The following points highlight the two main types of cell. The types are: 1. Eukaryotic Cell 2. Prokaryotic Cell.
Type # 1. Eukaryotic Cell:
In addition to the nucleus, the cytoplasm of the cell usually contains a number of distinct bodies or structures that presumably carry out one or more rather specific functions.
For purposes of discussion, the components of the cytoplasm may be broadly classified, on the basis of light and electron microscope studies as follows:
(1) Mitochondria
(2) Plastids
(3) Lysosomes
(4) Endoplasmic reticulum
(5) Microsomes
(6) Cytoplasmic matrix
(7) Golgi complex
(8) Cell membrane
(9) Vacuoles, and
(10) Cytoplasmic inclusions.
In principle, protoplasm may be considered to be primarily proteinaceous, and many of its properties are similar to those of colloidal dispersions of complex proteins.
A colloid may be defined as the dispersion of one substance in another, with chemical properties intermediate between those of true solutions and suspensions. Most of their reactions are surface rather than molecular and depend primarily on size, shape, and type of dispersion as well as electrical charge.
The most important type of colloid in living cells is apparently the coacervate, which may be somewhat over simply defined as a semi-flocculated colloid in which the dispersed particles tend to occur in aggregates. Most biologically important coacervates appear to be those which result from the interaction of two hydrophilic (water-loving) colloids of opposite charge, as, for example, histones and nucleic acids.
It would be a mistake to assume, however, that the properties of protoplasm in general could be duplicated by any test-tube colloidal complex. While certain kinds of colloidal systems may be used as models to demonstrate different protoplasmic properties especially with respect to flow phenomena and sol-gel transformations, protoplasm as already noted has structural qualities and organization not matched by any of the known synthetic colloids.
All structures, or organelles, of the cell have their own, important functions. These functions depend upon types of organelles.
The nucleus performs two important roles:
(i) To store and carry hereditary information from generation to generation of cells and individuals; and
(ii) To translate genetic information into the kind of protein characteristic of a cell and thus determine the cells specific role in the life process.
The nucleolus which remains present inside the nucleus contains ribonucleic acid (RNA) and may act as an intermediate between the code of the chromosomes and the execution of the code in the cytoplasm. In the mitochondria the energy-yielding oxidations from the breakdown of complex organic compounds are localized. This energy is stored in high-energy phosphate bonds and is utilized in biologic activities as needed.
The double-layered membranous endoplasmic reticulum (ergastoplasm) is supposed to contain enzymes that synthesize cholesterol and other non-proteins. Often associated with the endoplasmic reticulum are found granules (ribosomes). Actually these are sites of protein synthesis under the influence of RNA, which receives its coded genetic information from deoxyribonucleic acid (DNA) of the chromosomes.
The centrioles helps in determine the orientation of the plane of cell division probably, it also supply the basal granules, or kinetosomes, which are concerned with the formation of motile fibrillar structures, such as cilia and flagella at the surface of cells. Another type of granules is lysosomes which are somewhat larger than ribosomes. Lysosomes provide the sites of certain hydrolytic enzymes.
The Golgi complex represents the primary site for the packaging of the secretory products that are synthesized on the ribosomes. Here also carbohydrate molecules formed by the Golgi complex are added to the protein secretions to for glycoproteins before they are discharged for their various functions.
Plastids, found in plants, these are present some other organelles which are called plastids. These plastids serve as sites of synthesis of complex organic compounds from simpler substances, such as the formation of sugar from carbon dioxide and water.
The structure of every cell consists of chemical compounds, whatever function a cell may have depends on the properties of cellular compounds. With the electron microscope, it has been possible in recent years in learn a great deal about subcellular organization and to explain the relations between structure and function in terms of the interactions of macromolecules.
Among the exciting discoveries are:
(1) Information concerning the submicroscopic membrane systems of the cytoplasm and their roles in the synthesis, storage, and transport of metabolic products, and
(2) The structure and function of cellular constituents involved in the energy transformations of cells, such as the changing of oxidation energy into usable and packaged energy by mitochondria and the changing of light energy into chemical energy by plant chloroplasts.
Type # 2. Prokaryotic Cell:
(Gr., pro-primitive; karyon-nucleus). As the name indicates these cells are simple in structural organization and most primitive types.
