Class 9 The Fundamental Unit of Life Notes

Notes Important Questions NCERT Solutions MCQ Quiz – 1 MCQ Quiz – 2

The Fundamental Unit of Life Notes


A cell is the structural and functional unit of life.

  • Cell was discovered by Robert Hooke in 1665.

Unicellular organisms:-

The single celled organisms are called unicellular organisms. Ex:- Amoeba, Paramoecium, Bacteria, Chlamydomonas etc.

Multicellular organisms:-

The organisms that are composed of more than one cells are called multicellular organisms. Ex:- Some fungi, plants, animals etc.

  • The shape and size of the cells depend upon the specific function they perform.
  • Some cells like Amoeba has changing shape.

Plasma/Cell membrane:-

The outermost covering of the cell is known as cell membrane.

  • Cell membrane is made up of proteins & lipids.
  • Cell membrane controls the entry and exit of the materials in and out of the cell. So, cell membrane is called selectively permeable membrane.


The spontaneous movement of substances from the region of higher concentration to lower concentration through semi permeable membrane is known as diffusion. Ex:- Movement of carbon dioxide & oxygen across the cell membrane.


The spontaneous movement of water from the region of higher water concentration to lower water concentration through a semi permeable membrane is called osmosis.

Hypotonic solution:-

The solution in which the medium surrounding the cell has a higher water concentration than the cell is known as hypotonic solution.

  • In hypotonic solution, the net movement of water is from surrounding to the cell and so the cell will swell up.

Hypertonic solution:-

The solution in which the cell has higher water concentration than the medium surrounding the cell is known as hyper-tonic solution.

  • In hypertonic solution, the net movement of water is from cell to surrounding and so the cell will shrink.

Isotonic solution:-

The solution in which water concentration in the cell is equal to the water concentration of the medium surrounding the cell is known as isotonic solution.

  • In isotonic solution, there is no net movement of water.


The process by which the cells engulf in food & other materials due to the flexibility of the cell membrane is called endocytosis.

  • Amoeba acquires its food through endocytosis.

Cell Wall:-

The another rigid outer covering in addition to the plasma membrane found in plant cell is called cell wall.

  • Cell wall is mainly composed of cellulose which provides structural strength to plants.


When a plant cell loses water through osmosis there is shrinkage of contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.

  • Iodine solution, safranin solution or methylene blue solution can be used to stain the cells while observing under microscope.


Chromosomes are rod shaped structures found in nucleus which contain information for the transfer of characteristics from parents to next generation.

  • Chromosomes are composed of DNA (Deoxyribo Nucleic Acid) & protein.
  • Chromosomes are visible only when the cell is to divide.
  • DNA is present as part of chromatin which is visible as thread like structures when the cell is not to divide.

Chromatin:- Chromatin is thread like structure which is visible when the cell is not to divide.

Gene:- Functional segment of DNA is called gene.

Difference between prokaryotic and eukaryotic cell:-

S. No.Prokaryotic CellEukaryotic Cell
1.Nuclear membrane is absent.Nuclear membrane is present.
2.Its size is generally smaller.Its size is generally larger.
3.Single chromosome is present.More than one chromosomes are present.
4.Membrane bound cell organelles are absent.Membrane bound cell organelles are present.
5.Ex:- bacteria, cyanobacteria (blue green algae) etc.Ex:- Fungi, plants, animals etc.


The fluid content inside the plasma membrane is called cytoplasm.

Cell Organelles:-

Endoplasmic Reticulum (ER):-

  • There are two types of ER:-
    • RER (Rough Endoplasmic Reticulum)
    • SER (Smooth Endoplasmic Reticulum)
  • ER helps in transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus.
  • ER provides surface area for biochemical activities.
  • RER is rough due to the presence of ribosomes.
  • Protein is synthesized on RER.
  • SER helps in manufacturing of fats and lipids.
  • In liver cell, SER helps in detoxifying many poisons and drugs.
  • Some proteins and lipids function as enzymes and hormones.

Membrane Biogenesis :- The process of building cell membrane with the help of proteins and lipids is known as membrane biogenesis.

Golgi Apparatus:-

  • The materials synthesized near ER is packaged and dispatched to various targets inside and outside the cell through it.
  • Its functions include storage, modification & packaging of products in vesicles.
  • It helps in formation of lysosomes.


  • Lysosomes digest the unwanted materials & keep the cell clean.
  • Lysosomes contain powerful digestive enzymes.
  • These enzymes are made by RER.
  • Lysosomes are also known as suicide bags of a cell because if the cell gets damaged, lysosomes may burst & the enzymes will digest their own cell.


  • Mitochondria are known as powerhouses of the cell.
  • Mitochondria release energy in the form of ATP (Adenosine triphopshate).
  • ATP is known as energy currency of the cell.
  • Mitochondria have two membrane coverings.
  • Outer membrane is porous & inner membrane is deeply folded to produce more ATP.
  • They have their own DNA & ribosomes. So, they make some of their own proteins.


  • Plastids are present in only plant cell.
  • There are two types of plastids:-
    • Chromoplasts (coloured plastids)
    • Leucoplasts (white or colourless plastids)
  • Leucoplasts store materials such as starch, oils and protein granuels.
  • Chromoplasts containing the pigment chlorophyll (green pigment) are known as chloroplasts.
  • Chloroplasts also contain various yellow or orange pigments other than chlorophyll.
  • Plastids also have their own DNA & ribosomes.


  • Vacuoles are small sized in animal cells while large sized in plant cells.
  • They store, amino acids, sugar, various organic acids, & some proteins.
  • In single-celled organisms like amoeba, the food vacuole contains the food items that the amoeba has consumed.
  • In some unicellular organisms, specialised vacuoles also play important roles in expelling excess water and some wastes from the cell.

Differences between plant cell and animal cell :-

S.No Plant cellAnimal cell
(i)Plastids are present.Plastids are absent.
(ii)Cell wall is present.Cell wall is absent.
(iii)Single and large vacuole is present.More than one & small vacuoles are present.


  • SurajKumar

    Sir where is the important questions of bio

    • BrainIgniter

      Hello Suraj,
      Just click on the button of important questions given at the top of this post.

  • Sudhanshu

    Can you give me the important question of this chapter

    • BrainIgniter

      Hello Sudhanshu,
      Just click on the button of important questions given at the top of this post.


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