Mitochondria: Structure, Functions and Diagram

Overview

Mitochondria is a semi-autonomous cell organelle present in almost all the eukaryotic animal cells except for a few, like RBCs. Mitochondria are known as the Powerhouse of the cell because it is responsible for generating energy currency in the form of ATP which is later utilized by the cell for performing various functions. It is called as semi-autonomous because it contains its own genetic material in the form of Circular DNA. Human mitochondrial DNA is double-stranded and is passed from mother to her offsprings during reproduction. And hence, mitochondrial DNA can be traced back through the maternal progeny. Mitochondria contain its own Ribosome, therefore, it can synthesis its own proteins but majorly they are encoded by nucleus only (99%).

Structure of Mitochondria

Mitochondria are made up of following structures which play an essential role in all the processes taking place inside mitochondria.

• Outer mitochondrial membrane (The outer layer of the mitochondrial covering)

The outer membrane contains pores like structure called Porins through which, small, uncharged particles can pass inside and out of the mitochondria. For large, charged molecules to pass, special transmembrane proteins are required as they can not pass through the porins. The outer membrane is more permeable compared to the inner membrane

• Inner mitochondrial membrane (The inner layer of mitochondrial covering)

This is the site for the process of electron transport chain (ETC). Cristae, which are the foldings of the inner mitochondrial membrane, increase the surface area of the inner membrane. The increased surface area is required because most of the processes of ATP generation occur in this membrane only via oxidative phosphorylation, therefore increased surface area would give more space for the process to occur with more efficiency. The f0-f1 particles present on the cristae are the transmembrane proteins which are responsible for pumping protons (H+ ion) into the intermembrane space and thus generating ATPs. The inner membrane is very rigid and only allow particles through transporters.

• Inter membrane space (The space between the two membranes)

The inner membrane space contains enzyme ATP synthase which is responsible for generating ATP in the matrix.

• Matrix

Matrix is the fluid-filled space inside mitochondria containing important structures like DNA, Ribosomes, enzymes, proteins etc. In the matrix of mitochondria, important cycles like TCA (Kreb’s cycle), Beta Oxidation take place.

Functions of Mitochondria

Mitochondria are responsible for many major activities inside a cell:

• It is the main site for ATP synthesis and therefore it is called a powerhouse of the cell (Oxidative Phosphorylation). The cells like muscle cells, liver, indicate the higher rate of ATP utilization in these areas. In mature RBCs, mitochondria are absent in order to create more space for haemoglobin for oxygen binding.

• The inner membrane of mitochondria has proteins (F0-F1 particles) which are- involved in electron transport and ATP synthesis.

• Mitochondria has a vital role in uptake and release of Ca2+ which maintains the concentration of calcium in the cytoplasm of the cells.

• Mitochondria also play a very essential role in the process of  Apoptosis in mammalian cells. The proteins of BCL2 family regulates the release of cytochrome c from the inner and outer membrane of mitochondria which, once in cytoplasm, causes activation of other apoptotic signals, finally causing cell death.

• In adipose tissues, mitochondria release heat via electron transport chain.

Endosymbiotic Theory

Endosymbiotic theory states that the mitochondria and chloroplast are a result of endocytosis (engulfment) of the aerobic bacteria (prokaryote) by a eukaryotic cell. A large aerobic bacteria which would not have been able to survive otherwise, fused with the eukaryotic cell. There are many shreds of evidence to support the endosymbiotic theory of mitochondria, for example, it has its own circular DNA just like prokaryotic cells. Mitochondria divide by binary fission, independently of the cell they live in. The size of the mitochondria and prokaryotic cells are almost similar.