8.2 Cell Structure Answer Key Pdf

[Kusi Bertoldo]

Eachcell contains a fluid called the cytoplasm, which is enclosed by a membrane. Also present in the cytoplasm are several biomolecules like proteins, nucleic acids and lipids. Moreover, cellular structures called cell organelles are suspended in the cytoplasm.

Discovery of cells is one of the remarkable advancements in the field of science. It helps us know that all the organisms are made up of cells, and these cells help in carrying out various life processes. The structure and functions of cells helped us to understand life in a better way.

Ideas about cell structure have changed considerably over the years. Early biologists saw cells as simple membranous sacs containing fluid and a few floating particles. Today's biologists know that cells are infinitely more complex than this.

There are many different types, sizes, and shapes of cells in the body. For descriptive purposes, the concept of a "generalized cell" is introduced. It includes features from all cell types. A cell consists of three parts: the cell membrane, the nucleus, and, between the two, the cytoplasm. Within the cytoplasm lie intricate arrangements of fine fibers and hundreds or even thousands of miniscule but distinct structures called organelles.

Every cell in the body is enclosed by a cell (Plasma) membrane. The cell membrane separates the material outside the cell, extracellular, from the material inside the cell, intracellular. It maintains the integrity of a cell and controls passage of materials into and out of the cell. All materials within a cell must have access to the cell membrane (the cell's boundary) for the needed exchange.

The cell membrane is a double layer of phospholipid molecules. Proteins in the cell membrane provide structural support, form channels for passage of materials, act as receptor sites, function as carrier molecules, and provide identification markers.

The nucleus, formed by a nuclear membrane around a fluid nucleoplasm, is the control center of the cell. Threads of chromatin in the nucleus contain deoxyribonucleic acid (DNA), the genetic material of the cell. The nucleolus is a dense region of ribonucleic acid (RNA) in the nucleus and is the site of ribosome formation. The nucleus determines how the cell will function, as well as the basic structure of that cell.

The cytoplasm is the gel-like fluid inside the cell. It is the medium for chemical reaction. It provides a platform upon which other organelles can operate within the cell. All of the functions for cell expansion, growth and replication are carried out in the cytoplasm of a cell. Within the cytoplasm, materials move by diffusion, a physical process that can work only for short distances.

Cytoplasmic organelles are "little organs" that are suspended in the cytoplasm of the cell. Each type of organelle has a definite structure and a specific role in the function of the cell. Examples of cytoplasmic organelles are mitochondrion, ribosomes, endoplasmic reticulum, golgi apparatus, and lysosomes.

Bacteria have characteristic shapes (cocci, rods, spirals, etc.) and often occurin characteristic aggregates (pairs, chains, tetrads, clusters, etc.). Thesetraits are usually typical for a genus and are diagnostically useful.

Prokaryotes have a nucleoid (nuclear body) rather than an enveloped nucleus andlack membrane-bound cytoplasmic organelles. The plasma membrane in prokaryotesperforms many of the functions carried out by membranous organelles ineukaryotes. Multiplication is by binary fission.

Flagella: The flagella of motile bacteria differ in structure fromeukaryotic flagella. A basal body anchored in the plasma membrane and cell wallgives rise to a cylindrical protein filament. The flagellum moves by whirlingabout its long axis. The number and arrangement of flagella on the cell arediagnostically useful.

Cell Wall Peptidoglycans: Both Gram-positive and Gram-negativebacteria possess cell wall peptidoglycans, which confer the characteristic cellshape and provide the cell with mechanical protection. Peptidoglycans are uniqueto prokaryotic organisms and consist of a glycan backbone of muramic acid andglucosamine (both N-acetylated), and peptide chains highly cross-linked withbridges in Gram-positive bacteria (e.g., Staphylococcus aureus)or partially cross-linked in Gram-negative bacteria (e.g., Escherichiacoli). The cross-linking transpeptidase enzymes are some of thetargets for b-lactam antibiotics.

Teichoic Acids: Teichoic acids are polyol phosphate polymers bearinga strong negative charge. They are covalently linked to the peptidoglycan insome Gram-positive bacteria. They are strongly antigenic, but are generallyabsent in Gram-negative bacteria.

Lipoteichoic Acids: Lipoteichoic acids as membrane teichoic acidsare polymers of amphiphitic glycophosphates with the lipophilic glycolipid andanchored in the cytoplasmic membrane. They are antigenic, cytotoxic and adhesins(e.g., Streptococcus pyogenes).

Lipopolysaccharides: One of the major components of the outermembrane of Gram-negative bacteria is lipopolysaccharide (endotoxin), a complexmolecule consisting of a lipid A anchor, a polysaccharide core, and chains ofcarbohydrates. Sugars in the polysaccharide chains confer serologicspecificity.

Wall-Less Forms: Two groups of bacteria devoid of cell wallpeptidoglycans are the Mycoplasma species, which possess asurface membrane structure, and the L-forms that arise from either Gram-positiveor Gram-negative bacterial cells that have lost their ability to produce thepeptidoglycan structures.

