0610 · 12.2Aerobic respiration

Aerobic respiration and word equation

0:00When you walk upstairs, your muscles need energy to contract. But the glucose supplied to those muscles cannot simply turn straight into movement. Inside your cells, chemical reactions release energy from it. By the end of this video, you will understand how oxygen is involved, what substances are formed, and why the balanced equation contains the numbers it does. First, we need a small amount of chemistry. A molecule is a group of atoms joined together, and glucose is a nutrient molecule containing carbon, hydrogen and oxygen atoms.

0:28During a chemical reaction, atoms are rearranged into different substances. The substances we start with are called reactants, and the substances formed are called products. Here is the definition to know. Aerobic respiration is the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy. Notice that this tells us where the reactions happen, what they use, and what they achieve. They happen in cells, use oxygen, and release energy by breaking down nutrient molecules.

0:57For the equations in this topic, the nutrient molecule is glucose. Glucose stores chemical energy, and aerobic respiration releases some of that energy in a form the cell can use. Oxygen is needed for this process, but oxygen is not the fuel supplying that stored energy. Think of glucose as the fuel and oxygen as a necessary reactant that allows the energy releasing process to happen. This also explains why respiration is not another word for breathing.

1:21Breathing moves air into and out of the lungs, whereas aerobic respiration is a set of chemical reactions inside cells. Getting oxygen into the body helps supply those reactions, but it is not the reaction itself. You will see how oxygen enters the blood in the video on gas exchange in humans. The word equation for aerobic respiration is glucose plus oxygen gives carbon dioxide plus water. Glucose and oxygen are the reactants, so they belong before the arrow.

1:47Carbon dioxide and water are the products, so they belong after it. The arrow means that the reactants are changed into the products through chemical reactions. That equation summarises the overall change, rather than showing every individual reaction. Inside the cell, the change happens through a series of controlled steps, not one sudden burst like burning a sugar cube. Energy is released during the overall process, but energy is not a substance made from atoms.

2:12That is why the syllabus word equation lists carbon dioxide and water as the products, while the definition explains the release of energy.

Balancing the respiration equation

2:19Now let us connect those words to chemical formulae. Glucose is written as C six H twelve O six, meaning one glucose molecule contains six carbon atoms, twelve hydrogen atoms and six oxygen atoms. Oxygen gas is O two, because each oxygen molecule contains two oxygen atoms. Carbon dioxide is C O two, and water is H two O, so their formulae tell us their atom counts too. Simply replacing the names with formulae does not yet give a balanced equation.

2:46The reason is that chemical reactions rearrange atoms, and they do not make atoms appear or disappear. We therefore need the same number of atoms of each element on both sides. Let us build the balanced equation step by step, using one glucose molecule as our starting point. Begin with carbon, because all six carbon atoms on the left are in glucose. Each carbon dioxide molecule contains one carbon atom, so we need six carbon dioxide molecules on the right.

3:12We write a six in front of carbon dioxide to show that. A number in front multiplies the whole formula, so six carbon dioxide molecules also contain twelve oxygen atoms. Next, balance hydrogen. One glucose molecule contains twelve hydrogen atoms, and each water molecule contains two hydrogen atoms. Twelve divided by two is six, so we need six water molecules. We now have six carbon dioxide molecules and six water molecules on the product side, which together contain eighteen oxygen atoms.

3:41Finally, balance oxygen. The glucose already supplies six oxygen atoms, but the products contain eighteen, leaving twelve more oxygen atoms to account for. Each oxygen molecule supplies two, so twelve divided by two gives six oxygen molecules. That is why the balanced equation needs a six in front of oxygen, rather than twelve. The balanced chemical equation is C six H twelve O six plus six O two gives six C O two plus six H two O.

4:07Check it by counting atoms on both sides, which gives six carbon atoms, twelve hydrogen atoms and eighteen oxygen atoms. Notice that we changed the numbers in front of the formulae, not the formulae themselves. Changing a formula would change the identity of the substance, rather than balance the reaction.

Exam questions and recap

4:23Here is an exam style question using that reasoning. A student writes the glucose formula correctly but puts three oxygen molecules on the left, with six carbon dioxide molecules and six water molecules on the right. Explain why the equation is not balanced and state the correct number of oxygen molecules. First count oxygen on the left, where six atoms in glucose plus six in those three oxygen molecules gives twelve.

4:46Now count oxygen on the right, where twelve atoms in carbon dioxide plus six in water gives eighteen. A precise answer would be that there are twelve oxygen atoms on the left and eighteen on the right. The equation is not balanced because the numbers of oxygen atoms are unequal. The correct number of oxygen molecules is six. Another question might ask you to describe aerobic respiration. Use the definition directly, which is that aerobic respiration is the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy.

5:16If asked to state the word equation, give glucose plus oxygen gives carbon dioxide plus water. If asked to state the balanced chemical equation, include all three sixes in front of oxygen, carbon dioxide and water. Watch out for two other misconceptions. Respiration releases energy, and it does not produce or make energy, because the energy was already stored chemically in the nutrient molecules. Also, aerobic respiration is not limited to animal cells, because living plant cells carry it out too.

5:44The word aerobic tells you that oxygen is used, not that the organism must have lungs. Let us bring everything together. Aerobic respiration uses oxygen to break down nutrient molecules in cells and release energy, with glucose as the nutrient in our equations. Glucose and oxygen are changed into carbon dioxide and water through a series of chemical reactions. The balanced equation shows one glucose molecule reacting with six oxygen molecules to form six carbon dioxide molecules and six water molecules, conserving every atom.