A structure containing a mass of cytoplasm surrounded by semi-permeable membrane called plasma membrane is called a cell. It encloses cytoplasm, many cell organelles along with nucleus or nuclear material. On the basis of organization of membranes, variety and structure of cytoplasmic organelles and complexity of nuclear region, the cells are classified into two types: Prokaryotic cell and Eukaryotic cell. These terms were suggested by Hans Ris in 1960s.
Cell type and History of Origin
History and Origin: -
A cell was defined as “unit of biological activity delimited by a semi permeable membrane and capable of self-reproduction in a medium free of other living systems” by Loewy and Sinkovitz (1963). The study of cell has been made possible with the help of light microscope. Robert Hooke (1665) with the help of light microscope discovered that a section of cork is made up of small cavities surrounded by firm walls. He used the term “cell” for the first time to describe his investigations on the “texture of a piece of cork”. Later on, A. Van Leeuwenhoek (1632-1723) observed various unicellular organisms and cells like bacteria, protozoan’s, red blood cells and sperm etc. He observed nucleus in some erythrocytes, and all this were made possible with the improved microscopes. In 1809, Marble M. stated that all plant tissues are composed of cells. In the same year, importance of cells in living organisms was described by J.B. Lamarck. Robert Brown in 1831 observed nucleus in certain plant cells. Mimosa cells were boiled in nitric acid by Dutro Chet (1837) to separate the cells to conclude that all organic tissues are composed of globular cells, united by simple adhesive forces. “All living organism is composed of cells” was stated by Schwann, T. (1839) after examining a variety of animals and plant tissues.
BASIC COMPONENTS OF PROKARYOTIC AND EUKARYOTIC CELL: -
Prokaryotic Cells: -
Prokaryotic cells are the most primitive cells and have simple structural organization. It has a single membrane system. They include bacteria, viruses, blue-green algae, mycoplasmas, rickettsia, spirochetes etc. Cyanobacteria or blue green algae are the largest and most complex prokaryote, in which photosynthesis of higher plants type have evolved. Prokaryotes are included in the kingdom Monera and the super kingdom Prokaryotes. The Prokaryotes have the following characters:
1. The size of prokaryotic cells ranges between 1 to 10 µm. They occur in a variety of forms.
2. Prokaryotic cell consists of three main components:
(I) Outer covering: It is composed of inner cell or plasma membrane, middle cell wall and outer slimy capsule.
a. Cell membrane: Cell membrane made up of lipids and proteins, is thin and flexible and controls the movement of molecules across the cell. Respiratory enzymes are carried by it for energy releasing reactions. Mesosomes, the in-folds of plasma membrane bears respiratory enzymes, and these are considered analogous to mitochondria of eukaryotic cells. Similarly, the pigments and enzymes molecules that absorb and convert the light into chemical energy in photosynthetic cells are also associated with the plasma membrane’s in-folds called photosynthetic lamella. These lamellae are analogous to the chloroplast of eukaryotic cells. Plasma membrane plays role in replication and division of nuclear material. Since the in-folds remain continuous with the cell membrane, they are not considered as separate compartments. Thus, prokaryotic cell is non-compartmentalized.
b. Cell wall: It is a rigid or semi-rigid non-living structure that surrounds the cell membrane and its thickness ranges between 1.5 to 100 µm. Chemically it is composed of peptidoglycans... Some bacteria such as mycoplasmas lack cell wall.
c. Slimy capsule: A gelatinous coat outside the cell wall is the slimy capsule. It is composed of largely of polysaccharides and sometimes it may have polypeptides and other compounds also. It protects the cell against desiccation, virus attacks, phagocytosis and antibiotics
(II) Cytoplasm: Prokaryotic cytoplasm contains proteins, lipids, glycogen and inorganic ions along with enzymes for biosynthetic reactions and ribosomes, tRNA and mRNA for protein synthesis. Prokaryotic cytoplasm has some special features as follows:
a. It lacks cell organelles like endoplasmic reticulum, mitochondria, Golgi apparatus, Centrosomes, vacuoles, Lysosomes, microfilaments, intermediate filaments and microtubules.
b. The only cytoplasmic organelle found in prokaryotic cells is the ribosomes. They are smaller than eukaryotic ribosomes i.e., 70S and lie free in the cytoplasm. They form polyribosomes at the time of protein synthesis. They are the sites of protein synthesis.
c. Like eukaryotic cells, the cytoplasm of prokaryotic cell does not show streaming movement or cyclosis.
d. Gas vacuoles are also formed in some prokaryotic cells.
e. The cell does not show phagocytosis, pinocytosis and exocytose, substances enter and leave the cell through the cell membrane.
f. They may contain deposits of polysaccharides or inorganic phosphates.
