Overview
This lecture discusses the main characteristics used to define living things, how they differ from non-living objects, and why defining life can be complex and sometimes debated.
Defining Living Things
- Living organisms generally share several core characteristics, but exceptions and debates exist among scientists.
- The number and description of these characteristics can vary and are not universally fixed.
Characteristics of Life
Organization
- Living things are made of cells, the basic unit of life (cell theory).
- Organisms can be unicellular (one cell) or multicellular, with cells organized into tissues, organs, and organ systems.
Homeostasis
- Organisms maintain internal balance (homeostasis) such as temperature and water balance, necessary for biological processes.
- Both multicellular and unicellular organisms regulate homeostasis, but non-living things do not.
Metabolism
- Living things obtain and use energy through chemical reactions, collectively called metabolism.
- Autotrophs capture energy (e.g., plants via photosynthesis); heterotrophs consume food (e.g., animals).
- Cellular respiration breaks down glucose to produce ATP energy in living organisms.
Reproduction
- Living things can reproduce, either asexually (e.g., bacteria splitting) or sexually (combining sperm and egg).
- Non-living things do not reproduce.
Growth and Development
- Organisms grow and develop based on genetic instructions in their DNA.
- Increase in size alone (like a toy expanding in water) does not mean something is alive.
Response to Stimuli
- Living things react to internal and external stimuli (e.g., hunger, danger, plants bending toward light).
- Response may not always be immediately observable.
Evolution
- Populations of living organisms undergo evolution, changing gene frequencies over generations, often leading to adaptations.
Special Cases
- Viruses have some characteristics of life (genetic material, reproduction with a host, evolution) but lack others and are not classified as living by most scientists.
- The definition of life could change with new discoveries or the study of extraterrestrial life.
Key Terms & Definitions
- Cell Theory — The principle that all living things are made of cells.
- Homeostasis — The maintenance of stable internal conditions in an organism.
- Metabolism — All chemical reactions in an organism for energy use and maintenance.
- Autotroph — An organism that produces its own food (usually via photosynthesis).
- Heterotroph — An organism that consumes other organisms for energy.
- Stimulus — A change in the environment that elicits a response.
- Evolution — Change in gene frequencies in a population over generations.
- Virus — Infectious particle with genetic material; not classified as living by most scientists.
Action Items / Next Steps
- Consider examples of objects or organisms and identify which characteristics of life they possess.
- Review class notes on viruses and the debate over their classification as living or non-living.
- Stay curious and question how life might be defined in new discoveries or contexts.