What Makes Something Alive
Learn the six characteristics all living things share — cells, energy use, growth, reproduction, response, and homeostasis — and how to decide if something is truly alive.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on What Makes Something Alive, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
A candle flame moves, gives off heat, grows bigger when you feed it more wax, and dies when you smother it. So is fire alive? Most people say no right away, but explaining why takes real science. Biologists do not decide what is alive by how something looks or whether it moves. They use a checklist of characteristics that every known living thing shares.
In this lesson you will learn those six characteristics, practice using them as a set instead of one at a time, and work through the tricky cases that trip people up — seeds in a packet, a river carving a canyon, a robot that follows your voice, a virus. By the end you should be able to look at an unfamiliar object and build an argument, backed by evidence, about whether it is living, once-living, or never-living.
In this lesson you will learn those six characteristics, practice using them as a set instead of one at a time, and work through the tricky cases that trip people up — seeds in a packet, a river carving a canyon, a robot that follows your voice, a virus. By the end you should be able to look at an unfamiliar object and build an argument, backed by evidence, about whether it is living, once-living, or never-living.
The Six Characteristics Shared by All Living Things
Scientists describe an organism as anything that shows all six of these characteristics. Missing even one moves an object off the list.
Two of these are easy to mix up. Growth is getting bigger; development is changing into a different stage or form. A crystal in a cave gets bigger, but it never becomes a different kind of crystal.
The last one, homeostasis, is the one students most often forget to check, and it is often the deciding evidence. A car burns fuel and responds to the steering wheel, but nothing inside a car works to hold its temperature, water content, or chemistry steady. Living things spend energy constantly just to stay the same on the inside.
| Characteristic | What it means | Example |
|---|---|---|
| Made of cells | The body is built from one or more cells, the smallest units that can carry out life processes | An amoeba is one cell; you have trillions |
| Obtains and uses energy | Takes in food, sunlight, or chemicals and uses that energy to run life processes | A maple tree makes sugar from sunlight |
| Grows and develops | Gets larger by adding cells, and changes form over its life | A tadpole becomes a frog |
| Reproduces | Makes more organisms of the same kind and passes on genetic information | A bacterium splits in two |
| Responds to stimuli | Detects a change in the environment and reacts to it | A sunflower turns toward light |
| Maintains internal balance | Keeps conditions inside steady even when outside conditions change | You sweat when you are hot |
The last one, homeostasis, is the one students most often forget to check, and it is often the deciding evidence. A car burns fuel and responds to the steering wheel, but nothing inside a car works to hold its temperature, water content, or chemistry steady. Living things spend energy constantly just to stay the same on the inside.
Why One Characteristic Is Never Enough
Almost every nonliving thing you can name matches at least one item on the list. That is exactly why the list has to be used as a whole set.
Where students go wrong is treating movement as proof of life. Movement is not even on the list. Clams, coral, and oak trees barely move, and all three are clearly alive; wind-blown sand moves constantly and is not.
The second common error is arguing from a single match, such as "fire reproduces because one flame can start another flame." A new flame is not an offspring — no genetic information is copied and passed along, and the new flame is not built from cells that came from the parent flame. When you write an argument, name the characteristic, give the evidence, and then check the remaining five before you commit to a conclusion. A complete answer addresses the whole set, not the one that seems most obvious.
| Nonliving thing | Characteristic it seems to have | Characteristics it lacks |
|---|---|---|
| Fire | Uses energy, grows, spreads | No cells, no development, no homeostasis, no genetic material passed on |
| Salt crystal | Gets larger over time | No cells, no energy use, no response, no reproduction |
| River | Moves, changes shape, erodes rock | No cells, no reproduction, no internal balance |
| Robot vacuum | Responds to obstacles, uses electricity | No cells, does not grow, cannot make another vacuum |
| Cloud | Grows, changes, breaks apart | No cells, no energy use for life processes, no response to stimuli |
The second common error is arguing from a single match, such as "fire reproduces because one flame can start another flame." A new flame is not an offspring — no genetic information is copied and passed along, and the new flame is not built from cells that came from the parent flame. When you write an argument, name the characteristic, give the evidence, and then check the remaining five before you commit to a conclusion. A complete answer addresses the whole set, not the one that seems most obvious.
Cells: The Characteristic That Ties Everything Together
Of the six, "made of cells" carries special weight, because the other five actually happen inside cells. Energy is captured and released by structures inside cells. Growth happens when cells divide and enlarge. Reproduction passes DNA from parent cells to offspring cells. Nerve and sensory cells detect stimuli. Cell membranes control what enters and leaves, which is how organisms hold their internal conditions steady.
