Anaerobic Respiration in Plants: Products, Equation, and Importance
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Anaerobic respiration is the release of energy from glucose without oxygen, producing partially oxidised end products such as ethanol or lactic acid along with a small amount of energy. It allows plants, microorganisms, and animal muscle cells to keep generating energy when oxygen is unavailable, serving as a temporary or, in some organisms, a permanent metabolic pathway.
What is anaerobic respiration: definition and essence of the process
Anaerobic respiration is a form of cellular respiration that breaks down glucose to release energy in the absence of oxygen. Instead of drawing oxygen from the surrounding air, the cell rearranges the oxygen already present within the glucose molecule, which means the sugar is only partially oxidised. This is why anaerobic respiration yields far less energy than aerobic respiration and leaves behind energy-rich end products.
Plants survive primarily through ordinary respiration, but for a limited time they can live by anaerobic respiration. Plant anaerobic respiration switches on when the oxygen a plant needs is taken from organic compounds — mainly from sugar, the substance that normally serves as the starting material in standard respiration.
Aerobic and anaerobic respiration: key differences
The key difference between aerobic and anaerobic respiration is oxygen: aerobic respiration uses oxygen to fully oxidise glucose into carbon dioxide and water, while anaerobic respiration proceeds without oxygen and leaves partially oxidised products such as ethanol or lactic acid. Aerobic respiration in plant and animal cells occurs largely inside the mitochondria, the organelles where pyruvic acid is completely broken down; cells with high energy demands tend to contain a greater density of mitochondria.
- Oxygen: required for aerobic respiration, absent in anaerobic respiration.
- End products: carbon dioxide and water (aerobic) versus ethanol and carbon dioxide, or lactic acid (anaerobic).
- Energy released: about 686 kcal per molecule of glucose in full oxidation versus only around 48 kcal in anaerobic breakdown.
- Location: aerobic respiration finishes in the mitochondria; the anaerobic stage takes place in the cytoplasm.
Many organisms are facultative anaerobes — yeast cells and certain bacteria can switch between anaerobic and aerobic respiration depending on whether oxygen is available, using whichever pathway suits their environment.
Respiration and fermentation: what is the difference
Respiration is the cellular process of releasing energy from food molecules, whereas fermentation is the specific form of anaerobic respiration carried out by microorganisms that yields products such as ethanol or lactic acid. Respiration should also not be confused with breathing, or ventilation: ventilation is the physical movement of air in and out of an organism, while respiration is the chemical release of energy inside cells. An organism can ventilate its lungs yet still rely on anaerobic respiration in its tissues when oxygen supply cannot keep pace with demand.
Anaerobic respiration in plants
Anaerobic respiration in plants is a temporary substitute for oxygen respiration that keeps cells alive when oxygen cannot reach them. Because the process is wasteful and produces toxic by-products, higher plants cannot sustain it for long. It appears in well-defined situations: when soil is waterlogged, when a crust forms over the soil surface, or when grain is stored in large heaps that block air from the centre.
Distribution and breakdown of sugar in anaerobic respiration
In anaerobic respiration the sugar breaks down according to the scheme: C6H12O6 → 2C2H5OH + 2CO2 + 48 kcal. As the equation shows, the carbon of the sugar is only partly oxidised to carbon dioxide, while the remaining carbon is reduced to ethyl alcohol. No oxygen enters from outside, so the conversion of sugar happens purely through a redistribution of the oxygen already held inside its molecule.
Energy release: comparison with full oxidation
Anaerobic respiration releases only 48 kcal of energy, whereas full oxidation releases 686 kcal (for more detail, see The process of plant respiration). This difference exists because a large amount of potential energy remains locked in the alcohol, since oxidation does not run to completion. The result is that anaerobic respiration is an inefficient way to power a cell.
Anaerobic conditions and their effect on plants
Plants cannot live for long under anaerobic conditions. To obtain the same amount of energy it gains from ordinary respiration, a plant would have to spend a very large quantity of its stored reserves during anaerobic respiration. As a result, in anaerobic conditions plants quickly die from exhaustion and, in addition, from poisoning by the alcohol that accumulates in their tissues.
Where and when anaerobic respiration occurs in plants
Anaerobic respiration in higher plants is only a temporary replacement for oxygen respiration. It is observed in plants that remain for a long time in soil with excess moisture, where a crust has formed on the soil surface, or where grain is stored in large heaps. In each case oxygen cannot reach the living cells fast enough, forcing them onto the anaerobic pathway until normal conditions return.
Anaerobic respiration in microorganisms (fermentation)
For many lower plants and microorganisms, anaerobic respiration is the main process for obtaining the energy they need and can sustain their life indefinitely. In this case anaerobic respiration is called fermentation. Unlike higher plants, microorganisms do not use their own stored reserves for fermentation but instead draw nutrients from the surrounding environment, which is why bacteria and fungi such as yeast can ferment substances around them. This same chemistry underpins industrial applications of fermentation — from raising bread to brewing beer — where yeast cells convert sugars into ethanol and carbon dioxide.
