Lesson 3: Cell Structure and Taxonomy

Cells

• the fundamental units of life

• exhibit life characteristics: metabolism, growth, reproduction, response to stimuli, mutation

• can mutate, adapting to environmental changes

Prokaryotic Cells

• Simple cells, lacking a true nucleus and membrane-bound organelles

• E.g., bacteria and archaea

Eukaryotic Cells

• Complex cells, with a true nucleus (DNA enclosed by a nuclear membrane) and membrane-bound organelles

• E.g., algae, protozoa, fungi, plants, animals, and humans

• typically 10x larger than prokaryotic cells

Viruses

• Depends on host cells for reproduction

• Acellular (not composed of cells)

• Believed to be the result of regressive evolution

• Separate category due to acellular nature

• Understanding cell structure and metabolism helps in:

  • Identifying cells and microorganisms

  • Explaining how antimicrobial drugs selectively target pathogens without harming human cells

Cytology

• Study of cell structure and function

• Developed through advances in electron microscopy and biochemical research

Eukaryotic Cell | Cell Membrane

• Regulates entry and exit of substances due to selective permeability

• Composed of proteins and phospholipids

• Functions as a “skin” separating cell contents from the external environment

Eukaryotic Cell | Nucleus

• Functions as the command center of the cell

• Eukaryotic cells have a true nucleus, absent in prokaryotic cells

• Contains 3 main components:

  • Nucleoplasm

  • Chromosomes

  • Nuclear Membrane

Eukaryotic Cell | Cytoplasm

• A semifluid matrix where organelles and various substances are suspended

• Most of the cell’s metabolic reactions occur here

• all the organelles (except the nucleus), structures and fluid within the cell

• allow transport, maintain cell shape and structure, protection, storage and acts as the host to metabolic processes.

Eukaryotic Cell | Endoplasmic Reticulum (ER)

 network of membranes for transport within the cell

Eukaryotic Cell | Smooth ER

lacks ribosomes and is associated with lipid synthesis

Eukaryotic Cell | Rough ER

has ribosomes attached, involved in protein synthesis

Eukaryotic Cell | Ribosomes

• Involved in protein synthesis

• Consists of two subunits: 60S (large) and 40S (small), which combine to form an 80S ribosome

• made of rRNA and proteins

Eukaryotic Cell | Golgi Complex

• Known as the cell’s packaging plant

• Processes and packages proteins made in the ER

• Proteins are modified, packaged into vesicles, and sent to their destinations or secreted outside the cell

Eukaryotic Cell | Lysosomes

Contain digestive enzymes to break down foreign material, damaged cell parts, or the entire cell (in a process called autolysis

Components of Nucleus | Nucleoplasm

nucleus component that is gel-like material inside the nucleus

Components of Nucleus | Chromosomes

• a component of nucleus that is made of linear DNA proteins, containing genes

• holds genes that encode proteins or RNA

Components of Nucleus | Nuclear Membrane

• a component of nucleus which surrounds the nucleus and has pores for the passage of molecules

• serves as a skin around the nucleus which contains holes through which large molecules can enter and exit the nucleus

Nucleoulus

found inside the nucleus responsible for producing ribosomal RNA which forms ribosomes

Cytosol

fluid portion of the cytoplasm

Eukaryotic Cell | Peroxisomes

Involved in breaking down hydrogen peroxide into water and oxygen using the enzyme catalase

Eukaryotic Cell | Mitochondria

• Generates energy in the form of ATP through cellular respiration

• Known as the powerhouse of the cell

• Believed to have originated from bacteria that evolved to live inside eukaryotic cells

• about 0.5 to 1 µm (diameter) and 7 µm (length)

Eukaryotic Cell | Plastids

• Contain pigments for photosynthesis

• Found in plant cells and algae

Eukaryotic Cell | Cytoskeleton

• system of fibers

• Composed of microtubules, microfilaments, and intermediate filaments

• Functions:

  • Provide structural support (gives the cell its shape)

  • Plays a role in cell division, movement, and chromosome transport

Eukaryotic Cell | Cell Wall

• Found in plant cells, algae, fungi, and most bacteria providing rigidity, shape, and protection

• Made of different materials depending on the organism: cellulose (plants and algae) and chitin (fungi, exoskeleton of beetles and crabs)

Eukaryotic Cell | Cilia

• Shorter, more numerous than flagella, beating rhythmically to propel cells or move substances

• E.g., in the respiratory tract

• involved in locomotion and have a 9+2 arrangement of microtubules

Eukaryotic Cell | Flagella

• Long, whip-like structures that help certain cells (like sperm or protozoa) move

• have a 9+2 arrangement of microtubules

• involved in locomotion

Chloroplasts

 type of plastid, contains chlorophyll (green pigment) for converting light energy into chemical energy (stored in carbohydrates)

Prokaryotic Cell | Cytoplasm

• Contains a chromosome, ribosomes, and cytoplasmic particles

• Lacks internal membranes

• surrounded by a cell membrane, a cell wall (usually), and sometimes a capsule or slime later (makes up the bacterial cell envelope)

Prokaryotic Cell | Cell Envelope

Comprises a cell membrane of, cell wall (usually), and sometimes a capsule/slime layer

Prokaryotic Cell | Cell Membrane

• Encloses cytoplasm; selectively permeable and consists of proteins and phospholipids

