Answer key · Unit 4

Energy in living things.

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 4

Lesson 1 of 4 · About 20 minutes

Photosynthesis

How a plant builds sugar from carbon dioxide and water with light, where its mass and the oxygen it gives off come from, and what sets the rate.

Open the lesson

The 5 steps and their answers

Step 1 of 5
Open this step
From thin air
Practice · Choose
Question
An oak grown from an acorn weighs tonnes after fifty years. Where did most of its dry mass come from?

Correct answer
- Correct From carbon dioxide in the air
Setup, choices and feedback
The Scene at this step
Light
70 %
Carbon dioxide
60 %
Cut open
Chloroplast
Choices
Shown in a random order.
- Correct From carbon dioxide in the air
- Wrong From the soil
  Misconception: Thinks a plant builds its mass mainly from the soil.
  Feedback from the catalogue
  Most of a plant’s mass, apart from its water, is carbon, and that carbon comes from the air: from carbon dioxide the leaves take in by photosynthesis. From the soil the plant takes water and a little mineral. As early as the 17th century van Helmont grew a willow in a pot for five years: the tree gained about 74 kg, and the soil in the pot lost less than 100 g.
- Wrong From the water
  Feedback
  Water gives the sugar its hydrogen, and most of the tree’s fresh weight. But most of its dry mass is carbon, and that comes from the air.
- Wrong From sunlight
  Feedback
  Light is energy, not matter. It supplies the energy to build the sugar, but not the atoms.
Feedback for a right answer
From the air. Every carbon atom in the tree was once in a carbon dioxide molecule that entered a leaf. In the Scene they are the molecules arriving from the left.

Worked solution
1. A tree’s dry mass is mostly cellulose and wood: chains of sugar, $\mathrm{C_6H_{12}O_6}$ ⁠.
2. The carbon in the sugar comes from $\mathrm{CO_2}$ ⁠ the leaves take from the air.
3. So most of the dry mass comes from the air.
From the soil the plant takes only water and minerals, in small amounts.
Step 2 of 5
Open this step
Light is energy
Practice · Predict, then look
Question
The chloroplast is working in light at 70% of full sun, with plenty of carbon dioxide. Night falls: the light drops to zero. What will happen to the sugar making?

Correct answer
- Correct It will stop altogether
Setup, choices and feedback
The Scene at this step
Light
70 %fixed
Carbon dioxide
90 %fixed
Cut open
Chloroplast
Choices
- Correct It will stop altogether
- Wrong It will slow, running on stored sugar
  Feedback
  Stored sugar the plant can break down at night, by respiration. But building sugar takes light.
- Wrong It will not change, as there is plenty of carbon dioxide
  Misconception: Combines limiting factors, multiplying or averaging them, instead of taking the scarcest.
  Feedback from the catalogue
  The rate is set by the factor in shortest supply, the limiting factor, not by some combination of them all. Like a production line: with raw material for 60% of the rate and energy for 70%, the line runs at 60%. The spare energy does not help.
Once the prediction is locked in, the Scene changes
Light
0 %fixed
The Scene then shows: 0 · Rate of photosynthesis, per cent of the fastest: the scarcest of light, carbon dioxide and what the temperature allows/nodes/leaf/outputs/rate
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
It stops. Carbon dioxide and water keep arriving, but without the energy of light no sugar can be built from them. At night the plant only respires, breaking down the sugar it built by day.

Worked solution
1. Photosynthesis needs three things: carbon dioxide, water and the energy of light.
2. The rate is set by whichever is in shortest supply.
3. With no light that factor is zero, so the rate is zero.
Step 3 of 5
Open this step
Inside the chloroplast
Practice · Choose
Question
Which molecule does the oxygen a plant gives off in photosynthesis come from?

Correct answer
- Correct Water, $\mathrm{H_2O}$ ⁠
Setup, choices and feedback
The Scene at this step
Light
70 %
Carbon dioxide
60 %
Cut open
Inside
Choices
Shown in a random order.
- Correct Water, $\mathrm{H_2O}$ ⁠
- Wrong Carbon dioxide, $\mathrm{CO_2}$ ⁠
  Misconception: Thinks the oxygen a plant releases comes from the carbon dioxide.
  Feedback from the catalogue
  The oxygen a plant gives off comes from water. In the light-dependent stage, the light’s energy splits water molecules: their hydrogen and electrons go into building sugar, and the oxygen is released. The carbon dioxide’s oxygen ends up in the sugar. Experiments with water labelled with heavy oxygen showed this in the 1940s.
- Wrong Glucose, $\mathrm{C_6H_{12}O_6}$ ⁠
  Feedback
  Glucose is what photosynthesis makes, not what it starts from.
Feedback for a right answer
From water. In the stacks of discs inside the chloroplast, light splits water; its oxygen leaves as gas, and its hydrogen goes into building the sugar.

