Answer key · Unit 1

Cells and life.

Each lesson’s practice questions, their correct answers and the mistakes they spot. Answers read off the Scene use the same setting the learner sees.

Misconception catalogue

Answer key 7

Lesson 1 of 7 · About 25 minutes

The signs of life

What makes something alive: seven signs a living thing shows all at once, a flame that imitates life, a virus on the border, and one bacterium that becomes thousands.

Open the lesson

The 7 steps and their answers

Step 1 of 7
Open this step
Seven signs
No practice question in this step.
The Scene at this step
Specimen
Mouse
Time watched
0 h
Specimen / Evolution
Evolution
Step 2 of 7
Open this step
The candle flame
Practice · Predict, then look
Question
The candle flame is at the front. It feeds on wax, uses oxygen, grows as the wick is exposed, and even “reproduces” when it lights another candle. What will the verdict be once all seven signs are checked?

Correct answer
- Correct Not alive
Setup, choices and feedback
The Scene at this step
Specimen
Candle flamefixed
Time watched
0 h
Specimen / Evolution
Specimenfixed
Choices
- Correct Not alive
- Wrong On the border
  Feedback
  Only something that truly has at least one sign sits on the border. Count the flame’s blue lamps.
- Wrong Alive
  Misconception: Takes anything that moves, grows or uses fuel to be alive, like a candle flame.
  Feedback from the catalogue
  A flame moves, grows, “eats” wax and “breathes” oxygen, yet it is not alive. No single sign is enough: a flame is not made of cells, it does not keep itself steady when conditions change, and its “offspring” inherit nothing from it. A living thing shows all seven signs, first of all that it is made of one cell or more.
Once the prediction is locked in, the Scene changes
Specimen / Evolution
Evolutionfixed
The Scene then shows: 0 · 0 not alive, 1 on the border, 2 alive/nodes/bench/outputs/verdict
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Not alive: none of the flame’s seven lamps lights blue. Four light amber, signs it only imitates: it “uses energy”, “grows”, “reproduces” and holds a steady shape, but with no cells, no response and no heredity.

Hint
Mind the difference between a real sign and something that only looks like one.

Worked solution
1. The first sign is cells: a living thing is made of one cell or more. A flame has none.
2. A flame’s “feeding” and “breathing” are burning, a chemical reaction, not chemistry the thing runs and controls.
3. A flame “grows” and “reproduces”, but a new flame inherits nothing from the old one, so there is no evolution.
4. It has not one sign in full, so the verdict is: not alive.
If you chose “alive”, you let movement and growth decide. No single sign is enough.
Step 3 of 7
Open this step
Imitations
Practice · Calculate
Question
All seven of the flame’s lamps are lit now. In how many signs does it only imitate life (an amber lamp)?

Correct answer
4 ± 0.1
Setup, choices and feedback
The Scene at this step
Specimen
Candle flamefixed
Time watched
0 h
Specimen / Evolution
Evolutionfixed
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Signs it only imitates, or has only inside a host/nodes/bench/outputs/lookAlikes
Wrong answers it recognises
- 7 ± 0.1
  Misconception: Takes anything that moves, grows or uses fuel to be alive, like a candle flame.
  Feedback from the catalogue
  A flame moves, grows, “eats” wax and “breathes” oxygen, yet it is not alive. No single sign is enough: a flame is not made of cells, it does not keep itself steady when conditions change, and its “offspring” inherit nothing from it. A living thing shows all seven signs, first of all that it is made of one cell or more.
Feedback for a right answer
Four: energy (burning), growth, reproduction (lighting another candle) and holding a steady shape. For the other three, cells, response and evolution, the lamp is red.

Hint
Count only the amber lamps on the flame’s placard.

Worked solution
1. Go along the flame’s placard, sign by sign.
2. Cells: red. Energy: amber. Growth: amber. Response: red. Reproduction: amber. Homeostasis: amber. Evolution: red.
3. Amber lamps: $4$ ⁠.
If you answered $7$ ⁠, you counted everything the flame does as if it were a sign of life.
Step 4 of 7
Open this step
The virus
Practice · Predict, then look
Question
Now the virus is at the front. How many of the seven signs do you think it has in full (a blue lamp)?

Correct answer
1 ± 0.1
A number prediction counts as right within ± 0.1
Setup, choices and feedback
The Scene at this step
Specimen
Virusfixed
Time watched
0 h
Specimen / Evolution
Specimenfixed
Once the prediction is locked in, the Scene changes
Specimen / Evolution
Evolutionfixed
The Scene then shows: 1 · Signs of life the front specimen fully has, out of 7/nodes/bench/outputs/signs
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Just one: evolution. Viruses change over generations and the ones that copy best are selected, like each year’s new flu and coronavirus strains. They reproduce only inside a host cell, so that lamp is amber.

Hint
Think what happens to a virus left alone on glass for a week, and what happens to viruses over many generations.

Worked solution
1. Cells: none. A virus is genetic material in a protein coat.
2. Energy, growth, response and homeostasis: on its own it does none of these.
3. Reproduction: only with a host cell’s help, so that is a look-alike.
4. Evolution: yes. Its copies inherit its genetic material, with changes, and selection acts on them.
5. Signs in full: $1$ ⁠.
Step 5 of 7
Open this step
On the border
Practice · Choose
Question
Why does the virus sit on the border rather than with the living?

Correct answer
- Correct It has no cells and no chemistry of its own: it reproduces only inside someone else’s living cell.
Setup, choices and feedback
The Scene at this step
Specimen
Virus
Time watched
24 h
Specimen / Evolution
Evolution
Choices
Shown in a random order.
- Correct It has no cells and no chemistry of its own: it reproduces only inside someone else’s living cell.
- Wrong It is too small to be alive.
  Feedback
  Size does not decide. Some tiny bacteria are smaller than some large viruses, and they are fully alive. What a virus lacks is a cell of its own.
- Wrong It is a very small bacterium that has lost its tail.
  Misconception: Thinks a virus is a tiny cell, a kind of small bacterium.
  Feedback from the catalogue
  A virus is not a cell. It is genetic material in a protein coat, with no ribosomes and no chemistry of its own, tens of times smaller than a bacterium. It cannot grow or reproduce alone: it gets into a living cell and uses the cell’s machinery to make copies of itself. That is why it sits on the border between living and non-living, and why antibiotics, which attack the parts of bacterial cells, do nothing to viruses.
- Wrong It does not change over generations.
  Feedback
  That is the very sign it has: viruses change fast, which is why the flu vaccine is updated every year.
Feedback for a right answer
Right. A virus has genetic material and evolves, but it has no cell, ribosomes or energy of its own. Outside a cell it is an inert particle; inside one it takes over the cell’s machinery and copies itself.

Worked solution
1. The first condition for life in cell theory: made of cells. A virus has no cell.
2. With no ribosomes and no enzymes for energy, it can build nothing on its own.
3. It does carry genetic material and changes over generations, so it is not wholly non-living.
4. Hence “on the border”.
A virus is not a small bacterium: a bacterium is a whole, living cell.
Step 6 of 7
Open this step
One bacterium becomes many
Practice · Calculate
Question
The bacterium is at the front, and 2 h have passed since the start. At body temperature E. coli divides every 20 min. How many cells are there now, starting from one?

Correct answer
64 ± 0.5
Setup, choices and feedback
The Scene at this step
Specimen
Bacteriumfixed
Time watched
2 hfixed
Specimen / Evolution
Specimen
Accepted within
± 0.5
Computed from the Scene at this step’s setting
Cells grown from the one E. coli at the start/nodes/bench/outputs/bacteria
Wrong answers it recognises
- 6 ± 0.1
  Misconception: Gives the number of doublings as the number of cells.
  Feedback from the catalogue
  The number of doublings is not the number of cells. First count how many times the cells divided, $n$ ⁠, then work out the cells: $2^n$ ⁠. Six doublings make $2^6 = 64$ ⁠ cells, not $6$ ⁠.
- 12 ± 0.1
  Misconception: Adds a fixed number of cells each doubling time, instead of doubling the whole population.
  Feedback from the catalogue
  At each division every cell becomes two, so the number doubles; it does not grow by a fixed amount. After $n$ ⁠ divisions there are $2^n$ ⁠ cells: $2, 4, 8, 16, 32, 64$ ⁠. That is exponential growth, and it is why a colony from one cell becomes visible within hours.
- 7 ± 0.1
  Misconception: Adds a fixed number of cells each doubling time, instead of doubling the whole population.
  Feedback from the catalogue
  At each division every cell becomes two, so the number doubles; it does not grow by a fixed amount. After $n$ ⁠ divisions there are $2^n$ ⁠ cells: $2, 4, 8, 16, 32, 64$ ⁠. That is exponential growth, and it is why a colony from one cell becomes visible within hours.
Feedback for a right answer
$2^6 = 64$ ⁠ cells: six divisions in two hours, and the number doubles at each. The colony is still only 8 µm wide, far below what the eye can see.

