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Cell structure

Robert Hook (1655) was the first person who observed cells of cork (dead cells) under the microscope, which appeared like honey comb. Leeuwenhoek (1674) observed white blood cells with the help of a microscope with some organization inside. Schleiden and Schwann (1838-39) proposed the cell theory, according to which the cell is the unit of structure of living system (cell is the unit of life), i.e., all living organisms are made up of cells (except for viruses, which are living but have acellular organization and are composed of only proteins and DNA/RNA, some viruses also contain membranes) and the cell arises from the pre-existing cells.

 

CELL STRUCTURE

 

The cellular organisms have been divided into two categories, prokaryotes (e.g., archaebacteria, eubacteria, cyanobacteria, prochlorophytes) that are primitive and evolved around 3 billion years ago (Fig. 1.2); and more advanced eukaryotes that appeared around 1 billion years ago, e.g., algae (except for the algae belonging to classes Cyanophyta and Prochlorophyta), fungi, bryophytes, pteridophytes, gymnosperms, angiosperms, unicellular and multicellular animals (Fig. 1.1).

 

Fig. 1.1 The eukaryotic cells.

 

The cell size and shape varies from organism to organism. Some cells are visible to naked eyes whereas most of the cells can be observed through microscope. Generally, eukaryotic cells are l-2 µm to 1 mm and prokaryoyic cells 0.15-2.0 µm in size. The largest cell found is the ostrich egg having 15-20 cm diameter. Regarding the shape, the cells may be spherical, cylindrical, irregular, etc. Some plant (e.g., Euglena, Dunaliella, etc.) and animal (e.g., amoeba, leucocytes, etc.) cells keep on changing their shape.

 

Various parts of a cell are:

 

The Cell Wall

 

The plant cells contain the outermost boundary, the cell wall (some unicellular phytoplankton lack a cell wall in their vegetative cells, e.g., Dunaliella, Euglena, etc., but during their life cycle at sometime they form a cell wall, e.g., during spore formation) that is lacking in animals. The rigid, porous (pores, called plasmodesmata, allow movement of substances between the adjacent cells in multicellular plants) cell wall providing shape and protection to the cell and prevents the cell from bursting. Some cells have a capsule (glycocalyx) around the cell wall that is made up of polysaccharides (e.g., Diplococcus pneumoniae, Streptococcus pneumoniae), polypeptide (e.g ., Bacillus anthracis), mixture of both polysaccharides and proteins or waxy material (e.g., laprosy bacterium).

 

The compositions of the cell wall of prokaryotic and eukaryotic cells are as fallows.

 

  1. 1.      Prokaryotic Cell Wall. In some prokaryotes the vegetative cells are devoid of any cell wall, e.g., mycoplasma. In most of the prokaryotic cells, the cell wall is made up of peptidoglycan, a combination of proteins and carbohydrates. The peptidoglycan consists of alternating Nacetyl glucosamine (NAG) and N-acetyl muramic acid (NAM) sugars linked by β-1 ,4-glycosidic bonds (repeated disaccharides). NAM molecules are attached with a tetrapeptide chain of L-alanine, D-glutamic acid, L-lysine and D-alanine amino acids (Fig. 1.7). In G+ bacteria the peptidoglycan is quite thick and the two adjacent, parallel tetrapeptides are linked through a pentaglycine at the points of L-lysine and D-alaiiine (Fig. 1.3 and 1.6). In G- bacteria the peptidoglycan is thinner, having an outer membrane above it and the pentaglycene is absent, so that the terminal D-Ala is attached directly to the neighbouring tetrapeptide through L-Lys (or L-Lys in some cases may be replaced by diaminopimelic acid that is lysine like amino acid) (Fig. 1.4 and 1.5).

 

2. Eukaryotic Cell Wall. Different groups of eukaryotic cells have different composition of the cell wall. In green algae and higher plants the cell wall is mainly made up of cellulose fibres that are embedded in a matrix of pectin, lignin and hemicellulose; the middle lamella found between the adjacent cells is made up of pectin .. Sometimes, in higher plants, in addition to cellulose the outer cell wall also contains lignin (wood) and suberin (cork). Similarly, in various other groups of algae the cell walls have different compositions, e.g., in diatoms it is mainly made up of silica. In fungi, the main component of the cell wall is chitin.

 

Plasma Membrane

 

The plasma membrane (cell membrane) lies just beneath the cell wall in plant cells, whereas it makes the outermost boundary in case of animal cells. It is semi-permeable (selectively permeable) as it facilitates the transportation of selected substances through it and prevents the escape of cellular materials outside the cells. S. J. Singer and G. Nicholson (1972) proposed fluid mosaic model for the biological membranes (cell membrane and organeller membranes) according to which the biological membranes (about 75 A or 7.5 nm thick) are mainly composed of phospholipids and proteins that are arranged in a mosaic manner, and smaller amounts of oligosaccharides and other substances. A phospholipid molecule is made up of an alcohol (generally glycerol) attached with a phosphate group (hydrophilic polar head) and two fatty acids (hydrophobic non-polar tails) (Fig 1.9).

 

The phospholipids (for details see the chapter ‘Lipids”) form a double layer structure in which the polar heads (phosphate groups) are directed towards the two surfaces (outer and inner) of the membrane and the two non-polar fatty acid tails on the inner side (Fig. 1.10). Proteins are found either at the surfaces (extrinsic proteins) or embedded (partially or completely) inside this phospholipid bilayer (intrinsic proteins) (Fig. 1.8). Some proteins serve as the carriers (intrinsic proteins) for the transportation of water-soluble materials through the membrane, whereas others function as enzymes (e.g., various respiratory and photosynthetic enzymes are found in mitochondrial and chloroplastic membranes, respectively, in eukaryotes and in cytoplasmic and thylakoids membranes, respectively, in prokaryotic cells). In prokaryotes, the chromosomal DNA is found to be attached with the plasma membrane (the invaginated site is called mesosome) that helps in the separation of two DNA molecules formed as a result of replication to two daughter cells at the time of cell division.

