What Are Mitochondria?
Understanding the Organelles That Power and Coordinate Human Health
What Are Mitochondria?
Mitochondria are double-membrane organelles found in nearly every human cell. Descended from bacteria that formed a symbiotic partnership with an ancestral host cell well over a billion years ago, they retain their own genome and a bacterial character that still shapes how the immune system responds to them.¹ They are best known for oxidative phosphorylation, the process that converts energy from nutrients into adenosine triphosphate (ATP), yet energy production is only one part of their role. Mitochondria also supply metabolic building blocks, regulate calcium and redox signalling, communicate with the nucleus and other organelles, participate in innate immunity, and help determine whether cells adapt, differentiate, senesce, or die.¹⁻⁵
Because mitochondrial structure, abundance, and activity vary with tissue and demand, a healthy mitochondrial system is not defined by maximal ATP output, but by its capacity to match supply to need, signal appropriately, clear damaged material, and remodel as conditions change. Mitochondrial abnormalities are associated with inherited mitochondrial disorders, aging, and many common disease-related states, although association alone does not establish that mitochondrial dysfunction initiates disease.¹,²
Why Do Mitochondria Matter for Human Health?
Every cell must balance competing demands: usable energy, building blocks for growth and repair, accurate information about nutrients and stress, and safeguards that stop damaged components from compromising the whole. These demands shift within seconds during muscle contraction or neuronal firing, and over years with training, illness, and aging.
Mitochondria sit at the centre of this balancing act. Their pathways link carbohydrate, lipid, and amino-acid metabolism, and their outputs influence gene expression, inflammation, and cell fate, which is why current research describes them as dynamic processors of both energy and information rather than passive power plants.³⁻⁵
This broader view has direct clinical relevance. Symptoms such as fatigue or exercise intolerance are not specific to mitochondrial dysfunction, and no single blood test captures mitochondrial performance across the body, yet clearly defined mitochondrial defects can have serious consequences in high-demand tissues.
Why Do Cells Need Mitochondria?
Aerobic metabolism gains considerable efficiency by passing electrons from nutrient-derived carriers to oxygen, but that energy conversion must stay closely matched to demand while cells also produce lipids, nucleotides, amino acids, heme, and iron–sulfur clusters and preserve calcium balance and protein quality.¹,² Mitochondria must therefore resolve a coordination problem:
Too little respiratory capacity limits cellular work.
Excess or poorly matched flux disturbs membrane potential and redox balance.
Damaged mitochondrial DNA, proteins, or lipids must be repaired, diluted, or removed without stripping away the capacity the cell needs to meet demand.
What Is the Structure of Mitochondria?
The answer lies in compartmentalisation and adaptability. The outer membrane interfaces with the cytosol, the highly folded inner membrane generates the electrochemical gradient used for ATP synthesis, and the matrix houses metabolic enzymes and mitochondrial DNA (mtDNA). Far from being isolated bean-shaped structures, mitochondria form networks, move to regions of demand, exchange contents through fusion, and divide through fission.¹,⁴ Nuclear and mitochondrial genomes jointly build the mitochondrial proteome, while quality-control pathways decide whether components are repaired, recycled, or replaced.⁴,⁵
What Do Healthy Mitochondria Look Like?
Successful adaptation matches mitochondrial quantity, location, fuel use, and respiratory activity to the needs of the cell. When demand rises, existing mitochondria increase flux within seconds, while longer-term programmes drive biogenesis and tissue remodelling.¹,⁴
More, however, is not always better. Greater mitochondrial content, higher membrane potential, or increased reactive oxygen species may be beneficial, neutral, compensatory, or harmful depending on tissue, timing, and context. Success is better judged by coordinated physiology, such as restored respiratory reserve, improved tissue performance, or resilience to a defined stressor, than by any single marker.
How Do Mitochondria Work?
How Do Mitochondria Produce ATP?
Glucose, fatty acids, and selected amino acids are broken down to acetyl-CoA and other intermediates that enter the tricarboxylic acid (TCA) cycle. The cycle generates NADH and FADH₂, which donate electrons to the electron transport chain in the inner membrane. As electrons pass through complexes I to IV, protons are pumped into the intermembrane space, creating the proton-motive force that drives ATP synthase to convert ADP to ATP. Oxygen accepts the electrons at complex IV and is reduced to water.¹,²
The same gradient powers protein import and ion transport, so respiratory capacity depends not only on electron-transport machinery but on membrane integrity, substrate and oxygen supply, and cofactor availability.
