Short-answer box
The chemical equation for cellular respiration is:
C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy (ATP)
In words, one molecule of glucose reacts with six molecules of oxygen to produce six molecules of carbon dioxide, six molecules of water, and energy captured in ATP. ATP stands for adenosine triphosphate, the main energy carrying molecule used by cells.
The reactants are glucose and oxygen. The products are carbon dioxide, water, and ATP. This is the overall equation for aerobic cellular respiration, meaning respiration that uses oxygen. It summarizes many linked reactions, not a single step. Its three main stages are glycolysis, the citric acid cycle (also called the Krebs cycle), and oxidative phosphorylation, which includes the electron transport chain. One glucose molecule can yield an estimated 30 to 32 ATP in many eukaryotic cells. Fermentation without oxygen yields only 2 net ATP per glucose from glycolysis.
What the equation says in plain language
Cellular respiration is the set of reactions by which cells release chemical energy stored in food molecules and capture part of that energy in ATP. It is not the same as breathing, although breathing supplies oxygen and removes carbon dioxide in animals. Cellular respiration happens inside cells, and in eukaryotic cells much of it happens in mitochondria.
The equation is a summary because the full process has dozens of steps. Glucose is broken down step by step, electrons are transferred from carrier to carrier, and ATP is made at several points. Read the equation from left to right: everything on the left is used up, and everything on the right is produced. It describes aerobic respiration only. Without oxygen, cells cannot complete the same pathway, although they may still run glycolysis and then fermentation.
What are the reactants of cellular respiration?
The reactants of cellular respiration are glucose and oxygen. In the balanced equation they appear as one molecule of glucose, C6H12O6, and six molecules of oxygen, 6O2. A reactant is a substance consumed or changed in a reaction. Glucose is the fuel that is broken down. Oxygen accepts electrons at the end of the electron transport chain. If either is missing, the overall aerobic equation cannot run to completion.
Glucose: the fuel that is broken down
Glucose is a simple sugar with the formula C6H12O6: 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms per molecule. It is the molecule oxidized in the standard equation. Oxidation here means losing electrons, often together with hydrogen atoms, as the molecule is broken down into smaller pieces.
In animals, glucose comes from food, is carried in the blood to cells, and can also be drawn from glycogen stored mainly in the liver and in muscle. In plants, glucose made by photosynthesis may be used at once for respiration, stored as starch, or built into larger molecules such as cellulose. Glucose is not the only fuel cells can use. Fats and proteins can also be broken down, and their products enter the pathway at different points, so their equations and yields differ. Textbooks use glucose because it gives a clean example and is a central fuel in most organisms. Glucose is not burned in a flame inside the cell. Enzymes dismantle it in controlled steps, so energy is captured gradually rather than lost all at once as heat.
Oxygen: the final electron acceptor
Oxygen, written as O2, is the second reactant. The 6 in front of O2 means six molecules of oxygen gas, not six oxygen atoms. Each O2 molecule has two oxygen atoms, so six O2 molecules supply 12 oxygen atoms in total.
Oxygen does not react directly with glucose at the start. Glycolysis, the first stage, does not require oxygen. Oxygen acts mainly at the end, in the electron transport chain, where it accepts electrons and, together with hydrogen ions, forms water. Without a final acceptor, electrons would back up, electron carriers would stay loaded, and the chain would slow or stop. Glucose therefore supplies the electrons and the carbon skeleton, while oxygen provides the endpoint that lets the chain keep running and lets most ATP be made. In animals, oxygen reaches cells through breathing and blood circulation. In plants, it can come from the air or from photosynthesis within the plant.
What is not a reactant in the overall equation
ADP, phosphate, water, and enzymes are often expected in the list, but they do not appear as net reactants in the simplified equation. ATP is made from ADP and phosphate inside the cell, yet the simplified equation tracks only carbon, hydrogen, and oxygen from glucose and oxygen, and shows ATP as the useful outcome. Enzymes speed up reactions but are not consumed by them. Electron carriers such as NAD+ and FAD are recycled: they pick up electrons early in the pathway, drop them off at the electron transport chain, and return for more. Water is shown only as a net product, 6H2O, although detailed step by step accounting uses water in some reactions and makes it in others before canceling the shared amount to reach the net equation.
