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Samacheer Kalvi 8th Science Books Organisation of Life

Science : Chapter 18 : Organisation of Life


I.  Choose the best answer.

1. _____________ is tough and thick white sheath that protects the inner parts of the eye.

a) Sclera

b) Conjunctiva

c) Cornea

d) Iris

[Answer: (a) sclera]

2. ______________ cells are specialised cells that can be transformed into any kind of cells.

a) Nerve 

b) Stem 

c) Heart 

d) Bone

[Answer: (b) Stem]

3. Maintenance of constant internal environment of the body is known as

a) homeostasis

b) homeophytes

c) homeokinesis

d) homeophilics

[Answer: (a) Homeostasis]

4. In the absence of oxygen, glucose is broken down in to ______________.

a) lactic acid

b) citric acid

c) acetic acid

d) nitric acid

[Answer: (a) Lactic acid]

5. The process of air passing in and out the lungs is called ______________.

a) inhalation     

b) exhalation

c) breathing 

d) None of these

[Answer: (c) Breathing]

6. Osmosis isthe movement of water molecules from

a) higher concentration to a region of lower concentration.

b) lower concentration to a region of higher concentration.

c) Both of these 

 d) None of these

[Answer: (a) Higher concentration to a region of lower concentration]

7. The erythrocyte is placed in ______________solution which has lesser concentration of solutes and greater concentration of water than in the cytoplasm.

a) hypotonic

b) hypertonic

c) neutral

d) acidic

[Answer: (a) Hypotonic]

II. Fill in the blanks.

1. Cell is the structural and functional unit of living organisms.

2. The largest cell is egg of an ostrich.

3. Fermentation is a good example for anaerobic respiration.

4. Optic nerve is located at the end of the eyes behind the retina.

5. The size of the cells are measured in units of microns.

III. Match the following.

1. Carbohydrates- CO2, Water and Heat

2. Glucose-Amino acid

3. Protein- Glucose

4. Amino acids-Cholesterol and other steroid

5. Fatty acids- Enzymes, hormone, protein

[Answer: 1 – c, 2 – a, 3 – b]

1. Carbohydrates – (c) glucose

2. Glucose – (a) CO2, water and heat

3. Protein – (b) amino acid

4. Amino acids

5. Fatty acids

IV. State true or false. If false, correct the statement.

1. In hypotonic condition, concentration of the external and the internal solution of the organism are same . Answer: False.

Correct statement: The concentration of external solution is less compared to concentration of internal solution of the organism.

2. Diffusion is the movement of particles from an area of lower concentration to higher concentration . Answer: False.

Correct statement: Diffusion is the movement of particles from an area of higher concentration to lower concentration.

3. Human beings are warm blooded in nature. Answer: True.

4. The larynx has fold of tissue which vibrate with the passage of air to produce sound. Answer: True.

5. Aqueous humour plays an important role in maintaining the shape of the eye. Answer: True.

V. Answer very briefly.

1. What is cell differentiation?

Answer: Our body develops from a single cell called zygote. The zygote undergoes mitotic division to form many cells of different shape, size and content. These cells attain change in structure and function which is called differentiation. This form the foetus.

2. State different types of tissues.

Answer: Depending on the basis of their structure and function, tissues can be classified into four types.

(i) Epithelial (covering) tissue for protection. 

(ii) Muscular (contractile) tissue for movements and locomotion.

(iii) Connective (supporting) tissue for binding different structures of body.

(iv) Nervous tissue for conduction of nerve impulses.

3. Mention the function of ‘Alveoli’.


(i) Alveoli are tiny air sacs in the lungs.

(ii) They are the workhouses of the respiratory system.

(iii) The exchange of oxygen and carbon dioxide take place in alveoli of the lungs.

 4. Name the processes by which air enters and comes out of our lungs.


(i) The process of taxing air into the lungs is called inspiration or inhalation.

(ii) The process of expelling air from the lungs is called expiration or exhalation.

 5. Differentiate osmoconformers and osmoregulators.


There are two major types of Osmoregulation :


These organisms try to maintain the osmolality of their body matching with their surroundings. Most of the invertebrates, marine organisms are osmoconformers.


These organisms maintain their internal osmolality, which can be extremely different from that of the surrounding environment, through physiological processes.

 6. Define – Metabolism.


(i) Metabolism is the sum of chemical reactions by which living organisms sustain their life.

(ii) Metabolism consists of anabolism (the buildup of substances) and catabolism (the breakdown of substances).

VI. Answer briefly.

1. Define – Prokaryotic cell.

Answer: Organisms in which no true nucleus is seen are called prokaryotic. Ex : Bacteria.

