Mitochondria
Mitochondria (singular,mitochondrion) are typically tubular or sausage-shaped organelles about the size of bacteria that are found in all types of eukaryotic cells (figure below).Mitochondria areconspicuous organelles present in nearly all eukaryotic cells.They are diverse in size, number, and shape; some are rodlike,and others are nearly spherical. Mitochondria are present in all plants and animal cells except mature red blood cells.The mature red blood cells do not contain mitochondria at all.The absence of mitochondia in red blood cells leaves the space for millions of hemoglobin molecules needed in order to transport oxygen throughout the body.On the other hand muscle cells contain a large number of mitochondria needed to provide energy for muscle activity.They may be scattered uniformly throughout the cytoplasm or localized near cell surfaces and other regions of high metabolic activity. Within eukaryotic cells, mitochondria metabolize sugar to generate ATP.
Mitochondria are bounded by two membranes(phospolipid bilayer with embedded proteins): a smooth outer membrane, whereas the inner membrane is folded into numerous platelike or fingerlike projections called cristae(singular, crista) that play a key role in ATP generation. Which increase the internal surface area where chemical reactions occur.The outer membrane has many protein based pores that are big enough to allow the passage of ions and molecules,on the other hand inner membrane has much more restricted permeability,much like the plasma membrane of the cell.So the inner membrane form an effective barrier to even small molecules or ions.
Mitochondria are often called “powerhouses of cell ,” because these organelles generate power by converting energy into forms that are usable by cells.Mitochondira are the sites of cellular respiration.Through cellular respiration mitochondria produce the energy to perform the cellular activities such as cell division,cell growth or death.The enzymes located on the cristae catalyze the energy-yielding steps of aerobic metabolism. ATP (adenosine triphosphate), the most important energy-transfer molecule of all cells, is produced in this organelle.
Mitochondria are self-replicating. The cristae partition the mitochondrion into two compartments: a matrix, lying inside the inner membrane, and an outer compartment, or intermembrane space, lying between the two mitochondrial membranes. On the surface of the inner membrane, and also embedded within it, are proteins that carry out oxidative metabolism, the oxygen-requiring process by which energy in macromolecules is used to produce ATP.Matrix contains mitochondrial DNA,ribosomes and enzymes.
Mitochondria: The inner membrane of a mitochondrion is shaped into folds called cristae that greatly increase the surface area for oxidative metabolism. A mitochondrion in cross section and cut lengthwise is shown colored red in the micrograph.
Mitochondria have their own DNA,ribosomes and can make their own proteins.This circular DNA molecule contains several genes that produce proteins essential to the mitochondrion’s role in oxidative metabolism. Thus, the mitochondrion, in many respects, acts as a cell within a cell, maintaining its own genetic information specifying proteins for its unique functions. The mitochondria are not fully autonomous, however, because most of the genes that encode the enzymes used in oxidative metabolism are located in the cell nucleus.
A eukaryotic cell does not produce brand-new mitochondria each time the cell divides. Instead, the mitochondria themselves divide in two, doubling in number, and these are partitioned between the new cells. Most of the components required for mitochondrial division are encoded by genes in the nucleus and are translated into proteins by cytoplasmic ribosomes. Mitochondrial replication therefore is impossible without nuclear participation, and mitochondria thus cannot be grown in a cell-free culture.
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