1  What is Life?

1.1 Learning Objectives

By the end of this chapter, you should be able to:

  1. List and explain the seven characteristics common to all known living organisms
  2. Distinguish between living, non-living, and borderline cases using multiple criteria
  3. Explain why defining life is both scientifically important and conceptually challenging
  4. Describe how different historical perspectives have shaped our understanding of life
  5. Analyze how the “energy and information” framework provides a unifying perspective on life
  6. Apply scientific reasoning to evaluate whether unfamiliar entities should be considered alive

1.2 Introduction

What distinguishes a living organism from non-living matter? This question, fundamental to biology, has engaged scientists and philosophers for centuries. While we often recognize life intuitively—we know a dog is alive while a rock is not—creating a precise, scientific definition proves surprisingly difficult. This chapter examines the characteristics shared by all living things, explores borderline cases that challenge simple definitions, and introduces the conceptual framework that will guide our study of biology.

1.3 The Challenge of Definition

Defining life serves several important purposes in biology:

  1. Scientific Communication: Clear definitions enable precise discussion and research
  2. Origin of Life Studies: Understanding what life is helps identify how it might have arisen
  3. Astrobiology: Definitions guide the search for extraterrestrial life
  4. Synthetic Biology: Criteria help evaluate artificially created systems
  5. Medical Ethics: Definitions inform decisions about life support and abortion

Despite its importance, no definition of life has achieved universal acceptance. This difficulty arises because: - Life exhibits tremendous diversity - Some entities exist in gray areas between living and non-living - Definitions that include all known life often exclude potential artificial or extraterrestrial life - Life is a process, not a static state

1.3.1 Historical Perspectives on Life

1.3.1.1 Ancient and Medieval Views

  • Vitalism: The belief that living organisms contain a “vital force” (élan vital) distinct from physical matter
  • Spontaneous Generation: The idea that life regularly arises from non-living matter (e.g., maggots from rotting meat)
  • Scala Naturae: Aristotle’s “Great Chain of Being” organized all things hierarchically from minerals to God

1.3.1.2 Scientific Revolution (17th-18th Centuries)

  • Mechanistic View: Descartes proposed animals as complex machines
  • Experimental Refutation: Redi (1668) and later Pasteur (1864) disproved spontaneous generation
  • Cell Theory: Schwann and Schleiden (1839) proposed all organisms are composed of cells

1.3.1.3 Modern Synthesis (20th Century-Present)

  • Molecular Biology: Life understood through chemistry of nucleic acids and proteins
  • Evolutionary Biology: Life defined by capacity for Darwinian evolution
  • Systems Biology: Life emerges from complex interactions of components

1.4 Characteristics of Living Systems

While no single characteristic defines life, biologists recognize seven properties shared by all known living organisms. These characteristics, taken together, distinguish living from non-living matter.

1.4.1 Cellular Organization

All living things are composed of one or more cells—the basic structural and functional units of life.

Cell Theory:

  1. All living organisms are composed of one or more cells
  2. The cell is the basic unit of structure and function in organisms
  3. All cells arise from pre-existing cells through cell division

Examples:

  • Unicellular organisms: Bacteria, archaea, many protists
  • Multicellular organisms: Animals, plants, fungi

1.4.2 Metabolism

Living systems transform energy and matter through chemical reactions.

Two Types of Metabolic Processes:

  • Catabolism: Breaking down complex molecules into simpler ones, releasing energy Example: Cellular respiration breaks glucose into carbon dioxide and water
  • Anabolism: Building complex molecules from simpler ones, requiring energy Example: Protein synthesis assembles amino acids into proteins

Energy Sources:

  • Phototrophs: Capture energy from sunlight (plants, algae, some bacteria)
  • Chemotrophs: Obtain energy from chemical compounds (animals, fungi, many bacteria)

1.4.3 Homeostasis

Living organisms maintain a stable internal environment despite external changes.

Examples of Homeostatic Regulation:

  • Temperature regulation: Mammals maintain constant body temperature
  • pH balance: Blood pH remains near 7.4 despite dietary changes
  • Water balance: Kidneys regulate water concentration in body fluids

1.4.4 Growth and Development

Organisms increase in size and undergo programmed changes throughout their life cycle.

