ICSE Class 7 Biology Chapter 06 Respiration and Transpiration

Respiration and Transpiration

Syllabus

1. The respiratory system in human beings. Test for carbon dioxide (link with Chemistry - class VII) - getting rid of waste carbon dioxide/water vapour through exhalation. Difference between respiration and breathing (inhalation and exhalation).

2. The respiratory organs of some animals - e.g. fish, amphibians, insects.

3. Respiration in plants. Difference between respiration and photosynthesis. Transpiration in plants.

* Exhaled air contains carbon dioxide - breathing into fresh lime water (E).

* Experiment to show that soaked/germinating seeds respire (D).

Introduction to Respiration

Every cell of a plant, an animal or our own body requires energy for various activities. The muscle cells contract for movement, the brain cells receive and send messages, the root cells penetrate the soil and absorb nutrients, and so on. Even when we are sleeping, we need energy. How do we get this energy? We get it through respiration.

The breakdown of glucose occurs by utilising oxygen which we breathe in alongwith air. The plants release oxygen into the atmosphere during photosynthesis.

Some of the energy liberated in the breakdown of the glucose molecule, is in the form of heat, but a large part of it is converted into chemical energy in the form of a chemical substance called adenosine triphosphate (ATP). Any activity inside the cell is carried out by the energy released by this chemical.

What Is Respiration?

Respiration is the process of releasing energy by breaking down food (glucose) needed for various body activities.

Types of Respiration

There are two types of respiration:

(i) Aerobic respiration that utilizes oxygen.

(ii) Anaerobic respiration does not utilize oxygen.

Aerobic Respiration

Usually, all cells throughout life perform aerobic respiration which occurs utilizing oxygen to breakdown the food and get energy. In aerobic respiration, glucose is completely oxidised into carbon dioxide, water and energy. The excess energy gets stored in the cell in the form of ATP.

Aerobic respiration follows the following equation:

\[C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (38 ATP)}\]

(Glucose)

Anaerobic Respiration

Anaerobic respiration occurs usually in intestinal parasites, yeast, most bacteria and in our own muscles under certain special conditions when sufficient oxygen is not available to the body.

Anaerobic respiration follows the following equation:

\[C_6H_{12}O_6 \xrightarrow{\text{Enzymes}} 2C_2H_5OH + 2CO_2 + \text{Energy (2 ATP)}\]

(Glucose) (Ethanol)

In anaerobic respiration the food (glucose) is broken down into ethanol (in plants) or into lactic acid (in animals) and carbon dioxide, but the energy given out is less (only 2 ATP). Differences between aerobic and anaerobic respiration are given below in the table.

Anaerobic respiration in plants, like in the yeast, is also called fermentation. During physical exercise, our muscle tissues respire anaerobically and form lactic acid which accumulates in the muscle cells, causing toxic effect, fatigue and pain. Lactic acid during rest, slowly gets oxidised in the presence of oxygen into carbon dioxide.

Teacher's Note

When you feel tired after running, that's anaerobic respiration at work - your muscles needed energy faster than oxygen could be delivered, so they switched to the less efficient anaerobic pathway, producing lactic acid that makes your muscles ache.

Respiration in Humans

It occurs in three phases: (i) Breathing, (ii) Gaseous transport and (iii) Cellular respiration.

Breathing (External Respiration)

It involves inhalation (drawing in) of the air through the nostrils into the nasal cavity and finally into the lungs, and exhalation is forcing the air out of the lungs.

Gaseous Transport

Oxygen of the inhaled air is absorbed by the blood in the lungs where it combines with the haemoglobin in the red blood cells forming an unstable compound oxyhaemoglobin. The blood carries this oxygen from lungs to the body tissues or cells. It delivers the oxygen there and collects the carbon dioxide from the cells and tissues, transporting it back to lungs for its removal.

Cellular Respiration (Internal Respiration)

It is the process of breakdown of food in the cell with the release of energy. Cellular respiration takes place in the cells of all living organisms. It occurs in a series of chemical reactions. The energy released is in chemical form along with some amount of heat.

Respiratory System in Humans

In humans, the respiratory (breathing) organs include nose, pharynx, larynx, trachea, bronchi and the lungs.

Nose

It has two openings called nostrils, leading into nasal chamber. It has hairy lining to prevent dust particles from reaching into the lungs. The lining of the nasal chamber has mucous (a sticky fluid) which too traps germs and dust. The nasal chamber warms and moistens the air entering the lungs. One should always breathe through the nose and not through the mouth.

Pharynx

From the nose, the air passes into the pharynx or throat which is a common passage for the air and the food, behind the mouth. It leads into the air tube called wind pipe or trachea. Just at the entrance of the trachea, there is a voice box called larynx (or 'Adams apple' - you can feel it by your hand on the front of your neck). It contains two ligamentous folds called vocal cords. Air expelled forcibly through the vocal cords vibrates them to produce sound. The front opening (glottis) of the wind pipe is guarded by a muscular flap called epiglottis (Fig. 6.1). The epiglottis closes the wind pipe at the time of swallowing of food. Incomplete closure by the epiglottis during swallowing causes coughing.

