Introduction to Mitochondria:
Mitochondria are membrane-bounded cell organelles present in the cytoplasm. Found in all eukaryotic cells, produces adenosine triphosphate (ATP) i.e. the powerhouse of the cell which produces energy. Singular: mitochondrion, Mitochondria is the plural form. The term ‘mitochondrion’ is derived from the Greek words “mitos” and “chondrion” which respectively mean “thread” and “granules-like”. Carl Benda in the year 1898 coined the term "mitochondria". It was discovered by Richard Altman in 1886.
These are double-membrane organelle, functions as “digestive system” of the cell. They help in breaking down nutrients and generating energy for the cell. Cellular respiration and some other biochemical reactions take place within the mitochondria.
Structure of Mitochondria includes:
It is double-membraned and rod-shaped
found in both plant and animal cell.
Size ranges from 0.5 to 1.0 mm in diameter.
The structure comprises of an outer membrane, an inner membrane, and matrix(gel-like material).
The outer membrane and the inner membrane are made up of proteins and phospholipids
The two membranes are separated by the inter-membrane space.
The outer membrane covers the surface of the mitochondria
The out membrane also contains a large number of special proteins known as porins.
It is permeable to ions, nutrient molecules, energy molecules like the ADP and ATP.
The inner membrane of mitochondria is permeable has many folds. Folds help in increased surface area which creates more space for more reactions and increases its functionality. These folds form a layered structure called cristae. The inner membrane holds the proteins involved in the electron transport chain.
The cristae and the proteins of the inner membrane help in the production of ATP molecules. Matrix
Matrix is a viscous fluid that contains a mixture of enzymes and proteins. It is the inner space enclosed by the inner membrane. It also contains ribosomes, inorganic ions, mitochondrial DNA, nucleotide cofactors, and organic molecules. The enzymes present in the matrix play an important role in the synthesis of ATP molecules.
The matrix contains a variety of enzymes and proteins that are important for energy production. E.g.
• Enzymes. The mitochondrial matrix contains hundreds of enzymes that help in the Krebs cycle. e.g. tricarboxylic acid cycle (TCA). TCA helps in the production of energy in the form of adenosine triphosphate (ATP).
• Ribosomes. Mitochondria contain ribosomes. Basically 70S type ribosomes, which is found in prokaryotic cells. These ribosomes can help in protein synthesize.
• Mitochondrial DNA. Mitochondria have a small amount of DNA i.e. mtDNA.
Function of Mitochondria
Mitochondria help in ATP synthesis which produces energy through oxidative phosphorylation process. i.e. cellular respiration.
Helps in aerobic respiration, which requires oxygen.
Mitochondria is the work station for the citric acid cycle or Krebs cycle.
This cycle involves the oxidation of pyruvate. Pyruvate from glucose, help to produce acetyl-CoA molecule. Acetyl-CoA molecule is oxidized to produce ATP.
The citric acid cycle helps to reduce (NAD+) to NADH. NADH is used in the process of oxidative phosphorylation, which also takes place in the mitochondria. Electrons from NADH travel through protein complexes and are embedded in the inner membrane of the mitochondria. This set of proteins promotes the electron transport chain. Energy from the electron transport chain is then used to transport proteins back across the membrane, which gives them the power to form ATP.
The amount of mitochondria present in a cell depends upon how much energy that cell needs to produce. For example Muscle cells, have many mitochondria as they need to produce more energy for body movement, Red blood cells, which carry oxygen to other cells, do not need to produce more energy, as they work less.
Mitochondria are the powerhouse of the cell because it produce energy.
Mitochondria can store calcium, which maintains homeostasis of calcium levels in the cell.
It also regulates the cell’s metabolism and thermogenesis (heat production).
Regulates the metabolic activity of the cell Promotes cell growth and cell multiplication
Helps in detoxifying ammonia in liver cells Plays an important role in apoptosis i.e. cell death or degradation of cell. That’s why also referred to as the graveyard of the cell
Responsible for building certain parts of the blood cells and various hormones: testosterone and oestrogen Help in maintaining the concentration of calcium ions within the compartments of the cell involved in various cellular activities like cellular differentiation, cell signalling, cell senescence, controlling the cell cycle and cell growth.
Mitochondria help to maintain the concentration of calcium ions within the cell.
Cells having a high metabolic rate, viz: hepatocytes will have many mitochondria
Mitochondria also maintain the intracellular environment.
Mitochondria are involved in thermogenesis or production of heat. Occurs in the mitochondria present in brown adipose tissues, which is few in no. in human beings. These tissues contain a protein called thermogenin. Thermogenin help in transportation of protons into the mitochondrial matrix.
Apoptosis is the process of cell death. During apoptosis, shrinkage of the cell, fragmentation of the nuclear structure, condensation of chromatic, etc. occur.
Mitochondria play a dual role in apoptosis. They protect healthy cells and facilitate apoptosis.
During apoptosis, a special type of protein is produced called proapoptotic. These proteins enter into the mitochondrial membrane and form pores within it. As a result, the proteins present within the mitochondria enter the cytosol and initiate a series of biochemical processes that result in the death of the cell.
Storage of Calcium Ions
Calcium ions are stored in mitochondria and endoplasmic reticulum i.e. ER. Mitochondria store calcium for a short time, to maintain the cell homeostasis.
Diseases associated with Mitochondria:
Any irregularity in the way mitochondria functions can directly affect human health But it is difficult to identify because the symptoms differ from person to person. Disorders of the mitochondria can be quite severe and can lead to organ failure. Some examples of Mitochondrial diseases are Alpers Disease, Barth Syndrome, Kearns-Sayre syndrome (KSS).