Figure 1. Electron flow in a battery as an illustration of oxidation and reduction. In this schematic, electrons (e-) travel from the negative terminal (substance B) to the positive terminal (substance A), generating an electrical current that powers the fan and causes it to spin. Substance B, which donates electrons, is oxidized in the process, while substance A, which accepts the electrons, is reduced.
Introduction
Mitochondria-The Batteries of Our Cells
Every cell in the human body depends on a quiet, continuous stream of energy- a molecular current that keeps tissues alive, organs functioning, and the entire organism in motion. This energy does not appear spontaneously; it is extracted, transformed, and delivered through a beautifully coordinated system that begins with the nutrients we eat and culminates inside one of the most remarkable structures in biology: the mitochondrion.
Mitochondria are often described as the “batteries” of our cells, but this metaphor only hints at their sophistication. These tiny organelles convert the chemical potential stored in carbohydrates, fats, and proteins into adenosine triphosphate (ATP), the universal energy currency of life. Yet mitochondria cannot perform this task alone. Their ability to generate ATP depends on a fundamental chemical principle-the movement of electrons-a process known as oxidation–reduction (redox) chemistry (see Figure 1).
Redox reactions form the first bridge between nutrient breakdown and mitochondrial energy production. As molecules are metabolized, they donate electrons that are captured by specialized carriers, quietly setting in motion the electron flow that will ultimately drive ATP synthesis. Understanding this foundational redox logic is essential, because it prepares us for the deeper mitochondrial mechanisms-membrane gradients, oxidative phosphorylation, and metabolic integration-that we will explore in the articles ahead.
This introductory section offers a gentle but precise entry point into that world. It explains how electron transfer releases energy, how NAD and FAD shuttle those electrons toward the mitochondria, and why vitamins B2 and B3 support this system. With these concepts in place, we are ready to move from the surface of mitochondrial biology into its core- where the real complexity, beauty, and clinical relevance reside.
Every nutrient we eat-whether carbohydrate, fat, or protein-ultimately converges on a single, remarkable destination inside our cells: the mitochondria. These tiny structures are responsible for producing the vast majority of our ATP, the energy-rich molecule that powers nearly every biological function. No matter what food we consume, its usable energy is finally extracted and converted into ATP within these specialized organelles.
To understand how food becomes energy, we must understand mitochondria. They host many of the cell’s most essential metabolic pathways, including the Krebs cycle and the process that generates most of our ATP: oxidative phosphorylation. In every meaningful sense, mitochondria act as the batteries of our cells, transforming the chemical energy stored in nutrients into the usable form that keeps our bodies alive and functioning.
Figure 2. Mitochondrial structure. Mitochondria vary widely in shape and size, continually shifting their form, but they share a defining architectural feature: two membranes, with the inner membrane folded into numerous cristae. These folds greatly expand the membrane’s surface area, providing abundant sites for ATP generation- a point we will explore in later sections.
What Mitochondria Are
The word mitochondria (plural of mitochondrion) comes from Greek roots describing their appearance-“thread-like granules.” These organelles are tiny, typically around 1 micrometer in length, meaning one million mitochondria lined end-to-end would stretch one meter. Each mitochondrion is enclosed by membranes, much like the skin surrounding an egg yolk. These membranes are not just structural; they are critical for energy generation, as we will soon see.
Mitochondria vary widely in shape- from small spheres to long, spaghetti-like strands- and they constantly change size and form. Most importantly, they are indispensable. In complex organisms like humans, mitochondria occupy roughly 25% of the cell’s internal volume, a reflection of how much work they perform (see Figure 2).
What Mitochondria Do
Mitochondria coordinate a remarkable range of cellular functions. They participate in the metabolism of carbohydrates, fats, and proteins. They generate and detoxify free radicals. They regulate cell differentiation, cell death, and even programmed cell suicide. They influence immune responses, body weight, and heat production. Above all, they convert the energy stored in the molecules we eat into the usable form of ATP [1-2; 5].
In this role, mitochondria truly behave like biological batteries– not only because they supply energy, but because they generate it through mechanisms strikingly similar to the way physical batteries work (see Figure 1 and 3).
