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What is cell?

The cell is the smallest fundamental, structural and functional unit of living organisms or cell is the basic unit of life. The discovery and study of cells became possible after the invention of microscope. The microscope was invented by an Italian scientist Galilio in 1610.

The word cell was first used by a scientist Robert Hooke in 1665. He observed a piece of cork under microscope and found many small chambers in it. He named each chamber as a cell. Another scientist Robert Brown discovered a rounded body in the cells of orchids in 1831. This body was named as Nucleus.

Cell Theory

This theory was proposed Schleiden (1838), Schwann (1839) and Virchow(1858). It is a fundamental theory. The main points of this theory are as follows:

  1. All organisms are composed of one or more cells.
  2. The cell is the structural and functional unit of life.
  3. All the new cells are produced by the division of pre-existing cells.
  4. The cells contain a hereditary material, the nucleic acid. It transfers the hereditary characters into the new cells.
  5. Each cell have life. It takes energy from it’s environment. This energy used in the formation of important compound, protoplasm and other organisms.


Microscopes are the instruments which are used to observe microorganisms or small organisms.

Kinds of Microscope:

There are different kinds of microscopes. These kinds are as follows:

  1. Light microscope
    In this microscope visible light is used as a source of illumination. In 13th century convex lenses were invented. These lenses are used in the microscope. There is a stage on which slide of specimen is placed to observe by the help of lens. This microscope is of two types.

    1. Simple microscope of dissecting microscope:
    2. Compound microscope
  2. X-ray microscope
    In this microscope short wave length X-rays are used as source of light. This microscope is very efficient instrument to observe the three dimensional structure of cell parts. By the help of this microscope X-ray beams are focused by electromagnetic lenses or reflecting curved mirrors, the image of object is formedon the film.
  3. Electron microscope
    In this microscope electron beam is used for illumination. This microscope is designed by German scientists knol and rusk in 1932. Other scientists also contributed in it’s design, such as Marton of Belgium, prebus and Miller of Canada from 1932-1934.

Techniques to Isolate the component of the Cell

  1. To determine the chemical composition of various parts of a cell, its components are isolated, this process is known as fractionation.
  2. In fractionation many similar types of cells are placed in cold environment in a homogenizer and then spinning action is applied, called centrifugation.
  3. At low speed larger particles like cell nuclei are separated and these are settled down in the bottom, in the sediments.
  4. Smaller particles remain in the fluid, which are transferred into another test tube. These particles are centrifuged at high speed, it helps to separate these particles in various fractions. After their separation, they can be studied easily.

Cell Wall

The cell wall is present in plant cell on the outer side of plasma membrane. It is composed of cellulose, but it also contains lignin and pectin which make it stronger. In a younger cell, the cell wall is thin and delicate, but in a large cell it becomes thick and strong. The cell wall consists of three layers.

  1. Primary wall
    It is found around a young plant cell on the outerside of plasma membrane.
  2. Secondary wall
    It is formed on the inner side of primary wall in an old and large cell. After it’s formation the cell wall becomes thick and non-elastic.
  3. Middle lamella
    It is the layer between the two cells. It helps to attach the cells. It is made up of calcium and magnesium pectates.

Functions of Cell Wall

  1. It gives a definite shape and structural frame work to the cell.
  2. It protects the inner contents of the cell.
  3. It provides mechanical support to the cell.
  4. It is a permeable membrane for diffusion and helps in the absorption of minerals and solutes along with water in the cells of root hairs.


The nucleus was discovered by a scientist Robert Brown in 1831. It is spherical or oval in shape and is located in the centre in a young cell while in a mature cell it comes to lie on one side.

Structure of Nucleus

    The nucleus is surrounded by a thin, transparent membrane known as nuclear- membrane. It separates the cytoplasm from the nucleus. The nuclear membrane has numerous large pores which help in direct communication between cytoplasm and the nucleoplasm
    In the nucleus there is a dense but clear mass of protoplasm, called nuclear-sap or nucleoplasm. It contains enzymes and other complex substances which take part in the formation of DNA and RNA.
    In the neoplasm a network of fine loosely connected threads is present, called chromatin network. The chromatin is the hereditary material. In division of nucleus the chromatin forms a definite number of thread like structures, called chromosomes which contain hereditary units on their surface, called genes. Chemically chromatin consists of ribonucleic-acid (RNA) and Deoxyribonucleic acid (DNA).
    In each nucleus one or more globular bodies are present, called nucleolus. They consists of proteins and RNA. The nucleolus produces ribosomes, which take part in protein synthesis.

