Introduction to Cells PDF

Introduction to cells: Discovered by Robert Hook in the year 1665, cells are the structural and functional unit of life. Some cells are bound by membrane organelles while some are not. Depending on the internal structure of the cells and whether they are bound by a membrane or not, cells in organis...

Introduction to cells: Discovered by Robert Hook in the year 1665, cells are the structural and functional unit of life. Some cells are bound by membrane organelles while some are not. Depending on the internal structure of the cells and whether they are bound by a membrane or not, cells in organisms are of two types- Prokaryotic and Eukaryotic.  **Prokaryotic Cells** Known to be the very earliest in the world, prokaryotic cells are single-celled microorganisms, and include archaea and bacteria. These cells usually live freely by themselves or can be found in the gut of other organisms. The cells have a single membrane and consist of cytoplasm. Certain prokaryotic cells perform photosynthesis with the help of the cyanobacteria inside them.  **Common Characteristics of Prokaryotic Cells** Various Characteristic Features of Prokaryotic Cells are as follows: - There is no presence of a nuclear membrane. - Prokaryotic cells do not consist of mitochondria, Golgi bodies, chloroplast, or lysosomes.  - Their cell walls are composed of amino acids and carbohydrates.  - Prokaryotic cells do not have histone proteins. - A single chromosome consists of the genetic material.  - The genetic material is present on a single chromosome. - Prokaryotic cells use the method of binary fission to divide asexually or recombine as a sexual method of reproduction such as conjugation. - In Prokaryotic cells, the plasma membrane acts as the mitochondrial membrane carrying respiratory enzymes. **Main Components of Prokaryotic Cells** Prokaryotic Cells are Made up of the Following Components: - Cytoplasm- Present inside the cell and resembling a jelly-like structure, all cell organelles are suspended inside the cytoplasm. The cytoplasm also contains salts and enzymes.  - Plasma Membrane- This is the outer protective covering that consists of phospholipid molecules that act as a barrier between the surrounding [environment](https://www.vedantu.com/biology/environment) and the cell.  - DNA- This is a cell's genetic material. Circular in form, the DNA directs the proteins created by a cell and also regulates its actions.  - Ribosomes- This is the place where the protein synthesis of a cell occurs.  **Additional Structure of Prokaryotic Cells** While prokaryotic cells do not consist of nuclear membranes, mitochondria, or Golgi bodies, there are several other components that add to their structure. They are:  - Nucleoid- Spherical, spiral or shaped like a rod, the nucleoid is the genetic material inside the cytoplasm of a cell.  - Cell Wall- The outermost layer of prokaryotic cells, this lends shape and structure. - Capsule- In addition to the cell wall, a capsule protects bacterial cells, retains moisture, and helps cells attach themselves to various surfaces or nutrients.  - Flagella- Certain prokaryotic cells have long tail-like structures that help them go from one surface to another. These are called flagella.  - Pili- These are outgrowths of a cell that bear resemblance to fine, tiny hair and attach to the surface of other prokaryotic cells.  493,100+ Prokaryotic Cell Stock Photos, Pictures & Royalty \... - Difference Between Prokaryotic and Eukaryotic Cells Prokaryotic Cells Eukaryotic Cells ---------------------------------------------------------------------- --------------------------------------------------- Absence of nucleus Presence of nucleus Presence of a single chromosome, however, the chromosome is not true Presence of multiple chromosomes Unicellular in nature Multicellular in nature No microtubules Presence of microtubules Absence of mitochondria Presence of mitochondria Smaller ribosomes Bigger ribosomes No cytoskeleton Presence of cytoskeleton No Golgi apparatus Golgi apparatus present  Asexual reproduction Sexual and asexual reproduction Often has fimbriae and pili Does not have fimbriae and pili The cytoplasm where transcription takes place The nucleus is where transcription takes place Size is sub-microscopic Size is microscopic, though enclosed in membranes No presence of chloroplasts Chloroplasts are present in plants DNA arrangement is circular DNA arrangement is linear Cell division occurs through binary fission Cell division occurs through mitosis Examples include bacteria and