5. The Fundamental Unit of Life-page 1 Science Class 9 In English-CBSE Notes

5. Fundamental Unit of Life 

Biology: 

Cell : The functional and structural unit of life is called cell. 

• This is fundamental unit of all living things.
• All living things are made up of cells.
• For example - A brick works as structural unit to build a building similarily construction of a living thing body to add one by one cells to each others. 

Cells

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Tissues 

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Organs

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Body systeem

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Body

Cell is structural unit:

Cell provides structure to our body therefore it is structural unit of the body.

Cell is function unit:

All functions of the body take place at cell level, therefore It is called the functional unit of body.  

Discovery of cell: Cells were first discovered by Robert Hooke in 1665. He observed the cells in a cork slice with the help of a primitive microscope. 

Robert Brown in 1831 who discovered the nucleus in the cell.

Cell theory: 

All the plants and animals are composed of cells and that the cell is the basic unit of life, was presented by two biologists, Schleiden (1838) and Schwann (1839). The cell theory was further expanded by Virchow (1855) by suggesting that all cells arise from pre-existing cells.  

Type of organisms on the basis of Cell:

There are two kinds of organisms on the basis of cell-

(A) Unicellular Organisms : The organisms that are made up of single cell and may constitute a whole organism. such organisms are called unicellular organisms. Example - Amoeba, Chlamydomonas, Paramoecium and bacteria etc.

(B) Multicellular Organisms: 

Those organisms in which many cells group together in a single body and assume different functions in it to form various body parts. Such organisms are called multicellular organisms. Examle - fungi, plants, human being and animals etc. 

* Every multi-cellular organism has come from a single cell.

* Cells divide to produce cells of their own kind. 

* All cells thus come from pre-existing cells.

* Each living cell has the capacity to perform certain basic functions that                  are characteristic of all living forms.

The shape and size of cells are related to the specific function they perform.

(i) Some cells like Amoeba have changing shapes.

(ii) In some cases the cell shape could be more or less fixed and peculiar for a particular type of cell; for example, nerve cells have a typical shape.

Name of some cell present in human body:

(i) Nerve cell 

(ii) Blood cell

(iii) Fat cell

(iv) Bone cell

(v) Muscular Cell

(vi) Reproductive Cell

      (a) sperm

      (b) Ovum

Function of Cell: 

Parts Of Cell:

(i) Plasma Membrane Or Cell Membrane: 

It is outer most covering of the cell that seperates the contents of the cell from its
external environment. It is flexible and made up of doubled layer organic molecules like lipids and protien. 

Flexibility of Cell Membrane:  

The flexibility of cell membrane helps to obtain foods and other materials from their our environment in unicellular organisms like Ameoba. Due to this flexibility cells able to change thier shapes and they phagocyte the food materials with the help of pseudopodia.

The proccess of taking food in ameoba or other unicellular organisms by using flexibility of cell membrane is known as endocytosis or phagocytosis. 

Functions of cell membrane:  

(i) It seperates the cytoplasm from outer environment.  

(ii) It protects cell from foreign elements. 

(iii) It allows or permits the entry and exit of some selected materials like water, CO₂ and O₂ in and out of the cell. 

(iv) It also prevents movement of some other materials. 

(iv) The proccesses of diffusion and osomosis also take place through cell membrane.  

Plasma membrane is selectively permeable membrane: 

Plasma membrane allows or permits the entry and exit of some selected materials like water, CO₂ and O₂ in and out of the cell and it also prevents movement of some other materials, therefore is called a selectively permeable membrane. 

Some selected materials like carban dioxide and oxygen may transport  in and out through cell membrane by proccess of diffusion and water substances by osmosis. 

The law of movement of materials: Materials move high concentration to low concentration of material.  

Diffusion:  Diffusion is a proccess that takes place in cells in which the transportation of gaseous molecules like carbon dioxide and oxygen take place through selectively permeavle membrane. This proccess is known as diffusion. 

