Grade 7 Q&A: Chapter 11: Cell Structure and Micro-organisms

Concept Questions

Q1: What are cells? Who discovered them and when?

Answer: Cells are the basic structural and functional units of all living organisms. They were first discovered by Robert Hooke in 1665 when he observed slices of cork under his self-designed microscope.

Q2: What is the Cell Theory? Who proposed it?

Answer: The Cell Theory states that all plants are made of cells (proposed by Matthias Schleiden in 1838) and all animals are also made of cells (proposed by Theodor Schwann in 1839). Together, they put forth this theory, which postulates that cells are the fundamental units of all living organisms and new cells arise from pre-existing cells.

Q3: Name the three main components of a cell.

Answer: The three main components of a cell are the Cell Membrane, Cytoplasm, and Nucleus.

Q4: What is the function of the nucleus in a cell?

Answer: The nucleus acts as the control center of the cell. It contains the cell's genetic material (chromosomes/DNA) and regulates all cellular activities, including growth, metabolism, and reproduction.

Q5: List four important cell organelles and their functions.

Answer: 1. Mitochondria: Produce energy through cellular respiration (Powerhouse of the cell). 2. Chloroplasts: Site of photosynthesis in plant cells. 3. Ribosomes: Responsible for protein synthesis. 4. Vacuoles: Store water, nutrients, and waste products.

Q6: What are micro-organisms? Where are they found?

Answer: Micro-organisms (or microbes) are living organisms that are too small to be seen with the naked eye and can only be observed under a microscope. They are found almost everywhere: in air, water, soil, inside the bodies of plants and animals, and even in extreme environments like hot springs and polar regions.

Q7: Name the five major groups of micro-organisms.

Answer: The five major groups of micro-organisms are Bacteria, Fungi, Protozoa, Algae, and Viruses.

Q8: Why are viruses considered a link between living and non-living things?

Answer: Viruses are considered a link between living and non-living things because they exhibit characteristics of living organisms (like reproduction) only when they are inside a living host cell. Outside a host cell, they behave like non-living particles, showing no signs of life.

Q9: Give two examples of beneficial micro-organisms and their uses.

Answer: 1. Lactobacillus bacteria: Used in the production of curd and cheese from milk. 2. Yeast (a fungus): Used in baking to make bread and cakes rise, and in the production of alcoholic beverages through fermentation.

Q10: What are pathogens? Give two examples of diseases caused by pathogens in humans.

Answer: Pathogens are micro-organisms that cause diseases. Two examples of diseases caused by pathogens in humans are Tuberculosis (bacterial) and Influenza (viral).

Q11: List three methods of food preservation.

Answer: Three methods of food preservation are: 1. Chemical methods (using preservatives like sodium benzoate). 2. Preservation by sugar/salt (e.g., in jams, pickles). 3. Heat and cold treatments (e.g., boiling, refrigeration, freezing).

Q12: What is pasteurization? Who discovered it?

Answer: Pasteurization is a food preservation method, especially for milk, where milk is heated to about 70°C for 15 to 30 seconds and then rapidly cooled. This process kills harmful microbes without significantly altering the milk's taste or nutritional value. It was discovered by Louis Pasteur.

Q13: How do bacteria help in nitrogen fixation?

Answer: Certain bacteria, like Rhizobium (found in the root nodules of leguminous plants), convert atmospheric nitrogen into usable forms (nitrates and nitrites) that plants can absorb from the soil. This process is called nitrogen fixation and is vital for soil fertility and plant growth.

Q14: What are antibiotics? How are they useful?

Answer: Antibiotics are medicines that either kill or stop the growth of disease-causing micro-organisms (bacteria and some fungi). They are useful in treating various bacterial infections in humans and animals. Penicillin was the first antibiotic discovered.

Q15: Give an example of a viral disease in plants and an example of a fungal disease in animals.

Answer: * Viral disease in plants: Yellow Vein Mosaic of Okra (Bhindi). * Fungal disease in animals (or humans as animals): Ringworm.

Application-Based Questions

Q16: You observe a microscopic organism that moves using pseudopodia and consumes smaller food particles. To which group of micro-organisms does it most likely belong?

Answer: It most likely belongs to the group of Protozoa. Organisms like Amoeba, which move using pseudopodia (false feet) and engulf food particles, are classic examples of protozoa.