How Do Mitochondria Support Metabolism Beyond Energy?
Mitochondria are also biosynthetic hubs. TCA-cycle intermediates can be drawn off to build amino acids, lipids, and nucleotides, and mitochondria contribute to one-carbon and ketone metabolism, heme synthesis, and iron–sulfur cluster assembly, linking energy production to the material demands of growth and repair.¹,³
How Do Mitochondria Communicate With the Rest of the Cell?
Mitochondrial outputs also carry information. Calcium uptake stimulates key metabolic enzymes, aligning ATP production with workload. At physiological levels, reactive oxygen species activate adaptive transcriptional programmes, whereas excessive or mistimed production contributes to molecular damage. The AMP/ATP and NAD⁺/NADH ratios, along with metabolites such as acetyl-CoA, succinate, fumarate, and alpha-ketoglutarate, influence kinases, transcription factors, and chromatin-modifying enzymes.³⁻⁵
This communication runs both ways. Nuclear signals govern biogenesis and stress responses, mitochondrial stress can reprogramme nuclear gene expression, and contact sites with other organelles allow local exchange of metabolites, lipids, and ions.⁴,⁵
How Do Mitochondria Divide, Fuse and Renew Themselves?
Mitochondria continually fuse, divide, move, and remodel their inner-membrane folds (cristae). Fusion mixes contents and allows functional complementation, while fission aids distribution and isolates damaged regions for repair or removal. Protein quality control, mitochondrial-derived vesicles, and mitophagy clear damaged material, while biogenesis replaces lost capacity.¹,²
What Role Do Mitochondria Play in Immunity and Cell Death?
Their bacterial ancestry means mitochondria contain molecules the immune system recognises as foreign. When mtDNA, mitochondrial RNA, cardiolipin, or N-formyl peptides are inappropriately exposed, they can activate innate immune pathways. Mitochondria also initiate apoptosis by releasing cytochrome c, which helps remove damaged cells, but uncontrolled damage or incomplete clearance can amplify inflammation.²,⁵,⁶
How Do Mitochondria Differ Between Tissues?
Every mitochondrion runs on the same core machinery, yet the mitochondria in a muscle fibre, a heart cell, and a neuron look and behave quite differently. Each tissue tunes its mitochondria to its workload, fuel supply, and where in the cell energy is needed. Recognising these cell-type-specific mitochondrial phenotypes helps explain why mitochondrial problems often appear first, or most severely, in particular tissues.⁷
Skeletal Muscle: Mitochondria Built for Changing Demand
Few tissues face such sudden swings in energy demand as skeletal muscle, which can go from rest to many times its resting energy use within seconds - and its mitochondria are positioned to meet this:
One population sits just beneath the cell membrane, close to the blood supply (subsarcolemmal mitochondria).
Another lies between the contractile filaments, right where ATP is used (intermyofibrillar mitochondria).
Imaging studies suggest the two are linked in a connected network that acts like an internal power grid, distributing energy rapidly across the muscle fibre.⁸ Mitochondrial content also depends on fibre type: Slow-twitch fibres are packed with mitochondria and built for endurance, while fast-twitch fibres rely more on glycolysis, a quicker but less efficient route to ATP, for short bursts of power.⁹
Muscle mitochondria are also among the most adaptable in the body. Endurance training increases their number and capacity, while inactivity and aging reduce both.⁹ Strength training appears to work differently: rather than adding more mitochondria, it produces smaller mitochondria with more tightly folded inner membranes (cristae), increasing the surface available for energy production.¹⁰
The Heart: Mitochondria Built for Continuous Output
The heart never rests, and its mitochondria are built accordingly. In adult heart muscle cells, mitochondria take up at least 30% of the cell's volume and produce about 95% of its ATP.¹¹,¹² The heart stores only enough ATP for a few seconds of contraction, so production has to keep pace with every beat. Its main fuel is fat, which supplies roughly 40–60% of mitochondrial ATP. The heart can also draw on lactate, ketones, glucose and amino acids depending on what is available.¹²
Cardiac mitochondria are also unusually organised. Rather than moving freely, they are packed in tight rows between the contractile fibres. Fusion and fission happen far more slowly than in most cells and serve mainly to maintain mitochondrial quality.¹¹ Because the heart depends so heavily on this steady energy supply, it is particularly vulnerable when energy production, calcium handling or redox balance is disrupted. This is why mitochondria are an active focus of heart failure research.¹²,¹³
Neurons: Mitochondria Built for Distance and Precision
The brain makes up about 2% of body weight but uses roughly 20% of the body's oxygen, largely to maintain the electrical signals and synaptic activity that neurons depend on. Most of this energy comes from mitochondria.¹⁴ The challenge for neurons is distance: a single neuron can stretch over long distances, so mitochondria must be delivered to, and kept healthy at, sites far from the cell body. Motor proteins carry them along the cell's internal scaffolding, attached through adaptor proteins called Miro and TRAK. At any one time roughly a quarter of these mitochondria are on the move, while the rest are anchored where energy is needed most, such as at the junctions where neurons communicate.¹⁴
Neurons maintain these distant mitochondria through a combination of transport, local protein production and local quality control, including mitophagy.¹⁴ Because most neurons do not divide and must keep their mitochondria working for decades, problems with mitochondrial transport and maintenance have become a major focus of research into neurodegenerative disease.
