CBSE Class 8 Biology – 8. Cell : Structure and Functions (Notes)

 [h1]

Cell: – The basic structural and functional unit of a living organism is called cell.

All living beings are made up of cells.

Some organisms are made up of a single cell or of a few numbers of cells.

Some other organisms are made up of numerous cells.

Cell

Tissue

Organ

Organ System

Organism

Discovery of Cell:- Cell was first discovered by Robert Hooke in 1665. He observed that just like honeycomb, organisms are also composed of small compartments. He named these compartments as Cells.                                                                                                                                                                                                                                      Cell theory state that:                                                                                                                                                                                                                            All living organisms are composed of cells.                                                                                                                                                                                                            Cell is the fundamental unit of life.                                                                                                                                                                                                                  All new cells come from pre-existing cells.

Q.1 Who discovered cells and how?
Sol. It was Robert Hooke who discovered cells. He observed the cells in thin slices of cork. They appeared like small compartments when viewed through the microscope.

Q.2 Why is the cell called structural and functional unit of life?
Sol. A cell is capable of carrying out all the life functions; such as nutrition, excretion, respiration, etc. Hence a cell is called the functional unit of life. Additionally, the cell is the smallest unit of life and all the living beings are made up of cells. Hence a cell is called the structural unit of life.

                  Variation in cell number, shape and size:

Variation in Number:-Living beings show a large variation in number of cells. Large organisms; like humans, elephants, tiger, etc. are made up of trillions of cells but Amoeba and bacteria are made up of single cell.

        Organisms On the basis of number of cells,

Unicellular Organism:- An organism which is made up of a single cell is called unicellular organism.   Ex- bacteria, protozoa, yeast, etc

Multicellular Organism:-An organism which is made up of more than one cell is called multicellular organism   Ex- humans, elephants, tiger, banyan, snail, etc

Tissue:-A group of cells that works together to perform a particular function is called a tissue.

All body functions are done by single cell itself in a unicellular organism.

Different groups of cells perform different functions in a multicellular organism.

Shape and Size of cells

Cells vary in shape and size. A particular shape of a cell facilitates the particular function performed by that cell. Some examples are as follows:

Amoeba is of irregular shape. The shape of amoeba keeps on changing because of formation of pseudopodia.

Similarly, white blood cells (WBCs) in our body are of irregular shape.

Cells are generally round, spherical or elongated.

Smooth muscle cells are spindle-shaped. They are swollen in the middle and pointed at both ends.

Red Blood Cells are concave and round. RBCs can easily pass through narrow capillaries because of concave shape.

Cells of striated muscle are cylindrical in shape and are very long.

Nerve cells (neurons) are totally irregular like a star with long tail.

Size of Cells:- The Size of cell also varies in different animals and plants. Most of the cells are microscopic in size like red blood cells (RBC) while some cells are fairly large like nerve cells. Average size of cells varies from 0.5 to 20 micron.

Ostrich egg is the largest living cell and it measures 170 mm x 130 mm.

Type of cells

Prokaryotic cell	Eukaryotic cell
Primitive	1.Advance/ complex
Unicellular	2. Multicellular
Primitive nucleus	3. Well-developed nucleus
Nuclear membrane absent	4. Nuclear membrane present
[h1] Nucleolus absent	5. Nucleolus present
Monera ,bacteria, BGA	6.  Plant ,animal

Parts of Cell

        Cell Membrane   Cell wall     Cytoplasm Nucleus      Protoplasm          Vacuoles                                     Plastids        Golgi body        ER

Cell Membrane/Plasma membrane:-The outer boundary of a cell is called cell membrane.

Structure	Function
It is composed of protein and lipid. It is very thin, delicate, elastic and selectively permeable membrane. It is living part of the cell.	Cell membrane provides protection to the cell. It facilitates exchange of materials between cell and external environment. It separates the contents of the cell from external environment. Cell membrane is semi-permeable in nature due to it allows selected materials to pass through it.

Cell wall: Cell wall is outer most, non-living, thick and freely permeable covering.                                                              It is made up of cellulose.                                                                                                                                                                        Cell wall is available in eukaryotic plant cells and in prokaryotic cells.

Note:-plant cell lacks motility (locomotion) due to present of cell wall.

Functions:

          It determines the shape of the cell.

          It protects the plasma membrane.