This is why the cell test is such powerful evidence. If you put a sample under a microscope and find no cells at all, the other five characteristics almost certainly cannot be happening either, no matter how lifelike the object looks.
Organisms come in two arrangements. Unicellular organisms, such as bacteria and paramecia, do everything with a single cell. Multicellular organisms, such as mosses, mushrooms, and humans, have many specialized cells working together. Both count as fully alive — being small or simple does not make something less living.
A related mistake is assuming that anything too small to see must not be alive. A single drop of pond water can hold hundreds of living organisms. Another mistake is thinking body parts are separate organisms; your hair and fingernails are made of dead cells produced by living cells, so they are parts of an organism rather than organisms themselves.
This is also where the word once-living becomes useful. A wooden desk, a leather shoe, and a fallen log all came from organisms and still contain cell structures, but they no longer carry out life processes. Scientists sort objects into three groups — living, once-living, and never-living — and the middle group is the one students most often skip.
This is why the cell test is such powerful evidence. If you put a sample under a microscope and find no cells at all, the other five characteristics almost certainly cannot be happening either, no matter how lifelike the object looks.
Organisms come in two arrangements. Unicellular organisms, such as bacteria and paramecia, do everything with a single cell. Multicellular organisms, such as mosses, mushrooms, and humans, have many specialized cells working together. Both count as fully alive — being small or simple does not make something less living.
A related mistake is assuming that anything too small to see must not be alive. A single drop of pond water can hold hundreds of living organisms. Another mistake is thinking body parts are separate organisms; your hair and fingernails are made of dead cells produced by living cells, so they are parts of an organism rather than organisms themselves.
This is also where the word once-living becomes useful. A wooden desk, a leather shoe, and a fallen log all came from organisms and still contain cell structures, but they no longer carry out life processes. Scientists sort objects into three groups — living, once-living, and never-living — and the middle group is the one students most often skip.
Working Through the Hard Cases
Some objects sit right on the boundary, and reasoning through them is the best practice you can get.
A seed in a paper packet is alive. Its cells are intact, it responds to water and warmth, and it uses stored energy at a very low rate. It is dormant — life processes slowed way down — not dead. Plant it and it germinates. A roasted, ground-up seed cannot, because its cells were destroyed.
A virus is the famous borderline case. A virus has genetic material and can direct the making of more viruses, but it is not made of cells, cannot obtain or use energy on its own, does not grow or develop, and does not maintain internal balance. It only replicates by taking over a host cell. Because it fails four of six, most biologists classify viruses as nonliving, though the debate continues. Saying "scientists disagree, and here is the evidence on each side" is a stronger answer than picking a side with no reasons.
Yeast in a packet looks like beige powder, so students often call it nonliving. It is unicellular fungi in a dormant state — add warm water and sugar and it feeds, releases carbon dioxide, and buds off new cells.
A computer program that copies itself and responds to input matches two characteristics and no others. No cells, no energy obtained from food, no growth, no homeostasis.
The habit to build is simple: check all six, list which ones fail, and let the evidence decide. Do not let "it seems alive" or "it seems like a machine" decide for you.
A seed in a paper packet is alive. Its cells are intact, it responds to water and warmth, and it uses stored energy at a very low rate. It is dormant — life processes slowed way down — not dead. Plant it and it germinates. A roasted, ground-up seed cannot, because its cells were destroyed.
A virus is the famous borderline case. A virus has genetic material and can direct the making of more viruses, but it is not made of cells, cannot obtain or use energy on its own, does not grow or develop, and does not maintain internal balance. It only replicates by taking over a host cell. Because it fails four of six, most biologists classify viruses as nonliving, though the debate continues. Saying "scientists disagree, and here is the evidence on each side" is a stronger answer than picking a side with no reasons.
Yeast in a packet looks like beige powder, so students often call it nonliving. It is unicellular fungi in a dormant state — add warm water and sugar and it feeds, releases carbon dioxide, and buds off new cells.
A computer program that copies itself and responds to input matches two characteristics and no others. No cells, no energy obtained from food, no growth, no homeostasis.
The habit to build is simple: check all six, list which ones fail, and let the evidence decide. Do not let "it seems alive" or "it seems like a machine" decide for you.
Building an Evidence-Based Argument
Your teacher will usually want more than a yes or no. A strong response follows a claim, evidence, reasoning pattern.
