Key Points

The invention of the microscope, using curved glass lenses, revealed a hidden world of tiny living organisms that are invisible to the naked eye.

In 1665, scientist Robert Hooke observed a thin slice of cork under a microscope and was the first person to use the word 'cell' to describe the small, box-like structures he saw.

Antonie van Leeuwenhoek, known as the Father of Microbiology, was the first person to observe and describe single-celled living things like bacteria and blood cells using his improved microscopes.

The cell is the basic structural and functional unit of all living beings. All organisms, whether large or small, are made up of one or more cells.

A typical cell consists of three main parts: the cell membrane which is the outer boundary, the cytoplasm which is a jelly-like substance, and the nucleus which controls the cell's activities.

Plant cells differ from animal cells as they have an additional outer layer called the cell wall, chloroplasts for photosynthesis, and a large central vacuole for storage and support.

Cells vary in shape and structure because it is related to their specific function. For example, nerve cells are long and branched to transmit messages, while muscle cells are spindle-shaped for contraction.

In complex organisms, life is organized in levels: Cells group to form Tissues, tissues form Organs, organs form Organ Systems, and all systems together form an Organism.

Unicellular organisms like bacteria and Amoeba are made of a single cell. Multicellular organisms like plants and animals are made of many cells.

Microorganisms, or microbes, are very small living beings that can only be seen with a microscope. The main types are bacteria, fungi, protozoa, and some algae.

Viruses are microscopic but are not true cells. They are acellular and can only reproduce inside the living cells of a host organism.

Some microorganisms, like bacteria and fungi, act as decomposers. They break down dead organic matter and waste into simpler substances, cleaning the environment and recycling nutrients.

Yeast, a type of fungus, is used in baking to make bread soft and fluffy through fermentation. Bacteria like Lactobacillus are essential for making curd from milk.

Rhizobium bacteria live in the roots of leguminous plants like peas. They convert nitrogen from the air into a usable form for the plant, which increases soil fertility.

Microalgae are microscopic plant-like organisms that produce more than half of the Earth's oxygen. Some, like Spirulina, are also used as nutritious food supplements.