Characteristics of Life & Biological Diversity — Print · Grade 7 Science
Chapter: Scientific Inquiry & Diversity of LifeCharacteristics of Life & Biological Diversity
Learning goals
- I can explain characteristics of life & biological diversity.
- I can use evidence, a model, and stated assumptions to support a biological explanation.
Before this unit
Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
In 2005 a seed that had sat in a desert fortress at Masada for roughly two thousand years was planted, watered, and grew into a date palm. For twenty centuries it did nothing measurable: it did not move, grow, feed, or respond. Was it alive the whole time? Now hold that question next to a few others. A fire grows, consumes fuel, releases heat and spreads to new ground. A salt crystal in a saturated solution grows and takes on a regular form. A wind-up toy moves on its own. None of those is alive, and each one does something people instinctively associate with life. That is the problem this unit solves: no single characteristic separates the living from the non-living, and the question has to be answered with several at once.
The working definition uses a set. An organism is made of one or more cells; it takes in materials and uses energy; it responds to its surroundings; it grows and develops; it reproduces; and it maintains stable internal conditions. That last one is homeostasis, and it is the characteristic the fire and the crystal most clearly fail — a fire does not regulate its own temperature, it simply burns until the fuel runs out. The characteristics are applied together, and with an allowance for state: a dormant seed is not carrying them out at this moment but has the cellular organisation to do so, and the Masada seed proved it.
Organising the variety that results is classification, and the deepest division is one you have already met. Prokaryotic cells keep their DNA loose in the cytoplasm with no enclosed nucleus; bacteria are the familiar example, and they are by far the most numerous organisms on Earth. Eukaryotic cells keep their DNA in a nucleus, and that group covers animals, plants, fungi and the single-celled protists. Modern classification sorts organisms by shared ancestry, which is why a mushroom turns out to be more closely related to you than to a plant. One warning before the detail: the resulting tree is not a ladder from simple to advanced. Bacteria are not failed attempts at being complicated — they are superbly successful organisms that have been adapting for as long as everything else.
Build your understanding
Where this fits
Chapter 1 has been about method: designing a comparison, measuring it, summarising it, and arguing from it. This unit turns those tools on the first real question of the course — what counts as living, and how the enormous variety of living things can be organised. It also sets up Chapter 2, which asks what living things are built from.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Part 1 · Using several characteristics together
Take the characteristics one at a time against the awkward cases and you see why the set is needed. Growth: crystals grow, by adding material to the outside, while organisms grow by building new material inside cells and by making more cells. Movement: a toy moves, and most plants do not move about at all, so movement is neither necessary nor sufficient. Using energy: a fire does, spectacularly, and it does not regulate anything or reproduce in the biological sense.
Reproduction is the one most often misapplied. A fire spreading to the next field looks like reproduction and is not — no inherited information passes, and nothing about the new fire is determined by the old one. A mule is alive and cannot reproduce. So reproduction is a characteristic of life in general, applied to kinds of organism, not a test you can run on an individual.
Cellular organisation is the characteristic that does most of the work, which is why Chapter 2 spends five units on it. Everything we accept as living is made of cells, and no fire or crystal is. Even so, apply the set rather than the single test — viruses, which have genetic material and a protein coat but no cells and no metabolism of their own, sit on the boundary and biologists genuinely disagree about which side.
Pause and explain
A fire grows, uses fuel, releases energy and spreads. Why is it not alive?
Check your reasoning after answering
Because it fails most of the set. It is not made of cells, it does not maintain stable internal conditions, and its spread transmits no inherited information — the next fire is not built from instructions carried by the first. It satisfies growth and energy use, which is exactly why several characteristics have to be applied together rather than one.
Part 2 · A need is not the same as one way of meeting it
Every organism needs energy, materials, water and a way of dealing with waste. The mistake is to take the familiar solution to one of those needs and treat it as the need itself. Animals meet the energy requirement by eating other organisms; plants meet it by building their own glucose from light; some bacteria meet it by oxidising chemicals in rock, with no light involved at all. All three are feeding in the biological sense.
The same applies to gas exchange and to movement. You take in oxygen through lungs, a fish through gills, an insect through tubes that carry air directly to its tissues, and a plant through pores in its leaves. Lungs are a solution, not a requirement. Asking "how does this organism meet the need?" instead of "does it have the organ I know?" is what lets you reason about an unfamiliar one.
Homeostasis works the same way. You keep your temperature near 37 degrees by sweating, shivering and adjusting blood flow; a lizard does it by moving between sun and shade; a bacterium does not regulate its temperature at all but regulates its internal chemistry carefully. Different mechanisms, the same requirement — stable enough internal conditions for the chemistry of life to continue.
Pause and explain
A bacterium in a deep rock has no light and eats no other organism. How can it be alive?
Check your reasoning after answering
Because feeding means obtaining energy and materials, not eating in the familiar sense. Some bacteria obtain energy by driving chemical reactions with compounds in rock or water, building what they need from simple molecules. The need is energy and raw material; eating other organisms and photosynthesis are two solutions among several.
