Grade 7 Chapter 11: Cell Structure and Micro-organisms

Introduction to Cells

We see a variety of objects in our surroundings, both living and non-living. All living organisms, whether plants or animals, are made up of tiny fundamental units called cells. Just as a wall is made of bricks, the body of every living organism is made up of cells. The study of cells is known as Cell Biology or Cytology.

Discovery of the Cell

The cell was first discovered by Robert Hooke in 1665. He observed a thin slice of cork (the bark of a tree) under his self-designed microscope. He noticed box-like compartments, which reminded him of the small rooms in a monastery, called 'cella'. Hence, he named these compartments 'cells'.

Later, Anton von Leeuwenhoek (1674) observed living cells like bacteria and protozoa for the first time using a more advanced microscope.

In 1838, Matthias Schleiden proposed that all plants are made of cells, and in 1839, Theodor Schwann stated that all animals are also made of cells. Together, they put forth the Cell Theory.

Cell Structure

Despite their microscopic size, cells are complex structures. All cells, whether plant or animal, share some basic components:

1. Cell Membrane (Plasma Membrane)

This is the outermost covering of the cell (in animal cells) or found just inside the cell wall (in plant cells). It is very thin, delicate, elastic, and semi-permeable, controlling the entry and exit of substances. It is composed mainly of lipids and proteins.

2. Cytoplasm

It is the jelly-like substance found inside the cell membrane. It fills the space between the cell membrane and the nucleus. Most of the cell's metabolic activities take place here.

3. Nucleus

The nucleus is a dense, spherical, and centrally located organelle that acts as the control center of the cell. It is separated from the cytoplasm by a double-layered nuclear membrane, which has pores allowing the passage of substances. The nucleus contains thread-like structures called chromosomes, which carry genetic information in the form of DNA. Chromosomes contain genes, which are hereditary units responsible for transferring characteristics from parents to offspring.

4. Cell Organelles

Within the cytoplasm, various specialized structures called cell organelles perform specific functions. These include:

• Endoplasmic Reticulum (ER): A network of membranes involved in transport and synthesis of proteins and lipids.
• Golgi apparatus (Golgi complex): Involved in packaging and secreting proteins and lipids.
• Mitochondria: Known as the 'powerhouse of the cell' as they generate energy through cellular respiration.
• Ribosomes: Small organelles responsible for protein synthesis.
• Lysosomes: Contain digestive enzymes, often called 'suicidal bags' as they break down waste and cellular debris.
• Vacuoles: Storage sacs for water, nutrients, and waste products. They are larger in plant cells.
• Plastids: Found only in plant cells. They include chloroplasts (for photosynthesis), chromoplasts (for color), and leucoplasts (for storage).

Plant Cell vs. Animal Cell

While both plant and animal cells are eukaryotic, they have distinct differences:

• Cell Wall: Present only in plant cells, providing rigidity and protection. Animal cells lack a cell wall.
• Chloroplasts: Present only in plant cells, enabling photosynthesis. Animal cells lack chloroplasts.
• Vacuole: Plant cells typically have a large, central vacuole that occupies most of the cell volume. Animal cells have small, numerous vacuoles, or sometimes none.
• Shape: Plant cells usually have a fixed, rectangular shape due to the cell wall. Animal cells have irregular shapes.

Common components: Both have cell membrane, cytoplasm, and nucleus. They also have mitochondria, endoplasmic reticulum, Golgi apparatus, and ribosomes.

Micro-organisms (Microbes)

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

Study of Micro-organisms: The study of micro-organisms is called Microbiology.

Classification of Micro-organisms

Micro-organisms are broadly classified into five major groups:

1. Bacteria

• Single-celled (unicellular) organisms.
• Vary in shape: rod-shaped (bacilli), spherical (cocci), spiral (spirilla), or comma-shaped (vibrio).
• Can be found almost everywhere.
• Reproduce rapidly, often by binary fission.
• Examples: Lactobacillus (in curd), Rhizobium (in leguminous plants), E. coli (in intestines).

2. Fungi

• Can be unicellular (e.g., yeast) or multicellular (e.g., molds, mushrooms).
• Lack chlorophyll, so they are heterotrophic (depend on other organisms for food).
• Often grow on decaying organic matter, moist surfaces, and food items.
• Examples: Yeast (used in baking and brewing), Penicillium (produces antibiotic penicillin), Aspergillus (causes food spoilage).

3. Protozoa

• Unicellular organisms.
• Most are heterotrophic and live in water, moist soil, or as parasites.
• Can move from one place to another using pseudopodia (Amoeba), flagella (Euglena), or cilia (Paramecium).
• Examples: Amoeba, Paramecium, Plasmodium (causes malaria), Entamoeba (causes amoebic dysentery).

4. Algae

• Can be unicellular (e.g., Chlamydomonas, diatoms) or multicellular (e.g., Spirogyra, seaweeds).
• Contain chlorophyll and perform photosynthesis (autotrophic).
• Mostly aquatic, found in fresh or marine water.
• Examples: Spirogyra, Chlamydomonas, Volvox, diatoms.

