LECTURE -1 : CLASS VIII : SCIENCE : CHAPTER 8 : CELL STRUCTURE & FUNCTIONS

CLASS VIII   |    SCIENCE    |    CHAPTER 8

      notes prepared by subhankar Karmakar

Cell : Cells are the basic units of life. All the living things both plants and animals, are made from cells. Cells are tiny basic unit of structure and function of all living organisms. Cells are the building blocks of plants and animals. Therefore, we can say a cell is the smallest unit of life which has a definite structure and performs specific function.

Microscope: The instrument used to study cells is called microscope.

Discovery of cell: an English scientist named Robert hooke discovered cell in 1665.

In 1838, a German scientist Schleiden first suggested that all plants are made of cells. 

In 1839, another German scientist Schwann suggested that all animals are made of cells too. 

Cell division: all cells arise from pre-existing cells by a method which is called cell division.

Classification of cell: All cells are primarily categorised into two main types. Animal cells and plant cells. 

Structures of a cell: Each cell has a number of smaller parts or internal structures in it. Some of the structures are present in both animal and plant cells, while some structures are only present in plant cells, they are not present in animal cells. They are also called parts of a cell.

The parts of a cell present in both animal and plant cells: 

Cell membrane or plasma membrane, Cytoplasm, Nucleus and Mitochondria. These basic parts are present in in both animal as well as plant cells. 

.                  Parts of a cell

The parts of a cells present in plant cells only:

Cell wall, Chloroplasts, Large vacuole are present in plant cell only. 

.               Parts of a Plant Cell

1. Cell membrane or plasma membrane:

Every cell is covered by a thin sheet of skin which is called cell membrane or plasma membrane. The cytoplasm and nucleus are enclosed within the cell membrane. Therefore we can say, cell membrane encloses the living part of the cell which is known as protoplasm. 

The cell membrane protects the cell and also gives shape to the cell. The cell membrane has tiny pores in it. Through this pores, the cell membrane controls the movement of substances "into the cell" and "out of the cell" like the dissolved glucose and oxygen can enter into the cell whereas the waste products like carbon dioxide can go out from the cell through this pores of the cell membrane. 

Cell membrane separates the cell from other cells as well as from the surrounding medium. The cell membrane or plasma membrane is a living part of the cell.

2. Cytoplasm: 

Cytoplasm is a transparent jelly like material which fills the cell between nucleus and the cell membrane. It acts like a chemical factory of the cell. In cytoplasm, new substances are built from materials taken into the cell and energy is released and stored. It is a place where most of the chemical reactions which keep the cell alive take place. Cytoplasm has many internal structures which are known as organelles. The most prominent internal structure in the cytoplasm is nucleus. It also contains mitochondria, Golgi bodies, ribosomes etc. Cytoplasm for plant cells also contain chloroplast. Cytoplasm and the nucleus taken together is called protoplasm.

3. Nucleus:

Nucleus is a large spherical organelle present in all the cells. In animal cell, nucleus lies in the centre of the cell whereas in plant cells the nucleus maybe on the periphery of the cell.  Nucleus is separated from the cytoplasm by a membrane called nuclear membrane. Nucleus controls all the activities of cell. The transmission of characteristics from the parents to the offsprings is called inheritance and Nucleus plays a role in inheritance.  

Chromosome: Nucleus contains thread like structure called chromosome. The function of chromosomes is to transfer the characteristics from the parents to the offspring through the genes and it is called inheritance. 

Nucleolus: The nucleus also contains a tiny round structure called nucleolus. The nucleus containing chromosomes and nucleolus is bound by a membrane called nuclear membrane. Nuclear membrane has tiny pores for the exchange of materials with cytoplasm. Some cells like red blood cells do not have a nucleus.

Gene: Gene is a unit of inheritance in living organisms, which is transferred from parent to offspring during reproduction and determine some characteristic feature of the offspring. Thus genes control the transfer of hereditary characteristics from parents to offsprings. 

Protoplasm: All the living matter in a cell is called protoplasm. Protoplasm is a liquid substance which is present inside the cell membrane. Protoplasm includes cytoplasm, nucleus and other organelles. Most of the protoplasm is made up of compounds of only 4 elements Carbon, Hydrogen, Nitrogen and Oxygen. It also contains water, carbohydrates, proteins, nucleic acids, fats and mineral salts.

