Lesson Explainer: Aerobic Respiration Biology

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.

Key Term: Cellular Respiration

Cellular respiration is a complex chemical process in which nutrients and other molecules are broken down in order to release energy. The energy released in this process is transferred to a molecule called ATP.

Example 1: Identifying the Source of Glucose for Aerobic Respiration

Aerobic respiration in humans requires a supply of glucose. Where does this glucose come from?

Answer

Glucose is an especially important nutrient for humans because it is one of the reactants of cellular respiration. It is nearly as essential to the functioning of our cells as the other reactant in cellular respiration, oxygen. Unlike plants, which can make glucose through the process of photosynthesis, animals like humans cannot synthesize glucose on their own. Instead, we get glucose from the food we eat. That food is broken down into its components in the process of digestion and absorbed into the bloodstream. The blood carries the glucose to the cells that utilize it in the process of cellular respiration.

So, the glucose for cellular respiration comes from food in the person’s diet.

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.” 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.

Definition: Aerobic

Aerobic is a word that describes a process that involves oxygen.

Equation: Aerobic Respiration

Oxygenglucosecarbondioxidewaterenergy+++

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.

Definition: Exothermic

Exothermic is a word that describes a chemical reaction that releases energy into its surroundings.

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.

Key Term: ATP (Adenosine Triphosphate)

ATP is the molecule the cells use as an accessible energy source. One of the phosphate groups is broken off to release energy that the cell can use to function, which makes ADP. ADP is converted back into ATP by storing some of the energy released in cellular respiration.

Example 2: Identifying the Energy Storage Molecule in the Cell

During the process of aerobic respiration, energy is released from the breakdown of glucose.

This energy is stored in the form of a molecule in the cell. What is the name given to this molecule?

Answer

During aerobic respiration, the atoms in glucose and oxygen are rearranged into molecules of carbon dioxide and water. This process is an exothermic chemical reaction, which means that it releases energy as it progresses. This energy is then transferred to a molecule that the cells use as their available energy source. The molecule that the energy from cellular respiration is transferred to is called adenosine triphosphate, or ATP. ATP is a molecule with three phosphate groups attached to each other. When the cell needs energy to divide, or to make proteins, or to transport materials, one of the phosphate groups is broken off. This converts ATP into ADP, or adenosine diphosphate. During cellular respiration, ADP is converted back into ATP by reattaching a third phosphate group, a process that absorbs energy that the cell will use later.

Using this information, we can conclude that the name of the molecule that stores the energy released in the process of cellular respiration is ATP.

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 CHO6126. The chemical formula for oxygen is O2 because one molecule of oxygen is made of 2 atoms of oxygen bonded together. The chemical formula for water is HO2, 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 CO2 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.

Example 3: Stating the Balanced Chemical Equation for Cellular Respiration

While excluding energy/ATP, state the correct and balanced chemical (symbol) equation for aerobic respiration.

Answer

Cellular respiration is a chemical process that uses glucose and oxygen and rearranges the atoms into molecules of water and carbon dioxide. This complex chemical reaction releases energy that is transferred to a molecule called ATP. The chemicals that go into a chemical reaction are called reactants, and they are placed on the left side of the arrow in a chemical equation. The reactants in cellular respiration are glucose and oxygen. The chemicals that are produced in a chemical reaction are called products, and they are placed on the right side of the arrow in a chemical equation. The products of cellular respiration are carbon dioxide and water. We can represent the chemical equation for cellular respiration using words: glucoseoxygencarbondioxidewater++

If we replace the words with their chemical symbols, we get CHO+OCO+HO6126222

Notice that the question asks for the balanced chemical equation. This means that there must be the same number of carbon, hydrogen, and oxygen atoms on the left and the right side of the arrow. In order to balance this chemical equation, we use coefficients. Coefficients are numbers that are written in front of a chemical symbol, multiplying it. In this case, the coefficient is easy to remember because it is 6 for every component except for glucose. So, the balanced chemical equation for cellular respiration is written as follows: CHO+6O6CO+6HO6126222

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.

Example 4: Determining the Effect of Exercise on the Rate of Cellular Respiration

Fill in the blank using “increases,” “decreases,” or “stays the same”: As the intensity of exercise increases, the rate of cellular respiration .

Answer

Cellular respiration is a chemical process that uses glucose and oxygen and rearranges the atoms into molecules of water and carbon dioxide. This complex chemical reaction releases energy that is transferred to a molecule called ATP. ATP is a molecule with three phosphate groups attached to each other. When the cell needs energy, one of the phosphate groups is broken off. This converts ATP into ADP, or adenosine diphosphate. During cellular respiration, ADP is converted back into ATP by reattaching a third phosphate group, a process that absorbs energy that the cell will use later.

If a person is exercising, their muscle cells are using extra energy to move their body around and to pump their blood faster. This means that the muscle cells need more energy than normal to carry out these functions. The rate of ATP being used would increase as energy needs to increase within the cell. And if more ATP is being used, more ATP has to be continually produced to replace it. Since cellular respiration releases the energy needed to make ATP, the rate of cellular respiration would increase.

As the intensity of exercise increases, the rate of cellular respiration increases.

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

Key Points

  • The reactants of cellular respiration are oxygen and glucose. The products are water and carbon dioxide.
  • Cellular respiration is an exothermic reaction that releases energy.
  • The energy released in cellular respiration is transferred to ATP that provides energy for cellular functions.

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