What are the products of cellular respiration?
The products are carbon dioxide, water, and energy captured in ATP. Heat is also released, because no energy transfer is perfectly efficient, but textbooks write the useful outcome as ATP and mention heat separately.
Carbon dioxide, CO2, carries away the carbon atoms from glucose: 6 carbon atoms in glucose become 6 molecules of carbon dioxide, one carbon each. It is released during pyruvate oxidation and the citric acid cycle, not during glycolysis, which splits glucose into two three carbon pyruvate molecules without releasing carbon dioxide. In animals, carbon dioxide diffuses out of cells, travels in the blood to the lungs, and is exhaled. In plants, it can be reused in photosynthesis or leave through pores in leaves.
Water, H2O, forms mainly at the end of the electron transport chain, when oxygen accepts electrons and combines with hydrogen ions. The equation produces 6 molecules of water. This metabolic water is a small addition to the water already in most animal bodies. Its importance here is chemical: it accounts for hydrogen and oxygen atoms and shows that oxygen ends up reduced to water.
ATP, adenosine triphosphate, is the energy currency of the cell, not a fuel like glucose. It is a short term carrier that cells make, use, and remake constantly. When ATP is broken down to ADP, energy is released for work such as muscle contraction, active transport, building molecules, and nerve signaling. ATP is written as energy (ATP) with no fixed coefficient because the number made per glucose is not fixed by atom balance. It depends on cell type, shuttle system, and heat loss, so textbooks give it as an estimate, as this article does below. ATP is also not expelled as waste. It is made and used inside the cell.
Why the equation balances
A balanced equation has the same number of each kind of atom on both sides. This equation balances for carbon, hydrogen, and oxygen:
| Atom | Left side | Right side |
|---|---|---|
| Carbon | 6 in C6H12O6 | 6 in 6CO2, one per molecule |
| Hydrogen | 12 in C6H12O6 | 12 in 6H2O, two per molecule |
| Oxygen | 6 in C6H12O6 plus 12 in 6O2, total 18 | 12 in 6CO2 plus 6 in 6H2O, total 18 |
Oxygen is where most mistakes happen. Glucose supplies 6 oxygen atoms and six O2 molecules supply 12, for 18 on the left. Six CO2 molecules supply 12 and six H2O molecules supply 6, for 18 on the right. Coefficients, the numbers in front of each formula, mean molecules: 6 in front of O2, CO2, and H2O means six molecules of each, while glucose with no written coefficient means one molecule. Balance by changing coefficients only, never subscripts, because changing a subscript would change the substance itself. ATP is excluded from this atom count. Versions that add 30 to 32 ATP or 36 to 38 ATP are adding an estimated energy yield to an atom balanced core, not changing the atom balance.
A quick backward check helps: 6 carbons in glucose require 6 carbon dioxide molecules, and 12 hydrogens require 6 water molecules, since each water has two hydrogens. Oxygen then follows from the remaining count.
The three stages in plain language
Textbooks describe three main stages, plus a short bridging step between the first and second: glycolysis, the citric acid cycle, and oxidative phosphorylation. Pyruvate oxidation, the bridging step, is covered with the citric acid cycle because it prepares pyruvate for that cycle.
Stage 1: Glycolysis in the cytoplasm
Glycolysis takes place in the cytoplasm, outside the mitochondria in eukaryotic cells. It does not require oxygen and is the oldest, most widespread part of the pathway: almost all cells can run it, including cells without mitochondria. One six carbon glucose molecule is split into two three carbon molecules called pyruvate. The process uses 2 ATP to start and makes 4 ATP, for a net gain of 2 ATP per glucose, plus 2 NADH electron carriers for later use. Glycolysis releases no carbon dioxide and does not complete the breakdown of glucose. It is the only stage that can run without oxygen, which is why fermentation can still make a small amount of ATP when oxygen is absent.
