The Process of Plant Respiration: Equation, Types, and How Plants Breathe

Plant respiration is the process by which plants break down sugars to release the energy stored in them, taking in oxygen and giving off carbon dioxide. In the late eighteenth century several scientists established that plants not only absorb carbon dioxide but also release it, and this discovery became known as the process of plant respiration. Respiration happens in every living cell of a plant, day and night, and it is the reaction that powers growth, repair, transport, and reproduction. Plant respiration

What is the process of plant respiration?

Respiration is the oxidative breakdown of complex organic compounds — chiefly carbohydrates such as glucose — whose end products are carbon dioxide and water, accompanied by the release of energy. This energy is captured in ATP (adenosine triphosphate), the universal energy currency that cells spend on every activity that requires work. Plant respiration is a feature common to all living organisms, not a process unique to plants.

The value of respiration lies not only in liberating energy but also in the chain of intermediate compounds formed as carbohydrates break down step by step. These intermediates serve as building blocks for synthesizing proteins, fats, and other organic substances, which is why respiration is the central highway through which matter and energy are transformed inside the plant.

Plants need this energy for the same reasons every organism does: to drive chemical reactions, to power active transport across membranes, to fuel cell division and growth, and to maintain homeostasis. Unlike movement and muscle contraction in animals, plants spend most of their respiratory energy on biosynthesis, nutrient uptake against concentration gradients, and tissue maintenance.

Respiration is not the same as breathing. Breathing is the physical movement of air, while respiration is the biochemical release of energy from food inside cells. Plants have no specialized breathing organs; instead oxygen passes directly into each living cell, and the carbon dioxide produced diffuses straight out, helped by the large internal surfaces connected to the air.

How do plants breathe and exchange gases?

Plants exchange gases through specialized pores and openings spread across leaves, stems, and roots, because they have no lungs and no circulatory system to move air. Gas exchange in plants is driven entirely by diffusion across moist surfaces, so each living cell is close enough to an air space to obtain oxygen and shed carbon dioxide without dedicated organs.

• Stomata — microscopic pores in the leaf surface, the main route for gas exchange in green tissue. Each pore is flanked by two guard cells that swell or shrink to open and close it, regulating both gas exchange and water loss.
• Lenticels — small raised openings in the bark of stems and woody tissue that allow oxygen to reach living cells beneath the surface.
• Root hairs — fine extensions of root surface cells that absorb oxygen dissolved in the soil air and water, supplying root respiration.

Stomata and guard cells therefore control tissue-level gas exchange and stomatal conductance, balancing the oxygen needed for respiration against the water vapour lost to the atmosphere. In leaves, gas exchange shifts with light: during the day photosynthesis and respiration occur together, while at night only respiration continues, so leaves release carbon dioxide steadily after dark.

Respiration in roots depends on oxygen absorbed from soil pore spaces, which is why waterlogged soils — where oxygen availability is low — slow root respiration and can damage the plant. Roots and other non-green organs cannot photosynthesise because they lack chloroplasts and chlorophyll and receive no light; they depend entirely on sugars transported down from the leaves through the phloem, with water and minerals moving up through the xylem vessels.

History of the study of respiration chemistry

The chemistry of respiration was unravelled over more than a century by researchers who showed that both oxygen and hydrogen are activated during the oxidation of respiratory substrates. Their work established that respiration is not a single step but a sequence of enzyme-controlled reactions.

A. N. Bach's theory of oxygen activation

At the end of the nineteenth century A. N. Bach developed the theory of molecular oxygen activation. Molecular oxygen cannot combine directly with an oxidisable substance because both of its bonds are occupied, so to activate it those bonds must first be freed. Activated oxygen can then combine with the oxidisable substance to form a peroxide, which decomposes and carries out further oxidation.

Hydrogen activation in the work of V. I. Palladin

V. I. Palladin's research showed that respiration also involves the activation of hydrogen from the respiratory substance. Dehydrogenase enzymes strip hydrogen from the respiratory material, oxidising it, and the activated hydrogen then combines with oxygen. It is now generally accepted that respiration activates both oxygen and hydrogen.

