Notes For All Chapters – General Science Class 7
Cell Structure and Micro-organisms
1. Introduction
- All living things are made up of tiny units called cells.
- Some organisms have only one cell (unicellular), while others have many cells (multicellular).
- The study of cells and micro-organisms helps us understand how life functions at a small level.
2. Discovery of Cells
- Robert Hooke (1665) first discovered cells while looking at a thin slice of cork under a microscope.
- Anton van Leeuwenhoek (1673) observed living cells like bacteria and protozoa.
- M. J. Schleiden and Theodor Schwann proposed the cell theory in 1838, stating that:
- All living things are made up of cells.
- Cells are the basic unit of life.
- All cells come from pre-existing cells.
3. Structure of a Cell
A cell has three main parts:
(A) Cell Membrane (Plasma Membrane)
- It is the outer covering of the cell.
- It controls what enters and exits the cell (like oxygen, food, and waste).
(B) Cytoplasm
- The jelly-like substance inside the cell.
- Contains cell organelles that perform various functions.
(C) Nucleus
- The control center of the cell.
- It contains DNA, which has genetic information.
4. Differences Between Plant and Animal Cells
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell Wall | Present (gives shape and support) | Absent |
| Chloroplasts | Present (for photosynthesis) | Absent |
| Vacuoles | Large and central | Small and scattered |
| Shape | Regular and rectangular | Irregular and round |
5. Cell Organelles and Their Functions
| Organelle | Function |
|---|---|
| Nucleus | Controls cell activities and contains genetic material (DNA). |
| Mitochondria | Produces energy (powerhouse of the cell). |
| Ribosomes | Make proteins for the cell. |
| Golgi Body | Packages and transports proteins. |
| Lysosomes | Digests waste and unwanted materials (suicide bags). |
| Chloroplasts | Perform photosynthesis in plant cells. |
| Vacuoles | Store food, water, and waste. |
6. Micro-organisms
- Micro-organisms are tiny living things that cannot be seen with the naked eye.
- They are found everywhere – in water, air, soil, and even inside our bodies.
Types of Micro-organisms
| Type | Example | Importance |
|---|---|---|
| Bacteria | Lactobacillus, E. coli | Help in digestion, but some cause diseases. |
| Fungi | Yeast, Mushroom | Used in baking and medicine. |
| Protozoa | Amoeba, Paramecium | Some cause diseases like malaria. |
| Algae | Chlamydomonas, Spirogyra | Produce oxygen and food. |
| Viruses | Influenza, COVID-19 | Cause diseases but help in making vaccines. |
7. Role of Micro-organisms in Daily Life
(A) Useful Micro-organisms
In Food Production
- Yeast helps in making bread, idli, and alcohol.
- Bacteria help in making curd and cheese.
In Medicine
- Bacteria and fungi are used to make antibiotics like penicillin.
- Vaccines help in protecting against diseases like polio and measles.
In Agriculture
- Some bacteria increase soil fertility by fixing nitrogen.
- Fungi help in decomposing dead plants and animals.
(B) Harmful Micro-organisms
Cause Diseases
- Bacteria cause tuberculosis, cholera, and typhoid.
- Viruses cause flu, polio, and COVID-19.
- Protozoa cause malaria and dysentery.
Spoil Food
- Fungi and bacteria grow on food, causing food poisoning.
- Milk, bread, and fruits get spoiled if not stored properly.
Damage Crops
- Pests and fungal infections can destroy crops and cause famine.
8. Food Preservation Methods
To stop micro-organisms from spoiling food, we use:
| Method | Example |
|---|---|
| Refrigeration | Storing milk and vegetables in the fridge. |
| Drying | Removing water from grains, fish, and meat. |
| Boiling | Killing bacteria in water and milk. |
| Salting | Preserving pickles and fish with salt. |
| Canning | Sealing food in airtight tins. |
| Pasteurization | Heating milk to kill germs (discovered by Louis Pasteur). |
9. Importance of Vaccination
- Vaccination protects us from deadly diseases.
- It helps in building immunity against harmful bacteria and viruses.
- Example: Polio, Hepatitis, and COVID-19 vaccines.
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