The Chemistry of Plant Respiration: Aerobic and Anaerobic Oxidation Reactions

The chemistry of plant respiration describes the respiratory process as a sequence of consecutive transformations and chemical reactions through which plant cells break down sugars to release usable energy. In its simplest form, respiration takes glucose, combines it with oxygen, and produces carbon dioxide, water, and energy stored as ATP. The full process unfolds across two main phases — an anaerobic phase (glycolysis) and an aerobic phase — and involves dozens of enzymes, electron carriers, and phosphate compounds working in a coordinated chain. The chemistry of plant respiration

What is plant cellular respiration?

Plant cellular respiration is the metabolic process by which plant cells oxidise organic molecules — chiefly glucose — to release the chemical energy locked inside them and capture it in ATP. Every living plant cell respires continuously, day and night, in roots, stems, leaves, and flowers alike. The summary equation is the reverse of photosynthesis: glucose plus oxygen yields carbon dioxide, water, and energy. Respiration is what makes the energy fixed by photosynthesis actually available for growth, transport, repair, and the synthesis of new compounds.

Respiration matters because it powers life. The ATP produced fuels cell division, the uptake of mineral nutrients by roots, the loading of sugars into the phloem, and countless biosynthetic reactions. Without continuous respiration a plant could not maintain its tissues or grow, even if photosynthesis were running at full capacity.

How does respiration compare with photosynthesis?

Respiration and photosynthesis are complementary, opposite processes that together form the energy cycle of living organisms. Photosynthesis is the process in which chloroplasts use light energy, water, and carbon dioxide to build glucose and release oxygen; respiration breaks glucose back down with oxygen to release energy and carbon dioxide. The table below contrasts the two.

The relationship is more than a simple mirror image. Photosynthesis takes place in chloroplasts where chlorophyll captures light: the light-dependent reactions split water molecules across the thylakoid membranes through Photosystem II and Photosystem I, generating ATP and NADPH, while the light-independent reactions of the Calvin cycle use RuBisCo to fix carbon dioxide into sugar. The rate of photosynthesis depends on light intensity, temperature, and carbon dioxide availability. Respiration then consumes those sugars. During daylight a plant both photosynthesises and respires, so net gas exchange shows oxygen release; at night only respiration continues, so the plant takes up oxygen and releases carbon dioxide. This daily rhythm of gas exchange is governed largely by the stomata.

Autotrophs and heterotrophs

Plants are autotrophs, meaning they manufacture their own organic food from inorganic raw materials through photosynthesis, whereas heterotrophs such as animals and fungi must consume ready-made organic matter. This distinction underpins every food chain: the glucose and other compounds a plant builds become the energy source transferred to herbivores and, in turn, to predators. Even so, autotrophs still respire — they must break down a portion of the food they make to power their own living processes. Photosynthesis is therefore foundational to life on Earth, supplying both the oxygen most organisms breathe and the chemical energy that flows through ecosystems.

The chemical reaction of oxidation

Oxidation is the chemical reaction at the heart of respiration. In oxidation, oxygen is added to the substance being oxidised — for example, hydrogen oxidising to water: 2H₂ + O₂ → 2H₂O — or hydrogen is removed from the substance, or an electron is taken from it, raising its valency.

Types of oxidation reactions in respiration

Three principal forms of oxidation operate in plant respiration, and biological oxidation often relies on the latter two rather than the direct addition of oxygen:

• Addition of oxygen to the oxidised material, the most familiar form.
• Removal of hydrogen (dehydrogenation), frequently preceded by the attachment of a water molecule to the substrate, after which hydrogen is stripped away.
• Removal of an electron, which transfers reducing power to an acceptor and raises the valency of the donor.

A closely related variant is oxidation in which a molecule of phosphoric acid is first attached to the substrate, after which hydrogen is removed. This phosphorylation-linked oxidation is central to how respiration captures energy in ATP rather than wasting it as heat.

The role of electron acceptors

An electron acceptor — the substance that takes up an electron — acts as the oxidising agent in these reactions. For example, Fe⁺⁺ − e + A → Fe⁺⁺⁺ + Ae. The acceptor, denoted A, becomes reduced as it gains the electron (e), while the iron becomes oxidised because it has surrendered an electron. Substance A can then pass that electron on to a further acceptor, reducing it in turn. This relay of electrons from one carrier to the next is exactly the principle that drives the electron transport chain, where carriers such as NADH and FADH₂ hand their electrons down a series of acceptors to oxygen.

