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Scientists by the early 1800s had observed many different organisms’ cells. Several German scientists, including Theodor Schwann and Matthias Jacob Schleiden, recognised the importance of cells as the basic building blocks of all living things. The German physician Rudolf Virchow discovered around 1850 that cells divide and form new cells as they were observed under a microscope. Virchow established that living cells generate new cells by dividing. According to him, living cells can only arise from other living cells.
Virchow, Schwann, and Schleiden all contributed to the development of cell theory, which is a whole area of study in biology.
Cell theory states the following:
Cells are of many types. All living organisms are made up of two types of cells: eukaryotic and prokaryotic.
All humans are made up of eukaryotic cells. They can also be found in animals, plants, and fungi. How do they differ from other cells? They have a different structure.
Eukaryotic cells are distinguishable by having many parts and needing them all simultaneously. They execute many functions. Despite the diversity of structures in eukaryotic cells, the following components are standard.
1. Protective Membrane: Depending on the cell, this can be called the cytoplasmic membrane, cell wall, or cell membrane. Although it may be called many things, it is essentially a protective barrier separating the inside of the cell from the outside.
2. Nucleus: An important feature that makes eukaryotic cells unique is their brain or nucleus. In the cell, all the genetic material like deoxyribonucleic acid (DNA) is stored here, where all the magic happens.
3. Other Organelles: Different organelles are found in eukaryotic cells. Cells may differ slightly depending on whether they are plants, animals, fungi, or protists.
All living things are made up of eukaryotic cells, from the grass under your feet to the birds in the sky. Animals, plants, fungi, and all other forms would be on a long list of eukaryotes. It is a unique type of cell that contains living organisms. Algae and amoeba are examples of protozoa.
A prokaryotic cell’s simplicity is its key feature. A prokaryotic cell does not have a nucleus, unlike eukaryotic cells. Bacteria and archaea are the only types of prokaryotic cells. Whether a cell has a nucleus or not, it still has a brain. Rather than being enclosed in a nucleus, their DNA is found within the cells themselves.
A few examples of prokaryotic Cells
1. Coli, streptococcus and staphylococcus are prokaryotic cells that make us ill. Achaea, two common examples of Achaea include methanogens and thermophiles.
BACTERIA
Depending upon shape, bacteria are of following types :
(1) Coccus (Spherical) – Monococcus, Diplococcus, Staphylococcus, Streptococcus
(2) Bacillus (Rod-like) – Diplobacillus and Streptobacillus
(3) Vibrio (Comma shaped)
(4) Spirillum ( Spiral shaped )
Depending upon the number of flagella , bacteria are of following types :
1. Atricous
2. Monotrichous
3. Lophotrichous
4. Amphitrichous
5. Peritrichous
6. Amphilophotrichous
Structure of a Prokaryotic Cell :-
Cell Envelope: It is the outermost covering of protoplasm of a typical bacterial cell. It consists of three layers viz. glycocalyx, cell wall and plasma membrane.
Glycocalyx : It can be a loose sheath called Slime layer in some bacteria while in the form of hard covering called Capsule in others.
Slime layer is composed of glycoproteins and is loosely attached to the cell wall.
Capsule is made up of polysaccharides and is firmly attached to the bacterial cell wall.
Cell wall : In all eubacteria and cyanobacteria , cell wall is composed of peptidoglycan and murein.
Peptidoglycan is a repeating framework of long glycan strands (N-acetyl muramic acid and N-acetyl glucosamine) which are cross linked by short peptide chains.
Cell wall provides structural support to bacteria so that it doesn’t bursts or collapses.
All prokaryotes have cell wall surrounding them except mycoplasma.
Plasma Membrane : structurally the plasma membrane in prokaryotes is similar to that of the plasma membrane in eukaryotes. It is present inner to cell envelope in all the prokaryotic cells.
In general cholesterol is absent, but in some bacteria pentacyclic sterols called as hapnoids are present. Hapnoids stabilise the membrane.
Membranous structures:
Mesosomes : Mesosomes are formed by the infoldings of the plasma membrane. They bear respiratory enzymes. They are formed of vesicles, lamellae and tubules.
Types of Mesosomes
They are of two types as follows:
Septal Mesosome
They are seen associated with the chromosomes or cell wall of dividing cells. It is commonly involved in the cell wall formation, chromosome replication, and distribution of chromosomes to daughter cells.
Lateral Mesosome
They are not seen associated with the nucleoid. It possesses respiratory enzymes.
Functions of Mesosomes
The main functions of mesosomes are as follows:
· Cell wall formation.
· DNA replication.
· Distribution of replicated DNA to the daughter cells.
· Increasing surface area of the plasma membrane.
· Respiration.
· Secretory and enzymatic activity.
Chromatophores: These are the membranous extensions containing pigment molecules such as bacteriochlorophyll. These are abundantly found in cyanobacteria or blue green algae. Their main role is photosynthesis. They may be of different shapes like lamellar, tubular or vesicular in shape.
The cytoplasm is the internal cellular region enclosed by the cell membrane. This liquid matrix contains water, proteins, and a variety of biomolecules.
The nucleoid, an irregularly shaped region within prokaryotic cells, houses most or all genetic material. Unlike the eukaryotic nucleus, it lacks a surrounding nuclear membrane and typically contains a circular, double-stranded DNA molecule without histone proteins.
Deoxyribonucleic acid (DNA) carries genetic information necessary for an organism’s development and functioning. In prokaryotes, DNA is present freely as a closed circular structure within the central portion of the cytoplasm.
Bacterial cells may exhibit motility through flagella, thin filamentous extensions of the cell wall. Flagella’s number and arrangement vary among bacterial species, with distinct types such as monotrichous, amphitrichous, lophotrichous, peritrichous, and cephalotrichous.
Other Surface Structures
Biology Faculty
BDS from PT. B.D. Sharma University of Health Sciences, Rohtak
