Cell – Structure and Functions

All organisms are made of several smaller cells. The basic and structural unit of life is called a cell. Cells were first observed in cork i.e., the bark of a tree by Robert Hooke in 1665.

The Cell:

• The smallest living part of an organism.
• A small group of cells that have similar structure and perform the same functions is called a tissue.

Cell Characteristics:

Organisms differ in their shape and size. Also, their organs and cells vary in size, shape, and numbers.

Number of Cells: The number of cells varies in different organisms.

• Organisms made of more than one cell are called multicellular (multi-meaning many and cellular meaning cell) organisms like humans.
• Organisms made up of only one cell are called unicellular (uni meaning one and cellular meaning cell) organisms like amoeba.
• The single-cell performs all the basic functions of unicellular organisms while a variety of cells operate functions in multicellular organisms.

The shape of Cells: A wide variety of shapes is exhibited by a cell. The cell membrane encloses the components of the cell which further provides a rigid shape to cells.

• Amoeba has no definite shape. With the movement of the amoeba, these projections sometimes appear and then disappear.
• White blood cell (WBC) is a single cell found in human blood that can change its shape.
• Cells of blood, muscle, and nerve are of different shapes in human beings and have specific functions.
• Cells are round, spherical, or elongated.
• Spindle-shaped cells are long and pointed at both ends.
• Nerve cells or neurons are branched cells.

Size of Cells: A wide variety of sizes are exhibited by a cell. The size of a cell can be small as a millionth ometereter (micrometer or micron) or can be large as a few centimeters.

• Microscopic cells are not visible to the unaided eye. They are enlarged or magnified by a microscope to be seen.
• Hen’s egg can be seen through an unaided eye.
• The size of the smallest cell is 0.1 to 0.5 micrometers in bacteria. The egg of an ostrich measures  170 mm ×130 mm. 
• The size of the cell is not related to the size of the body of an animal or plant body.
• The size of the cell is correlated with the function. For example, nerve cells in the elephant or the rat, are long and branched and perform the same function i.e., transfer of messages.

Cell Structure:

Three main parts of cells: (1) the cell membrane, (2) cytoplasm which constitutes smaller components called organelles, and (3) the nucleus. The whole content of a living cell is known as protoplasm that includes cytoplasm and the nucleus. 

Cell Membrane: Also called the plasma membrane. The cell membrane encloses cytoplasm and nucleus. The cell membrane creates a partition between the inner cells membrane and the surrounding medium. 

• The porous nature of the plasma membrane allows the movement of substances both outside and inside. Cell membrane provides a rigid shape to the cell.
• In plants, an additional membrane called a cell wall covers the plant cell. The cell wall provides a rigid shape to the cell.

Cytoplasm: It is a jelly-like material that exists between both the cell membrane and the nucleus. The cytoplasm contains cell organelles such as mitochondria, Golgi bodies, ribosomes, and so on.

• Blank-looking structures in the cytoplasm are called a vacuole. Large vacuoles are seen in plant cells whereas vacuoles in animal cells are small in size. This is because plants need to store food and water, which is why they have enormous vacuoles that act as a reservoir in adverse conditions. 
• Plastids are small and colored bodies present in the cytoplasm of the leaf. They are of different colors. Green pigment-containing organelle is called chlorophyll.
• Green-colored plastids are called chloroplasts that provide green color to the leaves. Chlorophyll in the leaves is essential for photosynthesis.

Nucleus: Nucleus is present in the center of the cell and it is a dense round body. The membrane separates the nucleus from the cytoplasm. The membrane is porous and allows the movement of materials between the cytoplasm and the nucleus.

• Nucleolus – Smaller spherical body present in the nucleus.
• The nucleus includes chromosomes, which are thread-like structures that transport genes and aid in character inheritance from parents to kids. When a cell divides, the chromosomes are readily visible.

Gene: 

• Genes are the unit of inheritance in living organisms.
• Helps in the transfer of a hereditary character from parents to offspring.
• The different combination yields different characteristics of genes from parents.

Comparison of Plant and Animal Cells:

• Plant cells differ from animal cells in having an additional protective layer around the cell membrane termed cell wall.
• A plant cell has a big central vacuole whereas an animal cell has small vacuoles. 

Conclusion

As a result, the cell is the basic structural unit of all living things. It is also the fundamental unit of life’s functionality. A plasma membrane, made up of lipids and proteins, surrounds each cell. Plant, fungus, and bacterium cells may persist in the hypotonic medium without bursting because of the existence of the cell wall.