The history of studying respiratory chemistry

The modern understanding of respiratory chemistry was built on the work of A. N. Bach, V. I. Palladin, D. Keilin, O. Warburg, V. A. Engelhardt, D. M. Mikhlin, A. I. Oparin and other scientists. Their combined research established that respiration proceeds in two phases — anaerobic and aerobic — and identified the enzymes, cytochromes, and high-energy phosphate compounds that link them. Keilin's discovery of the cytochrome system and Warburg's work on respiratory enzymes were especially decisive in mapping how electrons travel toward oxygen.

The phases of the respiratory process

Respiration is divided into two phases: an anaerobic phase and an aerobic phase. The initial stage of sugar transformation — the anaerobic breakdown — runs identically in both respiration and fermentation. Through a series of consecutive transformations, the breakdown of a sugar molecule produces pyruvic acid, after which the subsequent reactions follow different routes depending on the enzyme systems present in the organism and on external conditions. In strictly biochemical terms the full breakdown of glucose comprises three stages: glycolysis, pyruvate oxidation and the citric acid cycle, and the electron transport chain.

The anaerobic phase: glycolysis

The anaerobic phase, called glycolysis, is the cycle of sugar transformation that ends with the formation of two molecules of pyruvic acid (CH₃COCOOH). It occurs in the cytoplasm, requires no oxygen, and is common to respiration and to fermentation alike. During the transformations of sugar in this first phase, ATP is produced and energy is stored within it — energy the cell can later mobilise for any of its living processes.

The role of phosphate compounds and ATP in glycolysis

Phosphate compounds play an enormous part in the transformation of organic substances during respiration. At the start of the anaerobic phase, a special enzyme attaches one residue of phosphoric acid from ATP to the glucose molecule:

Glucose + ATP → glucose-phosphate + ADP.

The glucose-phosphate then undergoes a series of complex, enzyme-driven transformations. In the course of these, inorganic phosphorus from phosphoric acid is taken up and diphosphoglyceric acid is formed, which carries one high-energy (macroergic) bond:

Phosphoglyceric aldehyde + H₂PO₄ → ^oxidation diphosphoglyceric aldehyde → diphosphoglyceric acid.

The diphosphoglyceric acid that forms reacts with ADP, transferring its high-energy bond to it, and so ATP and phosphoglyceric acid are produced:

Diphosphoglyceric acid + ADP → ATP + phosphoglyceric acid.

Diagram of anaerobic respiration in plants

Subsequently, after further complex transformations, phosphoglyceric acid forms pyruvic acid, and the residue of phosphoric acid again combines with a molecule of ADP to give a molecule of ATP:

Phosphoglyceric acid + ADP → ATP + pyruvic acid.

It should be emphasised that the schemes show only the final results of processes that in reality proceed through a series of complex intermediate steps. The process is depicted in more detail in the figure. Diagram of anaerobic respiration in plants

The formation of pyruvic acid

The anaerobic phase concludes with the formation of two molecules of pyruvic acid from each sugar molecule. This pyruvate is the pivotal branch point of respiration: under aerobic conditions it enters the mitochondria for full oxidation, while under anaerobic conditions it is diverted into fermentation. The net energy yield of glycolysis is modest — only two molecules of ATP per glucose — but it sets the stage for the much larger energy harvest of the aerobic phase.

The aerobic phase of respiration

The second phase of respiration is aerobic and begins with the conversion of pyruvic acid down to the final products, carbon dioxide and water. This conversion involves a whole complex of different enzyme systems and the formation of a series of organic acids — acetic, oxaloacetic, citric, oxalosuccinic, ketoglutaric and others. The aerobic phase takes place inside the mitochondria and accounts for the overwhelming majority of the ATP a plant cell generates.

The Krebs cycle (tricarboxylic acid cycle)

The Krebs cycle is the cyclical sequence of oxidations through which the carbon and hydrogen of pyruvic acid are progressively stripped away and oxidised. As organic acids form one after another, all of the carbon and hydrogen of pyruvic acid is oxidised. This cycle of pyruvic acid oxidation was investigated by Krebs and is named the Krebs cycle, also known as the citric acid cycle or the TCA cycle (tricarboxylic acid cycle). Before pyruvate enters the cycle it undergoes pyruvate oxidation, and within each turn of the cycle the energy of the substrate is captured as reduced carriers — NADH and FADH₂ — along with a small direct yield of ATP.