archaea Examples include plant and animal cells - The unit of measurement used in bacteriology is the micron (micrometer) which is one-thousandth of a millimeter. - Bacteria are, in general one-tenth the size of the eukaryotic cell. On average, the size of bacteria ranges from 0.5 to 5 µm. - However, they can be as tiny as 0.3 µm and as large as 0.7mm. - The limit of resolution with the unaided eye is about 200 microns, and as many bacteria are smaller than this size, they are not visible with naked eyes. - Among the largest bacteria is *Thiomargarita namibiensis*, which is up to half a millimeter long and Epulopiscium fishelsoni which is 0.75 mm long. - The smallest bacteria are members of genus *Mycoplasma* which are only 0.3 µm, as small as the largest viruses*.* - The bacteria that are oval or spherical in shape are included called cocci bacteria. - These may either remain single or attached to one another in groups. They appear flattened when placed in groups. - It is assumed that coccoid forms were derived from rod-shaped organisms through evolutionary time. - These are rod-shaped cells that also like cocci, remain either single or attached to other cells. - Bacilli bacteria are among the first bacteria to have arisen, and this shape is said to be not as advantageous as other shapes. This has been assumed upon the observation of the behavior of filamentous * E. coli *cells which, though motile and chemotactic, move slowly and cannot tumble to change direction. - This group includes bacteria that are either helical-shaped or curved (comma-shaped). - The bacteria can range from slightly curved to corkscrew-like spiral. - Cocci bacteria can be arranged either singly, in pairs, in groups of four, in chains, in clusters or cubes consisting of eight cells. - These cells remain attached during cell division. - This arrangement results when two bacterial cells occur as a pair (joined together). - Some of the cells in this arrangement might remain spherical while some might appear flattened, elongated, or bean-shaped. - Examples: *Streptococcus pneumonia, Moraxella catarrhalis, Enterococcus spp, Neisseria gonorrhea.* - Tetrad bacteria are arranged in a group of four cells that remain attached and grow in the attachment after cell division. - This arrangement results when the cells divide into two planes. - Examples: A*erococcus, Pediococcus, and Tetragenococcus.* - In this arrangement, the bacterial cells form a group of eight cells. - This happens when the cells divide in a perpendicular plane. - The common characteristic associated with these organisms is being strict anaerobe. - Examples: *Sarcina aurantiaca, Sarcina lutea, Sarcina ventriculi*. - Here, the bacteria are arranged in long chains. - These bacteria are present in family Streptococcaceae, which is characterized by a lack of motility and Gram-positive bacteria. - Examples: *Streptococcus pyogenes, Streptococcus pneumonia, Streptococcus mutans*. - This type includes bacteria that are arranged in grape-like clusters. - This results from cell division in both the planes and are characterized by organisms which are immotile and Gram-positive. - Examples: *Staphylococcus epidermidis, Staphylococcus haemolyticus, Staphylococcus aureus, Staphylococcus capitis.* - Bacilli are the bacteria which are rod-shaped and are present as single cells. - These bacteria can form endospores and are facultative anaerobes. - Examples: *Salmonella enterica subsp, Bacillus cereus, and Salmonella choleraesuis.* - As in Diplococci, Diplobacilli also exists in pairs. - After cell division, the two cells do not divide and grow in an attached arrangement. - Examples: *Coxiella burnetii, Klebsiella rhinoscleromatis, Moraxella bovis.* - In this group, bacteria are arranged in chains. - This results from cell division in a single chain. - Examples: *Streptobacillus moniliformis, Streptobacillus Levaditi, Streptobacillus felis, Streptobacillus hongkongensis.* - As the name suggests, coccobacilli resemble both cocci as well as bacilli. - These are shorter in size and thus, appear stumpy. - Examples: *Chlamydia trachomatis, Haemophilus influenza, Gardnerella vaginalis.* - Pallisades are the type of bacilli bacteria that resemble a picket fence structure as a result of the bent at the point of division during cell division. - They appear similar to Chinese letters. - Example: *Corynebacterium diphtheria that causes diphtheria.* - These are the slightly curved bacteria resembling a comma shape. - Examples: *Vibrio mytili, Vibrio anguillarum, Vibrio parahaemolyticus, Vibrio cholera.