ICSE Class 9 Biology Chapter 14 The Respiratory System

The Respiratory System

Syllabus: Respiratory System: Organs; mechanism of breathing; tissue respiration, heat production. Structures of the respiratory system. Differences between anaerobic respiration in plants and in man. Role of diaphragm and intercostal muscles in breathing to provide a clear idea of breathing process. Brief idea of gaseous transport and tissue respiration. Brief understanding of respiratory volumes. Effect of altitude on breathing: asphyxiation and hypoxia.

14.1 The Need For Respiration

Respiration is the chemical process of releasing energy by breaking down glucose for carrying out life processes.

This chemical breakdown occurs by utilizing oxygen and is represented by the following overall reaction:

\[C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}\]

There are five important points to remember about this chemical reaction in respiration.

1. This part of respiration, yielding energy, occurs inside the living cells and hence, it is better known as cellular or tissue respiration.

2. The breakdown of glucose (C₆H₁₂O₆) to carbon dioxide and water does not occur in a single step but in a series of chemical steps. Some of these steps occur in the cytoplasm of the cell and some inside the mitochondria.

3. Each breakdown step is due to a particular enzyme.

4. The energy liberated in the breakdown of the glucose molecule is not all in the form of heat, but a large part of it is converted into chemical energy in the form of ATP - a chemical substance called adenosine triphosphate. (As you have read in the case of plants when the energy in the form of ATP is used, it is converted to ADP (adenosine diphosphate) and again when more energy is available by further breakdown of glucose, the ADP is recovered to ATP and so it goes on. ATP is a sort of "currency of energy" inside the cell. One mole of glucose on complete oxidation yields 38 molecules of ATP.)

5. The essential steps of cellular respiration are same in plants and animals.

Why We Need Energy

Body cells need energy for a vast variety of activities in them. Some of these are:

1. Synthesis of proteins from amino acids

2. Production of enzymes

3. Contraction of muscles for movement

4. Conduction of electrical impulse in a nerve cell

5. Production of new cells by cell division

6. In keeping the body warm (in warm-blooded animals, i.e. birds and mammals)

14.2 Animals Need More Energy

The need for production of energy is greater in animals than in plants. This is because animals consume more energy in doing physical work.

They have to move about for obtaining food or run away to escape enemies.

They have to chew their food and have to look after their eggs or young ones, and so on.

Birds And Mammals Need Still More Energy

The birds and mammals including ourselves have also to produce a lot of heat for keeping the body warm. This heat comes through respiration in the cells. The amount of heat to keep the body warm is quite large. Think about the cold winter days when the outside temperature is far below our body temperature. We are constantly losing heat to the outside air, and more of it has to be continuously produced to make up the loss. Liver cells in particular produce much heat, and the muscle cells also contribute to it.

Shivering and clattering of teeth (when one feels too cold) is an emergency activity of the muscle cells to produce extra heat.

The energy used in all the cellular activities is obtained from the oxidation of glucose (C₆H₁₂O₆), a carbohydrate.

14.3 Glucose Has No Alternative For Respiration

If the simple carbohydrate (glucose) is not available directly, the cells may break down the proteins or fats to produce glucose for respiratory needs.

Think for a while about the wild animals which are totally flesh-eaters. The main constituent of their diet is protein with very little carbohydrates. The excess amino acids absorbed through protein digestion are broken down in the liver to produce sugar (glucose) and the nitrogenous part is converted into urea which gets excreted out. The glucose thus produced may be used immediately or may get stored in the liver cells as glycogen for future needs. A similar process takes place in humans if they take excessively protein-rich food.

Progress Check

1. Given below is the definition of respiration with a few blanks to be filled in. Write suitable words in the blanks:

Respiration is a process of releasing _____ by breaking down _____ for carrying out _____.

2. Write the overall chemical equation representing the above definition of respiration.

3. In what form is the energy liberated in respiration?

4. Give two examples of life activities which need energy.

14.4 Two Kinds Of Respiration - Aerobic And Anaerobic

In animals there is normally aerobic respiration using oxygen. Anaerobic respiration (in the absence of oxygen) is only exceptional in some cases as in the tapeworms living inside the human intestines.

