Virtually every living organism on Earth performs cellular respiration, from the largest whales to bacteria too small to see with the naked eye. The process is not limited to animals or even to oxygen-breathing life. Plants, fungi, protists, bacteria, and archaea all carry out their own versions of it, and collectively their respiration drives one of the planet’s largest fluxes of carbon dioxide into the atmosphere. The full picture of which organisms respire, and how, turns out to be far more varied than any textbook diagram of a single mitochondrion suggests.
Animals and Other Complex Multicellular Life
Animals are the organisms most people associate with cellular respiration, and for good reason. Every cell in an animal’s body depends on mitochondria to convert nutrients and oxygen into usable energy. This applies whether the animal is a hummingbird burning through sugar at an extraordinary rate or a deep-sea sponge processing oxygen that drifts down from above. The mitochondria that make this possible trace back roughly 1.5 billion years to an ancient event in which a prokaryotic cell took up residence inside another cell, eventually becoming the energy-producing organelle found in nearly all complex life today.1PubMed Central. Mitochondrial transfer between cells can rescue aerobic respiration That single partnership gave rise to the capacity for aerobic respiration and, eventually, to the entire lineage of multicellular organisms.
What makes animal respiration distinctive is its sheer demand. Muscle tissue during exertion, brain cells processing information, and the liver breaking down toxins all require a continuous supply of energy. When oxygen delivery to tissues falls short, cells can temporarily switch to fermentation to keep functioning, but this is a stopgap with much lower energy output. The overwhelming majority of an animal’s energy budget comes from oxygen-dependent respiration in mitochondria.
Plants Respire Too
One of the most common misconceptions about cellular respiration is that plants only photosynthesize. In reality, every plant cell respires, day and night. Photosynthesis captures light energy and stores it in sugars; respiration then breaks those sugars back down to release that stored energy for growth, maintenance, and reproduction. The two processes are complementary, not alternatives.
This happens not just in leaves but throughout the entire plant body. Root cells, which never see sunlight, depend entirely on respiration for their energy needs. Research measuring root respiration across ten woody plant species confirmed that roots actively consume oxygen as part of this process, and that the rate of oxygen consumption changes depending on nutrient availability.2Functional Ecology. Nutrient‐induced changes in root respiration in 10 woody plant species When soil is richer in nitrogen or phosphorus, roots ramp up their metabolic activity, consuming more oxygen and releasing more carbon dioxide.
Plant mitochondria also carry some unique equipment. Besides the standard respiratory pathway shared with animals, plant cells have an alternative oxidase pathway. This secondary route does not generate as much energy, but it serves a protective role: it helps lower the production of damaging reactive oxygen species during respiration.3PubMed. The alternative oxidase lowers mitochondrial reactive oxygen production in plant cells Experiments with genetically modified tobacco cells showed that plants with higher levels of this alternative oxidase produced fewer harmful byproducts. It is a built-in safety valve that animals do not have.
Fungi and Yeasts
Fungi, from the mushrooms in your yard to the yeast in bread dough, are full-fledged cellular respirers. Like animals, fungi are heterotrophs: they cannot make their own food from sunlight and instead break down organic material from their environment. The energy locked in those organic molecules is extracted through respiration.
Yeasts are a particularly well-studied example because they are metabolically flexible. In the presence of oxygen, yeast cells carry out aerobic respiration for maximum energy yield. When oxygen runs low, they switch to fermentation, producing ethanol and carbon dioxide as byproducts. This is what makes bread rise and beer ferment. But fermentation is a fallback, not the preferred mode. Given a choice, yeast cells will respire aerobically because the energy payoff is far greater.