Start with a claim: the object is living, once-living, or never-living. Then give evidence: specific observations tied to specific characteristics. Then give reasoning: explain why that evidence supports the claim, including which characteristics failed.
Compare a weak and a strong answer about a candle flame.
Notice three things the strong answer does. It names the characteristics by their scientific terms. It admits the characteristics the object does match instead of pretending it matches none. And it states the deciding evidence.
Where students go wrong most often: writing only about the characteristics that fail, and never explaining the near-misses. If your classmate says "but fire grows," your answer should already have handled that. Anticipating the strongest objection is what separates a complete argument from a list.
Start with a claim: the object is living, once-living, or never-living. Then give evidence: specific observations tied to specific characteristics. Then give reasoning: explain why that evidence supports the claim, including which characteristics failed.
Compare a weak and a strong answer about a candle flame.
| Weak answer | Strong answer |
|---|---|
| "Fire is not alive because it is not a plant or animal." | "The flame is nonliving. Under magnification there are no cells, and cells are required because all other life processes happen inside them. The flame does release energy from wax, and it does get larger, so it matches two characteristics, but it never develops into a different stage, it does not pass genetic information to new flames, and nothing inside it works to keep its temperature steady — it simply matches its fuel supply. Failing four of six means it is not alive." |
Where students go wrong most often: writing only about the characteristics that fail, and never explaining the near-misses. If your classmate says "but fire grows," your answer should already have handled that. Anticipating the strongest objection is what separates a complete argument from a list.
Key terms
- Organism.
- Any individual living thing that shows all six characteristics of life, from a single bacterium to a blue whale.
- Cell.
- The smallest unit that can carry out all life processes; every living thing is made of one or more cells.
- Stimulus.
- A change in the environment — light, temperature, touch, sound, chemicals — that an organism can detect and react to.
- Response.
- The reaction an organism makes to a stimulus, such as a plant bending toward a window.
- Homeostasis.
- The process of keeping internal conditions such as temperature, water, and chemical balance steady even as outside conditions change.
- Development.
- Change in an organism's form or structure over its life, such as a caterpillar becoming a butterfly; different from simply getting bigger.
- Dormant.
- A state in which an organism's life processes slow down dramatically but do not stop, as in a dry seed or packaged yeast.
- Once-living.
- Describes material that came from an organism and still shows traces of cells but no longer carries out life processes, such as a wooden board or a fallen leaf.
Worked example
A student finds a strange object on a shelf in the science room: a sealed glass jar of cloudy liquid labeled "pond sample, 3 days old." Under a microscope, small oval shapes swim, spin away from the bright light of the microscope lamp, and a few appear to be splitting in half. The jar itself is warm on the side facing the window. Is the pond sample living? Write a claim, evidence, and reasoning answer.
Step 1 — Separate the object from the container. The jar and the glass are never-living. The question is about the oval shapes in the liquid, so evaluate those.
Step 2 — Check each characteristic against the observations.
Made of cells: the oval shapes are visible only under a microscope and each is a single closed unit. That is consistent with unicellular organisms. Evidence supports this characteristic.
Responds to stimuli: they swim away from the lamp. The lamp light is the stimulus and moving away is the response. Supported.
Reproduces: some are splitting into two, which is how unicellular organisms reproduce. Supported.
Grows and develops: splitting cells must grow back to full size before splitting again, so growth is reasonable to infer, though the student did not directly observe it. Note this as inferred, not observed.
Obtains and uses energy: not directly observed, but swimming requires energy, and movement powered from the inside is evidence of energy use. Supported by inference.
Maintains internal balance: not directly observed in three days of watching. This one would need a longer investigation.
Step 3 — Check the trap. The jar is warm on the window side. A beginner might list that as evidence of body heat. It is not — that is sunlight heating the glass, an outside cause, not internal regulation. Rule it out explicitly.
Step 4 — Write the conclusion. Claim: the pond sample contains living organisms. Evidence: microscopic cells that swim away from light and divide in two. Reasoning: these observations match cells, response to stimuli, reproduction, and energy use, and the remaining characteristics can reasonably be inferred from cell division. The warmth of the jar is not evidence of life because it is explained by sunlight.
Step 2 — Check each characteristic against the observations.
Made of cells: the oval shapes are visible only under a microscope and each is a single closed unit. That is consistent with unicellular organisms. Evidence supports this characteristic.
Responds to stimuli: they swim away from the lamp. The lamp light is the stimulus and moving away is the response. Supported.