2. Tabulate the differences between aerobic and anaerobic respiration.



1. Aerobic respiration takes place in the presence of oxygen

2. The end products of aerobic respiration are carbon dioxide and water

3 Common in all higher plants and animals


1. Anaerobic respiration takes place in the absence of oxygen

2. The end products of anaerobic respiration are COand ethanol or lactic acid

3. Common in certain micro organisms and human muscle cell

3. Why the human eye is compared with camera?

Answer: The human eye can be compared to a camera as both functions by gathering, focusing, and transmitting the light through the lens for creating an image of an object.

(i) The iris of the eye controls the size of the pupil depending on the amount of light entering it.

(ii) The pupil is like the eyehole of a camera which allows light to come in.

(iii) Lens : It is a transparent, biconvex, and an adjustable part of an eye, made up of protein. The lens with the help of the cornea refracts light focused on the retina, therefore creating images on it.

(iv) The retina consists of photoreceptors and converts light rays into impulses to be sent to the brain. It is light sensitive.

The retina is compared to the film in a camera.

4. Which organ and organ system help to maintain homeostasis?


(i) Homeostasis is a property of a human biological system where the self-regulating process tends to maintain the balance for the survival.

(ii) Behavioural and physiological responses are two important regulating mechanisms that maintain the stability of homeostasis.

(iii) All the processes of integration and co-ordination of function are mediated by nervous and hormonal system. The liver, kidneys, and brain (hypothalamus), autonomic nervous system and the endocrine system help to maintain homeostasis.

Define Eukaryotic cell?

Answer: Organisms in which true nucleus is seen (presence of nuclear membrane) are called eukaryotic. Ex : Higher plants.

VII. Answer in detail.

1. Draw the struture of human eye and label its parts.


2. Explain osmosis with an example.

Answer: Osmosis is the movement of solvent particles across a semipermeable membrane from a dilute solution into a concentrated solution. The solvent moves to dilute the concentrated solution and equalize the concentration on both sides of the membrane.

The movement of liquids in and out cells is dependent on the concentration of the solution surrounding it. There are 3 types of situations in which this could vary:

(i) Isotonic : Here the concentration of external and internal solution of the organism are the same.

(ii) Hypotonic: Here the external solution concentration is less compared to the concentration of the inner solution of an organism. In this case water will rush into the organism.

(iii) Hypertonic: Here the external solution concentration is greater than the concentration of the inner solution of an organism. In this case the water will rush out of the organism.

Ex : Red blood cells.

(1) When red blood cells are placed in hypertonic solution, water flows out of the cell faster than it comes in. This results in shrivelling of RBC.

(2) On the other hand if RBC is hypotonic, more water will flow into the cell than out. This results in swelling of the cell and followed by bursting.

(3) If the RBC is placed in an isotonic solutions, the flow of water in and out of the cell will happen at the same rate.

3. Differentiate between inhalation and exhalation.



1. The muscles of the diaphragm contract.

2. The diaphragm goes downward.

3 The ribs move upwards and outwards.

4. The volume of thoracic (chest) cavity increases.

5. Air enters the lungs through the nose.


1. The muscles of the diaphragm relax.

2. The diaphragm goes upward.

3. The ribs move downwards.

4. The volume of thoracic (chest) cavity decreases.

5. Air goes out of the lungs through the nose.

4. List out the different types of metabolism with an example.


(i) Metabolism is the sum of chemical reactions by which living organisms sustain their life.

(ii) Metabolism consists of anabolism (the buildup of substances) and catabolism (the breakdown of substances).

Anabolism :

Anabolism or constructive metabolism, is all about building and storing: It supports the growth of new cells, the maintenance of body tissues, and the storage of energy for use in the future. During anabolism, small molecules are changed into larger, more complex molecules of carbohydrate, protein and fat.

For example,

Glucose → Glycogen and other sugars

Amino acids → Enzymes, hormones and proteins

Catabolism :

Catabolism or destructive metabolism, is the process that produces the energy required for all activity in the cells. In this process, cells break down large molecules (mostly carbohydrates and fats) to release energy. This energy release provides fuel for anabolism, heats the body, and enables the muscles to contract and the body to move.

Carbohydrates → Glucose

Glucose → CO2, Water and heat

5. Explain the mechanism of breathing.


(i) The process of taking air into the lungs is called inspiration or inhalation.

(ii) During inspiration, the sternum is pushed up and outward and the diaphragm is pulled down.

(iii) This increases the volume of the thoracic cavity and the pressure decreases.

(iv) The air outside the body flows into the lungs. Here exchange of gases takes place between the air and the blood.

(v) The process of expelling air from the lungs is called expiration or exhalation.

(vi) Upon exhalation, the lungs recoil to force the air out of the lungs.

(vii) The intercostal muscles relax, returning the chest wall to its original position. During exhalation, the diaphragm also relaxes, moving higher into the thoracic cavity.

(viii) This increases the pressure within the thoracic cavity relative to the environment.

(ix) Air rushes out of the lungs due to the pressure gradient. This movement of air out of the lungs is a passive event.