Worked solution
1. In the first, light-dependent stage, the light’s energy splits water molecules.
2. Every two water molecules release one oxygen molecule, $\mathrm{O_2}$ ⁠.
3. The carbon and oxygen of the $\mathrm{CO_2}$ ⁠ go into the sugar in the second stage.
Step 4 of 5
Open this step
The scarcer input
Practice · Calculate
Question
Light is at 70% of full sun, and Carbon dioxide at 60% of plenty. At a comfortable temperature, at what percentage of the top rate is photosynthesis running?

Correct answer
60 ± 1
Setup, choices and feedback
The Scene at this step
Light
70 %fixed
Carbon dioxide
60 %fixed
Cut open
Chloroplast
Accepted within
± 1
Computed from the Scene at this step’s setting
Rate of photosynthesis, per cent of the fastest: the scarcest of light, carbon dioxide and what the temperature allows/nodes/leaf/outputs/rate
Wrong answers it recognises
- 42 ± 1
  Misconception: Combines limiting factors, multiplying or averaging them, instead of taking the scarcest.
  Feedback from the catalogue
  The rate is set by the factor in shortest supply, the limiting factor, not by some combination of them all. Like a production line: with raw material for 60% of the rate and energy for 70%, the line runs at 60%. The spare energy does not help.
- 65 ± 1
  Misconception: Combines limiting factors, multiplying or averaging them, instead of taking the scarcest.
  Feedback from the catalogue
  The rate is set by the factor in shortest supply, the limiting factor, not by some combination of them all. Like a production line: with raw material for 60% of the rate and energy for 70%, the line runs at 60%. The spare energy does not help.
- 70 ± 1
  Misconception: Thinks more light always makes photosynthesis faster.
  Feedback from the catalogue
  Light speeds photosynthesis up only while it is the limiting factor. Once there is enough light, the rate stops at the next factor: carbon dioxide or temperature. Past that point the curve levels off, and more light changes nothing. That is why greenhouse growers add carbon dioxide, not just lamps.
Feedback for a right answer
60%: carbon dioxide is the limiting factor. The extra light does not help; in the Scene you can see light packets arriving unused.

Hint
Which of the two runs out first?

Worked solution
1. Light allows 70% of the rate, carbon dioxide 60%.
2. The rate is the smaller of them: $\min(70, 60) = 60$ ⁠.
3. Photosynthesis runs at 60%.
If you got $42$ ⁠, you multiplied; if $65$ ⁠, you averaged. The scarce factor decides alone.
Step 5 of 5
Open this step
The equation
Practice · Calculate
Question
A glucose molecule, $\mathrm{C_6H_{12}O_6}$ ⁠, holds six carbon atoms. How many carbon dioxide molecules does the plant need to build one glucose molecule?

Correct answer
6 ± 0.1
Setup, choices and feedback
The Scene at this step
Light
100 %
Carbon dioxide
100 %
Cut open
Chloroplast
Accepted within
± 0.1
A fixed value in the question
Feedback for a right answer
Six. The balanced equation: $6\,\mathrm{CO_2} + 6\,\mathrm{H_2O} \rightarrow \mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2}$ ⁠. Six carbon dioxide and six water, with light, give one sugar and six oxygen molecules.

Hint
Each $\mathrm{CO_2}$ ⁠ molecule holds one carbon atom.

Worked solution
1. Glucose holds $6$ ⁠ carbon atoms.
2. Each one comes from a different $\mathrm{CO_2}$ ⁠ molecule, which holds one carbon.
3. So it takes $6$ ⁠ molecules of $\mathrm{CO_2}$ ⁠.

Lesson 2 of 4 · About 25 minutes

Limiting factors

Light, carbon dioxide and temperature: the rate is set by whichever is scarce, and raising one factor helps only until another becomes limiting.

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The 6 steps and their answers

Step 1 of 6
Open this step
The weakest link
Practice · Predict, then look
Question
Light is at 30%, Carbon dioxide at 80%, and Temperature at 25 °C, comfortable for the enzymes. What is the limiting factor?

