Cell Structures - The Mitochondrion
Created | Updated Jan 28, 2002
Mitochondria are double-membraned cell organelles responsible for aerobic respiration within eukaryotic cells, and are found in both plants and animal cells. They convert pyruvic acid, from the breakdown of glucose, into the energy-yielding molecule adenosine triphosphate (ATP), and in general, are the main energy production centres within the cell. The number of mitochondria present in a cell may range from one to thousands, depending on the energy requirements of that cell, muscle cells will have more mitochondria than most other cell types. Mitochondria have their own DNA, separate from that of the cell nucleus.
Typically, Mitochondria are capsule shaped organelles, ranging in size from 0.5-1.0µm, although they may also be almost spherical, oval or kidney shaped. Their structure can be divided into four main areas:
The Outer Membrane
This is a smooth membrane containing many transport proteins allowing materials to be moved into and out of the mitochondrion.
The Intermembrane Space
A compartment, filled with liquid, which is used to store the protons necessary for ATP production.
The Inner Membrane
Unlike the outer membrane, this is highly convoluted into folds called cristae. This greatly increases the surface area where the membrane-bound synthesis of ATP takes place. ATP is formed by adding a phosphate group to Adenosine Diphosphate (ADP), this is termed oxidative phosphorylation
The Matrix
Contained within the inner membrane, this area is rich in enzymes and is the site of Kreb’s (or the citric acid) cycle. The matrix also contains several copies of the circular mitochondrial DNA genome, ribosomes for protein production, and transfer RNA.
After synthesis, the energy-rich ATP molecule is transported to the cell cytoplasm where it is used in nearly all processes that require energy input. Upon release of the energy stored in its third phosphate bond, the ATP is converted back to ADP, which is returned to the mitochondria for further phosphorylation.
ATP is produced anaerobically in the cell cytoplasm during the conversion of glucose into pyruvic acid (glycolysis), but only four ATP molecules can be gained from each glucose molecule in the absence of oxygen, and it takes two ATP to fuel glycolysis. However, under aerobic conditions, the pyruvic acid molecules that result from glycolysis can be converted by the mitochondria into a further 34 ATP molecules (dependent on conditions), plus Carbon Dioxide and Water.
Production of ATP is not the only function of the mitochondrion. Its DNA, apart from coding for proteins for its own use, also codes for important proteins in the nervous, circulatory, and other systems of mammals. Mutations in mitochondrial DNA have been associated with a number of genetic disorders including diabetes mellitus, Alzheimer’s and Parkinson’s diseases.
Mitochondria are thought to have arisen out of a symbiotic relationship which started when a prokaryotic (ie bacterial) cell was ingested, either by another prokaryote or by a eukaryote, but wasn’t digested. This ingestion may have led to the outer membrane of the mitochondrion, they can function without this membrane and are then called mitoplasts. They have many features in common with current prokaryotes, including the circular DNA, prokaryotic-like ribosomes, and division by binary fission.
In mammals mitochondrial DNA is inherited from the mother, as sperm mitochondria are contained in the energy-requiring tails, which are left out of the fertilised egg cell. Thus if a generation in a family fails to have any female offspring, that line of mitochondrial DNA inheritance will die out. One other consequence of the separate mitochondrial DNA is that animals cloned using a body cell from another animal as host, will have the nuclear DNA of the donor, but the mitochondrial DNA of the host cell.
Typically, Mitochondria are capsule shaped organelles, ranging in size from 0.5-1.0µm, although they may also be almost spherical, oval or kidney shaped. Their structure can be divided into four main areas:
The Outer Membrane
This is a smooth membrane containing many transport proteins allowing materials to be moved into and out of the mitochondrion.
The Intermembrane Space
A compartment, filled with liquid, which is used to store the protons necessary for ATP production.
The Inner Membrane
Unlike the outer membrane, this is highly convoluted into folds called cristae. This greatly increases the surface area where the membrane-bound synthesis of ATP takes place. ATP is formed by adding a phosphate group to Adenosine Diphosphate (ADP), this is termed oxidative phosphorylation
The Matrix
Contained within the inner membrane, this area is rich in enzymes and is the site of Kreb’s (or the citric acid) cycle. The matrix also contains several copies of the circular mitochondrial DNA genome, ribosomes for protein production, and transfer RNA.
After synthesis, the energy-rich ATP molecule is transported to the cell cytoplasm where it is used in nearly all processes that require energy input. Upon release of the energy stored in its third phosphate bond, the ATP is converted back to ADP, which is returned to the mitochondria for further phosphorylation.
ATP is produced anaerobically in the cell cytoplasm during the conversion of glucose into pyruvic acid (glycolysis), but only four ATP molecules can be gained from each glucose molecule in the absence of oxygen, and it takes two ATP to fuel glycolysis. However, under aerobic conditions, the pyruvic acid molecules that result from glycolysis can be converted by the mitochondria into a further 34 ATP molecules (dependent on conditions), plus Carbon Dioxide and Water.
Production of ATP is not the only function of the mitochondrion. Its DNA, apart from coding for proteins for its own use, also codes for important proteins in the nervous, circulatory, and other systems of mammals. Mutations in mitochondrial DNA have been associated with a number of genetic disorders including diabetes mellitus, Alzheimer’s and Parkinson’s diseases.
Mitochondria are thought to have arisen out of a symbiotic relationship which started when a prokaryotic (ie bacterial) cell was ingested, either by another prokaryote or by a eukaryote, but wasn’t digested. This ingestion may have led to the outer membrane of the mitochondrion, they can function without this membrane and are then called mitoplasts. They have many features in common with current prokaryotes, including the circular DNA, prokaryotic-like ribosomes, and division by binary fission.
In mammals mitochondrial DNA is inherited from the mother, as sperm mitochondria are contained in the energy-requiring tails, which are left out of the fertilised egg cell. Thus if a generation in a family fails to have any female offspring, that line of mitochondrial DNA inheritance will die out. One other consequence of the separate mitochondrial DNA is that animals cloned using a body cell from another animal as host, will have the nuclear DNA of the donor, but the mitochondrial DNA of the host cell.
