CELL: STRUCTURE AND FUNCTIONS

IMPORTANT TERMS

Cell                               A cell is defined as the smallest structural and functional unit of an organism.

Unicellular Organism    Organisms whose body consists of a single cell is called unicellular organisms. For example - Amoeba, Paramecium and bacteria etc.

Multicellular Organism  Organisms whose body consists of more than one cell are called as multicellular organisms. For example - Most plants and animals.

Organelles                   The living part of the cell that has a definite shape, structure and function are the organelles.

Cell Membrane             It is the outer covering of the cell. It is a porous membrane through which selected substance can enter or leave the cell. This membrane is also known as plasma membrane.

 

Cell Wall                     It is rigid wall found in plant cell which protects plants from changing atmosphere and surroundings.

 

Prokaryotic Cells         Cells that lack a well-defined nucleus surrounded by a nuclear membrane are called prokaryotic cells. For example - Bacteria, Blue-green algae and Virus.

 

Eukaryotic Cells          Cells that have a well-defined nucleus are called as eukaryotic cells.

 

Nucleus                   Nucleus is considered to be the brain of the cell. It is surrounded by a membrane called plasma membrane.

 

Nucleoplasm               The semi-solid substance filled inside the nucleus is called nucleoplasm.

 

Cytoplasm                  Cytoplasm is a jelly like fluid which fills up the part of the cell between the cell membrane and the nucleus.

 

Vacuole                      It is a fluid filled space enclosed in a membrane. Vacuole store excess water, useful minerals, pigments and many other substances.

 

 

Discovery of the Cell

A cell is defined as the smallest structural and functional unit of an organism.

Robert Hooke, an Englishman, first discovered the cell in 1665, while examining a slice of cork under a microscope. He observed the slice of cork is made up of tiny boxes or compartments. He called these compartments as ‘cells’. These boxes appeared like honey-comb.

Study of the cells of living organisms could be done after the discovery of improved microscope. The object to be viewed under microscope is generally referred as Specimen. A thin sheet of glass called a microscopic slide is used to hold a small sample called specimen.

The Cell

Cells in the living organism, are the basic structural units. The living organisms differ from one another but all are made up of cells.

The egg of Hen represents a single cell and is big enough to be seen by the unaided eye.

Cells: Variation in Number, Shape and Size

There are millions of living organisms. They are of different shapes and sizes. Their organs also vary in shapes, size and number of cells.

Organisms whose body consists of single cell are called as unicellular organisms. For Example- Amoeba, Paramecium, Euglena and Bacteria. Thus in a unicellular organism, all the activities are performed by a single cell.

Organisms whose body consists of number of cell are known as multicellular organism. For Example- Most plants and animals are multicellular organisms. Thus in multicellular organisms all the work is divided in between different cells.

An organism with billions of cells begins life as a single cell which is the fertilized egg.

Shapes of cells

Cells exist in different shapes. They can be disc-shaped, polygon, rectangular, branched or even irregular. The shape of cell depends on the specific function it performs.

• Amoeba has no definite shape, unlike other organisms. It keeps on changing its shape. Amoeba has pseudopodia, known as projections, which helps amoeba to move. These pseudopodia are known as ‘False Feet’. The change in shape is due to formation of pseudopodia which facilitates movement and help in capturing food.

 

• A white blood cell in human blood is another example of a single cell which can change its shape.
• Nerve Cells are the longest cell found in the human body. They carry messages between different parts of the body. Hence, they are elongated in shape.
• Muscle Cell helps in movement through contraction and relaxation. Hence, they are thin and long.
• Skin Cells covers a large area. Hence, they are flat in shape.

           Variation in Size

The size of cells in living organism may be as small as a millionth of a meter or as large as a few centimeters, but in general most of the cells are microscopic and are not visible to the unaided eyes. The smallest cell is 0.1 to 0.5 micrometer in bacteria and the largest cell measuring 170mm x 130mm, is the egg of an ostrich. The size of an egg has no relation with the size of body of an animal or plant. It is not necessary that the cells in elephant be much bigger than those of rat. The size of cell is related to its function. For example, Nerve cell in elephant and rat, are long and branched. They perform the same function that of transferring messages.

 

Cell Structure and Function

 

Different organs of an organism perform different functions. For example, roots help in the absorption of the water and minerals. Leaves are responsible for the synthesis of food.

 

Each organ is further made up of smaller parts called tissues.

 

A tissue is a group of similar cells performing a specific function.