Correct answer
- Correct Light
Setup, choices and feedback
The Scene at this step
Light
30 %fixed
Carbon dioxide
80 %fixed
Temperature
298.1 Kfixed
Chloroplast / Inside
Chloroplast
Choices
- Correct Light
- Wrong Carbon dioxide
  Feedback
  There is carbon dioxide for 80% of the rate, much more than the light allows.
- Wrong Temperature
  Misconception: Thinks photosynthesis keeps speeding up the hotter it gets.
  Feedback from the catalogue
  Heat speeds photosynthesis’ enzymes up only to the optimum temperature, about 25–30 °C in many plants. Above it the enzymes start to denature, and the rate falls; at 45 °C it is nearly zero. That is why many plants stop growing in a heatwave.
Once the prediction is locked in, the Scene changes
Chloroplast / Inside
Inside
The Scene then shows: 1 · The limiting factor: 1 light, 2 carbon dioxide, 3 temperature, 0 two or more at once/nodes/leaf/outputs/limiting
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Light: it allows only 30% of the rate, less than any other factor. More carbon dioxide or more warmth would change nothing; only more light.

Worked solution
1. Light: 30%. Carbon dioxide: 80%. A temperature of 25 °C: 100%.
2. The limiting factor is the one that allows the lowest rate.
3. Light.
Step 2 of 6
Open this step
More light
Practice · Reach the target
Question
Carbon dioxide is fixed at 40%, and the temperature is comfortable. Raise Light until photosynthesis reaches the highest rate these conditions allow, then press Check.

Goal
- 40 ± 0.5
  At the start of the step: 10
Setup, choices and feedback
The Scene at this step
Light
10 %
Carbon dioxide
40 %fixed
Temperature
298.1 Kfixed
Chloroplast / Inside
Chloroplast
Read from the Scene at
Rate of photosynthesis, per cent of the fastest: the scarcest of light, carbon dioxide and what the temperature allows/nodes/leaf/outputs/rate
One setting that reaches it
Light
40 %
Feedback for a right answer
40%: once the light reaches 40%, carbon dioxide becomes the limiting factor, and more light speeds nothing up. On a graph of rate against light this is where the curve levels off.

Feedback for a wrong answer
Not yet. How much light does it take for light to stop being the scarce factor?

Hint
Past a certain point, more light adds nothing.

Worked solution
1. The rate is the smallest of the factors. Carbon dioxide allows 40%.
2. While the light is below 40%, it is the limiting factor, and more light raises the rate.
3. From 40% up the rate is stuck at 40%. Raise Light to 40% or more.
Step 3 of 6
Open this step
A cold morning
Practice · Predict, then look
Question
Full light and plenty of carbon dioxide, at 25 °C: the rate is 100%. We will cool it to 15 °C. At what percentage of the top rate will photosynthesis run, if the enzymes slow by half for every 10 °C of cooling?

Correct answer
50 ± 2
A number prediction counts as right within ± 2
Setup, choices and feedback
The Scene at this step
Light
100 %fixed
Carbon dioxide
100 %fixed
Temperature
298.1 Kfixed
Chloroplast / Inside
Chloroplast
Once the prediction is locked in, the Scene changes
Temperature
288.1 Kfixed
The Scene then shows: 50 · Rate of photosynthesis, per cent of the fastest: the scarcest of light, carbon dioxide and what the temperature allows/nodes/leaf/outputs/rate
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
50%. Now temperature is the limiting factor: light and carbon dioxide are plentiful, but the enzymes that build the sugar work slowly. On a cold morning, even in full sun, a plant builds little.

Hint
A cooling of exactly 10 °C.

Worked solution
1. From 25 °C to 15 °C: a cooling of 10 °C.
2. The enzymes slow by half: $100 \div 2 = 50$ ⁠.
3. Light and carbon dioxide allow 100%, so temperature limits: 50%.
Step 4 of 6
Open this step
A hot day
Practice · Predict, then look
Question
Full light and plenty of carbon dioxide, at 25 °C. A hot day comes, 40 °C. What will happen to the rate?

