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CELL – THE BASIC UNIT OF LIFE




Introduction

  • The body of all organisms is made up of tiny microscopic units called cell.
  • A single cell as seen in unicellular organisms (e.g., bacteria and protozoa) is capable of performing all the vital activities of the body necessary for survival.
  • In multicellular organisms, although all cells of the body perform some basic functions like synthesis and respiration, they may form groups to collectively carry out some specific functions.
  • The cell is, thus, a fundamental, structural and functional unit of living organisms and basic 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.
  • Robert Brown (1831) discovered nucleus in the cells of orchid roots.
  • Purkinje (1839) coined the term ‘protoplasm’ for the fluid substance of the cell.

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 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.

Prokaryotic and Eukaryotic cells:

The cells can be divided into two types: Prokaryotic and Eukaryotic cells.

Prokaryotic cells:

  • Prokaryotic cells are primitive and incomplete cells.
  • Prokaryotes are always unicellular organisms.
  • Prokaryotic cells have all three basic structures of a typical cell but lack nuclear membranes around their genetic substances (DNA).
  • Nuclear material of a prokaryotic cell consists of a single chromosome which is in direct contact with cytoplasm.
  • The undefined nuclear region in the cytoplasm is called nucleoid.
  • In a prokaryotic cell other membrane bound organelles, such as Mitochondria, Golgi complex etc. are also absent.
  • Ribosomes are smaller and scattered randomly in the cytoplasm.
  • The prokaryotes include archaebacteria, bacteria and cyanobacteria (blue green algae).

Eukaryotic cells:

  • Eukaryotic cells are advanced and complete cells.
  • Eukaryotes include both unicellular and multicellular organisms.
  • These cells contain membrane bound nuclei.
  • The membrane bound cell organelles, like Mitochondria, Golgi complex etc. are present and perform specific functions.
  • Ribosomes are bigger. They are either attached to endoplasmic reticulum or are found free.
  • 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.”
  • During respiration, carbon dioxide is given out as a waste product and accumulates in higher concentration  inside the cell the concentration of carbon dioxide in the external environment of the cell is comparatively low. Due to the difference in concentration inside and outside the cell, carbon dioxide start moving out of the cell from the region of its higher concentration  to a region of lower concentration.
  • In a similar way, oxygen enters the cell (e.g. Amoeba) by the process of diffusion when the concentration of O2 inside the cell decreases.

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 ahs 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.
  2. 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.
  3. 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 plane 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 unfavorable conditions.
  4. It provides mechanical strength to the plant cells.

Nucleus:

  • The nucleus is a major, centrally located spherical cellular component. It is the centre from where all cellular activities are controlled. It is the carrier of hereditary material in the cell.
  • It is bounded by two membranes, both forming a nuclear envelope. The nuclear envelope contains many pores known as nuclear pores and encloses the liquid ground substance, the nucleoplasm.
  • The nucleopores allow transfer of materials between the nucleoplasm and the cytoplasm. Within nucleoplasm are embedded two types of nuclear structures- nucleolus and chromatin material. 

Nucleolus:

  • The nucleolus may be one or more in number ad is not bounded by any membrane.
  • Nucleolus consists of DNA, RNA and protein.
  • It is the site of ribosome formation. Nucleolus is known as factory of ribosomes.

Chromatin material:

  • The chromatin is a thin, thread-like intermingled mass of chromosome material and composed of the genetic substance DNA (deoxyribonucleic acid) and proteins (i.e., histones).
  •  DNA stores all the information necessary for the cell to function (metabolism), to grow and to reproduce further cells of the next generation.
  • The chromatin is condensed into two or more thick ribbon-like chromosomes during the division of cell.
  • Functional segments of DNA are called genes.
  • Genes carry information for protein synthesis, which in turn control the development of characters. Since genes are present on chromosomes and chromosomes pass from one generation to the next through, genes are called hereditary units.

Functions of nucleus:

  1. The nucleus controls all metabolic activities of the cell. If the nucleus is removed from the cell, the protoplasm ultimately dries up and dies.
  2. It regulates the cell cycle.
  3. It is the storehouse of genes which are concerned with the development and manifestation of all the body characters.
  4. Nucleus helps in the formation of ribosomes and RNA.
  5. It helps in the transmission of characters from one generation to the next.


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