The link between respiration and fermentation according to S. P. Kostychev

S. P. Kostychev demonstrated the connection between respiration and fermentation. The initial phase of sugar transformation proceeds identically in both processes and yields the same intermediate products. In respiration these intermediates are then oxidised to CO₂ and H₂O, while in fermentation they form alcohol and CO₂. Fermentation releases far less energy — only about 48 kcal per gram-molecule of sugar.

Many other scientists contributed to working out the chemistry of respiration. L. A. Ivanov demonstrated the role of phosphoric acid: it is not the free sugar molecule that is oxidised but its phosphate ester, showing that respiration both breaks down and synthesises complex organic compounds. A. Szent-Györgyi, H. Krebs, and S. M. Johnson investigated respiration in detail and revealed the role of organic acids — work that gave its name to the Krebs cycle, also called the citric acid or TCA cycle.

The equation for plant respiration (chemical formula)

The overall chemical equation for aerobic plant respiration is the oxidation of one molecule of glucose by six molecules of oxygen to yield carbon dioxide, water, and energy. In word equation form: glucose + oxygen → carbon dioxide + water + energy. This is the reaction every GCSE, A-level, and International Baccalaureate biology syllabus uses as the foundation for cellular respiration.

The summary equation for aerobic respiration

Aerobic respiration can be represented by the schematic: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 686 kcal. The oxidation is not as simple as this single line suggests, however; it proceeds through a series of intermediate stages. The byproducts of cellular respiration are carbon dioxide and water, while the released energy is conserved as ATP rather than lost entirely as heat.

The stages of cellular respiration: glycolysis, the Krebs cycle, and the respiratory chain

Cellular respiration in plants proceeds in three connected stages, each occurring in a different cellular location and releasing energy in controlled amounts. Splitting the overall reaction this way lets the cell capture energy efficiently as ATP instead of releasing it all as a single burst of heat.

1. Glycolysis — occurs in the cytoplasm and splits one glucose molecule into two molecules of pyruvate, producing a small net yield of ATP and reduced NADH from NAD+. Glycolysis requires no oxygen and is common to both aerobic and anaerobic respiration.
2. The Krebs cycle (citric acid cycle / TCA cycle) — takes place in the matrix of the mitochondria, where pyruvate is fully oxidised to carbon dioxide. Each turn reduces NAD+ to NADH and FAD to FADH₂, and generates the organic acid intermediates that double as raw materials for synthesising amino acids and other compounds.
3. The electron transport chain — embedded in the inner mitochondrial membrane, it passes electrons from NADH and FADH₂ along a chain of carriers to oxygen, the final electron acceptor, producing water. The energy released drives the bulk of ATP synthesis, which is why aerobic respiration yields so much more energy than fermentation.

The role of cytochromes and the alternative oxidase

Cytochrome c oxidase is the terminal enzyme of the main respiratory chain in plant mitochondria, transferring electrons to oxygen to form water while pumping the protons used to make ATP. Enzymes such as succinate dehydrogenase feed electrons into this chain from the Krebs cycle.

Plants also possess an alternative pathway through a second terminal enzyme, the alternative oxidase, which accepts electrons without conserving their energy as ATP. This alternative pathway releases more of the energy as heat and gives plants flexibility to keep respiration running when the main cytochrome pathway is restricted — for example under stress or when carbon supply is high.

Anaerobic respiration and fermentation in plants

Anaerobic respiration is the release of energy from glucose without oxygen, and it allows plant cells, yeast, and animal muscle to keep producing some ATP when oxygen runs short. Anaerobic respiration yields far less energy than aerobic respiration because the glucose is only partly broken down, leaving energy locked in the products.

The equation for alcoholic fermentation in yeast and plants

In yeast and in oxygen-starved plant tissue, anaerobic respiration takes the form of alcoholic fermentation, producing ethanol and carbon dioxide. The word equation for anaerobic respiration in yeast is: glucose → ethanol + carbon dioxide + energy. As a chemical equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + energy. This is the same reaction used in baking and brewing.

