What is a cell? What are the essential characteristics of cells?

A cell is the basic structural, functional, and biological unit of all living organisms. It is the smallest unit of life and is often referred to as the "building block of life." Cells carry out essential functions such as metabolism, energy production, and reproduction. Organisms can be unicellular (made of a single cell) or multicellular (composed of many cells).

Discovery of the Cell:

Robert Hooke discovered the cell structure in 1665 by observing a thin slice of cork, specifically from the cork cambium of the cork oak tree (Quercus suber). The cork cambium is a layer of tissue found in the bark of plants that produces cork. Under his microscope, Hooke saw small, hexagon box-like compartments, which he named "cells" because they resembled the small rooms. These compartments were the cell walls of dead plant cells, a key observation that laid the foundation for the study of cell biology.

Discovery of the First Living Cell:

The first living cell was discovered by Anton van Leeuwenhoek in 1674. Using his improved microscope, he observed single celled organisms, including bacteria and protozoa, in pond water. These microscopic organisms, which he called "animalcules," were the first living cells ever observed, marking a key moment in the development of microbiology and cell biology.

Types of Cells:

Cells can be classified in several ways:

01. On the Basis of Nucleus:

(A) Prokaryotic Cells: These cells lack a true nucleus. Instead, their genetic material (DNA) is located in a region called the nucleoid, but it is not enclosed by a membrane. Examples - Bacteria and Archaea.
  
(B) Eukaryotic Cells: These cells have a true, membrane-bound nucleus where their genetic material is stored. Examples - Plants, animals, fungi, and protists.

02. On the Basis of Cell Wall:

(A) Plant Cells: Plant cells have a rigid cell wall made of cellulose outside their cell membrane, which provides structure and support.

(B) Animal Cells: Animal cells lack a cell wall and are enclosed only by a flexible cell membrane.

3. On the Basis of the Number of Cells:

(A) Unicellular Organisms: These organisms are made up of only one cell. The single cell performs all life functions. Examples - Bacteria, Amoeba, and some algae.

(B) Multicellular Organisms: These organisms are composed of many cells that work together, with different cells specializing in various functions. Examples - Humans, plants, fungi, and animals.

Important Cell Discoveries:

  • First Cell: Robert Hooke discovered the cell structure in 1665 by observing a thin slice of cork, specifically from the cork cambium of the cork oak tree (Quercus suber).
  • First Living Cell: Discovered by Anton van Leeuwenhoek in 1674. He observed microorganisms (protists and bacteria) in pond water.
  • Nucleus: Discovered by Robert Brown in 1831. He observed it in orchid cells, realizing its central role in cellular function.
  • Sperm Cell: Discovered by Anton van Leeuwenhoek in 1677. He observed spermatozoa using his microscope and described their motion.
  • Egg Cell (Ovum): The mammalian egg cell was first described by Karl Ernst von Baer in 1827.

Sizes of Cells:

  • Mycoplasma (smallest cell) : 0.1–0.3 µm
  • Bacterial cells : 1–5 µm
  • Red blood cells : 6–8 µm
  • Plant cells : 10–100 µm
  • Human egg cell or ovum cell (largest human cell) : ~100 µm
  • Ostrich egg (largest cell) : ~120 mm (12 cm)
  • Acetabularia (largest unicellular green algae) : Up to 10 cm
  • Human nerve cells (longest human cell) : Up to 1 meter (1000 mm) in length
  • Sclerenchyma fibres (longest cells in plant) : 55 cm long

Essential Characteristics of Cells

Structural Unit of Life:
Cells are the basic units of structure and organization in living organisms. They provide the structural basis for the body's tissues and organs.

Functional Unit of Life:
Cells carry out all necessary functions to sustain life, including metabolism, energy production, and reproduction.

Surrounded by a Plasma Membrane:
Each cell is enclosed by a plasma membrane that separates its internal environment from the external environment, regulating the movement of substances in and out of the cell.

Contain Genetic Material (DNA):
Cells house DNA, the genetic blueprint that directs all cellular activities and ensures the transmission of genetic information from one generation to the next. In eukaryotic cells, DNA is contained within a nucleus, while in prokaryotic cells, it is located in the nucleoid region.

Cytoplasm:
The cytoplasm is a gel-like substance within the cell membrane that contains the organelles and is the site for most cellular processes. It is composed of cytosol and the organelles suspended in it.

Organelles:
Eukaryotic cells contain membrane-bound organelles, each with specific functions, such as the nucleus (genetic control center), mitochondria (energy production), endoplasmic reticulum (protein and lipid synthesis), and Golgi apparatus (modification and packaging of proteins). Prokaryotic cells lack membrane-bound organelles but still perform essential functions.

Energy Production and Metabolism:
Cells perform metabolic processes that convert nutrients into energy. In eukaryotic cells, mitochondria are the primary sites of cellular respiration and energy (ATP) production.

Reproduction and Heredity:
Cells have the ability to reproduce by dividing. Eukaryotic cells undergo mitosis (for growth and repair) and meiosis (for sexual reproduction), ensuring genetic continuity.

Response to Stimuli:
Cells can respond to environmental stimuli, a characteristic essential for survival. This includes responses to chemical signals, physical interactions, and changes in their environment.

Growth and Development:
Cells grow and develop according to their genetic instructions, contributing to the overall growth and development of the organism.

Cellular Communication:
Cells communicate with each other through chemical signals and physical contacts, coordinating activities within tissues and organs.


Read More:

SAQ 1

  1. What is a cell? What are the essential characteristics of cells?
  2. Explain the fluid mosaic model of the plasma membrane
  3. Which organelles are involved in photosynthesis?
  4. Why the mitochondria is called the powerhouse of the cell?
  5. Which organelle contains enzymes for cellular respiration?
  6. Why mitochondria and chloroplast are called semi-autonomous?
  7. Mention any two advantages of the extensive network of the endoplasmic reticulum
  8. What is the function of peroxisomes in plant cells?
  9. Explain the following terms: (a) chromatin network (b) chromosomes (c) Nucleosome (d) Solenoid Model
  10. What is the function of the nucleolus in the cell?

SAQ 2



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