The Cell

 

Cell: The basic structural unit of a living organism is called the 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 Theory

  • The cell is the basic structural and functional unit of life; all organisms are composed of cells.
  • cells are reproduced, multiplied in number by the division of existing cells. Each cell contains genetic material that is passed down during this process.
  • All basic chemical and physiological functions - for example, repair, growth, movement, immunity, communication, and digestion - are carried out inside of cells.
  • The activities of cells depend on the activities of subcellular structures within the cell Variety in cell number, shape and size:

Variety in Number: Living beings show a large variation in the 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 a single cell. On the basis of the number of cells, organisms can be divided into two groups, viz. unicellular and multicellular organisms.

  1. Unicellular Organism: An organism which is made up of a single cell is called unicellular organism, e.g. bacteria, protozoa, yeast, etc.
  2. Multicellular Organism: An organism which is made up of more than one cell is called multicellular organism, e.g. humans, elephants, tiger, banyan, snail, etc.

Tissue: A group of specialized cells made for a specific task is called a tissue. While a single cell is responsible for all the functions in a unicellular organism, different tissues perform different functions in a multicellular organism.

Variation in Shape

Cells show wide variations in terms of shape. A particular shape of a cell facilitates the particular function performed by that cell. Some examples are as follows:

  • Amoeba is a unicellular organism. It 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) appear like a star with the long tail.

Size of Cells: While most of the cells are small in size, cells show a wide variation in size as well. The smallest cell is 0.1 to 0.5 micron and it is a bacterium. Ostrich egg is the largest living cell and it measures 170 mm x 130 mm.

Cell Structure

Parts of Cell:

Following are the main parts of a cell:

 

 

Cell Membrane: Cell membrane is also called plasma membrane. It makes the boundary of a cell. It is composed of protein and lipid. It is porous and allows various materials to pass through it. The Cell membrane is semi-permeable in nature which means it allows selected materials to pass through it.

  • Cell membrane separates the contents of the cell from the external environment.
  • Cell membrane provides protection to the cell.
  • Cell membrane facilitates an exchange of materials between cell and external environment.

Cell Wall: Cell wall is present in plant cells and in bacteria. The cell wall in plants is made up of cellulose. Cell wall gives additional strength and protection to plant cells.

Cytoplasm: A jelly-like substance is present in the cell and nucleus. It is called cytoplasm. Different cell organelles are present in the cytoplasm. Examples of cell organelles are; mitochondria, chloroplast, etc.

Nucleus: This is a spherical structure which is present in the cell; usually at the centre of the cell. The nucleus is surrounded by a double membrane called nuclear membrane. A dot-like structure; called nucleolus is present inside the nucleus. The nucleus contains chromosomes which are thread-like structures. Genes are present on the chromosome. They are responsible for transferring characters from parents to their offspring.

  • Nucleus controls the function of a cell.
  • Nucleus controls inheritance of characters, i.e. transfer of characters from one generation to the next generation.

The nucleus contains genetic materials. But genetic materials may or may not be organized in the form of a nucleus. Based on this, organisms are divided into two groups, viz. prokaryotes and eukaryotes.

Prokaryotes: When genetic materials are not organized in the form of nucleus, the organism is called prokaryote. In other words, the nucleus is absent in prokaryotes. Bacteria and blue green-algae are the examples of prokaryotes.

Eukaryotes: When genetic materials are organized in the form of nucleus, the organism is called eukaryote. In other words, the nucleus is present in eukaryotes. Organisms; other than bacteria and blue-green algae; are eukaryotes. Animals, plants, fungi, etc. are eukaryotes.

Protoplasm: The fluid inside the cell is called protoplasm. Protoplasm is composed of cytoplasm and nucleoplasm. The fluid between cell membrane and nucleus is called cytoplasm. The fluid inside the nucleus is called nucleoplasm.

Vacuoles: Some fluid-filled bag-like structures can be seen inside a cell. These are called vacuoles. Vacuoles are very large and conspicuous in plant cells but are very small in animal cells. Vacuoles may be absent in most of the animal cells.

Plastids: Plastid is an important cell organelle and is found in plant cells only. Plastids perform some important functions in cells. The chloroplast is a type of plastid. It contains a green pigment called chlorophyll. It imparts the green colour to leaf and carries out photosynthesis.

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.