Cell, Cell Theory, Types of Cell – Biology XI
The term cell is Greek word (kytos=Cell, La, cella, hollow space) and it was first discovered by Robbert Hook in 1655, by describing his investigations on the structure of cork. He said that there is a hollow space named as cell. Later on Robert Brown in 1831 discovered a spherical body which was on the center of the cell only in animal cell, but in the plant cell it is slightly near to the cell wall because of a big vacuole and he termed it as nucleus and he observed it is the cell of Orchids.
The cell theory was proposed by two scientist, the one was a Zoologist named T.Schwann, both concluded that despite of all differences. They have proposed a comprehensive report. The theory of cell stats that;
- All the living organisms are composed of one or more cells.
- The cell is the structural and functional unit of life.
- Cell is a denovo-structure (Cooled arise from non-cellular component)
This was the cell theory proposed by the both, but it wasn’t convincing because of the 3rd point of the cell theory which states that cell is a de-novo structure. In 1855 Rudolf Virchow a German Pathologist further studied and finally made a convincing case and replaced the third point was, “Cell can arise only by the division of preexisting cell, it is not a “de-novo” structure”
Types of Cell:
There are two types of cell, (i) Eukaryotic Cell and (ii) Prokaryotic Cell.
(i). Eukaryotic Cell:
Eukaryotic are said to those organisms, which have a true & well-defined nucleus, also contains chromosomes and other cytoplasmic membranous organelles, such as mitochondria, Golgi body, lysosomes, plastids etc, in their cells. (Eu=true, Karyon-nucleus) may be unicellular or multi-cellular organism.
(ii). Prokaryotic Cell:
Prokaryotic are those organisms, who doesn’t have a true, well defined or membrane bounded nucleus in their cells and their nucleic material is usually Coiled. It doesn’t have any chromosome and membranous organelles. They only contain mesosomes, which are simple infoldings of plasma membrane, which is only responsible for respiration, photosynthesis and nitrogen fixation etc. they are unicellular organism like bacteria and cyanobacteria. It is simply known as earliest type of nucleus.
The Eukaryotic cell contains four (4) parts:
- Cell Membrane
- Cell Wall (In Plants, Bacteria & Fungi)
It is also called as;
- Plasma membrane; because it covers living material of the Cell called as plasma membrane.
- Unit membrane; it is covering layer of cell and many organelles.
- Selective membrane; it only allows selective things to pass
- Semipermeable membrane; because half of the things can pass through the membrane.
- Differentiated membrane; it differentiates a cell from others.
- Lipid bilayer; made up of layers of phospholipids.
Characteristics of Cell Membrane:
- It is an outer boundary wall of a cell.
- It separates the cell from outer environment.
- It is made up of phospholipids and proteins.
- Phospholipids forms two layers, hydrophilic ends of outer layer faces outside and hydrophilic ends of inner layer faces inside of cell.
- It also contains cholesterol
- Phospholipids are found in lipid states.
Integral Proteins (Intrinsic):
- Located inside the lipid membrane
- Embedded inside the lipid membrane.
- Incorporated inside the lipid membrane.
- Also, called as channel proteins between they provide passage.
Peripheral Proteins (Extrinsic):
- Located out of the cell membrane.
- Loosely connected to membrane, e.g: hemoglobin, receptors, antibodies etc.