(III) Nucleoid: Nuclear envelope is absent in prokaryotic cell and the genetic material lies directly into the cytoplasm. Such nuclear material is known as nucleoid. Nucleoid consists of greatly coiled single pro-chromosome. It shows the following special features:
a. A short and simple pro-chromosome is present which is attached at least at one point on cell membrane.
b. Mostly there is single copy of chromosome, the prokaryotic cell is haploid.
c. The DNA is naked as it is not associated with basic histone proteins. It is double stranded, helical and circular.
d. The amount of DNA is lesser than eukaryotic cell and it codes fewer proteins. Replication of DNA is continuous throughout the cell cycle. Transcription and translation occur in cytoplasm and processing of mRNA is not required.
e. The processes like meiosis, gamete formation or fertilization are absent. Conjugation is seen in some bacteria.
f. Mitotic apparatus absent.
g. There is no nucleolus.
h. Cell membrane folds or mesosomes help to segregate the replicated products of
chromosomes into daughter cells.
3. Plasmids: In some prokaryotic cells, in addition to nucleoid, a small circular double stranded DNA molecule is present. It is called plasmid. Plasmids have 1000 to 30,000 base pairs and they generally encode proteins required by the organism to resist antibiotic and other toxic material.
4. Flagellum: It is a whip like locomotory structure found in many bacteria. It is 150Å thick and 10 to 15µm long. As the flagellum does not have any surrounding membrane, it grows at the tip. It has two main parts: Filament and basal body.
(i) Filament- Filament extends out of cell into the medium and it is composed of many intertwined spiral chains of the subunits of a protein called flagellin. Flagellin differs from actins or tubulin.
(ii) Basal Body- The basal body attaches the flagellum to the cell and generates the force to rotate it. It is composed of many components and numerous proteins. It has two parts: shaft and hook.
Eukaryotic Cells: -
The internal organization of eukaryotic cell is more developed than prokaryotic cells from which they are believed to have been evolved. They are evolved to have double membrane system. Primary membranes are the one that surrounds the cell, celled cell or plasma membrane and the secondary membrane surround the nucleus and other cellular organelles. Eukaryotic cells occur in protists, fungi, plants and animals. Eukaryotic cells have the following characteristics:
1. Number- In multicellular organisms the numbers of cells are correlated with the body size. The human blood contains about 30 quadrillion (3 × 1015) corpuscles and a 60 kg human being has about 60 × 1015 cells
2. Shape- A cell may be spherical, cuboidal, oval, disc-like, polygonal, columnar, spindle like or irregular. Thus, cells acquire a variety of shapes not only in various organisms but also in different tissues of the same organism.
3. Size- Most of the eukaryotic cells is microscopic and their size ranges between 10 to 100µm. Sporozoites of malaria parasite (Plasmodium vivax) is among the smallest cells having the size equal to 2µm long.
4. Components of a cell- Three main components of the eukaryotic cells are cell membrane, cytoplasm and nucleus.
(i) Cell membrane- Cell membrane, plasma membrane or plasmalemma is a thin elastic living covering that surrounds the cell keeping the cell contents in place, provides shape to the cell and controls the transfer of materials across it. It is composed of lipid-protein complex. It lacks respiratory enzymes. In many protists and animal cells it allows endocytosis and exocytosis.
(ii) Cytoplasm- The cytoplasm or the cytostomes is a semi-fluid, homogeneous, translucent ground substance known as cytoplasmic matrix or cytosol which is present between the cell membrane and the nucleus.
a. Organelles: The organized structures having the specific functions and capacity of growth and multiplication in some cases are known as organelles.
