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Mitochondria

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Mitochondria is a double membrane organelle present in the cytoplasm of all eukaryotic cells. Mitochondria generates energy in the form of ATP because of which mitochondria is known as the “powerhouse of the cell”. In 1857 Albert von Kolliker first discovered the organelle and Carl Benda in 1898 gave the organelle its name Mitochondria.

What is Mitochondria?

The word mitochondria in Geek language stands for “mitos” and “chondrion” which means “thread” and “granules-like” respectively. Mitochondria (singular: mitochondrion) is present in the cytoplasm of most eukaryotic cells and has a double membrane covering. They have their own DNA and hence are called semi-autonomous organelles like the chloroplast. The mitochondria are called the powerhouse of the cell because their main function is to produce ATP for the cell so that the cell can perform metabolic activities.

Mitochondria plays a significant role in producing energy by breaking carbohydrates. Many important biochemical reactions occur in the mitochondria that are responsible for the control of cell growth, cell cycle, cell death, cell signaling, and cell differentiation other than the production of ATP. 

They appear ribbon-shaped, freely floating throughout the cytoplasm of the cell. In animals, the number of mitochondria can reach up to lakhs whereas a typical plant cell only has around hundreds of them. Mitochondria are very small organelle i.e. around 0.5 to 10 ┬Ám in diameter and require stain to observe under microscope.

Characteristics of Mitochondria

Following are some of the characteristics of mitochondria;

  1. They are either sausage-shaped or cylindrical-shaped.
  2. They appear ribbon-like.
  3. They are semi-autonomous organelle i.e. mitochondria has their own DNA, RNA and ribosomes.
  4. They have two membrane covering; inner membrane, and outer membrane
  5. The space inside the membrane is called the matrix.
  6. The inner membrane folds to form the cristae.
  7. The cristae are responsible for increasing the surface area.
  8. Their ribosome is 70S like the ones found in bacteria.
  9. Most of the stages of aerobic respiration takes place in the mitochondria.
  10. Human sperm cells have spiral-shaped mitochondria.

Diagram of Mitochondria

The diagram illustrating the structural feature of mitochondria:

Mitochondria

Structure of Mitochondria

Different shapes of mitochondria can be seen in different species of plants and animals. Proteins and phospholipids bilayers form the outer and inner membranes of mitochondria. Different functions are assigned to these two different membranes. ATP production increases in mitochondria due to the folding in its inner membrane. They have five distinct components due to double membrane organization. 

Outer Membrane

The outer membrane is thick i.e., 60-75 angstroms. A similar ratio of phospholipids to proteins is seen in mitochondria as in cell membranes. Porins are the integral membrane proteins that are present in their outer membrane that help in the transport of molecules. Between cytosol and the intermembrane, the proteins, ions, and metabolites are transported with the help of an anion channel called VDAC (Voltage-dependent Anion Channel). Cell death can be caused when the proteins get leaked into cytosol due to disruption in the outer membrane.

Intermembrane Space

The outer membrane and inner membrane have some space between them that is called the intermembrane space or Perimitochondrial space. The concentration of ions, small molecules, and sugar in this space is the same as in cytosol because the outer membrane is permeable to all these molecules. If we talk about large proteins then the ratio differs because large molecules are transported in the outer membrane by certain transporters. Cytochrome c is a protein that is brought into this space by transporters.

Inner membrane

The ratio of protein in the inner membrane is very high i.e., one-fifth of proteins. Cardiolipin is a phospholipid that is present abundantly in mitochondria. Porins are not present in the inner membrane. It is not permeable to all molecules like the outer membrane. The inner membrane is helpful in the electron transport chain as it has enzymes present that are required for ETC. The inner mitochondrial membrane is selectively permeable to oxygen and ATP. It consists of multiple folds called cristae that increase its surface area.

Cristae

Cristae are the folds in the inner membrane that form compartments in mitochondria. Due to these folds, the surface area of mitochondria increases which is helpful in more production of ATP. The folds of the inner membrane are five times more than the outer membrane in the cells where the need for ATP is more like in liver cells. The cells that have more demand for energy will have a greater number of cristae in them. Oxysomes or F1 particles are the small circled bodies attached to cristae.