Stage 2: Pyruvate oxidation and the citric acid cycle in the mitochondria
In eukaryotic cells, pyruvate moves into the mitochondria and is converted to a two carbon molecule called acetyl CoA. This bridging step releases one carbon dioxide per pyruvate, so two per glucose, and produces one NADH per pyruvate, so two per glucose, with no ATP made directly. Acetyl CoA then enters the citric acid cycle, also called the Krebs cycle, in the mitochondrial matrix. The cycle runs twice per glucose, once per acetyl CoA. Each turn releases two carbon dioxide molecules and produces 3 NADH, 1 FADH2, and 1 ATP (often made first as GTP, then converted to ATP). Across two turns, the cycle produces 4 carbon dioxide, 6 NADH, 2 FADH2, and 2 ATP. Together with pyruvate oxidation, this accounts for all 6 carbons from glucose as carbon dioxide. Most remaining energy is now held in NADH and FADH2, which the final stage converts into much more ATP. Cycle intermediates can also be drawn off to build amino acids and other molecules, so the cycle is part of a wider metabolic network, not only an ATP machine.
Stage 3: The electron transport chain and chemiosmosis
The final stage, oxidative phosphorylation, takes place in the inner mitochondrial membrane in eukaryotic cells and has two linked parts. First, NADH and FADH2 deliver electrons to protein complexes in the membrane. As electrons move along the chain, energy is released and used to pump hydrogen ions, also called protons, from the matrix into the space between the inner and outer membranes, creating a gradient. Second, in chemiosmosis, hydrogen ions flow back into the matrix through ATP synthase, like water through a turbine, driving it to join ADP and phosphate into ATP. Most ATP from respiration is made here. At the very end of the chain, oxygen accepts electrons and combines with hydrogen ions to form water. Without oxygen, this flow stops and oxidative phosphorylation cannot continue. That is why oxygen is essential for the full aerobic yield even though it takes no direct part in glycolysis or the citric acid cycle. In prokaryotic cells, which lack mitochondria, the chain sits in the cell membrane, but the principle is the same.
How much ATP does cellular respiration make?
Textbook totals differ, and the difference is informative. Yield depends on organism, cell type, the shuttle system moving electrons from the cytoplasm into mitochondria, and calculation assumptions. A widely used modern estimate for eukaryotic cells is 30 to 32 ATP per glucose: glycolysis gives a net 2 ATP, the citric acid cycle gives 2 ATP, and oxidative phosphorylation gives about 26 to 28 ATP. These stage figures are estimates, not exact counts.
Older textbooks often give 36 to 38 ATP. That range uses older assumptions: each NADH counted as about 3 ATP and each FADH2 as about 2 ATP. Newer accounting counts each NADH as about 2.5 ATP and each FADH2 as about 1.5 ATP, reflecting updated understanding of proton pumping and ATP synthase ratios. The biology did not change; the accounting did.
| Stage | Location | Net ATP per glucose, estimated |
|---|---|---|
| Glycolysis | Cytoplasm | 2 |
| Pyruvate oxidation | Mitochondrial matrix in eukaryotes | 0 directly |
| Citric acid cycle, two turns | Mitochondrial matrix in eukaryotes | 2 |
| Oxidative phosphorylation | Inner mitochondrial membrane in eukaryotes | About 26 to 28 |
| Total, aerobic, eukaryotic estimate | Whole pathway | About 30 to 32 |
The total still varies within 30 to 32 mainly because of the shuttle problem. NADH made in the cytoplasm cannot cross the inner mitochondrial membrane directly, so its electrons are shuttled in, and different shuttles deliver them at different points with different ATP consequences. Energy used for transport and lost as heat also lowers the realized yield, and prokaryotic cells, without the same transport costs, can differ again. State the assumption with the number: using modern accounting for a typical eukaryotic cell, aerobic respiration yields an estimated 30 to 32 ATP per glucose. An older textbook expecting 36 to 38 ATP is using older accounting for the same reaction, not describing a different reaction.