4. Mitochondria: 

Mitochondria are the tiny rod-shaped or spherical organelles which are found in all the cells. Mitochondria provide energy for all the activities of the cell. This energy is produced by the process of respiration in which food such as glucose is broken down by oxygen. Thus mitochondria use glucose and oxygen to produce energy. Mitochondria are found in large numbers in the cytoplasm in all the cells.

5. Cell wall: The plant cells have a thick cell wall around them outside the cell membrane. The cell wall is made of a tough material called cellulose. Cell wall gives shape and support to the plant cell. Cell walls also hold the plant cells together and give plants most of their strength. Plants need protection against high wind speed, variation in temperature and atmospheric moisture etc. Since plants are fixed at a place so they cannot move to protect themselves from the various changes in their surroundings. The tough cell wall present in plant cells provides protection to plants. Cell wall is not there in animal cells. The cell wall is a non living part of the plant cells.

6. Chloroplasts: Chloroplasts are the green coloured organelles present in the cytoplasm of plant cells. The process of food making by plants known as photosynthesis takes place in chloroplasts. The green colour of chloroplasts is due to the presence of a green pigment called chlorophyll in them. Chlorophyll can absorb sunlight energy. In the chloroplasts, carbon dioxide and water combine in the presence of sunlight energy to produce food such as glucose. And this process of food making is called photosynthesis. 

The organelles containing pigments or colour matter present in the cytoplasm of plant cells are called plastids. Plastids can be of many different colours. The plastids containing green pigment or chlorophyll are known as chloroplasts. Chloroplast are present in the cells of leaves. It makes the leaves green. The plastids are may be of different colour like red colours of tomatoes are due to red plastids. Different colours of flowers are also due to this plastids.

7. Large Vacoule: 

Vacoule is a space in the cytoplasm of a cell which is enclosed by a membrane and is usually contains substances dissolved in water. A vacoule appears as an empty space under the microscope. All the plant cells have a large vacoule. The vacuole is filled with a liquid called "cell sap" , which contains dissolve sugar and salts. The function of vacuole in a cell is to store various substances including waste products of the cell.

* In amoeba vacuoles contain food particles so they are called food vacuoles.

Lecture: 10 :CLASS VIII : SCIENCE : Chapter 2: PRESERVATION OF FOOD & NITROGEN FIXATION

CLASS VIII   |    SCIENCE    |    CHAPTER 2

      notes prepared by subhankar Karmakar

Preservation of food:

The process in which the food materials are given a suitable physical or chemical treatment to prevent their spoilage is called food preservation. Some of the methods for preserving foods are: 

1. Sun drying or dehydration

2. Heating

3. Cooling or refrigeration

4. Deep freezing

5. Addition of common salt

6. Addition of sugar

7. Addition of mustard oil and vinegar

8. Use of special chemical preservatives like sodium meta-bi-sulphite, sodium benzoate and citric acid.

9. Pasteurisation

10. Packing food in airtight packets.

1. Preservation of food by sun drying or dehydration:

Water content of the food is removed by sun drying. It is also known as dehydration.

Vegetables like spinach, methi leaves, cauliflower, peas are preserved in our homes by the sun drying method.

2. Preservation of food by heating:

Microorganisms in the food can be killed by heating the food. We boil milk to prevent it from spoilage.

3. Preservation of food by cooling or refrigeration:

The growth of microorganisms can be inhibited by keeping low temperature. 

The food materials like milk, kneaded flour, cooked food, fresh fruits and vegetables are kept in a cool place like refrigerator to prevent their spoilage.

4. Preservation of food by deep freezing:

At sub zero (below zero degree centigrade) temperature the growth of microorganism are completely stopped. 

Deep freezing method is used for the preservation of foods like meat, fish and their products; fruits and vegetables.

5. Preservation of food by adding common salt:

common salt prevents the growth of food spoilage microorganisms due to which it is used to preserve a number of food materials. It is one of the oldest practice for preservation of food like meat and fish. 

common salt is also used to preserve fruits such as raw mangoes lemon and amla as well as tamarind.

6. Preservation of food by adding sugar:

Sugar is used as a preservative in making jams and jellies from fruits. The fruits which are preserved in the form of jams and jellies by using sugar as preservative are Apple, ripe mango, Orange, strawberry, pineapple and guava etc.