Hint
How many divisions fit in two hours? And at each one, what happens to the number of cells?

Worked solution
1. Divisions: $\dfrac{120\ \mathrm{min}}{20\ \mathrm{min}} = 6$ ⁠.
2. Each division doubles the number: $1 \to 2 \to 4 \to 8 \to 16 \to 32 \to 64$ ⁠.
3. That is $2^6 = 64$ ⁠ cells.
If you answered $6$ ⁠, you gave the number of divisions. If you answered $7$ ⁠ or $12$ ⁠, you added a fixed number of cells each time instead of doubling.
Step 7 of 7
Open this step
A colony
Practice · Reach the target
Question
Change Time watched until the colony grown from the one bacterium holds 4,096 cells, then press Check.

Goal
- 4096 ± 0.5
  At the start of the step: 1
Setup, choices and feedback
The Scene at this step
Specimen
Bacteriumfixed
Time watched
0 h
Specimen / Evolution
Specimen
Read from the Scene at
Cells grown from the one E. coli at the start/nodes/bench/outputs/bacteria
One setting that reaches it
Time watched
4 h
Feedback for a right answer
Four hours: $12 \times 20 = 240$ ⁠ minutes, and $2^{12} = 4096$ ⁠. Four hours later there will be about 17 million.

Feedback for a wrong answer
Not yet. How many times must you double $1$ ⁠ to reach $4096$ ⁠?

Hint
$4096 = 2^{12}$ ⁠. How long do twelve divisions of 20 min take?

Worked solution
1. $4096 = 2^{12}$ ⁠, so $12$ ⁠ divisions.
2. Each division takes 20 min: $12 \times 20 = 240$ ⁠ minutes.
3. $240$ ⁠ minutes is 4 h. Set Time watched to 4 h.
At 3.75 h there are only $2^{11} = 2048$ ⁠ cells: one division short.

Lesson 2 of 7 · About 20 minutes

Inside a cell

Cell theory, and what is inside an animal cell: membrane, cytoplasm, nucleus, mitochondria and ribosomes, and the job of each.

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The 5 steps and their answers

Step 1 of 5
Open this step
Cell theory
Practice · Choose
Question
According to cell theory, where does every new cell come from?

Correct answer
- Correct From another cell that divided in two.
Setup, choices and feedback
The Scene at this step
Kind of cell
Animalfixed
Look at
Everything
Whole cell / Cut open
Whole cell
Choices
Shown in a random order.
- Correct From another cell that divided in two.
- Wrong From suitable non-living matter, like mud or rotting food.
  Misconception: Believes living things can arise from non-living matter, as maggots were once thought to come from rotting meat.
  Feedback from the catalogue
  Cell theory says every cell comes from another cell that divided. For centuries people thought flies “arose” from rotting meat and mice from rags, until experiments such as Louis Pasteur’s showed that nothing grows in a sealed, sterilised flask: life always comes from life that was there before, if only a single bacterium.
- Wrong From organelles that come together in the fluid between cells.
  Feedback
  Organelles do not live outside a cell, and do not assemble into new cells. A new cell gets them from its mother cell when it divides.
Feedback for a right answer
Right. Cell theory says three things: all living things are made of cells, the cell is the smallest unit of life, and every cell comes from an earlier cell. The bacterium on the last screen doubled exactly like that.

Worked solution
1. Cell theory: every living thing is made of cells, and the cell is the unit of life.
2. Experiments in sealed, sterilised flasks showed life does not appear from non-living matter.
3. So: every cell comes from an earlier cell that divided.
The idea that life arises by itself from rotting meat is “spontaneous generation”, and Pasteur disproved it.
Step 2 of 5
Open this step
The power station
Practice · Reach the target
Question
In Look at, pick the organelle where the cell breaks sugar down with oxygen and releases its energy, then press Check.

Goal
- 3 ± 0.1
  At the start of the step: 0
Setup, choices and feedback
The Scene at this step
Kind of cell
Animalfixed
Look at
Everything
Whole cell / Cut open
Cut open
Read from the Scene at
The part in focus: 0 everything, 1 membrane, 2 nucleus, 3 mitochondria, 4 ribosomes, 5 chloroplasts, 6 vacuole/nodes/cell/outputs/organelle
One setting that reaches it
Look at
Mitochondria
Feedback for a right answer
The mitochondria. Their inner membrane is folded into ridges, which leaves room for many respiration enzymes. A hard-working cell, like a muscle cell, has hundreds to thousands of them.

Feedback for a wrong answer
Not that one. The organelle that releases energy from food is sometimes called the cell’s power station.

Hint
Look for the long bodies, folded inside, scattered through the cytoplasm.

Worked solution
1. Respiration: sugar and oxygen become carbon dioxide and water, and the energy released is stored in ATP.
2. Most of those steps happen in the mitochondria.
3. Set Look at to Mitochondria.
The nucleus keeps the DNA and the ribosomes build proteins: neither releases energy from food.
Step 3 of 5
Open this step
The protein factories
Practice · Reach the target
Question
Now pick in Look at the structures where the cell joins amino acids into proteins, following the instructions that come from the nucleus, then press Check.

Goal
- 4 ± 0.1
  At the start of the step: 0
Setup, choices and feedback
The Scene at this step
Kind of cell
Animalfixed
Look at
Everything
Whole cell / Cut open
Cut open
Read from the Scene at
The part in focus: 0 everything, 1 membrane, 2 nucleus, 3 mitochondria, 4 ribosomes, 5 chloroplasts, 6 vacuole/nodes/cell/outputs/organelle
One setting that reaches it
Look at
Ribosomes
Feedback for a right answer
The ribosomes. Each one reads a copy of a gene, a messenger RNA, and joins the amino acids in the order it spells out. One cell holds millions.

Feedback for a wrong answer
Not yet. Look for the smallest and most numerous structures.

Hint
They are the smallest things in the picture, tiny dots, very many, some on the network round the nucleus.

Worked solution
1. A protein’s instructions are written in a gene, in the DNA in the nucleus.
2. A copy of the gene leaves the nucleus for the cytoplasm.
3. There the ribosomes read it and build the protein.
4. Set Look at to Ribosomes.
Step 4 of 5
Open this step
An organelle with no membrane
Practice · Calculate
Question
Look at is fixed on the ribosomes, and the cell is cut open. How many membranes wrap each ribosome?

Correct answer
0 ± 0.1
Setup, choices and feedback
The Scene at this step
Kind of cell
Animalfixed
Look at
Ribosomesfixed
Whole cell / Cut open
Cut open
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Membranes round the part in focus (0 with no focus, or when it is missing)/nodes/cell/outputs/membranes
Wrong answers it recognises
- 1 ± 0.1
  Misconception: Thinks every part of a cell is wrapped in a membrane, ribosomes included.
  Feedback from the catalogue
  Not every organelle has a membrane. A ribosome is a clump of proteins and RNA with no membrane round it at all, which is why bacteria, which have no membrane-bound organelles, still have ribosomes. The nucleus, mitochondria and chloroplasts have two membranes; the vacuole and the cell itself one.
- 2 ± 0.1
  Misconception: Thinks every part of a cell is wrapped in a membrane, ribosomes included.
  Feedback from the catalogue
  Not every organelle has a membrane. A ribosome is a clump of proteins and RNA with no membrane round it at all, which is why bacteria, which have no membrane-bound organelles, still have ribosomes. The nucleus, mitochondria and chloroplasts have two membranes; the vacuole and the cell itself one.
Feedback for a right answer
None. A ribosome is a clump of proteins and RNA with no membrane. That is why every cell has ribosomes, even bacteria, which have no membrane-bound organelles.