Correct answer
- Correct It will fall
Setup, choices and feedback
The Scene at this step
Light
100 %fixed
Carbon dioxide
100 %fixed
Temperature
298.1 Kfixed
Chloroplast / Inside
Chloroplast
Choices
- Correct It will fall
- Wrong It will rise, because heat speeds enzymes up
  Misconception: Thinks photosynthesis keeps speeding up the hotter it gets.
  Feedback from the catalogue
  Heat speeds photosynthesis’ enzymes up only to the optimum temperature, about 25–30 °C in many plants. Above it the enzymes start to denature, and the rate falls; at 45 °C it is nearly zero. That is why many plants stop growing in a heatwave.
- Wrong It will stay at 100%
  Misconception: Thinks more light always makes photosynthesis faster.
  Feedback
  Plenty of light and carbon dioxide is not enough: the enzymes have to work too.
Once the prediction is locked in, the Scene changes
Temperature
313.1 Kfixed
The Scene then shows: 33.33 · Rate of photosynthesis, per cent of the fastest: the scarcest of light, carbon dioxide and what the temperature allows/nodes/leaf/outputs/rate
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
It falls to about a third. Above 30 °C photosynthesis’ enzymes start to denature, and at 45 °C they hardly work at all.

Worked solution
1. Up to the optimum, heat speeds the enzymes up.
2. Above it the enzymes, being proteins, start to lose their shape.
3. At 40 °C the temperature allows only about a third of the rate, and it is the limiting factor.
Step 5 of 6
Open this step
All three
Practice · Reach the target
Question
A cold, cloudy morning: light 50%, carbon dioxide 30%, 10 °C. Change all three until photosynthesis runs at 80% of its top rate, then press Check.

Goal
- 80 ± 2
  At the start of the step: 30
Setup, choices and feedback
The Scene at this step
Light
50 %
Carbon dioxide
30 %
Temperature
283.1 K
Chloroplast / Inside
Chloroplast
Read from the Scene at
Rate of photosynthesis, per cent of the fastest: the scarcest of light, carbon dioxide and what the temperature allows/nodes/leaf/outputs/rate
One setting that reaches it
Light
80 %
Carbon dioxide
80 %
Temperature
295.1 K
Feedback for a right answer
Done. Each time you raised one factor, the next one in line became limiting, until all of them were enough. That is exactly how growers run a greenhouse.

Feedback for a wrong answer
Not yet. Which factor is limiting now?

Hint
All three factors must allow at least 80%, and at least one of them exactly 80%.

Worked solution
1. At 10 °C the temperature allows only about 35%. Warm it to 25 °C.
2. Now carbon dioxide, at 30%, limits. Raise it to 80%.
3. Now light, at 50%, limits. Raise it to 80%.
4. The rate: $\min(80, 80, 100) = 80$ ⁠.
Step 6 of 6
Open this step
In a greenhouse
Practice · Choose
Question
A winter tomato greenhouse has heating and strong lamps, and the windows shut. The grower adds carbon dioxide to the air, and the crop grows. Why?

Correct answer
- Correct With plenty of light and warmth, carbon dioxide was the limiting factor.
Setup, choices and feedback
The Scene at this step
Light
100 %
Carbon dioxide
30 %
Temperature
298.1 K
Chloroplast / Inside
Chloroplast
Choices
Shown in a random order.
- Correct With plenty of light and warmth, carbon dioxide was the limiting factor.
- Wrong The carbon dioxide strengthens the light the plants catch.
  Misconception: Thinks more light always makes photosynthesis faster.
  Feedback from the catalogue
  Light speeds photosynthesis up only while it is the limiting factor. Once there is enough light, the rate stops at the next factor: carbon dioxide or temperature. Past that point the curve levels off, and more light changes nothing. That is why greenhouse growers add carbon dioxide, not just lamps.
- Wrong The carbon dioxide warms the greenhouse, and heat always speeds things up.
  Misconception: Thinks photosynthesis keeps speeding up the hotter it gets.
  Feedback from the catalogue
  Heat speeds photosynthesis’ enzymes up only to the optimum temperature, about 25–30 °C in many plants. Above it the enzymes start to denature, and the rate falls; at 45 °C it is nearly zero. That is why many plants stop growing in a heatwave.
Feedback for a right answer
Right. In a shut greenhouse the plants use up the carbon dioxide fast, and its level falls. When light and warmth are already enough, it is what holds the rate back, so adding it pays.

Worked solution
1. In a heated, lit greenhouse, light and temperature allow a high rate.
2. With the windows shut the carbon dioxide runs low, and it becomes the limiting factor.
3. Adding the limiting factor, and only that, raises the rate.

Lesson 3 of 4 · About 25 minutes

Cellular respiration

How a cell breaks glucose down with oxygen: glycolysis, the Krebs cycle and the electron transport chain, and how much ATP each stage gives.

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The 5 steps and their answers

Step 1 of 5
Open this step
In every cell
Practice · Choose
Question
Where does cellular respiration take place?