Functions of Nucleus

  1. The nucleus controls all the vital activities of a cell, so it is considered as the brain of the cell.
  2. It produces chromosomes during cell division. The chromosomes transfer hereditary characters from parent cell to daughter cell.
  3. It directly takes part in cell division and reproduction.
  4. It produces DNA and RNA. DNA is a generic material and RNA takes part in protein synthesis.
  5. In the nucleolus of nucleus ribosomes are produced, which help in protein synthesis.

Endoplasmic Reticulum

It is a network of fine tube like structures, which extend from cell membrane to the nuclear membrane. It is of two types. It consists of lipoproteins.

  1. Agranulated or smooth endoplasmic reticulum (SER)
  2. Granulated or rough endoplasmic reticulum (RER)

Smooth endoplasmic reticulum does not contain ribosomes on their surface.It is found in steroid producing cells, like fat cells, liver and muscles.In skin the smooth endoplasmic reticulum converts cholesterol into vitamin D (a lipid compound). This vitamin helps to make bones strong and healthy. Rough endoplasmic reticulum contain ribosomes on their outer surface. This reticulum is found in the cells which take part in protein synthesis, such aspancreas and salivary glands of mammals.

Functions of Endoplasmic Reticulum

  1. It helps in the exchange of important materials between cytoplasm and nucleus.
  2. It is involved in protein synthesis due to the presence of ribosomes.
  3. It is the passage for RNA to transfer from nucleus to the cytoplasm.
  4. It takes part to neutralize the harmful effect of drugs.
  5. It helps in detoxification of chemicals.
  6. In skin smooth endoplasmic reticulum converts cholesterol into a lipid compound, called Vitamin-D in the presence of sunlight. This vitamin makes the bones healthy and strong.

Mitochondria OR Chonriosomes:

They are small spherical or plate like bodies present in the cytoplasm. Mitochondria are transferred from mother to the new generation. These are present in eggs and not in sperms, so mother transfers them into new generation. Mitochondria consist of 3 parts.

    It is smooth and consists of proteins and lipids.
    It forms numerous folds, called cristae. On these cristae enzymes and co-enzymes are present which help in the oxidation of starch, fatty acids and amino acid. These compounds are converted into CO2 and water. In this process energy is released in the form of ATP. This energy is stored in the mitochondria.
    It is the central granular part. It contains many organic compounds.

Functions of Mitochondira

Mitochondria are the main centres of the intercellular energy production, they arecalled power-house of the cell. Almost all the respiratory activities take place inmitochondria and they contain a number of enzymes.

Golgi Bodies OR Dictyosomes

The golgi bodies are found only in certain types of cells. In plant cells they arepresent throughout the cell, while in animal cell single golgi complex is present.Each golgi body consists of

  1. Flattened sac like structures, called cisternae,
  2. Outer network of connecting tubules and
  3. Certain vesicles or golgian vacuoles.

Functions of Golgi Bodies:

  1. Golgi bodies take part in the formation of protein and carbohydrates, called glycoproteins.
  2. In animal cell they are associated with the secretion of certain enzymes, hormones and other substances.
  3. In plant cell they are involved in the formation of cell wall and cell plate.

Lysosomal Storage Diseases

When the function of lysosomes is disturbed, it causes many abnormal conditions or diseases. W.G. Hers of Belgium in 1965 explained that when a lysosomal enzyme, a-glycosidase is not produced in the body, it results in the storage of undigested glycogen, it is accumulated in lysosome, due to which organelles become swollen and cells and tissues are damaged, such diseases in which there is deficiency of lysosomal enzyme and accumulation of undigested glycogen, are called lysosomal storage disorders.

There are more than 30 disorders of lysosomal storage. Some of them are as follows:

    It is a dangerous disease. It causes blindness and mental retardation. Due to this disease death of baby may occur by the age of three years.
    This disease causes mental retardation in infants. Due to this disease liver and spleen become enlarged and erosion of long bones takes place i.e. bones are damaged badly.
    This disease causes loss of myelin, mental retardation. It may cause death of baby by the age of two years.

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