Why mitochondria are the engines of life

From producing ATP to providing metabolites for macromolecules, your cells’ mitochondria govern a wide range of processes, which makes them crucial to muscle, heart, brain and gut health. Since your cells' mitochondria produce around 90% of the energy required for the human body, it is quite apt, then, that they are labelled as cellular engines.

Mitochondria are bean-shaped intracellular organelles (with a diameter of 0.5 μm) that float inside almost all of our 30 trillion cells—with the exception of red blood cells. In accordance with the Endosymbiotic Theory, which outlines how mitochondria were once phototrophic α-proteobacterian before they were endocytosed by eukaryotic cells approximately 1.5 billion years ago—mitochondria contain their own genomes. Mitochondria are the only structure in our cells with their own DNA, which is, notably, only inherited from mothers.

The DNA found in our cells’ nuclei and our mitochondrial DNA are what drive cellular processes. The majority of mitochondrial proteins are encoded by nuclear DNA and are imported from the cytoplasm, where they are synthesised. These include enzymes required for the citric acid cycle, the proteins involved in DNA replication and transcription, and ribosomal proteins.

Mitochondria produce Adenosine Triphosphate (ATP), which is the main source of chemical energy for our cells. Mitochondria are membrane-bound by inner and outer layers that enable the organelle to generate ATP through cellular respiration. The outer membrane contains protein-based pores that function as tunnels allowing ions and molecules to pass through. Meanwhile, the inner membrane contains folded projections called cristae.

Cellular respiration takes place in the mitochondria’s inner membrane, which facilitates the transport of metabolites through specialised carrier proteins. The conversion of substrates (nutrient molecules) into ATP (energy), which is called mitochondrial oxidative phosphorylation (OXPHOS), yields an energy production of 30 molecules of ATP per molecule of glucose. 

The exergonic process that pumps protons from the mitochondrial matrix into the intermembrane space creates an electrochemical gradient, much like water driving a turbine, which transforms adenosine diphosphate (ADP) and phosphate into ATP . Adenine nucleotide translocase (ANT) ensures that ATP is exported from mitochondria to the cytosol. 

Besides energy production, outer mitochondrial membranes activate multiple modulators in the immune system through anchoring proteins like MAVS. In addition, mitochondria release mtDNA and mtROS signals to regulate the transcription of immune cells. 

Mitochondria are also critical metabolic hubs for biomass production. They generate biosynthetic precursors for macromolecules and compartmentalise metabolites for the maintenance of redox homeostasis. More specifically, mitochondria export metabolic intermediates (citrate, nucleotides and amino acids) into the cytoplasm, which are then the foundations for proteins, lipids and DNA.

Furthermore, mitochondria, through cellular respiration, produce intercellular water that helps to hydrate our fascia. Fascia is an extensive web of connective tissue made of collagen fibres and ground substance. This occurs because mitochondria consume oxygen, electrons and protons via the electron transport chain and during cellular respiration, these elements are combined to create metabolic water. 

Metabolic water hydrates the cellular environment and supplies water for various hydrolytic reactions. In fact, most metabolic water is generated in mitochondria by cytochrome c oxidase during the electron transport process and by ATP synthase during oxidative phosphorylation.

Indeed, mitochondria are more than just cellular engines that convert nutrients and oxygen into ATP. Mitochondrial functions have multiple mechanisms that are the foundation of vitality, cellular signalling and metabolism.