I. Mitochondria: The rod like or globule shaped structures scattered in the cytoplasm are found singly or in groups. They are bounded by double membrane of lipoproteins. The inner membrane gives out finger like structure known as cristae which partially subdivide the inner chamber of mitochondrion. On the inner surface of cristae are present mushroom like structures, exosomes that are related to phosphorylation. The space between the membranes and its lumen is filled with mitochondrial matrix. Both the membranes and the matrix contain many oxidative enzymes and coenzymes. Since mitochondria contain DNA molecules and ribosomes, they synthesize certain proteins. They produce the energy and reserve it in the form of adenosine triphosphate (ATP). Due to the presence of its own DNA and ability of protein synthesis along with its duplication, the mitochondria are called semi-autonomous organelle. The DNA of mitochondria resembles that of bacterial cell; hence it is also called as endo-symbiotic organelle II. Centrosomes: (9+0) there is a clear zone around centrioles, near the nucleus, that includes a specialized portion of cytoplasm, called centrospheres. Its matrix is called Kino plasm that bears two rounded bodies the “centrioles”. Each centriole consists of nine fibrillar units and each of them is found to contain three microtubules arranged in a circle. Both the centrioles are arranged at right angle to each other.
III. Golgi bodies: These are the stack of flattened parallel-arranged sacs and vesicles found in association of endoplasmic reticulum. They are composed of many lamellae, tubules, vesicles and vacuoles. Their membranes are supposed to be originated from ER and are composed of lipoproteins. In plant cells the Golgi complex is called dictyosome that secretes required materials for the formation of cell wall at the time of cell division. It helps in the formation of acrosome of sperms,
release of hormones, enzymes and other synthetic materials
IV. Plastids: These organelles are found in plant cells and are absent in animal cells. They may be colored like chloroplast or chromoplasts or colorless like leucoplast.
V. Metaplasm: The particles like vacuoles, granules and other cytoplasmic bodies such as ribonucleoprotein molecules are represented by it.
VI. Cilia, basal bodies and flagella: Cilia are the minute structures covering the surface in some cells. Both cilia and flagella originate from the basal bodies or blepharoplast lying-in cytoplasm. They consist of nine outer fibrils with the two larger fibrils in the center.
VII. Microtubules: The ultra-fine tubules of protein (tubulin) traversing the cytoplasm of plant and animal cells providing the structural framework to the cell, determine the cell shape and general organization of the cytoplasm are known as microtubules.
IX. Ribosome’s: Ribosome is the minute spherical structures that originate in nucleolus and are found attached with the membrane of endoplasmic reticulum and in the cytoplasm. They are mainly composed of ribonucleic acids (RNA) and protein. They are mainly responsible for protein synthesis.
Self-Assessment Questions and Possible Answers
Multiple Choice Questions: -
1. There is no organized nucleus in:
(a) Bacterial cell (b) Green algae cell (c) Animal cell (d) Plant cell
2. The prokaryotic cells are characterized by:
(a) A distinct nuclear membrane (b)Absence of chromatin material (c)Distinct chromosome (d) Absence of nuclear membrane
3. In a prokaryotic cell, DNA is:
(a) Enclosed by nuclear envelop(b) Lacking(c)Not a genetic material(d)without a membrane
4. Cell wall is found around the: -
(a) Prokaryotic cell (b) Algal cells (c) Plant cell (d) All of these
5. Chemical energy of food stuffs is converted into biologically useful forms by:
(a) Ribosomes (b) Golgi complex (c) Mitochondria (d) Plastids
6. Sun radiant energy is converted into chemical energy of organic compound by:
(a) Mitochondria (b) Chloroplast (c) Ribosomes (d) Centrosomes
7. Which structure is present only in animal cell?
(a) Cell membrane (b) Lysosomes (c) Centrioles (d) Ribosomes
8. Single envelope system is characteristic of:
(a) Prokaryotic cell (b) Eukaryotic cell (c) None (d) Both
9. Prokaryote and eukaryotes have the common:
(a) Mitotic apparatus (b) Histone (c) Genetic code (d) Mitochondria
10. Unicellular microscopic organisms were first studied by:
(a) Robert Hooke (b) Priestley (c) Pasteur (d) Leeuwenhoek
ANSWERS: -
1. (a) 2. (d)3. (d) 4. (d) 5. (c) 6. (b) 7. (c) 8. (a) 9. (c) 10. (d)
Very Short Questions: -
1. What are prokaryotes? Give an example.
2. What are eukaryotes? Give few examples.
3. Cell is an open dynamic system. Is it correct?
4. Prokaryotic cells are haploid. Is it so?
5. What are cyanobacteria?
6. Give three essential characteristics of cell?
7. Where is nucleolus found?
8. What are the power houses of the cell?
9. Name the protein factories of prokaryotic and eukaryotic cells?
10. What is the control center of a cell?
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