Cell Characteristics, Prokaryotes and Eukaryotes, Human Cell

The cell:

                                Any individual living being, whether plant or animal, is called an organism. The living substance of all organisms is known as protoplasm. The smallest unit of protoplasm capable of independent existence is known as a cell. All organisms are made up of either one or several cells.

 Cell Characteristics

By definition, a cell is the smallest basic structural and functional unit of life, which is bounded by a membrane and contains the fundamental molecules of life. A cell may exist as an independent functional unit of life in unicellular organism or as a subunit in a multicellular organism.

Prokaryotes & Eukaryotes:

                                                    The simplest organisms (bacteria and archaea) are called prokaryotic organisms or prokaryotes. The prokaryotes are unicellular organisms and each of them consists of a prokaryotic cell. The prokaryotic cell neither contains a nucleus nor any organelles.

 The higher organisms (plants and animals) are known as eukaryotic organisms or eukaryotes. The eukaryotes are made up of one or more eukaryotic cells. Each eukaryotic cell contains at least one membrane - bound nucleus and many membrane - encased organelles (which are special, organized structures that perform specific functions for the cell).

Prokaryotes

Eukaryotes

The higher animals, including humans, can be regarded as a complex society of independent eukaryotic cells of many types that are specialized to carry out the functions essential for survival and reproduction of the animal as a whole. The human body is composed of trillions of eukaryotic cells. The cells serving similar function or functions are bound together by variable quantities of extracellular matrix to form tissues. Two or more tissues are combined to form large functional units called organs. Several organs having interrelated functions constitute an organ system; for example, the urinary system consists of kidneys, ureters, urinary bladder, and urethra.

Human Cell:

                       A human cell may be defined as a small mass of protoplasm which is enveloped by a membrane and contains at least one membrane - bound nucleus and many membrane- bound organelles. The protoplasm inside the nucleus is called karyoplasm, while the remaining protoplasm of the cell is known as cytoplasm. More than 200 different types of cells are found in the human body. Each living cell is capable of producing energy to carry out its normal functions. Each cell synthesizes macromolecules for its own use, while many cells also synthesize macromolecules meant for export from these cells. Generally, the human cells are capable of communicating with each other.

Human Cell

Shape and Size of the Human Cells:

• §  Shape: Free cells (e.g., white blood cells and ova) have a spherical or ovoid shape. In most issues, however the shape of the cells is modified, e.g., cuboidal, columnar, polygonal pyramidal, cylindrical, fusiform, or irregular, etc.
• §  Size: The human cells generally range from 5 to 50 μm in diameter. However, certain cells may be exceptionally large, e.g, the mature ovum (which has a diameter of 120 um) and some large neurons (which may measure up to 150 µm in diameter).