Table of Contents
Living things (organisms) differ from nonliving things in several fundamental ways. No single feature alone is enough; rather, it is the combination of several characteristics that defines life. In this chapter, we will outline these typical characteristics and explain what they mean in simple terms. Later chapters will go into the molecular, cellular, and evolutionary details behind them.
1. Cellularity: Built from One or More Cells
All known living things are made of cells. A cell is the smallest unit that can carry out the basic processes of life.
- Unicellular organisms (e.g., many bacteria, some algae, many protists) consist of a single cell that does everything needed for life.
- Multicellular organisms (e.g., plants, fungi, animals, humans) consist of many cells that often specialize for different tasks (e.g., muscle cells, nerve cells, leaf cells).
At this level, what matters is:
- The boundary: a cell membrane separates the inside of the cell from the environment and helps keep internal conditions relatively stable.
- The internal space: inside, chemical reactions occur in an organized way.
- The information: each cell contains genetic information (DNA) that guides its structure and function.
Nonliving structures, such as crystals or sand grains, may grow or change shape, but they are not composed of cells and do not show the other properties described below in an integrated way.
2. Metabolism: Exchange of Matter and Energy
Living things constantly exchange materials and energy with their environment. This overall set of chemical reactions is called metabolism.
Two broad aspects:
- Anabolism: building up; using energy and simple substances to make more complex molecules (e.g., building proteins, making cell components, creating energy stores).
- Catabolism: breaking down; splitting larger molecules to release energy and simpler building blocks (e.g., breaking down sugars or fats).
Key points specific to life:
- Metabolism is regulated and coordinated, not random.
- Organisms take in nutrients and energy sources (light, chemical compounds, food) and release waste products (e.g., carbon dioxide, urea).
- Energy flow through metabolism enables all other life processes: movement, growth, repair, active transport, and information processing.
Nonliving things can undergo chemical reactions, but they do not maintain a continuous, self-regulated network of reactions that sustains a stable, bounded system as organisms do.
3. Homeostasis: Maintaining Internal Stability
Despite changing external conditions, living organisms keep many internal conditions within narrow limits. This dynamic maintenance is called homeostasis.
Examples:
- Keeping body temperature within a certain range (in humans and other warm-blooded animals).
- Maintaining a stable internal salt and water balance (in many animals).
- Keeping the pH and ion concentrations inside cells within a range that allows enzymes to work.
Important aspects:
- Homeostasis is active: organisms use sensors (receptors), control mechanisms, and effectors (e.g., muscles, glands, transport processes) to correct deviations.
- It is dynamic: conditions fluctuate but are held around set ranges, not absolutely fixed.
Nonliving systems can show constant conditions only if the environment around them remains constant; they do not actively adjust themselves when conditions change.
4. Growth and Development
Living things typically grow and develop in an orderly way.
- Growth means an increase in size and often mass. In organisms, this is usually:
- Based on cell division (more cells) and/or
- Cell enlargement (bigger cells).
- Development means qualitative change: the organism becomes more complex or changes its form over time.
Examples:
- A fertilized egg cell dividing into many cells, then forming tissues and organs.
- A plant seed germinating and forming roots, stems, leaves, and eventually flowers.
- An insect passing through larval and pupal stages before becoming an adult.
Key points:
- Growth and development follow programs encoded in genetic information and influenced by the environment.
- They are not random enlargement, but highly ordered processes.
Nonliving things can increase in size (e.g., a crystal in a solution), but this occurs by simple addition of material from the environment and is not controlled by a genetic program or accompanied by differentiation into specialized parts.
5. Reproduction: Creating New Individuals
Living things reproduce, that is, they can generate new individuals that carry on the line of life.
Forms of reproduction (details belong to later chapters):
- Asexual reproduction: one parent produces genetically very similar or identical offspring (e.g., bacterial cell division, budding in yeast, certain plant offshoots).
- Sexual reproduction: two parents contribute genetic material to produce genetically unique offspring (e.g., most animals and many plants).
Important features:
- During reproduction, genetic information (mainly DNA) is copied and passed to offspring.