7. Preservation of food by mustard oil and vinegar: 

Mustard oil and vinegar are widely used as preservatives for the preservation of fruits and vegetables in the form of pickles.

Fruits like raw mango, amla and lemon are preserved using mustard oil and vinegar in the form of their pickles.

8. Preservation of food by using special chemicals as preservatives:

Sodium metabisulphite, sodium benzoate and citric acid are used as chemical preservatives. To preserve foods such as jams, jellies, juices and squashes sodium metabisulphite and sodium benzoate are used. To preserve confectionery citric acid is used as a preservative.

9. Preservation of food by pasteurisation:

Pasteurisation involves the process of heating followed by quick cooling. Packaged milk is always pasteurized.

10. Preservation of food by packing and air tight packets:

This days dry fruits and even vegetables are sold in sealed airtight packets to prevent the attack of microorganisms on them. This helps the dry fruits and vegetables to remained unspoiled for a longer time.

NITROGEN FIXATION:

The process of converting nitrogen gas of atmosphere or air into compounds of nitrogen which can be used by the plants is called nitrogen fixation.

The nitrogen gas of atmosphere or air can be fixed or converted into nitrogen compounds-

1. By certain nitrogen fixing bacteria present in the soil,

2. By Rhizobium bacteria present in the root nodules of leguminous plants,

3. Buy blue-green algae,

4. By lightning.

Rhizobium Bacteria:

The nitrogen fixing rhizobium bacteria live in in the root nodules of leguminous plants like peas, beans etc, also known as legumes.

Nitrogen fixation by lightning:

Nitrogen gas of atmosphere also gets fixed through the action of lightning in the sky. When lightning takes place in the sky during thunderstorm, a high temperature is produced in the atmosphere. At this high temperature, nitrogen gas of air combines with oxygen gas of air to form nitrogen compounds. These nitrogen compounds dissolve in rain water, fall to earth with rain water and go to the soil. Nitrogen of atmosphere can also be fixed by artificial methods.

THE NITROGEN CYCLE:

The circulation of nitrogen element through living things like plants and animals and non living environment like air soil and water, is called nitrogen cycle in nature.

"Nitrogen is required by both plants and animals for their growth and development. Nitrogen is an essential component of proteins, which make up the bodies of plant and animals. Nitrogen is also present in chlorophyll, nucleic acids and vitamins. The same nitrogen element is circulated again and again through living things like plants and animals and non living things like air soil and water. This whole process is known as nitrogen cycle."

                  Fig:  Nitrogen Cycle in nature    

The main steps of nitrogen cycle:

I. The atmosphere or air contains nitrogen gas. The nitrogen fixing bacteria present in the soil and in the root nodules of leguminous plants, blue green algae and lightning in the sky fix nitrogen gas from the atmosphere and convert it into compounds of nitrogen which go into soil.

II. The plant take compounds of nitrogen from the soil for their growth. The plants absorb the nitrogen compounds from the soil through their roots. The plants convert the compounds of nitrogen into plant proteins and other organic compounds which make up the body of plants.

III.  The plants are eaten by the animals as food. Animals convert plant proteins into animal proteins and other organic compounds which make up their body. Some animals also eat other animals to obtained nitrogen compound. Thus, animals obtain nitrogen compounds by eating plants as well as other animals.

IV.  When plants and animals die, the complex nitrogen compounds like proteins etc. present in their dead bodies are decomposed and converted into simple compounds of nitrogen by certain bacteria and fungi present in the soil. Animal excretions like urine etc. are also converted into simple compounds of nitrogen. All the simple compounds of nitrogen formed in this way go into the soil. In this way, the compounds of nitrogen which were taken by the plants from the soil during their growth are returned to the soil. From the soil these nitrogen compounds are again absorbed by the new plants for their growth and this part of nitrogen cycle is repeated endlessly.

V. Some of the compounds of nitrogen formed from the decay of dead plants and animals, are decomposed by denitrifying bacteria present in the soil to form nitrogen gas. This nitrogen gas goes back into the atmosphere from where it initially came. This process is the reverse of fixation of nitrogen. In this way the nitrogen gas which was removed from the atmosphere during fixation is put back into the atmosphere.

