the endosymbiotic hypothesis is more widely accepted.

So, let us talk about endosymbiosis in detail-

Mitochondria are thought to have originated from an ancient symbiosis when a nucleated cell engulfed an aerobic prokaryote. 

The engulfed cell became dependent on the protective environment of the host cell, and the host cell became dependent on the engulfed prokaryote for energy production. 

Over time, the descendants of the engulfed prokaryote developed into mitochondria, and the work of these organelles — using oxygen to create energy — became critical to eukaryotic evolution.

Modern mitochondria have striking similarities to some modern prokaryotes, even though they have diverged significantly since the ancient symbiotic event. 

For example, the inner mitochondrial membrane contains electron transport proteins like the plasma membrane of prokaryotes, and mitochondria also have their own prokaryote-like circular genome. 

One difference is that these organelles have lost most of the genes before carried by their prokaryotic ancestor. In present-day mitochondria synthesize some of it’s own proteins, but the majority of the proteins it requires are encoded by the nuclear genome.

Important points-

Plants have a less number of mitochondria compared to the animals, as you know why. (plants are metabolically less active).

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