Aerobic Cellular Respiration Overview

Aerobic cellular respiration is the series of reactions in cells that uses oxygen to break down glucose into carbon dioxide and water, releasing energy that is transferred to ATP. In brief: the word equation is glucose + oxygen → carbon dioxide + water + energy, and the balanced equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. The process starts in the cytoplasm and is completed in the mitochondria, where most of the ATP is made.

Respiration is the reason we need to eat and breathe. This guide gives an overview of the process: what goes in and what comes out, the three main stages, the role of mitochondria, a worked example that checks the equation and compares energy outputs, and the typical mistakes, such as confusing respiration with breathing.

What is cellular respiration?

Cellular respiration is a set of metabolic reactions in which cells release energy from nutrient molecules, mainly glucose, and use it to make ATP. It happens in all living cells, plant and animal, all the time. It is not the same as breathing, which is the exchange of gases between the body and the environment.

Respiration is a type of catabolic (breaking-down) metabolism. It is a controlled form of oxidation. The glucose is oxidized and the oxygen is reduced to water, and the energy that is released is captured step by step instead of in a single burst as in burning.

What is the equation for aerobic respiration?

Word equation: glucose + oxygen → carbon dioxide + water + energy

Balanced symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Aerobic respiration releases about 2 870 kilojoules of energy per mole of glucose (180 g) as heat and chemical energy. Part of this, around 40 percent, is transferred to ATP, and the rest is released as heat, which helps to keep the bodies of mammals and birds warm. Fats and proteins can also be used as fuel, after being broken down into units that enter the pathway.

What are the stages of aerobic respiration?

Aerobic respiration has four stages, which are described in more detail in the articles on advanced respiration. The overview below shows where each takes place.

1. Glycolysis (cytoplasm): one glucose (6 carbons) is split into two pyruvate molecules (3 carbons each). It makes a net gain of 2 ATP and 2 molecules of reduced NAD (NADH), a carrier of hydrogen. It does not need oxygen.
2. Link reaction (mitochondrial matrix): each pyruvate is converted into a 2-carbon acetyl group attached to coenzyme A. One carbon dioxide and one NADH are produced for each pyruvate.
3. Krebs cycle (mitochondrial matrix): acetyl groups are oxidized in a cycle of reactions. Carbon dioxide is released, and NADH, reduced FAD (FADH₂) and a little ATP are made.
4. Electron transport chain and oxidative phosphorylation (inner mitochondrial membrane): NADH and FADH₂ give up their electrons to a chain of proteins. The energy released pumps hydrogen ions across the membrane, and the ions flow back through ATP synthase, which makes ATP. Oxygen is the final electron acceptor, and it combines with electrons and hydrogen ions to form water.

Oxygen is needed only in the last stage, but because that stage cannot work without it, the other stages that depend on the carriers it regenerates also stop. This explains why we need oxygen to survive. The theoretical maximum yield is about 30 to 32 ATP per glucose in current estimates, and older textbooks give 36 or 38, so check the figure used in your course.

What is the role of mitochondria?

Mitochondria are the sites of the link reaction, the Krebs cycle and the electron transport chain. Their structure fits this function. The inner membrane is folded into cristae, which gives a large surface area for the electron transport chain and ATP synthase. The matrix contains the enzymes of the Krebs cycle, and the space between the two membranes stores the hydrogen ions that are pumped across.

Cells with high energy demands have many mitochondria. A heart muscle cell may have thousands, which take up about a third of its volume, while a skin cell has fewer.

Stages compared

Worked example: checking the equation and the energy

Check that the equation is balanced and estimate the energy captured in ATP.

1. Carbon: left side, 6 (in glucose); right side, 6 × 1 = 6 (in 6CO₂). Balanced.
2. Hydrogen: left side, 12 (in glucose); right side, 6 × 2 = 12 (in 6H₂O). Balanced.
3. Oxygen: left side, 6 (in glucose) + 12 (in 6O₂) = 18; right side, 6 × 2 = 12 (in 6CO₂) + 6 (in 6H₂O) = 18. Balanced.
4. Energy: if 30 ATP are made from one mole of glucose and each mole of ATP stores about 30.5 kJ (standard conditions), the energy captured is 30 × 30.5 = 915 kJ. The efficiency is 915 ÷ 2 870 ≈ 0.32, or 32 percent. With 50 kJ/mol for the conditions inside a cell, it becomes 1 500 ÷ 2 870 ≈ 52 percent.

These numbers show that respiration is fairly efficient compared with a petrol engine (about 25 to 30 percent), but a large share of the energy still becomes heat. The carbon dioxide produced can be shown by bubbling exhaled air through limewater, which turns milky, and a temperature rise in a flask of germinating seeds in a vacuum flask shows the release of heat.

How was respiration explained?

Antoine Lavoisier showed in the 1780s that respiration is a slow form of combustion in which oxygen is used and carbon dioxide produced. In the 1930s Hans Krebs described the cycle of reactions that bears his name, for which he received the Nobel Prize in 1953. Peter Mitchell’s chemiosmotic theory (1961) explained how ATP is made by the electron transport chain, and his work was recognized with the Nobel Prize in Chemistry in 1978.

Which mistakes are common?

Mistake 1: Confusing respiration with breathing

Breathing is gas exchange in the lungs. Respiration is the chemical process in cells.

Mistake 2: Saying that plants only respire at night

Plants respire all the time. In daylight, photosynthesis hides the effect.

Mistake 3: Saying that respiration produces energy

It transfers energy from glucose to ATP and heat.

Mistake 4: Thinking that the oxygen becomes the carbon dioxide

The oxygen in the released carbon dioxide comes from glucose and from water in the reactions. The oxygen that we breathe in ends up in water.