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Mitochondria Structure and Function Notes

In this mitochondria structure and function notes post we have briefly explained about mitochondria organelle definition, structure, morphology and functions.

Mitochondria Organelle

Mitochondria organelle (Gr., mito, thread; chondrion, granule) are thread like or granular structures of eukaryotic cells. These may assume rod-like shape called chondriosomes which may enlarge or aggregate to form massive spheroidal bodies called chondriospheres. These are not present in bacterial cells.

Mitochondria organelle are the ‘power plants’ which by oxidation release the energy contained in the fuel molecules or nutrients and make other forms of chemical energy. The main function of mitochondria organelle is oxidative phosphorylation, which is an exergonic reaction, meaning that it releases energy. In prokaryotes, oxidation of organic material is carried out by plasma membrane enzymes.

Definition

A cell has a compartment for energy production. It obtains energy from the food supplied by its environment. This energy then has to be converted into some form that can be distributed throughout the cell. The common solution is to store energy in the form of a common molecule that can be used whenever and wherever it is needed in the cell. The term ‘mitochondrion’ is derived from the Greek word ‘mitos’ which means ‘thread’ and ‘chondrion’ which means ‘granule’.

Mitochondria organelle lie freely in cytoplasm, possessing power of independent movement and may take the form of filaments. The number of mitochondria varies a good deal from cell to cell and from species to species. A few algae and some protozoan have only single mitochondria. In Amoeba (Chaos chaos), there may be as many as 50,000 mitochondria organelle. In rat liver cells, these are few in number, about 1000 to 1600. Some Oocytes contain as many as 3, 00,000 mitochondria.

History

Kölliker (1880) was the first who observed the mitochondria organelle in insects muscle cells. He called them as ‘sarcosomes’. Flemming (1882) named the mitochondria organelle as ‘fila’. Altmann in 1894 observed them and named them Altmann’s granules or bioblasts.

The term ‘mitochondria’ was applied by Benda (1897-98). They were recognized as the sites of respiration by Hogeboom and his co-workers in 1948. Lehninger and Kennedy (1948) reported that the mitochondria catalyze all the reactions of the citric acid cycle, fatty acid oxidation and coupled phosphorylation.

Morphology

Morphologically mitochondria organelle may be in the form of filaments or small granules. These may assume rod-like shape called chondriosomes which may enlarge or aggregate to form massive spheroid bodies called chondriospheres.

Mitochondria

Structure of a Mitochondria Organelle

Position

Mitochondria organelle lie freely in cytoplasm, possessing power of independent movement and may take the form of filaments. In some cells they can move freely, carrying ATP where needed, but in others they are located permanently near the region of the cell where more energy is needed. 

E.g., in the rod and cone cells of retina mitochondria are located in the inner segment, in cells of kidney tubules they occur in the folds of basal regions near plasma membrane, in neurons they are located in the transmitting region of impulse, in certain muscle cells (e.g. diaphragm), mitochondria organelle are grouped like rings or bracers around the I-band of myofibril. During cell division they get concentrated around the spindle.

Number

The number of mitochondria organelle varies a good deal from cell to cell and from species to species. A few algae and some protozoan have only single mitochondria organelle. Their number is related to the activity, age and type of the cell. Growing, dividing and actively synthesizing cells contain more mitochondria than the other cells. In Amoeba (Chaos chaos), there may be as many as 50,000 mitochondria organelle. In rat liver cells, these are few in number, about 1000 to 1600. Some Oocytes contain as many as 3, 00,000 mitochondria organelle.

Size

The average size of mitochondria organelle is 0.5-1.0µ in diameter and about 2-8 µ in length. In exocrine cells of mammalian pancreas they are about 10 µ long and in oocytes of amphibian Rana pipiens are 20-40µ long. Yeast cells have the smallest mitochondria.

Structure

The electron microscope shows the mitochondrion organelle as the vesicles bounded by an envelope of two unit membranes and filled with a fluid matrix.

Mitochondria Structure and Function Notes

Membranes

Both the inner and the outer mitochondrial membranes resemble the plasma membrane in molecular structure. Each of them is 60-70Å, trilamellar and composed of two layers of phospholipid molecules sandwiched between two layers of protein molecules.

The membranes may be connected at adhesion sites through which proteins are transferred from the outer to the inner membrane. The outer and the inner membrane are separated from each other by a narrow space called the inter-membrane space or outer chamber or peri-mitochondrial space. It is about 80Å wide. It contains a clear homogeneous fluid.  