CH2


   R = – CH2– CH2 – N

CH2

                       R ——————-> Organic|                                groupO|O=P- O|

O

|

CH2– CH2– CH2

|         |

O      O

|        |

C=O  C=O

|         |

CH2 CH2

|        |

CH2 CH2

|        |

CH2 CH2

|        |

CH2 CH2

|        |

CH2 CH2

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CH2 CH2

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CH2 CH2

|        |

CH2 CH2

|        |

CH2 CH2

|        |

CH2 CH2

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CH2 CH2

|        |

CH2 CH2

|        |

CH2 CH2

|        |

CH2 CH2

|        |

CH2 CH2

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CH2 CH2

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CH2 CH2

 

The transportation through membranes can be of two types (Fig. 1.l3): ·

 

1. Passive Transport. This transportation across the membrane involves the movement of solutes or gases (diffusion) and solvents (osmosis) (Fig. l.ll) along the concentration or pressure gradients (from a region of high concentration or pressure to a region of low concentration or pressure) till the concentration or pressure becomes equal on the two sides of the membrane and the molecules move equally in both the directions. This transportation does not require any energy.

 

Salt solution                  Pure water

(Low solvent concentration)               (High solvent concentration)

H2O     <- H2O

 

-> Biological membrane

 

Fig. 1.11 Osmosis

 

2. Active Transport This transportation of materials occurs against the concentration or electrochemical gradient (from a region of low concentration to a region of high concentration) and hence requires energy in the form of ATP. e.g., movement of Na+ and K+ (Fig. 1.12).

 

External medium

 

Cytoplasm

 

Fig. 1.12 Active transport of Na+ and K+ ions through cell membrane.

 

Fig. 1.13 Active and passive transports across the cell membrane.

 

Cytoplasm

 

Cytoplasm is the jelly-like fluid in between the plasma membrane and the nuclear membrane. It contains various organelles (like mitochondria, chloroplasts, Golgi bodies, endoplasmic reticulum in eukaryotes) or other membrane bound structures (like thylakoids, gas vacuoles in prokartotes), ribosomes, vitamins, various enzymes, proteins, etc.

 

Cyclosis is the cytoplasmic streaming found in eukaryotic cells that helps in the movement of organelles and other cell inclusions and in the intracellular distribution of nutrients, metabolites, enzymes, etc.

 

Nucleus

 

In eukaryotic cells the genetic material (several linear molecules of DNA) is found inside the nucleus and controls all the activities of a cell. Some cells are devoid of any nucleus, e.g., mature sieve cells of vascular plants that help in the transportation of substances, red blood cells of mammals, etc. Generally, the cells contain a single nucleus (uninucleate), but in some cases they may contain more than qne nuclei (multinucleate or coenocytic conclition) resulting due to repeated nuclear clivisions without any cytokinesis (division of cytoplasm), e.g., many algae (Vaucheria) and fungi (Rhizopus). Nucleus was discovered by Robert Brown ( 1833) and generally it is found in the centre of a cell, but in plant cells it may be located on one side due to the presence of a large central vacuole. The shape of the nucleus may be spherical, elliptical, elongated, kidney-shaped, lobed (in WBC), irregular, etc., and the size depends upon the type and activity of the cell.

 

The fact that nucleus contains hereditary information was shown by a Danish scientist

J. Hammerling (1953). He performed transplantation experiment using two species of a green alga Acetabularia crenulata and A. mediterrania that contains a single nucleus in their rhizoids and differ with respect to their cap (reproductive body) shape.

 

Parts of Nucleus

 

The nucleus consists of a surrounding nuclear membrane. nodeoplasm (the jelly-like substance inside the nuclear membrane), one or more nucleoli (singular: nucleolus) and chromatin or chromosomes (Fig. 1.14).

(i) Nuclear Membrane: (Nuclear- envelope). It is made up of two membranes that merge at the pores. The pores (diameter 30-100 nm) allow the transfer of materials between the nucleus and the cytoplasm, e.g., mRNA that is synthesized in nucleus and escapes into the cytoplasm through pores. In animal cells a diaphragm, called annulus, extends across each pore. The space between the two membranes is called perinuclear space (15 nm wide) that may be continuous with the lumen of the endoplasmic reticulum. The inner nuclear membrane is smooth, whereas the outer membrane may be attached with ribosomes at some locations to give a rough appearance.

 

(ii) Nucleolus: It is round or irregular in shape and found associated with a specific region (nucleolar-organizing region) of a chromosome called nucleolar-organizing chromosome that contains the gene for rRNA of which the nucleolus is made up of. The nucleolus and the nuclear membrane disappear at some stages (Metaphase and Anaphase) of the cell division.

Uii) Chromatin: The chromatin (at Interphase various uncoiled thin chromosomes appear as

a mass-like structure called chromatin) or the chromosomes (individuality of various chromosomes start appearing during the cell division due to increase in thickness arising because of coiling of chromatin fibres) are made up of DNA (33-43%), RNA (0.3-10%), histones (30-52%) and nonhistones (4-25%). Some regions of the chromatin are lightly stained called euchromatin whereas other regions appear as darkly stained regions known as heterochromatin.

 

The non-histones show more diversity and their number varies from 12-20 in different organisms and tissues within the same organism. It is expected that they play a role in gene regulation.