The oxidation of pyruvic acid releases three molecules of carbon dioxide, and since each sugar molecule yields two molecules of pyruvic acid, the total output is six molecules of carbon dioxide — exactly the figure given in the summary equation of respiration. Various oxidases take part in this oxidation.

The electron transport chain and oxidative phosphorylation

The electron transport chain (ETC) is the final stage of aerobic respiration, where the NADH and FADH₂ generated earlier deliver their electrons to a chain of carriers embedded in the inner mitochondrial membrane. As electrons pass from one carrier to the next toward oxygen, protons are pumped across the membrane; the resulting gradient drives ATP synthase to make ATP in a process called oxidative phosphorylation. The terminal enzyme, cytochrome c oxidase, hands the electrons to oxygen, which combines with hydrogen to form water — the source of the water that appears in the respiration equation.

Plants also possess an alternative pathway respiration that uses a cyanide-insensitive oxidase, bypassing part of the standard cytochrome route. This alternative pathway produces less ATP and releases more of the energy as heat, and it gives plants flexibility to keep respiration running under stress when the main pathway is restricted. Enzymes such as succinate dehydrogenase link the Krebs cycle directly into this electron-transport machinery.

The formation of organic acids

Depending on the plant's condition, its species, and the conditions of the external environment, one or another enzyme system may be engaged. The transient organic acids that build up during the Krebs cycle are not merely waste intermediates — many of them are siphoned off to serve as building blocks for amino acids, pigments and other compounds, which is one reason respiration is so tightly woven into a plant's wider metabolism.

ATP synthesis and energy storage in the cell

In the course of oxidising one glucose molecule all the way to CO₂ and H₂O, 38 molecules of ATP are formed — 2 in the first phase and 36 in the second phase of respiration. Because of this, the cell retains 380,000 calories, which amounts to 50–55% of the chemical energy contained in glucose. The remaining energy is dissipated as heat. This efficiency is remarkable for a biological system and explains why aerobic respiration is so much more productive than anaerobic breakdown alone.

ATP as the energy currency of the cell

ATP (adenosine triphosphate) is the universal energy currency of the cell, the form in which the energy released by respiration is held and spent. Each molecule of ATP stores energy in its high-energy phosphate bonds; when a cell needs energy it splits off a phosphate to give ADP, releasing the energy where it is required. Phosphorus is therefore indispensable to energy conversion — without an adequate supply of phosphate a plant cannot regenerate ATP efficiently. This continual cycling between ATP and ADP couples the energy-releasing reactions of respiration to the energy-consuming reactions of growth, transport and biosynthesis.

Aerobic and anaerobic respiration compared

Aerobic respiration fully oxidises glucose using oxygen and yields about 38 ATP per glucose, whereas anaerobic respiration operates without oxygen and yields only the 2 ATP produced in glycolysis. Aerobic respiration ends in carbon dioxide and water; anaerobic respiration leaves the carbon only partially oxidised, locked in compounds such as ethanol or lactic acid. The shared opening stretch of both routes is glycolysis — the divergence comes only after pyruvic acid is formed.

• Oxygen requirement: aerobic needs oxygen; anaerobic does not.
• Location: aerobic uses cytoplasm and mitochondria; anaerobic is confined to the cytoplasm.
• Energy yield: roughly 38 ATP versus 2 ATP per glucose.
• End products: carbon dioxide and water versus ethanol or organic acids plus carbon dioxide.

Fermentation as a form of anaerobic breakdown

Fermentation is the anaerobic route taken when oxygen is unavailable, regenerating the carriers needed to keep glycolysis turning. Because the aerobic phase cannot proceed without oxygen, pyruvic acid is instead converted to ethanol and carbon dioxide (alcoholic fermentation) or to lactic acid. Plant tissues fall back on fermentation under anaerobic conditions such as waterlogged soils, but the energy yield is meagre and toxic by-products accumulate, so it cannot sustain a plant for long.

Biosynthesis through respiratory processes

Respiration does far more than release energy: it supplies the carbon skeletons and reducing power for biosynthesis. The organic acids of the Krebs cycle serve as precursors for amino acids, nucleotides, fatty acids and secondary metabolites, while the NADH and NADPH generated provide the reducing equivalents many synthetic reactions demand. The three core functions of plant respiration are therefore energy production, the creation of biosynthetic precursors, and the maintenance of redox balance. Nutrient supply ties directly into this — nitrogen nutrition feeds the synthesis of the very enzymes that run respiration, and micronutrients such as sulfur, iron and magnesium are built into the cytochromes and cofactors of the respiratory chain.