Part 3 · Classifying cells and organisms from evidence
The prokaryote–eukaryote division rests on one observable feature with large consequences: whether the DNA is enclosed in a nucleus. Prokaryotic cells are generally small, from about 1 to 5 micrometres, with no membrane-bound organelles. Eukaryotic cells are typically ten to a hundred times larger in volume, with a nucleus and the organelles from Chapter 2 — and that internal compartmentation is what allows the division of labour that multicellular life depends on.
To classify something you need more than one observation, and you need to know what each one can establish. A cell with a wall could be a plant, a fungus or a bacterium, since all three build walls out of different materials. A green cell is probably photosynthetic but could be coloured another way. A cell with no visible nucleus under a light microscope might be prokaryotic, or might be a eukaryotic cell whose nucleus is out of the plane of focus, or a mature red blood cell that discarded one.
So a careful classification states which features it used and what it has not ruled out. "This is a prokaryote, on the evidence that DNA is present without an enclosing membrane in the electron micrograph, and that the cell is about 2 micrometres long" is a conclusion somebody can check. "It looks like a bacterium" is a hunch.
Pause and explain
Why is "no nucleus visible" weaker evidence for a prokaryote than "DNA present with no enclosing membrane"?
Check your reasoning after answering
Because invisibility has several causes. Under a light microscope a nucleus may simply be out of focus, unstained, or below the resolution of the instrument, and a mature mammalian red blood cell is eukaryotic with no nucleus at all. Observing DNA with no membrane around it, which needs an electron micrograph, establishes the defining feature directly rather than inferring it from an absence.
Part 4 · Organising diversity without building a ladder
Classification today groups organisms by shared ancestry, so the categories are meant to reflect history rather than resemblance. That produces results that surprise people. Fungi are more closely related to animals than to plants. Birds sit inside the group that contains the dinosaurs. A dolphin is closer kin to a cow than to a shark, despite looking far more like the shark, because similar bodies can evolve independently in similar conditions.
The picture to resist is the ladder — bacteria at the bottom, humans at the top, everything else arranged by how advanced it is. Every organism alive today has exactly the same length of ancestry behind it, and each is adapted to its own circumstances. Bacteria outnumber everything, inhabit places nothing else can, and have been evolving for the whole history of life. Simpler in structure is not earlier in time or worse at living.
The honest shape is a branching tree, where the question is not how high something sits but where its line split from another. That framing matters for the last chapter of this course, which is about the evidence for those splits, and it matters here because the shape of the diagram quietly teaches an idea whether or not anyone says it aloud.
Pause and explain
Why is it wrong to call bacteria the most primitive organisms alive?
Check your reasoning after answering
Because every living organism has the same length of evolutionary history behind it — bacteria alive now are not ancestors of anything alive now. They are simpler in cell structure and superbly adapted, occupying more habitats than any other group. Primitive confuses simple structure with being early in time, which the branching tree of shared ancestry does not support.
Apply the whole unit · Compare three records
Illustrative records, not real specimens: A is a seed made of cells, dormant at the time of observation, which later germinates. B is a battery-powered toy that moves across a table. C is a cell about 2 micrometres long with a membrane and DNA, but no enclosing membrane around that DNA.
Your task
Use several characteristics to decide the living status of each, and explain why movement alone and the presence of a nucleus alone are not sufficient tests.
What your response should show
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt
Apply the set to A: made of cells, with the organisation to grow, use energy and reproduce, which germination later demonstrates — dormancy is a state, not evidence against life.
Apply the set to B: it moves and uses stored energy, and it is not made of cells, does not grow, does not reproduce and regulates nothing, so it fails nearly every characteristic.
Apply the set to C: it is a cell with a membrane and DNA, so it is an organism; the absence of an enclosing membrane around the DNA classifies it as prokaryotic rather than placing it outside life.
State why single tests fail: movement is shown by B and absent in most plants, and a nucleus is absent in every bacterium and in mature mammalian red blood cells.
A and C are living under these records; B is not. A is made of cells and carries the organisation needed to grow, use energy and reproduce, and its later germination demonstrates that directly — dormancy is a state an organism can be in, not evidence against being one, as a date palm grown in 2005 from a two-thousand-year-old seed showed. C is a cell with a membrane and its own DNA, which makes it an organism; the fact that the DNA is not enclosed in a nucleus is a classification result, placing it among the prokaryotes, and not a reason to exclude it. Bacteria are enormously successful organisms and the most numerous on Earth. B moves and draws on stored energy, and that is all: it is not made of cells, does not grow or develop, does not reproduce, and maintains no internal conditions. The two tempting single tests both fail. Movement is shown by B, which is not alive, and is largely absent in plants, which are. A nucleus is absent in every prokaryote and in mature mammalian red blood cells, so its absence classifies rather than disqualifies. The records also leave things open: they do not say whether A's cells were still carrying out any measurable metabolism while dormant, and for C they report structure without reporting reproduction, which would be the direct evidence.
