Defining Life: Core Characteristics and Cellular Basis

Posted on Apr 11, 2025 in Biology

Core Characteristics of Living Things

All living things share fundamental characteristics:

  • Life Cycle: They have a life cycle that includes birth (origin) and death.
  • Chemical Composition: They share a similar chemical composition, primarily based on organic matter.
  • Cellular Structure: They are composed of one or more basic units called cells. Cells are the smallest microscopic units considered to be alive.
  • Life Functions: They perform essential life functions:

Nutrition

Nutrition is the set of processes by which living things obtain the energy and substances (primarily organic matter) they need to live. Based on their nutrition, living things are classified into two types:

Autotrophs

Autotrophs are capable of producing the organic substances they need from simple inorganic substances obtained from the environment (e.g., water, minerals from soil, carbon dioxide from the atmosphere). They typically use energy, such as solar energy via photosynthesis, to do this.

  • Examples: Plants, algae, and some bacteria.

Heterotrophs

Heterotrophs obtain the organic matter they need by consuming other living beings or their remains.

  • Examples: Fungi, animals (herbivores, carnivores, omnivores), and most other microorganisms.

Interaction (Relationship)

Interaction (or relationship) refers to the set of processes by which living things perceive stimuli and respond to or interact with their environment.

Reproduction

Reproduction is the set of processes by which living things create new individuals, perpetuating their species. There are two main types:

  • Asexual Reproduction: Involves only one parent individual.
  • Sexual Reproduction: Involves the combination of genetic material from two parent individuals.

Cell Theory

The fundamental principles of Cell Theory state that:

  • All living things are composed of one or more cells.
  • The cell is the smallest fundamental unit of structure and function in all living organisms.
  • All cells arise from pre-existing cells.

Organization of Living Things

Life exhibits a hierarchical organization:

  • Bioelements (e.g., Carbon, Hydrogen, Oxygen, Nitrogen – CHON) combine to form…
  • Biomolecules:
    • Inorganic: Water, mineral salts.
    • Organic: Carbohydrates (e.g., glucose), Lipids (e.g., cholesterol), Proteins (e.g., insulin), Nucleic Acids (e.g., DNA).
  • Biomolecules assemble into…
  • Cell Organelles, which form…
  • Cells: The basic unit of life. There are two main types: Prokaryotic and Eukaryotic.
  • In many Eukaryotes: Cells organize into…
  • Tissues (e.g., blood tissue), which form…
  • Organs (e.g., heart), which work together in…
  • Organ Systems (e.g., circulatory system), making up a complex…
  • Multicellular Organism.

Organisms, whether single-celled (like many prokaryotes) or multicellular (like eukaryotes with tissues, organs, systems), adapt to their environment.

Cellular Organization Types

All cells possess a cell membrane, cytoplasm, and genetic material. They are classified based on the organization of their genetic material:

Prokaryotic Cells

  • Examples: Bacteria.
  • Genetic material is not enclosed within a nucleus.
  • Structurally simpler.
  • Generally smaller than eukaryotic cells.

Eukaryotic Cells

  • Examples: Animal cells, plant cells, fungi, protists.
  • Genetic material is contained within a membrane-bound nucleus.
  • Structurally more complex, containing various organelles.
  • Generally larger than prokaryotic cells.

Differences Between Animal and Plant Cells

Both are eukaryotic, but have key differences:

  • Shape: Animal cells often have a variable shape; Plant cells typically have a fixed, often polyhedral shape due to the cell wall.
  • Cell Wall: Absent in animal cells; Present in plant cells (provides structural support).
  • Chloroplasts: Absent in animal cells; Present in plant cells (site of photosynthesis).
  • Vacuoles: Animal cells may have small, temporary vacuoles; Plant cells typically have a large, central vacuole (maintains turgor pressure, stores substances).

The Discovery of the Cell

The first simple microscopes were developed around 500 years ago. In the 17th century, the English scientist Robert Hooke used an improved microscope to observe thin slices of cork (plant tissue). He observed small, box-like compartments which he named ‘cells‘ because they reminded him of the small rooms (cellae) in a monastery or a honeycomb. By the 18th century, further observations led scientists to understand that all living things are composed of cells. These collective findings formed the basis of the Cell Theory.

Taxonomy: Classifying Life

Taxonomy is the biological science responsible for identifying, naming, grouping, and classifying living things. It establishes a hierarchical system based on shared characteristics, organizing organisms into ranks or categories called taxa (singular: taxon). Each level includes groups from the level below it. The broadest major taxonomic rank is the Kingdom.

An example of the main taxonomic ranks, from broad to specific (using a domestic cat as an example):

  • Kingdom: Animalia
  • Phylum: Chordata (includes vertebrates)
  • Class: Mammalia
  • Order: Carnivora
  • Family: Felidae
  • Genus: Felis
  • Species: Felis catus

The Five Kingdoms (Traditional System)

A common, though evolving, classification system groups organisms into five kingdoms:

  • Kingdom Monera: Consists of single-celled prokaryotic organisms (bacteria and archaea). They can be autotrophic or heterotrophic. (Note: Modern systems often split this into Kingdom Bacteria and Kingdom Archaea).
  • (The original text only listed Monera)