The cell is the fundamental, structural and functional unit of life. Cells were observed for the first time when in 1665, Robert Hooke examined a thin slice of cork under his crude microscope and observe honeycomb-like structures. He called the thick-walled, box-like compartments cells.
Cells have diverse shapes. They may be flat, oval, spherical, cylindrical, cubical, spindle-shaped or irregular in shape. Shape of a cell varies with the type of function it performs. This can be clearly seen in a nerve cell. A nerve cell has long threadlike projections which help it to conduct messages from one cell to another. Guard cells found in leaves are bean-shaped. Two guard cells are closely opposed to each other leaving a pore called stoma (plural stomata) through which exchange of gases takes place. Cells like amoeba and white blood cells extend portions of their body to engulf food particles and foreign bodies respectively. Thus, their shapes keep on changing.
Average size of cell varies from 1/10th to 1/100th of a millimeter. Most cells are microscopic. The size of a bacterium varies from 0.1 to 0.5 microns. However, a bird’s egg is also a single cell although a major part of it is occupied by stored food. An ostrich’s egg is the largest animal cell. It can be 17 cm long and 14 cm at its widest part. Human nerve cells are very alga which measures upto 10 centimeters.
All plant and animal cells possess three main components namely, cell membrane, cytoplasm and nucleus. Let us study the structure of a generalized cell in detail. A generalized cell possesses the following components:
1. Cell Wall:
The cell wall is the outermost rigid protective covering found only in plant cells. It is a non-living structure and provides shape and rigidity to the plant cell.
2. Cell Membrane:
The cell membrane is a delicate covering that surrounds the jelly-like cytoplasm present inside the cell. In animal cells, it is the outermost layer while in plant cells it is surrounded by the cell wall on the outside.
Cytoplasm is a jelly-like substance present inside the cell. It contains several structures called cell organelles floating in it.
Nucleus is a dense body found generally in the centre of the cell. It has a double-layered covering the nuclear membrane which encloses a jelly-like ground substance called nucleolus. Nucleoplasm contains certain thread-like structures called chromosomes. Chromosomes are responsible for the inheritance of characters from one generation to another. This accounts for the similarity between parents and their off springs. Nucleus controls all the metabolic activities of the cell. Most cells contain a single nucleus.
5. Cell Organelles:
Cell organelles are extremely small structures present in cytoplasm and concerned with cell function. Following cell organelles are commonly found in cells:
Endoplasmic Reticulum: Endoplasmic reticulum (E.R.) is a network of tubular channels present throughout the cytoplasm. The channels are connected to the nuclear membrane on the one end and to the cell membrane on the other, thereby forming a skeletal support system for the cell. E.R. also helps in the storage and transport of materials inside the cell.
Golgi Bodies: Golgi bodies, flattened sac-like structures, are involved in the secretion of certain substances in the cell. The golgi bodies are known as dictyosomes in plants.
Lysosomes: Lysosomes are tiny sacs containing digestive enzymes and are often called suicide bags because under adverse conditions they can eat the very cell in which they are present.
Centrosome: Centrosome is a small body present close to the nucleus in animal cells. It contains two tiny granular bodies’ centrioles. The centrioles help in cell division.
Plastids: Plastids are specialized bodies found in plant cells.
Mitochondria: Mitochondria (singular: mitochondrion) are tiny rod like structures found in cytoplasm. They release energy from the food we eat and are, therefore, called powerhouse of the cell.
Vacuole: A space within the cytoplasm of a plant cell, containing cell sap.
Growth and Division of Cell
You have seen a small seed grow into a big plant. How does this happen? The increase in size occurs mainly due to an increase in the number of cells. A cell grows to its normal size, and then divides into two. This process is called cell division.
The cells resulting from the division of a parent cell are called daughter cells. New cells are required in our body not only for growth but also for replacement of worn-out cells. First of all the nucleus divides into two, each one going to a daughter cell. This is followed by division of the cytoplasm and two daughter cells are formed. In multi-cellular organisms, cell division is a very complicated process involving several steps. Many cells in our body keep dividing throughout life. For example, skin cells continue to divide throughout life and new cells continuously replace the old worn-out cells. In contrast to this, nerve cells do not divide after the individual attains a certain age.