Cellular respiration equation, stages and diagram, explained simply

Short answer

The cellular respiration equation is: glucose + oxygen → carbon dioxide + water + energy. Written as a formula, that is C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy, and most of that energy is stored as ATP, the cell's "energy currency" (1, 6).

In plain words: your cells "burn" sugar slowly, in many small steps, so the energy can be caught in small packets instead of being lost as one big flash of heat. Complete breakdown of one glucose molecule gives about 30 ATP molecules (6). Only about 34% of the energy in glucose ends up captured; the rest leaves as heat, which is part of why your body is warm (4).

It happens in four stages: glycolysis (in the cell's jelly-like cytoplasm), pyruvate oxidation and the Krebs cycle (inside mitochondria), and the electron transport chain, where oxygen is used and most ATP is made (2, 3, 4).

If this topic felt like a wall of letters in class, finding it confusing is not weakness. Most textbooks start with the chemistry; this guide starts with the story and adds the chemistry one word at a time.

Note: this article is for general education and is not medical advice. If you are constantly tired for weeks without a clear reason, that is a reason to see a doctor, not to study biochemistry (see "When to see a doctor"). This guide is published by Welltory, which makes one of the apps mentioned.

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What is cellular respiration?

Cellular respiration is the way your cells turn food into usable energy. "Respiration" here does not mean breathing with your lungs. It means a chemical process inside each cell. Breathing and cellular respiration are linked, though: your lungs bring in the oxygen that cells use and carry away the carbon dioxide that cells make.

Think of it like this:

• Glucose (a sugar from food) is the fuel.

• Mitochondria are the power stations inside your cells.

• ATP (adenosine triphosphate) is a charged battery. Your cell "spends" ATP to move muscles, send nerve signals and build new parts. OpenStax describes ATP as working much like a rechargeable battery (1).

• Oxygen is the final "catcher" at the end of the line, without which the main power station stalls.

Your body never stores much ATP. It recharges the same batteries over and over, all day and all night. That is why you need a steady supply of fuel and oxygen even while you sleep.

What is the cellular respiration equation?

Here is the cellular respiration formula, also called the chemical equation:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (ATP and heat)

How to read it, piece by piece:

Why the sixes? The equation must balance: every atom on the left must show up on the right. Glucose has 6 carbon atoms, so it makes 6 carbon dioxide. Its 12 hydrogen atoms end up in 6 water molecules. Count the oxygen atoms and you get 18 on each side.

What does the word equation say? Glucose plus oxygen gives carbon dioxide plus water plus energy. If a test asks for "the word equation for cellular respiration," that is the line.

What are the reactants and products of cellular respiration?

Reactants are what goes in: glucose and oxygen. Products are what comes out: carbon dioxide, water and ATP (plus heat).

Based on 2, 3 and 4. The Krebs cycle numbers are per turn × 2, because one glucose makes two pyruvate.

Two new words in that table:

• NADH and FADH2 are "loaded delivery trucks." They pick up high-energy electrons in the early stages and drive them to the electron transport chain, where that energy is cashed in for ATP.

• Pyruvate is half of a glucose molecule after glycolysis has split it.

What is the purpose of cellular respiration?

The purpose of cellular respiration is to make ATP, the energy form that every part of the cell can use directly. Food energy is "locked" in chemical bonds; the cell cannot use it as it is. Cellular respiration unlocks it and repackages it into ATP (1).

Why is cellular respiration important? Without it:

• Muscles could not contract for more than a few seconds.

• Nerve cells could not keep their electrical charge, so signals would stop.

• Cells could not pump salts, build proteins or repair themselves.

It has a side job too. The middle stage, the Krebs cycle, also supplies building blocks the body uses to make other molecules (8). And the heat it releases helps keep your body temperature stable.

Stages of cellular respiration, simply

Here are the four stages of cellular respiration in order. Each one hands its product to the next, like a relay race.

1. Glycolysis — split the sugar. In the cytoplasm, one glucose (6 carbons) is split into two pyruvate (3 carbons each). The cell invests 2 ATP and gets 4 back, so the net gain is 2 ATP, plus 2 NADH. This stage does not use oxygen (2).

2. Pyruvate oxidation — the bridge. Pyruvate moves into the mitochondria and is trimmed into a 2-carbon piece called acetyl CoA. One carbon leaves as carbon dioxide, and NADH is made. CoA is built from vitamin B5 (3).

3. Krebs cycle — strip the carbons. Also called the citric acid cycle. In the mitochondrial matrix, acetyl CoA is taken apart in eight steps. Its carbons leave as carbon dioxide, and the energy is loaded into NADH and FADH2. The cycle makes very little ATP directly (3).

4. Electron transport chain and ATP synthase — cash in. On the inner mitochondrial membrane, NADH and FADH2 drop off their electrons. The electrons pass down a chain of proteins, which pump protons (hydrogen ions) across the membrane. The protons flow back through a tiny rotating machine called ATP synthase, which makes ATP. Oxygen catches the electrons at the end and becomes water. This step produces about 90% of the ATP from glucose (4).

