Mitochondria

Millions of years ago, when the life was still unicellular and eukaryotic cells had just started evolving, one eukaryotic cell had a prokaryotic cell for desserts. Lo and behold! The prokaryotic cell could carry out respiration for the predator, producing large amounts of energy; amounts that the predator had never seen before (anaerobic glycolysis was the only route of energy production till then). So the predator cell established a symbiotic association with its prey and in the course of evolution the prokaryotic cell has become a part of all the cells and has come to be known as the Mitochondrion. This widely accepted hypothesis of mitochondrial origin is called the endosymbiotic theory (All circumstantial evidences support this theory, though there is no direct experimental proof). Other hypotheses like the autogenic origin claims that mitochondria was slowly evolved by the eukaryotic cells themselves like other organelles to have more organized and efficient energy production . Yet another hypothesis is that it arose from endoplasmic reticulum(ER) and slowly acquired its features. Whichever had be the way mitochondrion originated , it has become a vital organelle for the very survival of any eukaryotic cell, not only producing energy, but also participating in an array of functions that a cell performs.

Appearance:

Mitochondria come in many shapes- oblong, branched, oval or circular; the shape varies with cell type and its health. But all of them have two membranes encircling them- the outer membrane and the inner membrane. Outer membrane allows free movement of molecules from cytoplasm. The inner side of inner membrane is the highly folded (like those wrinkles of your grandpa skin) cristae. These wrinkles provide a large surface area tucked into the small organelle (0.5-10 µm), since this is the site of almost all actions happening inside the cell’s energy generator. The inner membrane and the space enclosed by it (the matrix) contain all the proteins, and molecules (O2 being one of it) to carry out respiration.

Division:

Housed inside the matrix is the mitochondrion’s DNA which can help it make some of the enzymes that it requires to carry out the energy production (rest of the information is embedded in DNA in the nucleus). Mitochondrial DNA undergoes replication (make a photocopy of itself) and like a bacteria or yeast, a mitochondrion can undergo fission to form two mitochondria (with DNA distributed equally between them). If a mitochondrion grows old and loses its DNA it is eaten up by cell’s cleaning system called lysosomes (similar to phagocytes which are the scavenger cells in the body). In a given cell the number of mitochondria is maintained constant; not just by fission, but also by fusion where two or more mitochondria come together and form a new mitochondria.

Powering the cell:
The most important activity that the mitochondria do is energy production. The food that you eat is broken down in your digestive system to glucose and transported to the cells of your body. This glucose is used by the cells to produce energy for all its work. The energy in the cell is in the form of ATP (Adenosine triphosphate). Prokaryotic organisms like bacteria which lack mitochondria also use glucose and produce energy, so why have mitochondria in the first place? Mitochondria can help in producing more number of ATPs from the same number of glucose. One molecule of glucose can be used to make ~30 ATP molecules by the mitochondria (cells without mitochondria can make only 2 ATPs for every glucose molecule used)
Mitochondria are very similar to the power plants generating electricity. Glucose is like the coal used to boil water into steam; here glucose transfers its energy to NADH (a coenzyme involved in redox reactions) through a series of reactions called the glycolysis and the Krebs’ cycle. NADH in turn transfers its energy ( in the form of electrons) to the molecules forming the electron transport chain with O2 as the final electron acceptor . In the process a proton pool on the outer side of the inner membrane is generated. These protons (like the steam driving a turbine), drive ATP synthase (consider it as a fusion of turbine and generator), to synthesis ATP from ADP.

Calcium buffering:
Ca2+ is a very important signalling molecule for the cell, levels of which regulate an array of reactions happening inside the cell. Hence keeping a balance of its levels inside the cell is very important. Too much of Ca2+ roaming around freely in the cell can even kill a cell! Mitochondria like the other cell organelle endoplasmic reticulum can store Ca2+ within them temporarily and release it as and when the cell needs it.

Death signal:
For the good of the whole, sometimes a few cells which have grown old or are defective undergo death by suicide called apoptosis. When a cell senses (pro-apoptotic stimuli) something wrong with itself it embarks on a series of events that finally will lead to its death. Mitochondria play a significant role in sensing and participating in these changes. Mitochondrial inner membrane contains a molecule called cytochrome C (Cyt C). When mitochondria gets green signal for death, Cyt C is released from it. Cyt C then activates enzymes( caspases) which help in breaking down the cell starting from its DNA to other organelles packaging each of them into apoptotic bodies , which form the  neatly packed cell debris and
Which then are cleared by our body’s immune system.

To carry out all these functions, the mitochondria moves around in the cell wherever they are needed. Consider a protozoa swimming towards its prey, it requires more energy in the region where it is throwing its pseudopodia. So, more number of mitochondria will be taken to that area by the microtubules , so that there is no shortage or delay in the supply of ATPs. Further, new ones are brought to the scene of action, when the previously present mitochondria gets exhausted (these are moved back into the cell again where they are given time to rejenuvate). This double membrane organelle, however its origin might have been, is an indispensible protagonist in the cell, keeping the cell healthy and bubbling with energy.