Mitochondrion

Inner membrane

Electron transport chain
figure 7-11

Electron transport and chemiosmosis take place along the inner mitochondrial membrane and involve five steps.

The electrons in the hydrogen atoms from NADH and FADH2 are at a high energy level. In the electron transport chain, these electrons are passed along a series of molecules embedded in the inner mitochondrial membrane, as shown in Figure 7-11. In step Q, NADH and FADH2 give up electrons to the electron transport chain. NADH donates electrons at the beginning, and FADH2 donates them farther down the chain. These molecules also give up protons (hydrogen ions, H+). In step ©, the electrons are passed down the chain. As they move from molecule to molecule, they lose energy. In step ©, the energy lost from the electrons is used to pump protons from the matrix, building a high concentration of protons between the inner and outer membranes. Thus, a concentration gradient of protons is created across the inner membrane. An electrical gradient is also created, as the protons carry a positive charge.

In step O, the concentration and electrical gradients of protons drive the synthesis of ATP by chemiosmosis, the same process that generates ATP in photosynthesis. ATP synthase molecules are embedded in the inner membrane, near the electron transport chain molecules. As protons move through ATP synthase and down their concentration and electrical gradients, ATP is made from ADP and phosphate. In step Q, oxygen is the final acceptor of electrons that have passed down the chain. Oxygen also accepts protons that were part of the hydrogen atoms supplied by NADH and FADH2. The protons, electrons, and oxygen all combine to form water, as shown by the equation in step Q.

MITOCHONDRIA: Many Roles in Disease

Every cell contains very small organelles that are known as mitochondria. Mitochondria generate almost all of the ATP that fuels the activity in living organisms. Scientists have known for years that certain diseases are directly caused by mitochondrial dysfunction. However, new research shows that mitochondria may play roles in the symptoms of aging and may contribute to the development of Alzheimer's disease and cancer.

Mitochondrial Diseases

Mitochondria are very unusual organelles, because they have their own DNA. Mutations in mitochondrial DNA are responsible for several rare but serious disorders. Examples include Leigh's syndrome, a potentially deadly childhood disease that causes loss of motor and verbal skills, and Pearson's syndrome, which causes childhood bone marrow dysfunction and pancreatic failure.

Mitochondria in Aging

Mitochondria may play a role in causing some problems associated with aging. Chemical reactions of the Krebs cycle and electron transport chain sometimes release stray electrons that "leak out" of mitochondria into the cell.These electrons can combine with oxygen to form free radicals. Free radicals are especially reactive atoms or groups of atoms with one or more unpaired electrons. Free radicals quickly react with other molecules, such as DNA and protein; these reactions may disrupt cell activity. Biologists think that many characteristics of human aging, from wrinkles to mental decline, may be brought on partly by the damage caused by free radicals.

Mitochondria in Other Diseases

Recent research also shows that mitochondria may be important in diseases related to apoptosis, or programmed cell death. Scientists have shown that signals from the mitochondria are instrumental in starting and/or continuing the apoptosis process. Yet sometimes, mitochondria mistakenly push or fail to push the "self-destruct button" in cells. In cases of stroke and Alzheimer's disease, for example, mitochondria may cause too many cells to die, which may lead to mental lapses and other symptoms. In the case of cancer, mitochondria may fail to initiate apoptosis. This failure could allow tumor cells to grow and invade healthy tissues.

Promise of New Treatments

Researchers are now investigating mitochondria as targets for drug treatments to prevent or treat a variety of conditions. Conversely, researchers are also studying how certain conditions impair mitochondrial function. One day, scientists may use knowledge about mitochondria to help ease the symptoms of aging and to cure or prevent many diseases.

1. How do mitochondria contribute to free radical formation?

2. How could research on mitochondria be helpful to society?

3. Critical Thinking Evaluate the following statement: Mitochondria—we can't live with them; we can't live without them.

Mitochondria may play a role in programmed cell death, or apoptosis. A white blood cell undergoing apoptosis (right) looks very different from a normal white blood cell (left). (SEM 2,600x)

Mitochondria may play a role in programmed cell death, or apoptosis. A white blood cell undergoing apoptosis (right) looks very different from a normal white blood cell (left). (SEM 2,600x)

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