Biology · Unit 3

Cell Transport and Energy

Diffusion, photosynthesis and respiration

This is where cells stop being a diagram and start being a machine. Everything here is about traffic - what crosses the membrane, in which direction, and what it costs the cell to make that happen.

Photosynthesis and respiration are the two headline processes, and they are best learned together rather than apart, because one is very nearly the other run backwards. Diffusion, osmosis and active transport underpin both, and ATP is the currency all of it is priced in.

This unit breaks down into 20 short steps and 130 questions, starting at difficulty 1 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 20
Questions: 130
Difficulty: 1-3

What this unit covers

Photosynthesis
Cellular Respiration
ATP and Energy
Active Transport
Passive Transport and Diffusion

Where this fits

Needs The Cell first. Feeds directly into Molecular Biology, and into the digestion, lungs and circulation units, which are all transport problems at a larger scale.

Where people slip

Passive and active transport are separated by energy cost, not by direction or speed. Something moving into a cell is not automatically active transport - the question is whether it is going down its concentration gradient or being pushed up it.

How the unit is structured

Cell Transport and Energy runs as 20 short steps that unlock in order. 15 are practice rounds and 5 are challenge rounds that pull together everything before them. Questions start at difficulty 1 and climb to 3 as you progress.

Step 1 · easierStep 20 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 130 in the unit altogether.

ATP and Energy

Choose all that applyLevel 2
1. Which statements about ATP are correct? Select all that apply.
- ATP stores energy that cells can use quicklycorrect
- ATP is made when energy is released during respirationcorrect
- ATP provides the energy for active transportcorrect
- ATP is the gas that plants release in photosynthesis
ATP is made during respiration and stores energy that cells spend on jobs such as active transport; it is a molecule, not a gas.
Fill the blankLevel 3
2. ____ is the coenzyme that carries hydrogen in photosynthesis; it differs from NAD by having an extra phosphate group.
- NADPcorrect
- FAD
- ADP
- Coenzyme A
NADP is like NAD but with an additional phosphate, and it is the hydrogen carrier used in the light-dependent reactions and Calvin cycle.
Guess the numberLevel 3
3. Roughly how many ATP molecules does aerobic respiration release from a single glucose molecule?
Answer: 38 ATP
Aerobic respiration yields about 38 ATP per glucose, far more than the 2 from anaerobic respiration.
Multiple choiceLevel 3
4. In aerobic respiration, which stage generates by far the most ATP?
- Oxidative phosphorylation (the electron transport chain)correct
- Glycolysis
- The link reaction
- The Krebs cycle
Most ATP is produced by chemiosmosis during oxidative phosphorylation on the inner mitochondrial membrane.
Odd one outLevel 3
5. Three of these describe aerobic respiration. Which one does NOT belong?
- Releases only about 2 ATP per glucosecorrect
- Uses oxygen
- Produces carbon dioxide and water
- Releases a large amount of energy
Aerobic respiration releases lots of ATP; the low yield of about 2 ATP belongs to anaerobic respiration.
Picture questionLevel 3
6. 🔋 This rechargeable battery stands for the cell's short-term energy store, recharged by adding a phosphate to ADP during oxidative phosphorylation. Which molecule is it?
- ATPcorrect
- Glucose
- NADP
- Oxygen
ATP is recharged from ADP + Pi and hydrolysed again when energy is needed, acting like a rechargeable battery for the cell.

