7.11 MHCERT 7th General Science 11. Cell structure and micro organisms

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Solutions - 11th chapter General Science - class 7 Maharashtra State Board

1. Answer the following questions.

 

(a) What is a 'cell'?


Ans. The cell is the fundamental, structural and functional unit of living organisms.

 (b) Name the different organelles in a cell? 

Ans.

  1.  Nucleus,
  2.  Endoplasmic reticulum and the ribosomes attached to it,  
  3. Golgi bodies, 
  4. Mitochondria, 
  5. Plastids, 
  6. Lysosomes and 
  7. Vacuoles are the different organelles in a cell

(c) What are micro-organisms?

The living organisms which cannot be seen with our eyes but can only be observed under a microscope are called microorganisms.

(d) Which are the different types of micro-organisms? 



There are two types of microorganisms. 

1. unicellular microorganisms and
 2.  multicellular microorganisms 

Microorganisms are broadly classified into - 
algae,  fungi, protozoa,  bacteria and viruses 
This classification is based on the shape and the life processes


Q.2. 

Fill in the blanks with the proper word.

(a) The organelle called the plastids is present in plant cells only.

(b) Garbage is converted into fertilizer 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 microorganisms.

Q.3. 

What is the difference between us? 

(a) Plant cell and animal cell 



(A) Plant cell-

  1. It has a cell wall made up of cellulose.
  2. The cytoplasm is lesser granular and not dense.
  3. The large vacuole in the centre pushes the cytoplasm to one end.
  4. There are no lysosomes.
  5. The mitochondria are are few in number.
  6. Plastids are present.
  7. Vacuoles are more and large.
  8. The vacuoles are filled up with cell sap.

(B) Animal cell -


  1. It does not have a cell wall.
  2. The cytoplasm is more granular and dense.
  3. The cytoplasm is uniformly distributed in the cell.
  4. Lysosomes are present.
  5. The mitochondria are greater in number.
  6. Plastids are absent.
  7. The vacuoles are filled with food or excretory matter.

(b) Prokaryotic cell and eukaryotic cell

(A) Prokaryotic cell-

  1. The nucleus is not distinct. 
  2. They are 1 to 10 um in size.
  3. Nuclear material comes in contact with the cytoplasm.
  4.  The cell organelles are without a membrane.
  5.  There is a single chromosome.
  6.  Mitochondria are absent.
  7.  Chlorophyll is present in the plastids.
  8.  It is found in blue-green algae and bacteria.

(B) Eukaryotic cell-

  1.  The nucleus is distinct.
  2.  They are 5 to 100 um in size.
  3.  Nuclear material never comes in contact with the cytoplasm.
  4. The cell organelles are bound with membranes.
  5. There is more than one chromosome.
  6.  Mitochondria are present in it.
  7. Chlorophyll is present inside the chloroplast.
  8.  It is found in highly evolved plants and animals.

Q.4.

 Sketch and describe in your own words, the plant cell and animal cell.


Q.5

Explain the uses and the harmful effects of microorganisms.

The uses of microorganisms

  1. Some microorganisms convert the wet garbage into the best quality manure.
  2. Some microorganisms present in the soil convert atmospheric nitrogen into its compounds and increase the soil fertility and protein content of the soil.
  3.  Some microorganisms convert milk into yoghurt and other milk products.
  4. Some microorganisms cause the process of fermentation.
  5.  Some microorganisms are useful for making antibiotics for various diseases

The harmful effects of microorganisms


  1. Some microbes release toxic materials into the food and spoil the food.
  2. Some harmful microbes cause the diseases of the alimentary canal like amoebiasis, typhoid, cholera, hepatitis, gastro etc and the diseases of the respiratory tract for example common cold cough diphtheria pneumonia tuberculosis. 
  3. Insect borne diseases like malaria dengue elephantiasis yellow fever chikungunya Zika fever etc. are caused by harmful microbes which enter into the human bodies through insects.

Q.6. 

Give reasons.

(a) Diseases spread on a large scale during periods of heavy rainfall and floods.

Ans.
  1. During heavy rainfall, the logged water and puddles cause the growth of microorganisms and houseflies,  which spread the pathogens rapidly. 
  2. The puddles formed help in the breeding of mosquitoes. 
  3. All these things are the sources of insect-borne diseases.  Due to the moisture in the atmosphere, food also gets contaminated very easily and causes diseases of the alimentary canal. 
  4. These are the reasons for epidemic spread and diseases.

 (b) There is a possibility of food poisoning if we cat stale food

Ans.
 Stale food may have microbes that produce enterotoxins and they cause food poisoning. so there is a possibility of food poisoning if we eat stale food.

(c) Soil is turned over during tilling 

Ans.
  1.  There are some useful bacteria in the soil that help to decompose organic substances into inorganic materials. They produce fertilizers. 
  2. Some microbes help in nitrogen fixation which increases the nutritive value of the soil and proves to be beneficial. 
  3. Thus, this activity gives a better yield. So, the soil is turned over during tilling.

(d) Fungus grows quickly in moist humid conditions.

Ans. Air, warmth and moisture are very important things for the growth of any living organism. So, fungus grows very quickly in a moist or humid atmosphere as they get favourable conditions.

(e) A refrigerator is used in almost every home.



Ans.
  1. Air, warmth and moisture are the necessary things for the growth of microbes. 
  2. The harmful microbes do not grow in cold temperatures. 
  3. They can grow between 15 degrees Celsius to 35-degree celsius. 
  4. The refrigerator has a very low temperature. 
  5. So food can be stored there without spoiling. 
  6. So even perishable food items can also be stored for a long time. 
  7. So, a refrigerator is used in almost every home.

(f) Bread rises during baking

Ans.
  1. Yeast[a type of fungus] is added to the bread which undergoes a process and causes fermentation in the batter of bread. 
  2. This process releases carbon dioxide. 
  3. Therefore, bread rises during baking.

(g) Fodder is soaked in water before

Ans.
  1. Water helps in the process of fermentation. 
  2. The fermentation increases the vitamin content in fodder. 
  3. It also becomes easier for digestion. 
  4. So, fodder is soaked in water before offering to cattle. 

Q.7 

When will you use a simple microscope and when, a compound microscope? Explain.

Ans
A simple microscope is used for the observation of laser magnification like parts of flowers and insects and a compound microscope is used for the observation of microbes because its magnification is 100 to 450 times. for example, atoms or cells or tissues.

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