 

The histones are highly conserved and they have more structural role than the gene regulatory role (generally they repress the gene activity whenever bound to DNA). There are five types of histones found in eukaryotic chromatin and chromosomes. R.D. Kornberg and J.O. Thomas or 1974) gave the nucleosome model for the association of DNA and histones. According to this model two molecules each of the histones H2A, H2B, H3 and H4 fmm the octamer core and about 200 base pairs (bp) of DNA is found wrapped around this core to form 1 .75 turns. The histone H1 helps in the stabilization of this DNA coiling to form the nucleosome having 11 nm (110A) diameter (Fig. l.l5). This 11 nm fibre is again and again coiled to give rise to 30 nm (300 A) (solenoids) Fig. 1. 1 6) and thicker fibres that are stabilized by certain proteins, finally to give the Metaphasechromosomes having maximum thickness (therefore, chromosome counting for a cell is pe1formed at Metaphase). Thus, in humans the largest chromosome containing about 85,000 µm long DNA is packed into a 0.5 µm thick and 10 µm long Metaphase chromosome (Fig. 1.17).

Fig. 1.15 Assembly of histones and model of nucleosome.

Fig. 1.16 All nm thick fibre (left) and a 30 nm thick solenoid (right).

Fig. 1.17 Packaging of DNA into Metaphase chromosome (10,000 times).

(iv) Chromosomes: These are visible only during cell division and each is made up of two chromatids (half chromosome) . Each species has a characteristic number, size, shape, etc., of the chromosomes that is represented by a diagram called ideogram (Fig. 1 . 18).

 

Fig. 1.18 An ideogram of chromosomes of a male human cell.

At metaphase and anaphase the chromosomes are made up of coiled gene-bearing filament, chromonema, in the matrix that is surrounded by the pellicle (probably made up of nucleolar material :15 it is formed when the nucleolus disappears). The terminal regions of chromosomes are called telomeres (Fig. 1. 19). Strasburger ( 1 87 5) called these thread-like structures as chromosomes chroma means colour) because they have affinity for basic dyes. The diploid organisms have two sets of chromosomes, Haplopappus gracilis has lowest number of diploid chromosomes (2n = 4), whereas some pteridophytes contain more than 1,200 chromosomes in their diploid set.

 

Fig. 1.19 A cell (left) showing chromosomes and a metaphase chromosome (right).

 

The shape of the chromosomes is observed at anaphase where the location of the centromere (primary constriction) determines the shape because chromosome bending occurs only at the location of centromeres.The chromosomes appear rod

Chloroplast

 

Chloroplasts are found only in photosynthetic organisms like eukaryotic algae, Bryophytes, Pteridophytes, Gymnosperms and Angiosperms, in those parts that are exposed to sunlight. Three types of plastids are found in plant cells:

 

1. Leucoplasts that are colourless found in non-green plants,

 

2. Chromoplasts are variously coloured ranging from yellow to red and impart the colour to flowers and fruits.

 

3. Chloroplasts are green plastids that also multiply by the binary fission method. They contain the photosynthetic pigments, like chlorophyll a and b (green in colour), carotenoids (colour ranges from yellow to orange) and xanthophylls (yellowish coloured), that capture the sunlight for the synthesis of organic carbohydrates during photosynthesis.

 

The number of chloroplasts varies from 1 (e.g .. the green alga Ulothrix) to several hundred (e.g., green alga Chara) in a cell in different plants. Generally, each leaf cell contains 20-1.00 chloroplasts.

 

Slulpe: In higher plants the chloroplasts are generally disk shaped, but they may be rounded or club-shaped also. Their shape varies in different algae, e.g., ribbon-like in the green alga Spirogyra, cup-shaped in the green alga Chlorella, reticulate in the green alga Oedogoniwn, etc.

 

Size: The size of the chloroplast varies from 4-10 µm in length and 2- 4 µm in breadth.

 

Structure: Like mitochondria, chloroplasts are also semiautonomous bodies. They are enveloped within two smooth membranes, each being about 7.5 nm thick, that are separated by a space of 10-20 nm width. The outer membrane regulates the transport of various materials between the cytoplasm and the chloroplast, whereas the inner membrane is continuous to thylakoids at many places. The two membranes enclose the semifluid colloidal matrix, called stroma, containing chDNA (chloroplast DNA), RNA, ribosomes, enzymes including RuBisCo (Ribulose Bis-phosphate Carboxylase enzyme) that is responsible for the initial fixation of C02 in Pentose Phosphate Pathway (Fig. 1 .23)

 

Thylakoids: These are membrane bound flattened sac-like structures that run throughout the stroma to form a network. They contain the photosynthetic pigments and the components of electron transport chain. In higher plants at some places 2-100 disc-shaped thylakoids, called granal thylakoids, are closely stacked together to form the grana (like stacks of coins). Each chloroplast generally contains 40-60 grana that are connected by stromal thylakoids. The photosynthetic prokaryotes contain thylakoids for the light reaction of photosynthesis as they lack chloroplasts.

Fig. 1.23 Chloroplasts.

 

Endoplasmic Reticulum

 

The term endoplasmic reticulum was given by K. Porter ( 1953), which means fi ne network.

 

It is the net work of membranous tubules and flattened sacs in the cytoplasm of eukaryotic cells that are continuous with the outer nuclear membrane- of nucleus and the plasma membrane. In some regions. the outer surface of the endoplasmic retic.ulum (ER) is covered with ribosomes to form the rough endoplasmic reticulum (RER) where the ribosomes are engaged in protein synthesis that are delivered into the lumen (cavity) of the RER The other part s of ER that are not covered by any ribosomes are known as smooth endoplasmic reticulum (SER), which synthesizes lipids and sterols and it is continuous with the RER. Some regions of SER are continuous with Golgi apparatus.

