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Mitochondria

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  • Last Updated : 20 Oct, 2022
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In one word if the fundamental unit of life is to be described then that would be “Cell”. The cell is known as the building block for an organism. Cells have different organelle in them which perform different functions. There are trillions of cells present in our body. The specialized function is carried by the use of energy that the cells gain from the food that they eat. When food is metabolized through different processes then it produces energy which is used in performing the metabolic activities in an individual. 

Cell and Cell Organelles

The body has heredity information contained in cells that can replicate them on their own by the process called reproduction. Different functions are assigned to a different groups of cells. Many organelles are contained in a cell. First, is cytoplasm also known as cytosol. Fluid present in the cell that surrounds all the organelles in the cell is called cytosol. It has a jelly-like structure. Next up we have a cytoskeleton, as the name itself defines it gives a framework to the cell. The structural configuration of the cell is made of a network of long fibers. Shape determination, cell division, and movement of the cell functions are performed by the cytoskeleton of the cell. 

The organelles’ movement in the cell has a track-like system that guides them to their destination. The endoplasmic reticulum is a very important organelle in cells that are of two types one is the rough endoplasmic reticulum and the other is the smooth endoplasmic reticulum. RER is named so because it has a rough surface on its outer side as they have ribosomes attached to them. SER doesn’t have ribosomes on its surface so its surface is smooth. RER has ribosomes for the production of proteins. It also transports the molecules into vesicles to their final locations. Golgi apparatus is attached to the endoplasmic reticulum as the packaging of proteins is done here. The nucleus is the main center of cells that have hereditary material in the form of DNA. The nucleus is a double membrane organelle that is known as the brain of the cell. Here we will discuss mitochondria in brief which are known as the powerhouse of cells. 

Mitochondria

It is also known as the mitochondrion. Found in most eukaryotic cells have a double membrane. One of its popular names is the powerhouse of the cell. It is named so because its main function is to produce ATP for the cell so that cell can perform metabolic activities. ATP (Adenosine Triphosphate) which is used by the cell as chemical energy is produced by aerobic production. The voluntary muscles of the insect, it was first time discovered by Albert von Kolliker in 1857. In 1898 Carl Benda was the first person to name it the mitochondrion. 

Multicellular organisms have some cells which lack mitochondria like fully mature RBCs of mammalians. Mitochondria changes into other structures or is transformed in some unicellular organisms like diplomonads, microsporidia, parabasalids, etc. There are some species that have completely lost their mitochondria including Monocercomonoides. The size of this organelle can vary from 0.75 to 3-micrometer square. Not visible with naked eyes, need particular stain to see it. Control of cell growth, cell cycle, cell death, signaling, and differentiation is also performed by mitochondria besides the production of ATP in a cell. 

Along with these functions, mitochondrial biogenesis is associated temporarily. Due to its misfunctioning, there are many diseases caused in humans like cardiac dysfunction, autism, mitochondrial diseases, heart failure, etc. According to the cell type, organism and tissue number of mitochondria can differ for e.g., liver cells have more than 2000 mitochondria whereas mature RBCs have no mitochondria. Specialized functions are performed by different compartments of mitochondria. Cristae, intermembrane space, outer membrane, matrix, and inner membrane are the compartment or regions in mitochondria. Mitochondria have their own genetic material that resembles bacterial genomes but the main organelle that has genetic material i.e., DNA is known as the nucleus. Different species and tissues have different proteins.

Mitochondria Structure

 

Structure

Different shapes of mitochondria can be seen in different species. Proteins and phospholipids bilayers form the outer and inner membrane of mitochondria. Different functions are assigned to two different membranes. ATP production increases in mitochondria because of the folding in it. Microplasts are the peeled-off outer membrane of mitochondria. They have five distinct parts 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 its outer membrane. Between cytosol and intermembrane, the proteins, ions, and metabolites are transported with the help of an anion channel called as VDAC (Voltage-dependent Anion Channel). Specialized translocation complexes are present in the mitochondria to import pro-proteins. Oxidation of epinephrine, elongation of fatty acids, and degradation of tryptophan are the functions that are performed by the outer membrane as they have many enzymes present in it. 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 in them that space is known as intermembrane space. Perimitochondrial space is also the name given to intermembrane 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 which is helpful in ETC. Inner membrane protein OPA1 mediates the fusion of the inner membrane.

