Mitochondria - Structure and Function & Mitochondrial DNA
Introduction
We all eat our breakfast, dinner, and lunch, but have you ever wondered how we get energy from food? How can we use this energy? Well, it is due to mitochondria. Mitochondria are important cell organelles in our body and are known as the powerhouse of cells. Different types of cells have different numbers of mitochondria. For example, simple cells have one or two mitochondria, whereas complex cells, such as plant or animal cells, have numerous mitochondria as they require more energy.
Plant cells have hundreds of mitochondria, while the number goes to thousands and lakhs for animal cells. Interestingly, the human body has 1,00,000 to 6,00,000 mitochondria in each cell which can produce 90% of the energy in the body to perform daily tasks. In this article, we will discuss the mitochondria diagram, function of mitochondria, and structure of mitochondria.
Mitochondria
Mitochondria are the powerhouse of cells as they generate energy for cell functioning. The structure of mitochondria is unique. The mitochondrion is a rod or sausage-shaped structure found in animal and plant cells. It is a small organelle whose size is between 0.5 to 1 micrometre in diameter. Hence, it cannot be seen under a microscope unless stained. Unlike other organelles, it has two layers; inner and outer. Each layer performs different functions.
Let us understand the structure with the mitochondria diagram.
Mitochondria Structure
Outer Membrane - It is made of proteins. The membrane allows small protein-like molecules to pass through it.
Intermembrane Space - It is the space between outer and inner membranes.
Inner Membrane - This membrane is made of phospholipids and does not allow molecules to pass through it. Special transporters (carrier molecules) are required to transport substances. Here, ATP production takes place.
Cristae - These are the irregular folds of the inner membrane. They increase the space for chemical reactions to take place by increasing the surface area of the membrane.
Matrix - It is fluid within the inner membrane. This fluid has several enzymes required for ATP production. It also contains ribosomes, mitochondrial DNA, inorganic and organic molecules, etc.
Mitochondrial DNA
Mitochondrial DNA is a double stranded circular molecule, which is inherited from the mother in all multi-cellular organisms, though some recent evidence suggests that in rare instances mitochondria may also be inherited via a paternal route.
Typically, a sperm carries mitochondria in its tail as an energy source for its long journey to the egg. When the sperm attaches to the egg during fertilization, the tail falls off. Consequently, the only mitochondria the new organism usually gets are from the egg its mother provided.
There are about 2 to 10 transcripts of the mt-DNA in each mitochondrion. Compared to chromosomes, it is relatively smaller, and contains the genes in a limited number.
The size of mitochondrial genomes varies greatly among different organisms, with the largest found among plants, including that of the plant Arabidopsis, with a genome of 200 kbp in size and 57 protein-encoding genes.
The smallest mtDNA genomes include that of the protist Plasmodium falciparum, which has a genome of only 6 kbp and just 2 protein- encoding genomes. Humans and other animals have a mitochondrial genome size of 17 kbp and 13 protein genes.
Mitochondrial DNA consists of 5-10 rings of DNA and appears to carry 16,569 base pairs with 37 genes (13 proteins, 22 t-RNAs and two r-RNA) which are concerned with the production of proteins involved in respiration.
Out of the 37 genes, 13 are responsible for making enzymes, involved in oxidative phosphorylation, a process that uses oxygen and sugar to produce adenosine tri-phosphate.
The other 14 genes are responsible for making molecules, called transfer RNA (t-RNA) and ribosomal RNA (r-RNA). In some metazoans, there are about 100 – 10,000 separate copies of mt-DNA present in each cell.
Unlike nuclear DNA, mitochondrial DNA doesn’t get shuffled every generation, so it is presumed to change at a slower rate, which is useful for the study of human evolution.
Mitochondrial DNA is also used in forensic science as a tool for identifying corpses or body parts and has been implicated in a number of genetic diseases, such as Alzheimer’s disease and diabetes.
Changes in mt-DNA can cause maternally inherited diseases, which leads to faster aging process and genetic disorders.
Mitochondria convert the potential energy of food molecules into ATP by the Krebs cycle, electron transport and oxidative phosphorylation in presence of oxygen.
The energy from food molecules (e.g., glucose) is used to produce NADH and FADH2 molecules, via glycolysis and the Krebs cycle. The protein complexes in the inner membrane (NADH dehydrogenase, cytochrome c reductase, cytochrome c oxidase) use the released energy to pump protons (FT) against a gradient.
Mitochondrial marker enzymes
Mitochondrial marker enzymes are enzymes that are specifically present in mitochondria, in the mt-matrix, the inner mt-membrane, the inter-membrane space, or the outer mt-membrane.
Citrate synthase (mt-matrix)
NAD+ malate dehydrogenase (mt-matrix)
NAD+ glutamate dehydrogenase (mt-matrix)
Succinate cytochrome c reductase (inner mt-membrane)
Rotenone-sensitive NADH cytochrome c reductase (inner mt-membrane)
Adenylate kinase (intermembrane space)
Rotenone-insensitive NADH cytochrome c reductase (outer mt-membrane)
Monoamine oxidase (outer mt-membrane)
Kynurenine hydroxylase (outer mt-membrane)
Function of Mitochondria
The most common function of Mitochondria is energy generation. However, it performs several other vital functions of the body. These include the following.
Energy Generation
Mitochondria help produce ATP molecules which are the energy units of cells. Most energy production takes place in the cristae or folds of the inner membrane. It generates energy by converting chemical energy from food.
Cell Death
Apoptosis or cell death is an essential part of the regeneration of new cells. As cells damage or become old, they are destroyed by the mitochondria, and new cells are formed. It releases enzymes like Cytochrome C, which helps in cell degeneration.
Heat Production
In extreme colds, the body generates its heat by utilising tissue fat. Mitochondria release energy in the form of heat in cold climatic conditions.
Storing Calcium
Calcium is involved in several cellular processes. For example, releasing neurotransmitters for nerve conduction and muscle movement, blood clotting, fertilisation, hormone signalling, steroid synthesis, and cellular metabolism. As calcium is so important for the body, cells regulate it tightly. Mitochondria help in the absorption of calcium ions and store them until they are used.
New Things About Mitochondria
1.Mitochondria are the powerhouse of cells and produce energy.
2.It has a complex structure, and each performs distinct functions.
3.Like a nucleus, it has its DNA.
4.The outer membrane of the mitochondria has a protein called porins which forms protein channels to facilitate molecule transport.
5.Mature red blood cells have no mitochondria.
6.Liver cells have more than 2,000 mitochondria.
7.It has dividing and multiplying abilities.
8.In human sperm, mitochondria are spiral-shaped and provide energy for motion.
Summary
Mitochondria are popularly known for their ability to generate energy. They are a double membrane organelle and have a typical structure and are called the powerhouse of a cell. They carry numerous bodily functions and metabolic activities, including heat generation, apoptosis, neurotransmitter regulation, calcium uptake, etc. Interestingly, they have their DNA while other cell organelles do not possess DNA.
Changes in the mitochondrial DNA can lead to insufficient ATP or energy production and other critical diseases. The number of mitochondria varies from cell to cell or organism to organism. For example, human liver cells have more than 2000 mitochondria, whereas human red blood cells have no mitochondria. In the end, we have seen some facts about mitochondria. If there are any doubts related to the topic please ask in the comments.
