Lesson Explainer: Aerobic Respiration Biology • Second Year of Secondary School

In this explainer, we will learn how to recall the reactants and products of aerobic respiration and explain the importance of respiration for organisms.

When you hear the word respiration, you may first think of how you are breathing in and out right now. Our lungs take in oxygen from the air and release the carbon dioxide that our bodies produce as waste in an action called breathing, which is also known as respiration. But why do we need all this oxygen? Where does this carbon dioxide come from? The answer to both of these questions is given by the process of aerobic cellular respiration, which is a chemical reaction that is not the same as breathing, even though the two are related.

Our bodies are made of cells, and each of these many cells needs energy to do work. The work a cell does is different depending on the type of cell it is. Some cells need energy to move, most need energy to divide, and some cells use energy to communicate with each other. Cells need energy to carry out all the essential functions that keep us alive, like moving our bodies and digesting food. Cellular respiration provides the energy that these cells need to do their jobs. In eukaryotic cells, like the ones we are made of, cellular respiration is the function of the mitochondria.

All living organisms use some form of cellular respiration that breaks down nutrients to release and then store energy. In humans, the energy is usually released from a type of sugar called glucose and transferred to a molecule called ATP. You can see the chemical structure of ATP below. Notice that it is formed from the nitrogenous base adenine, a five-carbon sugar called ribose, and 3 phosphate groups. You can see this in Figure 1 below.

Glucose is a nutrient that we obtain from food. ATP stands for adenosine triphosphate, and it is the molecule that provides the energy for almost all cellular processes in almost all living things.

Glucose is not the only ingredient needed for cellular respiration. Our bodies also use the oxygen we breathe in through our lungs in this process. Because the cells need oxygen to carry out cellular respiration, we call it “aerobic” cellular respiration. Aerobic is a word that means “involving oxygen.” If cellular respiration does not utilize oxygen, we refer to it as anaerobic respiration. An- is a prefix that means not. However, our focus in this explainer is on aerobic respiration. Since both glucose and oxygen are needed for cellular respiration, we call these the “reactants.” You can see the reactants and the products of cellular respiration in Figure 2.

The oxygen molecules and the glucose molecules are rearranged in a process that releases energy. At the end, they are changed into molecules of carbon dioxide and water. We call these the “products” since they are produced by the reaction. The carbon dioxide made by cellular respiration is removed from our bodies by our lungs when we exhale. The water that is produced is largely recycled by the cell, but any excess will be removed from the body as waste. You can see this in Figure 3.

Most of the energy that is released in cellular respiration is carried in molecules of ATP. Some of the excess energy is given off as heat. A chemical reaction that releases energy is called an exothermic reaction. Exothermic reactions need an initial input of energy to occur, but the products have less stored energy than the reactants, which means that there is an overall loss of energy in the process. You can see this in Figure 4.

The energy released during cellular respiration is carried in a molecule called adenosine triphosphate, or ATP. The “tri-” in “triphosphate” means that there are three phosphate groups attached to each other in this molecule. When cells need energy to perform a function, they break off the last phosphate group and energy is released. This changes ATP into ADP, or adenosine diphosphate, as you can see in Figure 5. Di- is a prefix that means “two.”

During cellular respiration, the energy released from the breakdown of glucose is used to convert ADP back into ATP by attaching a third phosphate group to the molecule. Since ATP is needed continually by the cells to perform all of their important functions, cellular respiration occurs constantly. This means that, in general, ATP is made by the cells as quickly as it is used.

We have become familiar with the process of aerobic cellular respiration in a simplified form. We can also represent this process with a balanced chemical equation that shows the quantities of each of the molecules involved.

One molecule of glucose contains 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen. It is represented by the chemical formula . The chemical formula for oxygen is because one molecule of oxygen is made of 2 atoms of oxygen bonded together. The chemical formula for water is , meaning that one molecule of water is made of 2 hydrogen atoms bonded to 1 oxygen atom. And the chemical formula for carbon dioxide is because one molecule of carbon dioxide has 1 carbon atom and 2 oxygen atoms. You can see the unbalanced equation below in Figure 6.

Now that we know the chemical formulas for the products and the reactants in aerobic respiration, we are able to balance the chemical equation. There should be the same number of carbon, hydrogen, and oxygen atoms in the reactants as there are in the products. Luckily, this can be achieved simply by adding a coefficient of 6 to the symbols for oxygen, water, and carbon dioxide. This means that aerobic cellular respiration uses 1 molecule of glucose and 6 molecules of oxygen, producing 6 molecules of water and 6 molecules of carbon dioxide and releasing energy in the process. The amount of energy produced from the oxidation of one molecule of glucose is about 38 ATP molecules. The balanced chemical equation for aerobic cellular respiration can be seen in Figure 7.

Cellular respiration primarily takes place within the mitochondria. This also helps to explain why cells that are more active will have more mitochondria inside of them. For example, you could expect to find more mitochondria in a muscle cell than in a skin cell. This also helps to explain why as you do more physical activity, like exercise, your respiration rate and breathing rate will increase. When you are more physically active, you need to breathe in more oxygen so that your cells can make more ATP to keep your body moving.

Let’s summarize what we have learnt about aerobic respiration.