0610 · 12.3Anaerobic respiration

Anaerobic respiration and energy release

0:00Imagine sprinting to catch a bus, then stopping once you reach it. Your legs are no longer working so hard, but you are still breathing deeply and your heart is still beating quickly. Why does your body keep working after the exercise has stopped? The answer connects the energy your muscles need during the sprint with a process called anaerobic respiration. By the end of this video, you will understand how cells release energy without using oxygen, and why this releases less energy from each glucose molecule.

0:27We will compare what happens in yeast with what happens in your muscles. Then we will follow the recovery process after vigorous exercise, so that oxygen debt becomes something you can explain rather than just a phrase to memorise. Muscle contraction needs energy, so exercising muscles must release energy through respiration. Aerobic respiration uses oxygen and releases a relatively large amount of energy from each glucose molecule. But cells can also release energy through reactions that do not use oxygen.

0:56That alternative is the central idea of this video. Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen. Notice the wording without using oxygen. It tells us what the reactions require, rather than saying that the whole organism must contain no oxygen. During vigorous exercise, some anaerobic respiration can take place in muscles while aerobic respiration is also continuing. Why use this alternative at all?

1:24During a sprint, your muscles need energy very quickly, and the oxygen supplied may not be enough to meet their increased demand through aerobic respiration alone. Anaerobic respiration allows additional energy to be released without using oxygen. It helps the muscles continue contracting, but it releases much less energy per glucose molecule than aerobic respiration. The reason for that lower energy release is that glucose is not broken down as completely.

1:49The products of anaerobic respiration still contain chemical energy that has not been released. Think of it as getting only part of the available energy from each glucose molecule. When comparing the two processes, keep the comparison fair by comparing the energy released from the same amount of glucose.

Anaerobic respiration in yeast

2:06Let us start with yeast, a single celled organism. When yeast respires anaerobically, it breaks down glucose and forms alcohol and carbon dioxide. The alcohol is ethanol, so those two names refer to the same product here. The word equation is glucose gives alcohol plus carbon dioxide. The equation tells us which substance is used and which substances are formed. Glucose is the starting substance, called the reactant, while alcohol and carbon dioxide are the products.

2:34Oxygen is not included because these reactions do not use it. Energy is released during the process, but energy is not a substance that needs a chemical formula in the equation. A familiar example is yeast in bread dough. When yeast respires anaerobically, its carbon dioxide forms bubbles that help the dough rise. That makes one product of the reaction easy to visualise, even though the individual reactions happen inside tiny cells.

2:58For this topic, the important link is yeast breaking down glucose and producing alcohol and carbon dioxide.

Balancing the yeast equation

3:04For the extended syllabus, you also need the balanced chemical equation shown inside the yeast cell. The equation reads C six H twelve O six gives two C two H five O H plus two C O two. It says that one glucose molecule forms two ethanol molecules and two carbon dioxide molecules. The small numbers within each formula describe the atoms in a molecule. The larger numbers in front tell us how many molecules take part.

3:30Let us work through why those larger numbers are both two. A chemical reaction rearranges atoms rather than creating or destroying them, so each type of atom must have the same total on both sides. Start with carbon, because it appears in both products. One glucose molecule contains six carbon atoms, and one ethanol molecule contains two. Two ethanol molecules therefore contain four carbon atoms altogether. Two carbon dioxide molecules contain another two carbon atoms, giving six on the product side.

4:00That matches the six carbon atoms in glucose. We have now checked the carbon balance, but we must still check hydrogen and oxygen before deciding that the equation is balanced. Each ethanol molecule contains six hydrogen atoms, including the hydrogen written at the end of its formula. Two ethanol molecules therefore contain twelve hydrogen atoms, matching glucose. For oxygen, the ethanol molecules contribute two atoms altogether, and the two carbon dioxide molecules contribute four.

4:26That gives six oxygen atoms, so carbon, hydrogen and oxygen all balance. This is why balancing means changing the numbers in front of formulae, not changing the formulae themselves. Changing the formula would change the identity of the substance. Here, two ethanol molecules and two carbon dioxide molecules account for all the atoms from one glucose molecule. The balanced equation is therefore a precise description of the reaction, not just a pattern of numbers to remember.

