physicscatalyst.com logo






CELL – THE BASIC UNIT OF LIFE





Introduction

  • The body of all organisms is made up of tiny microscopic units called cell.
  • The cell is a fundamental, structural and functional unit of living organisms and basic unit of life.
  • All living organisms are made up of cells. Cells make tissues, tissues together make organ, organs make organ system and organ systems make body of organisms. So cell is called structural unit of life.
  • All the basic functions of the body like respiration, excretion etc. are carried out by cell through  its cell organelles, so cell is called functional unit of life.
  • Cell biology is the study of cells in all aspects of structure and functions.
 

Discovery of cell:

  • Cell was first discovered by Robert Hook in 1665. He observed the cell in a cork slice with the help of a primitive microscope.
  • Anton Von Leeuwenhoek (1674) was the first to observe free cells, like bacteria, protozoa, red blood cells and sperms in his home made microscope.

Cell theory:

  • The cell theory, that all the plants and animals are composed of cells and that the cell is basic unit of life, was presented by two biologists, M. Schleiden (1838) and T. Schwann (1839).
  • The cell theory was further expanded by a german physiologist, Rudolf Virchow (1855). He gave the phrase Omnis cellula-e-cellula, i.e., all cells arises from pre-existing cells.
  • Modified cell theory is termed as cell principle or modern cell theory which postulates that:
    • All living organisms are composed of cells or cell products.
    • All living cells arise from pre-existing cells.
    • All cells are basically alike in chemical composition and metabolic processes.
The functions of an organism as a whole is the result of the activities and interaction of the constituent cells.

Shape, size and number of cells:

Cell shape:


  • The shape of cells is related to the specific function they perform.
  • Some cells like Amoeba and WBCs have changing shapes.
  • 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.

Cell size:


  • The size of the cell also varies considerably in different animals and plants.
  • The average cell size varies from 0.5 to 20µ (µ = micrometer).
  • In human body, the smallest cell is RBC and the longest one is the nerve cells.

Cell number:


  • The number of cells in living beings differs from the one in unicellular organisms to many in multicellular forms.
  • The number of cells in not definite in multicellular organisms, and may increase along with the growth and volume of organism.


Types of Cells:


Differences between Prokaryotic and Eukaryotic cells:


S. No.

Prokayotic cell

Eukaryotic cell

1
primitive and incomplete cells
Advance and complete
2
Prokaryotes are always unicellular organisms.
 
Unicellular  and multicellular
3
Nucleus is not well defined and known as nucleoid
Well defined nucleus is present
4
membrane bound organelles, such as Mitochondria, Golgi complex etc. are absent.
 
membrane bound organelles, such as Mitochondria, Golgi complex etc. are also present.
 
5
Ribosomes are smaller and scattered randomly in the cytoplasm.
 
Ribosomes are bigger. They are either attached to endoplasmic reticulum or are found free.
 
6
The prokaryotes include archaebacteria, bacteria and cyanobacteria (blue green algae).
 
Eukaryotes include all living organisms, except bacteria and blue-green algae

Structure of cell:

Structurally the cell is formed of three major parts:
  1. Plasma membrane or cell membrane
  2. Cytoplasm and its contents
  3. Nucleus

Plasma membrane (cell membrane):

  • Plasma membrane is the outermost covering of the cell that separates the contents of the cell from its external environment.
  • Plasma membrane is living, thin, delicate, elastic, selectively permeable membrane.
  • The plasma membrane is flexible and made up of organic molecules called lipids and proteins.
Functions:
  • Plasma membrane permits the entry and exit of some materials in the cells. It also prevents movement of some other material. Therefore, the plasma membrane is called a selectively permeable membrane.
  • Substances can pass across a membrane by two processes- diffusion and osmosis.
    Diffusion: Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion.
    “Diffusion is the spontaneous movement of molecules from a region of its higher concentration to a region of its lower concentration.”
    Example: movement of carbon dioxide and oxygen during respirationin organisms.
    Osmosis:
    The movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration is called osmosis.
  1. Let us see what will happen if you put an animal cell or plant cells into solution of sugar or salt prepared in water?
  1. If the medium surrounding the cell has a higher water concentration than the cell, i.e., if solution is a very diluted solution, the cell will gain water by osmosis. Such a dilute solution is called hypotonic solution.
        Water molecule will pass across the plasma membrane in both directions, but more water molecules will enter the cell than will leave.     The cell will therefore, swell up and increase in volume. This process is called endosmosis.
  1. If the medium surrounding the cell is exactly the same water concentration as the cell, there will be no net movement of water across the plasma membrane resulting in no change in the size of the cell. Such a solution is called isotonic solution.
  2. If the medium has a lower concentration of water than the cell, i.e., if it is very concentrated solution, the cell will lose water by osmosis. Such a concentrated solution is called hypertonic solution.
      In this case too, water crosses the plasma membrane in both the directions, but this time more water leaves the cell than enter it. The cell will therefore, shrink and reduce in volume. This process is known as exosmosis.