1. Outer Membrane

The outer membrane is smooth permeable to most small molecules, having trans-membrane channels formed by the protein ‘porin’. It consists of about 50% lipid, including a large amount of cholesterol. It contains some enzymes but is poor in protein.

2. Inner Membrane

The inner membrane is selectively permeable and regulates the movement of materials into and out of the mitochondrion. It is rich in enzymes and carrier proteins permease. It has a very high protein/lipid ratio (about 4:1 by weight). It lacks cholesterol. Cardiolipin is closely associated with certain integral proteins and is apparently required for their activity.

Matrix

The space between the cristae called the inner chamber is filled with a gel like material termed the mitochondrial matrix. It contains proteins, lipids, some ribosomes, RNA, one or two DNA molecules and certain fibrils, crystals and dense granules.

Cristae

Mitochondria

Cristae in a mitochondria of an endothelial cell of human being

The inner mitochondrial membrane bears plate like infolding’s called the cristae. They extend inwards to varying degrees, and may fuse with those from the opposite side, dividing the mitochondrion into compartments.

They are arranged in a characteristic manner in different cells. Normally they run at right angles to the long axis of the rod shaped mitochondria. In cells of the proximal parts of the kidney tubules, the cristae are longitudinal folds parallel to the long axis of mitochondrion.

In many protozoans, in insect flight muscles cells and in adrenal endocrine cells the cristae are tubular. Cristae are lamellar in hepatocytes. In heart muscle cells cristae are zig-zag.

Oxysomes

Mitochondria

Detailed structure of a crista and an oxysome

The inner mitochondrial membrane bears minute regularly spaced particles known as the inner membrane subunits or elementary particles (EP) or oxysomes. An oxysome consists of three parts a rounded head piece or F1 subunit joined by a short stalk to a base piece or F0 subunit located in the inner membrane. There may be 100,000 to 1000,000 oxysomes in a single mitochondrion.

Functions

1. The mitochondria organelle can help the living cell to convert energy supplied by the environment into ATP, the common molecule, required for chemical reactions. ATP can be generated in two pathways: in the cytosol, and in mitochondrion. First pathway exists in the cytosol of an eukaryotic cell (or within a bacterial cell) where glycolysis degrades glucose to lactate and releases two molecules of ATP.

2. Second pathway is the main source of energy production as ATP (called oxidative phosphorylation and involves the electron transport chain). Pyruvate generated from glycolysis enters the matrix (lumen) of the mitochondrion, where it is degraded and combined with coenzyme A to form acetyl CoA. The acetyl part of the acetyl CoA is then degraded to carbon dioxide by the citric acid cycle, releasing hydrogen atoms. The hydrogen atoms are used to reduce the carrier NAD+ to NADH, and then oxidation of NADH releases a proton and an electron.

3. Mitochondria organelle help the cells to maintain proper concentration of calcium ions within the compartments of the cell. Mitochondria organelle also help in erythropoiesis and biosynthesis of hormones like testosterone and estrogen.

4. Enzymes involved in ammonia detoxification and urea synthesis (glutamate dehydrogenase, carbamoyl phosphate synthetase I and ornithine transcarbamylase) are exclusively expressed in the hepatocyte mitochondria.

5. The mitochondria organelle also play an important role in the process of apoptosis or programmed cell death. Abnormal death of cells due to the dysfunction of mitochondria can affect the function of an organ.

6. The mitochondria organelle are involved in other cellular activities like signalling, cellular differentiation and cell senescence. They also regulate the control of cell cycle and cell growth.

7. Unlike the outer membrane, the inner membrane is strictly permeable, it is permeable only to oxygen, ATP and it also helps in regulating transfer of metabolites across the membrane.

8. The matrix of the mitochondria organelle is a complex mixture of proteins and enzymes. These enzymes are important for the synthesis of ATP molecules, mitochondrial ribosomes, tRNAs and mitochondrial DNA.

9. Mitochondria organelle also affect human health. Mitochondrial disorders and cardiac dysfunction also play an important role in the aging process.

Further Readings

Reference

  1. https://www.britannica.com/science/mitochondrion
  2. https://www.ncbi.nlm.nih.gov/books/NBK545469/
  3. https://www.karger.com/article/fulltext/353883
  4. https://teachmephysiology.com/histology/cell-structures/mitochondria/
  5. https://byjus.com/biology/mitochondria/
  6. https://www.genome.gov/genetics-glossary/Mitochondria

Mitochondria Structure and Function Notes