Hint
Compare with the nucleus, round which you can see an envelope with pores.

Worked solution
1. Organelles like the nucleus and mitochondria are wrapped in a double membrane, and the vacuole in one.
2. A ribosome is not a bag: it is a molecular machine of proteins and RNA.
3. Membranes: $0$ ⁠.
If you answered $1$ ⁠ or $2$ ⁠, you assumed every part of a cell has a membrane.
Step 5 of 5
Open this step
The membrane
Practice · Choose
Question
What is the main job of the Membrane?

Correct answer
- Correct To choose what enters and leaves the cell.
Setup, choices and feedback
The Scene at this step
Kind of cell
Animalfixed
Look at
Membrane
Whole cell / Cut open
Whole cell
Choices
Shown in a random order.
- Correct To choose what enters and leaves the cell.
- Wrong To keep a rigid shape, like a wall.
  Misconception: Thinks the plant cell wall takes the place of the cell membrane.
  Feedback from the catalogue
  The wall does not replace the membrane; it wraps round it. The cellulose wall is strong and lets almost anything through: it gives the shape and stops the cell from bursting. The membrane, pressed against it from inside, is what chooses what enters and leaves the cell, in a plant as in an animal.
- Wrong To make the cell’s energy.
  Feedback
  The mitochondria release the energy. The membrane does use energy, to pump substances in against the flow.
- Wrong To keep the DNA.
  Feedback
  The nucleus keeps the DNA, inside its own envelope.
Feedback for a right answer
Right. The membrane is a thin double layer of fats with proteins in it that act as channels and pumps. Water, oxygen and carbon dioxide pass through; sugars and ions only through the proteins. It is flexible, not rigid.

Worked solution
1. The membrane wraps the whole cell and separates the cytoplasm from the surroundings.
2. It lets some substances through and not others: it is partially permeable.
3. So its job: to control traffic in and out.
The membrane is flexible. A plant cell’s rigid shape comes from its wall, outside the membrane.

Lesson 3 of 7 · About 20 minutes

Plant and animal cells

What a plant cell has that an animal cell lacks: a wall, chloroplasts and a large vacuole. And why a plant has mitochondria too.

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The 4 steps and their answers

Step 1 of 4
Open this step
The organelle that catches light
Practice · Reach the target
Question
Make the Scene show, in Look at, the organelle that catches light and builds sugar with it. Change Kind of cell too if you need to, then press Check.

Goal
- 5 ± 0.1
  At the start of the step: 0
- 1 ± 0.1
  At the start of the step: 1
Setup, choices and feedback
The Scene at this step
Kind of cell
Animal
Look at
Everything
Whole cell / Cut open
Cut open
Read from the Scene at
The part in focus: 0 everything, 1 membrane, 2 nucleus, 3 mitochondria, 4 ribosomes, 5 chloroplasts, 6 vacuole/nodes/cell/outputs/organelle
Read from the Scene at
1 when the part in focus is in this kind of cell, 0 when it is missing (chloroplasts or the vacuole in an animal cell)/nodes/cell/outputs/present
One setting that reaches it
Kind of cell
Plant
Look at
Chloroplasts
Feedback for a right answer
Chloroplasts, in a plant cell. Inside are stacks of green membrane discs, holding the chlorophyll that catches the light.

Feedback for a wrong answer
Not yet. Is the organelle you chose even in the kind of cell on screen?

Hint
An animal cell has no such organelle.

Worked solution
1. The organelle that catches light and builds sugar is the chloroplast.
2. Only plant cells (and algae) have chloroplasts.
3. Set Kind of cell to Plant and Look at to Chloroplasts.
If you chose chloroplasts in an animal cell, the focus points at something the cell does not have.
Step 2 of 4
Open this step
A plant needs energy too
Practice · Predict, then look
Question
The plant cell has chloroplasts that make sugar. When we switch to a plant cell and look at the mitochondria, what will we see?

Correct answer
- Correct The plant cell has mitochondria too.
Setup, choices and feedback
The Scene at this step
Kind of cell
Animalfixed
Look at
Mitochondriafixed
Whole cell / Cut open
Cut open
Choices
- Correct The plant cell has mitochondria too.
- Wrong It has none: the chloroplasts replace them.
  Misconception: Thinks plant cells have chloroplasts instead of mitochondria, so plants do not respire.
  Feedback from the catalogue
  Plant cells have chloroplasts and mitochondria. In the chloroplasts the plant builds sugar with light; in the mitochondria it breaks the sugar down to release energy the cell can use, like any other cell, by day and by night. A root, which has no chloroplasts at all, lives on that respiration.
Once the prediction is locked in, the Scene changes
Kind of cell
Plantfixed
Look at
Mitochondriafixed
The Scene then shows: 1 · 1 when the part in focus is in this kind of cell, 0 when it is missing (chloroplasts or the vacuole in an animal cell)/nodes/cell/outputs/present
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Right: the same eight mitochondria, in a plant cell as in an animal cell. The chloroplasts build sugar; the mitochondria break it down and release its energy for the cell to use.

Worked solution
1. Every living cell needs energy all the time, at night too, when there is no light.
2. Respiration releases that energy, in the mitochondria.
3. So plant cells have mitochondria too, beside their chloroplasts.
If you thought not, think of a root: it has no chloroplasts, and lives on sugar from the leaves burned in its own mitochondria.
Step 3 of 4
Open this step
Two membranes
Practice · Calculate
Question
This is a plant cell, and Look at is fixed on the chloroplasts. How many membranes wrap each chloroplast on the outside (not counting the discs inside)?

Correct answer
2 ± 0.1
Setup, choices and feedback
The Scene at this step
Kind of cell
Plantfixed
Look at
Chloroplastsfixed
Whole cell / Cut open
Cut open
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Membranes round the part in focus (0 with no focus, or when it is missing)/nodes/cell/outputs/membranes
Feedback for a right answer
Two, like a mitochondrion. That is one of the clues that chloroplasts and mitochondria were once free-living bacteria swallowed by a larger cell: they have a double membrane, their own looped DNA, and bacteria-like ribosomes.

Hint
Like the mitochondria and the nucleus.

Worked solution
1. A chloroplast is wrapped in an envelope of two membranes, outer and inner.
2. The green discs inside it (the thylakoids) are a separate membrane system and are not counted here.
3. The answer: $2$ ⁠.
Step 4 of 4
Open this step
Side by side
Practice · Choose
Question
Choose all the structures a plant cell has and an animal cell lacks.

Correct answer
- Correct A cell wall
- Correct Chloroplasts
- Correct A large central vacuole
Setup, choices and feedback
The Scene at this step
Kind of cell
Plant
Look at
Everything
Whole cell / Cut open
Cut open
Choices
Shown in a random order. More than one choice is correct: every one must be selected.
- Correct A cell wall
- Correct Chloroplasts
- Correct A large central vacuole
- Wrong Mitochondria
  Misconception: Thinks plant cells have chloroplasts instead of mitochondria, so plants do not respire.
  Feedback from the catalogue
  Plant cells have chloroplasts and mitochondria. In the chloroplasts the plant builds sugar with light; in the mitochondria it breaks the sugar down to release energy the cell can use, like any other cell, by day and by night. A root, which has no chloroplasts at all, lives on that respiration.
- Wrong A cell membrane
  Misconception: Thinks the plant cell wall takes the place of the cell membrane.
  Feedback from the catalogue
  The wall does not replace the membrane; it wraps round it. The cellulose wall is strong and lets almost anything through: it gives the shape and stops the cell from bursting. The membrane, pressed against it from inside, is what chooses what enters and leaves the cell, in a plant as in an animal.
- Wrong A nucleus
  Feedback
  Both kinds of cell have a nucleus: both are eukaryotic.
Feedback for a right answer
A wall, chloroplasts and a large central vacuole. Everything else, the membrane, nucleus, mitochondria and ribosomes, both kinds share.