They include:
1. Bacteria,
2. Pleuropneumonia-Like Organisms (PPLO),
3. Blue green algae,
In brief, they exhibit following characteristics:
a. Absence of Distinct Nucleus:
Definite nucleus is wanting in the prokaryotic cells. Basically these are made up of central nuclear components namely DNA and RNA molecules, nuclear proteins being surrounded by cytoplasmic matrix, i.e., nuclear membrane are absent. Genetic material is located on a single chromosome which comprises a circular double strand of DNA. Basic protein—histone (characteristic of eukaryotic cells) is absent here along with nucleolus and mitotic apparatus.
b. Absence of Membrane-Limited Structures:
The cytoplasm of prokaryotic cells lacks well defined cytoplasmic organelles such as endoplasmic reticulum, chloroplasts, mitochondria, Golgi complex, centrioles, etc. The bell wall is composed of carbohydrates and amino acids only. The plasma membrane which lies below the cell wall is usually produced into cytoplasmic matrix and acts as mitochondrial membrane carrying respiratory enzymes. In these primitive cells, streaming or amoeboid movements of the cytoplasm are lacking.
The prokaryotes or prokaryons are relatively simple cells. These have only cell membrane, which is usually surrounded by a rigid cell wall. Since they have no other membranes, they contain no nucleus and membranous organelles such as mitochondria or endoplasmic reticulum.
Some important prokaryotes are represented bacteria, blue green algae, spirochaetes, rickettsiae, mycoplasma or pleuropneumonia like organisms (PPLO). They contain only the chromosome, which consists of a single molecule of double helical DNA. The difference between prokaryotic cells and eukaryotic cells may be understood by taking an example of bacterial cell.
1. Bacterial Cell:
The bacteria are acellular, microscopic and sexually reproducing prokaryotes. They average 1.25 µ in diameter. The smallest bacterium is Dialister pneumosintes which measures about 0.15 to 0.3µ in length and the largest bacterium is Spirillum volutons which measures about 13 to 15µ in length.
Usually bacteria may occur in three forms, i.e. spheroidal (coccus), rod-shaped (bacillus) and spiral (Spirillum). Most of these bacteria occur in colonies. A typical bacterial cell consists of three parts, i.e. (a) outer coverings, (b) cytoplasm and its (c) constituents. Body of the bacteria is basically prokaryotic type.
Outer covering in the bacteria is made up of three layers namely:
(i) Capsule,
(ii) Cell wall and
(iii) Plasma membrane
(i) Capsule:
Most prokaryotes secrete mucilaginous material known as slime, which lies exterior to the cell wall.
In some organisms, slime forms only a simple amorphous coating known as a slime layer. In other organisms, slime is present as a layer of uniform thickness surrounding the cell. This type of slime layer is called the capsule (in case of bacteria) or a sheath (in case of blue-green algae).
Capsules and sheaths have no metabolic function. But it is known that bacteria with capsules are not as easily destroyed by white blood cells as bacteria that are not encapsulated. Capsules or sheaths are composed of polysaccharides, polypeptides or lipoproteins or combination of these and often contain traces of other substances.
(ii) Cell Wall:
Next of the capsule, there lies a strong rigid cell wall usually it is 10 μm or more in thickness. It occurs around the bacterial body except one small group of bacteria usually. It is composed of lipids, proteins, certain inorganic slats and specific amino acid-diaminopimelic acid (found only in bacteria and blue-green algae) and a glucose derivative called muramic acid.
Although the cell wall is a rigid structure, it does not possess the permeability properties of the plasma membrane, and very large molecules can pass through it. In colonial corms of blue-green algae, adjacent cells often appear to communicate with one another by means of fine cytoplasmic bridges called plasmodesmata, which pass through the cell wall and connect the plasma membranes of the two cells.
(iii) Plasma Membrane:
Bacterial plasma membrane is similar to plasma membrane of eukaryotic cells. But it contains respiratory chain enzymes.
The bacterial plasma membrane is modified to form varied structures:
(a) Mesosomes:
In Tubercle bacillus and Bacillus subtilis the plasma membrane is invaginated into whorls of convoluted membranes called mesosomes or chondroids. These contain enzymes of electron transport system. Mesosomes help in respiration, secretion and synthesis of material for cell wall; receive DNA during conjugation DNA-replication enzymes. These also help in the distribution of chromosomes (DNA) to daughter bacterial cells.
(b) Desmosomes:
In Thiovulum majus the plasma membrane sinks deep into cytoplasm forming multilayered structures called desmosomes.