* - Spirochetes are spiral bacteria having a helical shape. - These are flexible and have an axial filament which helps in motility. These filaments are essential distinguishing character between spirochetes and other bacteria. - These filaments run throughout the length of the bacteria and thus, help in twisting the motion of the bacteria. - Examples: *Leptospiraspecies (Leptospira interrogans), Treponema pallidum, Borrelia recurrentis.* - These bacteria are similar in structure with spirochetes but are more rigid. - They, too, have a flagellum but lack the endoflagella like in spirochetes. - Examples: *Campylobacter jejuni, Helicobacter pylori*, *Spirillum winogradskyi.* - **Crenarchaeota:**- The crenarchaeal is the kind of archaea that are found in a broad range of habitats. They can bear extreme heat and high temperatures due to the fact that they contain a special kind of protein that helps them to function at an absolute high temperature such as 230 degrees celsius. They are also found deep in the sea vents and also in hot springs. These consist of thermophiles, hyperthermophiles, and thermoacidophiles. - **Euryarchaeota:**- Unlike any living being on earth, they have the ability to produce methane and can survive under heavy alkaline conditions. This comprises methanogens and halophiles. - **Korarchaeota:**- They are believed to be the oldest living organisms on earth. They possess the genes that are common to Crenarchaeota and Euryarchaeota, hence all three are said to be descended from a common ancestor. They comprise hyperthermophiles. - **Thaumarchaeota**:- These are the typical kind of archaea that oxidize ammonia. - **Nanoarchaeota**:- It is the obligate symbiont of archaea and it belongs to the genus gonococcus. 1. Atrichous: There is no flagellum.\ Example: Lactobacillus  2. Monotrichous:\ Single polar flagellum can rotate both clocks and anti-clockwise resulting in forwarding movement and backward movement respectively.\ Example: Vibrio cholerae 2.  Amphitrichous:\ One flagellum is present on each end. Movements are like monotrichous flagella.\ Example: Alkaligens faecalis 2. Lophotrichous:\ Tufts of flagella present at one or both sides. Propagates clockwise and anticlockwise.\ Example: Spirillum 2. Peritrichous:\ Numerous flagella are present all over the bacterial body, anticlockwise rotation produces one-directional movement.\ Example: Salmonella Typhi 2. Cephalotrichous: several flagella are present at both ends. Movements are similar to monotrichous flagella.\ Example: Pseudomonas - Basal body - Hook - Filament - Flagella facilitates movement and locomotion in organisms. - Flagella can help detect changes in pH and temperature - They help eukaryotes to enhance their reproductive rates, they are present in the uterus of human females. - They help in the identification of certain types of organisms.  +-----------------------------------+-----------------------------------+ | | +--------------+--------------+ | | | | b)  | Specific | | | | | | motility | | | | | | membrane | | | | | | proteins | | | | +==============+==============+ | | | |   | Transmembran | | | | | | e | | | | | | proteins are | | | | | | attached to | | | | | | the host | | | | | | surface. | | | | | | This | | | | | | adhesion | | | | | | complex can | | | | | | either be | | | | | | specific to | | | | | | a certain | | | | | | type of | | | | | | surface like | | | | | | a certain | | | | | | cell type or | | | | | | generic for | | | | | | any solid | | | | | | surface. | | | | | | Motor | | | | | | proteins | | | | | | attached to | | | | | | an inner | | | | | | membrane | | | | | | force the | | | | | | movement of | | | | | | the internal | | | | | | cell | | | | | | structures | | | | | | in relation | | | | | | to the | | | | | | transmembran | | | | | | e | | | | | | proteins | | | | | | creating net | | | | | | movement. Th | | | | | | is | | | | | | is driven by | | | | | | the proton | | | | | | motive | | | | | | force. The | | | | | | proteins | | | | | | involved | | | | | | differ | | | | | | between | | | | | | species. An | | | | | | example of a | | | | | | bacterium | | | | | | that uses | | | | | | this | | | | | | mechanism | | | | | | would | | | | | | be *Flavobac | | | | | | terium*. | | | | | | | | | | | | c)  Poly | | | | | | saccharide j | | | | | | et | | | | | | ----- ---- | | | | | | ------------ | | | | | | ------------ | | | | | | ------------ | | | | | | ------------ | | | | | | ------------ | | | | | | ------------ | | | | | | ------------ | | | | | | ------------ | | | | | | ------------ | | | | | | ------------ | | | | | | ---------- | | | | | |   The | | | | | | cell release | | | | | | s a \'jet\' | | | | | | of polysacch | | | | | | aride materi | | | | | | al behind it | | | | | | propelling | | | | | | it forward. | | | | | | This polysac | | | | | | charide mate | | | | | | rial is left | | | | | | behind. | | | | +--------------+--------------+ | +-----------------------------------+-----------------------------------+ - Cell wall is an important structure of a bacteria. It give shape,rigidity and support to the cell. - On the basis of cell wall composition, bacteria are classified into two major group ie. Gram Positive and gram negative. 1. Peptidoglycan 2. Lipid 3. Teichoic acid 1. Peptidoglycan 2. Outermembrane: - Lipid - Protein - Lipopolysaccharide (LPS) - Peptidoglycan is porous cross linked polymer which is responsible for strength of cell wall. - Peptidoglycan is composed of three components. 1. Glycan backbone 2. Tetra-peptide side chain ( chain of 4 amino acids)  linked to NAM 3. Peptide cross linkage - Glycan backbone is the repeated unit of N-acetyl muramic acid (NAM) and N-acetyl glycosamine (NAG) linked by β-glycosidic bond. - The glycan backbone are cross linked by tetra-peptide linkage. The tetra-peptide are only found in NAM. - More than 100 peptidoglycan are known with the diversity focused on the chemistry of peptide cross linkage and interbridge. - Although the peptidoglycan chemistry vary from organism to organism the glycan backbone ie NAG-NAM is same in all species of bacteria. - L-alanine: 1^st^ position in both gm+ve and gm-ve bacteria - D-glutamic acid: 2^nd^ position - D-aminopimelic acid/ L-lysine: 3^rd^ position (variation occurs) - D-alanine: 4^th^ position - In gram negative bacteria, peptide cross linkage occur between  Diaminopamilic acid (3rd position) of one glycan back bone and D-alanine of adjacent  glycan back bone. - In gram positive bacteria, peptide cross linkage occur by peptide interbridge. The type and number of aminoacids in interbridge vary among bacterial species. - Teichoic acid is water soluble polymer of glycerol or ribitol phosphate. - It is present in gram positive bacteria. - It constitutes about 50% of dry weight of cell wall. - It is the major surface antigen of gram positive bacteria. - 3\. Outer membrane - It is an additional layer present in gram negative bacteria. - It is composed of lipid bilayer, protein and lipopolysaccharide (LPS) layer - Structure component of gram-ve cell wall - LPS is an endotoxin produced by gram --ve bacteria - Lipid-A is antigenic - LPS is attached to outer membrane by hydrophobic bond. LPS is synthesized in cytoplasmic membrane and transported to outer membrane. - LPS is composed of lipid-A and polysaccharide. - Lipid-A: it is phosphorylated glucosamine disaccharide. - Polysaccharide: it consists of core-polysaccharide and O-polysaccharide. - Gram positive bacteria - *Micrococcus* - *Staphylococcus* - Gram negative bacteria - *E. coli* - *Salmonella* - It is the semi-fluid structure within the plasma membrane where the cell's important parts float. Although prokaryotes lack membrane-bound organelles. - In prokaryotes, the cytoplasm has small structures associated with the plasma membrane called ribosomes. Here, proteins are made from messenger RNA. Prokaryotes have the 70S ribosome which is made of 30S (smaller) and 50S (larger) subunits. Ribosomes generally occur in groups called polysomes. **Inclusion Bodies** - They exist freely in the cytoplasm without a membrane. They act as storage for reserve material in prokaryotic cells. Special gas storing vacuoles are seen in cyanobacteria and other photosynthetic bacteria. **Nucleoid** Nucleoid is the genetic material in prokaryotes. It does not have a nuclear membrane. Many bacteria contain small circular DNA called plasmid. **Mesosomes:** 1. Prokaryotes have a specialized differentiated form of the cell membrane called the Mesosome. 2. It aids in breathing. 3. Mesosomes are organelles formed by plasma membrane infoldings. 4. They are found in the membranes of bacterial cells. 5. They\'re invaginated structures made up of vesicles and tubules with lamellar whorls. 6. Flat vesicles form the lamellae, which are connected to the cell membrane. 1. They play a role in the formation of cell walls. 2. They are involved in the replication of DNA in prokaryotes. 3. They increase plasma membrane surface area and enzymatic content. 4. They aid in the transfer of chromosomes to daughter cells. 5. Mesosomes are analogous to cristae in the mitochondria in eukaryotic cells. 