Some textbooks list three stages instead of four. They simply fold pyruvate oxidation into the Krebs cycle. Both versions are correct.

Where does cellular respiration take place?

Cellular respiration takes place in two parts of the cell: the cytoplasm and the mitochondria.

• Glycolysis: the cytoplasm, in all cells, including bacteria (2).

• Pyruvate oxidation and the Krebs cycle: the mitochondrial matrix, the inner "filling" of the mitochondrion (3).

• Electron transport chain: the inner mitochondrial membrane, folded into ridges called cristae so it can hold more machinery. In bacteria, which have no mitochondria, the chain sits in the cell's outer membrane (4).

A useful exception: red blood cells have no mitochondria, so they can only run glycolysis. Lactic acid fermentation is their only source of ATP (7). That is a neat design: the cells that carry oxygen do not use it up themselves.

Cellular respiration diagram, described in words

Most cellular respiration diagrams look busy, but they all follow the same layout. Here is how to read one from left to right:

1. Look for the glucose on the left or top. It is usually drawn as a hexagon or a chain of six circles, outside a large oval (the mitochondrion).

2. Find the glycolysis arrow in the cytoplasm. It splits glucose into two smaller shapes labeled pyruvate. Small labels show "2 ATP" and "2 NADH."

3. Follow pyruvate into the mitochondrion. An arrow crosses the membrane, and the label changes to acetyl CoA. A small "CO2" bubble floats away.

4. Find the circle. The Krebs cycle is drawn as a wheel inside the mitochondrion. CO2 arrows leave the wheel, and NADH and FADH2 labels point outward toward the membrane.

5. Find the membrane with the proteins. The electron transport chain is drawn as a row of blobs (often numbered I to IV) in the inner membrane, with H+ arrows going up through them and one big channel (ATP synthase) where H+ comes back down and ATP pops out.

6. Find the oxygen at the very end. O2 meets electrons and H+ and becomes H2O.

Tip for exams: if you can say the relay — sugar splits, carbons leave as CO2, electrons ride NADH to the membrane, oxygen catches them, ATP synthase spins — you can redraw any diagram from memory.

Aerobic cellular respiration vs anaerobic

Aerobic cellular respiration uses oxygen. It runs all four stages and gives about 30 ATP per glucose (6). That is the normal mode for most of your cells most of the time.

Anaerobic means without oxygen. When oxygen is short — for example in a muscle during a hard sprint — cells keep glycolysis running but stop there. Pyruvate is turned into lactate, which frees up the NAD+ that glycolysis needs to continue. This gives only 2 ATP per glucose, but it is fast (5, 7). The lactate is later carried to the liver and recycled (5).

Yeast does something similar but makes alcohol and carbon dioxide instead of lactate. That is how bread rises and beer ferments (7).

What is the role of oxygen in cellular respiration?

Oxygen is the final electron acceptor. At the end of the electron transport chain, oxygen picks up the "used" electrons and joins with hydrogen to form water (4, 9).

Why that matters: if nothing catches the electrons, the chain backs up like a traffic jam. Proton pumping stops, ATP synthase stops, and the Krebs cycle soon stalls because its NADH has nowhere to unload. That is why some poisons are so dangerous. Cyanide and carbon monoxide block the last protein in the chain, so cells cannot use oxygen even when there is plenty of it in the blood (9).

How many ATP does cellular respiration make?

Modern estimates are about 30 to 32 ATP per glucose (6, 7). You will still see 36 or even 38 in some textbooks (5). The difference comes from rounding: each NADH yields about 2.5 ATP and each FADH2 about 1.5, and some sources round these up to 3 and 2 (9). The real number also varies with how NADH from the cytoplasm gets into the mitochondria (4).

For a test, use the number your course uses. For real life, "about 30" is the honest answer.

How are photosynthesis and cellular respiration related?

They are two halves of one loop. Photosynthesis in plants uses carbon dioxide, water and sunlight to make sugar and release oxygen: six carbon dioxide and six water make one sugar and six oxygen (10). Cellular respiration takes that sugar and oxygen and turns them back into carbon dioxide and water, releasing the energy for the cell to use.

So the inputs of one are the outputs of the other. The carbohydrates plants make are the fuel animals use to make ATP through respiration (10). Both processes even use the same trick — a proton gradient driving ATP synthase — to make ATP (1).

Do plants do cellular respiration? Yes. Plants have mitochondria and respire day and night. During daylight they also photosynthesize, usually faster than they respire, so overall they release more oxygen than they use.

Fuel beyond sugar: fats and protein

Glucose is the textbook example, but it is not your only fuel. Fats and proteins are also broken down and fed into the same machine. They are converted into acetyl CoA, which enters the Krebs cycle, and during fasting fat breakdown supplies more of it (8). That is why the Krebs cycle is often called the "hub" of metabolism: almost every food ends up passing through it.

