Mitochondria

Mitochondria are specialized structures inside a cell that break down food and release energy. If a cell is like a tiny chemical processing plant, then the mitochondria are the power plants of the cell. Without mitochondria, a cell with a nucleus cannot use oxygen and cannot live. Mitochondria are found only in eukaryotic cells (those with a nucleus) and are not found in prokaryotic cells (those without a nucleus).

Mitochondria (singular, mitochondrian) are described as organelles. An organelle is a tiny structure inside a cell that is controlled by the nucleus and has a specific function to play in maintaining the life of the cell. For a cell, mitochondria play a most important role in carrying out aerobic respiration. In other words, mitochondria break down the food a cell takes in and release the energy it contains. Examined under a microscope, mitochondria appear as oval- or sausage-shaped structures that have a double membrane. The outer membrane is smooth and permeable to certain enzyme molecules, meaning that molecules of a certain size

can flow through its walls. The inner membrane consists of many folds that allow it to have an increased surface area and make it able to pack in many more enzymes than would be possible without the folds. A eukaryotic cell typically has anywhere from a dozen to a thousand mitochondria, and animal cells usually have more mitochondria than plant cells. Mitochondria are especially abundant in cells whose functions have high energy demands.

As the "powerhouse" of the cell, mitochondria carry out respiration. Respiration is the chemical process that breaks down food to release energy. Two key ingredients are necessary for respiration—sugar and oxygen. The sugar that cells use is usually in the form of glucose, which contains a great deal of stored energy. Oxygen is used to get the energy out of the glucose. This process takes place in the folds of the mitochondria where a substance known as adenosine triphosphate or ATP is created. ATP is the molecular storage form of energy that mitochondria produce. When a cell needs energy, it draws on the stored ATP, which releases the energy.

Here is a simplified example of how the human body uses mitochondria to release energy from food. For an organism like the human body, most of its energy comes from mitochondria, which are the body's power plant that burns the fuel to produce the energy to run it. The food a person eats is the fuel that is "burned" in the body's furnaces (mitochondria). The ATP produced by this process is the energy or "electricity" produced by the power plant (which powers the cells in the body). When a person breathes, he or she is taking in oxygen for the mitochondria to use to release the energy the body needs. This process is similar in all organisms, since all organisms must break down food to get energy. As a result, all life forms have cells that contain mitochondria.

[See alsoCell; Organelle ]