Scientific references for this unit
Parent / teacher teaching notes
Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.
The key idea
Living organisms are organized into cells and carry out interacting processes such as using matter and energy, responding to conditions, maintaining internal conditions, growing, and producing new organisms across their life cycle. No single visible action is a complete definition. A moving toy is not alive because it moves, and a dormant seed does not become nonliving simply because it currently shows little visible activity.
Worked examples
Example 1 · Decide a hard case
- Apply the characteristics one by one: it carries genetic material and can be replicated, but it is not made of cells, uses no energy of its own, and regulates nothing.
- Identify which failures are decisive: having no cells and no metabolism of its own are the two that matter most, since reproduction happens only by commandeering a host.
- State the position honestly: most biologists place viruses outside the definition, while noting that they evolve and carry inherited information, so the boundary is genuinely disputed.
A good answer here names which characteristics fail, not just the verdict.
Example 2 · Classify from two observations
- Use the defining feature: DNA present without an enclosing membrane makes the cell prokaryotic.
- Check the supporting observation: 3 micrometres is within the usual prokaryotic range, and the absence of membrane-bound organelles is consistent.
- State the open questions: the micrograph shows structure at one instant, not how the cell obtains energy, what it does, or which group of prokaryotes it belongs to.
Say what the evidence settles and what it leaves open; both belong in the answer.
Avoid these mistakes
- Mistake: movement shows something is alive. A battery toy moves and is not alive; most plants do not move about and are. Movement is neither necessary nor sufficient.
- Mistake: a dormant seed is not alive. The Masada date palm seed germinated after roughly two thousand years — dormancy is a state an organism can be in, not a failure of the characteristics.
- Mistake: no nucleus means not a cell. Every bacterium lacks one and is fully alive, and mature mammalian red blood cells discard theirs; absence of a nucleus classifies rather than disqualifies.
- Mistake: classification ranks organisms from primitive to advanced. Every organism alive today has the same length of ancestry behind it; the honest shape is a branching tree, not a ladder.
Check your understanding
Why does movement alone not establish life?
Discuss the answer
Nonliving objects can move through motors, wind, or gravity. Cellular organization and interacting life processes provide stronger evidence.
Why is “all living things eat other organisms” incorrect?
Discuss the answer
Photosynthetic organisms can build sugars using inorganic inputs and light. Organisms differ in how they obtain food and energy.
Words to know
- organism
- A living thing made of one or more cells that uses energy, responds, grows, reproduces and maintains stable internal conditions — characteristics applied as a set.
- homeostasis
- Keeping internal conditions stable enough for the chemistry of life to continue; the characteristic a fire or a crystal most clearly lacks.
- prokaryotic
- Having DNA loose in the cytoplasm with no enclosing nucleus, as bacteria do; typically 1 to 5 micrometres long.
- eukaryotic
- Having DNA enclosed in a nucleus, with membrane-bound organelles; animals, plants, fungi and protists.
- classification
- Organising organisms by shared ancestry, producing a branching tree rather than a ladder from simple to advanced.
Extend your scientific explanation
Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.
In the real world
Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.
Exit check
Does a classification key by itself show ancestry?
Check your explanation after answering
Not necessarily. It identifies using chosen features; an ancestry claim requires evidence supporting inherited relationships.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.
Problem 1
Answer and explanation
Nonliving objects can move through motors, wind, or gravity. Cellular organization and interacting life processes provide stronger evidence.
Problem 2
Answer and explanation
Photosynthetic organisms can build sugars using inorganic inputs and light. Organisms differ in how they obtain food and energy.
Problem 3
Answer and explanation
Yes. Bacteria and other prokaryotes lack a membrane-enclosed nucleus but carry genetic material and perform cellular processes.
Problem 4
Answer and explanation
Not necessarily. It identifies using chosen features; an ancestry claim requires evidence supporting inherited relationships.
Problem 5
Answer and explanation
A and C are living under the supplied records. A’s cellular organization and germination support that conclusion despite its earlier dormancy. C can function without an enclosed nucleus because it is prokaryotic. B’s movement alone does not establish cellular life. The comparison relies on the supplied biological observations, not a rule that every moving object is alive.
Identify cellular organization in A and C.
Use germination as evidence of a seed’s life process.
Separate powered toy motion from cellular activity.
Recognize C as a prokaryotic organism.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 6
Answer and explanation
No. Those two categories do not exhaust biological diversity. Fungi have their own characteristics; combine cell features, nutrition, and relationship evidence rather than forcing a two-category choice.
Response criteria
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Problem 7
- No DNA at all
- DNA and ribosomes but no membrane-enclosed nucleus
- A nucleus and no membrane
- Only a plastic motor
Answer and explanation
DNA and ribosomes but no membrane-enclosed nucleus
Prokaryotes contain genetic material and ribosomes without a membrane-enclosed nucleus.
Problem 8
Answer and explanation
Living status involves functioning cellular organization and interacting life processes. Reproduction characterizes life cycles and populations; it is not required from every individual at every time.