d:\data\newage~1\biote\bit-1.pm5/iiird proof/ The Cell Part A CELL BIOLOGY

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1 The Cell 1 Part A CELL BIOLOGY

2 2 Biotechnology

3 The Cell 3 1 THE CELL The cell is the smallest structural and functional unit of the all living organisms. Cell biology is the study of the cell. Cell biology is also known as cytology. The word cytology was derived from Greek word Kitos (cytos) means a halo vessel or a compartment and Logus means to discover or to study. Therefore cytology literally means a branch of biology which studies the different aspects of cells, like study of phenomenon of heredity, variation, evolution, neutrition, metabolism, growth, reproduction of cell, etc. History 1. In 1665, Robert Hook first time observed the cell. He observed a thin slice of cork under his own microscope. Under microscope, he observed a honey-comb like structure showing hallow empty spaces or compartments surrounded by firm cell wall. To each hallow space he called a cell. Actually what Robert Hook observed was dead cell. 2. Antony Van Leeuwenhoek ( ) observed first time the microorganisms (prokaryotic cell) under his compound microscope. He observed tiny microbes in a drop of pond water which he called animacules. 3. In 1831, Robert Brown, first time observed a nucleus in the cell. 4. In 1855 Rudolph Virchow observed that new cells arise from preexisting cells. 5. In 1938, M.J. Schleiden, a German botanist studied many plants section under microscope and came to conclusion that, all plants are ultimately made up of cell. 6. In 1938 T.S. Schwann, a German Zoologist came to the similar conclusion that, all the animals are ultimately made up of cell. Later, Schleiden and Schwann both together put forth a theory which is known as a cell theory. According to this theory all the

4 4 Biotechnology organisms i.e. plants and animals are made up of cells. In twentieth century, after the invention of modern microscopes and various technique, our knowledge of cell increased and a statement of cell theory was slightly modified. According to the latest cell theory, all the living organisms consists of cells and cell products. Therefore all the living organisms i.e. prokaryotic and eukaryotic organisms are made up of cells. Viruses are not made up of cells. In unicelluar organisms, all functions are carried out by the same cell, while in multicellular organisms, different functions are performed by different types of cells. The cell theory may be summarised as (a) Cells of organisms are differentiated into distinct cell types. (b) A cell is a feature of all organisms with the exception of viruses. (c) The protoplasm is the living content of the cell and determines the activity of the cell and thus the whole organism. (d) New cells originate from preexisting cells through division. In any case, cell never arise de novo. All living cells/organisms have a common ancestor. Therefore, all cells shows common simplicity in the molecular organisation and principle of molecular economy. Cell Size There seems no generalisation in size, shape and structure of the cell. The variety and diversity of cells are as many as their functions. The cell size broadly ranges from 0.2 µm to 2 mm (Table 1.1). Table 1.1 VARIOUS CELL TYPES AND THEIR SIZE Cell type Size (µ) 1. Mycoplasma (PPLO) Bacteria Small lymphocytes Erythrocytes Amoeba proteus Liver cells Euglena Human oocyte Ostrich egg Neurons (length)

5 The Cell 5 Cell shape Cell shape is controlled by certain physical, chemical and physiological factors. Majority of cells have characteristic fixed shape. While certain cells have no fixed shape called variable cells, for e.g., amoeba and leucocytes. Generally multicellular organisms are made up of different shaped cells (Table 1.2). Table 1.2 VARIOUS CELL SHAPES AND THEIR EXAMPLES Cell shape Example 1. Variable cells Amoeba, Leucocytes, Myxomycetes 2. Fixed cells (i) Spherical cells egg of many animals (ii) Flattened cells Squamous epithelium, endotheliums, and upper layer of epidermis (iii) Cuboidal cells Thyroid gland follicles (iv) Columner cells The cells lining the intestine. (v) Discoidal cells Erythrocytes (vi) Spindle shaped cells Smooth muscle fibres (vii) Elongated cells Nerve cells (viii) Branched cells Pigment cells of skin Depending upon their organisation, cells are basically of two types i.e., prokaryotic cells and eukaryotic cells. Prokaryotic cells are primitive type of cells. These cells lacks true nucleus and other cell organelle like mitochondria, chloroplasts, endoplasmic reticulum, golgi complex, etc. The word prokaryotes is derived from Greek word pro means primitive or old and karyotes means nucleus or main. Eukaryotic cells have true nucleus and it also contains cell organelles. The word eukaryotes is derived from Greek word eu means true and karyotes means nucleus. The main differences between prokaryotic and eukaryotic cells is enlisted in Table 1.3. Table 1.3 DIFFERENCE BETWEEN PROKARYOTIC AND EUKARYOTIC CELLS Prokaryotic cell Eukaryotic cell 1. A true nucleus is absent. A true nucleus is present. 2. Nuclear membrane is absent. Nuclear membrane is present. 3. Nucleolus is absent. Nucleolus is present. 4. Chromosome is single, circular. Chromosomes are many and linear. 5. Chromosome present freely in Chromosomes are enclosed inside the the cytoplasm. nuclear membrane. (Contd.)

