All organisms are composed of cells. Some are composed of a single cell and are called unicellular organisms while others like us are composed of many cells are called multicellular organisms.
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. Hence, the cell is the fundamental, structural and functional unit of all living organisms.
The first cell was discovered by Robert Hooke in a thin slice of Cork which was dead tissue. Anton Von Leeuwenhoek first saw and described a living cell.
Robert Brown’s letter later discovered the nucleus. The invention of the microscope and its improvement led to the high-quality electron microscope revealing all the structural details of the cell.
History of cell
In 1837, Matthias Jakob Schleiden viewed and stated that new plant cells are formed from the nuclei of old plant cells. Schwann remembered seeing similar structures that are “nuclei” in the cells of the notochord and instantly realized the importance of connecting the two phenomena that is the presence of nuclei and new cell formation.
The resemblance was confirmed without delay by both observers, and the results soon appeared in Schwann’s famous microscopic investigations on the accordance in the structure and growth of plants and animals, in which he declared that “All living things are composed of cells and cell products“. This became cell theory.
In 1855, pathologist Rudolf Virchow posed the maxim (aphorism) “Omnis cellula-e cellula” which means that every cell arises from pre-existing cells.
In 1838, Matthias Schleiden, a German botanist, examined 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, Theodore Schwann (1839) a British zoologist, studied different types of animal cells and reported that cells had a thin outer layer which is today 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 plant cells. Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells.
Schleiden and Schwann together formulated the cell theory. This theory, however, did not explain how new cells are formed. Rudolf Virchow (1855) first explained that cells are divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula). He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape.
Cell theory as understood today is—
- All living organisms 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.
Overview of cell
- A typical plant cell, has a distinct cell wall as its outer boundary and just within it is the cell membrane.
- Cells that have membrane-bound nuclei are called eukaryotic cells whereas cells that lack membrane-bound nuclei are prokaryotic cells.
- In both prokaryotic and eukaryotic cells, a semi-fluid matrix called cytoplasm which occupies the volume of the cell.
- The cytoplasm is the main arena (zone) of cellular activities in both plant and animal cells. Various chemical reactions occur in it to keep the cell in the ‘living state’.
- Besides the nucleus, the eukaryotic cells have other membrane-bound distinct structures called organelles like the endoplasmic reticulum (ER), the Golgi complex, Lysosomes, Mitochondria and Microbodies. The prokaryotic cells lack such membrane-bound organelles.
- Ribosomes are non-membrane-bound organelles found in all cells – both eukaryotic as well as prokaryotic cell. Within the cell, ribosomes are found not only in the cytoplasm but also within the two organelles that is Chloroplasts (in plants), Mitochondria, on Rough Endoplasmic Reticulum (RER) and Cytoplasm.
- Animal cells contain another non-membrane bound original called centriole which helps in cell division.