Mitochondria are a double-membrane organelle present in the cytoplasm of all eukaryotic cells. Mitochondria generate energy in the form of ATP, because of which mitochondria are 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.
Structure of Mitochondria
Different shapes of mitochondria can be seen in different species of plants and animals. Proteins and phospholipid bilayers form the outer and inner membranes of mitochondria. Different functions are assigned to these two different membranes. They have five distinct components due to double membrane organisation and one mitochondrial DNA.
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 and help in the transport of molecules.
- Between the cytosol and the intermembrane space, 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 leak into the cytosol due to a 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 Peri mitochondrial space.
- The concentration of ions, small molecules, and sugar in this space is the same as in the 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 the 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 circular bodies attached to cristae.
Mitochondrial Matrix
- Space surrounded by the inner membrane, having the fluid in it, is known as the mitochondrial matrix. It also contains a huge number of proteins, 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.
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, which has around 16500 base pairs, is called mitochondrial DNA or mtDNA.
- In most sexually reproducing organisms, it is obtained from the 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, ribosome synthesis and ribosomal functioning.
Characteristics of Mitochondria
The following are some of the characteristics of mitochondria;
- The word mitochondria in Geek language stands for "mitos" and "chondrion", which means "thread" and "granule-like" respectively.
- Mitochondria (singular: mitochondrion) are present in the cytoplasm of most eukaryotic cells and have 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 play a significant role in producing energy by breaking down carbohydrates.
- 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.
Functions of Mitochondria
The primary function of the mitochondria is to generate ATP via oxidative phosphorylation. Other functions of mitochondria are:
- Helps in ammonia detoxification in liver cells.
- Regulation of metabolic activity.
- Helps in cell growth and multiplication.
- Apoptosis or programmed cell death is also performed by mitochondria.
- Through the specialised somatic junctions status of microglia is controlled by neuronal mitochondria.
- Membrane potential and cellular metabolism are maintained by mitochondria.
- It also contributes to heme synthesis reactions and also in steroid synthesis.
- It is helpful in immune signalling and also in hormonal signalling.
- In cold climatic areas, the mitochondria utilise the stored fats in the cells to generate heat.
Mitochondrial Disease
If mitochondria are not working properly, it will directly affect humans. 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