Pathogenic fungi rely on the same fundamental process. Research on Histoplasma capsulatum, a fungus that causes lung infections in humans, found that it channels carbon heavily toward cellular respiration, producing carbon dioxide and energy through that route while also feeding certain biosynthetic side pathways.4PubMed Central. Mapping carbon utilization pathways in Histoplasma capsulatum through 13C-metabolic flux analysis Understanding how a pathogen powers itself has obvious implications for developing antifungal treatments: disrupt the energy supply, and the organism cannot thrive.
Bacteria, the Most Versatile Respirers
Bacteria are where cellular respiration gets creative. Many bacteria respire aerobically, using oxygen just as your cells do. But a vast number have evolved to respire without oxygen at all, using alternative molecules as the final destination for the electrons stripped from food. This is anaerobic respiration, and it is fundamentally different from fermentation despite both occurring in the absence of oxygen.
In aerobic respiration, oxygen is the terminal electron acceptor. In anaerobic respiration, other substances fill that role. Sulfate-reducing bacteria, for instance, use sulfate and produce sulfide as a waste product. The bacterium Desulfovibrio desulfuricans can use either sulfate or nitrate as its electron acceptor, and it actually grows faster and produces higher yields when nitrate is available instead of sulfate.5PubMed. Understanding the response of Desulfovibrio desulfuricans ATCC 27774 to the electron acceptors nitrate and sulfate – biosynthetic costs modulate substrate selection This kind of respiratory versatility allows bacteria to colonize environments where oxygen is scarce or absent entirely, from waterlogged soils to the deep subsurface.
The energetic difference between respiration and fermentation in bacteria has been directly measured. In Escherichia coli, the electrochemical force driving energy production across the cell membrane stays relatively stable whether the cell is respiring with oxygen, nitrate, fumarate, or dimethyl sulfoxide. Aerobic respiration produces the highest driving force (about 160 millivolts), and anaerobic respiration with various acceptors drops only modestly. Fermentation, by contrast, shows a much larger drop of roughly 40 millivolts compared to aerobic conditions.6PubMed Central. Changes in the proton potential and the cellular energetics of Escherichia coli during growth by aerobic and anaerobic respiration or by fermentation The takeaway: anaerobic respiration is a genuinely efficient energy strategy, much closer to aerobic respiration than it is to the relatively crude process of fermentation.
Cyanobacteria and Dual Metabolism
Cyanobacteria occupy a fascinating middle ground. These are the microbes that invented oxygenic photosynthesis billions of years ago, and they continue to be major producers of atmospheric oxygen today. But cyanobacteria also respire, and they do so using some of the very same cellular machinery they use for photosynthesis.
In cyanobacteria, the thylakoid membrane, the internal membrane where photosynthesis takes place, simultaneously hosts the respiratory electron transport chain.7PubMed Central. Distribution and dynamics of electron transport complexes in cyanobacterial thylakoid membranes The photosynthetic and respiratory chains share several protein components, meaning that electrons can flow through a variety of routes depending on the conditions.8PubMed. Co-existence of photosynthetic and respiratory activities in cyanobacterial thylakoid membranes During the day, photosynthesis dominates, but respiration still ticks along. At night, respiration takes over entirely. The shared infrastructure allows cyanobacteria to respond flexibly to changing light conditions without needing entirely separate systems for each task.
Archaea and Life at the Thermodynamic Edge
Archaea are often described as extremophiles, and while that reputation is somewhat unfair (many archaea live in ordinary soils and ocean water), their metabolic strategies are genuinely unusual. Some archaea respire aerobically, and researchers have isolated unusual electron-transporting protein supercomplexes from these aerobic species that do not look like anything found in bacteria or eukaryotes.9PubMed Central. Bioenergetics of the Archaea The basic principle of using a proton gradient across a membrane to generate energy holds true in archaea, but the molecular details are often distinct.
