Mitochondria – The power House of a Cell

The Cell Organelles – Mitochondria

 Introduction:

• Mitochondria is a double membrane-bound organelle.
• It is found in the cytoplasm of eukaryotic cells.
• ‘Mito’ means thread and ‘chondrion’ means granule-like.
• Carl Benda coined the term mitochondria.
• Richard Altman observed it for the first time in 1894.
• Mitochondria produces enzymes for the metabolic conversion of food to generate energy.

Powerhouse Of Cell

Mitochondria are primarily responsible for converting nutrients into energy.

They yield ATP molecules to fuel cell activities. As they do aerobic respiration, mitochondria are often referred to as the powerhouse of the cell. There are three stages of aerobic respiration.

Those three stages are:

1. Glycolysis
2. Citric acid (Krebs) cycle
3. Electron transport chain

Origin Of Mitochondria

• Endosymbiosis – It is a process by which mitochondria are derived from bacteria.
• About 2 billion years ago, mitochondria arose when a bacterium fused with an archaeal cell or established a symbiotic relationship with a primitive eukaryotic cell.
• Rickettsia – They are the closest, currently existing, relatives of bacteria that gave rise to mitochondria.
• Ivan Emanuel Wallin was the first person to recognize that mitochondria were descendants of endosymbiotic bacteria.
• Outer and inner membranes of mitochondria are composed of phospholipid bilayers and proteins. Both these membranes have different properties.

Structure Of Mitochondria

• Size – 0.05-1.0 micrometer
• Length – 1-10 micrometers long
• Shape – Normally, it is bean-shaped, while in fibroblast it is elongated and thread-like.
• Number – It depends on the type, size, and functional state of the cell. Example: An average liver cell contains around 1500 mitochondria.

• Structure:

1. Outer membrane
2. Inner membrane
3. Intermembrane space
4. Cristae
5. Matrix

Outer Membrane

It is a simple phospholipid layer. The mitochondrion is surrounded by this layer. It consists of the large number of integral protein structures called porins, which allow molecules to freely diffuse from one side of the membrane to another. Less than 5000 D [Dalton] molecules can be passed through porins. The substances that can pass through the outer membrane with ease are ions, nutrient molecules, ATP, ADP, etc. It consists of 50% phospholipids by weight and a variety of enzymes.

Intermembrane Space

• It is the space between the outer and inner membranes.
• It has high proton concentration.
• Its size is approximately 70 Angstroms.
• As the outer membrane is freely permeable to small molecules, the concentration of small molecules such as ions and sugars in the intermembrane space is the same as that of the cytosol.

Inner Membrane

• It is freely permeable only to oxygen, carbon dioxide, and water.
• It contains proteins that perform redox reactions in oxidative phosphorylation, ATP synthase, transport proteins, protein import machinery, mitochondria fission, and fusion protein.
• Many antiport systems are there that allow the exchange of anions.

Cristae

• These are folds of the inner mitochondrial membrane that expand its surface area and enhance its ability to produce ATP.
• They are covered with inner membrane spheres called stalked particles or knobs or heads.

Matrix

Matrix is the space enclosed by the inner membrane. It is dense, homogenous, and has a gel-like consistency. 2/3^rd of the total protein of mitochondria is present in the matrix. They contain enzymes, DNA genomes, ribosomes, tRNA, granules, fibrils, and tubules. With the aid of ATP synthase, the matrix is important in the production of ATP.

Mitochondrial DNA

It is a small, double-stranded, covalently closed, and circular molecule. It occurs in multiple copies. It has 16569 base pairs. Normally, it remains attached to the inner mitochondrial membrane. It stores biological information required for the growth and multiplication of mitochondria. It encodes RNAs and proteins that are essential for mitochondrial function. It can undergo replication and duplication. It is not absolutely autonomous and depends on nuclear DNA.

Functions Of Mitochondria

Mitochondria uses oxygen and complex molecules to produce a higher energy molecule known as ATP. This is known as aerobic respiration.

The cells that contain a lot of energy have more abundance of mitochondria. For example, muscle cells. Extra mitochondrial inheritance – mitochondrial DNA contains plasma genes. They synthesize mtDNA, RNA, and proteins.

Summary

▪ Mitochondria is a double membrane-bound organelle that is found in the cytoplasm of
eukaryotic cells.
▪ Enzymes for the metabolic conversion of food to generate energy are produced by
mitochondria.
▪ Mitochondria are primarily responsible for converting nutrients into energy and yielding
ATP molecule to fuel cell activities.
▪ As they do aerobic respiration, mitochondria are often referred to as the powerhouse of
the cell.
▪ Three stages of aerobic respiration are glycolysis, citric acid cycle, and electron transport
chain.
▪ The outer and inner membranes of mitochondria are composed of phospholipid bilayers
and proteins and both these membranes have different properties.