VIII. Higher Order Thinking Questions.

1. Why do we need instant energy? Does glucose give that energy? Explain.


(i) Energy is needed for performing day to day activities of the body which is got through intake of food. This provides energy for all organ systems.

(ii) Instant energy may be required in cases of extended physical activities like running or physical ailments like tiredness or giddiness.

(iii) Glucose is the simplest form of carbohydrate. Intake of glucose help it to solubilise in the blood immediately, and is carried to organs of the body thus helping to provide instant energy digestion of carbohydrates also converts it to glucose finally.

 2. How are we preparing pickles? What are the steps involved in that?


(i) Pickles are prepared usually by addition of excess salt. The salty solution creates a high concentration (hypertonic) in the external medium of the vegetable / fruit which is used for making pickles.

(ii) Therefore water comes out of the vegatable and it undergoes plasmolysis and begins to shrink.

(iii) Thus when water content is lost the pickle is able to retain its shelf life for a longer period.

IX. Value based questions.

1. Dr. Usha is a pulmonologist (Doctor for respiratory diseases). One day, a school student named Arjun, met her with respiratory problems. After diagnosis, the doctor advised him to go to playground daily and play football or basketball. She also advised him to do pranayamam in the morning.

a) Why did the doctor advise him to go to the playground?

b) What is the use of pranayamam?


(a) Playing is a good physical activity which helps to improve breathing and blood circulation in the body. It also helps to relieve anxiety.


(1) Pranayamam teaches us the proper way of breathing, slowly and deeply.

(2) It increases the capacity of the lungs and brings more oxygen into the body.

(3) It is especially very useful when one has respiratory problems and the breathing is irregular and unsteady.

(4) It improves blood circulation.

2. Explain why you are not able to breathe normally when you are in closed and crowded places?


In a closed and crowded place, the number of people are more. All of them breathe out carbon dioxide. Therefore the amount of CO2 in the air is much more than the amount of oxygen available for inhalation. Therefore we find it difficult to breathe in a closed and crowded place.

3. Shylesh is a school going kid studying standard VIII. He is crazy about playing video games in mobile phones. After couple of months, his eyes turned red and he felt severe pain in his eyes. His science teacher enquired about this and advised his parents to take him to an eye doctor.

 i) How does excessive usage of mobile phone affect our eyes?

ii) What are the values shown by the teacher?


Impact of excessive usage of mobile phones :


(1) Cell phone radiation can damage eyes and cause early cataract.

(2) It can also lead to cataract in lens apart from affecting retina, cornea etc.

(3) It strains the eye muscles.

(4) It also caused temporary problems like dry and itchy eyes, blurry vision, pain in eyes etc.


The teacher has shown values of

(1) Empathy

(2) Responsibility

(3) Personal care.

Student Activities

Activity 1

Boil a hen’s egg and remove the shell. What do you observe? A white material surrounds the yellow part. White material is albumin which solidifies on boiling. The yellow part is yolk. It is a part of the single cell. You can observe this single cell without any magnifying devices.

Activity 2

Stand erect and wave your hands in side wards. Take a deep breath and feel your rib movements. Then run some 100 metres and observe the rib movements. Discuss in the class room about what you observed.

Activity 3

Constructing a model of lungs.

Materials required

Y shaped tube, a large balloon, two small balloons, a one litre plastic bottle, cork.

Method of Construction

Cut off the plastic bottle in the middle. Fix two small balloons in both the ends of the Y-tube. Make a hole in the cork and fix the y-tube. Make a small hole in the cork and fix the y-tube through the hole as shown in the picture. Cut a large balloon into two halves and fix one half tightly around the open part of the bottle.

Method of Working

Hold the large balloon in the middle and pull it slowly downwards as shown in the picture. Observe the change in the balloons inside the bottle. Now leave the balloon free.

Our body is developed from a single cell called zygote. The Zygote undergoes continuous mitotic division and forms the foetus ­consisting multitude of cells of different shape, size and content. Foetal cells gradually attain change in structure and function. This process is known as cell differentiation.

On an average, an adult humanbeing at rest breathes in andout 15 – 18 times in a minute­. During heavy exercise, breathing rate can increase upto 25 times per minute.

Smoking damages lungs. Smoking is also linked to cancer. It must be avoided.

When you sneeze, you should cover your nose so that the foreign particles you expel are not inhaled by others.The mixing of foodstuffs and digestive juices in the gut occurs by diffusion. Exchange of respiratory gases (Oxygen and Carbondioxide) between blood and tissue fluids between tissue fluid and cells also occurs by diffusion.

More to know

Aerobic respiration releases 19 times more energy than anaerobic respiration from the same amount of glucose. In aerobic respiration each glucose molecules produce 36 ATPs.

More to know

Basal metabolism refers to the minimum energy required to maintain the normal activities of the body during complete rest in a warm atmosphere, 12 – 18 hours after the intake of food.

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