 

A cell has both living and non-living parts. The living parts of the cell that have a definite shape, structure and function are collectively called organelles.The main parts of a cell are cell membrane, cytoplasm and nucleus.

 

• Cell Membrane- It is the outer covering of a cell, which is a porous membrane through which selected substance can enter or leave the cell. The cell membrane is also called as plasma membrane.

In addition to plasma membrane there is another outer covering found only in the plants known as cell wall, which protects them from the change in their surroundings.

 

• Cytoplasm- It is the jelly-like substance present between the cell membrane and the nucleus. Several organelles are present in the cytoplasm.

 

• Nucleus- It is the important component of the cell, known as the brain of the cell, which controls all the activities of the cell. Nucleus is surrounded by a membrane called nuclear membrane and filled with semi-solid substance called nucleoplasm.

 

• Chromosomes- The thread like structure present inside the nucleus. They carry genes, which transfer the characteristics of a cell to the new cell that are formed during cell division. The chromosomes can be seen only when the cell divides.

 

• Nucleolus- A round granule present inside the nucleus is known as nucleolus, which further contains a network of fibrous material called chromatin fibres.

 

• Golgi Apparatus- It is made up of long tube like structure, vesicles and vacuoles and is responsible for the secretion of chemical substances such as enzymes, hormones and proteins.

 

• Vacuole- It is a fluid-filled space enclosed in a membrane. Vacuoles store excess water, useful minerals, pigments and many other substances.

 

• Protoplasm- The entire content of the living cell is known as protoplasm. It includes cytoplasm and the nucleus. Protoplasm is called the living substance of the cell.

 

• Lysosome- It contains chemical substance like enzymes that are capable of digesting cells. In time of emergency, lysosomes burst and destroy the cell. Hence they are also known as Suicide bags of the cell.

 

• Mitochondria- These are the tiny spherical or rod shaped bodies, which acts as sites of energy production, Hence are known as powerhouses of the cell.

 

• Endoplasmic Reticulum- It is a network of tubules and channels and is involved in the synthesis, storage and transport of the cell products.

 

• Ribosomes- These are small granular structures scattered in the cytoplasm that acts as sites of protein synthesis.

 

• Centrosomes- This structures present only in animals cells. The main function of the centrosomes is to initiate and regulate cell division.

 

• Plastid- These are present only in plant cells. Plastids contain certain pigments that have a specific role to play in the functioning of the plant. Depending on the pigment colour, the plastids are of three types.

 

1. Chloroplasts- It contains green pigment chlorophyll, which helps plants in making food.
2. Chromoplasts - It contains non-green pigment which gives colour to fruits and flowers.
3. Leucoplasts- These are colourless and stores food in the form of carbohydrates, fats and proteins.

 

Plant Cell	Animal Cell
Cell wall is present.	Cell wall is absent.
Cytoplasm is not as dense as in animal cell.	Cytoplasm is dense.
A large vacuole is present.	Vacuoles are generally absent, if present, they are small in size.
Plastids are usually present.	Plastids are absent.
Centrosome is absent.	Centrosome is present.

                                            

 

 

 

 

 

 

 

 

 

Prokaryotic Cells- The cells that have nuclear material without nuclear membrane or the cells that lacks a well defined nucleus surrounded by a nuclear membrane are termed as prokaryotic cells. Such organisms are called as prokaryotes (Pro; primitive and karyon; nucleus). The nucleus of the bacteria; cell is not well organized like the cell of multicellular organisms. For example- Bacteria, blue-green algae and viruses.

Eukaryotic Cells- The cells that have well-defined nucleus with a nuclear membrane are called as eukaryotic cells. Such organisms are called as eukaryotes (Eu; true and karyotes; nucleus).

Cell Division and Growth

The food that we eat leads to an increase in the size of the cells. After reaching a certain size, a cell divides into two by a process known as cell division. Increase in the number of cells in the body as a result of the cell division is responsible for the growth in organisms. Cell division also replaces the dead or damaged cells with the new ones and thus, responsible for healing wounds.

 

Some Interesting Facts

• Bacteria have been around for at least 3.5 billion years, making them the oldest known life-form on the planet.
• In every inch of skin, there are about 32 million bacteria.
• The tooth is the only part of the body that cannot repair itself.
• Nerve impulses to and from the brain travels as fast as 170 miles per hour.
• 300 billion new cells are produced every day. 300 million old cells die every minute.
• The acid in our stomach is strong enough to dissolve razorblades.