Reproduces: some are splitting into two, which is how unicellular organisms reproduce. Supported.
Grows and develops: splitting cells must grow back to full size before splitting again, so growth is reasonable to infer, though the student did not directly observe it. Note this as inferred, not observed.
Obtains and uses energy: not directly observed, but swimming requires energy, and movement powered from the inside is evidence of energy use. Supported by inference.
Maintains internal balance: not directly observed in three days of watching. This one would need a longer investigation.
Step 3 — Check the trap. The jar is warm on the window side. A beginner might list that as evidence of body heat. It is not — that is sunlight heating the glass, an outside cause, not internal regulation. Rule it out explicitly.
Step 4 — Write the conclusion. Claim: the pond sample contains living organisms. Evidence: microscopic cells that swim away from light and divide in two. Reasoning: these observations match cells, response to stimuli, reproduction, and energy use, and the remaining characteristics can reasonably be inferred from cell division. The warmth of the jar is not evidence of life because it is explained by sunlight.
Practice questions
A student argues that a river is alive because it grows during spring floods, moves, and "carries food" to the ocean. Which single piece of evidence most directly shows the river is not alive?
- The river changes speed at different points along its path
- The river is not made of cells, so no life processes can be carried out inside it
- The river is much larger than most organisms
- The river does not move at the same speed all year
Answer: The river is not made of cells, so no life processes can be carried out inside it
Size and changing speed have nothing to do with the characteristics of life — some organisms are enormous and all organisms change their activity over time. The cell test is the deciding evidence because the other five characteristics all happen inside cells. Once you establish there are no cells, energy use for life processes, development, reproduction, and homeostasis cannot be occurring either. Note that the river does appear to match "grows," which is why one matching characteristic is never enough.
Which statement correctly describes the difference between growth and development?
- Growth happens only in animals, while development happens only in plants
- Growth means getting larger, while development means changing form or structure over a lifetime
- Growth requires energy, while development does not
- Growth happens once, while development repeats every year
Answer: Growth means getting larger, while development means changing form or structure over a lifetime
Both plants and animals grow and develop, and both processes require energy. A tadpole growing longer is growth; a tadpole absorbing its tail and forming legs to become a frog is development. This distinction matters because nonliving things like crystals and clouds can get larger, but they never change into a different stage of the same thing.
A packet of dry baker's yeast looks like a beige powder and shows no movement at all. A classmate says it must be nonliving. Explain whether you agree, and describe one investigation that would gather evidence for your answer.
Answer: The yeast is living but dormant. An investigation: place the yeast in warm water with sugar and watch for bubbles of carbon dioxide, then examine a drop under a microscope for budding cells.
Dormancy means life processes have slowed almost to a stop, not that they have ended, so a lack of visible movement is not evidence of death. Yeast is a unicellular fungus. When you supply warm water and sugar, the cells obtain and use energy from the sugar and release carbon dioxide as a waste product, which you observe as foam and bubbles. Under a microscope you would see oval cells with small buds forming on them, which is reproduction. Together, energy use, reproduction, and the presence of cells give strong evidence that the powder was alive the whole time. This is the same reason a seed in a packet counts as living.
FAQ
- Are viruses alive?
- Most biologists say no. A virus has genetic material and can cause more copies of itself to be made, but it is not made of cells, cannot obtain or use energy on its own, does not grow or develop, and does not maintain any internal balance. It only replicates by hijacking a living host cell. Because it fails four of the six characteristics, it falls outside the definition of life, though scientists still debate the boundary.
- Why is movement not one of the characteristics of life?
- Because it does not sort things correctly. Oak trees, coral, and barnacles are firmly alive and barely move. Meanwhile wind, water, avalanches, and machines move constantly and are not alive. Movement can be a clue that something is responding to a stimulus, but the response itself is the characteristic, not the motion.
- What is the difference between nonliving and once-living?
- Never-living things, such as a rock, a glass marble, or water, never came from an organism. Once-living things, such as a wooden pencil, a cotton shirt, a fallen log, or a fossil, were produced by organisms and often still contain cell structures, but they no longer carry out life processes. Sorting an object into the right one of these three groups is often the real task in a lesson question.
- Do all six characteristics have to be present at the same moment?
- Not necessarily at every instant, but they must be part of what the organism does over its life. A dormant seed is not actively growing right now, and a mule cannot reproduce, yet both are made of cells, use energy, respond to stimuli, and maintain internal balance. Biologists look at the full pattern for that kind of organism rather than a single snapshot in time.
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