Introduction
Mitochondria are organelles found in nearly all eukaryotic cells. Their primary function is the synthesis of ATP in aerobic respiration. They are also involved in lipid synthesis.
Structure
Mitochondria are approximately 7 micrometres long and consist mainly of a double phospholipid envelope, the inner membrane of which is infolded into projections called cristae. The middle of the mitochondrion, called the matrix, contains small amounts of mitochondrial DNA and ribosomes. There are usually several mitochondria in each cell, depending on how much energy the cell requires.
Function
The primary function of the mitochondrion is the second half of the respiration process, where glucose and other sugars are converted into ATP, the body's "energy currency". Respiration can be divided into four main parts: glycolysis, the Krebs cycle, oxidative phosphorylation and the synthesis of ATP. The Krebs cycle, oxidative phosphorylation, and the synthesis of ATP all occur in the mitochondria. In anaerobic respiration only glycolysis occurs and mitochondria are not necessary.
The Krebs cycle:
The Krebs cycle is the process where the pyruvate created by glycolysis is broken completely down into carbon dioxide. It occurs in the matrix of the mitochondria. Although some ATP is created here, most of the energy from aerobic respiration is actually produced by the hydrogen atoms, which are removed from the cycle by hydrogen carrier molecules such as NAD or FAD, and enter the oxidative phosphorylation step.
Oxidative phosphorylation
The
hydrogen atoms taken from the
Krebs cycle are taken to the inner
membrane of the
mitchondrial envelope, where the
hydrogen atoms are split into
electrons and
hydrogen ions. The
electrons are passed along a chain of
electron carriers (usually
cytochromes) until the are recombined with the
hydrogen ion and
oxygen to create
water. Energy is produced by the
electron transfer chain which is used to pump
hydrogen ions into the
inter-membrane space for the
synthesis of
ATP.
Synthesis of ATP
Although some ATP has been produced by the earlier reactions, most of the ATP is produced by the movement of hydrogen ions from the intermembrane space, down the concentration gradient, into the matrix. The hydrogen ions can only pass through a protein in the membrane called ATP synthase, which uses the energy of the movement of hydrogen ions to add a phosphate group to ADP to create ATP.
Endosymbiotic theory
Several features of mitochondria have led to the endosymbiotic theory, which states that they were once separate simple single-celled organisms, which were completely absorbed by larger organisms, which then harnessed it's ability to aerobically respire to gain an evolutionary advantage. Evidence for this theory includes the presence of separate DNA and ribosomes in the mitochondria