What is Life?

Defining Life and Its Characteristics

Biology seeks to understand what distinguishes living organisms from non-living matter. Life is characterized by a set of properties and processes that all living things share.

• Order: Living organisms exhibit complex but ordered organization, from molecules up to entire organisms.

• Reproduction: All living things have the ability to reproduce, passing genetic information to their offspring.

• Growth and Development: Organisms grow and develop according to specific instructions coded in their DNA.

• Energy Processing: Life requires energy, which organisms obtain and transform to power their activities. Example: Plants convert sunlight into chemical energy via photosynthesis.

• Response to the Environment: Organisms detect and respond to environmental stimuli. Example: Shivering in response to cold to maintain body temperature.

• Regulation: Organisms regulate their internal environment to maintain a stable, constant condition (homeostasis). Example: Human body cooling down at night to conserve energy.

• Evolutionary Adaptation: Populations evolve over generations through adaptations that enhance survival and reproduction.

Additional info: Survival of the fittest refers to the ability of an organism to pass its genes to future generations.

Themes of Biology

Major Unifying Themes

Biology is organized around several major themes that help explain the diversity and unity of life.

Organization

• The biosphere: All life on Earth and the places where life exists.

• Ecosystems: Communities of living organisms and their physical environments.

• Communities: All organisms living in a particular ecosystem.

• Populations: Groups of individuals of the same species living in a specific area.

• Organisms: Individual living things.

• Organs: Body parts composed of different tissues performing specific functions.

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

• Cells: The basic unit of structure and function in living things.

• Organelles: Functional components within cells.

• Molecules: Chemical structures consisting of two or more atoms.

Information

• Genetic Information: DNA encodes the instructions for life. Differences in DNA sequences account for the diversity among organisms.

• DNA Stability: DNA is stable and not directly readable; it is transcribed into messenger RNA (mRNA) for protein synthesis.

• Protein Synthesis: The sequence of amino acids in proteins determines their function and interactions.

• Gene Regulation: Genes can be turned on or off, affecting when and how proteins are made. Mistakes in this process (mistranslation) can have significant effects, such as increased disease risk.

• Example: Humans and pigs have only about 5% difference in their DNA sequences.

Energy and Matter

• Conservation of Energy: Energy is not created or destroyed, only transformed from one form to another.

• Energy Use: Organisms cannot use all forms of energy (e.g., radiation) and can only store a limited amount (e.g., in chemical bonds).

• Example: An atomic bomb releases stored energy in atoms.

• Additional info: There is no net loss or gain of energy, but there can be a loss of usable (thermal) energy.

Interactions

• Biological Interactions: Organisms interact with each other and their environment, affecting survival and reproduction.

• Evolutionary Interactions: Traits that enhance survival are selected for over generations (natural and sexual selection).

• Example: Animals may select mates based on traits that signal fitness, such as bright coloration.

Evolution

• Definition: Evolution is the process by which populations change over time through genetic variation and natural selection.

• Example: Environmental factors can activate certain genes, leading to changes in an organism's traits (e.g., a pig in the wild developing boar-like features).

Classifying Life

Domains of Life

All living organisms are classified into three domains based on cellular organization and genetic differences.

Additional info: Prokaryotic cells lack a nucleus and membrane-bound organelles, while eukaryotic cells have both.

Unity in the Diversity of Life

Common Features and Diversity

Despite the vast diversity of life, all organisms share certain fundamental characteristics, such as the use of DNA as genetic material and similar cellular structures.

• Cilia Example: The cilia of Paramecium are used for movement, while in humans, cilia help keep bacteria out of the respiratory tract.

• Correlation vs. Causation: Just because two things occur together does not mean one causes the other.