Mitochondria
are often referred to as the powerhouses of the cell. They help turn the energy
we take from food into energy that the cell can use. But, there is more to
mitochondria than energy production.
Present
in nearly all types of human cell, mitochondria are vital to our survival. They
generate the majority of our adenosine triphosphate (ATP), the energy currency
of the cell.
Mitochondria are also involved
in other tasks, such as signaling between cells and cell death, otherwise known
as apoptosis.
In this article, we will look at
how mitochondria work, what they look like, and explain what happens when they
stop doing their job correctly.
The
structure of mitochondria
Mitochondria are small, often between 0.75 and 3
micrometers and are not visible under the microscope unless they are stained.
Unlike other organelles
(miniature organs within the cell), they have two membranes, an outer one and
an inner one. Each membrane has different functions.
Mitochondria are split into
different compartments or regions, each of which carries out distinct roles.
Some of the major regions include
the:
Outer
membrane: Small molecules can pass freely through the
outer membrane. This outer portion includes proteins called porins, which form
channels that allow proteins to cross. The outer membrane also hosts a number
of enzymes with a wide variety of functions.
Intermembrane
space: This is the area between the inner and
outer membranes.
Inner
membrane: This membrane holds proteins that have
several roles. Because there are no porins in the inner membrane, it is
impermeable to most molecules. Molecules can only cross the inner membrane in
special membrane transporters. The inner membrane is where most ATP is created.
Cristae: These
are the folds of the inner membrane. They increase the surface area of the
membrane, therefore increasing the space available for chemical reactions.
Matrix: This
is the space within the inner membrane. Containing hundreds of enzymes, it is
important in the production of ATP. Mitochondrial DNA is housed here (see
below).
Different cell types have
different numbers of mitochondria. For instance, mature red blood cells have
none at all, whereas liver cells can have more than 2,000. Cells with a high
demand for energy tend to have greater numbers of mitochondria. Around 40 percent of the cytoplasm in heart
muscle cells is taken up by mitochondria.
Although mitochondria are often
drawn as oval-shaped organelles, they are constantly dividing (fission) and
bonding together (fusion). So, in reality, these organelles are linked together
in ever-changing networks.
Also, in sperm cells, the
mitochondria are spiraled in the midpiece and provide energy for tail motion.
Mitochondrial DNA
Although most of our DNA is kept
in the nucleus of each cell, mitochondria have their own set of DNA.
Interestingly, mitochondrial DNA (mtDNA) is more similar to bacterial DNA.
The mtDNA holds the instructions
for a
number of proteins and other cellular support equipment across 37 genes.
The human genome stored in the
nuclei of our cells contains around 3.3 billion base pairs, whereas mtDNA
consists of less than 17,000.
During reproduction, half of a
child's DNA comes from their father and half from their mother. However, the
child always receives their mtDNA from their mother. Because of this, mtDNA has
proven very useful for tracing genetic lines.
For instance, mtDNA analyses
have concluded that humans may have originated in Africa relatively recently,
around 200,000 years ago, descended from a common ancestor, known as mitochondrial Eve.
What do mitochondria do?
Although the best-known role of mitochondria is
energy production, they carry out other important tasks as well.
In fact, only about 3
percent of the genes needed to make a mitochondrion go into its
energy production equipment. The vast majority are involved in other jobs that
are specific to the cell type where they are found.
Below, we cover a few of the
roles of the mitochondria:
Producing
energy
ATP, a complex organic chemical
found in all forms of life, is often referred to as the molecular unit of
currency because it powers metabolic processes. Most ATP is produced in
mitochondria through a series of reactions, known as the citric acid cycle or
the Krebs cycle.
Energy production mostly takes
place on the folds or cristae of the inner membrane.
Mitochondria convert chemical
energy from the food we eat into an energy form that the cell can use. This
process is called oxidative phosphorylation.
The Krebs cycle produces a
chemical called NADH. NADH is used by enzymes embedded in the cristae to
produce ATP. In molecules of ATP, energy is stored in the form of chemical
bonds. When these chemical bonds are broken, the energy can be used.
Cell
death
Cell death, also called
apoptosis, is an essential part of life. As cells become old or broken, they
are cleared away and destroyed. Mitochondria help decide which cells are
destroyed.
Mitochondria release cytochrome
C, which activates caspase, one of the chief enzymes involved in destroying
cells during apoptosis.
Because certain diseases, such
as cancer, involve a breakdown in normal
apoptosis, mitochondria are thought to play a role in the disease.
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