The proccess of diffusion in cells: some cellular wastes substance like CO₂ (which is cellular waste and requires to be excreted out by the cell) accumulates in high concentrations inside the cell. In the cell’s external environment, the concentration of CO₂ is low as compared to that inside the cell. As soon as there is a difference of concentration of CO₂ inside and outside a cell, CO₂ moves out of the cell, from a region of high concentration, to a region of low concentration outside the cell by the process of diffusion. Similarly, O₂ enters the cell by the process of diffusion when the level or concentration of O₂ inside the cell Thus, diffusion plays an important role in gaseous exchange between the cells as well as the cell and its external environment.

Osmosis: The movement of water molecules takes place through selectively permeable membrane such process is known as osmosis.

As gaseous exchanges occurs by the proccess of diffusion similarily osmosis follows the same rule. The movement of water molecules takes place through selectively permeable membrane from high concentration to low concentration.  

(ii) Nucleus: 

Nucleus is the largest organelle of cell which locates inside the cell. The nucleus has a double layered covering called nuclear membrane. 

Nuclear Membrane: This is a outer and double layer covering of nucleus. Nuclear membrane has pores which allow the transfer of material from inside the nucleus to its outside. 

Chromosomes: The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide. Chromosomes contain information for inheritance of features from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules.  

• Chromosomes are composed of DNA and protein.
• DNA molecules contain the information necessary for constructing and organising cells.
• Functional segments of DNA are called genes.
• In a cell which is not dividing, this DNA is present as part of chromatin material.

Chromatin: Chromatin material is visible as entangled mass of thread
like structures. Whenever the cell is about to divide, the chromatin material gets organised into chromosomes.

Cell Division: 

Cell division is a procsess in which a  single cell divides to form two new cells. 

The role of nucleus in cellular generation:

• Necleus divides into two parts to form new cells during cell division. 
• New cells have remain all charactristics of paraent cell. 
• It determines the development and maturity of a cell.  
• It also ditects chemical activities of the cell. 
• The nucleus plays a central role in cellular reproduction. 

Nucleoid: In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane. Such an undefined nuclear region containing only nucleic acids is called a nucleoid.

Functions of nucleus: 

(i) The nucleus plays a central role in cellular reproduction.

(ii) It divides itself to form new cells during cell division.

(iii) It carries chromosomes, DNA as well as gene with all informations in order to synthesise protien . 

(iv) It controlls all activities of cell therefore it is called the brain of cell. 

(A) Prokaryotes: Such organisms, whose cells lack a nuclear membrane, are called prokaryotes. E.g - bacteria etc.  

(B) Eukaryotes: Organisms with cells having a nuclear membrane are called eukaryotes. E.g all multicelluar organisms.  

There are two types of cell on the presence of nuclear membrane: 

(I) Prokaryotic cell: Cells those do not have nuclear membrane are called prokaryotic cells. 

(II) Eukaryotic cell: Cells those have clear nuclear membrane are called eukaryotic cells. Eg. Algee and cells of other all multicellular organisms.  

There are two types of cell on the basis of organisms:

(iii) Cytoplasm: A large  region of each cell enclosed by the cell membrane and filled with a special kind of fluid. It is called the cytoplasm. This region contains all the organelles. 

organells: Cytoplasm of each cell contains many specialised microscopic structures. Each of these microscopic structure performs a specific function for the cell. These are known as cell organelles. E.g - Mitochondria, Golgi aparatus, Ribosomes, Lysosomes and vacuoles.

Protoplasm: The region joining cytoplasm and nucleus together is called protoplasm.

Protoplasm contains all the aorganelles of a cell. 

Functions of Cell Organelles: 

(i) Building up new substances.

(ii) Cleaning up waste materials and removing other materails.

(iii) Storing energy for cell.  

There are the same organelles in various functioned cell whether it is any cell. 

The significance/utility of membranes : 

Viruses lack any membranes and hence do not show characteristics of life until they enter a living body and use its cell machinery to multiply.