Worked solution
1. Switch Kind of cell between Animal and Plant and watch what appears.
2. What appears: the fibrous wall outside, the green chloroplasts and the big vacuole that pushes the nucleus aside.
3. The membrane, nucleus and mitochondria stay in both.
A plant cell has a wall and a membrane, chloroplasts and mitochondria.

Lesson 4 of 7 · About 25 minutes

A bacterial cell

A cell with no nucleus: how a bacterium is built, splitting in two, growth that doubles itself, and the effect of temperature.

Open the lesson

The 5 steps and their answers

Step 1 of 5
Open this step
A different cell
Practice · Choose
Question
Look at the cut-open bacterium. Choose all the structures it has.

Correct answer
- Correct A cell membrane
- Correct A cell wall
- Correct Ribosomes
- Correct DNA
Setup, choices and feedback
The Scene at this step
Temperature
310.1 K
Ampicillin
0 mg/Lhidden
Newborn cell / Division
0.3 (Newborn cell → Division)
Choices
Shown in a random order. More than one choice is correct: every one must be selected.
- Correct A cell membrane
- Correct A cell wall
- Correct Ribosomes
- Correct DNA
- Wrong A nucleus
  Misconception: Thinks a bacterium keeps its DNA in a nucleus, like our cells.
  Feedback from the catalogue
  A bacterium has no nucleus. Its DNA, one long loop plus a few small plasmids, lies free in the cytoplasm, in a region called the nucleoid. Cells like that, with no nucleus and no membrane-bound organelles, are called prokaryotic; ours, and those of plants and fungi, with a nucleus, are eukaryotic.
- Wrong Mitochondria
  Feedback
  A bacterium has no mitochondria. It makes its energy at its own cell membrane, which holds the same enzymes our mitochondria do.
Feedback for a right answer
A membrane, a wall, ribosomes and DNA, plus flagella, pili and plasmids. It has no nucleus and no membrane-bound organelle at all: it is a prokaryotic cell.

Worked solution
1. The envelope: an outer membrane, a thin layer of wall and an inner membrane. There is a membrane and a wall.
2. Inside: cytoplasm crowded with ribosomes, and a tangle of DNA wrapped in no membrane.
3. There is no nucleus and no mitochondrion.
A bacterium’s DNA is not in a nucleus: it lies free in the cytoplasm.
Step 2 of 5
Open this step
Splitting in two
Practice · Predict, then look
Question
The bacterium is born 2 µm long. Before it splits in two it doubles everything inside it, its DNA included. How long will it be just before it divides?

Correct answer
4 µm ± 5%
A number prediction counts as right within ± 5%
Setup, choices and feedback
The Scene at this step
Temperature
310.1 Kfixed
Ampicillin
0 mg/Lhidden
Newborn cell / Division
Newborn cellfixed
Once the prediction is locked in, the Scene changes
Newborn cell / Division
Divisionfixed
The Scene then shows: 4 µm · Length of the rod now: 2 µm newborn, 4 µm as it divides/nodes/cell/outputs/length
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
4 µm: twice its birth length, so each of the two daughter cells starts life 2 µm long. This is binary fission: the cell copies its DNA, lengthens, and pinches in the middle.

Hint
Each daughter cell must be born the size its mother was born.

Worked solution
1. At the end there are two cells, each the length of a newborn, 2 µm.
2. The width stays 1 µm, so all the growth is in length.
3. Just before splitting: $2 \times 2 = 4$ ⁠, that is 4 µm.
Step 3 of 5
Open this step
Within an hour
Practice · Calculate
Question
Temperature is fixed at 37 °C, where E. coli divides every 20 min. How many cells will there be after an hour, starting from one?

Correct answer
8 ± 0.1
Setup, choices and feedback
The Scene at this step
Temperature
310.1 Kfixed
Ampicillin
0 mg/Lhidden
Newborn cell / Division
Newborn cell
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Cells grown from one in an hour under the dose (below 1: dying)/nodes/cell/outputs/cellsAfterHour
Wrong answers it recognises
- 3 ± 0.1
  Misconception: Gives the number of doublings as the number of cells.
  Feedback from the catalogue
  The number of doublings is not the number of cells. First count how many times the cells divided, $n$ ⁠, then work out the cells: $2^n$ ⁠. Six doublings make $2^6 = 64$ ⁠ cells, not $6$ ⁠.
- 6 ± 0.1
  Misconception: Adds a fixed number of cells each doubling time, instead of doubling the whole population.
  Feedback from the catalogue
  At each division every cell becomes two, so the number doubles; it does not grow by a fixed amount. After $n$ ⁠ divisions there are $2^n$ ⁠ cells: $2, 4, 8, 16, 32, 64$ ⁠. That is exponential growth, and it is why a colony from one cell becomes visible within hours.
- 4 ± 0.1
  Misconception: Adds a fixed number of cells each doubling time, instead of doubling the whole population.
  Feedback from the catalogue
  At each division every cell becomes two, so the number doubles; it does not grow by a fixed amount. After $n$ ⁠ divisions there are $2^n$ ⁠ cells: $2, 4, 8, 16, 32, 64$ ⁠. That is exponential growth, and it is why a colony from one cell becomes visible within hours.
Feedback for a right answer
$2^3 = 8$ ⁠. And in eight hours: $2^{24}$ ⁠, about 17 million, as the chart beside the Scene shows.

Hint
Three divisions. What happens to the number at each?

Worked solution
1. An hour holds $\dfrac{60}{20} = 3$ ⁠ divisions.
2. Each one doubles: $1 \to 2 \to 4 \to 8$ ⁠.
3. After an hour: $2^3 = 8$ ⁠ cells.
If you answered $3$ ⁠, you gave the number of divisions. If you answered $4$ ⁠ or $6$ ⁠, you added instead of doubling.
Step 4 of 5
Open this step
In the fridge
Practice · Predict, then look
Question
We move the bacterium to the Fridge, 4 °C. What will happen to it?

Correct answer
- Correct It stops dividing, but stays alive.
Setup, choices and feedback
The Scene at this step
Temperature
310.1 Kfixed
Ampicillin
0 mg/Lhidden
Newborn cell / Division
0.5 (Newborn cell → Division)
Choices
- Correct It stops dividing, but stays alive.
- Wrong The cold kills it.
  Misconception: Thinks a fridge kills bacteria.
  Feedback from the catalogue
  Cold does not kill most bacteria; it only stops or slows their dividing. In a fridge their enzymes work too slowly, and they wait. When the food warms up again, they start dividing again. That is why a fridge keeps food only for a while, and why to kill bacteria we cook or boil.
- Wrong It divides faster, because it likes the cold.
  Feedback
  Some bacteria like the cold, but E. coli lives in our gut at 37 °C. In the cold its enzymes almost stop.
Once the prediction is locked in, the Scene changes
Temperature
277.1 Kfixed
The Scene then shows: 1 · Cells grown from one in an hour under the dose (below 1: dying)/nodes/cell/outputs/cellsAfterHour
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
It is still one cell after an hour, and the chart’s line lies flat: below about 5 °C it does not grow at all. It is not dead: back in the warm, it will divide again. That is why a fridge slows food spoiling but does not sterilise it.

Worked solution
1. Division depends on enzymes, and cold slows them hugely.
2. In the model, below about 5 °C the rate drops to zero: after an hour there is still one cell.
3. The cell is not harmed; it waits.
Cold does not kill most bacteria. Killing them takes heat, as in cooking.
Step 5 of 5
Open this step
Warmest is fastest?
Practice · Reach the target
Question
Find the Temperature at which the bacterium divides fastest, the one that turns one cell into the most cells in an hour, then press Check.

Goal
- 9.18 ± 0.1
  At the start of the step: 1.636
Setup, choices and feedback
The Scene at this step
Temperature
293.1 K
Ampicillin
0 mg/Lhidden
Newborn cell / Division
0.5 (Newborn cell → Division)
Read from the Scene at
Cells grown from one in an hour under the dose (below 1: dying)/nodes/cell/outputs/cellsAfterHour
One setting that reaches it
Temperature
313.1 K
Feedback for a right answer
At 40 °C it divides about every 19 min: more than nine cells an hour. A little above that its enzymes start to come apart, and from 48 °C it does not grow at all.