Types of Growth:

  • Cell enlargement: Individual cells increase in size
  • Cell division: Production of new cells
  • Cell differentiation: Cells become specialized for specific functions

Development Patterns:

  • Direct development: Young resemble adults (birds, mammals)
  • Indirect development: Larval stages differ from adults (butterflies, frogs)

1.4.5 Reproduction

Organisms produce new individuals, transmitting genetic information to offspring.

Asexual Reproduction:

  • Single parent produces genetically identical offspring
  • Advantages: Rapid, no mate needed
  • Disadvantages: Little genetic variation
  • Examples: Binary fission in bacteria, budding in yeast

Sexual Reproduction:

  • Two parents contribute genetic material
  • Advantages: Genetic variation, evolutionary adaptability
  • Disadvantages: Requires mate, energy-intensive
  • Examples: Fertilization in animals, pollination in plants

1.4.6 Response to Stimuli

Organisms detect and respond to changes in their environment.

Types of Responses:

  • Taxis: Movement toward or away from a stimulus Example: Phototaxis (movement toward light) in photosynthetic bacteria
  • Tropism: Growth response in plants Example: Gravitropism (roots grow downward, stems upward)
  • Reflexes: Rapid, automatic responses Example: Withdrawal from painful stimuli

1.4.7 Evolutionary Adaptation

Populations of organisms change over generations through natural selection.

Key Components:

  • Variation: Individuals differ in their traits
  • Inheritance: Traits are passed from parents to offspring
  • Differential survival/reproduction: Some traits increase reproductive success
  • Time: Changes accumulate over many generations

Example: Antibiotic resistance in bacteria develops through evolutionary adaptation.

1.5 Borderline Cases and Definition Challenges

Several entities challenge simple definitions of life because they possess some but not all characteristics of living systems.

1.5.1 Viruses

Characteristics suggesting life:

  • Contain genetic material (DNA or RNA)
  • Evolve through natural selection
  • Reproduce (with host cell assistance)
  • Have complex, organized structure

Characteristics suggesting non-life:

  • No independent metabolism
  • No cellular structure
  • Cannot reproduce outside host cells
  • Do not grow or develop

Scientific consensus: Viruses exist in a gray area between living and non-living. Most biologists consider them non-living but acknowledge they share important properties with life.

1.5.2 Viroids

  • Description: Small, circular RNA molecules that infect plants
  • Comparison to viruses: No protein coat, much simpler structure
  • Life characteristics: Reproduce, evolve
  • Non-life characteristics: No metabolism, no cellular structure

1.5.3 Prions

  • Description: Misfolded proteins that cause normal proteins to misfold
  • Example: Prions causing mad cow disease (BSE) or Creutzfeldt-Jakob disease
  • Life characteristics: Reproduce (by converting normal proteins), evolve (different strains have different properties)
  • Non-life characteristics: No genetic material, no metabolism, no cellular structure

1.5.4 Mitochondria and Chloroplasts

These organelles have characteristics suggesting they were once free-living bacteria:

  • Contain their own DNA
  • Reproduce independently within cells
  • Have double membranes
  • Perform specialized metabolic functions

Endosymbiotic theory: These organelles originated through symbiotic relationships where one cell engulfed another.

1.5.5 Artificial and Synthetic Systems

Computer viruses: Reproduce, evolve, but lack physical substance Synthetic cells: Artificially created membrane-bound systems with synthetic genomes Self-replicating robots: Hypothetical machines that could reproduce and evolve

These cases demonstrate that our definitions of life may need revision as technology advances.

1.6 The Energy and Information Framework

Given the challenges of defining life through checklists of properties, many biologists now adopt a process-oriented framework that focuses on two fundamental aspects:

1.6.1 Energy Perspective

Life requires continuous energy flow to: 1. Maintain organization against the universal tendency toward disorder (entropy) 2. Perform work (movement, synthesis, active transport) 3. Grow and reproduce 4. Process information

Key principle: Living systems are open systems that maintain themselves far from thermodynamic equilibrium by exchanging energy and matter with their environment.