Q17: Why does bread dough rise when yeast is added to it?

Answer: Bread dough rises when yeast is added because yeast, a type of fungus, undergoes fermentation. During fermentation, yeast breaks down sugars in the dough and produces carbon dioxide gas. The carbon dioxide bubbles get trapped in the dough, causing it to expand and rise.

Q18: Explain how refrigeration helps in preserving food.

Answer: Refrigeration helps preserve food by significantly slowing down the growth and reproduction of most micro-organisms, including bacteria and fungi, that cause food spoilage. Low temperatures inhibit their metabolic activities, thus keeping food fresh for a longer period.

Q19: A farmer noticed that his citrus trees are showing symptoms of 'Citrus Canker'. What type of pathogen is causing this disease?

Answer: Citrus Canker is caused by bacteria.

Q20: Why is it important to wash fruits and vegetables thoroughly before eating them raw?

Answer: It is important to wash fruits and vegetables thoroughly before eating them raw because they might carry harmful micro-organisms (bacteria, viruses, parasites) from soil, water, or handling. Washing helps remove these microbes, as well as dirt, pesticides, and other contaminants, reducing the risk of foodborne illnesses.

Q21: You found a green, filamentous (thread-like) organism growing in a pond. When observed under a microscope, it showed chlorophyll. To which group does it belong?

Answer: This organism most likely belongs to the group of Algae. Algae can be filamentous (like Spirogyra), are aquatic, and contain chlorophyll to perform photosynthesis.

Q22: How do vaccines work to prevent diseases?

Answer: Vaccines work by introducing a weakened or dead form of a pathogen, or parts of it, into the body. This stimulates the immune system to produce antibodies and memory cells against that specific pathogen without causing the actual disease. If the body encounters the real pathogen later, the immune system can quickly recognize and fight it off, thus preventing the disease.

Q23: Why are decomposers (bacteria and fungi) crucial for the environment?

Answer: Decomposers like bacteria and fungi are crucial for the environment because they break down dead organic matter (dead plants, animals, and waste products). This process recycles essential nutrients back into the soil, making them available for new plant growth. Without decomposers, nutrients would remain locked in dead organisms, and the Earth would be covered in waste.

Q24: Explain why a doctor might prescribe an antibiotic for a bacterial infection but not for a viral infection like the common cold.

Answer: A doctor prescribes antibiotics for bacterial infections because antibiotics are specifically designed to kill or inhibit the growth of bacteria. However, antibiotics are ineffective against viruses. The common cold is caused by viruses, so an antibiotic would not help cure it and could potentially lead to antibiotic resistance if misused.

Q25: What is the importance of proper storage and packing in food preservation?

Answer: Proper storage and packing, such as using airtight containers, are important in food preservation because they prevent food from being exposed to air, moisture, and pests. Air and moisture provide favorable conditions for microbial growth and spoilage. Airtight packing creates a barrier, limiting contact with external contaminants and oxygen, thereby extending the food's shelf life.

Textbook Exercise Solutions

A. Fill in the blanks:

1. The basic structural and functional unit of all living organisms is the cell.
2. Robert Hooke first discovered the cell in 1665.
3. The jelly-like substance found inside the cell membrane is called cytoplasm.
4. Mitochondria is known as the 'powerhouse of the cell'.
5. Bacteria and fungi are examples of micro-organisms.
6. Diseases caused by micro-organisms are called infectious or communicable diseases.
7. The process of heating milk to 70°C for 15 to 30 seconds and then rapidly cooling it is called pasteurization.
8. Viruses are considered a link between living and non-living things.

B. State true or false:

1. All living organisms are made up of cells. True
2. Animal cells have a cell wall. False
3. Chloroplasts are found in animal cells. False
4. Micro-organisms can be seen with the naked eye. False
5. Antibiotics are effective against viruses. False
6. Malaria is caused by bacteria. False

C. Match the following:

Column A

1. Yeast
2. Rhizobium
3. Plasmodium
4. Penicillium
5. Lactobacillus

Column B

1. Curd formation
2. Antibiotic
3. Bread making
4. Malaria
5. Nitrogen fixation

Answers:

1. Yeast - Bread making
2. Rhizobium - Nitrogen fixation
3. Plasmodium - Malaria
4. Penicillium - Antibiotic
5. Lactobacillus - Curd formation

D. Answer the following questions briefly:

Q1: Who discovered the cell and when?

