Mitochondria - Turning on the Powerhouse
Mitochondria are known as the powerhouses of the cell. They are organelles that act like a digestive system which takes in nutrients, breaks them down, and creates energy rich molecules for the cell. The biochemical processes of the cell are known as cellular respiration. Many of the reactions involved in cellular respiration happen in the mitochondria. Mitochondria are the working organelles that keep the cell full of energy.
Mitochondria are small organelles floating free throughout the cell. Some cells have several thousand mitochondria while others have none. Muscle cells need a lot of energy so they have loads of mitochondria. Neurons [cells that transmit nerve impulses] don’t need as many. If a cell feels it is not getting enough energy to survive, more mitochondria can be created. Sometimes a mitochondria can grow larger or combine with other mitochondria. It all depends on the needs of the cell.
The folding of the inner membrane increases the surface area inside the organelle. Since many of the chemical reactions happen on the inner membrane, the increased surface area creates more space for reactions to occur. If you have more space to work, you can get more work done. Similar surface area strategies are used by microvilli in your intestines.
What’s in the matrix? It's not like the movies at all. Mitochondria are special because they have their own ribosomes and DNA floating in the matrix. There are also structures called granules which may control concentrations of ions. Cell biologists are still exploring the activity of granules.
Using Oxygen to Release Energy
How does cellular respiration occur in mitochondria? The matrix is filled with water and proteins [enzymes]. Those proteins take organic molecules, such as pyruvate and acetyl CoA, and chemically digest them. Proteins embedded in the inner membrane and enzymes involved in the citric acid cycle ultimately release water [H2O] and carbon dioxide [CO2] molecules from the breakdown of oxygen [O2] and glucose [C6H12O6]. The mitochondria are the only places in the cell where oxygen is reduced and eventually broken down into water.
Mitochondria are also involved in controlling the concentration of calcium [Ca2+] ions within the cell. They work very closely with the endoplasmic reticulum to limit the amount of calcium in the cytosol.
Related Video...
Chalk Talk: Mitochondria [US-NSF Video]
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The cytoplasm of nearly all eukaryotic cells contain mitochondria, although there is at least one exception, the protist Chaos [Pelomyxa] carolinensis. They are especially abundant in cells and parts of cells that are associated with active processes. For example, in flagellated protozoa or in mammalian sperm, mitochondria are concentrated around the base of the flagellum or flagella. In cardiac muscle, mitochondria surround the contractile elements. Hummingbird flight muscle is one of the richest sources of mitochondria known. Thus, from their distribution alone one would suspect that they are involved in energy production. Multicellular organisms probably could not exist without mitochondria. The inability to remove electrons from the system and the buildup of metabolic end products restrict the utility of anaerobic metabolism. Through oxidative phosphoryation mitochondria make efficient use of nutrient molecules. They are the reason that we need oxygen at all.
The double-membraned mitochondrion can be loosely described as a large wrinkled bag packed inside of a smaller, unwrinkled bag. The two membranes create distinct compartments within the organelle, and are themselves very different in structure and in function.
The outer membrane is a relatively simple phospholipid bilayer, containing protein structures called porins which render it permeable to molecules of about 10 kilodaltons or less [the size of the smallest proteins]. Ions, nutrient molecules, ATP, ADP, etc. can pass through the outer membrane with ease.
The inner membrane is freely permeable only to oxygen, carbon dioxide, and water. Its structure is highly complex, including all of the complexes of the electron transport system, the ATP synthetase complex, and transport proteins. The wrinkles, or folds, are organized into lamillae [layers], called the cristae [singlular: crista]. The cristae greatly increase the total surface area of the inner membrane. The larger surface area makes room for many more of the above-named structures than if the inner membrane were shaped like the outer membrane.
The membranes create two compartments. The intermembrane space, as implied, is the region between the inner and outer membranes. It has an important role in the primary function of mitochondria, which is oxidative phosphorylation.
The matrix contains the enzymes that are responsible for the citric acid cycle reactions. The matrix also contains dissolved oxygen, water, carbon dioxide, the recyclable intermediates that serve as energy shuttles, and much more [see "other functions"]. Diffusion is a very slow process. Because of the folds of the cristae, no part of the matrix is far from the inner membrane. Therefore matrix components can diffuse to inner membrane complexes and transport proteins within a relatively short time.
Electron micrographs have revealed the three dimensional structure of mitochondria. However, since micrographs are themselves two dimensional, their interpretation can be misleading. Texts frequently show a picture of a 'typical' mitochondrion as a bacteria-sized ellipsoid [perhaps 0.5 by 1 micrometer]. However, they vary widely in shape and size. Electron micrographs seldom show such variation, because they are two-dimensional images.
Isolated mitochondria, such as from homogenized muscle tissue, show a rounded appearance in electron micrographs, implying that mitochondria are spherical organelles.
Mitochondria in situ can be free in the cytoplasm or packed in among more rigid structures, such as among the myofibrils of cardiac muscle tissue. In cells such as muscle, it is clear that mitochondria are not spherical, and often are not even ellipsoid. In some tissues, the mitochondria are almost filamentous, a characteristic that two dimensional micrographs may fail to reveal.
A planar section cuts through one or several parts of the organelle, making a single organelle appear to be more than one. The image we see of a circular or ellipsoidal organelle may disguise the true nature of the mitochondrion.