6. Mesosome is a convoluted membranous structure formed in a prokaryotic cell by the invagination of the plasma membrane. Its functions are as follows: 7. These extensions help in the synthesis of the cell membrane, replication of DNA, and protein synthesis. They also help in the equal distribution of chromosomes into the daughter cells during cytokinesis. 8. It also increases the surface area of the plasma membrane to carry out various enzymatic activities. 9. It helps in secretion processes as well as in bacterial respiration. **Plasmids:** Plasmids are small circular DNA fragments, double-stranded, self-replicating extra chromosomal structures found in many microorganisms.  The term Plasmid was coined by Joshua Lederberg in 1952. Plasmids are important as genetic tools, which are used to introduce, manipulate or delete certain genes from the host cell. Properties of Plasmids: - They are extra chromosomal DNA fragments present in the cell. - They are double stranded structures. Exceptions are the linear plasmids in bacteria *Streptomyces spp *and *Borrelia spp.* - They can replicate independently. - The absence of a plasmid in the cell does not affect cell functioning, but the presence of a plasmid in the cell is usually beneficial.  - Plasmids are also known as sex factors, conjugants, extra chromosomal replicons, or transfer factors.  - Copy number -- the copy number refers to the number of copies of plasmid present in the bacterial cell. Usually, small plasmids are present in high numbers and large plasmids are present in few numbers.  - Compatibility of plasmids -- this refers to the ability of two different plasmids to coexist in the same bacterial cell.  **Structure of Plasmids:** **1. Every plasmid has certain essential elements. These are as follows --** - Origin of replication (OR) -- This refers to a specific location in the strand where the replication process begins. In plasmids, this region is A=T rich region as it is easier to separate the strands during replication.  - Selectable marker site -- This region consists of Antibiotic resistance genes which are useful in the identification and selection of bacteria that contain plasmids.  - Promoter region -- this is the region where the transcriptional machinery is loaded. - Primer binding site -- this is the short sequence of single-strand DNA which is useful in DNA amplification and DNA sequencing. - Multiple cloning sites -- This site contains various sequences where the restriction enzymes can bind and cleave the double stranded structure. 3\. It is the extrachromosomal element of the cell which is not required for the growth and development of the cell.  4\. Most of the plasmids contain the TRA gene, which is the transferred gene and is essential in transferring the plasmid from one cell to another.  **Cytoskeleton:** The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including bacteria and archaea. It extends from the cell nucleus to the cell membrane and is composed of similar proteins in the various organisms. **Prokaryotic Cytoskeleton:** **FtsZ:** FtsZ was the first protein of the prokaryotic cytoskeleton to be identified. Like tubulin, FtsZ forms filaments in the presence of guanosine triphosphate (GTP), but these filaments do not group into tubules. During cell division, FtsZ is the first protein to move to the division site, and is essential for recruiting other proteins that synthesize the new cell wall between the dividing cells. **MreB and ParM:** Prokaryotic actin-like proteins, such as MreB, are involved in the maintenance of cell shape. All non-spherical bacteria have genes encoding actin-like proteins, and these proteins form a helical network beneath the cell membrane that guides the proteins involved in cell wall biosynthesis. Some plasmids encode a separate system that involves an actin-like protein ParM. Filaments of ParM exhibit dynamic instability, and may partition plasmid DNA into the dividing daughter cells by a mechanism analogous to that used by microtubules during eukaryotic mitosis. **Crescentin:** The bacterium Caulobacter crescentus contains a third protein, crescentin, that is related to the intermediate filaments of eukaryotic cells. Crescentin is also involved in maintaining cell shape, such as helical and vibrioid forms of bacteria, but the mechanism by which it does this is currently unclear. Additionally, curvature could be described by the displacement of crescentic filaments, after the disruption of peptidoglycan synthesis. (for prokaryotic Cytoskeleton)