Teacher's Note

When you get food stuck in your windpipe and start coughing, that's your epiglottis doing its job imperfectly - it's trying to protect your lungs but something got through anyway.

Trachea (Wind pipe)

The trachea or the wind pipe emerges from the larynx voice box down below in the neck. It runs in the middle of the chest upto a short distance between the two lungs where it divides into two branches called bronchi (Fig. 6.2). The wall of the trachea is strengthened by C-shaped rings of cartilage to keep it distended permanently.

Bronchi (Singular: bronchus)

The two bronchi lead into right and left lungs respectively. Each bronchus is further divided into smaller and smaller branches called bronchioles. At the end of these bronchioles are the microscopic air sacs called alveoli. Each lung contains millions of alveoli (singular: alveolus) (Fig. 6.3). These are richly supplied with blood capillaries covering their walls. Their walls are extremely thin and moist for allowing diffusion of gases.

Lungs

The lungs are a pair of spongy, elastic organs. They lie in the thoracic cavity, protected by rib cage and rest on the diaphragm. The diaphragm is a muscular sheet which internally divides the body cavity into two cavities - the chest cavity or thoracic cavity in front, and the abdominal cavity behind.

The lungs are protected from outside by two membranes called the outer and inner pleura. The space between the membranes is filled with a fluid. The two lungs are more or less cone shaped, broader at the bottom. The left lung is slightly smaller with two lobes (to accommodate heart in between) and the right lung has three lobes.

The air which reaches the alveoli through respiratory passages (nose, pharynx, trachea, bronchi, bronchioles) is rich in oxygen. So, diffusion of gases occurs through the blood capillaries surrounding the air sacs. The oxygen diffuses into the blood, and is picked up by red blood cells. Carbon dioxide diffuses out into the lungs and is exhaled out through the same passage.

Mechanism of Breathing

The process, by which the air is drawn in from atmosphere into the lungs and forced out from lungs back into the atmosphere, is called breathing. Breathing involves two steps, inhalation (inspiration) and exhalation (expiration).

Inhalation is caused by two agencies - firstly, movement of ribs upwards and outward, and secondly, diaphragm (upwardly arched/dome - shaped) becomes flattened (Fig. 6.4A). The two movements increase the volume of chest cavity, and the lungs expand. Thus, there is a fall in air pressure inside the lungs and it causes the outside air to move inward, getting into the lungs (high pressure to low pressure).

Exhalation involves downward and inward movement of ribs back to their original position on their own and with assistance from rib muscles. Diaphragm muscles relax, moves up and back into its convex (dome) shape partly assisted by the contraction of abdomen. These movements compress the chest cavity and lungs contract (Fig. 6.4B). The pressure on the air inside the lung increases, which forces the air out through the respiratory passages.

Teacher's Note

You can feel your diaphragm working right now by placing your hand on your belly and taking a deep breath - that bulge you feel is your diaphragm moving down to make room for your lungs to expand.

Breathing normally is an involuntary act, which means, it is not under your control. But, breathing can be controlled within certain limits - you can hold your breath for a minute or two, you can have shorter breaths, or you can take deeper breaths.

Activity 1

Exhaled air contains more carbon dioxide than in atmospheric air: Set up an apparatus as shown here - clip 'C' is opened and clip 'D' is closed. Air is sucked in with the mouth through the tube at the centre. Atmospheric air rushes in flask 'A' bubbling through the lime water. Next, clip 'C' is closed and clip 'D' is opened. Now blow the exhaled air through the same central tube. This time the exhaled air is forced into flask 'B' bubbling through its lime water. Repeat this process 8-10 times. The lime water in flask 'B' turns milky much faster than in flask 'A'.

This proves that expired air contains more carbon dioxide than the atmospheric air.

Activity 2

Try It: Put your hands on your chest, and take a gradual deep breath. You can feel the rising of the ribs and bulging of your abdomen. The abdomen bulges due to the lowering of the diaphragm pressing the intestine downwards.

Activity 3

You can easily demonstrate the action of diaphragm. Take a bell-jar as shown in the figure given here. Fix two rubber balloons tied at the ends of two small arms of a Y-shaped glass tube. Insert the long arm of the glass tube through the cork and fix the cork tightly on the mouth of the jar. Tie a sheet of rubber at the wide mouth of the bell-jar.

Now pull the rubber sheet downwards. What happens to the balloons?

Demonstration of breathing process

Activity 4

Record your own breathing rate. Take a wrist watch. Lie down on a bed in a relaxed position and count the number of times your chest rises and falls in a minute. Next, record the breathing rate after running for about ten minutes. Do you find any difference? If yes, can you tell the reason for the difference?

Similarly, you can count the breathing rate of a person while he is sleeping, sitting at rest, and after he has climbed stairs. Likewise, you would find some difference in the breathing rate of a young boy and an old person.

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