International A Level Biology / Cellular respiration
Aerobic respiration: the overall reaction
Edexcel International A Level Biology (YBI11), chapter 20: Cellular respiration, specification point 7.1. Free lesson.
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Aerobic respiration breaks down glucose using oxygen and releases energy that cells use to make ATP. Its balanced equation shows where the carbon, hydrogen and oxygen end up, while its four stages show how the process happens in controlled steps.
What the overall equation shows
The balanced equation for aerobic respiration is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, releasing energy. Glucose is the usual respiratory substrate, meaning the molecule that is broken down. Cells can also respire other substrates, such as fats. Oxygen comes from the air you breathe in and joins with hydrogen to form water. Every carbon atom in the carbon dioxide comes from glucose. Carbon dioxide is a waste product that leaves the cell and is breathed out. Water forms when hydrogen removed from glucose joins back up with oxygen.
What happens to glucose, carbon and hydrogen
Respiration first splits the substrate, then reunites its hydrogen with oxygen. Enzymes inside the cell break glucose down a bit at a time. The hydrogen is carried by the coenzymes NAD and FAD. A coenzyme is a small molecule that helps an enzyme work. When hydrogen meets oxygen at the end, water forms and energy is released for making ATP. The carbon from glucose leaves as carbon dioxide, a waste product. The energy is not made during respiration. It was stored in glucose and is released for the cell to use. Photosynthesis does the opposite in this respect: light splits water and its…
Why respiration happens in small steps
Glucose can burn in one fast reaction, but cells break it down through many small reactions. The overall equation and total energy are the same, but the stepped process lets the cell use the energy more effectively and control the rate. Each step is catalysed by a specific intracellular enzyme, meaning an enzyme that works inside a cell. The enzymes do not need to be named here. Enzymes allow respiration to happen at body temperature. Burning needs a very high temperature. Respiration releases energy in small amounts. More can be used to make ATP, and less is lost as heat than when energy…
The four stages and where they happen
Aerobic respiration has four stages. Glycolysis is the only stage outside the mitochondrion. Without oxygen, only glycolysis runs, so very little ATP is made. Glycolysis happens in the cytoplasm. One glucose molecule splits into two pyruvate molecules, each with three carbon atoms. In the link reaction, pyruvate enters the mitochondrial matrix and becomes acetyl CoA. The first carbon dioxide is released. The Krebs cycle also happens in the matrix. It releases the rest of the carbon dioxide and removes most of the hydrogen. Oxidative phosphorylation happens at the inner mitochondrial…
How to explain the advantage of small steps
For a question about why respiration happens in many enzyme-controlled steps, link the energy released to ATP, then explain the role of enzymes.