(c) Cytoplasm:
The bacterial cytoplasm contains granules of fats, glycogen and proteins; poly β-hydroxy butyric acid and volutin granules etc. Certain photosynthetic bacteria have bacteriochlorophyll or some other photosynthetic pigments (carotenoids). The pigment and the enzymes are found associated with the internal membranes which are arranged either as lamellae or tubules or vesicles. These form the so called chromatophores. It contains 70S ribosomes, which occur either freely in the cytoplasm or form polyribosomes. Each bacterial ribosome consists of subunits 30 S and 50 S.
(d) Nuclear Material:
Interior of cytoplasm contains DNA molecules often called bacterial chromosomes or genophores located in a distinct nuclear region— nucleoid. Nucleoid is devoid of any limiting membrane and has one circular molecule of DNA. This structure may be compared to eukaryotic nucleus.
(e) Other Structures:
Many bacteria consist of whip like cellular outgrowths called flagella meant for movement. These flagella are most common in rod- shaped bacteria. Each bacterial flagellum is made up of single fibril (about 100-200 A0 thick and several microns in length) having flagellin protein.
Likewise, some bacteria contain hair-like extra-cellular outgrowths—pilli or fimbriae, meant for attachment. These are composed of helically arranged subunits of pilin protein. Cytoplasm of prokaryote cells also contains gas vacuoles, volutin granules (polyphosphate granules), glycogen granules, lipid droplets, polyhedral bodies.
2. Pleuropneumonia-Like Organisms (PPLO):
These are bacteria-like organisms but are without cell wall and mesosomes. The plasma membrane is about 75 Å. The cytoplasm contains enzymes involved in protein synthesis and in the biosynthesis of ATP by anaerobic break down of sugars. It also contains enzymes required in DNA replication, transcription and translation. The nucleus is absent. The nuclear area contains double-stranded DNA molecule, which may be circular or in the form of fibrils.
W.V. Iterson (1969) placed PPLO with bacteria in the group Mycoplasmata cell but latter Novikoff and Holtzman (1970) have excluded them from bacteria and considered them as simplest prokaryotic cells. Smallest PPLO belongs to Mycoplasma laidlawii and measure about 0.1µ in diameter. It is saprophytic form found in sewage, compost and earth, etc. Another species Mycoplasma gallisepticum measures about 0.25µ in diameter and occurs as parasite in the cells and cell exudates of respiratory organs of warm blooded animals. It causes various respiratory diseases in animals.
3. Blue-Green Algae (Cyanobacteria):
It is another group of prokaryotes resembling bacteria in many aspects. Some of the blue-green algae forms are unicellular but most species are multicellular forming colonies of prokaryotic cells.
General structure of a typical blue green alga cell is as follows:
(a) Slimy Layer or Gelatinous Sheath:
It is the outermost covering present outside the cell wall. It is gelatinous in nature.
(b) Cell Wall:
It resembles the cell wall of bacteria. Basically it is composed of lipoproteins, lipopolysaccharides and mucoproteins. Next to cell wall is present innermost lipoproteinous plasma membrane. Blue-green algae and also purple and green bacteria have elaborate internal membranes to which photosynthetic pigments and other constituents of photosynthetic apparatus are attached.
These membranes are in the form of sacs of thylakoids of typical ‘unit membrane’ structure. Thylakoids exist usually as lamellae arranged throughout the peripheral regions of the cell in parallel, concentric rings. Thylakoids are the sites of photosynthesis. The interior of thylakoid membranes contains light capturing pigments, cytochromes and other compounds for electron transfer during the process of photosynthesis. Carotenoids lie in the cell matrix.
(c) Cytoplasm:
The cytoplasm lacks ER, Golgi complex, mitochondria and lysosomes. The ribosomes are freely distributed in the cytoplasm and form polyribosomes during protein synthesis.
The cytoplasm also contains the photosynthetic pigments, i.e. chlorophyll and carotenoid. These occur in concentrically arranged flattened sacs called lamellae. These forms have oxygen evolving photosynthetic system like higher plants, and produce oxygen. Cytoplasm also contains blue pigment phycocyanin, and red pigment phycoerythrin. These two pigments are collectively known as phycobilin and are found inside small granules called cynosomes or phycobilisomes.
Each cynosome measures about 400 Å in diameter and contains about 10 subunits, each about 130 Å in diameter. Thus, plastid-like structures (characteristic of eukaryotes) are lacking here. In the cells of blue-green algae, cilia and flagella are absent but they have capacity of movement by gliding and rotatory motions. Blue-green algae in presence of sunlight synthesize all their cell substance from CO2 water and salts.