Cell | Definition, Types, Functions, Diagram & Cell Theory
Unicellular organisms are capable of
- Independent existence
- Performing the essential functions of life
Anything less than a complete structure of a cell does not ensure independent living. Thus, cell is the fundamental structural and functional unit of all living organisms.
Robert Hooke studied and discovered the cell from a thin slice of cork but that was the ‘dead cell’. Anton Von Leeuwenhoek was the first person who observed few living cells capable of moving, such as bacteria, protozoa, spermatozoa and red blood corpuscles under his own design microscope. Later, Robert Brown Discovered the nucleus of a cell. The invention of the microscope and its improvement leading to the electron microscope revealed all the structural details of the cells.
In 1838, Matthias Schleiden, a German botanist studied a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant. At about the same time, another scientist Theodore Schwann (1839), a British zoologist, studied different types of animal cells and reported that cells had a thin outer layer which is now known as the ‘Plasma Membrane’. He also concluded, based on his studies on plant tissues, that the presence of cell wall is a unique character of the plant cells. On the basis of this, he proposed a hypothesis that the bodies of plants and animals are composed of cell and their products. Schleiden and Schwann together formulated the cell theory. This theory however, did not explain as to how new cells were formed. Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula). He modified the hypothesis of Schleiden and Schwann to give cell theory a final shape.
Cell theory is understood as
- All living organism are composed of cells and products of cells.
- All cells arise from pre-existing cells.
- Activities of an organism are the outcome of sum total of activities and interaction of its constituent cells.
An Overview of Cell
Let us collect onion structure. The onion cell which is a typical plant cell, has a distinct cell wall and its outer boundary and just within it is the cell membrane. The cells of the human cheek have an outer member and as the delimiting structure of the cell. Inside each cell is a dense membrane bound structure called nucleus. This nucleus contains the chromosomes which in turn, contain the genetic, DNA.
Cells that have membrane bound nuclei are called eukaryotic whereas cells that lack a membrane-bound nucleus are prokaryotic. In both prokaryotic and eukaryotic cells, semi fluid matrix called cytoplasm occupies the volume of the cell. The cytoplasm is the main arena of cellular activities in both the plant and animal cells.
Besides nucleus, the eukaryotic cells have other membrane bound distinct structures called organelles like the endoplasmic reticulum (ER), the Golgi complex, lysosomes, mitochondria, microbodies and vacuoles. The prokaryotic cells lack such membrane bound organelles.
Ribosomes are non membrane bound organelles found in all cells — both eukaryotic as well as prokaryotic. Within the cell, ribosomes are found not only in the cytoplasm but also within the two organelles — chloroplasts in plant and mitochondria and no rough ER. Animal cells contain another non membrane bound organelles called centriole which helps in cell division.
Cell differ greatly in size, shape and activities. For example Mycoplasma, the smallest cell, are only 0.3 ųm in length while bacteria could be 3 to 5 ųm. The largest isolated single cell is the egg of an ostrich. Among multicellular organisms, human red blood cells are about 7 ųm in diameter. Nerve cells are some of the largest cells. Cell also vary greatly in their shape. They may be disc like, polygonal, columnar, cuboid, thread like or even irregular. The shape of the cell may vary with the function they perform.