Aerobic versus anaerobic respiration
Aerobic respiration uses oxygen and completes the full pathway. Anaerobic processes do not, and they produce much less ATP.
| Feature | Aerobic respiration | Fermentation without oxygen |
|---|---|---|
| Oxygen required | Yes, as final electron acceptor | No |
| Main stages | Glycolysis, citric acid cycle, oxidative phosphorylation | Glycolysis followed by fermentation |
| Carbon dioxide | 6 molecules per glucose in the full equation | Alcoholic fermentation, yes, but not in the same pathway or amount; lactic acid fermentation, no |
| Water from the chain | Yes | No |
| Net ATP per glucose, estimated | About 30 to 32 in many eukaryotic cells | 2 from glycolysis |
| Purpose when oxygen is absent | Not applicable | Recycle NAD+ so glycolysis can continue |
Fermentation is not the same as anaerobic respiration in the strict sense. Fermentation regenerates NAD+ by transferring electrons from NADH to pyruvate or a derivative, producing lactate in animals and some bacteria, or ethanol and carbon dioxide in yeast and some other microbes. It uses no electron transport chain and does not fully oxidize glucose. Anaerobic respiration, in the stricter sense used for some prokaryotes, does use an electron transport chain, but with a final acceptor other than oxygen, such as sulfate in some microbes. It can yield more ATP than fermentation, but it is not what happens in human muscle during intense exercise, where the short term fallback is lactic acid fermentation. There, glycolysis speeds up, pyruvate becomes lactate, and NAD+ is regenerated so glycolysis can continue. Lactate can then be carried to the liver and converted back, or used by other tissues. Yeast, by contrast, converts pyruvate to ethanol and carbon dioxide without oxygen, which is why it is used in bread making, where carbon dioxide raises dough, and in brewing, where ethanol is the desired product. In both cases the yield stays at the small glycolysis figure.
Photosynthesis as the reverse process
The overall equation for photosynthesis is:
6CO2 + 6H2O + light energy -> C6H12O6 + 6O2
Compare it with respiration: C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy (ATP). The substances swap sides. Photosynthesis takes in carbon dioxide and water, uses light energy, and builds glucose while releasing oxygen. Respiration breaks glucose down with oxygen and releases carbon dioxide and water while capturing energy in ATP.
The reversal holds for net inputs and outputs, but not for the reactions themselves. Photosynthesis and respiration use different pathways, enzymes, and organelles: chloroplasts for photosynthesis in plants and algae, mitochondria for much of respiration in eukaryotes. A plant cell does both, and in daylight may do both at once, with the balance depending on light, temperature, and need. At ecosystem level, photosynthetic organisms capture light energy in organic molecules, and consumers, plus plants themselves through respiration, release that energy for work, returning carbon dioxide and water. The cycle is not perfectly closed in the short term, since organisms build biomass, store carbon, and lose energy as heat, but the paired equations explain why oxygen and carbon dioxide levels track the balance of the two processes.
What the equation leaves out, and mistakes to avoid
The summary leaves out intermediates such as pyruvate and acetyl CoA, carriers such as NADH and FADH2, enzymes, ADP and phosphate, and the proton gradient that drives most ATP synthesis. It also leaves out heat and regulation: when ATP is plentiful, key enzymes slow down, and when ADP builds up, the pathway speeds up. None of that control appears in the balanced equation, which states what can happen, not how fast. The equation is written for glucose, not for fat or protein, whose intermediates and yields differ. It is also not a single reaction that could be run by mixing glucose and oxygen in a test tube. Without enzymes and the staged pathway, glucose and oxygen do not react at a useful rate in a cell. The staging is the biology; the equation is only the bookkeeping.
Avoid these common mistakes. Do not write oxygen as O instead of O2: oxygen gas is diatomic, two atoms per molecule. Do not forget the coefficients: C6H12O6 + O2 -> CO2 + H2O is not balanced. Do not treat ATP totals as atom balanced counts: they are estimates that vary with assumptions and cell type. Do not say glycolysis requires oxygen, or that it releases carbon dioxide: it does neither. Do not confuse cellular respiration with breathing: breathing moves gases at whole organism level, while cellular respiration is the chemical process inside cells.
