Cell Number, Shape and Size

Organisms can be classified based on the number of cells they possess:

1. Unicellular organisms: Single-celled organisms capable of independent existence, carrying out all life functions within one cell. Examples include Amoeba and Euglena.

2. Multicellular organisms: Organisms composed of more than one cell, with specialized cells performing different functions.

Example: In multicellular organisms, cells vary in size and shape depending on their specific functions. For instance, parenchyma cells in plants are polyhedral and used for storage, while sclerenchyma cells are spindle-shaped and provide mechanical support.

The shape of cells is closely tied to their function:

• Nerve cells: Long and branched, conducting nerve impulses
• Red blood cells: Biconcave, optimized for carrying oxygen
• Muscle cells: Cylindrical or spindle-shaped, facilitating body movement

Highlight: The பொலிகோனல் நுட்பமான செல் அமைப்பு (polygonal intricate cell structure) of parenchyma cells in plants allows for efficient storage and metabolic functions.

Cell size varies greatly, ranging from less than a micron to several centimeters:

• Smallest living cell: PPLO (Pleuro Pneumonia Like Organism) at 0.1μm
• Largest living cell: Ostrich egg, 170 to 180 mm in diameter
• Bacteria: 0.1 to 0.5 μm
• Sclerenchyma fibre: Up to 60 cm in length

This diversity in cell size and shape allows organisms to adapt to various environmental conditions and perform specialized functions.

Components of a Cell

Cells are composed of various structures and substances, each playing a vital role in cellular function:

1. Cell Membrane: The outer boundary of the cell, controlling substance movement.

2. Cytoplasm: The gel-like substance within the cell, housing organelles and cellular components.

3. Nucleus: The control center of the cell, containing genetic material.

4. Cell Organelles: Specialized structures performing specific functions, divided into membrane-bound and non-membranous organelles.

Membrane-bound organelles include:

• Endoplasmic Reticulum
• Golgi Complex
• Mitochondria
• Plastids (in plant cells)
• Lysosomes

Non-membranous organelles include:

• Ribosomes
• Cytoskeleton
• Centrioles (in animal cells)

5. Cell Inclusions: Various substances stored or produced by the cell, including:

• Storage products (e.g., starch, fats, proteins)
• Secretory products (e.g., enzymes, hormones, pigments)
• Excretory products (e.g., inorganic salts, latex, tannin)

This complex organization allows cells to carry out the diverse functions necessary for life.

Differences Between Plant and Animal Cells

While plant and animal cells share many similarities, there are key differences in their structures:

Plant cells have:

• Cell wall
• Chloroplasts
• Central vacuole

Animal cells have:

• Centrosome with centrioles
• Lysosomes
• Flagella (in some cases)

These differences reflect the unique functions and adaptations of plant and animal cells. For example, the cell wall in plant cells provides structural support, while chloroplasts enable photosynthesis.

Highlight: The presence of விலங்கின் செல்கள் மற்றும் கொள்கலன் நீரிணைகள் (animal cells and container fluids) in animal cells allows for greater flexibility and specialized functions compared to plant cells.

Understanding these differences is crucial for comprehending the diverse ways in which plants and animals carry out life processes at the cellular level.

What is a Cell?

A cell is the fundamental structural and functional unit of all living organisms. This concept was first observed and named by Robert Hooke in 1665 when he examined cork under a microscope and saw honeycomb-like structures he called "cells".

Highlight: Anton Von Leeuwenhock first described a living cell in 1667, while Robert Browne discovered the nucleus in 1833.

The cell theory, proposed by Mathias J Schleiden and Theodore Schwann in the late 1830s, established three key principles:

1. All living organisms are composed of cells and cell products
2. All cells arise from pre-existing cells through cell division
3. The body of living organisms is made up of one or more cells

This theory forms the foundation of modern cell biology and our understanding of life at the cellular level.