Samacheer Class 8 Science - Organisation of Life

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.

Depending on the basis of their structure and function, tissues can be classified into four types.
Epithelial (covering) tissue for protection.
Muscular (contractile) tissue for movements and locomotion.
Connective (supporting) tissue for binding different structures of body.
Nervous tissue for conduction of nerve impulses.

Alveoli are tiny air sacs in the lungs.
They are the workhouses of the respiratory system.
The exchange of oxygen and carbon dioxide take place in alveoli of the lungs.

There are two major types of Osmoregulation:
Osmoconformers:
These organisms try to maintain the osmolality of their body matching with their surroundings. Most of the invertebrates, marine organisms are osmoconformers.
Osmoregulators:
These organisms maintain their internal osmolality, which can be extremely different from that of the surrounding environment, through physiological processes.

Metabolism is the sum of chemical reactions by which living organisms sustain their life.
Metabolism consists of anabolism (the buildup of substances) and catabolism (the breakdown of substances).

S. No
Aerobic
Anaerobic
1.
Aerobic respiration takes place in the presence of oxygen
Anaerobic respiration takes place in the absence of oxygen
2.
The end products of aerobic respiration are carbon dioxide and water
The end products of anaerobic respiration are CO 2 and ethanol or lactic acid
3
Common in all higher plants and animals
Common in certain micro organisms and human muscle cell

The epithelial tissue is a tissue which acts as a covering and protects the underlying tissues. It is classified into the following types.
Squamous epithelium.
Cuboidal epithelium.
Columnar epithelium.
Ciliated epithelium.
Glandular epithelium.

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.
The iris of the eye controls the size of the pupil depending on the amount of light entering it.
The pupil is like the eyehole of a camera which allows light to come in.
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.
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.

Homeostasis is a property of a human biological system where the self – regulating process tends to maintain the balance for the survival.
Behavioural and physiological responses are two important regulating mechanisms that maintain the stability of homeostasis.
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.

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:
1. Isotonic:
Here the concentration of external and internal solution of the organism are the same.
2. 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.
3. 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.
Example:
Red blood cells.
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.
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.
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.

Inhalation:
The muscles of the diaphragm contract.
The diaphragm goes downward.
The ribs move upwards and outwards.
The volume of thoracic (chest) cavity increases.
Air enters the lungs through the nose.
Exhalation:
The muscles of the diaphragm relax.
The diaphragm goes upward.
The ribs move downwards.
The volume of thoracic (chest) cavity decreases.
Air goes out of the lungs through the nose.

The process of taking air into the lungs is called inspiration or inhalation.
During inspiration, the sternum is pushed up and outward and the diaphragm is pulled down.
This increases the volume of the thoracic cavity and the pressure decreases.
The air outside the body flows into the lungs. Here exchange of gases takes place between the air and the blood.
The process of expelling air from the lungs is called expiration or exhalation. Upon exhalation, the lungs recoil to force the air out of the lungs.
The intercostal muscles relax, returning the chest wall to its original position. During exhalation, the diaphragm also relaxes, moving higher into the thoracic cavity.
This increases the pressure within the thoracic cavity relative to the environment.
Air rushes out of the lungs due to the pressure gradient. This movement of air out of the lungs is a passive event.

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

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.
Therefore water comes out of the vegatable and it undergoes plasmolysis and begins to shrink.
Thus when water content is lost the pickle is able to retain its shelf life for a longer period.

1. A. Why did the doctor advised him to go to the playground?
2. 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.
(b)
Pranayamam teaches us the proper way of breathing, slowly and deeply.
It increases the capacity of the lungs and brings more oxygen into the body.
It is especially very useful when one has respiratory problems and the breathing is irregular and unsteady.
It improves blood circulation.

In a closed and crowded place, the number of people are more. All of them breathe out carbon dioxide. Therefore the amount of CO 2 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.

Impact of excessive usage of mobile phones:
(i)
Cell phone radiation can damage eyes and cause early cataract.
It can also lead to cataract in lens apart from affecting retina, cornea etc.
It strains the eye muscles.
It also caused temporary problems like dry and itchy eyes, blurry vision, pain in eyes etc.
(ii) The teacher has shown values of:
Empathy
Responsibility
Personal care.