Anaerobic archaea push energy metabolism to its absolute limits. Methanogens, for example, produce methane as a byproduct of their energy metabolism. The energy available from methanogenesis is tiny compared to aerobic respiration, so these organisms have evolved remarkably efficient molecular machinery. Different groups of anaerobic archaea, including methanogens, Thermococcus, Pyrococcus, and Ignicoccus, have developed different ways to energize their membranes, often using sodium ions rather than hydrogen ions as the coupling agent. They share an adapted ATP synthase enzyme whose rotor structure shows extraordinary variation, reflecting the demands of operating at what researchers describe as the thermodynamic limit of life.10FEMS Microbiology Reviews. Adaptations of anaerobic archaea to life under extreme energy limitation
Some archaea also form intimate partnerships with other organisms. Certain anaerobic ciliates, single-celled eukaryotes, harbor methanogenic archaea inside their cells. Research has shown that each ciliate strain typically hosts a single dominant population of methanogenic archaea, and the identity of the methanogen is closely linked to both the host’s taxonomy and the environment it inhabits.11PubMed Central. Methanogenic symbionts of anaerobic ciliates are host and habitat specific The ciliate provides a sheltered, low-oxygen environment and waste hydrogen; the methanogen uses that hydrogen to power its own respiration and produces methane in return. These partnerships illustrate how tightly interwoven different organisms’ respiratory strategies can become.
Single-Celled Eukaryotes
Protists, the catchall group of eukaryotes that are not animals, plants, or fungi, are enthusiastic respirers. Amoebae, paramecia, and countless other unicellular organisms depend on mitochondria for aerobic respiration just as your cells do. What differs is the physical challenge of getting oxygen to those mitochondria. In a large animal, circulatory systems deliver oxygen directly to cells. A protist must rely on diffusion alone.
Modeling of oxygen gradients in heterotrophic protists has revealed an elegant adaptation: mitochondria in these cells tend to cluster near the outer membrane rather than being spread evenly throughout the cell.12PubMed. Respiration in heterotrophic unicellular eukaryotic organisms Mitochondria make up roughly ten percent of cell volume regardless of cell size, but their positioning near the surface ensures they have first access to incoming oxygen. For small and medium-sized protists, this arrangement means that oxygen supply is not a limiting factor, and respiration rates scale closely with cell volume. Larger protists tend to adopt flattened or elongated shapes that increase their surface area relative to volume, further easing the oxygen supply problem.
Deep-Sea Life Without Sunlight
At hydrothermal vents on the ocean floor, entire ecosystems thrive in total darkness, sustained not by solar energy but by chemical energy from the Earth’s interior. The microorganisms that form the base of these food webs are chemolithotrophs: they harvest energy from inorganic chemicals like hydrogen sulfide, hydrogen gas, or iron compounds, and they do so through respiratory electron transport chains, not through photosynthesis. Studies of deep-sea hydrothermal vent communities have found a diverse mix of chemolithoautotrophic, heterotrophic, and mixotrophic microbes, each with versatile metabolic strategies suited to the variable chemistry of vent fluids.13PubMed Central. Microorganisms from deep-sea hydrothermal vents
Some of these vent bacteria go further still. Species like Shewanella oneidensis can transfer electrons directly to solid minerals or electrodes outside their cells, a process called extracellular electron transfer. Research measuring the energy losses during this process found that between roughly 44 and 68 percent of the available chemical energy is lost as potential across the interface between the bacterial biofilm and the external surface.14Journal of Power Sources. Potential losses in extracellular electron transfer with Shewanella oneidensis That sounds wasteful, but it is enough to sustain life in environments where no other energy source exists. This capability also underpins the concept of microbial fuel cells, devices that harness bacterial respiration to generate small amounts of electricity.
Parasites That Rewired Their Mitochondria
Parasitic organisms face an unusual energy challenge: they often live inside a host’s body where oxygen levels are low or unpredictable. Some have responded by modifying their mitochondria to work in reverse compared to the standard setup. In the parasitic roundworm Ascaris suum and the malaria parasite Plasmodium falciparum, a key mitochondrial enzyme complex operates as a quinol-fumarate reductase rather than in the typical oxidative direction.15Biochimica et Biophysica Acta (BBA) – Bioenergetics. Role of complex II in anaerobic respiration of the parasite mitochondria from Ascaris suum and Plasmodium falciparum This modification lets the parasite carry out anaerobic respiration using fumarate as the electron acceptor, generating energy even in the near-absence of oxygen deep inside host tissues.
