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Overview:

Introduction

Mitochondria are essential parts of cells which are found in nearly all human cells, and are responsible for producing energy in the form of adenosine triphosphate (ATP), the molecule that powers most cellular processes.

Modern research shows that the mitochondria also regulate cell survival, metabolic signalling, and inflammation in the body.
They are known as the ‘powerhouses of the cell’, and their importance is highlighted when exploring defects in mitochondrial function, which contributes to a wide range of human diseases.

Origin

Several billion years ago, a large primitive cell engulfed a smaller bacterium, and instead of the cell digesting it, the bacterium survived and supported the host cell by producing energy.

The two became dependant on one another for survival. This relationship is called a symbiotic relationship, and through the course of evolution, this bacteria became what we now know as mitochondria.

Mitochondrial Structure

Mitochondria have a distinctive double membrane. The outer membrane encloses the mitochondria, protecting it and maintaining the internal environment, whilst the inner membrane folds into cristae, which are the tightly folded structures that increase the available surface area for chemical reactions to occur between proteins and enzymes to generate ATP.

This dramatically increases the surface area for ATP synthesis. The cristae contain protein complexes that generate ATP through a process called oxidative phosphorylation. The internal compartment, called the matrix, contains mitochondrial DNA (mtDNA) and ribosomes; these aspects were conserved from the bacterial ancestor species they descended from. Human mitochondrial DNA contains 37 genes, and 13 of these genes make proteins required for the electron transport chain, the system that converts oxygen and sugar into usable energy for the cells. The other 24 genes produce RNA tools needed for protein synthesis: transfer (tRNA) and ribosomal (rRNA), and together the tRNA and rRNA help the mitochondria make proteins. The tRNA are helpers that deliver amino acids to the ribosomes, and the rRNA form the structural core of ribosomes. The core functions as the ‘workbench’ and the ‘machinery’ that assembles proteins from amino acid building blocks.

Core Functions of Mitochondria

1. Energy Production

The primary energy currency used by most human tissues is ATP. The mitochondria produce ATP through oxidative phosphorylation, a multi-step process in which the electron transport chain and the resulting proton gradient across the inner mitochondrial membrane drive the conversion of adenosine diphosphate (ADP) into ATP, the cell’s usable energy. In mitochondria, electrons act as fuel. As they move through the electron transport chain, their energy is used to pump protons to one side of the inner membrane which creates a high-pressure, high-concentration gradient.
This imbalance drives protons back across the membrane toward the lower concentration side to restore balance. Protons flow through a narrow channel inside an enzyme called ATP synthase. This enzyme acts like a tiny rotary turbine, as the directed proton flow makes the molecular turbine rotate, and that rotation powers the ATP synthase to attach a phosphate group to ADP, producing ATP.

2. Calcium Regulation

Mitochondria buffer intracellular calcium to prevent toxic overload and maintain normal signalling pathways. They remove excess calcium when concentrations rise, preventing toxic overloads, and release it in controlled amounts to support normal cell signalling pathways such as muscle contraction and hormone release.

3. Apoptosis and Cell Fate

Mitochondria release signalling molecules that co-ordinate the removal of damaged or unnecessary cells through controlled apoptosis, known as programmed cell death. A cell has a built-in self-destruct programme so it can disappear without harming nearby healthy cells. If a cell accumulates too much DNA damage rendering it incapable of safe repair, the mitochondria will trigger apoptosis to prevent cells from becoming cancerous due to mutations.

4. Metabolism

Mitochondria influence whole-body metabolic health through participation in amino acid metabolism, lipid oxidation and synthesis of key biomolecules. They break down amino acids to supply materials needed for energy production and cellular repair, and fatty acids are broken down to fuel ATP generation, and synthesise essential molecules. One of these is heme, which is the component of haemoglobin that enables red blood cells to carry oxygen throughout the body.

Mitochondrial Dynamics: Fusion, Fission and Quality Control

Mitochondria continually undergo processes that help maintain their shape and function. Fusion is the merging of two mitochondria, allowing them to share contents and dilute any damage. This maintains a healthy mitochondrial network for efficient ATP production. Fusion enables cells to adapt to metabolic demands during stress or high-energy activity.

Fission is the division of one mitochondrion into two, isolating any damaged fragments as part of quality control within a cell. During cell division, which is how your body makes new cells, fission ensures new cells receive sufficient mitochondria so tissues can repair and renew themselves, keeping them functioning properly. The damaged mitochondrial fragments are removed through mitophagy, a specialised form of cell cleaning that occurs whilst the cell is alive. Think of it as cellular ‘spring cleaning’. Disruptions in mitophagy are associated with neurodegenerative diseases and metabolic disorders, meaning effective mitochondrial quality control is central to preventing disease and maintaining normal cellular function.

Why This Matters for Human Health Research

Understanding the structure, function, and dynamics of mitochondria is essential for therapeutic research into how mitochondrial quality control mechanisms can be leveraged to restore healthy mitochondrial networks. Mitochondrial dysfunction contributes to conditions such as Parkinson’s disease, diabetes, and cardiovascular disorders, to name a few. This is because the mitochondria influence energy metabolism, inflammation, and cell survival. As research continues, the mitochondria remain central to understanding human biology and disease.

Disclaimer 

This article is for general information purposes only and should not be taken as medical advice. For concerns about health or disease, consult a qualified healthcare professional.

References

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