Answer: The cell was discovered by Robert Hooke in 1665.

Q2: What is cytoplasm?

Answer: Cytoplasm is the jelly-like substance present inside the cell membrane. It fills the space between the cell membrane and the nucleus, and most cellular activities occur within it.

Q3: Name any two cell organelles.

Answer: Two cell organelles are Mitochondria and Nucleus.

Q4: What are micro-organisms?

Answer: Micro-organisms are tiny living organisms that are so small they cannot be seen with the naked eye and require a microscope to be observed.

Q5: Name the five major groups of micro-organisms.

Answer: The five major groups of micro-organisms are Bacteria, Fungi, Protozoa, Algae, and Viruses.

Q6: Give two examples of beneficial micro-organisms.

Answer: Two examples of beneficial micro-organisms are Lactobacillus (used in curd production) and Yeast (used in bread making and alcohol production).

Q7: What are antibiotics?

Answer: Antibiotics are medicines produced by certain micro-organisms that can kill or stop the growth of other disease-causing micro-organisms (especially bacteria).

Q8: Name two common methods of food preservation.

Answer: Two common methods of food preservation are refrigeration and preservation by using salt/sugar.

E. Answer the following questions in detail:

Q1: Describe the structure of a cell with the help of a well-labelled diagram. (Note: Diagram cannot be generated, but components are described.)

Answer: A typical cell, whether plant or animal, consists of three main parts:

• Cell Membrane (Plasma Membrane): This is the outer boundary of the cell. It is thin, elastic, and semi-permeable, controlling the movement of substances into and out of the cell. In plant cells, it is located just inside the cell wall.
• Cytoplasm: This is the jelly-like substance that fills the cell and surrounds the nucleus. It is where most of the cell's metabolic activities and chemical reactions occur. Various cell organelles are suspended in the cytoplasm.
• Nucleus: Often considered the control center of the cell, it is a dense, usually spherical structure. It is enclosed by a nuclear membrane with pores. The nucleus contains chromosomes, which carry the genetic material (DNA) in the form of genes, controlling heredity and cell functions.

Cell Organelles: Within the cytoplasm, specialized structures called organelles perform specific functions:

• Mitochondria: Convert food into energy for the cell.
• Endoplasmic Reticulum: Network involved in transport and synthesis.
• Golgi apparatus: Modifies, sorts, and packages proteins.
• Ribosomes: Sites of protein synthesis.
• Lysosomes: Contain digestive enzymes to break down waste.
• Vacuoles: Store water, nutrients, and waste (much larger in plant cells).
• Plastids (e.g., Chloroplasts): Found in plant cells, contain chlorophyll for photosynthesis.

Q2: Differentiate between a plant cell and an animal cell.

Answer:

Feature	Plant Cell	Animal Cell
Cell Wall	Present, rigid, outer covering.	Absent.
Chloroplasts	Present, contain chlorophyll for photosynthesis.	Absent.
Vacuole	Usually one large, central vacuole.	Small, numerous, or sometimes absent.
Shape	Generally fixed, rectangular shape.	Irregular or round shape.
Centrioles	Absent (except in some lower plants).	Present.

Q3: Explain the different groups of micro-organisms with examples.

Answer: Micro-organisms are classified into five major groups:

1. Bacteria: These are single-celled organisms, very diverse in shape (rod-shaped called bacilli, spherical called cocci, spiral called spirilla, comma-shaped called vibrio). They reproduce rapidly and are found almost everywhere.
   • Examples: *Lactobacillus* (in curd), *Rhizobium* (in legumes).
2. Fungi: Fungi can be unicellular (like yeast) or multicellular (like molds and mushrooms). They lack chlorophyll and are heterotrophic, obtaining food from decaying organic matter or living hosts.
   • Examples: Yeast (baking), *Penicillium* (antibiotic source), *Aspergillus* (food spoilage).
3. Protozoa: These are unicellular animal-like organisms. Most are heterotrophic and live in water, moist soil, or as parasites. They often show motility using pseudopodia, flagella, or cilia.
   • Examples: Amoeba, Paramecium, *Plasmodium* (causes malaria).
4. Algae: Algae can be unicellular (like Chlamydomonas) or multicellular (like Spirogyra, seaweeds). They contain chlorophyll and are autotrophic, meaning they produce their own food through photosynthesis. Most are aquatic.
   • Examples: Spirogyra, Volvox, Diatoms.
5. Viruses: These are the smallest micro-organisms. They are unique because they are only considered living when inside a host cell, where they reproduce. Outside a host, they are inert. They cause many diseases in plants, animals, and humans.
   • Examples: Influenza virus, HIV, Polio virus, Coronavirus.