Alcoholic fermentation: enzymes and intermediate products
Anaerobic respiration in plants resembles alcoholic fermentation. Under anaerobic conditions, a series of enzymes form the same intermediate products as in fermentation, in particular pyruvic acid. In aerobic conditions pyruvic acid is fully oxidised to carbon dioxide and water, while in anaerobic conditions during alcoholic fermentation it breaks down into CO2 and alcohol.
The relationship between normal aerobic respiration and anaerobic alcoholic fermentation is shown in the scheme below.
During alcoholic fermentation, the enzyme carboxylase destroys the carboxyl group of pyruvic acid, releasing carbon dioxide and forming acetaldehyde. The enzyme dehydrogenase then transfers two hydrogen atoms to the acetaldehyde, reducing it to ethyl alcohol. The final products of alcoholic fermentation are therefore ethanol and carbon dioxide.
The role of pyruvic acid in respiration and fermentation
Pyruvic acid is the key branching point that connects respiration and fermentation. As the scheme shows, the processes of respiration and fermentation are identical up to the formation of pyruvic acid. During respiration, no oxygen is needed to form pyruvic acid, so this phase of respiration is itself anaerobic. When oxygen is available and a system of oxidative enzymes is present, pyruvic acid is oxidised completely; when oxygen is absent, the same pyruvic acid is diverted into fermentation and converted into ethanol and carbon dioxide.
Anaerobic respiration in animals and humans
Anaerobic respiration in animals breaks glucose down into lactic acid rather than ethanol, releasing a small amount of energy without oxygen. Animal cells, and especially muscle cells, switch to this pathway when their oxygen supply runs short. The end product differs from that in plants and yeast: instead of ethanol and carbon dioxide, animal cells produce lactic acid.
Lactic acid fermentation in muscles during exercise
During intense exercise, muscle cells respire anaerobically and convert pyruvic acid into lactic acid. When breathing and circulation cannot deliver oxygen quickly enough to working muscles, the muscle cells rely on lactic acid fermentation to keep contracting. The build-up of lactic acid is associated with the burning sensation and muscle fatigue that accompany heavy effort. Researchers measure lactate levels using laboratory tools such as the Amplite Fluorimetric D-Lactate Assay Kit to study muscle metabolism.
Anaerobic respiration and athletic performance
Anaerobic respiration powers short, explosive bursts of athletic performance such as sprinting and weightlifting, where energy is needed faster than oxygen can be supplied. Because it does not depend on oxygen delivery, the anaerobic pathway provides energy rapidly, but the accumulation of lactic acid limits how long this output can be maintained. Trained athletes adapt to tolerate and clear lactic acid more efficiently, extending the time they can perform at high intensity.
Oxygen debt (EPOC) after exertion
After anaerobic exertion, the body takes in extra oxygen to recover, a phenomenon known as EPOC — excess post-exercise oxygen consumption, or oxygen debt. This additional oxygen is used to convert the accumulated lactic acid back into glucose or fully oxidise it, restore energy reserves, and return the muscles to their resting state. The heavy breathing that continues after a sprint is the body repaying this oxygen debt.
End products of anaerobic respiration in plants and animals
The end products of anaerobic respiration depend on the organism: plants and yeast produce ethanol and carbon dioxide, while animals and humans produce lactic acid. This is the central difference between plant and animal anaerobic respiration. In both cases the products are toxic if they accumulate — ethanol poisons plant tissues, and lactic acid contributes to muscle fatigue — which is why anaerobic respiration cannot continue indefinitely in higher organisms.
Comparative table: products in plants, microorganisms, and animals
| Organism | End products | Duration possible |
|---|---|---|
| Higher plants | Ethanol + carbon dioxide | Temporary only — death from exhaustion and alcohol poisoning |
| Microorganisms (yeast, bacteria) | Ethanol + carbon dioxide (alcoholic fermentation) | Indefinite — main energy source |
| Animals and humans | Lactic acid | Short term — limited by fatigue and oxygen debt |
The importance of anaerobic respiration in nature and for humans
Anaerobic respiration matters because it lets life continue when oxygen is scarce and underpins valuable industrial processes. In nature it keeps waterlogged plants and stored grain alive through short periods without oxygen, and it allows muscle cells to sustain bursts of activity. For humans, the fermentation carried out by yeast cells and bacteria is harnessed to bake bread, brew beer, and produce ethanol, making anaerobic metabolism a cornerstone of both biology and industry. The topic is a standard part of the grade 11 biology curriculum, where students compare aerobic and anaerobic pathways across plants, microorganisms, and animals. For more science explainers, browse our Agronomy and articles on nature and science.