• Site for metabolic reactions; may have mesosomes for cellular respiration

• Frequently observed in TEM of bacteria

Prokaryotic Cell | Chromosome

• Single, supercoiled circular DNA molecule; no nucleoplasm or nuclear membrane

• control center of the bacterial cell

• ONLY ONE FOR BACTERIA

• DNA location: bacterial nucleoid, length is 500-750 times the cell length

• Contains 450-8000 genes; plasmids may also be present

Prokaryotic Cell | Cytoplasm Composition

Semiliquid; includes water, enzymes, nutrients, and metabolic materials

Prokaryotic Cell | Ribosomes

• Sites of protein synthesis

• Smaller than eukaryotic ribosomes (70S)

  • 30S subunit and 50S subunit

• Approximately 15,000 ribosomes E. coli

Prokaryotic Cell | Bacterial Cell Wall

• Rigid, complex structure made primarily of peptidoglycan (aka murein)

• Variations:

  • Gram-positive (thick peptidoglycan)

    • Stain is purple

  • Gram-negative (thin peptidoglycan + outer membrane)

    • Stain is reddish to pinkish

Prokaryotic Cell | Pili and Fimbriae

• Hair-like structures aiding in attachment and genetic material transfer (sex pili)

• Import for bacterial pathogenicity

• Adhere or attaches to the surface

• Organelles of attachment

Prokaryotic Cell | Flagella

• Threadlike appendages for motility; structure differs from eukaryotic flagella

• Classification based on arrangement:

  • monotrichous

  • ampitrichous

  • lohotrichous

  • peritrichous

Prokaryotic Cell | Glycocalyx

• Extracellular layer; consists of slime layers (unorganized) and capsules (organized)

• Capsules protect against phagocytosis and aid in disease virulence

Prokaryotic Cell | Spores (endospores)

• Thick-walled spores formed under stress (nutrient depletion)

• Resistant to extreme conditions; germination leads to new vegetative cells

• Bacterial survival

Five-Kingdom System (Whittaker)

• Procaryotae (bacteria and archaea), Protista, Fungi, Plantae, Animalia

• Viruses are not included as they are acellular

Three-Domain System (Woese)

• Divided life into Archaea, Bacteria, and Eukarya based on rRNA differences

• Gaining popularity in microbiology

Genome

complete collection of genes

Gene

• beads on a string

• particular segment of the DNA molecule

• contains genetic information that enables cell to produce one or more gene products

• products are: protein and ribonucleic acid (RNA)

S

• refers to Svedberg units b

Sedimentation Coefficients

• expresses the rate at which a particle or molecules moves in centrifugal filed

• determined by the size and shape of the particle or molecule

Phagocytosis

engulfing of large particles my amebas and certain types of WBC (called phagocytes)

Autolysis

act of destroying the entire cell if the cell is damaged or deteriorating

Catalase

a type of enzyme that catalyzes (speeds up) the breakdown of hydrogen peroxide into water and oxygen

Cellular Respiration

• energy is released from glucose molecules and other nutrients to drive other cellular functions

• light energy is converted to chemical energy

Binary Fission

• simple division of one cell into two cells, after DNA replication and the formation of a separating membrane and cell wall

• bacteria’s way of production

Mesosomes

• inward foldings of cell membrane

• where cellular respiration takes place in bacteria

• some things it is nothing more than artifacts created during the processing of bacterial cells for electron microscopy

Bacterial Nucleoid

DNA-occupied space within a bacterial cell

Plasmids

• extrachromosomal DNA

• small, circular molecules of double-stranded DNA that are not part of chromosome

Peptidogylcan

• cell wall of bacteria

• murein

• complex macromolecular polymer

• only found in bacteria

• consists of many polysaccharide chains linked together by small peptide (protein) chains

Mycoplasm

bacteria that does not contain a cell wall

Glyococalyx | Slime Layers

• type of glycocalyx that is unorganized

• easily detaches from the cell wall and drifts away

• e.g., Pseudomonas (causes Pseudomonas species)

• enable certain bacteria to glide or slide along solid surfaces, and seem to protect bacteria from antibiotics and desiccation

Glycocalyx | Capsule

• organized

• may be combined with lipids and proteins (depending on the bacterial species)

• serves as an antiphagocytic function (protecting the encapsulated bacteria from being phagocytized by phagocytic WBC)

Vegetative Cells

active, metabolizing, growing bacterial cells

Taxonomy

• science of classification of living organisms

• has 3 areas:

  • classification

  • nomenclature

  • identification

Taxonomy | Classification

arrangement of organisms into taxonomic groups on the basis of similarities or relationships

Taxonomy | Nomeclature

assignment of names to the various taxa according to international rules

Taxonomy | Identification

process of determining whether an isolate belongs to one of established, named taxa or represents a previously unidentified species

Ultrastructure

• detailed structure beyond the resolving power of a compound light microscope

  • knowledge gained though electron microscopy

Flagella | Peritrichous Bacteria

a type of bacteria that has a flagella all over their surface

Flagella | Lophotrichous Bacteria

a type of bacteria that has a tuft of flagella on its one end

Flagella | Amphitrichous Bacteria

a type of bacteria that has one or more flagella at each end

Flagella | Monotrichous Bacteria

a type of bacteria that has a flagella on its single polar only

true nucleus

DNA enclosed by a nuclear membrane