Feedback for a wrong answer
Not yet. Try a few degrees each way and see when the chart’s line is steepest.

Hint
A little above body temperature. Watch the chart as you move it.

Worked solution
1. Start at room temperature and warm slowly.
2. The rate rises with warmth, as the enzymes work faster, up to a peak near 40 °C.
3. Past the peak it collapses, as its proteins begin to lose their shape.
4. Set Temperature to 40 °C, that is 313 K.

Lesson 5 of 7 · About 25 minutes

Magnification and scale

How a microscope magnifies, how to work out true size from a magnified picture, and why magnification alone shows no detail smaller than light's wavelength.

Open the lesson

The 6 steps and their answers

Step 1 of 6
Open this step
Two lenses
Practice · Calculate
Question
In the light microscope the eyepiece magnifies 10 times, and the objective in use now magnifies 10 times. What is the total magnification?

Correct answer
100 ± 1%
Setup, choices and feedback
The Scene at this step
Magnification (powers of ten)
2fixed
Light’s wavelength
550 nm
Microscopes / Beam paths
Beam paths
Accepted within
± 1%
Computed from the Scene at this step’s setting
Total magnification, times/nodes/scopes/outputs/magnificationTimes
Wrong answers it recognises
- 20 ± 0.1
  Misconception: Adds the eyepiece and objective magnifications instead of multiplying them.
  Feedback from the catalogue
  The two lenses’ magnifications multiply; they do not add. The objective makes an enlarged picture, and the eyepiece enlarges that picture again. A 40× objective and a 10× eyepiece give $40 \times 10 = 400$ ⁠, not $50$ ⁠.
Feedback for a right answer
$10 \times 10 = 100$ ⁠: 100 times. With the 40× objective it would be 400 times, and with the oil objective 1,000 times.

Hint
The eyepiece enlarges the picture the objective has already enlarged.

Worked solution
1. Total magnification = eyepiece × objective.
2. Substitute: $10 \times 10 = 100$ ⁠.
3. The magnification: 100 times.
If you answered $20$ ⁠, you added the magnifications instead of multiplying them.
Step 2 of 6
Open this step
Real size
Practice · Calculate
Question
The light microscope’s picture is printed 20 mm wide, at a magnification of 100 times. How wide is the patch of specimen it really shows?

Correct answer
200 µm ± 2%
Setup, choices and feedback
The Scene at this step
Magnification (powers of ten)
2fixed
Light’s wavelength
550 nm
Microscopes / Beam paths
Microscopes
Accepted within
± 2%
Computed from the Scene at this step’s setting
Width of the specimen both pictures show/nodes/scopes/outputs/fieldWidth
Any unit of the same quantity is accepted; it is converted before checking.
Wrong answers it recognises
- 2000000 µm ± 2%
  Misconception: Multiplies the image size by the magnification to get the real size, instead of dividing.
  Feedback from the catalogue
  The picture is bigger than the real thing, so the real size is smaller than the picture: divide by the magnification, don’t multiply.
  
  $\text{real size} = \frac{\text{image size}}{\text{magnification}}$ ⁠
  
  Check the answer for sense: a cell that shows 20 mm wide in a magnified picture cannot be metres across.
Feedback for a right answer
$\dfrac{20\ \mathrm{mm}}{100} = 0.2\ \mathrm{mm}$ ⁠, that is 200 µm: about three cheek cells side by side.

Hint
$\text{real size} = \dfrac{\text{image size}}{\text{magnification}}$ ⁠, then convert to µm: 1 mm = 1000 µm.

Worked solution
1. The givens: image size 20 mm, magnification $100$ ⁠.
2. Real size = image size ÷ magnification: $\dfrac{20}{100} = 0.2\ \mathrm{mm}$ ⁠.
3. Convert: $0.2 \times 1000 = 200$ ⁠, that is 200 µm.
If you got 2000 mm, you multiplied by the magnification: the real patch is smaller than the picture, not bigger.
Step 3 of 6
Open this step
Choosing a magnification
Practice · Reach the target
Question
A cheek cell is about 60 µm across, and a bacterium about 2 µm long. Change Magnification until the 20 mm picture shows a patch exactly 20 µm wide, then press Check.

Goal
- 0.00002 m ± 3%
  At the start of the step: 0.0005024 m
Setup, choices and feedback
The Scene at this step
Magnification (powers of ten)
1.6
Light’s wavelength
550 nmfixed
Microscopes / Beam paths
Microscopes
Read from the Scene at
Width of the specimen both pictures show/nodes/scopes/outputs/fieldWidth
One setting that reaches it
Magnification (powers of ten)
2.99
Feedback for a right answer
1,000 times: $\dfrac{20\,000\ \mu\mathrm{m}}{20\ \mu\mathrm{m}} = 1000$ ⁠. In such a patch a bacterium shows as a 2 mm rod, and a cheek cell is bigger than the whole picture.

Feedback for a wrong answer
Not yet. Magnification in the Scene is in powers of ten: 2 is 100 times, 3 is 1,000 times.

Hint
$\text{magnification} = \dfrac{\text{image size}}{\text{real size}}$ ⁠. First put both sizes in the same unit.

Worked solution
1. Image size: 20 mm = 20,000 µm. Real size: 20 µm.
2. Magnification = image size ÷ real size: $\dfrac{20\,000}{20} = 1000$ ⁠.
3. $1000 = 10^3$ ⁠: set Magnification to 3.
Step 4 of 6
Open this step
From a photograph
Practice · Calculate
Question
In a photograph taken at 400 times, a cheek cell shows 24 mm across. What is its real width?

Correct answer
60 µm ± 2%
Setup, choices and feedback
The Scene at this step
Magnification (powers of ten)
2.6
Light’s wavelength
550 nm
Microscopes / Beam paths
Microscopes
Accepted within
± 2%
A fixed value in the question
Any unit of the same quantity is accepted; it is converted before checking.
Wrong answers it recognises
- 9600000 µm ± 2%
  Misconception: Multiplies the image size by the magnification to get the real size, instead of dividing.
  Feedback from the catalogue
  The picture is bigger than the real thing, so the real size is smaller than the picture: divide by the magnification, don’t multiply.
  
  $\text{real size} = \frac{\text{image size}}{\text{magnification}}$ ⁠
  
  Check the answer for sense: a cell that shows 20 mm wide in a magnified picture cannot be metres across.
Feedback for a right answer
$\dfrac{24\ \mathrm{mm}}{400} = 0.06\ \mathrm{mm} = 60\ \mu\mathrm{m}$ ⁠. That is just where the skin cell sits on the size ladder beside the Scene.

Hint
Divide by the magnification, then convert from mm to µm.

Worked solution
1. Real size = image size ÷ magnification.
2. $\dfrac{24}{400} = 0.06$ ⁠, that is 0.06 mm.
3. $0.06 \times 1000 = 60$ ⁠: 60 µm.
If you got a huge number, nearly 10 m, you multiplied by the magnification instead of dividing.
Step 5 of 6
Open this step
The smallest detail
Practice · Calculate
Question
At 1,000 times, in green light of 550 nm, you work on the oil objective, whose numerical aperture is $\mathrm{NA} = 1.25$ ⁠. What is the finest detail the light microscope separates, $d = \dfrac{0.61\,\lambda}{\mathrm{NA}}$ ⁠?

Correct answer
268.4 nm ± 2%
Setup, choices and feedback
The Scene at this step
Magnification (powers of ten)
3fixed
Light’s wavelength
550 nmfixed
Microscopes / Beam paths
Microscopes
Accepted within
± 2%
Computed from the Scene at this step’s setting
Smallest detail the light microscope shows, 0.61 λ / NA/nodes/scopes/outputs/lightResolution
Any unit of the same quantity is accepted; it is converted before checking.
Wrong answers it recognises
- 550 nm ± 2%
  Misconception: Thinks more magnification always shows more detail.
  Feedback
  $550$ ⁠ is the wavelength itself. The formula multiplies it by $0.61$ ⁠ and divides by the aperture.
Feedback for a right answer
$d = \dfrac{0.61 \times 550}{1.25} \approx 268\ \mathrm{nm}$ ⁠, about a quarter of a micrometre. A bacterium, 2 µm, shows; a ribosome, 25 nm, does not.