 

The RER and SER are involved (Fig. 1.24) in the intracellular transport of materials, like proteins and lipids.

l = Nuclear membrane, 2 = Pore, 3 = Endoplasmic reticulum, 5 = Ribosome, 6 =Primary vesicle, 8 = Golgi complex.

 

 

Fig. 1.24 Endoplasmic reticulum.

 

Golgi Apparatus

 

Golgi apparatus (complex) is found in eukaryotic cells and is made up of stacks, called dictyosomes, of 4-8 fluid-filled parallel-flattened sacs and vesicles bounded by single membrane, called cisternae. These cisternae have continuity with each other and are separated by thin layer of cytoplasm (Fig. 1.25). The apparatus is involved in the processing of products transported either directly through ER or by fusion of the vesicles budded off from the ER with the Golgi apparatus.

 

The vesicles budded off from the ER and Golgi apparatus also give rise to the membranous organelles lysosomes and peroxysomes. The lysosomes (secretary vesicles) ultimately release their contents outside the cell by exocytosis and, thus, they also participate in recycling of the plasma membrane by adding new streches of the membranes during this process. Stretches of plasma membrane pinched off by pinocytosis (liquid substances outside the cell get enclosed within membranous vesicles formed by the invagination of the cell membrane) to form pinosomes or by phagocytosis (formed in a similar manner as pinosomes except for containing solid substances) to form phagosomes fuse with the lysosomes for digestion.

Fusion

Fig. 1.25 Golgi apparatus

 

Ribosomes

 

They are made u p of rRNAs (ribosomal RNAs) and proteins and have about 23 nm average diameter. They have two subunits, one larger and the other smaller and at the time of translation process of protein synthesis, the two subunits bind with the mRNA. The mRNA attached with a number of ribosomes that are involved in the synthesis of the same protein constitutes a polysome (polyribosome).

 

In prokaryotes the ribosomes have 66% rRNAs and 34% proteins and are of 70S (S is the

Swedberg unit indicating the sedimentation coefficient) type with 30S smaller (16S rRNA and about 21 proteins) and 50S larger (23S rRNA, SS rRNA and about 31 proteins) subunits (Fig. 1.26).

 

In eukaryotes, the ribosomes in the cytoplasm contain about 60% rRNA and 40% proteins and are of 80S type with smaller 40S (18S rRNA and about 33 proteins) and larger 60S (28S rRNA, 5.8S rRNA, SS rRNA and about 45 proteins) subunits.

 

These ribosomes are found either freely in the cytoplasm or attached with the endoplasmic reticulum (rough endoplasmic reticulum). In the organelles- mitochondria and chloroplasts (in photosynthetic organisms), the ribosomes are generally of 70S type, as in prokaryotes.

 

Fig. 1.26 Structure of ribosomes.

 

Lysosomes

 

 

Lysosomes are formed by budding off of the vesicles from Golgi apparatus and SER in

eukaryotes and their diameter ranges from 0.2-0.8 µm Their lumen is bounded by a single membrane and contains the hydrolytic enzymes, like nucleases, proteases, lipases, glycosidases, etc. that help – the digestion of the food matedal in the lysosome. Therefore, they are also known as suicide bags their accidental lysis (breakage) may lead to the break down of various cellular constituents. Lysosomes of WBC (white blood cells) help in the defense mechanism. Acrosome of sperms in animals contains lysosomes that help in the penetration of egg during fertilization. Lysosomes are of four types:

 

(i) Primary Lysosomes. They are formed by pinching off of the membranous vesicles from Golgi apparatus and contain hydrolytic enzymes.

 

(ii) Secondary Lysosomes. Fusion of a primary lysosome with a phagosome leads to the formation of a secondary lysosome, where the solid food is acted upon by the hydrolytic enzymes and thus, the digested food diffuses into the cytoplasm.

 

(iii) Residual Bodies. After digestion of the food in secondary lysosome, the vesicle with leftover undigested food is called residual body that passes outward to fuse with the plasma membrane to throw out the waste products outside the cell by exocytosis.

 

(iv) Autophagosomes. They are formed by the fusion of primary lysosomes with degenerated intracellular organelles and help in the digestion of old and useless organelles. They also help in the digestion of stored food to provide energy during starvation.

Fig. 1.27 Different types of lysosomes.

 

Vacuoles

 

The eukaryotic cell contains one or more central vacuoles (sap vacuoles) for the storage of nutrients and wastes, rapid exchange of solutes and gases between the cytoplasm and the adjoining fluid. They are more common and larger to restrict the cytoplasm into a thin layer in plants than in animals where they are fewer and smaller in size. It is covered by a semipermeable membrane, called tonoplast, to enclose the cell sap that contains minerals, sugars, amino acids, waste products, etc.

 

Some protozoa contain contractile vacuoles that help in osmoregulation and excretion. Many prokaryotes (e.g., many cyanobacteria) contain air vacuoles that gives buoyancy and stores metabolic gases.

