Mitochondria: Cellular Power Plants
Blink. Breathe. Wiggle your toes. These subtle movements—as well
as the many chemical reactions that take place inside organelles—require
vast amounts of cellular energy. The main energy source in your body is
a small molecule called ATP,
for adenosine triphosphate.
ATP is made in organelles called mitochondria.
Let's see if we can find some. They look like blimps about as long as pickup
trucks but somewhat narrower. Oh, a few of them are over there. As we get
nearer, you may hear a low whirring or humming sound, similar to that made
by a power station. It's no coincidence. Just as power plants convert energy
from fossil fuels or hydroelectric dams into electricity, mitochondria convert
energy from your food into ATP.
Like all other organelles, mitochondria are encased in an outer membrane.
But they also have an inner membrane. Remarkably, this inner membrane is
four or five times larger than the outer membrane. So, to fit inside the
organelle, it doubles over in many places, extending long, fingerlike folds
into the center of the organelle. These folds serve an important function:
They dramatically increase the surface area available to the cell machinery
that makes ATP. In other words, they vastly increase the ATP-production
capacity of mitochondria.
The mazelike space inside mitochondria is filled with a strong brew of
hundreds of enzymes, DNA (mitochondria are the only organelles to have their
own genetic material), special mitochondrial ribosomes, and other molecules
necessary to turn on mitochondrial genes.
| ACTUAL SIZE (AVERAGE) | PERCEIVED SIZE WHEN MAGNIFIED 3 MILLION TIMES | |
|---|---|---|
| Cell diameter | 30 micrometers* | 300 feet |
| Nucleus diameter | 5 micrometers | 50 feet |
| Mitochondrion length | Typically 1–2 micrometers, but can be up to 7 micrometers long | 18 feet |
| Lysosome diameter | 50–3,000 nanometers* | 5 inches to 30 feet |
| Ribosome diameter | 20–30 nanometers | 2–3 inches |
| Microtubule width | 25 nanometers | 3 inches |
| Intermediate filament width | 10 nanometers | 1.2 inches |
| Actin filament width | 5–9 nanometers | 0.5–1 inch |
*A micrometer is one millionth (10-6) of a meter. A nanometer
is one billionth (10-9) of a meter.
When you think about food, protein, and energy, what may come to mind is the quick meal you squeeze in before racing off to your next activity. But while you move on, your cells are transforming the food into fuel (ATP in this case) for energy and growth.
As your digestive system works on an apple or a turkey sandwich, it breaks the food down into different parts, including molecules of a sugar called glucose. Through a series of chemical reactions, mitochondria transfer energy in conveniently sized packets from glucose into ATP. All that's left are carbon dioxide and water, which are discarded as wastes.
This process is extremely efficient. Cells convert nearly 50 percent of the energy stored in glucose into ATP. The remaining energy is released and used to keep our bodies warm. In contrast, a typical car converts no more than 20 percent of its fuel energy into useful work.
Your body uses ATP by breaking it apart. ATP stores energy in its chemical bonds. When one of these bonds is broken, loosing a chemical group called a phosphate, energy is released.
ATP is plentifully produced and used in virtually every type of cell. A typical cell contains about 1 billion molecules of ATP at any given time. In many cells, all of this ATP is used up and replaced every 1 to 2 minutes!
Peter Mitchell, a British scientist, proposed the theory of chemiosmosis in 1961 to describe the way in which ATP is actually synthesized; he received the Nobel Prize for his work in 1978. In prokaryotes, it occurs at the cell membrane. In eukaryotic animal cells, chemiosmosis takes place in the cristae of the mitochondria, and in autotrophic cells, it occurs across the thylakoid membranes in the chloroplasts. Since chemiosmosis is the same for all three membranes, in the following discussion we will not need to differentiate between the membranes.
On one side of the membrane (we will call this side the "inside") is a supply of hydrogen atoms. Special carrier molecules use the energy released in the electron transport chain to bring hydrogen atoms close to the membrane and separate the hydrogen into an H+ ions and electrons. The electrons are brought back to the inside of the membrane while the H+ ions are forced to the other side (the "outside"). As more and more H+ ions accumulate on the outside of the membrane, two gradients are formed. First, a pH gradient is formed. This means that the outside is more acidic (because it has H+ ions) than the inside of the membrane. Also, an electrical gradient forms, since H+ ions have a positive charge.