Anaerobic respiration may occur even in our own body in the fast-working skeletal muscles during continuous physical exercise as in fast running, walking over long distances, swimming, wrestling, weight-lifting, etc., our muscles work too fast but not getting enough oxygen. In this situation, the muscles are working in the absence of oxygen (anaerobic respiration) to provide extra energy. The product of anaerobic respiration in such muscles is lactic acid. Accumulation of lactic acid gives the feeling of fatigue. This is a condition which may be called oxygen-debt. When you rest after such exercise, the lactic acid gets slowly oxidised by the oxygen later available and then the "debt is cleared" producing carbon dioxide in the process.

Chemical Steps In Respiration

Aerobic Respiration In Animals

The chemical changes taking place in aerobic respiration in animals are the same as in the aerobic respiration in plants. The overall chemical change can be represented by the equation:

\[C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + 686 \text{ kcal/mole}\]

Glucose - Oxygen - Carbon dioxide - Water - Energy

The above equation depicts the chemical substances in mole. Thus by taking 180 g of glucose the energy released is 686 kilocalories. If expressed in kJ (kilojoules) the energy released is about 2890 (686 × 4.2) kJ.

In the above equation we can represent energy in the form of ATP as follows:

\[C_6H_{12}O_6 + 6O_2 \xrightarrow{\text{enzymes}} 6CO_2 + 6H_2O + 38\text{ATP} + 420 \text{ kcal}\]

(One mole of ATP requires 7 kcal, so 38 ATP are produced by consuming 38 × 7 = 266 kcal, the rest of the energy i.e. 686 - 266 = 420 kcal is released as heat.)

Anaerobic Respiration In Animals

In animal cells, particularly in the skeletal muscle cells, anaerobic respiration may occur when they have to work very fast with insufficient oxygen. The overall chemical reaction in anaerobic respiration is summarised as follows:

\[C_6H_{12}O_6 \xrightarrow{\text{enzymes}} \text{lactic acid} + 2\text{ATP} + \text{heat energy}\]

Special points to note in the above chemical reaction in anaerobic respiration in animals, are as follows:

1. It is a slow process.

2. The reaction cannot continue for long time. The product lactic acid has a toxic effect on cells, which causes muscle fatigue and aches.

3. No CO₂ is produced.

4. Total energy released per mole of glucose is much less compared to aerobic respiration.

The basic steps in cellular respiration are same in plants and animals. However, the anaerobic respiration is different in the two in some respects.

14.5 Parts Of Respiration

In humans (as in most other animals) there are four major parts of respiration:

1. Breathing: This is a physical process in which the atmospheric air is taken in and forced out of the oxygen-absorbing organs, the lungs.

2. Gaseous Transport: The oxygen absorbed by the blood in the lungs is carried by the RBCs as oxyhaemoglobin throughout the body by means of arteries. The carbon dioxide from the tissues is transported to the lungs by the blood by means of veins in two ways:

(i) as bicarbonates dissolved in plasma, and partly,

(ii) in combination with the haemoglobin of RBCs as carbamino-haemoglobin.

3. Tissue Respiration: The terminal blood vessels, i.e., the capillaries deliver the oxygen to the body cells or tissues where oxygen diffuses through their thin walls and in a similar way, the capillaries pick up the carbon dioxide released by them (Fig. 14.1).

4. Cellular Respiration: The complex chemical changes which occur inside the cell to release energy from glucose.

Where Does Respiration Occur In A Cell?

The cellular respiration occurs in two main phases at two different locations inside the cell:

1. Glycolysis (breakdown of glucose):

- occurs in cytoplasm outside the mitochondria

- breakdown into pyruvic acid which further breaks down into ethanol in plants and lactic acid in animals.

- anaerobic (not requiring oxygen)

- very little energy released.

2. Krebs Cycle:

- occurs inside mitochondria

- needs oxygen

- step by step break down of pyruvic acid/lactic acid to produce ATP and CO₂

- H+ ions released in the cycle are removed through the oxygen supplied by forming H₂O.

- much energy produced

- So now you know what for our body needs oxygen - Yes, to remove the H+ ions.

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