 

Cytoskeleton

 

In eukaryotic cells the cytoskeleton facilitates changes of cell shape, regulates distribution and orientation of organelles through protoplasmic streaming, produces structures related to cell division like spindles (colchkines disrupts the microtubules of spindles to inhibit the movement of chromosomes during Anaphase and, thus, induces polyploidy) and organelles like centrioles, basal bodies, cilia and flagella and bas role in amoeboidaJ movement. It is made up of three types of filaments consisting of proteins:

 

1. Microtubules. They are elongated , unbranched, cylindrical, hollow tubes of about 25 nm diameter occurring singly or in bundles in the cytoplasm. They also form the skeleton of cilia and flagella in motile organisms and of spindle in the dividing cells. Each microtubule encloses a hollow core of about 15 nm diameter and the wall is made up of 13 longitudinal filaments consisting of helically arranged two types of subunits, α and β of a globular protein tubulin. Fig 1.28 see Appendix.

 

They produce spindles and organelles like centrioles, basal bodies, cilia and flagella and facilitate changes in cell shape, protoplasmic streaming and cell movements along with microfilaments.

 

2. Microfalaments. They are 4-6 nm thin, cylindrical, rod-like filaments forming an extensive network in the cytoplasm and are associated with protoplasmic streaming, cell motion like amoeboid movements, change of cell shape, etc. They are mainly composed of the protein actin that may be associated with some filaments of the protein myosin and other proteins related to the process of contraction of microfilaments. Motility of non-muscle cells involves the interaction of actin and myosin in the microfilaments.

 

3. Intermediate Filaments. They are tough and durable, 8-10 nm thick, rope-like protein fibres (Fig. 1.29). One type of intermediate filament forms a network below the inner nuclear membrane, whereas, other types extends across the cytoplasm and gives mechanical strength to the cell.

Fig. 1.29 Intermediate filaments.

Centrioles

 

Centrioles are generally found in pairs, arranged at right angles to each other near one pole of the nucleus only in (eukaryotes) animal cells and flagellated plant cells (e.g., motile algae, motile sex cells of some primitive plants) (Fig. 1.30). During cell division, the two centrioles migrate to two opposite poles to form spindles. Each centriole is 0.5 µm long and 0.2 µm wide cylindrical, nonmembranous structure open at both the ends. Their wall is made up of 9 groups (tilted at an angle of 40) of triplets of microtubules that are arranged in a circle (9 + 0 arrangement). Each microtubule is made up of the tubulin protein. The triplets of microtubules are connected to a proteinaceous hub in the centre through radial proteinaceous spokes. They are responsible for the formation of spindles at the time of cell division (plant cells lack centrioles and the spindles are formed without their aid) and, cilia (the centrioles divide many times and orient themselves along the surface of the cell to form numerous basal bodies of cilia) and flagella (centrioles move to the periphery of the cell to become basal body of the flagella).

Fig. 1.30 Two pairs of centrioles in a dividing cell (Metaphase).

 

Cilia and Flagella

 

Cilia. In eukaryotes, cilia are 3-10 µm long and 0.5 µm thick, hair-like, numerous outgrowths that extend into the surrounding medium and move in a rowing manner for locomotion of some isolated cells, e.g., some protozoa like Paramoecium. Cilia of tracheal and bronchial cells of respiratory tract move the inhaled dust particles towards the pharynx (outwards) to minimize their entry into the lungs (cigarette smoking may reduce or inhibit the ciliary movements of these cells) (Fig. 1.31).

Flagella. It have the same structure as that of cilia except for having larger length of about 100-200 µm and they are one or few in number in a cell. These are also used for the locomotion of cells by the wave-like motion of the flagella (the organism is driven in the opposite direction to that of the location o f flagellum), e.g., flagellar movement o f spermatozoa propel them within the fluid medium of the female reproductive tract (Fig. 1.32).

Fig. 1.32 Differential distribution of flagella over the cell surface.

Fig. 1.33 Basal body of the eukaryotic flagellum (9 + 0 arrangement of microtubules).

 

The basal bodies (9+0 arrangement) are located at the base of cilia and flagella, and arise from the centrioles (in eukaryotes). Their structure is similar to that of centrioles (Fig. 1.33). The axis of cilia and flagella, called axoneme, is surrounded by a membrane (that is the extension of plasma membrane) and consists of rnicrotubules that are arranged as 9 doublets at the periphery of a circle with 2 single rnicrotubules in the centre (9 + 2 arrangement). The sliding movement of microtubules (that requires energy in the form of ATP) causes the movement of cilia and flagella (see Appendix Fig. 1.34).

 

In prokaryotes, the motile bacteria possess flagella emerging from the cell wall and are not covered within any membrane, as in eukaryotes. Their number varies from one (monotrichous) to two to many where the flagella may be arranged in a cluster at one pole (lophotrichous) or two clusters at two poles (amphitrichous) or all over the surface (peritrichous).

 

The prokaryotic flagellum has three basic parts:

 

(i) Basal Body: It is made up of a small rod with 2 (in G+ bacteria) (Fig. 1.35) or 4 (in Gbacteria) (Fig. 1.36) rings and helps in the attachment of flagellum to the cell wall and plasma membrane.

Fig.l.35 A Gram positive bacterial flagellum

Fig. 1.36 A Gram negative bacterial flagellum.

 

(ii) Hook: It emerges outside the cell wall and is made up of different proteins.

 

(iii) Shaft: A long shaft (filament), extending from the hook is comprising of the globular protein flagellin, that are arranged helically around a hollow core.

 

Pili

 

In certain G- bacteria, pili (fimbriae) are short (0.2-2.0 µm long), thin, straight, hair-like appendages found at poles or all over the surface of cell wall. They originate from the cytoplasm and

Fig. 1.37 Role of sex pilus in the process of conjugation in bacteria.

are not involved with motility of the cells. Their formation is generally governed by plasmids and is made up of a protein pilin. The sex pili help in the conjugation of two bacteria (one having the F factor or p+ and the other lacking it or r) where they join together through their sex pili to form a conjugation tube. The fertility factor (F factor that is a plasmid) passes from p+ to p- bacterium through the conjugation tube (Fig. 1.37) and (Fig. 1.38 see Appendix). In other bacteria, the pili imparting adhesive properties help in the attachment of the bacterium to a substrate or with other cells to form aggregates.