When these gradients become sufficiently intense, they force the H+ ions through a channel (sometimes called the F0 channel) in the membrane in a tremendous gush. The ions end up in a large structure called the F1 unit where an enzyme called ATP synthetase is located. Also present in the F1 are ADP molecules and phosphate molecules. As the H+ ions rush by, they provide the energy which brings the ATP synthetase, ADP, and phosphates together. The ATP synthetase bonds the ADP and phosphate molecules, forming ATP. The H+ ions, now on the inside of the membrane, can be transported by the carrier molecules across the membrane so that the process may be repeated when enough energy is released from the electron transport chain.
Got Energy?
Energy from the food you eat is converted
in mitochondria into ATP. Cells use ATP to power their chemical
reactions. For example, muscle cells convert ATP energy into physical
work, allowing you to lift weights, jog, or simply move your eyeballs
from side to side.
When you think about food, protein, and energy, what may come to mind is the quick meal you squeeze in before racing off to your next activity. But while you move on, your cells are transforming the food into fuel (ATP in this case) for energy and growth.
As your digestive system works on an apple or a turkey sandwich, it breaks the food down into different parts, including molecules of a sugar called glucose. Through a series of chemical reactions, mitochondria transfer energy in conveniently sized packets from glucose into ATP. All that's left are carbon dioxide and water, which are discarded as wastes.
This process is extremely efficient. Cells convert nearly 50 percent of the energy stored in glucose into ATP. The remaining energy is released and used to keep our bodies warm. In contrast, a typical car converts no more than 20 percent of its fuel energy into useful work.
Your body uses ATP by breaking it apart. ATP stores energy in its chemical bonds. When one of these bonds is broken, loosing a chemical group called a phosphate, energy is released.
ATP is plentifully produced and used in virtually every type of cell. A typical cell contains about 1 billion molecules of ATP at any given time. In many cells, all of this ATP is used up and replaced every 1 to 2 minutes!
Peter Mitchell, a British scientist, proposed the theory of chemiosmosis in 1961 to describe the way in which ATP is actually synthesized; he received the Nobel Prize for his work in 1978. In prokaryotes, it occurs at the cell membrane. In eukaryotic animal cells, chemiosmosis takes place in the cristae of the mitochondria, and in autotrophic cells, it occurs across the thylakoid membranes in the chloroplasts. Since chemiosmosis is the same for all three membranes, in the following discussion we will not need to differentiate between the membranes.
On one side of the membrane (we will call this side the "inside") is a supply of hydrogen atoms. Special carrier molecules use the energy released in the electron transport chain to bring hydrogen atoms close to the membrane and separate the hydrogen into an H+ ions and electrons. The electrons are brought back to the inside of the membrane while the H+ ions are forced to the other side (the "outside"). As more and more H+ ions accumulate on the outside of the membrane, two gradients are formed. First, a pH gradient is formed. This means that the outside is more acidic (because it has H+ ions) than the inside of the membrane. Also, an electrical gradient forms, since H+ ions have a positive charge.
When these gradients become sufficiently intense, they force the H+ ions through a channel (sometimes called the F0 channel) in the membrane in a tremendous gush. The ions end up in a large structure called the F1 unit where an enzyme called ATP synthetase is located. Also present in the F1 are ADP molecules and phosphate molecules. As the H+ ions rush by, they provide the energy which brings the ATP synthetase, ADP, and phosphates together. The ATP synthetase bonds the ADP and phosphate molecules, forming ATP. The H+ ions, now on the inside of the membrane, can be transported by the carrier molecules across the membrane so that the process may be repeated when enough energy is released from the electron transport chain.
Plastids
Plastids are organelles which are found only in autotrophic cells. In many respects, plastids are similar to entire prokaryotic organisms; they contain their own DNA and can replicate themselves. Like the mitochondria, plastids have an outer membrane which separates them from the cytoplasm and a highly folded inner membrane. Plastids come in three different types: leucoplasts, chloroplasts, and chromoplasts.
Leucoplasts are colorless plastids which store starch (a carbohydrate), proteins, and lipids. These materials are released from the leucoplast when the cell requires them.
Chloroplasts are the organelles in which photosynthesis takes place. Photosynthesis is an important process by which autotrophic cells manufacture their own food. Chloroplasts contain the green pigment chlorophyll (this is why plant leaves are green) which absorbs light to provide the energy necessary to complete photosynthesis.
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Chloroplasts have two membranes: an outer membrane and an inner membrane. A solution called the stroma fills the part of the chloroplast inside of the inner membrane. In this area, there are stacks of flattened vesicles. The stacks themselves are known as grana, and the vesicles are called thylakoids. The thylakoids are where photosynthesis actually occurs.
Chromoplasts are very similar to chloroplasts, but they do not contain the green pigment chlorophyll.
Instead, they contain other pigments which give color to flowers and
to leaves during the fall. These other pigments absorb colors of light
than chlorophyll.
