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

Cytology

The cell and its structures are studied under a branch of biology called cytology.

Cell

• The structural & functional unit of living beings is called a cell.
• A mass of protoplasm bounded by a plasma membrane.
• A unit of biological activity, delimited by a differentially permeable membrane and capable of self-reproduction.
  
  Cell Biology
• A cell is called the structural unit of a living being because the body of all living organisms is made up of one or more cells.
• The cell is called the functional unit of all organisms, because all the vital activities or physiological activities, i.e. respiration, digestion, excretion, circulation, etc are performed at the level of the cell.

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

Basic Criteria for Defining the Cell

(i) Presence of genetic material.

(ii) Presence of limiting plasma membrane.
(iii) Presence of metabolic machinery.

The cell is called Structural & Functional Unit of Life because:

(i) All living organisms are composed of one or more cells.
(ii) All the cells have a similar basic structure.
(iii) Similar cell organelles of different cells perform similar functions.

Cell Theory

• Cell theory was presented by Schleiden (Botanist) (1838) and Schwann (Zoologist) (1839). Accordingly, all plants and animals are composed of cells and the cell is the basic unit of life.
  
• Later discoveries led to the modification of cell theory. Modified cell theory is termed as Cell Principle or Modern Cell Theory.

Postulates of Modern Cell Theory 

• All living beings are composed of minute units, the cells.
• A cell is a mass of protoplasm containing a nucleus and bounded by a cell membrane, and in many cases by a cell wall also.
• All cells are alike in structure and metabolic activities.
• The functions of an organism as a whole are the result of the activities and interactions of the constituent cells.
• All living cells arise from pre-existing cells.

Size of Cell

Normal size in humans 20 µm to 30 µm in diameter.

• Largest Cell: In animals - Ostrich egg (15 cm in diameter), In plants - Acetabularia (6 - 10 cm).
• Longest Cell: In animals - Nerve cell (up to 1m), In plants - Ramie's fibre.
• Smallest Cell: PPLO  (Pleuro Pneumonia Like Organism), Mycoplasma (0.1 to 0.5 µm).

Shape of Cell

The shape of a cell mainly depends upon the specific function it performs.

• Elongated - Nerve cell 
• Discoidal/saucer - RBC
• Spindal - Muscle cell 
• Spherical - Eggs
• Branched - Pigment cell of the skin
• Slipper shaped - Paramecium
• Cuboidal - Germ cells of gonads
• Polygonal - Liver cells

Units of Measurement Used in Cell Biology

• The cells and their organelles are measured by fractions or multiples of a millimetre.
• A millimetre is one-thousandth of a metre. Its symbol is mm. 
  1 mm (millimetre) = 10^-3 metre
• One-thousandth of a millimetre is called a micron, or micrometre. Its symbol is mm and it is equal to 10^-6 m. 
  1 mm (micrometre) = 10^-3 mm
• One-thousandth of a micron is termed millimicron or nanometre. Its symbol is nm. 
  1 nm or 1 mm (millimicrometre) = 10^-3 mm or 10^-6 mm
• One-tenth of a millimicron is known as Angstrom. Its symbol is Å.
  1 Å (Angstrom) = 10^-1 nm or 10^-1 mm or 10^-7 mm

Prokaryotes and Eukaryotes

• All living organisms are made of cells. And based on the kind of cells they are made up of, they are divided into two groups: Prokaryotes and Eukaryotes.
  Structure of Prokaryotes and Eukaryotes
• Prokaryotes, which include bacteria and blue-green algae (cyanobacteria), appeared (on the earth) about 3,500 million years ago. 
• All other organisms are eukaryotes, and they probably evolved from prokaryotes.

Differences Between Prokaryotic & Eukaryotic Cells

Types of Organisms on Basis of Number of Cells

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

(i) Unicellular: Single-celled
Example: Amoeba, Paramecium, Euglena, Plasmodium- Malarial parasite, Chlamydomonas.

(ii) Multicellular: Numerous cells 
Example: Higher Plants & Animals.

Diversity of Cells

• The majority of multicellular organisms have a large body composed of numerous cells. The number of cells in such organisms is correlated to the size of the body. 
  Example: A human being weighing about 60 kg has about 60 × 10¹⁵ cells, and the blood (vascular tissue) in a human being contains about 30 × 10¹⁵ corpuscles. 
• There are also instances wherein the number of cells are fixed in multicellular organisms, e.g., rotifers and nematodes. In such organisms, mitosis stops during embryonic development. 
• The phenomenon of having a fixed, genetically constant number of cells is called eutely.