 

Peroxysomes

 

They are spherical, single membrane bound 0.5-1.0 µm diameter structures found in eukaryotic cells and contain the enzymes for peroxide synthesis as well as catalase for metabolism of H202. In animals, they are involved in the metabolism of lipids, whereas in plants they perform photorespiration m association with mitochondria and chloroplast in which the carbohydrates and 02 are used (in photorespiration) in the presence of sunlight to produce CO?

 

Glyoxysomes

 

They are found only in fat-rich parts of plants (e.g., common in germinating oil seeds) and contain enzymes for the µ-oxidation of fatty acids to produce acetyl coenzyme-A that is metabolild in glyoxylate pathway and for try glyceride metabolism to produce carbohydrates.

 

Some important differences between prokaryotic and eukaryotic cells

 

S.No.                                       Prokaryotic Cell                                           Eukaryotic Cell

1.                                 Nuclear Properties

(i) An organized nucleus is absent, i.e.,          (i) A well-organized nudeus with:

(ii) Single, circular, coiled, naked DNA         (ii) Many linear, coiled D NA molecules, called nucleoid, that is not    called chromosomes, are found to be

found to be associated with any histone        associated with histones and nonand

non-histone proteins, lies in the                      histones.

cytoplasm.

 

(iii) Nuclear envelope, nucleoplasm and         (iii)Nuclear membrane, nucleoplasm

nucleolus are absent.                                      nucleolus are present.

(iv) DNA is attached with mesosome that is  (iv) Mesosome is absent.

an invagination of the cell membrane.

 

2.                                 Cytoplasmic Properties

 

(i) Cell organelles like mitochondria,  (i) Cell organelles mitoc hondria,

chloroplast, endoplasmic reticulum, Golgi    chloroplasts, endoplasm, reticulum,

apparatus, central vacuole, etc, are not           Golgi apparatus, centralvacuoles, etc.

found in the cells. In some prokaryotes          are present.

gas vacuoles are found that provide buoyancy and help in floating of the cells in aquatic environment.(ii) Respiratory enzymes and other components are found in the cell membrane.

 

(iii) In photosynthetic prokaryotes, the photosynthetic components are found in the membranous sacs or tubes, called thylakoids.

 

 

(iv) Microtubules and microfilaments are absent.

 

 

 

 

(v) Centrioles are absent.

 

 

 

 

 

 

(vi) Ribosomes are of 70S type.

 

 

 

 

3.  Cytoplasmic Movement

 

Cyclosis is absent.

 

 

 

4.  Cell Wall

 

 

 

 

5. F1agella

 

In motile prokaryotes flagella, if present, are single-stranded consisting of flagellin protein. Here, the flagellum is not enveloped within any membrane.

 

 

 

6. Cell Division

Mitosis and meiosis are absent and the cell divides by simple binary fission method in which the DNA replicates into two that are equally distributed into two daughter cells with the help of mesosome, as the spindles are absent.

 

7. Protein Synthesis

(i) Transcription (DNA to mRNA rRNA or tRNA) and translation (mRNA to protein) occur simultaneously in the cytoplasm.

 

(ii) During transcription, a gene in DNA leads to the formation of mRNA where the number ofnucleotides is exactly same as in the corresponding gene in the DNA.

 

 

 

 

 

 

 

 

 

 

 

8. Sexual Reproduction

True sexual reproduction is lacking, as gametes are not formed in these organisms. Though, some kind of parasexuality through transformation, conjugation or transduction may occur that involves exchange of some genetic material.

 

 

 

(ii) Double membrane bound mitochondria are meant for aerobic respiration.

 

(iii) Double membrane bound chloroplasts with grana! and stromal thylakoids perform the photosynthesis in photosynthetic eukaryotes.

 

(iv) The cytoplasm contains a network of microtubules and microfilaments that are made up of proteins and function as the cytoskeleton.

 

(v) In animal and motile cells a pair of centrioles, arranged right angles to each other and consisting of microtubules, are found at one side of the nucleus that produce spindle fibres during cell division.

 

(vi) Ribosomes are of 80S type in the cytoplasm, whereas, the cell organelles have generally 70S type of ribosomes.

 

Cyclosis, the cytoplasmic streaming, is present in eukaryotic cells that helps in the homogenous distribution of various components and movement of organelles.

 

Cell wall, if present, has different composition in different eukaryotes, e.g. cellulose, hemicellulose, pectin, chitin (in fungi), etc.

 

In motile cells flagella, if present, are 11- stranded (9+2 arrangement) in the shaft portion and 9-stranded (9+0 arrangement) in the basal portion, where each strand is made up of α and β- tubulin proteins. Moreover, the flagellum is enveloped in a membrane.

 

Cell divides by mitosis and for sexual reproduction by meiosis, where the spindle fibres help in the separation of chromosomes into the daughter cells.

 

 

 

 

(i) Here, the transcription occurs in the nucleus, whereas, the translation in the cytoplasm.

 

 

(ii) In eukaryotes many genes in DNA are split genes containing exons and introns, where the transcription leads to the production of RnRNA that is quite longer than the actual mRNA. Thus, the mRNA is produced after the processing of HnRNA where the intervening introns and other unwanted nucleotide sequences from the 5′ and 3′ ends are removed. Moreover, a cap of 5-methyl guonasine at the 5′ end and a tail of poly A at the 3′ end of mRNA is also added.

