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Textbook: Essential Cell Biology

Function of Mitochondria

To understand the function of mitochondria is helps to know about the structure of mitochondria. Click here for a labelled diagram of the structure of mitochondria

Function of mitochondria:

Simplest answer:

  • "Provides energy for the cell".

They do this by ...

  • Production of adenosine triphosphate (ATP) via

  • The TCA Cycle
    ... also known as ...
    the Krebs Cycle and
    the Citric Acid Cycle.

Mitochondria are present in eukaryotic cells - incl. animal cells and plant cells. The number of mitochondria per cell varies from just one mitochondrion to 10,000 mitochondria in some specialized types of cells. A "typical" number of mitochondria per cell is around 200. The quantity of mitochondria per cell depends on the type of cell, the precise details of cell structures depending on cell functions.


The main function of mitochondria is the production of energy during the production of adenosine triphosphate (ATP) via the TCA Cycle (which is also as the Krebs Cycle and the Citric Acid Cycle). That process is an important metabolic pathway (see also what is metabolism). It is described in detail on the pages about the TCA Cycle - which is often included in introductory biology courses e.g. A-Level Biology, but not necessarily in first-level anatomy & physiology e.g. ITEC A&P.

For further general knowledge about cell biology and the functions of cell organelles such as mitochondria it is useful to be aware of some of the other functions of mitochondria. Mitochondria have an important role (or roles) in many other metabolic tasks, including ...

List of Functions of Mitochondria

1.

Production of energy
ATP Synthesis

The main function of mitochondria is the production of ATP.

According to many textbooks: Energy production within cells is the main function of mitochondria. However, some students are told not to state that mitochondria "produce" or "create" energy. Strictly, the energy is released from storage (in chemical bonds) by reactions that occur in mitochondria. According to the Law of Conservation of Energy, the total amount of energy in an isolated system remains constant over time. A-Level Biology students can say that mitochondria are the site of aerobic respiration in eukaryotic cells.

This occurs by a process of cellular respiration, also known as aerobic respiration, which is dependent on the presence of oxygen. (When oxygen is limited, the chemicals that would otherwise be oxidized are, instead, metabolized by anaerobic respiration, via a process that is independent of the mitochondria.)
The 3 main stages in the overall process of aerobic cellular respiration are:

  1. Glycolysis - splitting sugar molecules
  2. TCA Cycle
  3. Electron Transport

For further detail see the page about the TCA Cycle.

2.

Production of heat:
Non-shivering thermogenesis

The term thermogenesis refers to heat production in living organisms, mainly mammals. It is classified according to how the heat production is initiated:

  • Exercise-associated thermogenesis - due to movement e.g. shivering.
  • Non-exercise activity thermogenesis - incl. non-shivering thermogenesis, see below.
  • Diet-induced thermogenesis - i.e. heat generated by the body following the digestive processes. See also metabolic rate.

Non-shivering thermogenesis is due to facilitated diffusion of protons into the mitochondrial matrix. This is called "proton leak" or "mitochondrial uncoupling" and only occurs in certain circumstances. When it does happen, this results in the unharnessed potential energy of the proton electrochemical gradient being released as heat.

Non-shivering thermogenesis occurs mainly in brown adipose tissue, also known as "brown fat" (adipose tissue is "fat tissue") because the process is controlled by a protein present in brown fat. The protein is thermogenin, also known as UCP1, and acts as a proton channel that sometimes enables protons (i.e. H+ ions) to enter the mitochondrial matrix without contributing to ATP synthesis.

Brown adipose tissue is produced in various amounts by different types of mammals and is at its highest levels in early life and in hibernating animals e.g. bears living in cold environments. Adult humans usually only have small brown fat deposits throughout the body - brown fat is present in humans at birth, but decreases with age.

3.

Role as independent units within eukaryotic cells:
Mitochondria have mitochondrial DNA (mtDNA)

Mitochondria contain their own genetic material - which is independent of the cell in which they are located.

