11. Cell Structure and Micro-organisms
Complete solutions to the Exercises. Maharashtra State Board (Balbharati), Grade Seven General Science.
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Exercises
(a) What is a ‘cell’?
A cell is the basic structural and functional unit of all living organisms. The bodies of all living things are made up of cells. All life processes take place with the help of cells. The word ‘cell’ was given by Robert Hooke.
(b) Name the different organelles in a cell.
Nucleus, endoplasmic reticulum, ribosomes, Golgi bodies, lysosomes, mitochondria, vacuoles and plastids
(c) What are micro-organisms?
Living organisms which cannot be seen with naked eyes are called micro-organisms. They can be seen only with a microscope. Examples: bacteria, amoeba, yeast, viruses.
(d) Which are the different types of micro-organisms?
Micro-organisms are of five types: algae, fungi, protozoa, bacteria and viruses.
(a) The organelle called the chloroplast (plastid) is present in plant cells only.
(b) Garbage is converted into manure by micro-organisms.
(c) In the cell, photosynthesis is carried out with the help of chloroplasts.
(d) An electron microscope is necessary for the study of viruses.
(a) Plant cell and animal cell.
| Plant cell | Animal cell |
|---|---|
| Cell wall is present. | Cell wall is absent. |
| It has a definite shape. | It has no definite shape. |
| Chloroplasts are present. | Chloroplasts are absent. |
| It has one large vacuole. | It has many small vacuoles. |
| Nucleus is on one side. | Nucleus is in the centre. |
(b) Prokaryotic cell and eukaryotic cell.
| Prokaryotic cell | Eukaryotic cell |
|---|---|
| It has no true nucleus. | It has a true nucleus with a membrane. |
| Membrane-bound organelles are absent. | Membrane-bound organelles are present. |
| It is very small. | It is larger. |
| Example: bacteria. | Example: plant and animal cells. |
Plant cell and animal cell (simplified).
Plant cell: It has a hard cell wall on the outside. So it has a definite shape. Inside the cell wall is the cell membrane. The cell is filled with cytoplasm. It has one large vacuole. The nucleus is pushed to one side. Chloroplasts make food. Mitochondria produce energy.
Animal cell: It has no cell wall. The cell membrane is its outer covering. So it has no definite shape. The nucleus is in the centre. It has many small vacuoles. There are no chloroplasts. Mitochondria produce energy.
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Uses of micro-organisms:
- They decompose garbage and convert it into manure.
- They increase the fertility of soil by fixing nitrogen.
- They convert milk into curd, butter and cheese. They make bread and idli batter rise.
- They are used to make alcohol, vinegar and vitamins.
- They are used to make medicines like antibiotics and vaccines.
- They produce biogas from waste.
Harmful effects of micro-organisms:
- They spoil food and cause food poisoning.
- They cause diseases like typhoid, cholera and hepatitis through food and water.
- They cause cold, tuberculosis and pneumonia through air.
- Mosquitoes spread malaria and dengue.
- They cause diseases in plants and animals.
- Fungi spoil clothes, paper and leather.
(a) Diseases spread on a large scale during periods of heavy rainfall and floods.
During floods, dirty water mixes with drinking water. So diseases like cholera and typhoid spread. Water collects in many places. Mosquitoes breed there and spread malaria and dengue. Moist weather helps microbes to grow. So diseases spread on a large scale.
(b) There is a possibility of food poisoning if we eat stale food.
Micro-organisms grow on stale food. They release poisonous substances into it. Eating such food causes food poisoning. So we should not eat stale food.
(c) Soil is turned over during tilling.
Turning over the soil loosens it. Air reaches deep into the soil. Useful micro-organisms in the soil need oxygen to grow. They increase the fertility of the soil. Roots also get enough air.
(d) Fungus grows quickly in moist or humid conditions.
Fungi need moisture to grow. Their spores are present in the air. In moist conditions, the spores get water and grow quickly. So fungus grows fast in humid weather.
(e) A refrigerator is used in almost every home.
Micro-organisms grow slowly at low temperature. The temperature inside a refrigerator is low. So food does not spoil quickly. It stays fresh for a longer time.
(f) Bread ‘rises’ during baking.
Yeast is added to the dough of bread. Yeast causes fermentation. Carbon dioxide gas is produced. The gas gets trapped in the dough. So the dough swells and the bread rises. It becomes soft and spongy.
(g) Fodder is soaked in water before offering to cattle.
Dry fodder is hard. Soaking makes it soft. Microbes in it start breaking it down. So cattle can chew and digest it easily.
Simple microscope: It is used to see small objects in a little enlarged form. Example: fungus on bread, parts of a flower, small insects.
- Keep the object in good light.
- Hold the lens close to the object.
- Move the lens up or down until the object looks clear and big.
Compound microscope: It is used to see very small objects which cannot be seen with naked eyes. Example: onion peel cells, micro-organisms in pond water.
- Keep the specimen on a glass slide. Add a drop of water and stain. Cover it with a cover slip.
- Keep the microscope on a steady table. Adjust the mirror so that light falls on the stage.
- Place the slide on the stage and fix it with clips.
- Use the low-power lens first. Focus with the coarse knob.
- Make the image clear with the fine knob.
- Observe and draw the diagram.
Process of making bread in a bakery:
- Flour, water, yeast, sugar and salt are mixed.
- The mixture is kneaded into a soft dough.
- The dough is kept in a warm place. Yeast ferments it and it rises.
- The dough is shaped and kept in trays.
- It is baked in a hot oven.
- The bread is cooled and packed.
Making pav at home (with an elder’s help): Mix yeast and sugar in warm water. Add flour, salt and oil and knead a soft dough. Keep it covered in a warm place until it doubles. Make small balls and keep them in a greased tray. Bake them in an oven until golden brown.