Respiration can, however, also take place in the absence of oxygen (of air), though it is very rare. This means that oxidation of food to obtain energy can occur in the presence of oxygen as well as in the absence of oxygen. Based on this, we have two types of respiration : aerobic respiration and anaerobic respiration.
1. Aerobic Respiration
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The respiration which uses oxygen is called aerobic respiration. It is called aerobic respiration because it uses air which contains oxygen (‘aerobic’ means ‘with air’). In aerobic respiration, the glucose food is – completely broken down into carbon dioxide and water by oxidation. Aerobic respiration produces a considerable amount of energy for use by the organism which gets stored in the ATP molecules. The breaking down of glucose (food) during aerobic respiration (which is carried out by most of the organisms) can be represented as follows:
Please note that during aerobic respiration (shown above), 1 molecule of glucose (food) produces 38 energy-rich ATP molecules (Please do not worry about the name ‘Kreb’s cycle written in the above equation. We will study this in higher classes). All the organisms which obtain energy by aerobic respiration, cannot live without oxygen (of air). This is because if there is no oxygen, they cannot get energy from the food which they eat. Mitochondria are the sites of aerobic respiration in the cells. Thus, the breakdown of pyruvate to give carbon dioxide, water and energy takes place in mitochondria.
The energy released during aerobic respiration is used by the organism. Most of the living organisms carry out aerobic respiration (by using oxygen of air). For example, humans (man), dogs (see Figure 46), cats, lions, elephants, cows, buffaloes, goat, deer, birds, lizards, snakes, earthworms, frogs, fish, and insects (such as cockroach, grasshopper, houseflies, mosquitoes and ants, etc.) and most of the plants carry out aerobic respiration by using oxygen of air (to obtain energy)
2. Anaerobic Respiration
The respiration which takes place without oxygen is called anaerobic respiration. It is called anaerobic respiration because it takes place without air which contains oxygen (‘anaerobic’ means ‘without air’). The microscopic organisms like yeast and some bacteria obtain energy by anaerobic respiration (which is called fermentation). In anaerobic respiration, the micro-organisms like yeast break down glucose (food) into ethanol and carbon dioxide, and release energy. This energy is then used by the micro-organisms. Anaerobic respiration produces much less energy which gets stored in the ATP molecules. The breaking down of glucose (food) during anaerobic respiration carried out by yeast (plants) can be represented as follows:
Please note that during anaerobic respiration (shown above), 1 molecule of glucose (food) produces only 2 energy-rich ATP molecules. A few organisms such as yeast plants and certain bacteria (called anaerobic bacteria) can obtain energy from food in the absence of oxygen by the process of anaerobic respiration. Please note that all the organisms which obtain energy by anaerobic respiration can live without oxygen (of air). For example, the single-celled, non-green plant called ‘yeast can live without oxygen because it obtains energy by the process of anaerobic respiration. From this discussion we conclude that all the cells do not use oxygen to produce energy. Energy can be produced in cells even without oxygen. Please note that the whole process of anaerobic respiration takes place in the cytoplasm of cells.
We can carry out the fermentation of sugar by using the anaerobic respiration of yeast as follows: Take some sugar solution (or fruit juice) in a test-tube and add a little of yeast to it. Close the mouth of the test-tube with a cork and allow it to stand for some time. Now, open the cork and smell. A characteristic smell of ethanol (ethyl alcohol) is obtained from the test-tube. A gas is also evolved during this process. When this gas is passed through lime-water, the lime-water turns milky showing that it is carbon dioxide gas. This experiment tells us that the products of fermentation of sugar brought about by yeast are ethanol and carbon dioxide.
we (the human beings) obtain energy by aerobic respiration. But anaerobic respiration can sometimes take place in our muscles (or the muscles of other animals). For example, anaerobic respiration takes place in our muscles during vigorous physical exercise when oxygen gets used up faster in the muscle cells than can be supplied by the blood. When anaerobic respiration takes place in human muscles (or animal muscles), then glucose (food) is converted into lactic acid with the release of a small amount of energy. The breaking down of glucose (food) during anaerobic respiration in muscles can be represented as follows:
The sudden build up of lactic acid in our muscles during vigorous physical activity can cause muscular ‘cramps’. (The painful contractions of muscles are called cramps). Let us discuss this in a little more detail. During heavy physical exercise (or any other heavy physical activity), most of the energy in our muscles in produced by aerobic respiration. Anaerobic respiration in muscles provides only some extra energy which is needed under the conditions of heavy physical activity (like running very fast or running for a long time) (see the people running a long distance race).
During vigorous physical exercise (such as running a long distance race) leg muscles respire anaerobically (without oxygen) to produce extra energy. The man on the right side in the above picture has just got a cramp in the leg due to the accumulation of lactic acid in the leg muscles produced by the anaerobic respiration which took place in leg muscles during marathon (a long-distance running race).
The anaerobic respiration by muscles brings about partial breakdown of glucose (food) to form lactic acid. This lactic acid accumulates in the muscles. The accumulation of lactic acid in the muscles causes muscle cramps. Thus, muscle cramps occur due to the accumulation of lactic acid in muscles when the muscles respire anaerobically (without oxygen) while doing hard physical exercise. We can get relief from cramps in muscles caused by heavy exercise by taking a hot water bath or a massage. Hot water bath (or massage) improves the circulation of blood in the muscles. Due to improved blood flow, the supply of oxygen to the muscles increases. This oxygen breaks down lactic acid accumulated in muscles into carbon dioxide and water, and hence gives us relief from cramps. The anaerobic respiration does not take place only in the muscles of human beings, it also takes place in the muscles of other animals such as lion, tiger, cheetah, deer, and many other animals when they run very fast and require much more energy than normal.
Just imagine what would happen if the fast running deer trying to escape from the clutches of cheetah gets a leg cramp at this very moment (due to accumulation of lactic acid in its leg muscles produced during anaerobic respiration). The cheetah is sure to get enough lunch or dinner!
This means that even the animals like lion, tiger, cheetah and deer, etc., can get leg cramps due to the accumulation of lactic acid in leg muscles if they run very fast for a considerable time. Please note that:
(a) the anaerobic respiration in plants (like yeast) produces ethanol and carbon dioxide as end products.
(b) the anaerobic respiration in animal muscle tissue produces lactic acid as the end product.
The similarity between aerobic respiration and anaerobic respiration is that in both the cases, energy is produced by the breakdown of food like glucose. The main differences between aerobic respiration and anaerobic respiration are given below.
Differences between Aerobic and Anaerobic Respiration
| Aerobic respiration | Anaerobic respiration |
| 1. Aerobic respiration takes place in the presence of oxygen. | 1. Anaerobic respiration takes place in the absence of oxygen. |
| 2. Complete breakdown of food occurs in aerobic respiration. | 2. Partial breakdown of food occurs in anaerobic respiration. |
| 3. The end products in aerobic respiration are carbon dioxide and water. | 3. The end products in anaerobic respiration may be ethanol and carbon dioxide (as in yeast plants), or lactic acid (as in animal muscles). |
| 4. Aerobic respiration produces a considerable amount of energy. | 4. Much less energy is produced in anaerobic respiration. |
Let us answer one question now.
Question: The breakdown of pyruvate to give carbon dioxide, water and energy takes place in :
(a) cytoplasm
(b) mitochondria
(c) chloroplast
(d) nucleus
Answer: (b) mitochondria
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