Cristae

Cristae are the folds in the inner membrane which 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 they need for ATP is more like in liver cells. The cells which have more need for energy will have a greater number of cristae in them. Oxysomes or F1 particles are the small circled bodies attached to cristae.

Matrix

Space in the inner membrane is known as a matrix. It also contains a huge number of proteins in it i.e., two third part of proteins. ATP synthase is present in the inner membrane so the inner membrane is very important for the production of ATP. Many copies of 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.

Functions

  • The most important function that is assigned to mitochondria is the production of energy in the cell.
  • ATP is needed to carry out all the metabolic activities in a cell like respiration, transportation, and many others.
  • The citric acid cycle, Kreb’s cycle, and oxidative phosphorylation are the reactions that are helpful in ATP production.
  • Mitochondria is helpful in the pyruvate and citric acid cycle to produce pyruvate molecules in glycolysis.
  • It is also helpful in the conversion of energy.
  • Many energy-releasing reactions occurs in mitochondria.
  • As many processes occur in mitochondria at the same time so a lot of heat is also produced by mitochondria.
  • It also has a storage function i.e., it gives a space to store calcium ions in it.
  • Mitochondria also play a very crucial role in the regulation of cellular proliferation.
  • The apoptosis which is a cell death program is also performed by mitochondria.
  • Helpful in calcium signaling and also in regulation of cellular metabolism.
  • Through the specialized somatic junctions status of microglia is controlled by neuronal mitochondria.
  • It is helpful in immune signaling and also in hormonal signaling.
  • Membrane potential and cellular metabolism are maintained by mitochondria.
  • It also contributes to heme synthesis reactions and also in steroid synthesis.

Frequently Asked Questions

Question 1: Explain different facts about mitochondria.

Answer:

There are many interesting facts about mitochondria that are as follows-

  • They are motile and can move wherever they want so they change shapes very quickly and move to that place where there are needed the most in a cell
  • Bacteria and mitochondria show some similar properties in ancient times mitochondria were mistaken for bacteria because complex cells usually absorb them.
  • Carbon dioxide is also produced by mitochondria along with ATP.

Question 2: Do mitochondria grow and if yes then how?

Answer:

Yes, mitochondria can grow. The number of mitochondria can be increased and decreased in a cell depending on the need for energy in the cell. If the requirement of energy is more in the cell, then mitochondria will start reproducing and will divide themselves to fulfill the demands of energy by the cell. If the scenario is the opposite means if the energy requirement is less then mitochondria will start dying and will reduce their number and will become inactive.

Question 3: How do mitochondria get their popular name ‘Powerhouse of the cell’?

Answer:

Powerhouse means giving energy for them to do their work. The same is the function of the mitochondria i.e., the production of energy in the cell. It fulfills the energy requirement of the cell by cellular respiration and due to mitochondria, the metabolic activities of the cell are performed with full efficiency. As it provides ATP to the whole cell that is why they are known as the ‘Powerhouse of the cell’.

Question 4: Where are the mitochondria found in maximum number?

Answer: 

Different cells have a different number of mitochondria present in them because the need for ATP by every cell is different. The more the need for energy in a cell more will e the number of mitochondria present in its heart cells has 5000 mitochondria per cell whereas in the liver 2000 mitochondria are present in each cell and mature RBCs don’t have mitochondria at all. So, the number is purely dependent on the need for the cell.

Question 5: What is the structure of mitochondria and what is contained?

Answer:

Mainly mitochondria have five parts in which it is structurally divided. The outer membrane is thicker and it contains many enzymes which perform different functions. Provide protection to mitochondria but if it gets disrupted the proteins can leak from it and can cause cell death. Inner membrane space is the space between the outer and inner membranes containing proteins that are to be transported into the inner membrane. The inner membrane is not permeable to all molecules like the outer membrane. Here for the entry and exit of every molecule, there are some transporters that help the molecules to move in and out of the inner membrane. The folds or compartments in the inner membrane are called cristae and the fluid in it is called matrix.


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