5. Viruses

• Smallest of all micro-organisms, much smaller than bacteria.
• Unique because they are considered a link between living and non-living things.
• They can only reproduce inside the living cells of other organisms (hosts). Outside a host cell, they behave like non-living particles.
• Cause a wide range of diseases in plants, animals, and humans.
• Examples: Influenza virus (causes flu), HIV (causes AIDS), Polio virus (causes polio), Coronavirus (causes COVID-19).

Beneficial Micro-organisms

Not all micro-organisms are harmful; many are essential for life and play vital roles:

• In Food Industry:
  • Curd and Cheese: Lactobacillus bacteria convert milk into curd and cheese.
  • Bread and Cakes: Yeast (a fungus) is used in baking to make dough rise.
  • Alcoholic Beverages: Yeast is used in fermentation to produce alcohol.
• In Medicine:
  • Antibiotics: Medicines that kill or stop the growth of disease-causing micro-organisms. Penicillin was the first antibiotic, discovered by Alexander Fleming from the fungus *Penicillium*.
  • Vaccines: Weakened or dead microbes are used to produce vaccines, which build immunity against diseases.
• In Agriculture and Environment:
  • Nitrogen Fixation: Bacteria like Rhizobium in legumes convert atmospheric nitrogen into usable forms for plants.
  • Decomposition: Bacteria and fungi act as decomposers, breaking down organic waste and dead organisms, recycling nutrients in the soil.
  • Cleaning the Environment: Microbes help in sewage treatment and bioremediation (cleaning up pollutants).

Harmful Micro-organisms (Pathogens)

Micro-organisms that cause diseases are called pathogens or disease-causing microbes. They can cause diseases in humans, animals, and plants.

Diseases in Humans caused by Microbes:

• Bacteria: Tuberculosis, Cholera, Typhoid, Diphtheria, Tetanus, Pneumonia.
• Viruses: Common Cold, Influenza (Flu), Polio, Measles, Chickenpox, AIDS, COVID-19.
• Protozoa: Malaria (by Plasmodium), Amoebic dysentery (by Entamoeba).
• Fungi: Ringworm, Athlete's foot, skin infections.

Diseases in Animals caused by Microbes:

• Anthrax: Bacterial disease in cattle and sheep (caused by *Bacillus anthracis*).
• Foot and Mouth Disease: Viral disease in cattle.
• Bird Flu (Avian Influenza): Viral disease in birds.

Diseases in Plants caused by Microbes:

• Citrus Canker: Bacterial disease affecting citrus fruits.
• Rust of Wheat: Fungal disease affecting wheat.
• Yellow Vein Mosaic of Okra (Bhindi): Viral disease.

Food Preservation

Since micro-organisms can spoil food, various methods are used to preserve food:

• Chemical Methods: Using preservatives like sodium benzoate and sodium metabisulphite to prevent microbial growth.
• Sugar/Salt Preservation: Adding salt to meat/fish or sugar to jams/jellies. High concentrations of salt/sugar draw out water from microbes, preventing growth.
• Oil/Vinegar Preservation: Using oil or vinegar (acetic acid) in pickles, as microbes cannot survive in such environments.
• Heat and Cold Treatments:
  • Boiling/Heating: Kills microbes (e.g., boiling milk).
  • Refrigeration: Slows down microbial growth (e.g., storing food in fridge).
  • Freezing: Stops microbial growth completely (e.g., freezing vegetables).
• Pasteurization: Heating milk to about 70°C for 15 to 30 seconds and then rapidly cooling it. This kills harmful microbes without significantly affecting taste or nutritional value. This process was discovered by Louis Pasteur.
• Storage and Packing: Storing food in airtight containers to prevent contact with air and moisture, which can promote microbial growth.

Summary

• Cell: Basic structural and functional unit of all living organisms, discovered by Robert Hooke.
• Cell Organelles: Specialized structures within the cell performing specific functions (e.g., nucleus, mitochondria, chloroplasts, vacuoles).
• Plant vs. Animal Cell: Plant cells have cell wall, chloroplasts, and a large central vacuole, unlike animal cells.
• Micro-organisms: Organisms too small to be seen with naked eye, studied in Microbiology.
• Types of Micro-organisms: Bacteria, Fungi, Protozoa, Algae, and Viruses.
• Viruses: Act as a link between living and non-living, only reproduce in host cells.
• Beneficial Microbes: Used in food (curd, bread), medicine (antibiotics, vaccines), agriculture (nitrogen fixation, decomposition).
• Harmful Microbes (Pathogens): Cause diseases in humans (e.g., cholera, flu, malaria), animals (e.g., anthrax), and plants (e.g., citrus canker).
• Food Preservation Methods: Chemical methods, sugar/salt, oil/vinegar, heat/cold treatments (pasteurization), and proper storage/packing.