Micro Chapter 4

Chapter 4: A Survey of Prokaryotic Cells and Microorganisms

4.1 Basic Characteristics of Cells and Life Forms

Characteristics of Cells and Life

  • Cells

    • Definition: The smallest functional unit of life.

    • Types of Cells:

    • Eukaryotic Cells:

      • Characteristics:

      • Contain membrane-bound organelles.

      • Contain a nucleus.

      • Examples: Fungi, algae, protozoa.

    • Prokaryotic Cells:

      • Characteristics:

      • Lack a nucleus and other membrane-bound organelles.

      • Examples: Bacteria and Archaea.

4.2 Life-Defining Properties

Overview of Life-Defining Properties

  • Reproduction:

    • Importance: Necessary for the continuation of a species' line of evolution.

    • Types:

    • Asexual Reproduction:

      • Example: One cell divides into two new cells through fission or mitosis.

    • Sexual Reproduction:

      • Involves the union of sex cells from two parents.

    • Example: Reproductive spores of Aspergillus fumigatus.

  • Transport:

    • Definition: A system controlling material flow into and out of cells.

    • Importance:

    • Movement of nutrients and water into the cell; removal of waste.

    • Cell membrane acts as a gatekeeper for cellular activities.

    • Example: Water vacuoles in amoebas expelling collected water.

  • Heredity:

    • Definition: The transmission of an organism's genome to the next generation via chromosomes.

    • Importance: Chromosomes carry DNA, the molecular blueprint of life.

  • Metabolism:

    • Definition: The thousands of chemical reactions necessary for cell functioning.

    • Functions:

    • Synthesis of new cell components or energy release for activities.

    • Supported and regulated by enzymes like alpha-amylase, which digests starch and glycogen to produce glucose and maltose.

  • Growth:

    • Definitions:

    • Increase in population size through reproduction.

    • Enlargement of an organism during maturation.

    • Example: Mycobacterium tuberculosis in electron micrographs.

  • Responsiveness:

    • Definition: Capacity to interact with external factors.

    • Illustrations:

    • Cells react to environmental stimuli (light, chemicals, other cells).

    • Communication through signaling; self-propulsion with locomotor structures.

    • Example: Human neutrophil ingesting Methicillin-resistant Staphylococcus aureus to protect the body.

Characteristics of Life

  • All living organisms share the following traits:

    • Heredity: Genome composed of DNA in chromosomes.

    • Reproduction: Ability to produce offspring both sexually or asexually.

    • Growth and Development: Capacity for increase in size and complexity.

    • Metabolism: Chemical and physical life processes are essential for function.

    • Responsiveness: Ability to respond to internal and external stimuli.

    • Transport: Regulation of nutrient and waste flow; maintaining homeostasis.

Application to Microbiology

  • Microbial Traits:

    • Microbes reproduce rapidly.

    • Possess hereditary material (DNA).

    • Exhibit growth and development.

    • Respond to environmental changes (temperature, pH).

    • Require transport mechanisms for nutrient uptake.

  • Viruses:

    • Note that viruses do not meet many of the fundamental properties of life.

4.3 The Cell Envelope: The Outer Boundary Layer of Bacteria

4.4 Bacterial Internal Structure

4.5 Bacterial Shapes, Arrangements, Sizes

Bacterial Structure

  • Bacteria: Historical Presence

    • Existed for approximately 3.5 billion years, demonstrating versatility and adaptability.

  • Essential Structures:

    • Cell Membrane: Encases the cytoplasm, regulates material flow.

    • Cytoplasm: Gel-like substance within the cell.

    • Ribosomes: Sites for protein synthesis.

    • Chromosome(s): Genetic material encoding proteins.

  • Additional Structures:

    • Cell wall, glycocalyx, flagella, pili, fimbriae, capsule or slime layer, S layer, cytoskeleton, endospores.

Structure of a Bacterial Cell

In All Bacteria

  • Cell (Cytoplasmic) Membrane:

    • Description: A thin sheet of lipid and protein surrounding the cytoplasm, controlling material flow.

In Some Bacteria

  • S Layer:

    • Description: Monolayer of protein providing protection and/or attachment.

  • Bacterial Chromosome (Nucleoid):

    • Composition: Condensed DNA coding for all cell proteins.

  • Fimbriae:

    • Description: Fine hair-like structures assisting in adhesion to other cells.

  • Ribosomes:

    • Composition: 60% ribosomal RNA and 40% protein; located throughout the cytoplasm for protein synthesis.

  • Cytoplasm:

    • Definition: Water-based solution filling the cell.

  • Outer Membrane:

    • Description: An extra membrane similar to the cell membrane containing lipopolysaccharides, controlling material flow and toxic when released.

  • Cell Wall:

    • Function: Provides structural support and shape, primarily composed of peptidoglycan.

  • Actin Cytoskeleton:

    • Function: Protein fibers encircling the cell for shape maintenance.

  • Pilus:

    • Function: Appendage for DNA transfer between bacteria.

  • Capsule (Glycocalyx):

    • Function: Protective, adhesive layer composed of external sugars/proteins.

    • Types:

    • Slime Layer: Loosely organized.

    • Capsule: Tightly attached, thicker; often enhances pathogenicity.

  • Inclusion/Granule:

    • Definition: Bodies storing nutrients like fat, phosphate, or glycogen for later use.

  • Endospore:

    • Description: Dormant body formed by some bacteria for survival in adverse conditions.

  • Bacterial Microcompartments:

    • Function: Protein-coated packets that localize enzymes in the cytoplasm.

  • Plasmid:

    • Definition: Small, double-stranded DNA carrying extra genetic traits, such as antibiotic resistance.

  • Intracellular Membrane:

    • Function: Functions as compartmentalization for cellular processes.