Other Specialised Mitochondria
Other tissues show equally striking specialisations. In brown adipose tissue, uncoupling protein 1 (UCP1) lets protons flow back across the inner membrane without producing ATP, releasing the energy as heat.¹⁵ In immune cells, mitochondria help switch cells between functional states: activated inflammatory cells shift towards glycolysis, long-lived memory T cells depend on oxidative metabolism, and mitochondrial metabolites act as signals that shape the immune response.¹⁶ Mature red blood cells, by contrast, contain no mitochondria and rely entirely on glycolysis, so they do not consume the oxygen they carry.
Mitochondria at a Glance
| Tissue | How mitochondria are organised | Main priority | Why it matters |
|---|---|---|---|
| Skeletal muscle | Subsarcolemmal and intermyofibrillar populations forming a connected network; content varies by fibre type | Rapidly scaling ATP supply during contraction | Highly responsive to training, disuse, and aging; the main tissue for studying mitochondrial adaptation in humans |
| Heart | Densely packed rows between myofibrils, occupying at least 30% of cell volume; limited movement and slow fusion and fission | Continuous ATP supply, mainly from fatty acid oxidation | Heavy reliance on oxidative metabolism makes energy supply, calcium, and redox balance critical |
| Neurons | Transported along axons and anchored at synapses and other high-demand sites | Supporting electrical signalling and synaptic transmission far from the cell body | Long-lived cells that depend on transport and local quality control |
| Brown adipose tissue | Abundant mitochondria expressing UCP1 | Heat production through uncoupled respiration | Shows that mitochondria can be tuned for purposes other than ATP production |
| Immune cells | Mitochondrial activity shifts with activation state | Supporting activation, memory, and immune signalling | Mitochondrial metabolism helps determine how immune cells behave |
Exercise as Mitochondrial Adaptation
Exercise brings these principles together. At the onset of muscular work, ATP turnover rises sharply, and changes in calcium, AMP/ATP balance, oxygen delivery, and redox state signal the new demand. Existing mitochondria respond within seconds by increasing fuel oxidation, while signalling to the nucleus begins to coordinate recovery. With repeated, appropriately dosed training, skeletal muscle gains mitochondrial content, respiratory capacity, and endurance, showing how transient energetic and redox stress can act as information rather than damage alone.
The example also shows the limits of simple interpretation. Suppressing all oxidant signalling may blunt adaptation, while extreme workloads without adequate recovery can overwhelm repair capacity. For practitioners, the relevant endpoint is improved function within a safe, progressive training programme rather than change in any single mitochondrial marker.
What Happens When Mitochondrial Function Becomes Dysregulated?
Mitochondrial dysfunction describes a family of problems rather than a single measurable state:
Energy production may be compromised through impaired oxidative phosphorylation, altered fuel use, or loss of membrane potential.
Signalling can be disrupted through calcium mishandling or oxidants produced in excess or in the wrong place.
The machinery that maintains mitochondria may also break down, with defective protein import, damaged mtDNA, disordered dynamics, too little mitophagy, or impaired biogenesis.
Mitochondria can send inappropriate immune and cell-death signals.¹,²,⁵
These problems compound one another: impaired respiration shifts redox balance and ATP supply, damaged membranes disrupt ion handling, and released mitochondrial components fuel inflammation. Identifying these changes does not mean mitochondria caused the problem, as they may also reflect the body adapting to stress or the disease itself. In common conditions with many contributing factors, mitochondrial dysfunction is best seen as one part of the picture rather than the sole cause.
Because each tissue relies on its mitochondria differently, the same underlying problem can affect muscle, heart, and brain in different ways. The conditions most often linked to mitochondrial dysfunction, and how they are assessed, are covered in our Mitochondrial Dysfunction article.
