Science textbook solutions for Class 7 science Chapter 11 Cell Structure and Micro-organisms


Important points to remember :
  1. cell  is the fundamental  structural  and functional  unit of living  organisms.
  2. In 1665, a scientist,  Robert  Hooke observed a thin section of cork under a microscope. In this section, he saw hexagonal compartments like those of a bee-hive.  
  3. He named  those compartments ‘cells’.  This term is derived from the Latin word ‘cella’ which means ‘small room. 
  4. In 1838, the two scientists M. J. Schleiden  and Theodore Schwann  formulated a  theory about cellular structure   which stated, ‘All the living organisms are made up of cells and the cell is the fundamental component of living  organisms’. 
  5. In 1885, Rudolph Virchow stated  that  all  cells  are formed from  pre-existing cells. 
  6. In 1673, Anton van Leeuwenhoek assembled various lenses to construct a microscope.
  7. There is great variation  in the shapes  of cells. Their shapes  are mainly related to their function. organisms which cannot be seen with our eyes but can only be observed under a  microscope  are called  micro-organisms. 
  8. Fermentation The chemical  process of conversion of one type of carbon compound into another type of carbon compound by the action of micro-organisms is called fermentation. 


Question 1: Answer the following questions.
a) What is cell ?
Answer : Cell is the smallest unit of life. They are the structural, functional and biological uits of life. The discovery of cells was first made by Robert Hooke. While examining a section of a cork under the microscope, he observed small compartment-like structures and named them cells. However, Hooke observed dead cells under the microscope as cork is made up of dead cells. Cells exhibit different types of properties like:
  1. It is the basic structural and functional living unit of life.
  2. It is so small that it is not visible to the naked eye.
  3. The shape of the cell varies in different organisms and within an organism.
  4. Size of cells also differs.
  5. All living cells exhibit certain basic characteristics like respiration, growth, metabolism, etc.

b) Name the different organelles in a cell ?
Answer : The cells may conatin the following cell organelle
Cell membrane, cell wall, vacuole, plastids, endoplasmic reticulum, chloroplast, golgi apparatus, ribosomes, mitochondria, nucleus, lysosomes, centrosome and inclusion bodies.

c. What are Microorganisms
Answer : Those Living organisms which are not visible seen by naked eye are known as micro-organisms. They are living organisms that can be seen only with a microscope.

d) which are the different types of micro-organisms ?
Answer : Microorganisms can be divided into 5 major groups like:

Bacteria- Bacteria are the most primitive or prokaryotic unicellular organisms found in living world. Because of their unique biochemical pathways and properties, they are of high economic importance to the human beings.

Fungi- Fungi is a group of eukaryotic, unicellular as well as multicellular, non-photosynthetic organisms that includes mushrooms, moulds and yeasts.

Algae- Algae are a diverse group of aquatic organisms that have the ability to conduct photosynthesis.

Protozoa- Protozoa are a group of unicellular micoorganism which are non photosynthetic in nature. Many protozoa cause disease in animals and humans.

Viruses- Viruses are ultra microscopic, obligatory parasites. They have simple structure having genetic material coated by protein coat. They are obligate parasites and highly pathogenic.

Question 2: Fill in the blanks with the proper word.

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

b) Garbage is converted into compost by micro-organisms.

c) In the cell, photosynthesis is carried out with the help of chloroplast.

d) An electron microscope is necessary for the study of cells .


Question 3: What is difference between us ?
a) Plant cell and animal cell.
Answer : All living things are made up of cells . Cells are the basic unit of a living organism and where all life processes are carried out. Animal cells and plant cells share the common components of a nucleus , cytoplasm , mitochondria and a cell membrane. Plant cells have three extra components, a vacuole , chloroplast and a cell wall .
Animal cells
  1. Animals are made up of millions of cells. Animal cells have an irregular structure and are made up of four key parts:
  2. Nucleus – This contains genetic material (DNA), and controls the cell's activity.
  3. Cell membrane – A flexible layer that surrounds the cell and controls the substances that enter and exit.
  4. Cytoplasm – A jelly-like substance where the chemical reactions happen.
  5. Mitochondria – This is where energy is released from the food molecules.

