The singular of mitochondria is a mitochondrion. This is a cell organelle with a double-skinned membrane that exists in almost every mammalian cell. Mitochondria are often referred to as the powerhouse of the cell. They are responsible for the majority of ATP synthesis and are, therefore, important to function both microscopically and macroscopically.

The outer membrane of this double-skinned cell contains proteins known as porins, which allow the migration of ions into and out of the cell body. Enzymes involved in the elongation of fatty acids and the oxidation of adrenaline also exist on the outer membrane.

The space within the inner membrane of the cell is known as the matrix, which contains the enzymes of the Krebs (TCA) and fatty acid cycles, alongside DNA, RNA, ribosomes and calcium granules.

The Anatomy of Mitochondria

The inner membrane is arranged into cristae to increase the surface area available for energy production via oxidative phosphorylation.

Mitochondria Diagram
A basic diagram of a mitochondrion
Source: Medical News Today

The mitochondrion is the site of ATP synthesis, which involves the energy production provided by the oxidation of “metabolic fuels” such as carbohydrates, lipids, and proteins. It is then used to sustain energy-dependent processes, such as the synthesis of macromolecules, muscle contraction, active ion transport, or thermogenesis.

The number of mitochondria found in a cell is a good indicator of the cell’s rate of metabolic activity. Cells that are metabolically active, or energy-hungry, such as brain cells, have many mitochondria.

In brown adipose tissue, mitochondria have the function of heat production using the electron transport chain. This is why mammals are warm-blooded.

The DNA of mitochondria have maternal lineage which means their DNA is passed from mother to child with little change.

The biochemical processes of the cell are known as cellular respiration. The function of mitochondria can be compared to the digestive system in that take in nutrients, breaks them down, and creates energy-rich molecules for the cell.

Because many of the chemical reactions happen on the inner membrane, this is folded many times to increase the surface area.

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Cellular Respiration

The matrix is filled with water and proteins. These proteins take organic molecules, such as pyruvate and acetyl CoA, and chemically digest them.

Proteins embedded in the inner membrane and enzymes involved in the citric acid cycle ultimately release water (H2O) and carbon dioxide (CO2) molecules from the breakdown of oxygen (O2) and glucose (C6H12O6). The mitochondria are the only places in the cell where oxygen is reduced and eventually broken down into water.

Mitochondria are bacteria-sized organelles (about 1 × 2 μm in size), which are found in large numbers in almost all eukaryotic cells (cells of animals, plants and fungi). Typically, there are about 2000 mitochondria per cell, representing around 25% of the cell volume.

The most important function of the mitochondria is to produce energy. Charged molecules combine with oxygen and produce ATP molecules in a process known as oxidative phosphorylation.

Mitochondria also play an important role in apoptosis or the process of programmed cell death.

But, the primary function of the mitochondrion is to generate large quantities of energy in the form of adenosine triphosphate (ATP). In addition to producing energy, mitochondria store calcium for cell signalling activities, generate heat, and mediate cell growth and death.

The number of mitochondria per cell varies,  for example, in humans, erythrocytes (red blood cells) do not contain any mitochondria, whereas liver cells and muscle cells may contain hundreds or even thousands.

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Prokaryote Ancestry

Mitochondria differ from other cellular organelles. In that, they have two distinct membranes and a unique genome and reproduce by binary fission. These features indicate that mitochondria share an evolutionary past with prokaryotes (single-celled organisms).

Mitochondria Genome
Mitochondria Genome
Source: National Human Genome Research Institute

The matrix contains the deoxyribonucleic acid (DNA) of the mitochondrial genome and the enzymes of the tricarboxylic acid (TCA) cycle (also known as the citric acid cycle, or Krebs cycle), which metabolizes nutrients into by-products the mitochondrion can use for energy production.

The ETC (electron transport chain) uses a series of oxidation-reduction reactions to move electrons from one protein component to the next. Ultimately producing free energy that is harnessed to drive the phosphorylation of adenosine diphosphate (ADP) to ATP.

Mitochondrial DNA (mtDNA) is highly susceptible to mutations. Because it does not possess the DNA repair mechanisms common to nuclear DNA.

The mitochondrion is a notable site for the production of reactive oxygen species (ROS; or free radicals) due to the high propensity for uncontrolled release of free electrons.

While several different antioxidant proteins within the mitochondria scavenge and neutralize these free radicals. Some free radicals may inflict damage to mtDNA. Certain chemicals, infectious agents, or alcohol abuse, can damage mtDNA.

Function of Mitochondria

Mitochondria produce ATP (adenosine triphosphate) through the process of cellular respiration, specifically, aerobic respiration, which requires oxygen.

The citric acid cycle or Krebs cycle involves the oxidation of pyruvate, which comes from glucose, to form the molecule acetyl-CoA. Acetyl-CoA is in turn oxidized and ATP is produced.

So, mitochondria produce energy and different cells have different energy needs. Muscle cells have many mitochondria whereas red blood cells need none.

Mitochondria are analogous to a furnace in the cell. Because, like furnaces, mitochondria produce energy from basic components (in this case, molecules that have been broken down by the digestive system so that they can be used).

However, mitochondria have other functions. They store calcium, which maintains the homeostasis of calcium levels in the cell. They regulate the cell’s metabolism and have a role in apoptosis (controlled cell death), cell signalling, and thermogenesis (heat production).

Biological energy conversion in mitochondria is carried out by the membrane protein complexes of the respiratory chain and the mitochondrial ATP synthase in the inner membrane cristae.

Why the body needs food

Your metabolism is the collection of chemical reactions that occur in your cells to sustain life.

Some of these reactions use stored energy. To build things up, which we call anabolism, while other reactions break things down, which we call catabolism.

Living things break down the three major categories of foods (proteins, fats, and carbohydrates) into their constituent parts for two reasons:

  1. Once the food atoms and groups of atoms (molecules) are broken down. They can be built back up into the specific kinds of things the organism needs. Like bone, muscle, skin, hair, feathers, fur, bark, leaves, etc.
  2. Breaking down the food molecules releases the energy that was holding them together. And that released energy is temporarily stored by the cell for the re-building process.

In summary

The mitochondrion is the cell organelle of life. It provides life energy which every living creature needs.

Food is the fuel of the life process and the digestive system and mitochondria work in concert to achieve this remarkable result.

The digestive system breaks down food into basic molecules, glucose and proteins.

The lungs extract oxygen from the air in the environment. And blood transports the oxygen and the glucose molecules and proteins around the body to the mitochondria in the cells.

Finally, the mitochondria use glucose, proteins and oxygen to produce energy.

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The Anatomy of Mitochondria (the powerhouse of life)

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