Cell, Types of Cell and Organisms | Science & Technology for UPSC CSE PDF Download

Cytology

Cytology is the branch of biology that deals with the study of cells and their structures.

Cell

• The cell is the structural and functional unit of life.
• It is a mass of protoplasm enclosed within a plasma membrane.
• It performs all life processes like respiration, digestion, and excretion.
• Cells are capable of self-reproduction and carry genetic information.

Why is the Cell a Basic Unit of Life?

• All living organisms are made up of one or more cells.
• All life processes occur inside cells.
• Different cells perform different functions in multicellular organisms.

Discovery of Cell

1. Robert Hooke (1665): An English man and first curator of Royal Society of London.

• Observed a thin transverse section of the bark of a tree under a self-designed microscope.
• He noticed honey-comb-like compartments.
• He coined the term cell.
• He wrote a book - Micrographia.
• He observed dead cells.
  Image drawn by Hooke and the Cork Tissue

2. Antony Van Leeuwenhoek (1674): was the first to observe living cells like Bacteria (from tartar of teeth), Erythrocytes (fish), Sperms, and Protozoans (Vorticella).

3. N. Grew (1682): Proposed cell concept which states that a cell is the unit of structure of organisms.

4. Rudolf Virchow (1858): Proposed that new cells formed from the pre-existing cells.

5. Knoll & Ruska (1931): Designed the electron microscope which was employed to study the ultrastructure of a cell and various cell organelles.

Microscope designed by Knoll and Ruska

Cell Theory

Classical Cell Theory (1838-1839):

• Proposed by Schleiden and Schwann.

• All living organisms are made of cells.

• Cell is the basic structural and functional unit of life.

Modern Cell Theory:

1. All living beings are composed of cells.

2. Cells arise only from pre-existing cells.

3. Cells perform all essential life activities.

4. Cells contain hereditary material (DNA).

5. All cells are basically similar in structure and function.

Size and Shape of Cell

Units of Measurement Used in Cell Biology

• Millimetre (mm) = 10⁻³ m
• Micrometre (µm) = 10⁻⁶ m
• Nanometre (nm) = 10⁻⁹ m
• 1 µm = 10⁻³ mm
• 1 nm = 10⁻³ µm
• Angstrom (Å) = 10⁻¹ nm = 10⁻⁷ mm

Prokaryotes and Eukaryotes

Prokaryotes

Prokaryotes are organisms whose cells lack a nuclear membrane, so the nuclear region is poorly defined and is called a nucleoid. They also lack most membrane-bound cytoplasmic organelles. In photosynthetic prokaryotic bacteria, chlorophyll is present in membranous vesicles, not in plastids.

Eukaryotes

Eukaryotes are organisms whose cells have a well-defined nucleus enclosed by a nuclear membrane. Their cells also contain membrane-bound organelles, such as plastids in plant cells, where chlorophyll is stored for photosynthesis.

Types of Organisms on Basis of Number of Cells

Based on the number of cells the organisms can be categorised as:

1. Unicellular organisms: Made of one cell (e.g., Amoeba, Paramecium, Chlamydomonas).

2. Multicellular organisms: Made of many cells (e.g., Plants, Animals).

Eutely: Fixed number of cells in an organism (e.g., rotifers, nematodes).