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Discovery of Cell, Cell Shape, Cell Size, Cell Numbers and Parts of Cell- Biology Guide for Class 8

Information about Discovery of the Cell and Parts of Cell

Title

Discovery of Cell, Cell Shape, Size and Characteristics and Parts of Cell

Class

Class 8

Subject

Class 8 Biology

Topics Covered

  • Discovery of the Cell
  • Cell Number
  • Cell Shape
  • Cell Size
  • Parts of a Cell


There are a large variety of organisms on this earth that are all distinct in their form and structure. However, they all possess similarity in their basic structure and functions. Just as a building is made up of bricks, similarly, the 'bodies' of all plants and animals are made up of cells. From microscopic bacteria, or Amoeba, to large organisms, like elephants, whales or gigantic trees, all are made up of 'cells', the basic units of all organisms.

Discovery of the Cell

Cells are the basic 'structural unit' of all living beings. They remained undiscovered for a long time because the majority of the cells are too small to be seen by the unaided eye.
  • Robert Hooke was the first scientist who observed thin slices of cork (obtained from the bark of a tree) through his self-designed microscope , in 1665.
  • He observed that they had honey-comb like structures consisting of little compartments (in Latin, 'cell' means 'a little room').
  • It was later explained that these 'compartments' were actually 'dead cells, bound by a 'cell wall'. 
  • The cells have the same basic structure, but they are different, with respect to their number, shape and size, in different living organisms.

Cell Number

  • An Amoeba and an earthworm are of different sizes. This difference, in the size of the organism, is due to the number of cells present in them.
  • While Amoeba is a living organism consisting of a single cell, an earthworm has millions of cells.
  • On the basis of their 'number of cells, living organisms can be classified into two categories
    (i) unicellular
    (ii) multicellular.

Cell Shape

  • The shapes of cells differ not only in different organisms but also in different organs of the same organism.
  • They may be oval, spherical, cuboidal, fibre-like or polygonal.
  • These differences in shapes are due to their location and function in the tissue. For instance, a nerve cell has to transmit nerve impulses to organs located in different parts of the body. Hence, they possess a long fibre-like structure. 
  • The smallest cell PPLO (Pleuro pneumonia-like organism), also called mycoplasma, is about 0.1 micron (denoted as 'p') in diameter (1 p = 10-6m).
  • The ostrich egg, considered to be the largest cell, is (nearly) 170 mm in diameter. The hen's egg also represents a single cell; it is big enough to be seen with the unaided eye.

The life span of a red blood cell is about 120 days

Activity on Hen's Egg

  • Take a hen's egg.
  • Gently break its shell and transfer the contents to a flat plate.
  • You will observe two clear portions.
  • The central yellow mass is the yolk. It is surrounded by a transparent white jelly-like fluid, called albumen.
  • Albumen and yolk represent the reserve food material in the cytoplasm.
  • Hen's egg is a single cell. Its different parts have been labelled in the diagram given here.

Activity on Cheek Cell

  • To observe animal cells make a temporary mount of cheek cells.
  • Take a clean toothpick.
  • Scratch it gently on the inner side of your cheek.
  • Some frothy material appears on the toothpick.
  • Rub it in the centre of a clean glass slide.
  • Put a drop of methylene blue.
  • Let it stain for a minute.
  • Put a cover slip and observe it under the microscope.
You will observe polygonal, isolated cells, or cells in clusters. Observe the darkly stained nucleus in each cell. 

Activity on Cheek Cell

  • Follow the instructions given below to make a slide of onion peel. (Onion peel is the thin membrane-like layer present around fleshy scale leaves of onion.)
  • Put a drop of water on a glass slide.
  • Place a small piece of neatly cut onion peel on it.
  • Put a drop, or two, of saffranin.
  • Stain for a minute. 
  • Put a cover slip and observe it under the microscope.
You will see that the cells here are arranged in rows. Observe their boundaries. There is a dark structure in the centre of each cell. It is the nucleus. 

Parts of a Cell

A cell consists of a living protoplasm surrounded by a cell membrane. The protoplasm consists of the cytoplasm and the nucleus.
Cytoplasm contains a number of structures, which are called cell organelles. Organelles are, therefore, structures present within a cell that help it to perform its relevant functions.

Cell Membrane 

  • All living cells are bound by a membrane called the plasma membrane, or the cell membrane.
  • It surrounds its inner gel-like material called protoplasm.
  • The plasma membrane controls the entry and exit of substances as per the requirements of the cell.
  • Cell wall: The cells of plants, fungi and bacteria have an additional outer covering called the cell wall.
    The cell wall is an important covering in plant cells; it provides rigidity and protection to the cell against variations in the environment. It also gives a definite shape, size and support to the cell. 

Cytoplasm

The portion of the protoplasm, lying inner to the cell membrane but outside the nuclear membrane, is called cytoplasm [kytos (hollow), plasma (liquid)].
It acts as a 'ground substance' for all cell activities. It is made up of carbohydrates, proteins, fats, minerals and vitamins, along with a large proportion of water.
All these components work together to provide a unique living nature to the protoplasm.

Nucleus

  • It is the most important part of the cell. It generally lies in the centre of the cell, however, in some cases, it may also occupy peripheral positions.
  • It controls all the activities of the cell.
  • The nucleus is a dense structure bound by a nuclear membrane.
  • The protoplasm of the nucleus is called nucleoplasm.
  • Chromatin: It has a thread-like network called chromatin.
  • Chromosomes: When the cell is ready to divide, this chromatin condenses to form thicker, thread-like structures, called chromosomes.
  • These chromosomes are the structures responsible for the characters (genes) inherited by one generation from the earlier generations. Many small living structures are present in the cell.
  • These are equivalent to the organs of the body. Hence, they are named as 'cell organelles'.

The number of chromosomes in a cell differs in different organisms. Some are shown below.

Organisms

Chromosome number

Man

46

Dog

78

Pigeon

80

Yeast

32

Wheat

42


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