Vitamins matter here too. CoA is built from vitamin B5 (3), and vitamin B1 (thiamine) is a required helper for key steps around the Krebs cycle (8). This does not mean extra vitamins give you extra energy — a true deficiency is a medical problem, while extra pills on top of a normal diet do not speed up the machine.

What this means for your energy — and what you can see in your data

You cannot feel ATP, and no wearable or app measures it. But you can see the demand side of cellular respiration in everyday signals:

• Heart rate rises when you move. Working muscles need more oxygen for aerobic respiration, so your heart pumps faster to deliver it. When you sprint beyond what oxygen delivery can cover, muscles lean on fast anaerobic glycolysis. Training zones are built on this idea; see our guide to heart rate zones.

• Fitness is partly an oxygen-use skill. Regular aerobic training helps your body deliver and use oxygen better, which is what VO2 max estimates.

• Resting heart rate and HRV reflect recovery, not ATP. After poor sleep, illness, a hard workout or stress, many people see a higher resting heart rate and lower heart rate variability (HRV). These are signs of how your nervous system is handling load (what HRV measures), not a readout of your mitochondria.

• Sleep is when a lot of repair happens. If your energy dips by afternoon, look at sleep first; our article on how sleep affects daily energy explains why.

One honest rule: feeling tired is rarely about "slow cellular respiration." It is much more often about sleep, stress, illness, iron, thyroid or other causes a doctor can check (11).

Common misconceptions

• "Respiration means breathing." In biology, cellular respiration is a chemical process in cells. Breathing supplies its oxygen and removes its carbon dioxide.

• "Plants photosynthesize, animals respire." Plants do both.

• "Glycolysis needs oxygen." It does not; only the later stages depend on oxygen (2).

• "The Krebs cycle makes most of the ATP." It makes very little directly. Most ATP comes from the electron transport chain (3, 4).

• "Lactic acid causes next-day soreness." Lactate is cleared and recycled by the liver (5); next-day soreness has other causes.

• "All the energy in food becomes ATP." Only about a third does; the rest is heat (4).

Living with a body that runs on cellular respiration: the questions people ask next

What organisms perform cellular respiration?

Almost all living things: animals, plants, fungi and many bacteria. Some microbes live without oxygen and use only fermentation or other pathways.

Why do I breathe harder when I exercise?

Muscles burn more fuel, so they need more oxygen and make more carbon dioxide. Your brain senses the rising carbon dioxide and speeds up your breathing to clear it.

Why does cellular respiration make heat?

About two-thirds of glucose energy is not captured as ATP and is released as heat (4). That is part of why you feel warm during exercise.

Can you live without cellular respiration?

No. Blocking it, as cyanide does, is quickly fatal because cells cannot make enough ATP (9).

When to see a doctor

Cellular respiration is a school topic, but people often search for it when they feel drained. See a doctor if you have felt tired for a few weeks and don't know why, if tiredness is affecting your daily life, or if it comes with weight loss, mood changes or loud snoring with gasping or choking in your sleep (11). Common, checkable causes include poor sleep, stress, depression, hormonal changes, iron deficiency anemia, diabetes and thyroid problems (11).

When you go, describe your tiredness in concrete terms: since when, how many hours you sleep, what you can no longer do. Ask directly whether blood tests for iron, thyroid and blood sugar make sense for you.

How Welltory helps — and what it cannot do

The limits first. Welltory is a general wellness product, not a medical device. It does not measure ATP, mitochondria, oxygen use inside cells or lactate, and it does not diagnose any condition.

What it can do is help you see how your body responds to sleep, training and stress — the everyday load that cellular respiration has to keep up with.

Heart rate and HRV with your phone camera. You can take a spot measurement with your phone camera and flash — a short, deliberate reading, not 24/7 tracking (camera measurements). Measure at the same time each morning for comparable results (accurate HRV measurements).

Connect a watch or health app. If you already wear a tracker, you can connect it as a data source (data sources).

Tag what you change. When Welltory notices a stress or recovery episode and asks "What happened?", you can add a tag such as "hard workout", "bad night" or "skipped lunch" — then, on iOS, look back in My Patterns to see what tends to come before your low-energy days (My Patterns).

A record for your doctor. If tiredness persists, you can export your data as CSV from the web app and bring it to your appointment (export guide).

How we made it

We based this article on open-access textbooks — OpenStax Biology 2e and Anatomy and Physiology 2e, Molecular Biology of the Cell on NCBI Bookshelf — and StatPearls reviews of glycolysis, the Krebs cycle and the electron transport chain, plus NHS guidance on tiredness. Search questions came from what people in the United States most often ask about cellular respiration. For the product section, we checked every function against the Welltory Help Center on November 9, 2026.

Made with AI tools, then edited and fact-checked by the Welltory team. See our Editorial & AI policy.

Jane Smorodnikova, co-founder of Welltory, built the methodology behind how we read physiological data.

Written by Tatsiana Yashyna.

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