cell structure and taxonomy

• The cell is the fundamental unit of any living organism
• There are two categories of cells: eukaryotic and prokaryotic
• Some microbes are prokaryotes (bacteria and archaea), some are eukaryotes (algae, protozoa, fungi), and some are not composed of cells (viruses, prions, viroids)
• Eukaryotic cells contain a “true” nucleus, whereas prokaryotic cells do not
• Eukaryotic cells possess a complex system of membranes and membrane-bound organelles, whereas prokaryotic cells do not
• Both eukaryotic and prokaryotic cells possess a cell membrane with selective permeability
• Eukaryotic Nucleus:

  • "Command center" of the cell
  • Components: nucleoplasm, chromosomes, and a nuclear membrane
  • Chromosomes are embedded in the nucleoplasm
  • Eukaryotic chromosomes consist of linear DNA molecules and proteins
  • Genes are located along chromosomes, forming the genotype or genome
  • Genes can code for proteins or two types of ribonucleic acid (rRNA and tRNA)
  • Human diploid cells have 46 chromosomes (23 pairs)
  • Estimated human genome consists of 20,000 to 25,000 genes
• • Cell wall: provides shape, protection, and rigidity
  • Flagella and cilia: organelles of locomotion with microtubules
• Other Eukaryotic Cell Structures:

  • Cytoplasm: semifluid, gelatinous matrix with storage granules and organelles
  • Endoplasmic reticulum (ER): transport network with rough ER (ribosomes) and smooth ER
  • Ribosomes: sites of protein synthesis
  • Golgi complex: completes protein transformation and packaging
  • Lysosomes and peroxisomes: contain enzymes for digestion and H2O2 breakdown
  • Mitochondria: produce ATP molecules
  • Plastids: membrane-bound structures for photosynthesis, including chloroplasts
  • Cytoskeleton: system of fibers essential for cell activities
• Prokaryotic Cell Structure:

  • Prokaryotic cells are about 10 times smaller than eukaryotic cells
  • Reproduce by binary fission
  • Cytoplasm is not filled with internal membranes
  • Surrounded by a cell membrane, cell wall, and sometimes a capsule or slime layer
  • Cell membrane is selectively permeable with attached enzymes for metabolic reactions
  • Chromosome is a single, long, circular DNA molecule
  • Cytoplasm consists of water, enzymes, nutrients, and waste products
  • Ribosomes are sites of protein synthesis
• • Bacterial cell wall is chemically complex, with peptidoglycan as the main constituent
• Main constituent of most bacterial cell walls is peptidoglycan (found only in bacteria)
• Gram-positive bacteria have a thick layer of peptidoglycan; Gram-negative bacteria have a much thinner layer
• Mycoplasma spp. do not have a cell wall; they are pleomorphic
• Some bacteria possess glycocalyx, a slimy, gelatinous material produced by the cell membrane and secreted outside the cell wall
• There are two types of glycocalyx: slime layer (loosely connected to the cell wall) and capsule (highly organized and firmly connected)
• Pseudomonas spp. produce a slime layer
• Klebsiella pneumoniae, Neisseria meningitidis, and Streptococcus pneumoniae possess a capsule, which serves an antiphagocytic function
• Motile bacteria possess flagella, whip-like appendages composed of threads of protein called flagellin
• Number and arrangement of flagella are characteristic of a particular species
• Four basic types of flagellar arrangement on bacteria:

  • Peritrichous bacteria: flagella over entire surface
  • Lophotrichous bacteria: tuft of flagella at one end
  • Amphitrichous bacteria: one or more flagella at both ends
  • Monotrichous bacteria: single polar flagellum
• Pili (also called fimbriae) are hairlike structures, most often observed on Gram-negative bacteria
• Pili are composed of polymerized protein molecules called pilin
• Pili enable bacteria to anchor themselves to surfaces
• Some bacteria possess a sex pilus for conjugation
• A few genera (e.g., Bacillus and Clostridium) are capable of forming thick-walled spores as a means of survival
• Spores have been shown to survive for many years and are resistant to heat, cold, drying, and most chemicals
• Endospores can be visualized using a spore stain
• Eukaryotic cells contain a true nucleus, prokaryotic cells do not
• Eukaryotic cells contain membranous structures and many membrane-bound organelles; prokaryotic cells possess no membranes other than the cell membrane that encloses the cytoplasm
• Prokaryotic cells reproduce by binary fission, where one cell splits in half to become two daughter cells
• Before a prokaryotic cell divides in half, the chromosome must be duplicated
• The time it takes for binary fission to occur is called the generation time
• Generation time varies from one species to another and depends on growth conditions
• Taxonomy is the science of classification of living organisms
• Taxonomy consists of classification, nomenclature, and identification
• Classification is the arrangement of organisms into taxonomic groups (known as taxa)
• The Five-Kingdom System of Classification:

  1. Bacteria and archaea – Kingdom Prokaryotae

  2. Algae and protozoa – Kingdom Protista

  3. Fungi – Kingdom Fungi

  4. Plants – Kingdom Plantae

  5. Animals – Kingdom Animalia
• The Three-Domain System of Classification:

  1. Archaea (prokaryotic)

  2. Bacteria (prokaryotic)

  3. Eucarya (all eukaryotic organisms)
• The most widely used technique for gauging diversity or “relatedness” of organisms is rRNA sequencing