1.6.2 Information Perspective

Life depends on accurate information processing to: 1. Store genetic instructions 2. Regulate cellular processes 3. Enable adaptation through evolution 4. Coordinate multicellular development

Key principle: Biological information exists at multiple levels of organization and must be accurately stored, transmitted, and interpreted.

1.6.3 Integration of Energy and Information

The interaction between energy flow and information processing defines living systems:

ENERGY ACQUISITION
    ↓
ENERGY TRANSFORMATION
    ↓
INFORMATION PROCESSING
    ↓
BIOLOGICAL FUNCTION
    ↓
REPRODUCTION & EVOLUTION

Energy provides the capacity for work, while information provides the instructions for that work. Their coordinated interaction enables living systems to persist, grow, and adapt.

1.7 Life as an Emergent Property

Life cannot be reduced to any single molecule or chemical reaction. Instead, it emerges from the complex interactions of many components organized in specific ways.

1.7.1 Levels of Biological Organization

Each level exhibits properties not present at lower levels:

  1. Molecular Level (atoms → molecules → macromolecules) Example: DNA nucleotides form genes with informational properties

  2. Cellular Level (organelles → cells) Example: Cellular metabolism emerges from coordinated enzyme reactions

  3. Organismal Level (tissues → organs → organ systems → organisms) Example: Consciousness emerges from neural networks

  4. Ecological Level (populations → communities → ecosystems → biosphere) Example: Ecosystem stability emerges from species interactions

1.7.2 Emergent Properties in Living Systems

  • Metabolism: Emerges from coordinated enzyme pathways
  • Homeostasis: Emerges from feedback regulation systems
  • Consciousness: Emerges from neural network interactions
  • Ecosystem resilience: Emerges from biodiversity and species interactions

Understanding emergent properties helps explain why studying isolated components cannot fully explain life.

1.8 Scientific Approaches to Studying Life

Biology employs multiple complementary approaches to understand living systems:

1.8.1 Observation and Description

  • Field studies of organisms in natural habitats
  • Microscopic examination of cells and tissues
  • Anatomical studies of organism structure
  • Example: Darwin’s observations during the HMS Beagle voyage

1.8.2 Hypothesis Testing

  • Controlled experiments with manipulated variables
  • Statistical analysis of experimental data
  • Peer review and replication of findings
  • Example: Meselson-Stahl experiment confirming DNA replication mechanism

1.8.3 Model Building

  • Mathematical models of population dynamics
  • Computer simulations of molecular interactions
  • Conceptual models of biological systems
  • Example: Lotka-Volterra equations modeling predator-prey dynamics

1.8.4 Comparative Studies

  • Comparing different species to identify patterns
  • Analyzing evolutionary relationships (phylogenetics)
  • Studying developmental patterns across organisms
  • Example: Comparing embryonic development across vertebrate species

1.8.5 Reductionist and Holistic Approaches

  • Reductionism: Understanding complex systems by studying their components
  • Holism: Understanding systems through the interactions of their parts
  • Modern biology integrates both approaches

1.9 Chapter Summary

1.9.1 Key Concepts

  1. Defining life is challenging due to diversity of life forms and existence of borderline cases
  2. Seven characteristics are shared by all known living organisms: cellular organization, metabolism, homeostasis, growth and development, reproduction, response to stimuli, and evolutionary adaptation
  3. Borderline cases like viruses, prions, and viroids challenge simple definitions
  4. The energy and information framework provides a unifying perspective on life as process
  5. Life is an emergent property that arises from complex interactions at multiple organizational levels
  6. Biology employs multiple scientific approaches including observation, experimentation, modeling, and comparison

1.9.2 Comparison of Life Characteristics Across Entities

Characteristic Human Cell Virus Prion Crystal Computer Program
Cellular Organization Yes No No No No
Metabolism Yes No No No No*
Homeostasis Yes No No No No
Growth & Development Yes No No Yes (growth) Yes*
Reproduction Yes Yes (with host) Yes No Yes
Response to Stimuli Yes Limited No No Yes
Evolutionary Adaptation Yes Yes Yes No Yes*

* Depends on program design

1.10 Review Questions

1.10.1 Level 1: Recall and Understanding

  1. List the seven characteristics common to all living organisms.
  2. What are the three principles of cell theory?
  3. Distinguish between catabolism and anabolism.
  4. Define homeostasis and provide two examples.
  5. What is the difference between asexual and sexual reproduction?