Practitioner Lens: Mitochondrial Supplements in Clinical Practice
Mitochondrial disturbances tend to show up first in tissues with the highest and least flexible energy demands, which is why muscle weakness, exercise intolerance, cardiac involvement, and neurological symptoms are common in primary mitochondrial disease.² Blood-based markers do not necessarily reflect what is happening in muscle, heart, or brain. And because most human evidence on mitochondrial adaptation comes from skeletal muscle biopsies, findings in muscle should not be assumed to apply to every tissue.
A useful starting point with any supplement is a simple question: what is it meant to act on, and does the formulation that was tested actually reach that target in people? A promising result in cells or animals is not the same as a change in human biomarkers, and neither guarantees an improvement in how patients feel or function. It is also worth resisting the pull of shared labels: two products described as “antioxidants” or “mitochondrial support” may have very different evidence behind them.
Practitioner Interpretation
Mitochondrial biology is a valuable framework for clinical reasoning, but it does not replace diagnosis. Fatigue, brain fog, weakness, and slow recovery are common complaints with many possible causes, so it is important to rule out conventional explanations and check for red flags before attributing symptoms to mitochondrial dysfunction. For most patients, evidence-based exercise, sleep, nutrition, smoking cessation, and management of cardiometabolic risk factors form the foundation on which any targeted intervention should be built.
What Does the Current Evidence Show?
Mitochondria are essential well beyond energy production, and their structure and activity adapt to the body’s demands. Inherited variants in mitochondrial or nuclear genes show that severe disruption of these pathways can cause human disease, while exercise studies show that mitochondrial content and function in skeletal muscle can be improved.¹⁻⁵,⁹
Researchers are still mapping how mitochondria differ within a single tissue, how their contacts with other organelles coordinate metabolism, and how stress responses play out in people. In many common diseases, it is unclear whether mitochondrial changes set the problem in motion, make it worse, help the body compensate, or simply follow from it. Closing these gaps will depend on biomarkers that show an intervention is reaching its target and predict whether it will make a meaningful difference.
Evidence Hierarchy
| Evidence type | What it can establish | What it cannot establish alone |
|---|---|---|
| Mechanistic or cell | Molecular relationships, target plausibility, and pathway response | Human safety, whole-body physiology, or clinical benefit |
| Animal model | Causality and integrated physiology within a defined model | Equivalent effects in humans or across heterogeneous disease |
| Human biomarker or tissue study | Association, target engagement, and tissue-specific biological change | That the change caused symptom or clinical improvement |
| Human functional or clinical trial | Physiological or clinical relevance in the studied population and intervention | Generalisability beyond the tested formulation, dose, duration, and population |
How Is Mitoquinol Relevant to Mitochondrial Biology?
Mitoquinol is the reduced (ubiquinol) form of the active component of MitoQ, in which ubiquinone is linked to a lipophilic triphenylphosphonium cation. This design, developed by Professor Mike Murphy and Dr Robin Smith, uses the mitochondrial membrane potential to concentrate the compound within mitochondria, making mitoquinol a valuable tool for exploring mitochondrial oxidative stress and the redox processes that take place at mitochondrial membranes.¹⁷
Mitoquinol Human Evidence
In healthy older adults, six weeks of MitoQ supplementation improved brachial artery flow-mediated dilation, a measure of vascular endothelial function, and reduced selected markers of oxidative stress.¹⁸ In separate studies, MitoQ attenuated exercise-induced mitochondrial DNA damage and improved 8 km cycling time-trial performance in trained participants.¹⁹,²⁰ To date, these findings centre on vascular and skeletal muscle outcomes, the tissues most accessible to study in people. They span different populations and endpoints, and the wider evidence base also includes null and mixed findings that condition-specific summaries should report alongside positive results.
Key Takeaways for Practitioners and Researchers
Mitochondria are integrated hubs of energy production, metabolism, signalling, quality control, immunity, and cell fate, not ATP factories alone.
Healthy mitochondria adapt flexibly to tissue demand rather than maximising any single marker.
Mitochondria are specialised by tissue: muscle mitochondria are built for rapidly changing demand, cardiac mitochondria for continuous output, and neuronal mitochondria for distance and precision.
Mitochondrial dysfunction may be primary or secondary, and causal, compensatory, or downstream of disease.
Measurements in one tissue, particularly blood, may not reflect mitochondrial function in another.
Mitoquinol is a mitochondria-targeted redox tool with promising human findings in vascular function, exercise-induced mtDNA damage, and performance.
