Mitochondria

I

Introduction

Mitochondria, small cellular structures, or organelles, found in the cytoplasm of eukaryotic cells (cells with a nucleus). Mitochondria are responsible for converting nutrients into the energy-yielding molecule adenosine triphosphate (ATP) to fuel the cell's activities. This function, known as aerobic respiration, is the reason mitochondria are frequently referred to as the powerhouse of the cell.

Mitochondria are unusual organelles in that they contain deoxyribonucleic acid (DNA), typically found in the cell’s nucleus, and ribosomes, protein-producing organelles abundant in the cytoplasm. Within the mitochondria, the DNA directs the ribosomes to produce proteins, many of which function as enzymes, or biological catalysts, in ATP production. The number of mitochondria in a cell depends on the cell's function. Cells with particularly heavy energy demands, such as muscle cells, have more mitochondria than other cells.

II

Mitochondrial Structure

A mitochondrion is typically long and slender, but it can appear bean-shaped or oval-shaped under the electron microscope. Ranging in size from 0.5 micrometer (0.00005 in) to 1 micrometer (0.0001 in) in length, a mitochondrion has a double membrane that forms a sac within a sac. The smooth outer membrane holds numerous transport proteins, which shuttle materials in and out of the mitochondrion. The region between the outer and inner membranes, which is filled with liquid, is known as the outer compartment. The inner membrane has numerous folds called cristae. Cristae are the sites of ATP synthesis, and their folded structure greatly increases the surface area where ATP synthesis occurs. Transport proteins, molecules called electron transport chains, and enzymes that synthesize ATP are among the molecules embedded in the cristae. The cristae enclose a liquid-filled region known as the inner compartment, or matrix, which contains a large number of enzymes that are used in the process of aerobic respiration.

III

Mitochondrial Function

The chief function of the mitochondria is to create energy for cellular activity by the process of aerobic respiration. In this process, glucose is broken down in the cell's cytoplasm to form pyruvic acid, which is transported into the mitochondrion. In a series of reactions, part of which is called the citric acid cycle or Krebs cycle, the pyruvic acid reacts with water to produce carbon dioxide and ten hydrogen atoms. These hydrogen atoms are transported on special carrier molecules called coenzymes to the cristae, where they are donated to the electron transport chain.

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The electron transport chain separates the electron and proton in each of the ten hydrogen atoms. The ten electrons are sent through the electron transport chain and some eventually combine with oxygen and the protons to form water.

Energy is released as the electrons flow from the coenzymes down the electron transport chain to the oxygen atoms, and this energy is trapped by the components of the electron transport chain. As the electrons flow from one component to another, the components pump random protons from the matrix to the outer compartment. The protons cannot return to the matrix except by one pathway—through the enzyme ATPase, which is embedded in the inner membrane. As the protons flow back into the matrix, ATPase adds a phosphate group to a molecule in the matrix, adenosine diphosphate (ADP). The addition of a phosphate group to ADP forms ATP.

Aerobic respiration is an ongoing process, and mitochondria can produce hundreds of thousands of ATP molecules each minute in a typical cell. The ATP is transported to the cytoplasm of the cell, where it is used for virtually every energy-requiring reaction it performs. As ATP is used, it is converted into ADP, which is returned by the cell to the mitochondrion and is used to build more ATP.

IV

Origin of Mitochondria

Mitochondria have significant features that resemble those of prokaryotes, primitive cells that lack a nucleus. Mitochondrial DNA is circular, like the DNA of prokaryotes, and mitochondrial ribosomes are similar to prokaryotic ribosomes. Mitochondria divide independently of the cell through binary fission, the method of cell division typical of prokaryotes.

The prokaryote-like features of mitochondria lead many scientists to support the endosymbiosis hypothesis. This hypothesis states that millions of years ago, free-living prokaryotes capable of aerobic respiration were engulfed by other, larger prokaryotes but not digested, possibly because they were able to resist digestive enzymes. The two cells developed a symbiotic, or cooperative, relationship in which the host cell provided nutrients and the engulfed cell used these nutrients to carry out aerobic respiration, which provided the host cell with an abundant supply of ATP. The engulfed cells evolved into mitochondria, which retain the DNA and ribosomes characteristic of their prokaryotic ancestors.

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