What are the products of cellular respiration?

What is respiration?

In biology, respiration is the chemical process that releases energy from food inside every living cell. It happens all the time, in plants as well as animals, in bacteria and fungi too. Cellular respiration is the same thing, with "cellular" added to make clear it happens in cells.

It is not the same as breathing. Breathing, or ventilation, moves air in and out of the lungs. It brings in the oxygen that respiration uses and gets rid of the carbon dioxide it makes, but the respiration itself happens in your cells, mostly inside mitochondria.

What are the reactants of cellular respiration?

The reactants, the substances that go in, are glucose (C₆H₁₂O₆) and oxygen (O₂). Glucose comes from digested food in animals and from photosynthesis in plants, and the blood carries both glucose and oxygen to your cells. Cells can also respire fats and proteins by feeding their breakdown products into the later stages.

What are the products of cellular respiration?

Strictly, energy isn't a substance, so some teachers list only carbon dioxide and water as the products and say energy is released. The useful part of that energy is stored in ATP. Each ATP molecule carries a small, ready-to-use packet of energy, and cells spend ATP the way you spend coins.

Checking the equation balances

Count each kind of atom on both sides. Carbon: 6 on the left, 6 in 6CO₂. Hydrogen: 12 on the left, 12 in 6H₂O. Oxygen: 6 in glucose plus 12 in 6O₂ makes 18, and the right has 12 in 6CO₂ plus 6 in 6H₂O, which is also 18. Every atom in the glucose and oxygen ends up in the carbon dioxide and water.

Try balancing it yourself, then press Mark to check it in our Cell Biology course.

What is the purpose of cellular respiration?

The purpose of cellular respiration is to release the energy stored in glucose and capture it as ATP, which the cell can use straight away. Glucose holds a lot of energy, but in a form the cell can't spend directly. Respiration breaks it down in small, controlled steps, rather than all at once as in burning, so the energy can be trapped in ATP instead of escaping as a flash of heat.

ATP then powers:

• Movement: muscle contraction, and the beating of cilia and flagella.
• Active transport: pumping substances into or out of cells against a concentration gradient.
• Building molecules: making proteins from amino acids, and starch, glycogen and DNA from smaller units.
• Nerve impulses: resetting the ion balance across nerve cell membranes.
• Keeping warm: in mammals and birds, the heat released helps hold body temperature steady.

The stages of cellular respiration

The one-line equation hides four stages. Each one takes the glucose apart a little more.

So the carbon dioxide comes from stages 2 and 3, 6 molecules per glucose in all, and the water from stage 4. NADH and FADH₂ are carrier molecules that collect hydrogen and electrons from the earlier stages and deliver them to the electron transport chain.

Stage 4 is where oxygen matters. As electrons move along the chain, the proteins pump hydrogen ions across the inner membrane. The ions flow back through an enzyme called ATP synthase, which uses that flow to make ATP, much as water turns a turbine. OpenStax notes this process, chemiosmosis, makes about 90% of the ATP from aerobic respiration. Oxygen is the final electron acceptor: without it, the chain backs up and stops.

How much ATP does one glucose make?

Modern estimates are about 30 to 32 ATP per glucose, and StatPearls gives approximately 32. Many older textbooks say 36 or 38, because they assumed each NADH makes 3 ATP and each FADH₂ makes 2, figures that turned out to be too high. The exact number varies with the cell. Check which figure your course or exam board uses.

Aerobic vs anaerobic respiration

Aerobic respiration uses oxygen. When oxygen runs short, cells can still respire anaerobically, but they only run glycolysis and get far less energy.

During hard exercise, your muscles need ATP faster than your blood can deliver oxygen, so some of their respiration becomes anaerobic and lactate builds up. Afterwards you keep breathing hard to supply extra oxygen, which is used to deal with the lactate and restore the muscles. This extra oxygen is often called the oxygen debt.

Photosynthesis and respiration

The overall equation for photosynthesis is respiration in reverse: carbon dioxide and water, using light energy, make glucose and oxygen. Plants do both. They photosynthesise only in the light, but they respire all the time, day and night. In bright light, photosynthesis runs faster than respiration, so a plant takes in carbon dioxide overall. In the dark, it only gives carbon dioxide out.

Experiments that show cells respire

You can't see respiration happen, but you can detect what it uses and what it makes. These are the classic classroom experiments.

• Limewater test: carbon dioxide turns limewater cloudy (milky). Breathe out through limewater and it clouds quickly. Put germinating peas in a sealed tube with limewater below them and it clouds too, while a tube of boiled, dead peas stays clear.
• Yeast and a balloon: mix yeast, sugar and warm water in a bottle and stretch a balloon over the top. Carbon dioxide from the yeast slowly inflates it. Warm water works faster than cold, but very hot water kills the yeast.
• Respirometer: seeds or small animals such as woodlice sit in a sealed tube with soda lime, which absorbs carbon dioxide. As they use oxygen, the gas volume shrinks and a drop of coloured liquid moves along a thin tube. The distance it moves each minute measures the rate of respiration. A tube with glass beads instead of living things is the control.

Respiration is controlled by enzymes, so its rate rises with temperature up to an optimum and then falls as the enzymes start to denature. That is why the yeast in the balloon experiment works best when warm, not hot.

Worked examples

Example 1: mass of carbon dioxide

What mass of carbon dioxide does a cell make by fully respiring 90 g of glucose? Glucose has a molar mass of 180 g/mol and carbon dioxide 44 g/mol.

1. Moles of glucose: 90 ÷ 180 = 0.5 mol.
2. The equation gives 6 CO₂ for every glucose, so 0.5 × 6 = 3 mol of CO₂.
3. Mass: 3 × 44 = 132 g of carbon dioxide.

The same method gives 0.5 × 6 × 18 = 54 g of water. The products weigh more than the glucose because oxygen joined in too.

Example 2: how much better is aerobic?

Using 32 ATP for aerobic respiration and 2 for anaerobic, how many times more ATP does aerobic respiration make per glucose? 32 ÷ 2 = 16 times. That is why a sprint can't last: anaerobic respiration burns through glucose sixteen times faster for the same energy.

Example 3: the respiratory quotient

The respiratory quotient (RQ) is the carbon dioxide made divided by the oxygen used. For glucose, the equation gives 6 CO₂ for 6 O₂, so RQ = 6 ÷ 6 = 1.0. Fats need more oxygen for every carbon dioxide they make, so their RQ is lower, about 0.7. Measuring a person's RQ shows roughly which fuel they are using.

Common mistakes

• Saying respiration is breathing. Breathing moves air. Respiration is the chemical reaction in cells.
• Saying plants don't respire. They respire all the time. They also photosynthesise when it's light.
• Saying respiration produces energy. Energy can't be made. Respiration releases energy stored in glucose and transfers it to ATP.
• Putting carbon dioxide in the products of anaerobic respiration in animals. In animals the only product is lactic acid. Carbon dioxide is made in yeast and plants.
• Saying glycolysis happens in the mitochondria. It happens in the cytoplasm. Only the later stages are in the mitochondria.

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