General Science Notes Chapter 11 Class 7 Maharashtra Board

Notes For All Chapters – General Science Class 7

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:
1. All living things are made up of cells.
2. Cells are the basic unit of life.
3. 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.