1.10.2 Level 2: Application and Analysis

  1. A newly discovered entity from a Martian meteorite has a complex structure, contains organic molecules, and appears to change shape in response to light. Using the characteristics of life, design experiments to determine if it is alive.
  2. Explain why viruses are considered borderline cases between living and non-living.
  3. Compare mitochondria to bacteria in terms of characteristics of life. What does this similarity suggest about the origin of mitochondria?
  4. How does the concept of emergent properties help explain why we cannot understand life by studying only individual molecules?

1.10.3 Level 3: Synthesis and Evaluation

  1. NASA defines life as “a self-sustaining chemical system capable of Darwinian evolution.” Evaluate this definition. What are its strengths and weaknesses?
  2. Some scientists argue that ecosystems should be considered living entities. Do you agree? Support your position with evidence.
  3. If we discovered silicon-based life on another planet, how might our current definitions of life need to be modified?
  4. How does the scientific study of life differ from philosophical approaches? What unique insights does each provide?

1.11 Key Terms

  • Biology: The scientific study of life
  • Cell: The basic unit of structure and function in living organisms
  • Metabolism: The sum of all chemical reactions in an organism
  • Homeostasis: The maintenance of a relatively stable internal environment
  • Reproduction: The production of new individuals
  • Evolution: Change in the genetic composition of populations over generations
  • Virus: A non-cellular entity containing genetic material that requires a host cell to reproduce
  • Prion: An infectious protein that causes normal proteins to misfold
  • Emergent Property: A characteristic that arises from the interaction of components and is not present in the individual components
  • Reductionism: Understanding complex systems by studying their individual components
  • Holism: Understanding systems through the interactions of their parts

1.12 Further Reading

1.12.1 Books

  1. Schrödinger, E. (1944). What is Life? Cambridge University Press. (Classic work connecting physics and biology)
  2. Nurse, P. (2020). What is Life? David Fickling Books. (Modern perspective by Nobel laureate)
  3. Margulis, L., & Sagan, D. (1995). What is Life? Simon & Schuster. (Focus on microbial life and symbiosis)

1.12.2 Scientific Articles

  1. Cleland, C. E., & Chyba, C. F. (2002). Defining ‘life’. Origins of Life and Evolution of the Biosphere, 32(4), 387-393.
  2. Trifonov, E. N. (2011). Vocabulary of definitions of life suggests a definition. Journal of Biomolecular Structure and Dynamics, 29(2), 259-266.
  3. Benner, S. A. (2010). Defining life. Astrobiology, 10(10), 1021-1030.

1.12.3 Online Resources

  1. NASA Astrobiology Institute: The Nature of Life
  2. Stanford Encyclopedia of Philosophy: Life
  3. HHMI BioInteractive: What is Life?

1.13 Practical Investigation: Analyzing Borderline Cases

Objective: Apply the characteristics of life to evaluate different entities.

Materials: Information sheets on five entities: Escherichia coli (bacterium), Influenza virus, Prion (PrP^Sc), Crystallizing salt solution, Computer virus

Procedure:

  1. For each entity, determine whether it exhibits each of the seven characteristics of life
  2. Create a data table summarizing your findings
  3. Calculate a “life-likeness” score for each entity (1 point per characteristic)
  4. Rank the entities from most to least life-like
  5. Write a paragraph explaining which entities you would classify as alive and why

Discussion Questions:

  1. Were any entities difficult to classify? Why?
  2. Did any characteristics seem more important than others in determining if something is alive?
  3. How might your classification change if we discovered life on other planets?
  4. What additional information would help you make better classifications?