          It helps in the transport of various substances in and out of the cell.
Protoplasm: The jelly-like, viscous, colourless semi-fluid substance that present in cell is called protoplasm.
          Protoplasm consists two parts: cytoplasm and nucleoplasm.
Cytoplasm: The part of protoplasm that present between the cell and nucleus is called cytoplasm.
          Different cell organelles are present in the cytoplasm.
Nucleoplasm: The part of protoplasm that present inside the nucleus is called nucleoplasm.
Functions:

         Protoplasm acts as a store of vital chemicals like amino acids, proteins, sugars etc.

         It is the site of some metabolic reactions.
Golgi body: first discovered by Camillo Golgi.                                                                                                                                         Characteristics: Also known as Golgi complex/ Golgi apparatus:                                                                                         It consists of membrane bound structures arranged parallel to each other.
Functions:

      Golgi bodies store, modify, package and dispatch the substances.
Ribosomes: 
    Characteristics:                                                                                                                                                                      Ribosomes are extremely small, dense and round bodies in the cytoplasm.                                                       Ribosomes are made up of ribonucleic acid(RNA) and proteins.                                                                                In prokaryotic cell, ribosomes are of 70 types.                                                                                                            In eukaryotic cells, ribosomes are of 80 types.
Function:                                                                                                                                                                                        synthesis of proteins.
Mitochondria: 
      Mitochondria are small rod-shaped organelles.
      Structure: Consist of  Outer membrane , Inner membrane , Cristae ,DNA ,Ribosome
Functions:

     They provide energy for the vital activities of living cells.

They store energy releases during reactions, in the form of ATP (Energy currency of the cell).       Therefore it is also called power house of the cell.

   Plastids:

       The organelle that contains pigment is called plastid. It are usually spherical in shape.

It is found in plant cells only.

Plastics are of three types:
Chloroplasts: Green coloured plastids containing chlorophyll.
Chromoplasts: Colourful plastids.
Leucoplasts: Colourless plastids.
  Structure: They are double membrane bound organelles.
  Function:
Chloroplasts: manufacturing food by the process of photosynthesis.
Chromoplasts: provide color to flower and fruits.
Leucoplasts: help in storage of various substances like starch ,protein and fats
Lysosomes:  They are small, spherical, sac like structures which contain several digestive enzymes enclosed in a membrane.                                                                                                                                                                                         They are found in eukaryotic cells mostly in animals.
Functions:

       Lisosomes help in digestion of foreign substances and worn-out cell organelles.

       They provide protection against bacteria and virus.

       They help to keep the cell clean.

During the disturbance in cellular metabolism, for eg; when the gets damaged, lysosomes     may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as suicide bags of a cell.
   Vacuoles:
The bag like structure that is seen inside a cell is called vacuoles.                                                          Fluid-filled inside the vacuoles
In plant cells, vacuoles are large and permanent.
In animal cells, vacuoles are small in size and temporary.
In mature plant cell, it occupies 90% space of cell volume. Due to its size, other organelles including nucleus shift towards plasma membrane.
Endoplasmic Reticulum: It is a network of tube like structures from nuclear membrane to plasma membrane.
  It is absent in prokaryotic cells and matured RBCs of mammals.
  There are two types of endoplasmic reticulum:
Rough ER:  Ribosomes are present on the surface.
Smooth ER: Ribosomes are absent.
Functions:

         It gives internal support to cell.

         It helps in transport of various substances from nuclear membrane to plasma membrane or  vice versa..

Function:-

Rough ER helps in synthesis and transportation of proteins.

Smooth ER helps in synthesis and transport of lipids.
Nucleus: The dense and spherical organelle which is present inside the centre of cell is called    nucleus.
   Nucleus is bounded by double membranes, both forming nuclear envelope.
   Nuclear envelope contains many pores known as nuclear pores.
   The fluid which is inside the nucleus is called nucleoplasm.
Nucleus contains chromosomes which are thread like structures. And chromosomes contain   genes which are the carriers of genetic information.
Functions:
Nucleus controls the function of cell.
Nucleus controls inheritance of characters.
Nucleus is the storehouse of genes. Without nucleus, cell can neither survive nor shows   specialized activities.

Difference Between Plant Cell and Animal Cell
Plant Cell	Animal Cell
Cell wall is present.	Cell wall is absent.
Chloroplast is present.	Chloroplast is absent.
Vacuoles are large.	Vacuoles are much smaller or absent.