Mitochondrial DNA (mtDNA) is maternally inherited. At fertilization only nuclear DNA enters from the sperm because although the egg contains mitochondria, sperm cells do not. Sperm are so tiny that mitochondria would hamper their passage toward the egg. (Therefore exercise capacity e.g. for endurance sports tends to be maternally inherited. Maternal ancestral history can also be traced via mtDNA.)

mtDNA accounts for about 1% of the total cellular DNA - recall that the number of mitochondria per cell varies considerably with the type and function of the cell. mtDNA exists in a circular arrangement in the mitochondrial matrix. Mutations of errors in some mitochondrial genes can result in certain diseases - mitochondrial diseases. Mitochondria self-replicate (divide) by fission, as is also true of bacteria cells - which are also known as prokaryotic cells.

4.

 

Role in the process of apoptosis: (Programmed cell death)

 

Two types of cell death occur in multicellular organisms:

  • Apoptosis - the process of programmed cell death (PCD), in which biochemical changes lead to cell changes such as cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation, the ultimate consequence of which is death of the cell.
  • Necrosis - traumatic (not pre-planned or "programmed") cell death e.g. due to acute cellular injury.

Apoptosis is advantageous to organisms for many reasons. For example, during development it is necessary for some cells to die in order for normal tissue and organ formation to proceed, e.g. the differentiation of fingers in a human embryo occurs by apoptosis of the cells between the fingers, resulting in separate fingers. Death of abnormal cells such as cells that are cancerous or virally infected is also good for the organism. Unlike necrosis, apoptosis results in cell fragments called apoptotic bodies that phagocytic cells can engulf and quickly remove before the contents of the cell spills over surrounding cells which could cause tissue damage.

Mitochondria help to safeguard cell survival while appropriate and to facilitate apoptosis when necessary. When apoptosis of a cell is stimulated: proapoptotic proteins insert into the mitochondrial membrane, forming pores in the membrane.

  1. Proapoptotic proteins insert into the mitochondrial membrane, forming pores in the membrane.
  2. A protein called cytochrome leaves the intermembrane space of the mitochondrion via the pores in the mitochondrial membrane.
  3. Cytochrome emerges from the mitochondrion into the cytosol (i.e. the intracellular fluid) of the eukaryotic cell.
  4. Cytochrome in the cytosol of the cell stimulates a cascade of biochemical changes that lead to apoptotic death of the cell.

5.

Storage of Ca2+ ions

Calcium (Ca2+) has many important functions in the biochemistry of cells and in physiology generally, e.g. re.

Storage of Calcium (Ca2+):

In the case of mammals, including humans, bone tissue is the main mineral storage site. Release and re-absorption of Ca2+ from and into bone is regulated by hormones. At a cellular level, Ca2+ is held within two types of organelles, the endoplasmic reticulum and mitochondria. The endoplasmic reticulum (SER & RER) is the main site of cellular storage of calcium. Mitochondria can also transiently store calcium - contributing to the cell's homeostasis of calcium by acting as "cytosolic buffers" for calcium.

The above list of functions of mitochondria is not comprehensive but a general introduction likely to be sufficient for introductory courses that include some cell biology e.g. types of cells and the structures and functions of cell organelles.

Other functions of mitochondria

Many other functions of mitochondria are specific to certain types of cells,
i.e. some of the functions of mitochondria depend on the type of cells in which they are located.

Examples of such functions include:

  • contribution to synthesizing, breaking-down, and recycling biochemicals needed for cell functioning e.g. components of genetic material.
  • contribution to synthesizing certain hormones e.g. oestrogen and testosterone.
  • role in cholesterol metabolism ("metabolism" refers to chemical changes due to reactions in the body incl. energy required & released)
  • role in neurotransmitter metabolism ("metabolism" refers to chemical changes due to reactions in the body incl. energy reqd & released)
  • detoxification of ammonia in the urea cycle (ammonia is among the chemicals in cigarettes)


See also structure of mitochondria, list the functions of organelles in eukaryotic cells & compare prokaryotic with eukaryotic cells.

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