Quiz 8 — What Is Life?

The Guide

Harder to define than you would think

Everyone can tell a cat from a stone. But write down a rule that separates them, and awkward cases start arriving: a fire, a seed in a packet, a virus. Biology's answer is not one rule. It is a short list of features, and something counts as alive when it shows all of them.

The features of living things

Made of cells. One cell or many. A living thing made of a single cell is called unicellular (uni- means one). One made of many cells is multicellular (multi- means many). This is the feature that does most of the work of ruling things out.
Takes in fuel and releases energy. Food, or sunlight. Everything else on this list costs energy.
Grows and develops. Not just larger — a caterpillar becomes something a caterpillar was not.
Responds to its surroundings. Light, heat, touch, hunger, danger.
Makes copies of itself. Reproduction, by whatever route.
Keeps its inside steady. Temperature, water, salt held within narrow limits while the outside moves around. This is called homeostasis, and Quiz 19 is about it.

Room one, Alive or Not?, adds a seventh sign: over many generations, a kind of living thing changes to suit its surroundings. That is called evolutionary adaptation. It is true of every living kind, but you cannot see it in one plant or animal on one day, so the table in this quiz leaves it out.

Why the list needs every line. Take any single feature and something non-living has it. A fire grows, spreads, and responds to a draft. A crystal grows. A machine takes in fuel and releases energy. No one feature separates living from non-living, which is exactly why the list is a list.

The awkward case worth knowing. A virus is a package of genetic material in a protein coat. It has no cells, takes in no fuel, and does nothing at all on its own. Put it inside a living cell — a host cell — and it uses that cell's machinery to make copies of itself. So it does one thing on the list and only with borrowed equipment. Most biologists do not classify viruses as living, and the argument is still running. Being able to say why it is a hard case matters more than picking a side.

The levels of organization

A body is not a bag of cells. The cells are grouped, and the groups are grouped, and each level does something the level below could not. Similar cells working together make a tissue. Different tissues working together make an organ. Organs working together make an organ system. The systems together make the organism.

A single cell has no tissues. An organism made of one cell does everything with the equipment inside that one cell — no tissues, no organs, no systems. The ladder starts and finishes on the same rung. Most living things on Earth are like this.

Worked Examples

Two questions, worked through

Example 1. A dry bean seed sits in a paper packet on a shelf for a year. Is it alive?

Run the list. It is made of cells — yes. Is it taking in fuel, growing, responding, or making copies? None of those, not while it sits there. On a strict reading it fails four of six.

But give it water and warmth and it starts doing all of them within days. Nothing was added except conditions. The seed was alive the whole time, running so slowly that the activity was hard to detect.

The lesson: the features can be paused. A dormant seed, a hibernating animal, and a frozen embryo are all alive. Ask what the thing does when conditions allow, not only what it is doing at this moment.

Example 2. A maple leaf is made of layers of similar cells that carry out photosynthesis. Which level of organization is a leaf?

Work up the ladder, naming each rung. One photosynthesis cell is a cell. A layer of similar cells working together is a tissue. A leaf is several different tissues — the outer skin, the green middle layer, the veins — combined into one working part.

A leaf is an organ. The test of an organ is not size or importance; it is whether more than one kind of tissue is working together on one job. By that test a leaf and a heart are the same kind of thing.

The Quiz

Ten questions

Answer all ten, then press the button at the bottom. Nothing is timed.

Table — questions 1 to 4

	Made of cells	Takes in fuel, releases energy	Grows	Responds to surroundings	Makes copies of itself
Rabbit	Yes	Yes	Yes	Yes	Yes
Wood fire	No	Yes	Yes	Yes	Yes
Salt crystal	No	No	Yes	No	No
Virus	No	No	No	No	Only inside a host cell

Four things checked against five features of living things.

1.According to the table, which feature does the rabbit have that the other three lack?

AGrows larger over time
BIs made of cells
CMakes copies of itself
DResponds to its surroundings

2.A student says the fire must be alive, since it grows, spreads, and responds to a draft. What does the table show the fire lacks?

AIt is not made of cells, and living things are.
BIt does not grow, and growth is needed for life.
CIt does not respond to the air around it at all.
DIt cannot take in fuel or release any energy.

3.Biologists disagree about whether a virus is alive. Which row of the table best explains the difficulty?

AA virus copies itself, but only inside a host cell.
BA virus grows larger over time, as a crystal does.
CA virus responds to light but is not made of cells.
DA virus takes in fuel and releases energy by itself.

4.What does the table as a whole suggest about deciding whether something is alive?

AA single feature, such as growth, is enough to settle it.
BAnything that can make copies of itself must be alive.
CNothing outside biology can be compared with living things.
DLiving things show all of these features, not just one.

Diagram — questions 5 to 7

5.According to the diagram, which level comes directly above tissue?

ACell
BOrgan
COrgan system
DOrganism

6.The diagram gives the heart, the blood vessels, and the blood together as an example of which level?

ACell
BOrgan
COrgan system
DTissue

7.The lining of the stomach is a sheet of similar cells working together. Which level is that?

ACell
BOrgan
COrgan system
DTissue

Passage — questions 8 to 10

A student puts a drop of pond water under a microscope. She finds a small oval organism, made of one cell, swimming steadily toward a bright spot at the edge of the slide.

Over the next hour it takes in a smaller particle from the water, becomes slightly larger, and then pinches in the middle and separates into two organisms of equal size, which swim apart.

