Mitochondria – structure , Functions , Defination

Mitochondria Structure and Functions 

Mitochondria (Singular : 
 Mitochondrion) :
These are important cell organelles
involved in aerobic respiration. Mitochondria are absent in prokaryotic cells and red blood corpuscles (RBCs). Their shape may be oval or spherical or spiral strip-like. It is a double membrane-bound
organelle. The outer membrane is permeable to various metabolites due to the presence of a protein -Porin or Parson’s particles. The inner membrane is selectively permeable to a few substances only. Both membranes are separated by a space-outer chamber.

Inner membrane shows several finger-like
or plate-like folds- cristae. Inner membrane
bears numerous particles- exosomes and
cytochromes/electron carriers. Inner membrane encloses a cavity- inner
chamber, containing a fluid- matrix. The matrix
contains a few coils of circular DNA, RNA, 70S types of ribosomes, lipids, and various enzymes of the Krebs cycle and other pathways.

Cristae increases surface area for ATP generating chemical reactions (Electron Transport Chain). The inner compartment is filled with a fluid called matrix. The matrix of mitochondria contains enzymes of Kreb’s cycle. Matrix has a single circular DNA molecule and ribosomes (the 70S); hence they can make their own proteins. Therefore mitochondria are regarded as semi-autonomous cell organelle.

Oxysomes / F1 Particle :
These are also known as elementary particles or FO-F1 complex. These particles are associated with the inner membrane. F1 component of Oxysome lies towards the inner chamber. Each particle consists of 3 parts: head, stalk, and base piece. Oxysome represents ATPase enzyme which is concerned with ATP formation.

How is energy produced in Mitochondria?
 ATP is the main energy currency for all organisms. It does this through cellular respiration and regulating cell metabolism. A few chemical processes take place within the mitochondria to convert glucose (obtained from food) into energy.

Pyruvate (a 3-C sugar) is obtained from glucose (a 6-C sugar) during glycolysis in the cytoplasm. This pyruvate then goes through a “prep-step” or pyruvate conversion in the matrix of the mitochondria. The matrix is the space within the inner membrane.

The Krebs cycle then takes this newly formed acetyl-CoA from the prep step and forms it into electron carriers such as NADH, and FADH. This NADH and FADH are then sent to the Electron Transport Chain. The ETC takes the electron carriers and makes ATP.

Functions of Mitochondria

  1. The most prominent roles of mitochondria are to produce the energy currency of the cell, ATP (i.e., phosphorylation of ADP), through respiration, and to regulate cellular metabolism. The central set of reactions involved in ATP production is collectively known as the citric acid cycle, or the Krebs cycle.
  2. Mitochondria are crucial for advanced life on earth. A lot of energy is required to make the cells work, and the mitochondria produce that energy.
  3. Mitochondria are used in breakdown incoming nutrients and output energy via a biochemical process called oxidative phosphorylation.
  4. Mitochondria generate ATP from glycolysis-derived pyruvate, fatty acids, and amino acids through oxidative phosphorylation to conduct biological functions.
  5. Mitochondria act as biosynthetic hubs- metabolites produced in mitochondria can get into anabolic pathways to generate macromolecules such as glycogen, lipids, proteins, and nucleotides.
  6. Mitochondria also act as signaling organelles and regulate many biological processes such as cell proliferation, cell death, and metabolic adaptation through different mechanisms including, but not restricted to, the release of reactive oxygen species (ROS), and metabolites, such as acetyl CoA, calcium, cytochrome C, and mtDNA. 
  7. The mitochondrial outer membrane also serves as a scaffold for many proteins critical for cellular signaling.
  8. Mitochondria also synthesize heme which is a component of hemoglobin, necessary for red blood cells to carry oxygen around the body.
  9. Maintenance of proper concentration of calcium ions in the cell.
  10. The mitochondria of Liver cells have certain enzymes that help in the detoxification of Ammonia.
  11. The mitochondria also help in apoptosis or programmed cell death. death of mitochondria can lead to premature cell death and ultimately organ failure

Frequently Asked Questions 

Why is the mitochondrion called the powerhouse of a cell?
Answer: ATP is the energy currency of the cell is generated here. The Krebs cycle occurs in mitochondria through which ATP is generated. So it is known as the powerhouse of the cell.

Where are mitochondria found?
Answer: Mitochondria are found in all body cells, except for a few. There are usually multiple mitochondria found in one cell, depending upon the function of that type of cell.

Why mitochondria are absent in RBCs?
Answer: RBC doesn’t have mitochondria or some organelles because organelles need Oxygen to perform various functions. Now, if there are any organelles present, then they could utilize the oxygen carried by the RBC. In the image, we can see how mitochondria might utilize Oxygen.

How do cells utilize the energy made by the mitochondria?
Answer: Mitochondria produce energy through the process of cellular respiration. Respiration is another word for breathing. The mitochondria take food molecules in the form of carbohydrates and combine them with oxygen to produce ATP.

Is Mitochondrial DNA is Similar to nuclear DNA 
Answer: The mitochondria possess a completely different DNA than the cell’s nuclear DNA where protein synthesis occurs. Symbiotic theory suggests that the prehistoric cell engulfed the mitochondrial millions of years ago. Now they live in symbiosis. The mitochondria DNA is passed down from the egg so a child or offspring shares the same mitochondrial DNA as their mother.

What is the significance of mitochondrial DNA?
Answer: Mitochondria contain 37 genes and replicate independently of the cells. Those 37 genes mainly code for enzymes that comprise the electron transport chain which produces the vast majority of ATP needed by our cells. With fewer ATP cells would metabolize slowly and multicellular lifeforms would be poorly developed.

Mitochondria are inherited from the maternal side; research has demonstrated that nearly all humans are descended from a single female ancestor.

Do all mitochondria have a similar shape?
Answer: Yes, in general mitochondria do have an overall similar shape and similar size. This is because mitochondria must maintain the surface area to volume ratio that would allow metabolic reactions to take place at an optimal rate.

Why are proteins coded by genes in mitochondria?
Answer: The 13 mitochondrial gene-encoded proteins all instruct cells to produce protein subunits of the enzyme complexes of the oxidative phosphorylation system, which enables mitochondria to act as the powerhouses of our cells.

Why are mitochondria called semi-autonomous?
Answer: There are the following reasons, which make mitochondria semi-autonomous in nature-

  1. It contains their own DNA.
  2. Contains its own proteins, which are responsible for its many structural and functional activities.
  3. Having their own genome, responsible for coding.
  4. Having the ability to produce energy (through oxidative phosphorylation, crabs cycle& citric acid cycle) as a form of ATP, used by their own as well other organelles.

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