The equation for anaerobic respiration (lactic acid fermentation)

In animal muscle, anaerobic respiration produces lactic acid rather than ethanol. The word equation for anaerobic respiration in muscle is: glucose → lactic acid + energy, and as a chemical equation: C₆H₁₂O₆ → 2C₃H₆O₃ + energy. Lactic acid fermentation occurs in eukaryotic muscle cells during hard exercise, while many prokaryotic organisms rely on fermentation routes as their normal mode of energy release.

Comparison of aerobic and anaerobic respiration

Respiration and photosynthesis: comparing the two processes

Respiration and photosynthesis are opposite processes: photosynthesis builds glucose and stores energy, while respiration breaks glucose down and releases that energy. Comparing the summary equations shows that photosynthesis creates organic substances using solar energy, and plant respiration then frees the energy accumulated in that organic matter. Respiration and photosynthesis

The equations for photosynthesis and respiration side by side

Placed together, the two equations are mirror images of one another:

• Photosynthesis: 6CO₂ + 6H₂O → (light energy 686 kcal / chlorophyll) → C₆H₁₂O₆ + 6O₂
• Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 686 kcal

Photosynthesis takes place in chloroplasts and depends on chlorophyll to capture sunlight, splitting water and using photosynthesis to fix carbon dioxide into glucose, with oxygen released as a byproduct and NADPH carrying the captured reducing power. The rate of photosynthesis is governed by light intensity, carbon dioxide concentration, and temperature, and the oxygen it produces can be demonstrated practically by collecting the gas given off by an aquatic plant such as Cabomba. Roots cannot perform this because their structure contains no chloroplasts and they sit in darkness.

The energy balance of the two processes

In producing one gram-molecule of sugar by photosynthesis, 686 kcal of solar energy is consumed, and the very same amount is released when that sugar is oxidised in respiration. Energetically, therefore, respiration is the exact opposite of photosynthesis. Over a plant's life only part of the carbon fixed by photosynthesis is retained as growth; the rest is returned to the air by respiration, making the balance between the two the key to whether a plant accumulates biomass. Photosynthesis is also the foundation of nearly every food chain on Earth, since plants are the primary producers that feed almost all other life.

Unlike animals, plants have no special respiratory organs, and oxygen enters each living cell directly. Because their surfaces are so extensively developed and closely linked to air-based nutrition, air reaches every cell easily, so no additional organs are needed to take in oxygen and remove the carbon dioxide produced. The respiration of different green plants and their organs is not uniform and is compared by intensity — the amount of carbon dioxide released per unit weight per unit time.

Factors affecting the intensity of plant respiration

Plant respiration rate is controlled by both intrinsic factors inside the plant and external environmental factors. Intrinsic controls include the plant's age, growth rate, and the proportion of living, metabolically active tissue; external controls include temperature, oxygen and carbon dioxide concentration, water supply, and nutrient availability.

Respiration is closely linked to growth, so the more vigorously a plant grows the stronger its respiration. Intensity also depends on age: in young plants respiration proceeds more energetically, and it declines as the plant matures. The change in respiratory intensity during individual development (after B. A. Rubin) is shown below.

The respiratory intensity of different plant organs depends on how much living content their cells hold. Flowers respire most intensely of all. In massive flowers such as the Amazon water lily Victoria regia, respiration raises the temperature inside the flower to as much as 12° above the surrounding air. During the respiration of Victoria regia water-lily flowers the temperature inside rises above air temperature

Plant respiration can be observed vividly in germinating wheat seeds, which also respire very intensely. Placed in a well-insulated container such as a Dewar flask, they cause a marked temperature rise reaching 30–50°. Germinating wheat seeds in a Dewar flask

At such temperatures the seeds may even be killed by the heat they generate. Nutrient supply also shapes respiration: nitrogen is needed to build the enzymes that catalyse each step, phosphorus is essential for synthesising ATP and the phosphate sugar esters L. A. Ivanov described, and micronutrients act as cofactors in the respiratory chain — which is why nutrient balance matters in managed systems such as hydroponic growing.