- Reproduction is not required of a single individual to call it living (e.g., a sterile worker ant is still alive), but it is a property of the species or lineage.
Nonliving objects do not produce offspring; they may be manufactured or assembled from outside, but they do not self-reproduce using encoded instructions.
6. Irritability: Perception and Response to Stimuli
Living organisms can sense changes in their environment (stimuli) and respond to them in specific ways. This property is sometimes called irritability, responsiveness, or excitability.
Stimuli can be:
- Physical: light, heat, sound, touch, gravity, mechanical pressure.
- Chemical: presence of certain substances, changes in pH, signals from other organisms.
- Biological: presence of predators, prey, competitors, or potential mates.
Responses include:
- Moving toward or away from something (e.g., a plant bending toward light, a bacterium swimming toward nutrients).
- Changing internal processes (e.g., adjusting metabolic rate, opening or closing stomata in leaves).
- Behaviors in animals (e.g., escape, foraging, communication).
Important aspects:
- Responses are often adaptive, increasing survival or reproductive success.
- Even very simple organisms show signal detection and reaction at the cellular level.
Nonliving objects can change under external forces (e.g., a rock rolling downhill), but they do not process information about stimuli or generate purposeful responses.
7. Adaptation: Fit to the Environment Through Evolution
Living things are adapted to their environments. An adaptation is a heritable trait that increases an organism’s chances of surviving and reproducing in a given environment.
Examples:
- Thick fur or fat layers for insulation in cold climates.
- Streamlined bodies in aquatic animals to reduce resistance in water.
- Specialized leaves in desert plants to reduce water loss.
- Enzymes that function best at the temperature and pH typical of the organism’s habitat.
Key points:
- Adaptations arise and spread in populations over many generations through evolutionary processes, especially natural selection.
- Individual organisms can also adjust within limits during their lifetime (acclimatization), but the term adaptation in biology usually refers to genetically based traits shaped by evolution.
Nonliving systems may fit an environment by chance (e.g., a stone having a shape that fits a depression in the ground), but they do not have heritable traits refined over generations by differential survival and reproduction.
8. Evolutionary Potential: Capacity for Change Over Generations
Living populations are not fixed; they can evolve. Evolution is a change in the genetic composition of a population over generations.
Underlying features:
- Genetic variability: individuals within a species differ in their genetic makeup.
- Inheritance: genetic traits are passed from parents to offspring.
- Selection and other factors: some traits become more common, others rarer, due to survival, reproduction, mutation, migration, and random processes.
This evolutionary capacity is a key characteristic of life as a whole:
- It explains how new forms arise and how living things diversify.
- It allows populations to track changing environments over long time scales.
Nonliving systems can change in appearance or arrangement, but they do not show heredity, variation, and selection in the biological sense.
9. Organization and Complexity
Living organisms show a high degree of ordered complexity:
- Components (molecules, organelles, cells, tissues, organs) are arranged in a specific, hierarchical manner.
- The whole is more than the sum of its parts: interactions among parts create new properties (emergent properties), such as consciousness in humans or coordinated movement in animals.
This organization is:
- Built and maintained using information stored primarily in DNA.
- Constantly renewed and repaired through metabolism and growth.
Nonliving complex systems exist (e.g., machines, computers), but they are designed and built from the outside; they do not self-construct and self-maintain based on internal genetic instructions.
10. Summary: Life as a Combination of Characteristics
To summarize, typical characteristics of living things include:
- Being made of one or more cells.
- Having regulated metabolism (exchange and transformation of matter and energy).
- Maintaining homeostasis (stable internal conditions).
- Undergoing growth and development according to genetic programs.
- Being capable of reproduction and passing on genetic information.
- Showing responsiveness to stimuli.
- Possessing adaptations shaped by evolution.
- Having the capacity to evolve as populations over generations.
- Exhibiting a high degree of organized complexity based on genetic information.
No single property alone absolutely defines life; some viruses, for example, show genetic information and evolution but depend on host cells for metabolism and reproduction, making them “borderline cases.” Nevertheless, the combined presence of these characteristics provides a practical and widely used way to distinguish living from nonliving systems.