Cell wall:

  • In plant cells, there occurs a rigid cell wall which lies outside the plasma membrane.
  • Cell wall is thick, non-living and permeable covering made up of cellulose. Cellulose is a kind of carbohydrate (polysaccharide) and it provides structural strength to the plant.
Functions:
  1. Cell wall protects cell membrane and the internal structures of the cell.
  2. It provides rigidity and determines the shape of the plant cell.
  3. It prevents drying of the cell and helps it to bear unfavourable conditions.
  4. It provides mechanical strength to the plant cells.
Protoplasm:
  • All cells contain living substance called protoplasm. It is jelly-like, viscous, colourless semi-fluid substance in which various cell organelles and inclusion remains in colloidal form.
  • Protoplasm can be distinguished in two forms:
    1. Cytoplasm: it is that part of protoplasm which surrounds the nucleus.
    2. Nucleoplasm: it is that part of protoplasm which is located inside the nucleus.
                                           Protoplasm = cytoplasm + nucleoplasm

Cytoplasm

The part of the cell which occurs between the plasma membrane and nucleus envelop is called the cytoplasm
  • Cytoplasm consists of an aqueous ground substance, the cytosol, containing a variety of cell organelles and other inclusions such as insoluble waste and storage product (starch, lipid, etc.).
  • Chemically cytoplasm contains about 90% water,, 7% proteins, 2% carbohydrates and lipids and 1% inorganic minerals, minerals vitamins, etc.

Test your Knowlegde

Fill in the blanks
1) The cell is a fundamental, structural and functional __________________ of living organisms and basic __________________ of life.
2) All __________________  are made up of cells.
3) Cells make tissues, tissues together make organ, organs make organ __________________ and organ __________________s make body of organisms.
4) Cell is called structural unit of __________________.
5) All the basic functions of the body like respiration, excretion are carried out by __________________ through  its __________________ organelles, so __________________ is called functional unit of life.
6) Cell was first discovered by _____ in _____.
7) Two biologists ______ and _______presented the cell theory which says that all the plants and animals are composed of cells and that the cell is basic unit of life
8) All cells are basically alike in chemical __________________ and metabolic processes.
9) Some cells like __________________ and WBCs have changing shapes.
10) The smallest cell in human body is __________________ and the longest one is the ______
11) The prokaryotes include archaebacteria, bacteria and cyanobacteria (blue green __________________).
12) __________________ membrane is living, thin, delicate, elastic, selectively permeable membrane and is flexible and made up of organic molecules called lipids and proteins.
13) Substances can pass across a membrane by two processes- __________________ and _____
14) __________________ is the spontaneous movement of molecules from a region of its higher concentration to a region of its lower concentration.
15) The movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration is called __________________.
16) If the medium surrounding the cell has a higher water concentration than the cell, then the cell will gain water by osmosis. Such a dilute solution is called ____________
17) The swelling up and increase of volume of cell as a result of water movement inside the cell is known as ____________
18) if medium surrounding the cell is exactly the same water concentration as the cell, there will be no net movement of water across the plasma membrane resulting in no change in the size of the cell. Such a solution is called ______
19) If the medium surrounding the cell has a lower concentration of water than the cell, then cell will lose water by osmosis. Such a concentrated solution is called _________________
20) The shrinking and reduce in volume of cell as a result of water movement outside is known as__________
21)__________________ is a kind of carbohydrate (polysaccharide) and provides structural strength to the plant.
22) _____________protects cell membrane and the internal structures of the cell
23)All cells contain living substance called __________________ which is jelly-like, viscous, colourless semi-fluid substance in which various cell organelles and inclusion remains in colloidal form.
24)________ is part of protoplasm which surrounds the nucleus.
25) ______ is part of protoplasm which is located inside the nucleus.
26) Chemically cytoplasm contains about 90% _____, 7% _____, 2% _________ and lipids and 1% inorganic minerals, minerals vitamins






Class 9 Maths Class 9 Science