 

 

Sexually reproducing eukaryotes produce gametes that unite to form the zygote (true sexual reproduction).

 

Important differences between G + and G- bacteria:

S. No.1.

 

 

 

 

 

 

 

 

2.

 

 

 

3.

 

 

 

 

 

 

 

4.

 

 

5.

 

 

 

6.

 

 

7.

 

8.

 

9.

 

                                     c+ BacteriaDuring Gram’s staining* they retain crystal

violet colour after the treatment with alcohol.

 

 

 

 

 

 

Outer membrane in the cell wall is absent.

 

 

 

Peptidoglycan in the cell wall is thicker

(20-80 n m), therefore, susceptibility to

antibiotics (e.g., penicillin) that act upon the peptidoglycan is high.

 

Pentaglycine links the two adjacent tetrapeptides in peptidoglycan.

 

Cell wall has low lipid content due to absent

 

Periplasmic space is absent.

 

 

 

Flagella, if present, contain two rings in the basal body.

 

Cell wall usually contains teichoic acid.

 

These bacteria are mostly spore-fonning.

 

Pili over the cell surface are absent.

 

                     c- BacteriaThey do not retain crystal violet after

alcohol treatment and get decolourised (as the outer membrane containing phospholipids gets degraded and the violet dye passes out of the cells). They are counter

stained with reddish safranin to observe under the microscope.

 

Outer membrane, consisting of phospholipids and proteins, is found above the peptidoglycan.

 

Peptidoglycan is comparatively quite thinner (8-12 nm) and thus, the susceptibility to penicillin-like antibiotics is low Pentaglycine in peptidoglycan is absent.

 

 

 

Cell wall has high lipid because of the presence of outer membrane.

 

A periplasmic space is present between outer membrane and plasma membrane.

 

Flagella, if present, have four rings in basal body.

 

Teichoic acid is absent in the cell wall

 

They are generally non-sporous.

 

Some G- bacteria have pili (made up of protein pilin) over the cell wall that help in conjugation with other cell and attachment with some substratum or with other cells in aggregates

 

 

*Gram’s staining: Different steps of Gram’s staining are:

 

(i)                 A thin smear of the bacterial cells at the slide is made.

(ii)               The smear is stained with crystal violet dye.

(iii)             Now, it is treated with a mordant (Kl), which fixes the crystal violet colour in the cells.

(iv)             The smear is washed with 95% alcohol.

(v)               After the alcohol treatment, which dissolves the lipids present in cell wall, the o+ bacteria retain the violet colour (see Appendix Fig. 1.39), whereas, the a-lose it and become colourless.

(vi)             The decolourised cells of o- bacteria are again stained with reddish coloured safranin dye (see Appendix Fig. 1.40).

 

Bacteria

 

Bacteria are p1-yokaryotic organisms devoid of any organized nucleus and membrane bound organelles like mitochondria, chloroplast, endoplasmic reticulum, central vacuole, etc. Whittaker ( 1 969) has grouped all autotrophic as well as heterotrophic prokaryotic organisms into the group Monera and according to Bergey ‘s Manual of Systematic Bacteriology the main groups of prokaryotic organisms are:

 

1 . Bacteria (Eubacteria): They are unicellular microorganisms and have appeared on the Earth after the evolution of archaebacteria. They may be:

 

(a) Photosynthetic (purple or green coloured), where the main photosynthetic pigments are bacteriochlorophy and they do not evolve any O2 during photosynthesis as the electron

ource are H2S and other compounds (e.g., Rhodospirillum, Chlorobium, etc.),

 

(b) Chemoautotrophic, deriving their energy from chemical reactions (e.g., Nitrsomonas,

Nitrobacter that oxidize NH4+ to NO2- and NO2- to NO3-, respectively). (c) Heterotrophic (e.g., Escherichia coli).

 

2. Archaebacteria: They are more primitive and ancient bacteria generally living in harsher environments, e.g., Methanobacterium (strict anaerobic methanogenic), Halobacterium (extremely halophytic), Thermococcus (S04 2 – metabolizing thermophilic)

 

3. Actinomycetes: They are filamentous bacteria, e.g .. Streptomyces, Frankia, etc.

 

4. Mycoplasma: These bacteria are devoid of a cell wall and are pleuromorphic in shape

(variable shapes) and generally form colonies, e.g .. Mycoplasma.

 

5. Cyanobacteria: They are commonly known as blue-green algae and their structure ranges from unicellular, filamentous (branched or unbranched) to colonial organization. They are

photosynthetic and have photosynthetic pigments chlorophyll a, carotenoids, xanthophylls and blue coloured phycocyanin with or without red phycoerythrin. They evolve O2 during photosynthesis as the source of electron is H2O.

 

Bacteria are unicellular microorganisms and were first discovered by Antony van Leeuwenhoek ( 1 677) in pond water and the tarter scrapped from the teeth. They are universally distributed in water, soil as well as in air in moderate (mesophilic), very cold (psychrophilic, upto -22°C), hot (thermophilic, upto 90°C, e.g., Thermus aquaticus), acidic (acidophilic upto pH l), alkaline (alkalopbilic, upto pH 11 ) or saline (halophilic, e.g., Halobacterium) environments. They may be free-living, (Azotobacter, E. coli, etc.) or symbiotic (e.g., Rhizobium. Bradyrhizobium. etc.), photoautotrophic, (e.g., Rhodospirillum, Chlorobiwn , etc.), chemoautotrophic, (Nitrosomonas, Nitrobacter, Thiobacillus, etc.), heterotrophic saprophytic, (e.g., E.coli, Bacillus, etc.), heterotrophic parasitic, (e.g., Vibrio cholerae, Diplococcus pneumonae, etc.) in nature.