  • Flagellum:

    • Description: Specialized appendage for motility, rotates to push the cell forward.

The Cell Envelope

  • Definition: External covering of the cell.

  • Components:

    1. Cell wall

    2. Cell membrane

Cell Walls

  • Function:

    • Determines cell shape and provides structural integrity.

    • Found in cells of algae, fungi, plants, and bacteria.

    • Protects the cell.

  • Composition:

    • Primary component is peptidoglycan, a polymer of cross-linked strands.

Cell Membrane

  • Composition:

    • Phospholipid bilayer with embedded proteins.

  • Properties:

    • Fluid nature allows protein movement and cell growth.

    • Selectively permeable; regulates substance entry and exit.

  • Functions:

    • Sites for energy reactions, nutrient processing, and synthesis.

    • Control flow of materials into and out of the cell.

Cell Membrane Diagrams

  • Includes integral proteins, glycolipids, carbohydrate receptors, transport proteins, actin filaments, and lipoproteins.

External Structures

  • Appendages:

    • Various types with distinct functions including:

    • Flagella: Motility and environmental signal response.

    • Pili: Connection and DNA transfer.

    • Glycocalyx: Adherence and protection functions.

Flagella

  • Purpose:

    • Motility and self-propulsion.

    • Can sense and respond to environmental signals.

    • Variation in number: One or many flagella.

    • Distribution:

    • Single (e.g., Vibrio cholerae)

    • Bundled at one end (e.g., Helicobacter pylori)

    • Spread across the surface (e.g., Proteus mirabilis)

Pili

  • Definition: Flexible tubular structures made of protein (pilin), shorter and thinner than flagella.

  • Functions:

    • Adherence to surfaces.

    • Connecting bacterial cells for DNA transfer.

  • Comparison with Fimbriae:

    • Fimbriae are finer and more hairlike.

Glycocalyx

  • Description: External coating made of sugars/proteins.

  • Functions:

    • Adherence: Helps cells stick to surfaces.

    • Protection: Guards against dehydration and nutrient loss.

  • Types:

    • Slime Layer: Loosely attached.

    • Capsule: Tightly attached, thicker, enhances pathogen capabilities.

Nucleoid

  • Description: Region where bacterial DNA aggregates without a nuclear membrane.

    • Bacterial Chromosome:

    • Singular, circular, double-stranded DNA molecule containing essential genetic information.

Plasmids

  • Definition: Small, circular pieces of DNA additional to the bacterial chromosome.

  • Function: Carry extra genes for advantageous traits, such as antibiotic resistance.

Bacterial Ribosomes

  • Composition:

    • 60% ribosomal RNA and 40% protein.

  • Location: Found throughout the cytoplasm; often in chains or attached to the cell membrane.

  • Function: Sites of protein synthesis, essential for all cells.

Inclusion Bodies

  • Definition: Intracellular storage bodies holding nutrients until needed.

  • Characteristics:

    • Vary in size, number, and content, used when external nutrient sources are depleted.

Cytoplasm

  • Characteristics:

    • Solution inside the cell membrane.

    • Composed of 70-80% water, along with sugars, amino acids, organic molecules, salts.

    • Holds all cell content (chromosomes, ribosomes, etc.).

Cytoskeleton

  • Definition: Network of protein filaments defining cell shape.

  • Importance: Critical for cell division and motility; provides structural support.

Bacterial Shapes

  • Variety: Bacteria can vary in shape, size, and arrangement.

  • Basic Shapes:

    • Coccus (cocci): Spherical.

    • Bacillus (bacilli): Rod-shaped.

    • Coccobacillus: Variant shape.

    • Vibrio: Gently curved.

    • Spirillum and Spirochetes: Spiral shapes with different rigidity.

    • Branching Filaments: Grow in branched configurations.

    • Pleiomorphic Bacteria: Irregular shapes and sizes.

Diplococci

  • Morphology: Cocci appear in pairs called diplococci.

  • Chains: Cocci can also appear in chains (streptococci).

Archaea: The Other Prokaryotes

Overview of Archaea

  • Characteristics:

    • Single-celled microbes evolutionarily distinct from bacteria and eukaryotes; the third domain of life.

    • Considered the most ancient life forms.

    • Capable of surviving in extreme environments (high acidity, temperature, salt, pressure).

    • Examples of habitats: Volcanoes, submarine vents, deep ocean sulphur vents.

Comparison of Three Cellular Domains

  • | Characteristic | Bacteria | Archaea | Eukarya |

  • |------------------------------------|---------------|------------------------------|------------------------------|

  • | Cell Type | Prokaryotic | Prokaryotic | Eukaryotic |

  • | Chromosomes | Single, or few, circular | Single, circular | Several, linear |

  • | Types of Ribosomes | 70S | 70S (similar to 80S) | 80S |

  • | Unique Ribosomal RNA Signature | Yes | Yes | Yes |

  • | Number of RNA Sequences with Eukarya | One | Three | - |

  • | Presence of Peptidoglycan | Yes | No | No |

  • | Cell Membrane Lipids | Fatty acids, ester linkages | Long-chain hydrocarbons, ether linkages | Fatty acids, ester linkages|

  • | Sterols in Membrane | No (some exceptions)| No | Yes |

Review Questions

  1. Draw a coccus bacterial shape.

  2. Compare the appearance of bacteria with capsules versus those without.

    • Does the capsule make bacteria more or less pathogenic?

  3. Illustrate a bacterium featuring one flagellum at one end.

    • What is the purpose of the flagellum?

  4. Sketch a nucleoid in a prokaryotic cell.

    • Is it enclosed within a nucleus?

  5. List one characteristic of life.

  6. Explore whether the cell membrane contains proteins.