Respiration and ATP

After a hard sprint up a few flights of stairs, your legs may burn and you may keep breathing hard for a few minutes. Meanwhile, bread dough in a warm kitchen puffs up with bubbles, and in summer, parts of the deep Chesapeake Bay hold very little oxygen.

Each of these is about how cells get usable energy out of food, and what happens when oxygen is short. One equation and one small molecule explain all three.

Explore: run a cell’s energy budget

In the simulation you run one cell’s energy budget. You choose whether oxygen is present and how much glucose arrives each second. Labelled bars show the ATP made per glucose, the ATP made per second, and which products pile up: carbon dioxide and water, lactate, or ethanol and carbon dioxide.

Try this.

1. Predict how the ATP made per glucose will change when you switch oxygen off. Write your guess, then switch it off.
2. With oxygen off, choose a muscle cell and then a yeast cell. Predict which products each one shows before you look.
3. Keep oxygen off and double the glucose supply. Predict whether ATP per second changes, and whether ATP per glucose changes.
4. Turn oxygen back on. Predict how many glucose molecules the cell needs to make 60 ATP, then check.

Explain: how cells get energy out of glucose

Food holds energy in the way its atoms are bonded. The main example is glucose, the sugar from Capturing Sunlight.

Cellular respiration is the process in which cells break down glucose and other food molecules and use the released energy to build ATP. It goes on in the cells of plants, animals, fungi and most microbes.

When oxygen is available, the overall result fits in one line:

glucose + oxygen → carbon dioxide + water + energy (much of it captured in ATP)

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

Check the atoms. There are 6 carbons on each side, 12 hydrogens on each side, and 18 oxygens on each side. Matter is rearranged, not created or lost.

This equation is a summary. The real process runs as many small steps, each sped up by an enzyme (see Enzymes at Work). The first steps happen in the cytoplasm. Most of the ATP is made in the mitochondria (one is a mitochondrion): organelles in plant, animal and fungal cells where the later steps of aerobic respiration happen. Bacteria have no mitochondria. In bacteria that use oxygen, the electron transport steps, which make most of the ATP, run on the cell membrane instead.

How much ATP does one glucose give? Modern estimates say about 30 to 32 ATP per glucose. Older textbooks say 36 to 38. Real cells vary, because some energy leaks away along the way.

Part of the energy is released as heat, one reason you warm up when you exercise. No energy is made or destroyed: it moves from glucose into ATP and heat.

Explain: ATP carries energy to where it is used

Every working cell does jobs that cost energy. A muscle cell pulls, a nerve cell pumps charged particles across its membrane, and every cell builds proteins.

ATP (adenosine triphosphate) is a small molecule that carries energy from the breakdown of food to the jobs that use energy. It has three phosphate groups in a row. When ATP reacts with water, the end phosphate group comes off. What is left is ADP (adenosine diphosphate), the same molecule with only two phosphate groups.

ATP + water → ADP + phosphate + released energy (used for cell work)

The energy comes from the whole reaction, not from snapping one bond. Breaking any chemical bond takes energy. The products, ADP and phosphate, end up in a more stable arrangement than ATP and water started in, and that difference is the energy the cell can use.

Running the reaction backwards, building ATP from ADP and phosphate, takes energy. Cellular respiration supplies it. So ATP and ADP go round in a cycle: respiration turns ADP back into ATP, and cell work turns ATP into ADP. A cell keeps only a small supply of ATP, so it rebuilds it all the time.

ATP works a little like a rechargeable battery that is charged by respiration and drained by work. The analogy breaks in one place: a battery stays the same object, while ATP really changes into a different molecule and a loose phosphate, and is then rebuilt.

Explain: with oxygen and without it

Aerobic means using oxygen. Anaerobic means without oxygen. In your muscles during a sprint, and in yeast inside dough, cells can keep making some ATP without oxygen.

The SOL calls this anaerobic respiration. In your muscles and in yeast the process is called fermentation: a way of making a small amount of ATP from glucose without oxygen. Some microbes carry out anaerobic respiration in a stricter sense, using another substance such as nitrate or sulfate in place of oxygen.

• In muscle cells, fermentation turns glucose into lactate, a small molecule that builds up in working muscle.
• In yeast, fermentation turns glucose into ethanol and carbon dioxide. The carbon dioxide forms the bubbles that make bread dough rise.

Fermentation gets far less ATP from each glucose, but it can make ATP quickly. A 2024 analysis estimated that, for the same mass of enzyme, the fermentation route made ATP about 2.1 times as fast as respiration in yeast and 3.1 times as fast in mammal cells. In the bacterium E. coli, though, respiration was faster. Scientists still debate this result, because another 2024 study found respiration at least as efficient in its tests.

During a sprint your muscles use ATP faster than aerobic respiration alone can rebuild it, so fermentation adds ATP and lactate builds up. The burn is not caused by lactate alone; experiments point to a mix of acid, lactate and other chemicals. Soreness the next day is a different effect, and lactate is not its cause. After the sprint you keep breathing hard partly because your cells use extra oxygen while they recover.

Respiration and photosynthesis mirror each other. Photosynthesis uses light energy to build glucose from carbon dioxide and water and releases oxygen. Respiration breaks glucose back down with oxygen into carbon dioxide and water. Plants do both. The atoms go round and round between them, while energy arrives as light and leaves as heat (see Matter Cycles).

Elaborate: the Bay’s summer oxygen gap

In spring and summer, algae grow in the sunlit surface water. Many die and sink. Bacteria near the bottom break them down by aerobic respiration, which uses up the oxygen dissolved in the water. Water with less than 2 milligrams of oxygen per liter (2 mg/L) is called hypoxic.

Source: Virginia Institute of Marine Science (William & Mary), December 2025 dead-zone report, Table 1. Each value is a computer-model estimate for the whole Bay on that year’s worst day, not a reading at one station. One cubic kilometer (km³) is a trillion liters.

1. Which year had the smallest peak volume? About what fraction of the 2022 peak was it?
2. Find the mean of the six values, rounded to the nearest whole number.
3. Use the respiration equation: why would more sinking algae make the bottom water lose oxygen faster?

Design problem

Plan an investigation to test whether bread yeast needs to be alive to make gas from sugar. You have dry yeast, sugar, warm water, small plastic bottles, balloons and a string with a ruler.

Evaluate: check your thinking

Your sprinting muscles use ATP faster than aerobic respiration alone can rebuild it, so fermentation adds quick ATP and lactate builds up, and the burn comes from a mix of chemicals. Yeast in dough ferments sugar and releases the carbon dioxide that makes the bubbles. In the deep Bay, bacteria break down sinking algae by aerobic respiration and use up the oxygen faster than it returns.

Science practices you used: making predictions and testing them with a model, reading a data table and finding a mean, planning a fair test with a control and repeated trials, and backing a claim with evidence while naming the limits of the data.