Q4: Describe the role of micro-organisms in food preservation and in medicine.

Answer: Role in Food Preservation: Micro-organisms are often responsible for food spoilage. To prevent this, various methods are employed:

• Chemical Methods: Preservatives like sodium benzoate and sodium metabisulphite are used to inhibit microbial growth in jams, squashes, etc.
• Sugar/Salt Preservation: High concentrations of sugar (in jams, jellies) or salt (in pickles, meat, fish) create an environment that draws out moisture, making it difficult for microbes to grow.
• Oil/Vinegar Preservation: Used in pickles, oil and vinegar create an acidic or anaerobic environment unsuitable for microbial survival.
• Heat and Cold Treatments:
  • Boiling/Heating: Kills most microbes (e.g., boiling milk or water).
  • Refrigeration: Low temperatures slow down microbial growth.
  • Freezing: Stops microbial growth completely.
• Pasteurization: Specific heat treatment for milk (70°C for 15-30 seconds, then rapid cooling) to kill harmful microbes while retaining nutrients.
• Storage and Packing: Airtight containers prevent contact with air and moisture, reducing microbial contamination and growth.

Role in Medicine: Micro-organisms play a crucial role in medicine, primarily through:

• Antibiotics: Many antibiotics, which kill or inhibit the growth of disease-causing bacteria, are derived from micro-organisms (e.g., Penicillin from *Penicillium* fungus).
• Vaccines: Vaccines are prepared using weakened or dead forms of microbes, or their components. When administered, they stimulate the body's immune system to produce antibodies, thereby providing immunity against specific diseases (e.g., polio vaccine, measles vaccine).

Q5: Give reasons for the following:

a) We should not keep bread open in moist conditions for a long time.

Answer: We should not keep bread open in moist conditions for a long time because moisture provides an ideal environment for the growth of fungi (like bread mold) and bacteria. These micro-organisms thrive in moist conditions, leading to rapid spoilage of the bread and making it unfit for consumption.

b) Curd is set by adding a small amount of old curd to milk.

Answer: Curd is set by adding a small amount of old curd (called inoculum or starter) to milk because old curd contains millions of beneficial bacteria, primarily *Lactobacillus*. When added to warm milk, these bacteria multiply rapidly, converting the lactose sugar in the milk into lactic acid. The lactic acid causes the milk proteins to coagulate, resulting in the formation of curd.

c) Antibiotics are not effective against cold and flu.

Answer: Antibiotics are not effective against cold and flu because these illnesses are caused by viruses. Antibiotics are specifically designed to target and kill or inhibit the growth of bacteria. Since viruses have a different cellular structure and mechanism of action than bacteria, antibiotics have no effect on them. Using antibiotics for viral infections is ineffective and can contribute to antibiotic resistance.

d) Food is often stored in airtight containers.

Answer: Food is often stored in airtight containers to prevent contact with air, moisture, and micro-organisms present in the environment. Air and moisture provide favorable conditions for the growth of bacteria, fungi, and other spoilage microbes. Airtight containers create a barrier that restricts the entry of these elements, thereby extending the shelf life of the food and keeping it fresh.

e) Mosquitoes must be kept away.

Answer: Mosquitoes must be kept away because they act as vectors (carriers) for various disease-causing micro-organisms. For example, female Anopheles mosquitoes transmit the *Plasmodium* protozoan, causing malaria, and Aedes mosquitoes transmit viruses causing dengue and chikungunya. By keeping mosquitoes away, we can prevent the spread of these serious diseases.

References

1. Maharashtra State Board 7th Standard Science Syllabus (Based on current curriculum)
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