Figure 3. Oxidation and reduction as drivers of electron flow in the mitochondrial inner membrane. In this illustration, electrons move from the carriers NAD and FAD to oxygen through the protein complexes of the electron transport chain. As the electrons pass along these membrane-embedded proteins, their energy is used to pump protons (H⁺) from the mitochondrial interior into the intermembrane space, establishing an electrochemical gradient that stores energy for ATP synthesis. During this process, NAD and FAD are oxidized and recycled to collect new electrons in the Krebs cycle, while oxygen is reduced to form water.
How Mitochondria Generate Energy
A battery produces energy by driving oxidation-reduction (redox) reactions that create a positive and negative side. The difference in charge between these sides is what powers the devices connected to the battery.
Mitochondria operate on the same principle. They orchestrate a series of redox reactions that create a charge difference across their membranes, forming the electrochemical gradient needed to synthesize ATP. This gradient is the driving force behind oxidative phosphorylation, the process that produces the vast majority of ATP in human cells.
How Mitochondria Use Redox Reactions to Generate Energy
To understand how mitochondria generate ATP, we must first understand redox reactions. In simple terms, redox reactions involve the transfer of electrons from one molecule to another. Electrons are small, mobile components of atoms and molecules that carry a negative charge. When a molecule gains electrons, its overall charge becomes more negative- a process known as reduction. Conversely, when a molecule loses electrons, it becomes oxidized. The term “oxidized” originates from oxygen’s strong ability to pull electrons away from other substances- rust being a familiar example (see Figure 1).
The key point is not the terminology, but the energy flow these reactions create. When electrons move from a molecule with lower electron affinity to one with higher affinity, energy is released. In rusting metal, that energy dissipates as heat. In batteries, it drives electrical current. In our bodies, the energy released from redox reactions is captured and used to power cellular processes (see Figure 3).
As glucose and other nutrients are broken down into CO₂, they are simultaneously oxidized, meaning they lose electrons. These electrons are removed during glycolysis, the formation of acetyl-CoA, and most prominently during the Krebs cycle. Two specialized molecules- nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD)– collect these electrons. NAD and FAD act as electron shuttles, picking up electrons from partially degraded nutrients and delivering them to the mitochondrial membrane, where the major ATP-generating reactions occur. Importantly, NAD and FAD carry electrons not only from carbohydrate metabolism but also from the breakdown of proteins and lipids, a topic we will explore in future articles (see Figure 3) [3-4].
NAD and FAD are synthesized inside the body using nicotinamide (vitamin B3) and riboflavin (vitamin B2) as building blocks. Vitamins are essential molecules we cannot produce ourselves and must obtain through diet. Fortunately, most individuals with healthy, varied diets obtain sufficient amounts of these vitamins to support NAD and FAD production. Except in medically identified cases of deficiency or special need, vitamin supplements offer no additional benefit, and in some situations may even be harmful.
(Cf. previous blog entitled as: “The Maternal Spark: How Mitochondrial Inheritance Shapes Life, Health, and the Developing Brain.”)
Take-Home Message
- Mitochondria are the cell’s central energy hubs, converting the chemical potential in nutrients into ATP-the usable energy that sustains life.
- Their ability to generate ATP depends entirely on electron flow, making mitochondria the final destination for the energy extracted from carbohydrates, fats, and proteins.
- Redox reactions quietly drive this energy flow, releasing power whenever electrons move from one molecule to another.
- Oxidation and reduction are simple electron shifts– molecules losing electrons are oxidized, and those gaining electrons are reduced.
- Nutrients donate electrons as they break down, and this electron donation is the first step toward ATP production.
- NAD and FAD serve as essential electron couriers, collecting electrons from metabolic pathways and delivering them to the mitochondrial membrane.
- Vitamins B2 and B3 support this electron-carrying system, and balanced diets typically provide all that is needed for healthy NAD and FAD formation.
- This redox-driven electron flow is the foundation of mitochondrial energy production, setting the stage for the deeper mechanisms- membrane gradients, oxidative phosphorylation, and metabolic integration- that will be explored in the next articles.
Summary and Conclusions
Mitochondria serve as the central energy hubs of the cell, converting the chemical potential stored in nutrients into ATP- the molecule that sustains nearly every biological function. Their ability to generate this energy depends on a fundamental chemical principle: the controlled movement of electrons through oxidation–reduction (redox) reactions. These reactions quietly release energy whenever electrons shift from one molecule to another, forming the essential bridge between nutrient breakdown and mitochondrial ATP production.