The effect of temperature on respiration

Temperature is one of the strongest external controls on respiration: within a plant's tolerable range, respiration rate rises sharply as temperature increases, roughly doubling for each 10°C until enzymes begin to denature at high temperatures. The self-heating of germinating wheat seeds in a Dewar flask is a direct demonstration of how fast respiration accelerates as warmth builds up. Because warming the planet raises ecosystem respiration, the temperature sensitivity of this process is central to predicting how global warming will affect the carbon balance of vegetation.

The effect of CO₂ and oxygen concentration

Oxygen availability and carbon dioxide concentration both regulate respiration, since aerobic respiration needs oxygen as its final electron acceptor and is inhibited when oxygen is scarce, as in waterlogged roots. Elevated atmospheric CO₂ can affect plant respiration both directly and indirectly: direct effects involve influence on mitochondrial enzymes, while indirect effects work through changes in tissue chemistry and growth as plants respond to higher CO₂. These responses are species-specific and difficult to measure cleanly — research published in Annals of Botany has highlighted that measurement artifacts can distort apparent respiratory responses to high CO₂. High carbon dioxide levels are also exploited deliberately in Modified Atmosphere Packaging, which slows the respiration of harvested produce to extend its shelf life.

Respiration and the accumulation of biomass

Respiration determines how much of a plant's fixed carbon is retained as biomass, because the carbon a plant respires away cannot be added to its tissues. Plant physiologists separate total respiration into growth respiration, which powers the synthesis of new tissue, and maintenance respiration, which keeps existing tissue alive — and this growth-versus-maintenance partitioning explains why fast-growing young plants respire so much more than mature ones. Specific respiration rates (per unit of tissue) differ from whole-plant rates because the proportion of living, active cells changes as the plant builds up woody and storage tissue, and biomass is allocated between shoots and the root system. Water stress and drought reduce respiration by limiting metabolism, further shifting how carbon is divided between growth and survival.

Respiration and the role of ecosystems as carbon sinks

Plant respiration scales up from individual cells to whole ecosystems and is a major term in the global carbon cycle, because the carbon dioxide released by vegetation partly offsets the carbon captured by photosynthesis. Whether a terrestrial ecosystem acts as a carbon sink or a carbon source depends on the balance between photosynthetic uptake and the combined respiration of plants and soil organisms.

Root respiration is a key part of soil carbon flux, returning carbon to the atmosphere from below ground, and its share of the total can be quantified using the oxygen isotope technique. Researchers including Miquel A. Gonzalez-Meler at the University of Illinois at Chicago have studied how respiration scales from tissues to ecosystems and how it responds to a changing climate. Frameworks such as WBE theory (the West–Brown–Enquist metabolic scaling theory) attempt to predict how respiratory processes scale with body size from cells to whole organisms and ecosystems.

Because respiration grows more temperature-sensitive than photosynthesis in many systems, warming can tip a terrestrial ecosystem's carbon balance toward releasing more CO₂, weakening its capacity to act as a carbon sink. Understanding the plant-level and ecosystem-level carbon budget — including the partitioning between root and shoot respiration in crops such as soybean (Glycine max) — is therefore essential for forecasting how vegetation will influence atmospheric carbon under global warming.

The significance of respiration for the synthesis of organic substances

Respiration matters not only because it releases energy but because its intermediate products are the raw materials for building the plant's complex molecules. As carbohydrates break down step by step, they generate a range of intermediate compounds — the organic acids of the Krebs cycle and the phosphorylated sugars identified by L. A. Ivanov among them — that feed into the synthesis of proteins, fats, and many other organic substances.

The oxidative breakdown of complex organic compounds is thus the main channel through which matter and energy are transformed in the plant. By coupling energy release to ATP production and supplying carbon skeletons for biosynthesis, respiration links the harvest of stored solar energy to growth, maintenance, and the formation of new tissue — making it as fundamental to plant life as photosynthesis itself. If you found this explanation useful, you may also enjoy our other articles on agronomy and the natural sciences.