Properties of Life

The 8 characteristics that define living organisms — and the things that blur the line

BIO 111 — Chapter 1
The real question isn't "what is alive?" — it's "what does life do?" Biology answers with eight properties. But viruses, prions, and fire have been arguing with that list for decades.
01
🧱
Made of cells
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All living things are made of one or more cells — the basic structural and functional unit of life. This is the cell theory. Unicellular organisms (like bacteria) are a single cell doing everything. Multicellular organisms (like you) have trillions of specialized cells working together.

Example ✓
A bacterium is one cell. A human is ~37 trillion cells, each with the same DNA but different jobs.
Edge case ✗
Viruses are NOT made of cells — they're just protein + genetic material. This is why they're not considered alive by most definitions.
Cell theory Unicellular Multicellular
02
⚡
Uses and processes energy (metabolism)
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Living things take in energy from their environment and use it to do work — building molecules, moving, maintaining structure. Metabolism is the sum of all chemical reactions in an organism. Anabolism builds things up; catabolism breaks things down. Both require energy.

Example ✓
You eat food → cells break it down → ATP is produced → powers every cellular process in your body.
Edge case ✗
Fire also uses and releases energy — but it's not metabolism. Fire doesn't regulate or direct energy use; it just burns.
Metabolism ATP Anabolism Catabolism
03
📈
Grows and develops
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Living things increase in size and complexity over their lifetime, following a genetically programmed pattern of development. Growth isn't random — it's directed by DNA. Development refers to the ordered sequence of changes from simpler to more complex forms (like a fertilized egg becoming a human).

Example ✓
A single fertilized egg undergoes billions of cell divisions in a precise, programmed sequence to form a complete organism.
Edge case ✗
Crystals "grow" by adding molecules to their surface — but this isn't genetically directed, and they don't develop.
Cell division DNA-directed Development
04
🧬
Reproduces
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Living things produce offspring — either sexually (combining DNA from two parents) or asexually (one parent, genetically identical offspring). Reproduction isn't required for individual survival, but it's required for a species to persist. The ability to reproduce is a defining feature of life even if an individual organism never does it.

Example ✓
Bacteria reproduce asexually by binary fission — one cell splits into two identical daughter cells. Can happen every 20 minutes under ideal conditions.
Edge case ✗
Viruses can replicate — but only by hijacking a host cell's machinery. They can't reproduce independently.
Sexual Asexual Binary fission Heredity
05
🌡️
Maintains homeostasis
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Living things actively regulate their internal environment to keep it stable despite changes in external conditions. Temperature, pH, water balance, blood glucose — all kept within narrow ranges through feedback mechanisms. Homeostasis requires constant energy expenditure. When it fails, the organism gets sick or dies.

Example ✓
Your body temperature stays at ~98.6°F whether you're in Alaska or Florida. Sweating and shivering are homeostatic responses.
Lab connection 🔬
Yogurt fermentation depends on bacterial homeostasis — LAB regulate their internal pH to survive acid buildup. When it fails, spoilage organisms win.
Feedback loops Internal regulation Temperature pH balance
06
⚡
Responds to stimuli
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Living things detect and respond to changes in their environment — light, temperature, touch, chemicals, sound. Responses can be immediate (pulling your hand from heat) or long-term (tanning in the sun). Even single-celled organisms respond to stimuli — bacteria swim toward nutrients and away from toxins.

Example ✓
A plant grows toward light (phototropism). A bacterium moves toward a food source (chemotaxis). You flinch when startled.
vs. Homeostasis
Homeostasis = maintaining internal balance. Response to stimuli = reacting to external changes. They overlap but aren't the same.
Irritability Chemotaxis Phototropism
07
🌿
Adapts through evolution
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Populations of living things change over generations through natural selection. Individuals with traits better suited to their environment survive and reproduce more, passing those traits to offspring. Evolution doesn't happen to individuals — it happens to populations over time. This is what makes life adaptive rather than static.

Example ✓
Antibiotic resistance: bacteria that survive treatment pass on resistance genes. Over generations, the population becomes resistant. This is evolution happening in real time.
Lab connection 🔬
Pathogen evolution is why food safety standards have to keep updating. Listeria and Salmonella strains evolve. Your HACCP plan has to account for this.
Natural selection Populations Adaptation Heredity
08
🔗
Has organized complexity (DNA & heredity)
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All living things contain DNA (or RNA in some viruses) that carries genetic information and passes it to offspring. Life is organized in a hierarchy of complexity — from atoms → molecules → organelles → cells → tissues → organs → organisms → populations → ecosystems. Each level has emergent properties not present at the level below.

Example ✓
Your entire body plan — every cell type, every protein, every developmental stage — is encoded in ~3 billion base pairs of DNA in every nucleus.
Edge case ✗
Prions are misfolded proteins that can cause other proteins to misfold — no DNA involved. They reproduce in a sense but have no hereditary information of their own.
DNA Heredity Biological hierarchy Emergent properties

Click any property to expand · All 8 are fair game on exams

Pick a subject below and see how it scores against each of the 8 properties. This is exactly the kind of analysis your bio exams will ask for.

Each result shows whether the subject meets, partially meets, or fails each property

All 8 at a glance

🧱 Cells ⚡ Metabolism 📈 Growth 🧬 Reproduction 🌡️ Homeostasis ⚡ Response 🌿 Evolution 🔗 DNA

Mnemonic: C-MERGE-D — Cells, Metabolism, Evolution, Reproduction, Growth, Energy, DNA (+ homeostasis & response)

The hard cases

Viruses 🦠
Have DNA/RNA and can replicate — but not independently, not made of cells. Most biologists say not alive. Your exam may call them "on the boundary."
Fire 🔥
Grows, uses energy, "reproduces" (spreads), responds to environment. But no cells, no DNA, no metabolism. Definitively not alive.
Crystals 💎
Grow in a structured way and have organized complexity. But no energy use, no cells, no reproduction. Not alive.
Prions 🧠
Misfolded proteins that "spread" by making other proteins misfold. No DNA, no cells. Not alive — but cause serious disease (mad cow).

Exam reminders

You need to know all 8 properties — and be able to apply them to novel examples, not just memorize the list.
Viruses are the classic exam trap. They meet some properties but not all — the key missing ones are cells and independent reproduction.
Homeostasis and response to stimuli are different things. Know the distinction.
Evolution happens to populations, not individuals. Never say an organism "evolves" to survive — its offspring do.
The biological hierarchy (atom → molecule → cell → tissue → organ → organism) is fair game too.