From the atmosphere nitrogen gas is used again during nitrogen fixation and the nitrogen cycle is repeated in nature again and again. As a result of nitrogen cycle in nature, the percentage of nitrogen gas in the atmosphere or their remains constant.

 

Lecture: 8 :CLASS VIII : SCIENCE : Chapter 2: GOOD MICRO-ORGANISMS

Friendly microorganisms/ beneficial microorganism:

Not all microorganisms are bad for human. Some of them are useful too. Beneficial effects of microorganisms are as follows:

1. In the making of card, bread and cake microorganisms are used.

2. In the production of alcohol, wine, acetic acid or vinegar microorganisms are used.

3. Microorganisms are used to produce antibiotics.

4. Microorganisms are used to increase the fertility of the soil by nitrogen fixation.

5. Microorganisms are used to decompose the organic waste matter.

a. Process to make curd:

Milk is is turned into curd by using the bacteria Lactobacillus. 

Milk contains sugar called lactose. Lactobacilli bacteria convert the lactose sugar into lactic acid. This lactic acid then converts milk into curd.

b. Process to make bread:

Yeast is used in the baking industry for making bread. When Yeast is mixed in dough, it reproduces quickly and produces carbon dioxide during respiration. The bubbles of carbon dioxide gas filled the dough and increases volume. This is the secret of increasing the volume of bread. It makes the bread light, soft and spongy.

c. Commercial use of microorganisms:

Microorganism are used for the large scale production of alcohol and acetic acid. Yeast is the microorganism used for the large scale production of alcohol.

The process of conversion of sugar into alcohol by the action of yeast is called fermentation. 

Bacteria can turn alcohol into acetic acid. A dilute solution of acetic acid is called vinegar.

The bacteria which convert alcohol into acetic acid is called Acetobacter.

d. Medicinal use of microorganism:

A medicine which stops the growth of, or kills the disease causing microorganisms is called antibiotic. Antibiotics medicine are made from microorganisms. 

Example: some of the common antibiotics which are made from fungi and bacteria are: penicillin, streptomycin, erythromycin, tetracycline.

Antibiotics are effective for bacteria causing diseases but they are ineffective against the diseases caused by virus.

Precautions that to be observed in the use of antibiotics:

1. Antibiotics should be taken only on the advice of a qualified doctor.

2. A person must complete the full course of antibiotics prescribed by the doctor.

3. The antibiotics should be taken in proper doses as advised by the doctor.

4. Antibiotics should not be taken unnecessarily, if taken unnecessarily it may kill the useful bacteria in the body and harm us.

Vaccine:

A vaccine is a special kind of preparation or medicine which provides immunity or protection against a particular disease.

A vaccine contains the dead or weakened but alive microorganisms of a disease. Find the vaccine containing dead or alive microorganisms introduced into the body of a healthy person either orally or by injection the body of that person responds by producing some substances called antibodies in the blood. These antibodies kill the disease causing microorganism in the body. Thus a vaccine developes the immunity from a disease.

Vaccination: 

Vaccination is the process of giving a vaccine orally or by injection which provides protection against a particular disease.

The diseases which can be prevented by vaccination of children at proper age are: polio, smallpox, cholera, typhoid, hepatitis, tuberculosis, tetanus, measles, rabies, diphtheria and whooping cough.

Polio in India is prevented by giving oral polio vaccine or OPV under the pulse polio programmes of government of India.

e. Increasing soil fertility by microorganisms:

Some bacteria and blue green algae are able to fix nitrogen gas from the atmosphere to enrich the soil with nitrogen compounds and increase its fertility. The nitrogen fixing bacteria and blue green algae are called biological nitrogen fixers. One of the nitrogen fixing bacteria is rhizobium. Rhizobium bacteria present in the root nodules of leguminous plant (like peace, beans etc) also fix atmospheric nitrogen and increase soil fertility.

f. Cleaning the environment by microorganisms:

Some microorganisms decompose the harmful and smelly dead remains of plants and animals and animal wastes (like faeces, urine etc) into harmless materials and thus they clean the environment. In addition to the cleaning the environment, the microorganisms also help in recycling the nutrients present in dead plants, dead animals and animal wastes in nature which can then bi used as food by Green plants.