How do external factors affect respiration?

The rate of plant respiration is shaped by both intrinsic and external factors. Intrinsic factors include the plant's species, tissue type, age and metabolic demand, while the leading external factor is temperature. Respiration rate rises steeply with warmth: the Q10 effect describes how the rate roughly doubles for every 10 °C increase, up to the point where heat begins to damage mitochondrial enzymes. Water stress and drought tend to suppress respiration, while wounding and other stresses can sharply raise it as the plant mobilises energy for defence and repair.

Different organs respire at different rates and contribute differently to the soil and atmosphere. Roots respire actively and depend on oxygen drawn from soil air spaces; root hairs absorb that oxygen directly, which is why well-aerated, loose soil rich in organic matter supports healthy root respiration, whereas waterlogged or compacted soil starves roots of oxygen and forces them into inefficient fermentation. Woody stems breathe through lenticels — small pores in the bark — because the bark itself is largely impermeable to gases, while leaves exchange gases through stomata regulated by guard cells, and gases move within the leaf by diffusion through internal air spaces.

The effect of oxygen and carbon dioxide levels

Oxygen and carbon dioxide concentrations directly influence respiration rate. Low oxygen, as in flooded soils or poorly aerated hydroponic systems, limits aerobic respiration and pushes roots toward fermentation; ensuring adequate oxygen at the root zone is therefore essential in hydroponics, and ventilation matters in any closed growing environment where carbon dioxide can build up. Elevated atmospheric carbon dioxide has both direct effects — often a modest, sometimes inconsistent suppression of respiration rate, partly an artefact of measurement methods — and indirect effects, as enhanced photosynthesis under high carbon dioxide increases tissue mass and changes how plants partition resources. Postharvest, respiration continues in harvested produce, so cooling and Modified Atmosphere Packaging (MAP) slow it down to extend shelf life. Aquatic plants such as Cabomba are used as pond oxygenators, while cacti and other succulents store gases and time their stomatal opening to limit water loss.

Respiration at the cell, tissue and whole-plant scales

Respiration can be measured and understood at several nested scales — biochemical, cellular, tissue, organ and whole-plant — and the picture changes at each level. A single mitochondrion runs the Krebs cycle and electron transport chain; a tissue integrates the respiration of many cells; and the whole plant sums the contributions of leaves, stems and roots, each with its own rate and rhythm. Roots, which carry out little or no photosynthesis, depend entirely on sugars delivered from the leaves and so are net consumers of energy. Scaling cleanly from biochemistry to the whole organism remains a challenge, and frameworks such as WBE Theory attempt to relate metabolic rate to body size across plants.

Respiration and biomass accumulation

Respiration and biomass are linked because the energy respiration releases is what builds and maintains plant tissue. Physiologists divide respiration into growth respiration, which fuels the synthesis of new biomass, and maintenance respiration, which keeps existing tissue alive. How a plant partitions its resources between these — and between roots, shoots and reproductive structures — determines its growth rate and the efficiency with which it turns photosynthate into harvestable yield. Manipulating respiration through metabolic engineering is an active avenue for improving crop productivity, and species-specific responses mean different crops react differently to the same conditions.

Plant respiration and the global carbon cycle

Plant respiration is a major flux in the global carbon cycle, returning to the atmosphere a large share of the carbon that photosynthesis removes. At the ecosystem scale, the balance between photosynthetic uptake and respiratory release determines whether terrestrial ecosystems act as carbon sinks or sources. Root respiration drives much of the carbon flux from soils, and the carbon budget of terrestrial biosphere–atmosphere exchange is a central variable in climate models. Because warming accelerates respiration via the Q10 effect, rising temperatures could increase the rate at which ecosystems release carbon dioxide — a feedback with significant implications for climate change and for the role plants play in mitigating it.

Conclusion

The chemistry of plant respiration is the orderly oxidation of glucose through glycolysis, the Krebs cycle and the electron transport chain, capturing roughly half of the energy in sugar as ATP and releasing the rest as heat. It is the mirror image of photosynthesis, the engine behind growth and biosynthesis, and a process that scales from a single mitochondrion to the carbon balance of the entire biosphere. Understanding it clarifies everything from why soil aeration matters to how harvested produce stays fresh and how forests influence the climate. For more on the natural sciences, explore our Agriculture and Astronomy sections, or return to the main page of articles about travel, nature, science and life.