Plant cells
Plants are also made up of millions of cells. Plant cells have a nucleus, cell membrane, cytoplasm and mitochondria too, but they also contain the following structures:
  1. Cell wall – A hard layer outside the cell membrane, containing cellulose to provide strength to the plant.
  2. Vacuole – A space inside the cell that is used to store substances and help the cell keep its shape.
  3. Chloroplasts – Structures that contain the green pigment chlorophyll , which are a key part of photosynthesis .


b) Prokaryotic cell and eukaryotic cell.
Answer : 
prokaryotic cell :
  1. They lack a well-defined nucleus, have a nucleoid instead. 
  2. Usually have double-stranded, circular DNA. 
  3. Do not have mitochondria. 
  4. Have 70S ribosomes. 
  5. Golgi apparatus and endoplasmic reticulum also absent.
Eukaryotic cell :
  1. Have a well-defined nucleus enclosed in the nuclear membrane. 
  2. Have a linear double-stranded nucleus. 
  3. Mitochondria are present. 
  4. Have 80S ribosomes. 
  5. They do have Golgi apparatus and endoplasmic reticulum.


Question 4: Sketch and describe in your own words, the plant cell and animal cell.
Answer :

Plant cell : Plant cells are eukaryotic cells, that are found in green plants, photosynthetic eukaryotes of the kingdom Plantae which means they have a membrane-bound nucleus. They have a variety of membrane-bound cell organelles that perform various specific functions to maintain the normal functioning of the plant cell. A typical plant cell organelles include Cell wall, Cell membrane, Cytoskeleton, Plasmodesmata, Chloroplast, Vacuoles, endoplasmic reticulum, Golgi bodies, Mitochondria, Ribosomes, peroxisomes, Nucleus, Nucleolus.

Cell wall : It is the rigid outer cover of the plant cell with a major role of protecting the plant cell, giving it, its shape.

plasma membrane :
This is a lipid membrane that is made up of protein subunits and carbohydrates, with a characteristic semi permeability factor.

Chloroplast : Plastids are specialized organelles found specifically in plant and algal cells. They have a double-layered membrane.

Animal cell :
Animal cells are the basic unit of life in organisms of the kingdom Animalia. They are eukaryotic cells, meaning that they have a true nucleus and specialized structures called organelles that carry out different functions. Animal cells do not have cell walls or chloroplasts, the organelle that carries out photosynthesis. The cell has a variety of different parts. It contains many different types of specialized organelles that carry out all of its functions. Not every animal cell has all types of organelles, but in general, animal cells do contain most if not all of the following organelles.

Nucleus : The nucleus contains a cell’s deoxyribonucleic acid (DNA), its genetic material. DNA contains instructions for making proteins, which controls all of the body’s activities.

Ribosomes : Ribosomes are where proteins are synthesized. They are found within all cells, including animal cells.

Mitochondria : The process of cellular respiration occurs in the mitochondria. During this process, sugars and fats are broken down and energy is released in the form of adenosine triphosphate (ATP).


Question 5: Explain the uses and the harmful effects of micro-organisms.
Answer : Useful effects of microorganisms are as follows
  1. Some microorganisms are naturally found in our body, called normal flora that work as protector by living on our skin and gut. 
  2. In gut, they help us to digest food and regulate the production of vitamins and nutrients that are essential to keep our body strong and healthy.
  3. Some microorganisms are used for production of yogurt and some probiotic drinks.
  4. Some are used for production of vaccines.
  5. Some bacteria transforms harmful substances into simpler harmless substitutes.
Harmful effects of microorganisms are as follows
  1. Some microorganisms are harmful and even cause diseases in both plants and animals. 
  2. They helps in decaying fruits, vegetables and other food items in plants. 
  3. In human, it can cause many diseases such as typhoid, diarrhea, cholera etc. 
  4. These microorganisms are known as pathogens. 
  5. They are especially adapted to infect and multiply within a host.
  6. Some microorganisms need carrier to transfer diseases such as Anopheles mosquitoes, which are the carriers used by the protozoan Plasmodium vivax to transfer disease to the host.
  7. Some microorganisms enter the  body with the help of contaminated food, water, air and also diseases caused via small cuts on our skin.

