Diagram of Mitochondria

This cell organelle is oval-shaped and the size generally ranges between 0.5 to 10 μm. The theory of endosymbiosis states that all mitochondria were free-living organisms and used aerobic respiration. Large anaerobic cells engulfed these organelles to use their energy which gave rise to complex cells.

The Structure of Mitochondria

Mitochondria is a membrane-bound cell organelle which can be found in eukaryotic cells.  They also contain their own genetic material which is why they can self replicate. The level of organisation differs between cells based on organisms. The name mitochondria was coined by Carl Benda, a microbiologist who was researching developing sperms.

There are five major components in the structure of mitochondria. These are the outer membrane, the inner membrane, cristae and the matrix. Below are brief summaries of each of the component:

Outer membrane: The outer portion of the mitochondria includes proteins called porins which have channels to pass small proteins and similar molecules to pass through. The outer membrane also hosts many enzymes which have a wide range of functions.

Intermembrane space:  This is the area between the inner and the outer membranes which are separated by 10 to 40 nm structures called cristae junctions. The intermembrane space is clearly subdivided into two compartments, the lumen and the intra-cristae space. The function of the intermembrane space includes modification of proteins, transportation of molecules and coordination of apoptosis. 

Inner membrane: There are no porin channels in the inner membrane which makes it impermeable to many molecules. Only some specific molecules can enter this region with the help of unique transporters. This is the region where most of the ATP molecules are formed.

Cristae: Cristae are the folds observed in the inner membrane. By increasing the surface area of the inner membrane they increase the space of chemical reaction within the mitochondria. These are similar to the microvilli in the intestine and the electron transport chain takes place here.

Matrix: The space within the inner membrane is called the mitochondrial matrix. This space houses hundreds of enzymes and the mitochondrial DNA. It is a gel-like region where the Krebs cycle takes place.

The matrix side of the folded membrane has certain structures which are sometimes called as stalked particles. In these, light-bulb like structures, Adenosine di Phosphaphate is converted to Adenosine tri Phosphate. 

A well-labelled diagram of mitochondria is given below for your better understanding of the structure.

Labelled Diagram of a Mitochondrion

Image will be updates soon

Characteristics of the Mitochondrial DNA/ Genome:

• The mitochondrial DNA is circular and is made up of 16,569 DNA base pairs.

• It contains 37 genes out of which 13 code for proteins, 22tRNAs and 2 rRNAs.

• The 13 mitochondrial genes that are encoded for proteins instruct cells to produce protein subunits of the enzyme complexes that are utilised in the oxidative phosphorylation system. This enables mitochondria to act as the powerhouses of cells.

• One mitochondrion can have many copies of the mt Genome.

• The mtDNA is not enveloped and is not packaged in chromatin.

• The mode of inheritance for mtDNA is strictly maternal.

• The genome also contains few non-coding DNA sequences.

The Function of Mitochondria

The major function of mitochondria is to generate energy by breaking down carbohydrates and fatty acids. The energy produced is used in the form of ATP.

ATP generation takes place in the mitochondrial matrix. After glucose is converted to pyruvate outside the organelle it is transported to the matrix. For fatty acids, they enter the matrix in their original form.

In the first step, the enzymes present in the matrix convert pyruvate and fatty acids into acetyl CoA which uses the starting material for the TCA or Krebs cycle. The products of this reaction are CO2, FADH2 and NADH. It should be noted that FADH2 and NADH are electron-rich products and in the second step, they move to the inner mitochondrial membrane. In the third step, oxidative phosphorylation begins and NADH and FADH2 donate their electrons and contribute towards the formation of the energy molecules ATP.

Mitochondria also synthesize their own proteins and they can start the transcription of RNA and DNA, translation of RNA to protein units known as amino acids without the help of any other component of the cell.

Mitochondria also functions as a calcium storage unit, It can absorb, hold and release calcium which can initiate the release of hormones from endocrine cells or a neurotransmitter from any nerve cell.

Mitochondria can also generate heat through a process called proton leak. This function of mitochondria is known as non-shivering thermogenesis. The heat is generated from a tissue called brown fate which can be found at a higher level in babies.