Correct answer
- Correct In every living cell in the body, mainly in its mitochondria.
Setup, choices and feedback
The Scene at this step
Oxygen supply
100 %
Glucose / Electron chain
Glucose
Choices
Shown in a random order.
- Correct In every living cell in the body, mainly in its mitochondria.
- Wrong In the lungs.
  Misconception: Takes cellular respiration to be breathing with the lungs.
  Feedback from the catalogue
  Breathing with the lungs is moving air in and out: it brings oxygen to the blood and removes carbon dioxide. Cellular respiration is a chemical reaction inside every cell, in which sugar and oxygen become carbon dioxide and water, and the energy is stored in ATP. The first serves the second. Bacteria and plants, which have no lungs, respire too.
- Wrong In the blood, as it carries oxygen.
  Misconception: Takes cellular respiration to be breathing with the lungs.
  Feedback from the catalogue
  Breathing with the lungs is moving air in and out: it brings oxygen to the blood and removes carbon dioxide. Cellular respiration is a chemical reaction inside every cell, in which sugar and oxygen become carbon dioxide and water, and the energy is stored in ATP. The first serves the second. Bacteria and plants, which have no lungs, respire too.
Feedback for a right answer
Right. The lungs and blood only deliver the oxygen; respiration itself, breaking down sugar and making ATP, happens inside every cell, in a muscle cell as in a skin cell.

Worked solution
1. Cellular respiration: $\mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2} \rightarrow 6\,\mathrm{CO_2} + 6\,\mathrm{H_2O}$ ⁠, with the energy stored in ATP.
2. Every cell needs ATP, so every cell respires.
3. The first stage is in the cytoplasm, the rest in the mitochondria.
Step 2 of 5
Open this step
Stage by stage
Practice · Predict, then look
Question
Glycolysis is done: one glucose has split into two pyruvates, and the tally reads 2 ATP. The pyruvates now enter the Krebs cycle. How many ATP will the tally show, with oxygen, once the cycle is done?

Correct answer
4 ± 0.5
A number prediction counts as right within ± 0.5
Setup, choices and feedback
The Scene at this step
Oxygen supply
100 %fixed
Glucose / Electron chain
Glycolysisfixed
Once the prediction is locked in, the Scene changes
Glucose / Electron chain
Krebs cyclefixed
The Scene then shows: 4 · ATP the traced glucose has made so far with oxygen, as the tally counts it/nodes/mito/outputs/atpWithOxygen
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Only $4$ ⁠: the Krebs cycle adds 2 ATP directly. But it strips all the carbon off as $\mathrm{CO_2}$ ⁠ and loads electron carriers, whose energy the electron chain will turn into another 28 ATP.

Hint
The Krebs cycle is not the stage that makes most of the ATP.

Worked solution
1. After glycolysis: 2 ATP.
2. The Krebs cycle adds 2 ATP directly per glucose (one per pyruvate).
3. In all: $2 + 2 = 4$ ⁠.
Step 3 of 5
Open this step
The chain
Practice · Choose
Question
Which stage of cellular respiration makes most of the ATP?

Correct answer
- Correct The electron transport chain
Setup, choices and feedback
The Scene at this step
Oxygen supply
100 %fixed
Glucose / Electron chain
Electron chain
Choices
Shown in a random order.
- Correct The electron transport chain
- Wrong The Krebs cycle
  Misconception: Thinks the Krebs cycle, the famous stage, makes most of the ATP.
  Feedback from the catalogue
  The Krebs cycle makes only 2 ATP directly per glucose. Its main job is to strip the carbon and load up electron carriers. Most of the ATP, about 28 of 32, comes from the electron transport chain in the mitochondrion’s inner membrane, at whose end the electrons reach oxygen.
- Wrong Glycolysis
  Feedback
  Glycolysis makes only 2 ATP, but it is the only stage that works without oxygen too.
Feedback for a right answer
The chain: about 28 of 32. The electrons pass along the folded inner membrane to oxygen, and the energy they release spins the enzyme ATP synthase like a turbine.

Worked solution
1. Glycolysis: 2 ATP.
2. Krebs cycle: 2 ATP.
3. Electron transport chain: about 28 ATP.
4. Most of it, $\dfrac{28}{32}$ ⁠, comes from the chain.
Step 4 of 5
Open this step
What comes out
Practice · Calculate
Question
There is plenty of oxygen, and all the pyruvate is burned. How many carbon dioxide molecules are released from each glucose, $\mathrm{C_6H_{12}O_6}$ ⁠?

