Characteristics of Living Things

Defining Features of Life

All living organisms share a set of fundamental characteristics that distinguish them from non-living matter. These features are essential for the maintenance, survival, and evolution of life.

• Cellular Organization: All living things are composed of one or more cells, which are the basic units of life.

• Growth and Development: Organisms increase in size and complexity over time.

• Reproduction: Ability to produce new individuals, either sexually or asexually.

• Metabolism: The sum of all chemical reactions that provide energy and build cellular components.

• Response to Environment (Irritability): Ability to sense and respond to stimuli.

• Homeostasis: Regulation of internal conditions to maintain a stable environment.

• Adaptation/Evolution: Capacity to evolve and adapt to changing environments over generations.

Viruses

• Not considered alive by most definitions because:

  • No cellular structure (acellular).

  • Cannot reproduce independently; require host cell machinery.

  • Do not metabolize energy.

• Borderline: Viruses do evolve and have genetic material (DNA or RNA).

Bacterial Cell Walls

Structure and Function

The bacterial cell wall provides structural support, maintains cell shape, and protects against osmotic lysis. The composition of the cell wall is a key factor in bacterial classification and antibiotic susceptibility.

• Function: Maintains shape, protects against osmotic pressure.

Gram-Positive Bacteria

• Thick peptidoglycan layer (many layers).

• Teichoic acids present (give negative charge, help with ion transport).

• Stains purple with Gram stain.

• More resistant to drying and physical disruption.

Gram-Negative Bacteria

• Thin peptidoglycan layer.

• Outer membrane present (contains lipopolysaccharide, LPS).

• Periplasmic space between membranes.

• Stains pink/red with Gram stain.

• More resistant to antibiotics (outer membrane barrier).

Glycocalyx

Capsule and Slime Layer

The glycocalyx is an external layer found in many bacteria, providing protection and aiding in attachment to surfaces.

• Capsule:

  • Organized, firmly attached to cell wall.

  • Protects against phagocytosis (e.g., Streptococcus pneumoniae).

  • Acts as a virulence factor.

• Slime Layer:

  • Loosely attached, unorganized.

  • Helps with attachment to surfaces and protects from dehydration.

Lipid A & LPS

Structure and Biological Role

LPS (Lipopolysaccharide): A major component of the outer membrane of Gram-negative bacteria, contributing to structural integrity and protection.

• Components:

  • Lipid A: Toxic portion, embedded in the membrane.

  • Core polysaccharide: Provides structural support.

  • O antigen: Variable, used in serotyping (e.g., E. coli O157:H7).

• Lipid A:

  • Released when bacteria die or divide.

  • Acts as an endotoxin.

  • Causes fever, inflammation, vasodilation, shock, and can lead to endotoxic shock.

Bacterial Flagellum

Structure and Function

The flagellum is a whip-like appendage that enables motility in many bacteria, allowing them to move toward nutrients or away from harmful substances (chemotaxis).

• Structure:

  • Filament: Long, whip-like, made of flagellin protein.

  • Hook: Connects filament to basal body.

  • Basal body: Anchors flagellum into cell wall/membrane; powered by proton motive force.

• Function: Motility (chemotaxis).

Fimbriae and Pili

Surface Appendages

Fimbriae and pili are hair-like structures on the surface of many bacteria, involved in attachment and genetic exchange.

• Fimbriae:

  • Short, numerous, hairlike projections.

  • Function: Attachment to surfaces, biofilm formation.

• Pili:

  • Longer, fewer in number.

  • Function: DNA transfer between bacteria (conjugation).

Endosymbiotic Theory

Origin of Eukaryotic Organelles

The endosymbiotic theory proposes that mitochondria and chloroplasts in eukaryotic cells originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Evidence:

  • Own DNA (circular, like bacteria).

  • 70S ribosomes (similar to prokaryotes).

  • Double membranes.

  • Divide by binary fission.

Biofilms

Microbial Communities

Biofilms are structured communities of microorganisms attached to a surface and encased in a self-produced matrix.

• Matrix composed of polysaccharides, proteins, and DNA.

• Commonly found on teeth (plaque), medical devices, catheters, and water pipes.

• Protect microbes from antibiotics and the immune system.

• Cells within biofilms communicate via quorum sensing.