Biologists Study Life on Many Scales

Characteristics of Living Organisms

Biologists define life by a set of shared characteristics that distinguish living organisms from non-living matter.

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

• Reproduction: Living organisms can reproduce, passing genetic information to offspring.

• Homeostasis: The ability to maintain a stable internal environment despite external changes.

• Growth: Living things grow and develop according to specific instructions coded in their DNA.

• Response to Stimuli: Organisms can respond to environmental changes.

• Use of Energy: All living things require energy to carry out life processes.

Ecological Scales

Life is organized into hierarchical levels, from the broadest to the most specific, to study interactions and relationships in biology.

• Biosphere: The entire Earth, including all living organisms and their environments.

• Ecosystem: All living (biotic) and non-living (abiotic) components interacting within a given area.

• Community: Interactions among different species living in a given area.

• Population: Interactions among individuals of the same species in a given area.

Organismal Scales

Within an individual organism, biological organization is structured from the simplest to the most complex levels.

• Organism: An individual living entity.

• Organ Systems: Groups of organs that work together to perform complex functions (not present in unicellular organisms).

• Organs: Structures composed of different tissues working together for specific functions.

• Tissues: Groups of similar cells performing a common function.

• Cells: The smallest functional unit of life.

Example: In humans, the digestive system (organ system) includes the stomach (organ), which is made of muscle tissue (tissue), composed of muscle cells (cells).

Cell Types: Prokaryotic vs. Eukaryotic

Cells are classified based on the presence or absence of a nucleus and other membrane-bound organelles.

• Prokaryotic Cells: "Before nucleus"; DNA is free within the cytosol, not enclosed by a membrane. Example: Bacteria.

• Eukaryotic Cells: "True nucleus"; DNA is separated from the cytosol by a nuclear membrane. Examples: Plants, animals, fungi, and protists.

Unicellular vs. Multicellular: Prokaryotes are typically unicellular, while eukaryotes can be unicellular or multicellular.

Chemical Scales

Biological organization extends below the cellular level to molecules and atoms.

• Molecules: Atoms bonded together (e.g., DNA, H2O).

• Atoms: The smallest units of matter (e.g., carbon, hydrogen, oxygen).

Taxonomy and Biological Classification

Modifications in Taxonomy Due to Scientific Advancement

Taxonomy, the science of classifying organisms, has evolved as new scientific discoveries have been made.

• Pre-1866: Classification included only plants and animals.

• 1866: Protists were added as a separate group.

• 1925: Distinction between eukaryotes and prokaryotes was established.

• 1969: The five-kingdom system was proposed (Monera, Protista, Fungi, Plantae, Animalia).

• 1977: Eubacteria and Archaebacteria were recognized as separate groups.

• Modern (3 Domains): Classification based on molecular analysis (especially rRNA) led to the three-domain system: Bacteria, Archaea, and Eukarya.

Example: The three-domain system groups organisms based on genetic and molecular evidence, showing that Archaea are more closely related to Eukarya than to Bacteria.

Additional info: Modern taxonomy uses molecular data, such as ribosomal RNA sequences, to determine evolutionary relationships among organisms.