Correct answer
6 ± 0.1
Setup, choices and feedback
The Scene at this step
Oxygen supply
100 %fixed
Glucose / Electron chain
Electron chain
Accepted within
± 0.1
Computed from the Scene at this step’s setting
CO₂ molecules given off per glucose: 6 × the share burned/nodes/mito/outputs/carbonDioxidePerGlucose
Feedback for a right answer
Six, one for each carbon atom: $\mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2} \rightarrow 6\,\mathrm{CO_2} + 6\,\mathrm{H_2O}$ ⁠. That is photosynthesis run backwards. The blood carries this carbon dioxide to the lungs, and you breathe it out.

Hint
Each carbon atom of the glucose leaves in a $\mathrm{CO_2}$ ⁠ molecule of its own.

Worked solution
1. Glucose holds $6$ ⁠ carbon atoms.
2. In full respiration all the carbon leaves as $\mathrm{CO_2}$ ⁠, one carbon in each.
3. So $6$ ⁠ molecules of $\mathrm{CO_2}$ ⁠.
Step 5 of 5
Open this step
Plants too
Practice · Choose
Question
When does a plant carry out cellular respiration?

Correct answer
- Correct All the time, day and night.
Setup, choices and feedback
The Scene at this step
Oxygen supply
100 %
Glucose / Electron chain
Krebs cycle
Choices
Shown in a random order.
- Correct All the time, day and night.
- Wrong Only at night, when there is no photosynthesis.
  Misconception: Thinks plants photosynthesise instead of respiring, or respire only at night.
  Feedback from the catalogue
  Plants respire all the time, day and night, in every living cell, as animals do: they break sugar down with oxygen in their mitochondria to make ATP. By day photosynthesis outpaces respiration, so on balance the plant gives out oxygen; at night only respiration is left, and it gives out carbon dioxide.
- Wrong Never: plants photosynthesise instead of respiring.
  Misconception: Thinks plants photosynthesise instead of respiring, or respire only at night.
  Feedback from the catalogue
  Plants respire all the time, day and night, in every living cell, as animals do: they break sugar down with oxygen in their mitochondria to make ATP. By day photosynthesis outpaces respiration, so on balance the plant gives out oxygen; at night only respiration is left, and it gives out carbon dioxide.
Feedback for a right answer
Right. Every living cell needs ATP all the time. By day a plant both photosynthesises and respires, and photosynthesis is faster, so on balance it gives out oxygen. At night it only respires.

Worked solution
1. Photosynthesis builds sugar; respiration breaks it down and makes ATP.
2. Plant cells need ATP all the time, and they have mitochondria.
3. So they respire day and night. By day you just don’t notice, as photosynthesis masks it.

Lesson 4 of 4 · About 25 minutes

Without oxygen

What happens when there is not enough oxygen: glycolysis alone, lactate in muscles, alcohol and carbon dioxide in yeast, and the ATP sums when the oxygen is partial.

Open the lesson

The 7 steps and their answers

Step 1 of 7
Open this step
Cut off
Practice · Predict, then look
Question
With plenty of oxygen each glucose gives 32 ATP. We will cut off the oxygen supply completely. How many ATP will each glucose give?

Correct answer
2 ± 0.5
A number prediction counts as right within ± 0.5
Setup, choices and feedback
The Scene at this step
Oxygen supply
100 %fixed
Glucose / Electron chain
Glucose
Once the prediction is locked in, the Scene changes
Oxygen supply
0 %fixed
The Scene then shows: 2 · ATP the cell gets per glucose at this oxygen supply: 2 + 30 × the share burned/nodes/mito/outputs/atpPerGlucose
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Only $2$ ⁠, from glycolysis. Without oxygen the electron chain stops, and the Krebs cycle with it. The pyruvate becomes lactate, so that glycolysis can keep going.

Hint
Which stage needs no oxygen?

Worked solution
1. Oxygen is the last acceptor of the chain’s electrons. Without it, the chain stops.
2. Without the chain, the Krebs cycle stops too.
3. Only glycolysis is left, in the cytoplasm: 2 ATP.
Step 2 of 7
Open this step
How many times more
Practice · Calculate
Question
The supply is full. How many times more ATP does a glucose give by respiration with oxygen than without?

Correct answer
16 ± 0.1
Setup, choices and feedback
The Scene at this step
Oxygen supply
100 %fixed
Glucose / Electron chain
Electron chain
Accepted within
± 0.1
Computed from the Scene at this step’s setting
ATP the cell gets per glucose at this oxygen supply: 2 + 30 × the share burned/nodes/mito/outputs/atpPerGlucose
Feedback for a right answer
16 times. So a muscle short of oxygen must burn sixteen times the glucose for the same energy, which is why you can sprint only briefly.

