The Fundamental Unit of Life 9th Class Full Chapter

In 1665, the English physicist Robert Hooke looked at a slice of cork through a microscopic lens and noticed some “Pockets” in it. Robert Hooke believed these pores had served as containers for the ‘plant juice’. He called these pores as ‘cells’ meaning ‘little rooms’.

Cells are very small structures so can not be seen by the naked eye. Hence, the development of microscopes has been an important factor in our understanding of the cell.

As per modern knowledge, we know that the cells are the basic unit of life. It is the first structure in which life can exist.

Cell- (Basic Unit of Life)

The basic structural and functional unit of life is the cell. ‘Cell’ is a Latin word which means ‘a little room’. All Living organisms are made up of cells/ cells. As a home is made up of bricks, an organism is made up of cells.

Different organisms have different types of cells. Even cells of an individual are not the same. Nevertheless, all cells have some fundamental common features such as

  • All cells have a definite boundary or cell membrane
  • The structure of the cell membrane is the same in all cells.
  • All cells have genetic material (usually as DNA or sometimes as RNA).
  • Various functions such as the synthesis of nucleic acids (replication) and proteins (transcription and translation) Occur exactly in similar manner.
  • The mechanism of energy production is the same in all cells.
  • All cells transmit genetic information to its successor cells.
  • Exchange of energy, material, and other basic metabolic processes are also the same.

Cell Theory

At the beginning of the 19th Century, several discoveries were made. These finally led the german botanist Mathias Jacob Schleiden (1838) to establish the cell theory. Schleiden observed large varieties of plant tissues and found all of them were composed of cells. He concluded that the cells are the ultimate basic units that form the structure of the plant tissues.

Schwann at the same time critically examined many types of animal tissues. He was able to see the nuclei, but not the cell walls. (as animal cell lack cell walls, they have only a very thin limiting membrane which we call today as plasma membrane).

Schwann found that interior of both animal and plant cells was almost the same. Both had a nucleus, surrounded by clear substance around it. After his investigations, Schwann proposed his hypothesis and Stated the concept of the cell as a mass of protoplasm, limited in space by a limiting membrane and possessing a nucleus.

Both Schleiden and Schwann compared their finding in 1839 and jointly proposed cell theory. Important points of this theory are given below :

  • All living beings are composed of cell.
  • All Cells are basically similar in chemical composition and metabolic activities.
  • The functions of the organisms are due to the interactions of the constituent cells
  • New cells develop inside the old ones (Cytoblast)
  • Cell could be generated spontaneously like a crystal

Modern Cell Theory (Cell Principle)

The Cell theory proposed by Schleiden and Schwann was not perfect. It had many shortcomings. Hence it was modified time to time with new discoveries.

One Important modification was proposed by Rudolf Virchow (1855). He stated the concept “Ommins Cellula e cellula”, which means that ” the new cells always arise from pre-existing cells”. This proposal is called the theory of lineage.

This concept was also proved experimentally by Louis Pasteur (1862). Two most important features of modern cell Theory are:

  1. All Living organisms are composed of cells and their products.
  2. All Cells arise from pre-existing cells

Cell Principle (Modern Cell Theory)

  • Cell is the basic, Structural, and functional unit of life
  • Cell is the smaller unit in which life can exist.
  • All Living beings are composed of cells.
  • Each cell consists of a protoplasmic mass, containing a nucleus and surrounded by a cell membrane with or without a cell wall.
  • A Cell is able to maintain its vitality and individuality.
  • All cells are closely similar in their structure, chemical composition, and basic activities.
  • New cells always arises from a pre-existing cell.
  • A Cell can be specialized to perform a particular function according to the needs of the individual.
  • All cell contains all the required information in the form of genetic material
  • All present-day cells have a common origin as they have arisen from a single cell that was evolved for the first time on the earth.

Exceptions to Cell Theory

  • There are many cells, which do not have a nucleus. Exanple-Bacteria, Mycoplasma Etc.
  • Viruses do not have cell structure, in spite of this, they can show some living properties
  • Mammalian RBCs, sieve tube cells of plants do not have any nucleus.
  • Many Fungi (Rhizopus) and algae (Vaucheria) have cells that contain many nuclei.

Features Common to All Cells

There is a great biodiversity. Different organisms have different types of cells. Even all cells of an individual are not the same. Nevertheless, all cells have some fundamental common features, These are:

  • All Cells have genetic material (usually as DNA or Sometimes as RNA).
  • All Cells have a definite boundary or cell membrane is same in all cells
  • Synthesis of nucleic acids (replication) and proteins (transcription and translation) occurs on exactly similar plan.
  • Cells have exactly are same genetic cides
  • Mechanism of aerobic respiration is the same in all cells
  • All Cells transmit genetic information to its successor cells.
  • Exchange of energy, material and other basic metabolic processes are also same.

Unicellular and Multicellular Organisms 

Organisma which consists of a single cell are called Unicellular (uni=single) organisms. While those organisms which Consists of more than one cells are called multicellular organisms.

The Single cell in unicellular organisma behave completely as an individual and carries out all the vital life activities and there is no differentiation. Since the Organisms is made up of single cell, the grade of body organisation of unicellular organisms is acellular.

In multicellular organisms, there can be several grades of body organisation. Various cells in these organisma get specialised for a particular function. This process of getting specialised for a particular function is called differentiation.

 

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