These modified respiratory chains are significant for medicine because they differ enough from the host’s own mitochondria to serve as drug targets. If you can selectively block the parasite’s unusual version of the enzyme without affecting the host’s normal version, you have a potential treatment. This is one reason why studying the fine details of which organisms respire, and exactly how they do it, has practical implications beyond pure biology.
The Evolutionary Origins of Respiration
The current understanding of how eukaryotic respiration began centers on a pivotal symbiotic event. The hydrogen hypothesis proposes that the original host was an archaeon, not a primitive eukaryote, and the symbiont was a facultatively anaerobic bacterium capable of both oxygen-based respiration and hydrogen-producing fermentation under low-oxygen conditions.16PubMed Central. Endosymbiotic theories for eukaryote origin This partnership, rooted in anaerobic syntrophy (where one partner feeds on the metabolic waste of the other), eventually gave rise to mitochondria and the entire eukaryotic lineage.
Given this history, it might seem like mitochondria and cellular respiration are inseparable from eukaryotic identity. But at least one organism has proved otherwise. Monocercomonoides, a gut-dwelling protist found in chinchillas, has completely lost its mitochondria. Genome sequencing confirmed that this organism lacks all hallmark mitochondrial proteins, making it the first known eukaryote without any form of a mitochondrion.17PubMed. A Eukaryote without a Mitochondrial Organelle The loss was secondary, meaning Monocercomonoides descended from ancestors that did have mitochondria and gradually shed them, likely because its oxygen-free gut environment made aerobic respiration unnecessary.
Monocercomonoides is an extreme case, but reduced forms of mitochondria are actually common among microbial eukaryotes in low-oxygen habitats. These mitochondrion-related organelles, including structures called hydrogenosomes and mitosomes, share ancestry with mitochondria but do not carry out aerobic respiration. Some produce ATP through alternative pathways; others produce no ATP at all and serve other cellular functions.18PubMed Central. The curious case of vanishing mitochondria The anaerobic lifestyle has evolved multiple independent times across the eukaryotic tree, suggesting that the transition away from aerobic respiration is not especially difficult when the environment selects for it.19PubMed Central. Multiple secondary origins of the anaerobic lifestyle in eukaryotes
How Respiration Shapes the Global Carbon Cycle
The collective respiration of all these organisms adds up to an enormous planetary force. Every organism that respires takes in organic carbon and releases carbon dioxide, and when you sum this across all the bacteria, fungi, plant roots, and soil invertebrates in the world’s soils alone, the numbers are staggering. Soil respiration is the second-largest carbon flux between land ecosystems and the atmosphere, trailing only gross primary productivity (the total amount of carbon fixed by photosynthesis).20PubMed. Contribution of soil respiration to the global carbon equation
To put that in perspective, the respiratory carbon dioxide flux from soils alone overshadows the annual carbon dioxide emissions from fossil fuel burning by roughly tenfold.21PubMed Central. The role of soil microbes in the global carbon cycle: tracking the below-ground microbial processing of plant-derived carbon for manipulating carbon dynamics in agricultural systems This does not mean soil microbes are a bigger climate problem than fossil fuels: the soil flux is largely balanced by the carbon that plants fix through photosynthesis and deposit into soils. But it does mean that small shifts in the rate of microbial respiration, driven by rising temperatures, changing rainfall patterns, or shifts in land use, can have outsized effects on atmospheric carbon dioxide levels. The question of which organisms respire, and how fast, is not just a matter of biology. It is one of the key variables in climate science.