Hint
Substitute: $0.61 \times 550$ ⁠, divided by $1.25$ ⁠.

Worked solution
1. The givens: $\lambda = 550\ \mathrm{nm}$ ⁠, $\mathrm{NA} = 1.25$ ⁠.
2. The formula: $d = \dfrac{0.61\,\lambda}{\mathrm{NA}}$ ⁠.
3. Substitute: $d = \dfrac{0.61 \times 550}{1.25} = \dfrac{335.5}{1.25} \approx 268$ ⁠.
4. $d \approx 268\ \mathrm{nm}$ ⁠.
Violet light, a shorter wave, separates slightly finer detail; red, coarser.
Step 6 of 6
Open this step
Empty magnification
Practice · Predict, then look
Question
The light microscope is at 1,000 times. We will magnify a hundred times more, to 100,000. What will the light picture show?

Correct answer
- Correct A bigger blur, with no new detail.
Setup, choices and feedback
The Scene at this step
Magnification (powers of ten)
3fixed
Light’s wavelength
550 nmfixed
Microscopes / Beam paths
Microscopes
Choices
- Correct A bigger blur, with no new detail.
- Wrong Finer detail, such as the viruses on the bacteria.
  Misconception: Thinks more magnification always shows more detail.
  Feedback from the catalogue
  Magnification makes the picture bigger, but it adds no detail. What sets the smallest detail is the resolution, and in a light microscope the wavelength of light limits it: two points closer than about 0.2 µm merge into one blur. Beyond about a thousand times, you only enlarge the blur. To see a virus you need a shorter wave: electrons.
Once the prediction is locked in, the Scene changes
Magnification (powers of ten)
5fixed
The Scene then shows: 1 · For Callouts: 0 more magnification still shows more, 1 empty magnification, 2 cut open/nodes/scopes/outputs/lightNote
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
A bigger blur: “empty magnification”. The finest detail light can separate stays about 270 nm, and a virus of about 100 nm is smaller than that. In the electron picture, at the same magnification, the viruses are sharp.

Worked solution
1. A light microscope’s resolution: $d = \dfrac{0.61\,\lambda}{\mathrm{NA}}$ ⁠, and above a thousand times it is already on the oil objective, $\mathrm{NA} = 1.25$ ⁠.
2. In green light: $d = \dfrac{0.61 \times 550}{1.25} \approx 268\ \mathrm{nm}$ ⁠.
3. Magnification does not change $d$ ⁠: it only enlarges the blur.
4. A virus, about 100 nm, is smaller than $d$ ⁠, so it will not show.

Lesson 6 of 7 · About 25 minutes

Life in a drop

One drop of pond water under a microscope: what the eye alone can make out, which magnification suits each creature, single cells bigger than whole animals, and three ways to get food: hunting, grazing and making it from light.

Open the lesson

The 6 steps and their answers

Step 1 of 6
Open this step
A drop of pond water
Practice · Choose
Question
The eye alone can just make out something 100 µm long. Which of the six creatures in the drop could you see, at least as a speck, in a jar held up to the light? Choose all of them.

Correct answer
- Correct Amoeba, about 500 µm
- Correct Paramecium, about 250 µm
- Correct Rotifer, about 300 µm
- Correct Tardigrade, about 350 µm
Setup, choices and feedback
The Scene at this step
Magnification (powers of ten)
1
Follow
Paramecium
Watch it feed
Outside
Choices
Shown in a random order. More than one choice is correct: every one must be selected.
- Correct Amoeba, about 500 µm
- Correct Paramecium, about 250 µm
- Wrong Euglena, about 50 µm
  Feedback
  Half the eye’s limit: a euglena is invisible without a microscope.
- Wrong Diatom, about 60 µm
  Feedback
  Smaller than 100 µm: a diatom is invisible to the eye.
- Correct Rotifer, about 300 µm
- Correct Tardigrade, about 350 µm
Feedback for a right answer
The amoeba, the paramecium, the rotifer and the tardigrade, all longer than 100 µm, show as tiny white specks. The euglena and the diatom are below the limit, and the bacteria, 2 µm, have no chance. Almost everything alive in the drop is invisible.

Hint
Compare each length with 100 µm.

Worked solution
1. The eye’s limit: 100 µm, a tenth of a millimetre.
2. Longer than that: amoeba ( $500$ ⁠), tardigrade ( $350$ ⁠), rotifer ( $300$ ⁠), paramecium ( $250$ ⁠).
3. Shorter: diatom ( $60$ ⁠) and euglena ( $50$ ⁠).
4. The first four show, just, as specks; the last two only under a microscope.
Step 2 of 6
Open this step
Zoom in to the euglena
Practice · Reach the target
Question
The euglena is about 50 µm long. Change Magnification (powers of ten) until it spans a tenth of the field’s width, and press Check.

Goal
- 0.1 ± 0.005
  At the start of the step: 0.009953
Setup, choices and feedback
The Scene at this step
Magnification (powers of ten)
1.6
Follow
Euglenafixed
Watch it feed
Outsidefixed
Read from the Scene at
Share of the field’s width the organism spans/nodes/drop/outputs/fieldShare
One setting that reaches it
Magnification (powers of ten)
2.58
Feedback for a right answer
At 400× the field is half a millimetre across, and 50 µm is a tenth of it. That is the magnification at which you can see the euglena’s shape and its flagellum, while still seeing enough of the drop round it to follow it as it swims.

Feedback for a wrong answer
Not yet. If the euglena takes up less than a tenth of the field, magnify more; if more, less.

Hint
If the euglena is a tenth of the field, the field is 500 µm, half a millimetre, across. At what magnification does 200 mm divided by it come to half a millimetre?

Worked solution
1. The euglena should be a tenth of the field, so the field must be $10 \times 50 = 500$ ⁠ micrometres, half a millimetre.
2. The field is 200 mm divided by the magnification: $\dfrac{200}{M} = 0.5$ ⁠.
3. $M = 400$ ⁠, the 400× preset, about $2.6$ ⁠ on the slider ( $10^{2.6} \approx 400$ ⁠).
Step 3 of 6
Open this step
One cell or a thousand
Practice · Predict, then look
Question
The amoeba, about 500 µm long, is one cell. The rotifer is shorter, about 300 µm. Predict how many cells the rotifer is built of.

Correct answer
- Correct About a thousand cells
Setup, choices and feedback
The Scene at this step
Magnification (powers of ten)
2fixed
Follow
Amoeba
Watch it feed
Outside
Choices
- Wrong One cell, since it is smaller than the amoeba
  Misconception: Thinks a bigger creature must be made of more cells, so anything bigger than a many-celled animal is many-celled too.
  Feedback from the catalogue
  Size does not tell you how many cells something has. An amoeba half a millimetre long is one cell, and a rotifer 0.3 mm long is built of about $1000$ ⁠ cells. A single cell can grow far bigger than a typical cell in your body, and a tiny animal’s cells can be very small. To know, you look for cell membranes, nuclei and organs, not at its length.
- Correct About a thousand cells
- Wrong Millions of cells, like any animal
  Feedback
  Big animals are made of billions, but an animal the size of a full stop has no room for millions: about a thousand small cells do the job.
Once the prediction is locked in, the Scene changes
Follow
Rotifer
The Scene then shows: 1000 · Cells in its body: 1 for the protists, about 1,000 for the rotifer and the tardigrade/nodes/drop/outputs/cells
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
About a thousand cells. The rotifer is a whole animal, with a mouth, jaws, a gut, muscles and a brain, packed into less than a third of a millimetre. The bigger amoeba is one cell. Size does not tell you how many cells something has.

Worked solution
1. The amoeba is one cell grown to half a millimetre: a single cell can be very large.
2. The rotifer has organs: a crown of cilia, a mouth, jaws and a gut. Organs are built of many cells.
3. So it is many-celled, though smaller than the amoeba: about $1000$ ⁠ cells.
4. If you chose “one cell”, you assumed smaller means fewer cells. Here it is the other way round.
Step 4 of 6
Open this step
Not bacteria
Practice · Choose
Question
The paramecium is one cell, and so are the bacteria it eats. What does the paramecium have that a bacterium does not?

