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Mitochondria are elongated, fluid-filled sacs 2-5 um long, with a tendency to change size and shape somewhat. These

organelles house most of the biochemical reactions that extract energy from the nutrient molecules in digested food. Mitochondria move

slowly in the cytoplasm and can divide. A mitochondrion contains a small amount of DNA (37 genes) that encodes information for making a few types of proteins and specialized RNA. However, most proteins that mitochondria use are encoded in the DNA of the nucleus, synthesized elsewhere in the cell, and then enter the mitochondria.

A mitochondrion has two layers—an outer membrane that surrounds the organelle and an inner membrane. The inner membrane folds extensively inward, forming shelflike partitions called cristae, which enclose the fluid-like substance, or matrix, of the mitochondrion. Cristae greatly increase the surface area making contact with the matrix, which permits the residence of more protein complexes and enzymes essential to capturing energy and transferring newly released energy into the chemical bonds of the molecule

sphate (ATP). It is the enzymes in the cristae, and other enzymes dissolved within the matrix, that control the chemical reactions that release energy from glucose and other nutrients and produce carbon dioxide and water (with assistance from oxygen) in a process called cellular respiration. Cells use ATP to power many cellular activities and can therefore be thought of as "cellular energy." Because the mitochondrion supplies ATP, it is nicknamed the “powerhouse” of the cell.

A typical cell has about 1,700 mitochondria, but cells with very high energy requirements, such as skeletal muscle cells, have many thousands of mitochondria. This abundance is why common symptoms of illnesses affecting mitochondria are exercise intolerance, fatigue, and weak, flaccid muscles. Some cells, such as red blood cells, do not have mitochondria. Mitochondria from the male are excluded from

the fertilized ovum. Therefore, all offspring inherit mitochondria only from the mother.

Lysosomes are the "garbage disposals" of the cell, where enzymes dismantle debris. The process of a cell's disposing of its own

trash is called “autophagy," which means "eating self.”

Lysosomes can be difficult to identify because their shapes vary, but many are small, membranous sacs. Lysosomes bud off of sections of Golgi membrane. Their powerful enzymes break down proteins, carbohydrates, and nucleic acids, including those of foreign particles that the cell has taken in. For example, lysosomes digest bacteria that certain white blood cells engulf. In liver cells, lysosomes break down cholesterol, toxins, and drugs.

Lysosomes destroy worn cellular parts. In certain scavenger cells they may engulf and digest entire smaller body cells that have been damaged. How the lysosomal membrane is able to withstand being digested is not well understood, but this organelle sequesters enzymes that can function only under very acidic conditions, preventing them from destroying the cellular contents around them. Human lysosomes

contain 43 types of enzymes. An abnormality in just one type of lysosomal enzyme can be devastating to health.

Peroxisomes are membranous sacs that resemble lysosomes in size and shape. They are in all human cells but are most abundant in cells of the liver and kidneys. Peroxisomes contain enzymes, called peroxidases, that catalyze metabolic reactions that release hydrogen peroxide (H2O2), which is toxic to cells. Peroxisomes also contain an enzyme called catalase, which decomposes hydrogen peroxide.

The outer surface of a peroxisomal membrane contains some forty types of enzymes, which catalyze a variety of biochemical reactions, including:

• synthesis of bile acids used in fat digestion

• breakdown of lipids called very-long-chain fatty acids

• degradation of rare biochemicals

• detoxification of alcohol