6 6 Biotechnology Prokaryotic cell Eukaryotic cell 6. Histone proteins are absent in Chromosomes are well organised with the organisation of chromosome. histone protein. 7. In photosynthetic cells, chloro- In photosynthetic cells, the chlorophyll phyll pigments are present in pigments are present in the plastids cell membrane. chloroplast. 8. Ribosomes are 70s type. Ribosomes are 80s type. 9. Cell organells are absent. Cell organells are present. 10. Streaming or amoeboid move- Streaming or amoeboid movement of ment of cytoplasm absent. cytoplasm present. 11. Flagella if present, do not have Flagella shows typical typical arrangement. arrangement. 12. Enzymes necessary for respi- Enzymes necessary for respiration are ration are present in plasma- present in mitochondria. membrane. 13. Cell division by the process of Cell division by the process of mitosis amitosis i.e., mitosis and meiosis and meiosis. is absent. 14. mrna will not have the 5' mrna having 5' methyl cap and 3' methyl cap and 3' poly A tail. poly A tail. 15. Transcription and translation are Transcription and translation are not combined process, both takes combined process. Transcription takes place in cytoplasm. place in nuclers, while translation occurs in cytoplasm. 16. In genes non-coding regions are In genes non-coding region (i.e., absent. introns) are present. 17. After transcription there is no After transcription, post transcriptional post transcriptional modification modifications occurs in mrna. in mrna. 18. Polycistronic mrna. Mono cistronic mrna. 19. The cell wall contains amino When cell wall present, it doesnot consugars and muramic acid. tain amino sugar and muramic acid. Ultrastructure of Eukaryotic cell Eukaryotic cells are highly evolved cells and show striking characters in contrast to prokaryotes. It contains specialised membrane bound cell organelles. The most important cell organelle is nucleus, which is membrane bound and includes filamentous chromosomes. Other membrane bound organelles include chloroplants, mitochondria, lysosomes, golgi complex, endoplasmic reticulum, etc. The cell organelles provides micro environments to subcellular metabolic units. The eukaryotic organisms are very diverse, few are unicellular, where all the functions are carried out by same cell, while many are multicellular organisms. Multicellular eukaryotes contains variety of cell

7 The Cell 7 types originate as a result of cell differentiation and it s organised aggregates, forming tissue systems. Although there is a basic pattern of organisation, several functional specializations are acquired by cells. Eukaryotes are grouped into phototrophs (e.g., plants) and chemotrops (e.g., animals). The phototrophs have a photosynthetic apparatus and a rigid cell wall of cellulose. Photosynthetic Eukaryotes The eukaryotes, which are able to synthesize their own food in presence of the sunlight is called Photosynthetic Eukaryotes. As an example of this we can consider a typical plant cell. A plant cell has a thick outer most coating called cell wall, which protect the cell from various osmatic stresses. Cell wall is made up of cellulose. Inside the cell wall, there is a plasma membrane, which encloses the cytoplasm. At several locations, two adjacent cells have intercellular channels called plasmodesmata, which help cellular communication. The cytoplasm of plant cell is semifluid, in which many cell organelles micro and macroneutrients and enzymes are suspended. All cell organelles are bound by double membrane. Different cell organelles which are present in plant cell are as follows (Fig. 1.1) (a) Nucleus Nucleus is bounded by double layered nuclear membrane. It contains filamentous chromosomes, which contains genetic information. (b) Endoplasmic reticulum The endoplasmic reticulum forms a network of tubules whose one end is connected with nuclear membrane. There are two types of endoplasmic reticulum smooth endoplasmic reticulum and rough or granular endoplasmic reticulum. The main function of endoplasmic reticulam is synthesis and storage of protein/ enzymes. (c) Golgi complex It is made up of cisternae and vesicles. It s main function is storage. (d) Mitochondria Mitochondria synthesize ATP molecule (energy curency of cell) by oxidative phosphorylation in the absence of light by breaking down organic molecules. (e) plastids Plastids are characteristic of plant cells only. Plastids are of two types leucoplasts (with out pigment) and chromoplasts (with pigments). The most important plastid is with green chlorophyll pigment called chloroplasts. Chloroplasts participate in the photosynthetic activity. (f) Lysosomes It contains hydrolytic enzymes for digestion. (g) Peroxysomes It neutralise metabolic peroxides in the cell. (h) Glyoxysomes It is involved in glyoxylate metabolism.

