With the exception of a few metabolically inert types, such as the red blood cells of many higher animals, eukaryotic cells of animals, plants, fungi, and protozoa contain mitochondria.
Most cells contain several hundred. The efficiency of mitochondria in adenosine triphosphate (ATP) production provides the energy source that powers all the varied activities of eukaryotic cells. For these reasons, mitochondria have been aptly termed the “powerhouses” of the cell.
In most cells, mitochondria appear as spherical, elongated bodies about 0.5 micrometer in diameter and 1 to 2 micrometers in length. Their size is roughly the same as that of bacterial cells. In some cells mitochondria may measure several micrometers in diameter, with lengths up to 10 micrometers.
The name “mitochondrion” comes from Greek words meaning “thread” (mitos) and “granule” (chondros). Mitochondria appear in either elongated, thread like forms ormore spherical, granular shapes. Under the light microscope, mitochondria can be seen to grow, branch, divide, and fuse together.
Mitochondria are surrounded by two separate membranes. The outer membrane is a continuous, unbroken membrane that completely covers the surface of the organelle. It is smooth in appearance. The innermembrane is also continuous and unbroken. It is convoluted, with infoldings called cristae that greatly increase its surface area.
The outer and inner membranes separate the mitochondrial interior into two distinct compartments. The intermembrane space is located between the outer and inner membranes. The innermost compartment, enclosed by both membranes, is the mitochondrial matrix. Each membrane and compartment carries out specific functions important to the production of ATP.
The outermembrane is more permeable than the inner membrane and allows molecules up to the size of small proteins to pass freely from the surrounding cytoplasm into the intermembrane space.
Larger proteins are prevented from escaping into the surrounding cytoplasm, and certain cytoplasmic proteins, such as potentially destructive enzymes, are prevented from entering.
The innermost compartment, the matrix, contains a battery of enzymes that catalyze the oxidation of fuel substances of many types, including simple sugars, fats, amino acids, and other organic acids. The primary goal of oxidation is the removal of high-energy electrons and the use of them to perform chemical work.
High-energy electrons are used in the membrane that immediately surrounds the matrix, the inner membrane. This membrane contains a group of proteins that work as electron-driven protons (hydrogen ions or H+) pumps.
The proteins, called electron transport carriers, accept electrons at a higher energy level and release the mat a lower level. The energy released by the electrons causes the shape of the carrier proteins to change, which allows them to transport protons across the inner membrane.
The various electron transport carriers accept and release electrons at different energy levels, allowing them to act in a series called the electron transport chain. The electrons released by one carrier have sufficient energy to power the pumping activity of the next one in line.
When operating at peak efficiency, a mitochondrion of average size conducts about 100,000 electrons through the electron transport chan per second. After passing through several carriers, most of the energy of the electrons has been tapped off.
As a consequence of proton pumping, protons become depleted in the matrix and become more concentrated in the intermembrane space. This creates an electrochemical gradient between the intermembrane space and the matrix.
It is called an electrochemical gradient because there is a concentration gradient across the inner membrane and because protons have an electrical charge, so there is also an electrical potential (a charge difference) across the membrane. Electrochemical gradients represent a form of stored energy capable of doing work and in this case is used to drive the synthesis of ATP.
At the very end of the electron transport chain, the electrons have so little energy that the last carrier molecule donates these low-energy electrons to oxygen that is already in the matrix. The oxygen eukaryotes need to survive thus has its primary biological role as the final acceptor of spent electrons released by the electron transport chain.
When an oxygen molecule receives four electrons, it then picks up two protons from the matrix, and water (H2O) is produced. The protons used in this process cause an additional reduction in proton concentration in the matrix, increasing the electrochemical gradient across the inner membrane.
A protein complex embedded in the innermembrane uses the proton gradient to make ATP. This complex, called ATP synthase, is a molecular “machine” that acts as a hydrogen-driven ATP synthesizer. It takes ADP (adenosine diphosphate) from the matrix and combines it with inorganic phosphate (Pi) to make ATP.
The proteins of the inner mitochondrial membrane thus work in two coordinated groups. One, the electron transport chain, uses the energy of electrons removed in oxidative reactions to pump protons hydrogens from the matrix to the intermembrane space. The second group, the ATP synthases, uses the proton gradient created by the electron transport chain as an energy source to make ATP from ADP and Pi.
ADP comes from the matrix itself and from other regions of the cell in which ATP is used to facilitate cellular activities such as growth and movement. Much of the ATP produced in this way is then transported out of the mitochondria for use in other parts of the cell.
mRNA and DNA
When examined with an electron microscope, a number of structures are visible in mitochondrial matrix, including granules of various sizes, fibrils, and crystals. Among the granules are ribosomes.
These structures, like their counterparts in the cytoplasm, are capable of protein assembly, using directions encoded in messenger ribonucleic acid (mRNA) molecules as a guide. The mitochondrial ribosomes are more closely related in structure and function to prokaryotic ribosomes than to ribosomes in the cytoplasm.
Also in the mitochondrial matrix are molecules of deoxyribonucleic acid (DNA). Mitochondrial DNA (mtDNA) stores the information required for synthesis of some of the proteins needed for mitochondrial functions. Unlike nuclear DNA, which is linear, mtDNA is circular, like bacterial DNA.
The presence of bacteria like DNA and ribosomes inside mitochondria has given rise to the endosymbiotic theory, which proposes that mitochondria may have evolved from bacteria that invaded the cytoplasm of other prokaryotic cells and established a symbiotic relationship.
Over long periods of time, these bacteria are believed to have lost their ability to live independently and gradually became transformed into mitochondria. The evolutionary advantage to the host provided by the bacterial invaders may have been greater efficiency in ATP production.