Correct answer
- Correct A nucleus, and organelles wrapped in membranes
Setup, choices and feedback
The Scene at this step
Magnification (powers of ten)
2.6
Follow
Paramecium
Watch it feed
Outside
Choices
Shown in a random order.
- Correct A nucleus, and organelles wrapped in membranes
- Wrong A cell membrane
  Feedback
  Every cell has a membrane, a bacterium too: without one there is no cell.
- Wrong DNA
  Feedback
  A bacterium has DNA too, but loose in its cytoplasm, not inside a nucleus.
- Wrong Nothing, it is just a big bacterium
  Misconception: Thinks every single-celled organism is a bacterium.
  Feedback from the catalogue
  Single-celled does not mean bacterium. A bacterium has no nucleus and no organelles wrapped in membranes. An amoeba, a paramecium, a euglena and a diatom are protists: each one cell, but with a nucleus, mitochondria and vacuoles like the cells of your body, and a hundred times bigger than a bacterium.
Feedback for a right answer
A nucleus. The paramecium is a protist, a single-celled eukaryote: it has a nucleus, mitochondria and vacuoles wrapped in membranes, like your cells. A bacterium has no nucleus, and its DNA lies loose in the cell. The paramecium is also about a hundred times longer.

Worked solution
1. What every cell has: a membrane, cytoplasm, ribosomes and DNA. Those do not tell them apart.
2. What sets a eukaryote apart from a prokaryote: a nucleus and membrane-wrapped organelles.
3. The paramecium has a nucleus, food vacuoles and contractile vacuoles: it is eukaryotic. A bacterium has none.
4. Single-celled is the number of cells, not the kind of cell.
Step 5 of 6
Open this step
Bailing out water
Practice · Choose
Question
Why does the paramecium need the contractile vacuoles that fill and empty every few seconds?

Correct answer
- Correct Water seeps in all the time by osmosis, and they pump it out before it bursts
Setup, choices and feedback
The Scene at this step
Magnification (powers of ten)
2.6
Follow
Parameciumfixed
Watch it feed
0.3 (Outside → Feeding)
Choices
Shown in a random order.
- Correct Water seeps in all the time by osmosis, and they pump it out before it bursts
- Wrong They digest the bacteria it swallows
  Feedback
  That is the food vacuoles’ job; they form at the end of the oral groove and are carried round the cell.
- Wrong They squirt a jet of water that pushes it along
  Feedback
  The paramecium rows with its cilia. The vacuole’s squirt is too small to move it.
- Wrong They take in oxygen from the water
  Feedback
  Oxygen diffuses in through the whole membrane; a small cell needs no breathing organ.
Feedback for a right answer
The paramecium’s inside is saltier than the pond, so water flows in all the time by osmosis. It has no cell wall to stop it swelling, so the contractile vacuoles collect the extra water and squeeze it out. In salty water they almost stop.

Worked solution
1. The cell holds more dissolved substances than the pond water.
2. Water crosses the membrane from the dilute side to the concentrated one: inwards.
3. A plant has a wall that stops it swelling; the paramecium has none.
4. So she must push the extra water out actively: that is the contractile vacuoles’ job.
Step 6 of 6
Open this step
Three ways to eat
Practice · Choose
Question
Which of the drop’s creatures make their own food, from light? Choose all of them.

Correct answer
- Correct Euglena
- Correct Diatom
Setup, choices and feedback
The Scene at this step
Magnification (powers of ten)
3
Follow
Euglena
Watch it feed
Feeding
Choices
Shown in a random order. More than one choice is correct: every one must be selected.
- Correct Euglena
- Correct Diatom
- Wrong Amoeba
  Feedback
  The amoeba is a hunter: it wraps an alga in pseudopods and digests it.
- Wrong Paramecium
  Feedback
  The paramecium grazes bacteria with its cilia; it has no chloroplasts.
- Wrong None: only plants make food from light
  Misconception: Thinks only plants make food by photosynthesis.
  Feedback from the catalogue
  Photosynthesis is not just for plants. Any cell with chloroplasts can make sugar from carbon dioxide and water in light: euglenas, diatoms and algae, and even cyanobacteria, which have no chloroplasts at all. Diatoms alone make about a fifth of the oxygen produced on Earth.
Feedback for a right answer
The euglena and the diatom. Both have chloroplasts, green in the euglena and gold-brown in the diatom, that make sugar from carbon dioxide and water in light. The euglena stores it as paramylon grains and the diatom as oil droplets. They are single cells, not plants.

Worked solution
1. Food from light means photosynthesis, which needs chloroplasts.
2. In the Scene, at Feeding: the euglena and the diatom have chloroplasts that glow and stored food that appears.
3. The amoeba, the paramecium, the rotifer and the tardigrade eat other creatures.
4. So two: the euglena and the diatom.

Lesson 7 of 7 · About 25 minutes

The hidden fungus

What comes up above the ground is only the fruit: the web of threads below digests leaves outside its body, works faster in warmth and slower when the soil is too dry or too wet, and trades sugar for water with the tree’s roots.

Open the lesson

The 6 steps and their answers

Step 1 of 6
Open this step
Fruit of a hidden body
Practice · Choose
Question
Choose Underground under Look underground. What, really, is the red fly agaric standing on the litter?

Correct answer
- Correct The fruiting body of a fungus that lives in the soil as a web of threads
Setup, choices and feedback
The Scene at this step
Soil moisture
60 %
Soil temperature
283.1 K
Look underground
Forest floor
Choices
Shown in a random order.
- Correct The fruiting body of a fungus that lives in the soil as a web of threads
- Wrong The whole fungus; the threads in the soil are its roots
  Misconception: Thinks the mushroom above the ground is the whole fungus.
  Feedback from the catalogue
  The mushroom above the ground is a fruiting body, built for a few days to scatter spores, like an apple on a tree. The creature itself is the mycelium, a web of fine threads in the soil or the litter that lives there for years and can spread over a huge area. Pick the mushroom, and the mycelium stays.
- Wrong A small plant with no leaves
  Misconception: Thinks fungi are plants that make their food by photosynthesis.
  Feedback from the catalogue
  Fungi have no chlorophyll and cannot make food from light. They feed on food that already exists: their threads release enzymes that digest it outside the body, and absorb what dissolves. Their cell walls are chitin, not cellulose, and in the tree of life they are closer to animals than to plants.
- Wrong A part of the birch that grew up from its root
  Feedback
  The birch and the fungus are two different creatures. They only trade with each other below the ground.
Feedback for a right answer
A fruiting body. The creature itself is the mycelium, a web of hyphae that lives in the soil for years. When conditions suit it, it raises a fruiting body for a few days, and the gills under the cap shed millions of spores. The threads are not roots: they are the fungus itself.

Worked solution
1. Below the ground there is a web of threads that runs far beyond the mushroom’s stem.
2. That web, the mycelium, is the fungus’s body, and it lives all year.
3. The fly agaric above the ground comes up for a few days, sheds spores and rots: that is a fruit’s job.
4. So it is the fruiting body, not the whole creature.
Step 2 of 6
Open this step
Eating from outside
Practice · Choose
Question
A fungus has no mouth and no chlorophyll. How do the bonnets’ threads feed on the fallen leaves?

Correct answer
- Correct They grow into the leaf, release enzymes that digest it outside their body, and absorb what dissolves
Setup, choices and feedback
The Scene at this step
Soil moisture
60 %
Soil temperature
283.1 K
Look underground
Underground
Choices
Shown in a random order.
- Correct They grow into the leaf, release enzymes that digest it outside their body, and absorb what dissolves
- Wrong They make sugar from light, like the leaves themselves
  Misconception: Thinks fungi are plants that make their food by photosynthesis.
  Feedback from the catalogue
  Fungi have no chlorophyll and cannot make food from light. They feed on food that already exists: their threads release enzymes that digest it outside the body, and absorb what dissolves. Their cell walls are chitin, not cellulose, and in the tree of life they are closer to animals than to plants.
- Wrong They wrap round bits of leaf and digest them inside their cells
  Feedback
  That is how an amoeba eats. A fungus’s cells have a stiff wall of chitin, and they cannot swallow anything.
- Wrong They drink water from the leaves, and that is enough
  Feedback
  Water is not food: it holds no energy. The threads need the sugars in the leaf.
Feedback for a right answer
Digestion outside. The threads grow into the leaf and release enzymes that break down cellulose and even lignin, the tough stuff that stiffens plant cell walls. Fungi are the main living things that can break down lignin. Then the threads absorb the small sugars through their walls.

