Mitochondria are cellular structures that generate the energy required by cells for life processes. These organelles are mostly found in Eukaryotic cells. Their name comes from the Greek word- ????? (mitos or thread) and ???????? (chondrion or granular).

The number of mitochondria can vary from a few hundred to more than 2000/per cell (hepatocytes). The number and distribution of mitochondria are proportional to the energy requirement of the cells. Cells like red muscles that do sustained work are abundant in mitochondria. Similarly, the liver has high energy demand. Therefore, each hepatocyte can have in excess of 2000 . On the other hand, the white muscles have a small number of these organelles despite consuming a large amount of energy. Most of the energy used by the white muscles comes from anaerobic metabolism of glycogen. Therefore, the number of mitochondria is also small. Some cells like the red blood cells do not contain any mitochondria.

We have previously learned about aerobic and anaerobic metabolism. Mitochondria are exclusively involved in aerobic metabolism.

These organelles are 0.5 to 1.0 micrometres in size. They have a double layer of lipid membrane and the proteins and enzymes of the mitochondrial electron transport system are embedded within the two layers of the mitochondrial membrane.

mitochondria

Glucose, the principal source of energy for the cell is degraded in the cytosol by anaerobic metabolism to produce pyruvate. The pyruvate is taken up by the mitochondrial membrane by an active process and this (pyruvate) is further degraded by the enzymes in the mitochondrial matrix. The process of degradation of pyruvate results in the production of water, carbon dioxide and energy. The energy released by the breakdown of glucose is captured in the phosphate bonds of Adenosine triphosphate (ATP). ATP is used by the cell for its energy requirements. As oxygen is required for this process, this (breakdown of glucose) is called as aerobic metabolism or respiration.

Mitochondria are unique in many aspects. They can replicate, i.e. it can divide and reproduce in response to increasing energy demand. They also contain DNA, the genetic code of life. Defects or problems in the mitochondrial DNA can cause many diseases- mitochondrial disease. The musculoskeletal system is commonly involved by disorders of the mitochondria. These disorders are called mitochondrial myopathies. The pattern of inheritance of mitochondrial disorders is also unique. As the zygote (the fertilised ovum) receives all its mitochondria from the mother, all mitochondrial disorders are transferred from the mother to the offspring.

In addition to generating energy, these organelles also regulate many cellular functions like cell signalling, differentiation, control of cell cycle and apoptosis.

In conclusion, mitochondria are vital organelles that generate energy for maintenance of life processes.

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