The structure and functions of mitochondria
In this essay I will be discussing the structure and function of mitochondria. Mitochondria are membrane-bound organelles found in the cytoplasm of most cells. They are absent in prokaryotes but present in eukaryotic cells. Eukaryotic cells are all living things that are not bacteria or archaea. Mitochondria are believed to have evolved from bacteria. Mitochondrion is the second largest organelle. There are several scientists associated with mitochondria. Albert Von Kolliker first observed mitochondria in muscles of insects in 1857. He described mitochondria as granule like structures present in the muscle cells. Robert Altman and Walter Fleming then discovered mitochondria in 1886. Altman established them as cell organelles and described them as bioblasts. The name Mitochondria was given by Carl Benda in 1897. (Giezen 2011)
Mitochondria is a site of aerobic respiration in the cell. Aerobic respiration is where glucose is broken down to release energy. They are also known as the powerhouse of the cell because they are responsible for forming most of the cell’s energy which is stored in the form of adenosine triphosphate. This energy powers the different types of processes that take place within the eukaryotic cell. Mitochondria are also responsible for cytoplasmic inheritance, cell death also known as apoptosis, cellular differentiation as well as calcium signalling. Apoptosis is the ability of the mitochondria to release a molecule that essentially eliminates our cell. Apoptosis plays an important role by maintaining our health by killing old and unhealthy cells. Calcium signalling happens when the outer membrane of the mitochondria can associate with the membrane of the endoplasmic reticulum creating different types of important signalling pathways using calcium. The outer membrane and the endoplasmic reticulum works together to limit the amount of calcium in the cytosol. Mitochondria are also involved in thermogenesis. This takes place in adipose tissues in the mitochondria of the cells. Adipose tissues contain thermogenin which is a protein that plays a role in the transportation of protons into the mitochondrial matrix. Most mitochondria proteins are transported into the mitochondria through specialised proteins translocators complexes. Mitochondria are present in both plants and animal cells. Their quantity, the shape and the size depend on the type and the physiological activity of the cell. If the cell is more active it will have more number of mitochondria and it will have less number if it is less active. (Thompson 2017)
Each mitochondrion consists of two phospholipid bilayers; an inner and an outer membrane. The space between the inner and the outer membrane is known as intermembrane space or the peri mitochondrial space. The outer membrane is permeable to small ions and molecules, the concentration of these molecules and ions is the same in the cytosol and the peri mitochondrial space. The inner membrane is made up of many proteins that play an important role in producing ATP. These proteins can shuttle molecules in and out of the mitochondrial matrix. The inner membrane has a rough appearance due to finger-like projections known as cristae. The cristae increase the surface area for the complexes and proteins that help in producing ATP. The increased surface area creates more space for reactions to take place and hence allows the mitochondria to able to produce more ATP.ATP is a molecule that stores energy for the cell. The inner membrane is made up of a lipid bilayer, but it is impermeable to most ions and small molecules because it does not have porin proteins. The innermost part of the mitochondria is known as the mitochondrial matrix. It is called the matrix because it has a higher protein concentration than the outside of the mitochondria. These proteins are important for the synthesis of ATP molecules. They act as enzymes in the citric acid cycle during ATP synthesis. Matrix has DNA molecules which have a higher denaturation temperature molecules, ribosomes and components of protein synthesis. Matrix is also rich in divalent ions such as magnesium ions. These ions are the activators of the enzymes which take part in the cellular respiration. The mitochondrial circular DNA and the mitochondrial RNA are found in the matrix. The inner membrane contains an electron transport system that is made up of protein and lipids in a ratio of 2:1. These are the series of proteins that are responsible for setting up an electrochemical gradient between the mitochondrial matrix and the inter membrane space for the synthesis of ATP molecule. The inner membrane has structures known as the oxysomes or elementary particles. There are about 104 to 105 oxysomes in a mitochondrion. Oxysomes play an important role in the protein synthesis. The outer membrane of the Mitochondria is a phospholipid bilayer. It has less protein and it is freely permeable. It is smooth, and it is also composed of protein and lipids in a ratio of 1:1. This membrane is permeable to very small molecules such as nutrients molecules and ions. The outer membrane also contains integral proteins called porins.Porins are what allows certain types of small molecules to pass through via the process of facilitated diffusion without using any type of energy source. (Kuhlbrandit 2015)
Cellular respiration is the process by which we make ATP.Cellular respiration can be written as an overall equation.
This means one glucose molecule plus 6 oxygen will yield 6 carbon dioxide and water. The first step of cellular respiration is glycolysis. Glycolysis is the only step that occurs in the cytoplasm of the cell. During glycolysis, glucose molecule splits into two molecules of pyruvate. This process also leads to the production of two nicotinamide Adenine dinucleotide or NADH and two ATP.During the PDH process, the pyruvate molecules enters the mitochondria. Inside the matrix, the two molecules convert to two Acetyl Co. Acetyl CoA can then enter the Krebs cycle and generate ATP. (Wecker 2016)
The Krebs cycle takes place in the mitochondria matrix of the eukaryotes and in the cytosol of the prokaryotes. Krebs cycle is also known as the TCA cycle which is tricarboxylic cycle. t is named after its discoverer Sir Hans Adolf Krebs. The Krebs cycle is used to generate energy through the oxidation of Acetyl Co. Pyruvate acid which was synthesised because of glycolysis in cytoplasm diffuses in the mitochondria. It is then converted into Acetyl Co. Acetyl CoA combines with oxaloacetic acid to form a six-carbon acid called citric acid. Several chemical reactions take place during the Krebs cycle. One of the reactions releases enough energy to synthesis ATP molecule. Krebs cycle is an entry point of other catabolic mechanisms. Proteins can be broken down into amino acids and the amino acids can then be broken down into Acetyl -CoA and fats can be turned into glucose which could eventually go through cellular respiration. In the process of glycolysis and the Krebs cycles, FADH 2 and 10 NADH molecules is produced. These molecules play a role in transporting electrons from glucose into the electron transport chain which is on the inner mitochondrial membrane. The electrons move through complexes in the inner membrane. As they move across, protons are pumped outside the matrix into the inner membrane space. When these protons come back into the inner membrane space, more ATP is produced. Oxygen is produced and then broken down into water. These electrons are used to reduce Oxygen and form water. In the process, complexes all release protons. These protons are pumped from the mitochondrial matrix into the intermembrane space. The creation of electrochemical gradient creates a high amount of charge in the intermembrane space and that allows ions to move down the electrochemical gradient from a region of high concentration to low concentration. When these ions move ATP, molecules are synthesised.3 ATP molecules are formed from one NADH from citric acid cycle and 2 ATP from NADH coming from Glycolysis. (Brown 2013)
In conclusion, mitochondria are important organelles which are responsible for many processes of metabolism. They take fat, sugar and protein from our food and combine it with oxygen converting it into energy for our cells and tissues such as brain and muscle. Cells with an increased need for energy contain greater numbers of these organelles than cells with lower energy needs.