Muscles and oxygen debt

4:54Now switch from yeast to human muscle cells during vigorous exercise. The starting nutrient is still glucose, but the product is different. The word equation is glucose gives lactic acid. There is no alcohol and no carbon dioxide in this muscle equation, so the yeast equation cannot simply be reused for muscles. Both examples are anaerobic respiration because both release energy from glucose without using oxygen. However, different cells carry out different reactions and produce different substances.

5:23In yeast, the products are alcohol and carbon dioxide. In muscles during vigorous exercise, the product is lactic acid, and the reactions release energy needed for contraction. During vigorous exercise, lactic acid builds up in the muscles and blood, causing an oxygen debt. Oxygen debt refers to the additional oxygen needed after exercise to deal with this build up of lactic acid. It is not a store of missing oxygen trapped inside a muscle.

5:47It is a useful way of describing why an increased oxygen supply is still needed during recovery. After the exercise stops, the demand for muscle contraction falls, but the accumulated lactic acid still needs to be dealt with. The heart rate remains fast, helping transport lactic acid in the blood from the muscles to the liver. Notice the destination, because it is important. For the explanation required here, the liver is where the lactic acid is respired aerobically.

6:14Breathing also remains deeper and faster after exercise. This supplies oxygen for the aerobic respiration of lactic acid. The lactic acid is then respired aerobically in the liver, removing the oxygen debt. So the heart maintains transport, breathing maintains the oxygen supply, and the liver carries out the required aerobic respiration. Those three roles explain why recovery is not instantaneous. Stopping the sprint does not immediately remove the lactic acid that has already built up.

6:41Your circulation and breathing therefore remain above their resting rates while recovery takes place. As the oxygen debt is removed, there is less need for that continued increase in heart rate and breathing.

Exam answers and key mistakes

6:51Let us apply that sequence to an exam style question. The question says, a student finishes a short, vigorous sprint but continues breathing deeply and rapidly. Outline how the oxygen debt is removed after exercise. Before answering, connect the observation to the process. Vigorous exercise can cause lactic acid to build up, so recovery requires its transport and aerobic respiration. The command word outline asks you to give the main features of the process.

7:18Organise your answer around transport, oxygen supply and what happens in the liver. Saying only that the student needs more oxygen leaves out why that oxygen is needed. Saying only that the heart beats faster leaves out what the blood is carrying and where it is going. Here is a concise model answer. The heart rate remains fast to transport lactic acid in the blood from the muscles to the liver.

7:39Breathing remains deeper and faster to supply oxygen for the aerobic respiration of lactic acid. Lactic acid is respired aerobically in the liver, removing the oxygen debt. You could also be asked to state an equation or compare energy release. For an equation, identify the organism or tissue before choosing the products. For a comparison, state that anaerobic respiration releases much less energy per glucose molecule than aerobic respiration. That final phrase matters because it shows that you are comparing equal amounts of the same starting nutrient.

8:11There are a few common mistakes worth clearing up. First, anaerobic does not mean that no energy is released, because releasing energy is the purpose of these reactions. Second, respiration does not make or produce energy, but releases energy from nutrient molecules. Third, you do not need to stop breathing for your muscles to respire anaerobically, because their oxygen supply may simply be insufficient for their increased demand. Another mistake is assuming that all anaerobic respiration has the same products.

8:40Yeast produces alcohol and carbon dioxide, whereas muscles during vigorous exercise produce lactic acid. Finally, oxygen debt is not removed just by taking a few extra breaths without any connection to the blood or liver. The oxygen supply, transport of lactic acid and its aerobic respiration must be linked together. Let us bring the whole idea together. Anaerobic respiration breaks down nutrient molecules in cells to release energy without using oxygen.

9:07It releases much less energy per glucose molecule than aerobic respiration because the breakdown is less complete. Yeast forms alcohol and carbon dioxide, while muscles during vigorous exercise form lactic acid. Lactic acid builds up in the muscles and blood during vigorous exercise, causing an oxygen debt. Afterwards, the continued fast heart rate transports lactic acid to the liver, while deeper and faster breathing supplies oxygen. The lactic acid is respired aerobically in the liver.

9:35That is why your body can still be working hard on recovery even after your sprint has finished.