As carbohydrates, fats, and proteins are metabolized, they donate electrons, becoming oxidized in the process. These electrons are collected by the specialized carriers NAD and FAD, which transport them to the mitochondrial membrane. There, the delivered electrons initiate the downstream reactions that ultimately drive ATP synthesis. The formation of NAD and FAD depends on vitamins B3 and B2, respectively- nutrients that most individuals obtain adequately through balanced diets. Thus, the redox system operates continuously and efficiently under normal physiological conditions, providing the foundational electron flow required for mitochondrial energy production.
This introductory article establishes the core logic of redox chemistry as the first step in understanding how mitochondria generate ATP. By clarifying how electrons are transferred, how energy is released, and how NAD and FAD function as electron couriers, we set the stage for exploring the deeper mechanisms of mitochondrial biology- including membrane gradients, oxidative phosphorylation, and the intricate architecture of the electron transport chain.
Gaps in Knowledge
Despite the clarity of these basic principles, several important questions remain open:
- How variations in NAD and FAD availability influence mitochondrial efficiency under different physiological or pathological conditions is not fully understood.
- The precise regulatory mechanisms that determine how many electrons are captured, transferred, or lost during nutrient breakdown remain incompletely mapped.
- Individual differences in vitamin metabolism– including absorption, transport, and utilization- may subtly affect electron-carrier formation, but these relationships are still being investigated.
- How early redox disturbances shape later mitochondrial function is an emerging area of study, particularly in metabolic and neurodevelopmental disorders.
Future Directions
Future articles in this series will build directly on this redox foundation by exploring:
- How mitochondria convert electron flow into ATP through oxidative phosphorylation.
- How membrane gradients and proton motive force are established, maintained, and regulated.
- How mitochondrial structure, dynamics, and distribution influence energy production across different tissues.
- How redox chemistry integrates with broader metabolic pathways, including lipid oxidation, amino acid metabolism, and reactive oxygen species management.
These next steps will deepen our understanding of mitochondrial biology, revealing how the quiet movement of electrons becomes the driving force behind cellular life.
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Further Reading
- Nunnari J, Suomalainen A. Mitochondria: in sickness and in health. Cell. 2012 Mar 16;148(6):1145-59. doi: 10.1016/j.cell.2012.02.035. PMID: 22424226; PMCID: PMC5381524.
https://pubmed.ncbi.nlm.nih.gov/22424226/
https://www.cell.com/action/showPdf?pii=S0092-8674%2812%2900235-8
(Provides the foundational mitochondrial perspective that the introduction builds upon.) - Jones DP. Redefining oxidative stress. Antioxid Redox Signal. 2006 Sep-Oct;8(9-10):1865-79. doi: 10.1089/ars.2006.8.1865. PMID: 16987039.
https://pubmed.ncbi.nlm.nih.gov/16987039/
(Defines redox reactions, electron flow, and cellular energy logic- the heart of the article.) - Verdin E. NAD⁺ in aging, metabolism, and neurodegeneration. Science. 2015 Dec 4;350(6265):1208-13. doi: 10.1126/science.aac4854. PMID: 26785480.
https://pubmed.ncbi.nlm.nih.gov/26785480/
https://www.thehealthedgepodcast.com/wp-content/uploads/2018/11/NAD-Neurodegen2015Sciencereview.pdf
(Explains NAD⁺ as a central electron carrier derived from vitamin B3- directly supporting the NAD narrative.) - Powers HJ. Riboflavin (vitamin B-2) and health. Am J Clin Nutr. 2003 Jun;77(6):1352-60. doi: 10.1093/ajcn/77.6.1352. PMID: 12791609.
https://pubmed.ncbi.nlm.nih.gov/12791609/
https://ajcn.nutrition.org/article/S0002-9165(23)05794-5/pdf
(Covers riboflavin’s biochemical role in forming FAD and FMN- essential for mitochondrial redox chemistry.) - Sies H, Jones DP. Reactive oxygen species (ROS) as pleiotropic physiological signalling agents. Nat Rev Mol Cell Biol. 2020 Jul;21(7):363-383. doi: 10.1038/s41580-020-0230-3. Epub 2020 Mar 30. PMID: 32231263.
https://pubmed.ncbi.nlm.nih.gov/32231263/
https://www.ozoderm.co/a/fotos/p43.pdf
(Provides the modern, high-impact synthesis of redox couples, electron flow, and metabolic regulation- the perfect conceptual capstone.)