Hint
$32$ ⁠ with oxygen, $2$ ⁠ without.

Worked solution
1. With oxygen: 32 ATP per glucose. Without: 2.
2. The ratio: $\dfrac{32}{2} = 16$ ⁠.
3. 16 times.
Step 3 of 7
Open this step
Half the oxygen
Practice · Calculate
Question
The muscle gets only half the oxygen it needs, so only half the pyruvate is burned in the mitochondria. How many ATP does each glucose give, on average?

Correct answer
17 ± 0.1
Setup, choices and feedback
The Scene at this step
Oxygen supply
50 %fixed
Glucose / Electron chain
Electron chain
Accepted within
± 0.1
Computed from the Scene at this step’s setting
ATP the cell gets per glucose at this oxygen supply: 2 + 30 × the share burned/nodes/mito/outputs/atpPerGlucose
Wrong answers it recognises
- 16 ± 0.1
  Misconception: Halves the aerobic ATP for half the oxygen, forgetting the 2 ATP glycolysis makes without oxygen.
  Feedback from the catalogue
  Glycolysis makes 2 ATP from every glucose, with oxygen or without. Only the other 30 depend on oxygen. With half the oxygen, half the pyruvate is burned: $2 + 30 \times 0.5 = 17$ ⁠, not $\dfrac{32}{2} = 16$ ⁠.
Feedback for a right answer
$2 + 30 \times 0.5 = 17$ ⁠. Glycolysis does not depend on oxygen; only the 30 ATP of the Krebs cycle and the chain scale with the oxygen.

Hint
Glycolysis gives $2$ ⁠ either way. What depends on oxygen?

Worked solution
1. Glycolysis: 2 ATP, always.
2. Krebs and the chain: 30 ATP when all the pyruvate is burned; here only half: $30 \times 0.5 = 15$ ⁠.
3. In all: $2 + 15 = 17$ ⁠.
If you answered $16$ ⁠, you halved all $32$ ⁠, including glycolysis’s $2$ ⁠, which do not depend on oxygen.
Step 4 of 7
Open this step
Twenty-six
Practice · Reach the target
Question
Change Oxygen supply until each glucose gives 26 ATP, then press Check.

Goal
- 26 ± 0.5
  At the start of the step: 2
Setup, choices and feedback
The Scene at this step
Oxygen supply
0 %
Glucose / Electron chain
Electron chain
Read from the Scene at
ATP the cell gets per glucose at this oxygen supply: 2 + 30 × the share burned/nodes/mito/outputs/atpPerGlucose
One setting that reaches it
Oxygen supply
80 %
Feedback for a right answer
80%: $2 + 30 \times 0.8 = 26$ ⁠. A fifth of the pyruvate becomes lactate.

Feedback for a wrong answer
Not yet. How many ATP above glycolysis’s $2$ ⁠ do you need, and what share of $30$ ⁠ is that?

Hint
$26 = 2 + 30 \times f$ ⁠, where $f$ ⁠ is the share of the pyruvate burned.

Worked solution
1. $26 - 2 = 24$ ⁠ ATP must come from the Krebs cycle and the chain.
2. Out of a possible $30$ ⁠: $\dfrac{24}{30} = 0.8$ ⁠.
3. Set Oxygen supply to 80%.
Step 5 of 7
Open this step
Lactate
Practice · Calculate
Question
At half the oxygen, how many lactate molecules form from each glucose, on average? (A glucose splits into two pyruvates.)

Correct answer
1 ± 0.05
Setup, choices and feedback
The Scene at this step
Oxygen supply
50 %fixed
Glucose / Electron chain
Glycolysis
Accepted within
± 0.05
Computed from the Scene at this step’s setting
Lactate molecules made per glucose from the pyruvate not burned/nodes/mito/outputs/lactatePerGlucose
Wrong answers it recognises
- 2 ± 0.05
  Misconception: Thinks a cell is either fully aerobic or fully anaerobic, never partly.
  Feedback from the catalogue
  A cell does not flip a switch. When only part of the oxygen it needs arrives, in a muscle during a sprint for instance, some of the pyruvate is burned in the mitochondria and some becomes lactate. The less oxygen, the bigger the share that becomes lactate.
- 0 ± 0.05
  Misconception: Thinks a cell is either fully aerobic or fully anaerobic, never partly.
  Feedback from the catalogue
  A cell does not flip a switch. When only part of the oxygen it needs arrives, in a muscle during a sprint for instance, some of the pyruvate is burned in the mitochondria and some becomes lactate. The less oxygen, the bigger the share that becomes lactate.
Feedback for a right answer
One: of the two pyruvates, one is burned in the mitochondria and the other becomes lactate. The muscle works partly with oxygen and partly without, and the less oxygen, the more lactate builds up.