8 8 Biotechnology (i) Ribosomes Plant cell contains 80s types ribosomes. Ribosomes participates in protein synthesis. (j) Vacuoles The plant cell contains a well defined, large vacuoles. It is usually filled with liquid material. Unlike other cell organelles, vacuoles are bound by single membrane called tonoplast. In plant cell, large volume is made up of vacuole generally central and cytoplasm and other cell organells occupy little space, generally peripheral. Mitochondrion Cellulose cell wall Chloroplast Cell membrane (plasmatemma) Central vacuole Leucoplast containing starch grain Ground substance of cytoplasm with granular inclusions Plasmodesma Small vacuole Golgicomplex (dictyosome) Nucleolus Nucleoplasm containing chromatin Endoplasmic reticulum with attached ribosomes Fig. 1.1 A typical plant cell. Chemosynthetic Eukaryotes The eukaryotes which are not able to synthesise their food in the presence of sunlight are called chemosynthetic eukaryotes. e.g.. Animal cell. Animal cells are devoid of a cell wall, and the plasma membrane is the outermost cell boundary, which carries out catalytic as well as osmotic protection functions. The animal cell contains all membrane-bound cell organelles in cytoplasm similar to those of plant cells, with the exception of plastids. Other than

9 The Cell 9 cell organelles, animal cells have centrosomes, locomotory organelles and an elaborate cytoskeleton (Fig. 1.2). Region containing microtubules and microfibrils Pinocytic vesicles Golgi apparatus Secretion vacuole Phospholipid storage granule Tysosome Centrosome Nucleus Nucleolus Ribosomes attached to endoplasmic reticulum Nuclear membrane Neutral lipid storage granule Cytoplasmic matrix Mitochondrion Plasma membrane Fig. 1.2 A typical animal cell. Centrosomes are usually one per cell, which is located near the nuclear surface. It is a complex structure with a dense outer granular mantle that encloses a less granular material around two pairs of cylindrical bodies called centrioles. The centrioles participate in mitosis and also anchor the flagellum in sperm cell. The cytoplasmic matrix consists of a complex network of microtubules and microfilaments, forming the cytoskeleton. The cytoskeleton helps in the movement of cell organelles, change in cell shape, amoeboid locomotion, cytokinesis, etc.

10 10 Biotechnology Summary The cell is the smallest structural and functional unit of the all living organisms. Cell biology is the study of cell. In 1938, M.J. Schleiden and T.S. Schwann proposed the cell theory. There is no generalisation in cell size, shape and structure. Basically cells are of two types namely prokaryotic cell and eukaryotic cell. Prokaryotic cell is more primitive cell. Eukaryotic cells depending upon its capacity to produce their own food in presence of sunlight is divided into photosynthetic eukaryotes (e.g. plant cell) and chemosynthetic eukaryotes (e.g. animal cell). The main characteristic of plant cells is it contains chloroplasts and large vacuoles. The main characteristic of animal cells is it contains centrosomes, locomotory organelles, cytoskeleton. The remaining cell organelles commonly present in both plant and animal cells are nucleus, mitochondria, golgi complex, 80s ribosome, endoplasmic reticulum, lysosomes, peroxysomes. EXERCISE I. 1. What is cell? Describe cell theory. 2. Explain broad classification of cell. II. Differentiate between the following: 1. Eukaryotic cells and Prokaryotic cells. 2. Plant cells and animal cells.