Plasma Membrane: The bacterial plasma membrane is composed primarilyof protein and phospholipid (about 3:1). It performs many functions, includingtransport, biosynthesis, and energy transduction.

Organelles: The bacterial cytoplasm is densely packed with 70Sribosomes. Other granules represent metabolic reserves (e.g.,poly-β-hydroxybutyrate, polysaccharide, polymetaphosphate, andmetachromatic granules).

Endospores:Bacillus and Clostridium species can produceendospores: heat-resistant, dehydrated resting cells that are formedintracellularly and contain a genome and all essential metabolic machinery. Theendospore is encased in a complex protective spore coat.

Bacteria have characteristic shapes. The common microscopic morphologies are cocci(round or ellipsoidal cells, such as Staphylococcus aureus orStreptococcus, respectively); rods, such asBacillus and Clostridium species; long,filamentous branched cells, such as Actinomyces species; andcomma-shaped and spiral cells, such as Vibrio cholerae andTreponema pallidum, respectively. The arrangement of cells isalso typical of various species or groups of bacteria (Fig. 2-1). Some rods or cocci characteristically grow inchains; some, such as Staphylococcus aureus, form grapelikeclusters of spherical cells; some round cocci form cubic packets. Bacterial cells ofother species grow separately. The microscopic appearance is therefore valuable inclassification and diagnosis. The higher resolving power of the electron microscopenot only magnifies the typical shape of a bacterial cell but also clearly resolvesits prokaryotic organization (Fig. 2-2).

Prokaryotic and eukaryotic cells were initially distinguished on the basis ofstructure: the prokaryotic nucleoidthe equivalent of the eukaryotic nucleusisstructurally simpler than the true eukaryotic nucleus, which has a complex mitoticapparatus and surrounding nuclear membrane. As the electron micrograph in Fig. 2-2 shows, the bacterial nucleoid, whichcontains the DNA fibrils, lacks a limiting membrane. Under the light microscope, thenucleoid of the bacterial cell can be visualized with the aid of Feulgen staining,which stains DNA. Gentle lysis can be used to isolate the nucleoid of most bacterialcells. The DNA is then seen to be a single, continuous, "giant" circular moleculewith a molecular weight of approximately 3 X 109 (see Ch. 5). The unfolded nuclear DNA would beabout 1 mm long (compared with an average length of 1 to 2 m forbacterial cells). The bacterial nucleoid, then, is a structure containing a singlechromosome. The number of copies of this chromosome in a cell depends on the stageof the cell cycle (chromosome replication, cell enlargement, chromosome segregation,etc). Although the mechanism of segregation of the two sister chromosomes followingreplication is not fully understood, all of the models proposed require that thechromosome be permanently attached to the cell membrane throughout the variousstages of the cell cycle.

Bacterial chromatin does not contain basic histone proteins, but low-molecular-weightpolyamines and magnesium ions may fulfill a function similar to that of eukaryotichistones. Despite the differences between prokaryotic and eukaryotic DNA,prokaryotic DNA from cells infected with bacteriophage ?, whenvisualized by electron microscopy, has a beaded, condensed appearance not unlikethat of eukaryotic chromatin.

Two types of surface appendage can be recognized on certain bacterial species: theflagella, which are organs of locomotion, and pili (Latin hairs), which are alsoknown as fimbriae (Latin fringes). Flagella occur on both Gram-positive andGram-negative bacteria, and their presence can be useful in identification. Forexample, they are found on many species of bacilli but rarely on cocci. In contrast,pili occur almost exclusively on Gram-negative bacteria and are found on only a fewGram-positive organisms (e.g., Corynebacterium renale).

Structurally, bacterial flagella are long (3 to 12 m), filamentoussurface appendages about 12 to 30 nm in diameter. The protein subunits of aflagellum are assembled to form a cylindrical structure with a hollow core. Aflagellum consists of three parts: (1) the long filament, which lies external tothe cell surface; (2) the hook structure at the end of the filament; and (3) thebasal body, to which the hook is anchored and which imparts motion to theflagellum. The basal body traverses the outer wall and membrane structures. Itconsists of a rod and one or two pairs of discs. The thrust that propels thebacterial cell is provided by counterclockwise rotation of the basal body, whichcauses the helically twisted filament to whirl. The movement of the basal bodyis driven by a proton motive force rather than by ATP directly. The ability ofbacteria to swim by means of the propeller-like action of the flagella providesthem with the mechanical means to perform chemotaxis (movement in response toattractant and repellent substances in the environment). Response to chemicalstimuli involves a sophisticated sensory system of receptors that are located inthe cell surface and/or periplasm and that transmit information tomethyl-accepting chemotaxis proteins that control the flagellar motor. Geneticstudies have revealed the existence of mutants with altered biochemical pathwaysfor flagellar motility and chemotaxis.

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