Hint
What happens to the half of the pyruvate that is not burned?

Worked solution
1. Each glucose gives $2$ ⁠ pyruvates.
2. At half the oxygen, half is burned: $2 \times 0.5 = 1$ ⁠ pyruvate.
3. The rest, $2 - 1 = 1$ ⁠, becomes lactate.
A cell need not be all aerobic or all anaerobic: the split follows the supply.
Step 6 of 7
Open this step
Yeast
Practice · Choose
Question
Dough with yeast rises in a covered bowl, even after its oxygen has run out. What puffs it up?

Correct answer
- Correct Carbon dioxide from the yeast’s alcoholic fermentation
Setup, choices and feedback
The Scene at this step
Oxygen supply
0 %
Glucose / Electron chain
Glycolysis
Choices
Shown in a random order.
- Correct Carbon dioxide from the yeast’s alcoholic fermentation
- Wrong Lactate, as in our muscles
  Feedback
  Lactic fermentation, as in muscles, releases no gas at all. Yeast breaks pyruvate down differently: into alcohol and carbon dioxide.
- Wrong Oxygen the yeast makes
  Feedback
  Yeasts are fungi: they do not photosynthesise or make oxygen.
Feedback for a right answer
Right. Without oxygen, yeast turns pyruvate into alcohol (ethanol) and carbon dioxide. The gas is trapped in the dough and puffs it up, and the alcohol evaporates in the oven. The same fermentation makes wine and beer.

Worked solution
1. Without oxygen, yeast too relies on glycolysis and 2 ATP per glucose.
2. To keep glycolysis going, it breaks pyruvate down into ethanol and $\mathrm{CO_2}$ ⁠: alcoholic fermentation.
3. The $\mathrm{CO_2}$ ⁠ is the gas that puffs up the dough.
Step 7 of 7
Open this step
The currency
Practice · Choose
Question
Which statement about ATP is true?

Correct answer
- Correct It carries energy round the cell: built in respiration, and broken down wherever energy is needed.
Setup, choices and feedback
The Scene at this step
Oxygen supply
100 %
Glucose / Electron chain
Electron chain
Choices
Shown in a random order.
- Correct It carries energy round the cell: built in respiration, and broken down wherever energy is needed.
- Wrong It is made only in mitochondria.
  Misconception: Thinks ATP is made only in mitochondria.
  Feedback from the catalogue
  Glycolysis makes ATP in the cytoplasm, with no mitochondria and no oxygen. That is how red blood cells, which have no mitochondria at all, get by, and many bacteria too. And chloroplasts make ATP of their own with light. Mitochondria make most of the ATP, not all of it.
- Wrong The body keeps a large store of it for hard times.
  Feedback
  The body holds only a few tens of grams of ATP at any moment, built and broken down again all the time. Long-term energy is stored as glycogen and fat.
Feedback for a right answer
Right. ATP is the cell’s “currency”: when its last bond breaks, it becomes ADP and releases energy for a muscle to contract, for pumping ions and for building proteins. Respiration turns the ADP back into ATP. A person breaks down and rebuilds roughly their own body weight in ATP every day.

Worked solution
1. Energy from food does not go straight into work: it is first stored in a bond of ATP.
2. ATP is also made in glycolysis, in the cytoplasm, not only in mitochondria.
3. It is broken down where energy is needed, and rebuilt by respiration.

Across the unit · 11 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

Photosynthesis

From thin air

Question

Correct answer

The Scene at this step

Choices

Worked solution

Light is energy

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

Inside the chloroplast

Question

Correct answer

The Scene at this step

Choices

Worked solution

The scarcer input

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

The equation

Question

Correct answer

The Scene at this step

Hint

Worked solution

Limiting factors

The weakest link

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

More light

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

A cold morning

Question

Correct answer

The Scene at this step

Once the prediction is locked in, the Scene changes

Hint

Worked solution

A hot day

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

All three

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

In a greenhouse

Question

Correct answer

The Scene at this step

Choices

Worked solution

Cellular respiration

In every cell

Question

Correct answer

The Scene at this step