 

Many bacteria can form spores that help in their survival under unfavourable conditions of temperantre, pH, water availability, etc.

According to the requirements and non-requirements of 02, bacteria are also divided into following:

 

(i)                 Aerobic, (e.g., Azotobacter, Pseudomonas, etc.) requiling O2 for respiration,

(ii)               Anaerobic, (e.g., Clostridium) where the 02 becomes toxic and they grow in the absence of O2

(iii)             Facultative, aerobic or anaerobic, that do not require O2 for the growth but can use it whenever available in the environment, (e.g., Shigella, Salmonella, etc.) or microaerophilic requiring low levels of O2 for growth.

 

The bacteria may have different shapes (Fig. 1 .4 1 ):

 

(i)                 Cocci, that are spherical or ovoid cells occurring singly- or in groups (e.g., Staphylococcus) or chains (e.g., Streptococcus), groups of 2 (e.g., Diplococcus pneumoniae), 4 or many.

(ii)               Bacilli, are rod-like cells found singly or in groups, e.g., Bacillus.

(iii)             Spirillium are spirally coiled cells, e.g., Azospirillium.

(iv)             Vibrio are comma-shaped bacteria, e.g., Vibrio cholerae causing cholera.

(v)               Mycelial bacteria are filamentous and aseptate, e.g., Actinomycetes, Streptomyces.

(vi)             Pleuromorphic bacteria can occur in many shapes at different stages, e.g. Rhizobium.

 

 

Fig. 1.41 Different shapes of bacterial cells.

 

The bacteria may or may not have flagella that help in the motility of cells (Fig. 1 .32).

 

The cell wall of most of the bacteria is made up of peptidoglycan and on the basis of Gram’s staining they have been classified into G+ and G bacteria. In Gram’s staining the bacteria are first stained violet with the dye, crystal violet. Then the dye is fixed in the bacterial cells by treatment with s) 0.5% KI solution. When the cells are washed with absolute alcohol, some bacteria having low lipid content in their cell wall retain the crystal violet dye to remain violet and are known as G+ bacteria (e.g., Bacillus subtilis. Lactobacillus, etc); whereas other bacteria are classified as Gbacteria (e.g., Escherichia coli, Rhizobium, etc.) as they have high lipid content in their cell wall due to presence of outer cell membrane and they lose the dye to become colourless and are counter stained with safranin to attain reddish colour.

 

They reproduce asexually by binary fission (Fig. 1 .42) method and true sexual reproduction,

Fig. 1.42 Division of cell by binary fission method in E. coli.

 

by the union of male and female gametes is lacking. Some kind of genetic exchange may takes place by:

 

(i) Conjugation, where the free plasmid or plasmid inserted in bacterial chromosome is transferred from donor to the recipient cell through a conjugation tube (Fig. 1 .37).

 

(ii) Transduction, where during infection by bacterio phages a small segment of the bacterial chromosome may get incorporated into the progeny phage genetic material and transferred to another host bacterial cell during the fresh infection by these progeny phages (see Appendix Fig. 1 .43).

 

(iii) Transformation, in which the free DNA segments present in the external medium may be taken up by the bacteria and get integrated into their chromosome. The first experiment on transformation was conducted by Avery, Me Cleod and Me Carthy (1944) in the bacterium Diplococcus pneumoniae.

 

Though, most of the bacteria are harmful for human beings as they spoil the food (food ining by Clostridium botulinum, Staphylococcus aureus, etc.) and drinking water, cause varies diseases to humans (e.g., Staphylococcus aureus, Salmonella typhi, Vibrio cholerae, Shigella .,enteriae, Mycobacterium tuberculorisis, etc.), animals (e.g., Bacillus anthracis) and plants “‘., Xanthomonas citrii, Agrobacterium tumefaciens, etc.). Many bacteria (e.g., denitrifying bacteria) may lead to loss of fertility of soils by converting fixed NH4 + back into the inert N2 gas that finally escapes into the atmosphere (e.g., Th iobacillus denitrificans). Many bacteria are economically important and have been used successfully in various fields like industries, agriculture, medicines (vaccines, antibiotics, interferons, insulin, growth hormone, etc.), sewage treatment, bio-mining (e.g., Cu, Au, Fe, U, etc. by Thiobaciltus ferroxidans), fuels (e.g., bio gas that mainly contains CH4 gas by the activity of the anaerobic methanogens like Methanobacterium), etc.

 

In industry, they are used in food industry (e.g., dairy products, pickles, vinegar, lactic and citric acids, idli, etc.), anaerobic retting of fibres (like jute, flax, hemp, etc. by the bacteria Clostridium, Pseudomomas fluorescence, etc.), removal of hairs from the hides in leather industry, commercial production of various useful chemicals (like enzymes, amino acids, organic acids, etc.), etc. In agriculture, many bacteria have played important role in reclamation of alkaline and saline soils, composting, nutrient cycling and, thus, maintaining the fertility of soil by making available many nutrients, like NO3– (e.g., Rhizobium, Azotobacter, etc. convert the non-utilizable atmospheric N2

into NH/, Nitromonas, etc, oxidize NH/ into N02-, Nitrobacter, etc. further oxidize the NO2– to

NO3-, etc.), HPO/- (rock phosphate solubilising bacteria like Pseudomonas, Bacillus, etc.), SO4 etc. for the growth of crop plants. Many bacteria are commercially produced as biofertilizer, like

Rhizobium, Azotobacter, Azospirillum, Pseudomonas, etc. that replace the harmful chemical fertilizers to some extent.