Worked solution
1. No chlorophyll, no photosynthesis: the fungus needs ready-made food.
2. Its cells have walls of chitin, so it cannot swallow particles.
3. What is left: break the food down before it comes in, with enzymes the threads release outside.
4. The small molecules that result are absorbed through the wall.
Step 3 of 6
Open this step
Warmer and faster
Practice · Predict, then look
Question
In moist soil at 10 °C a fallen leaf’s half-life is about $1.5$ ⁠ years. The fungi work $2.2$ ⁠ times faster for every 10 °C. Predict the half-life, in years, when Soil temperature rises to 20 °C.

Correct answer
0.6931 ± 0.1
A number prediction counts as right within ± 0.1
Setup, choices and feedback
The Scene at this step
Soil moisture
60 %fixed
Soil temperature
283.1 K
Look underground
Forest floor
Once the prediction is locked in, the Scene changes
Soil temperature
293.1 K
The Scene then shows: 0.6931 · Years for a fallen leaf to lose half its mass (Infinity if it never rots)/nodes/floor/outputs/halfLife
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
About $0.7$ ⁠ of a year, a little over eight months. Fungi that work $2.2$ ⁠ times faster get through each leaf in $2.2$ ⁠ times less time. The reverse happens as autumn cools: the work slows, and the year’s litter stays on the floor until spring.

Worked solution
1. The half-life is the inverse of the rate: $t_{1/2} = \dfrac{\ln 2}{k}$ ⁠, and $k$ ⁠ grows with how fast the fungi work.
2. From 10 °C to 20 °C the rate grows $2.2$ ⁠ times.
3. $1.52 \div 2.2 \approx 0.69$ ⁠ years.
4. The chart now shows about $8$ ⁠ months.
If you predicted $3$ ⁠ years, you multiplied instead of dividing: a faster rate means a shorter time.
Step 4 of 6
Open this step
The best soil
Practice · Reach the target
Question
Soil temperature is locked at Autumn. Change Soil moisture until the leaves rot as fast as they can, and press Check.

Goal
- 1.525 ± 0.02
  At the start of the step: 6.523
Setup, choices and feedback
The Scene at this step
Soil moisture
25 %
Soil temperature
283.1 Kfixed
Look underground
Forest floor
Read from the Scene at
Years for a fallen leaf to lose half its mass (Infinity if it never rots)/nodes/floor/outputs/halfLife
One setting that reaches it
Soil moisture
60 %
Feedback for a right answer
60%, in the middle. Below it the water films round the grains are too thin for enzymes and food to move; above it water fills the spaces air needs. That is the moisture at which almost all soil life works best.

Feedback for a wrong answer
Not yet. If the pace still rises as you add water, keep going; if it is already falling, come back a little.

Hint
Follow the dot on the upper curve, or the half-life on the lower chart; fastest means the highest pace and the shortest half-life.

Worked solution
1. At 25%, dry soil, the pace is low: there is not enough water for the enzymes to work.
2. Add water: the pace rises, and the half-life shortens.
3. At about 60% the pace peaks, $0.45$ ⁠ times the pace at 20 °C, and the half-life is about $1.5$ ⁠ years.
4. Beyond that it falls again: air runs short.
Step 5 of 6
Open this step
Too wet
Practice · Predict, then look
Question
The soil is moist, 60%. Predict what happens to the fallen leaves when Soil moisture rises to 95%, waterlogged.

Correct answer
- Correct They rot far more slowly, and litter piles up
Setup, choices and feedback
The Scene at this step
Soil moisture
60 %
Soil temperature
283.1 Kfixed
Look underground
Forest floor
Choices
- Wrong They rot faster: more water, more work
  Misconception: Thinks the wetter the soil, the faster fungi rot leaves, with no limit.
  Feedback from the catalogue
  Water helps only up to a point. The threads breathe oxygen, and the soil’s air sits in the same spaces the water fills. Above about 60% of the spaces full, oxygen starts to run short, and in waterlogged soil rotting slows to about a seventh of its best. That is why bogs preserve leaves.
- Correct They rot far more slowly, and litter piles up
- Wrong Nothing changes: there is enough water either way
  Feedback
  The water does not just suffice: it takes up room. In waterlogged soil there is no room for air.
Once the prediction is locked in, the Scene changes
Soil moisture
95 %
The Scene then shows: 5.528 · Years for a fallen leaf to lose half its mass (Infinity if it never rots)/nodes/floor/outputs/halfLife
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Far more slowly. The half-life leaps from about a year and a half to over five years, and litter builds up to nearly 3 kg a square metre. The threads breathe oxygen, and the water has driven the air out of the spaces between the grains. That is how peat bogs keep leaves, and even bodies, for thousands of years.

Worked solution
1. Fungi need water to work, but oxygen to breathe as well.
2. Water and air share the same spaces in the soil.
3. In waterlogged soil 95% of the spaces are full: almost no air, and the pace falls to about a seventh of its peak.
4. Slow rotting while the leaves keep falling: $L = F/k$ ⁠ grows, and the litter piles up.
Step 6 of 6
Open this step
Trading with the tree
Practice · Choose
Question
Below the ground, the fly agaric’s threads sheathe the birch’s root tips. What passes between them?

Correct answer
- Correct The tree gives the fungus sugar, and the fungus gives the tree water and nutrients
Setup, choices and feedback
The Scene at this step
Soil moisture
60 %
Soil temperature
283.1 K
Look underground
Underground
Choices
Shown in a random order.
- Correct The tree gives the fungus sugar, and the fungus gives the tree water and nutrients
- Wrong The fungus steals sugar from the tree and gives nothing back
  Misconception: Thinks a fungus living on a tree’s roots must be harming it, as a parasite.
  Feedback from the catalogue
  A fungus on a root is not necessarily a parasite. In a mycorrhiza both gain: the threads, far finer than roots, gather water, phosphorus and nitrogen from pores a root cannot reach and pass them to the tree, and the tree pays in sugar from its leaves. Most land plants have such partners, and many trees grow poorly without them.
- Wrong The fungus gives the tree sugar, and the tree gives it water
  Feedback
  The fungus has no chlorophyll and cannot make sugar. The sugar comes from the tree’s leaves.
- Wrong The fungus rots the roots, as it does the leaves
  Feedback
  The bonnets rot the dead leaves. The birch’s roots are alive, and the fly agaric feeds through them without rotting them.
Feedback for a right answer
A trade. Amber sugar leaves the root tips along the threads to the fruiting body; sky-blue water and nutrients move towards the roots. The threads are far finer than a root and reach into tiny pores, and the tree pays for it with a tenth or more of the sugar it makes. The fly agaric’s fruiting bodies are built from the birch’s sugar.

Worked solution
1. Who makes sugar? Only the tree, in its leaves, by photosynthesis.
2. Who is good at gathering water and phosphorus from the soil? The threads, far finer than roots.
3. In the Scene: amber dots (sugar) from the roots to the fungus, sky-blue dots (water and nutrients) from the fungus to the roots.
4. Both sides gain: a mycorrhiza, not parasitism.

Across the unit · 20 misconceptions

Misconception catalogue

Each common mistake this unit recognises, in a few words. Open one for what the learner reads when an answer matches it, and where it is asked.

Index

The signs of life

Seven signs

The candle flame

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Hint

Worked solution

Imitations

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

The virus

Question

Correct answer

The Scene at this step

Once the prediction is locked in, the Scene changes

Hint

Worked solution

On the border

Question

Correct answer

The Scene at this step

Choices

Worked solution

One bacterium becomes many

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

A colony

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

Inside a cell

Cell theory

Question

Correct answer

The Scene at this step

Choices

Worked solution

The power station

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

The protein factories

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

An organelle with no membrane

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

The membrane

Question

Correct answer

The Scene at this step

Choices

Worked solution

Plant and animal cells

The organelle that catches light