References
Moura JP, Oliveira PJ, Urbano AM. Mitochondria: an overview of their origin, genome, architecture, and dynamics. BBA – Molecular Basis of Disease. 2025;1871:167803. doi:10.1016/j.bbadis.2025.167803.
Harrington JS, Ryter SW, Plataki M, Price DR, Choi AMK. Mitochondria in health, disease, and aging. Physiological Reviews. 2023;103:2349-2422. doi:10.1152/physrev.00058.2021.
Lee-Glover LP, Picard M, Shutt TE. Mitochondria – the CEO of the cell. Journal of Cell Science. 2025;138:jcs263403. doi:10.1242/jcs.263403.
Meichsner A, Bader V, Winklhofer KF. Mitochondria as sources and targets of cellular signaling. Molecular Cell. 2026;86:503-526. doi:10.1016/j.molcel.2026.01.008.
Vringer E, Tait SWG. Mitochondria and cell death-associated inflammation. Cell Death & Differentiation. 2023;30(2):304-312. doi:10.1038/s41418-022-01094-w.
Picca A, et al. Mitophagy in human health, ageing and disease. Nature Metabolism. 2023;5:2047-2061. doi:10.1038/s42255-023-00930-8.
Monzel AS, Enríquez JA, Picard M. Multifaceted mitochondria: moving mitochondrial science beyond function and dysfunction. Nature Metabolism. 2023;5(4):546-562. doi:10.1038/s42255-023-00783-1.
Glancy B, Hartnell LM, Malide D, et al. Mitochondrial reticulum for cellular energy distribution in muscle. Nature. 2015;523:617-623. doi:10.1038/nature14614.
Hood DA, Memme JM, Oliveira AN, Triolo M. Maintenance of skeletal muscle mitochondria in health, exercise, and aging. Annual Review of Physiology. 2019;81:19-41. doi:10.1146/annurev-physiol-020518-114310.
Botella J, Schytz CT, Pehrson TF, et al. Increased mitochondrial surface area and cristae density in the skeletal muscle of strength athletes. Journal of Physiology. 2023;601(14):2899-2915. doi:10.1113/JP284394.
Piquereau J, Caffin F, Novotova M, et al. Mitochondrial dynamics in the adult cardiomyocytes: which roles for a highly specialized cell? Frontiers in Physiology. 2013;4:102. doi:10.3389/fphys.2013.00102.
Lopaschuk GD, Karwi QG, Tian R, Wende AR, Abel ED. Cardiac energy metabolism in heart failure. Circulation Research. 2021;128:1487-1513. doi:10.1161/CIRCRESAHA.121.318241.
Brown DA, Perry JB, Allen ME, et al. Mitochondrial function as a therapeutic target in heart failure. Nature Reviews Cardiology. 2017;14(4):238-250. doi:10.1038/nrcardio.2016.203.
Misgeld T, Schwarz TL. Mitostasis in neurons: maintaining mitochondria in an extended cellular architecture. Neuron. 2017;96(3):651-666. doi:10.1016/j.neuron.2017.09.055.
Cannon B, Nedergaard J. Brown adipose tissue: function and physiological significance. Physiological Reviews. 2004;84(1):277-359. doi:10.1152/physrev.00015.2003.
Mills EL, Kelly B, O’Neill LAJ. Mitochondria are the powerhouses of immunity. Nature Immunology. 2017;18(5):488-498. doi:10.1038/ni.3704.
Murphy MP, Smith RAJ. Targeting antioxidants to mitochondria by conjugation to lipophilic cations. Annual Review of Pharmacology and Toxicology. 2007;47:629-656. doi:10.1146/annurev.pharmtox.47.120505.105110.
Rossman MJ, Santos-Parker JR, Steward CAC, et al. Chronic supplementation with a mitochondrial antioxidant (MitoQ) improves vascular function in healthy older adults. Hypertension. 2018;71:1056-1063. doi:10.1161/HYPERTENSIONAHA.117.10787.
Williamson J, Hughes CM, Cobley JN, et al. The mitochondria-targeted antioxidant MitoQ attenuates exercise-induced mitochondrial DNA damage. Redox Biology. 2020;36:101673. doi:10.1016/j.redox.2020.101673.
Broome SC, Woodhead JST, Merry TL, et al. Mitochondria-targeted antioxidant supplementation improves 8 km time trial performance in middle-aged trained male cyclists. Journal of the International Society of Sports Nutrition. 2021;18:58. doi:10.1186/s12970-021-00454-0.