Question 6: Give reasons.
a) Diseases spread on a large scale during periods of heavy rainfall and floods.
Answer : Floods can potentially increase the transmission of the following communicable diseases: Water-borne diseases, such as typhoid fever, cholera, leptospirosis and hepatitis A. Vector-borne diseases, such as malaria, dengue and dengue haemorrhagic fever, yellow fever, and West Nile Fever.

b) There is a possibility of food poisoning if we eat stale food.
Answer : There is a possibility of food poisoning if we eat stale food as this is likely to course food poisoning as a result of bacteria which are present in the food.

When food is stored for a long period of time, bacterial growth occurs in the food and as a result of the reaction of the immune responses.

c) Soil is turned over during tilling.
Answer :  Tilling for Severely Compacted Soil. Compacted soil that is heavy in clay makes it difficult for roots to push through and grow. To break up compacted soil, you will need to till the soil and then work organic matter into the soil so it retains its loosened texture.

d) Fungus grows quickly in moist or humid conditions.
Answer : Fungus grows quickly on moist or humid conditions. This statement is 100 percent correct because moisture and humidity are the basic requirements for the fungus to grow. We know that temperature of the humid places is mostly higher, which is ideal condition for fast reproduction of mold spores and thus fungus.

e) A refrigerator is used in almost every home.
Answer :  It is almost used in every home. Explanation: A refrigerator is a device used to maintain temperature of stored product lower than that of surrounding atmosphere.

f) Bread 'rises' during baking.
Answer : This organism lies dormant until it comes into contact with warm water. Once reactivated, yeast begins feeding on the sugars in flour, and releases the carbon dioxide that makes bread rise (although at a much slower rate than baking powder or soda).

g) Fodder is soaked in water before offering to cattle.
Answer : Fodder still contain a few seeds that remain even after harvest. When fodder is soaked in water it aids in sprouting the seeds / grains that still remain. The sprouts are richer in nutrients than compared to them remaining as grains.

Question 7: When will you use a simple microscope and when, a compound microscope? Explain in detail how you will use them.
Answer : 
Simple microscope: A simple microscope is used at a basic level, where there is no rigorous requirement of research. There are 3 to 5 objective lenses in a compound which helps in magnifying algae, fungi and bacterium. Its total magnification is limited to the magnification of the single lens used.

Compound microscope :
Typically, a compound microscope is used for viewing samples at high magnification (40 - 1000x), which is achieved by the combined effect of two sets of lenses: the ocular lens (in the eyepiece) and the objective lenses (close to the sample)


# Use your brain power 
1. Where do the life processes of unicellular  organisms take place ?
Answer : All the life processes of unicellular organisms take place in a single cell. Their feeding, digestion,excretion all take place through one cell. During reproduction the same cell divides into two. All life processes of unicellular organisms take place in single cell.

2. How do the cells  acquire definite  shapes ? 
Answer : Cells acquire definite shape due to different building material. In plant cell, the shape is determined by the cellulose wall.

3. How are cells  protected ? 
Answer : Animals cells are protected by the plasma membrane and cell coat called glycocalyx (composed of hyaluronic acid and proteins). However, the plant cells are protected by cell wall present outside the cell membrane. The cell wall is outermost layer of plant cell and is composed mainly cellulose.

4. What are the needs of cells ?
Answer : Cells require several kinds of resources:
  1. water;electron acceptor; for aerobic respiration, 
  2. it is oxygen; for anaerobic respiration, 
  3. there are various molecules to serve;supply of energy.
  4. supply of carbon. 
  5. supply of nitrogen and phosphorus, for synthesis of biomolecules ; minerals.

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