8.Which feature of living things does the organism show by swimming toward the bright spot?

AIt grows and develops over several hours.
BIt makes copies of itself.
CIt responds to its surroundings.
DIt takes in fuel from the water.

9.What is an organism called when it carries out all its life processes inside one cell?

AMulticellular organism
BOrgan system
CTissue
DUnicellular organism

10.What does this organism show about the levels of organization?

ALiving things need tissues and organs to stay alive.
BA single cell can do all the work of staying alive.
COrgan systems form whenever a cell divides in two.
DOrganisms with organs are the ones that sense light.

Send this line to your teacher

The line records which questions you missed and which answer you chose. That is more useful to your teacher than the score, because it shows where a question went wrong. If a question felt unclear even though you got it right, add its number with a question mark — for example 5? — before you send it.

Score ______ / 10 Missed — write the question number and the letter you chose:
______________________________________________________________

The Key

Answers, and the trap in each one

1. B — is made of cells. Read down each column and look for the one where the rabbit says yes and the other three say no. Only the first column does that. A, C, and D are all things the rabbit does, which is what makes them tempting. The question asked which one the others lack, and the fire matches the rabbit on four of the five columns.

2. A — it is not made of cells. The student's three observations are correct and the table agrees with all of them. The fire fails on one column only, and that is the column that settles it. B, C, and D each deny something the table says the fire does. If a wrong answer contradicts the stimulus, it is wrong no matter how sensible it sounds — and a fire that grows and spreads can look alive.

3. A — it copies itself, but only inside a host cell. A virus fails four columns outright. The fifth is neither yes nor no, and that is the whole argument: it does the thing on the list using equipment it does not own. B, C, and D each claim the virus does something the table marks as no. Read the virus row before choosing — three of these four options describe a virus that does not exist.

4. D — living things show all of these features. Every column has at least one non-living thing saying yes. Growth alone lets in the crystal. Copying alone lets in the fire. The list works as a whole and not one line at a time. A is the way most people reason before meeting the table, and the table exists to break it. C sounds cautious but the whole page is a comparison between living and non-living things, and it works.

5. B — organ. Follow the arrow up from tissue. Similar cells make a tissue; several kinds of tissue working on one job make an organ. A is the level below rather than above. When a diagram runs bottom to top, "above" on the page and "later in the sequence" mean the same thing here — but check the arrows rather than assuming, since some diagrams run the other way.

6. C — organ system. The example sits on that rung in the diagram. A system is organs working together: heart, blood vessels, and blood, all on one job. B is the rung below, where the heart sits by itself. The heart is one organ; the circulatory system is the heart plus everything it pumps through.

7. D — tissue. Similar cells, working together, forming a sheet. That is the definition of a tissue, and this item is asking you to apply it to something the diagram never mentioned. B tempts because the stomach is an organ and the word "stomach" is in the question. The lining is not the stomach. An organ is built from several tissues, and this is one of them.

8. C — it responds to its surroundings. Light appeared at the edge of the slide and the organism moved toward it. Something outside changed, and the organism acted because of it. A, B, and D are all things this organism does later in the passage. They are true and they answer a different question. Check which sentence the item pointed at before choosing.

9. D — unicellular organism. One cell doing the whole job. The word breaks into "uni," meaning one, and "cellular," meaning of cells. A is the opposite word, which is why it tempts. Most living things on Earth are unicellular, which makes this the ordinary case rather than the exception.

10. B — a single cell can do all the work of staying alive. This organism took in fuel, responded to light, grew, and reproduced, with no tissues, no organs, and no systems anywhere in it. The ladder is how large bodies are built, not a requirement for being alive. A is the belief the passage was written to correct, and it is easy to hold after studying a diagram full of hearts and systems. The diagram describes an animal. It does not describe every living thing.

Your Score

What the number means

8 to 10	Solid. Check topic 8 on your map and go on to Quiz 9, on the cell itself.
6 or 7	Close. Read the whole key, then take this again in a few days before you check the box.
5 or fewer	Worth another pass. The list of features and the five rungs are short enough to write out from memory, and doing that once fixes most of it.

Misses on 1, 5, or 6 mean a table or diagram was read too quickly. Misses on 2, 3, or 4 mean the list of features is not yet solid, and writing the six lines out by hand will fix that faster than rereading them. A miss on 7 or 10 is the useful kind: both ask you to take a rule off the page and use it on something the page never mentioned, which is what the test does most.

The room that walks the same ground

This quiz has a companion you work with your hands: Alive or Not?, the first of the three Life Science rooms. It opens with two films of the inside of a cell, sets out the same features of life this guide does, and then hands you eight cases to sort — a candle flame, a virus, a seed, a salt crystal, a robot vacuum, a mule, a bacterium, a mushroom — and checks each one against the whole list. The awkward cases in the guide above are the ones the room makes you decide. Either order works: the room first, for the arguing, or this quiz first, for the reading.

The plate for this quiz

Part II's gallery image is one of Ernst Haeckel's marine plates from Art Forms in Nature, published in 1904 — a page of radiolarians, single-celled organisms that build glass skeletons around themselves, arranged by Haeckel in perfect symmetry because he could not resist. They are drawn from real specimens dredged out of the ocean, and they are also compositions.

They belong here for a reason beyond beauty. Every organism on that page is one cell. No tissues, no organs, no systems, and yet each builds a mineral shell more intricate than anything in this quiz. Public domain by age; the plates are widely available at high resolution.