Frequently asked questions
What is the chemical equation for cellular respiration?
C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy (ATP). In words, glucose plus oxygen yields carbon dioxide plus water plus energy captured in ATP, summarized across glycolysis, the citric acid cycle, and oxidative phosphorylation.
What are the reactants of cellular respiration?
Glucose (C6H12O6) and oxygen (O2). One glucose molecule reacts with six oxygen molecules. Glucose is the fuel broken down, and oxygen is the final electron acceptor in the electron transport chain.
What are the products of cellular respiration?
Carbon dioxide (CO2), water (H2O), and energy captured in ATP: six molecules each of carbon dioxide and water per glucose, plus ATP as the useful outcome. Heat is also released but is not the product used for work.
Why is the equation written with 6O2 and 6CO2?
To balance atoms. Glucose has 6 carbons, requiring 6 carbon dioxide molecules, and 12 hydrogens, requiring 6 water molecules. Six O2 molecules supply 12 oxygen atoms, which with the 6 in glucose give 18 oxygen atoms on each side.
Does cellular respiration require oxygen?
Aerobic respiration requires oxygen for its full pathway, as the final electron acceptor. Glycolysis does not require oxygen, so cells can still make 2 net ATP by glycolysis followed by fermentation without oxygen, but not the full aerobic yield.
How many ATP are produced by cellular respiration?
A modern estimate for a typical eukaryotic cell is 30 to 32 ATP per glucose from aerobic respiration. Older textbooks give 36 to 38 ATP under older NADH and FADH2 accounting. Both are estimates varying with cell type and shuttle system. Fermentation alone yields 2 net ATP per glucose.
Where does cellular respiration take place?
Glycolysis takes place in the cytoplasm. In eukaryotic cells, pyruvate oxidation and the citric acid cycle take place in the mitochondria, and oxidative phosphorylation takes place in the inner mitochondrial membrane. In prokaryotic cells, the electron transport chain is in the cell membrane.
How is cellular respiration different from photosynthesis?
Photosynthesis builds glucose from carbon dioxide and water using light energy and releases oxygen. Respiration breaks glucose down with oxygen, releasing carbon dioxide and water and capturing energy in ATP. Their net equations reverse each other, but they use different pathways, enzymes, and organelles, and a plant cell can do both.
Why is ATP shown without a coefficient in the equation?
Because its number per glucose is an estimate varying with cell type and assumptions, not a number fixed by atom balance. The atom balanced core is C6H12O6 + 6O2 -> 6CO2 + 6H2O, with ATP totals such as 30 to 32 added as estimated yields.
Sources & references
- OpenStax, Biology 2e: cellular respiration as the process of making ATP using the chemical energy in glucose and other nutrients, mitochondria as the site of ATP production in eukaryotic cells, oxygen use and carbon dioxide production, and anaerobic metabolism including lactic acid fermentation when oxygen is insufficient; Biology 2e Chapter 7 material on metabolism without oxygen and on glycolysis, including the general respiration formula and the roles of oxygen and carbon dioxide.
- NCBI Bookshelf, StatPearls, Biochemistry, Oxidative Phosphorylation: electron transfer from NADH to oxygen coupled to ATP synthesis, chemiosmosis and the proton motive force, ATP synthase as the final complex, and approximately 30 to 32 ATP from the electron transport chain compared with 2 ATP from glycolysis alone.
- NCBI Bookshelf, StatPearls, Physiology, Adenosine Triphosphate: ATP as the primary energy source for cellular use and storage, ATP structure and hydrolysis, and approximately 32 ATP per glucose molecule oxidized during cellular respiration in the account summarized there.
- Biology LibreTexts, ATP Yield: cellular respiration in a eukaryotic cell metabolizing one glucose molecule into 30 to 32 ATP, glycolysis producing 2 ATP with the rest produced during the electron transport chain, and variation in yield by species, proton pumping, and mitochondrial shuttle system.