Mitochondrial Matrix

Space surrounded by the inner membrane having the fluid in it is known as a mitochondrial matrix. It also contains a huge number of proteins in it i.e., two-thirds of the total proteins. ATP synthase is present in the inner membrane so the inner membrane is very important for the production of ATP. The mitochondrial genome, mitochondrial ribosomes, tRNA, and many enzymes are present in the matrix. Fatty acid oxidation, citric acid cycle, and oxidation of pyruvate are the main functions performed by the inner membrane.

Also Read: Difference between Mitochondria and Plastid

Functions of Mitochondria

The primary function of the mitochondria is to generate ATP via oxidative phosphorylation. Other functions of mitochondria are:

  1. Helps in ammonia detoxification in liver cells.
  2. Regulation of metabolic activity.
  3. Helps in cell growth and multiplication.
  4. Apoptosis or programmed cell death is also performed by mitochondria.
  5. Through the specialized somatic junctions status of microglia is controlled by neuronal mitochondria.
  6. Membrane potential and cellular metabolism are maintained by mitochondria.
  7. It also contributes to heme synthesis reactions and also in steroid synthesis.
  8. It is helpful in immune signaling and also in hormonal signaling.
  9. It helps in the regulation of calcium ion concentration in the cell.
  10. Helps in various other cellular activities like cell-to-cell signaling, regulation of the cell cycle, etc.
  11. In cold climatic areas, the mitochondria utilize the stored fats in the cells to generate heat.

Mitochondrial Disease

If mitochondria are not working properly it will directly affect the human. Symptoms of mitochondrial disease vary from person to person. In some cases, ineffective mitochondria lead to organ failure in severe cases. Examples of Mitochondrial disease are:

  • Alpers Disease
  • Barth Syndrome
  • Alzheimer’s Disease
  • Muscular Dystrophy
  • Diabetes

Mitochondrial DNA

A genetically distinct form of DNA that is independent of the nuclear DNA is found in the mitochondria. This double stranded DNA is either circular or linear and codes for all the genetic information required for the mitochondria. This specific DNA of mitochondria that has around 16500 base pairs is called mitochondrial DNA or mtDNA. In most of the sexually reproducing organisms it is obtained from mother and does not have any paternal information. Compared to the nuclear chromosome it is very small and regulates only the mitochondrial functions related to RNA formation, ribsome synthesis and ribosomal functioning.

Key Points on Mitochondria

Following are some of the key points on mitochondria;

  1. It is a semi-autonomous organelle i.e. it has its own DNA, RNA, and ribosomes.
  2. It is double membraned.
  3. It is energy producing organelles of eukaryotic cells.
  4. Most of the processes of cellular respiration occurs in mitochondria.
  5. It’s structure is divided into five distinct components; outer membrane, inter membrane space, inner membrane, cristae, and matrix.
  6. It store many essential enzymes.
  7. It performs multiple metabolic activities.
  8. Its ribsomes resembles to that of the prokaryotes.
  9. Disorder in mitochondria may lead to multiple disorders.
  10. It’s DNA is small double stranded and circular in nature.

FAQs on Mitochondria

1. What is the other name of Mitochondria?

Answer:

Mitochondria is also known as the “power house of the cell” due to its energy producing capability.

2: Do Mitochondria have DNA?

Answer:

Yes, mitochondria carried DNA and because of this there are known as semiautonomous organelles.

3. Where are the Mitochondria Found?

Answer: 

Mitochondria is a double membrane organelle that is found in every cell cytoplasm. Their number varies according to the type of cells.

4. What is the Structure of Mitochondria?

Answer:

Mainly mitochondria have five main components in which it is structurally divided; the outer membrane, intermembrane space, inner membrane consisting of the cristae, and the matrix.

5. Which Organ of the Human body has the most Mitochondria?

Answer:

The human heart with a staggering number of around 5000 mitochondria is the organ having the most mitochondria.



Last Updated : 10 Oct, 2023
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