Active Transport

Fact or fibLevel 1
7. Active transport requires energy from the cell to move substances.
Answer: True
Active transport moves substances against their concentration gradient, so it needs energy supplied by ATP.
Multiple choiceLevel 1
8. Root hair cells take in mineral ions from soil that is more dilute than the cell. Moving them uphill against the gradient is called...
- Active transportcorrect
- Osmosis
- Simple diffusion
- Evaporation
Active transport uses energy to move substances against their concentration gradient.
Build the sentenceLevel 3
9. Arrange the words to describe how glucose is taken up by co-transport in the small intestine.
Answer: glucose enters with sodium ions through a co-transporter
In co-transport, glucose enters the cell coupled to sodium ions moving through the same carrier protein.
Choose all that applyLevel 3
10. Which statements about the sodium-potassium pump are correct? Select all that apply.
- It moves 3 sodium ions out for every 2 potassium ions it brings incorrect
- It uses energy from ATPcorrect
- It moves sodium and potassium in opposite directionscorrect
- It works by simple diffusion without any energy
The pump uses ATP to move 3 sodium ions out and 2 potassium ions in per cycle, carrying the ions in opposite directions against their gradients.
Guess the numberLevel 3
11. For each ATP molecule it uses, how many potassium ions does the sodium-potassium pump move into the cell?
Answer: 2 ions
Each cycle of the pump uses one ATP to move 2 potassium ions in and 3 sodium ions out.
Match the pairsLevel 3
12. Match each type of bulk transport to what it means.
Answer: Phagocytosis = Engulfing large solid particles; Pinocytosis = Taking in droplets of liquid; Exocytosis = Releasing materials from vesicles; Endocytosis = Bringing material in inside vesicles
Bulk transport uses vesicles and ATP: phagocytosis and pinocytosis take material in, while exocytosis releases it and endocytosis is the general term for uptake.

Cellular Respiration

Fact or fibLevel 1
13. Respiration happens in every living cell, in both plants and animals.
Answer: True
All living cells respire all the time to release the energy they need to stay alive.
Fill the blankLevel 1
14. Most aerobic respiration in a cell takes place in the ____.
- mitochondriacorrect
- nucleus
- cell wall
- ribosomes
The mitochondria are the site of most aerobic respiration, which is why they are often called the powerhouse of the cell.
Build the sentenceLevel 2
15. Build the word equation for fermentation (anaerobic respiration) in yeast.
Answer: glucose ethanol carbon dioxide
In yeast, glucose is converted to ethanol plus carbon dioxide with no oxygen used.
Choose all that applyLevel 3
16. For each pyruvate that goes through the link reaction, which products are made? Select all that apply.
- Acetyl coenzyme Acorrect
- Carbon dioxidecorrect
- Reduced NADcorrect
- ATP
Each pyruvate is converted to acetyl coenzyme A, releasing one carbon dioxide and reducing one NAD; no ATP is made in the link reaction.
Guess the numberLevel 3
17. In the balanced equation for aerobic respiration, how many molecules of oxygen react with one molecule of glucose?
Answer: 6 molecules
The balanced equation is C6H12O6 + 6O2 to 6CO2 + 6H2O, so six oxygen molecules react with one glucose molecule.
Match the pairsLevel 3
18. Match each molecule involved in respiration to its role.
Answer: Reduced NAD = Carries hydrogen to the electron transport chain; Coenzyme A = Carries the acetyl group in the link reaction; Oxaloacetate = Regenerated at the end of each Krebs cycle turn; ATP synthase = Makes ATP as protons pass through it
Reduced NAD delivers hydrogen to the chain, coenzyme A carries the acetyl group, oxaloacetate is regenerated each Krebs turn, and ATP synthase makes ATP from the proton flow.

Passive Transport and Diffusion

Fill the blankLevel 1
19. A good gas exchange surface, like the lining of an air sac, is very ____ so gases only diffuse a short distance.
- thincorrect
- thick
- dry
- hard
A thin surface shortens the diffusion distance, so substances cross it quickly.
Choose all that applyLevel 2
20. Which changes would increase the rate of diffusion across a surface? Select all that apply.
- A steeper concentration gradientcorrect
- A higher temperaturecorrect
- A larger surface areacorrect
- A thicker barrier to cross
A steeper gradient, more heat and more surface area all speed diffusion, while a thicker barrier slows it.
Multiple choiceLevel 2
21. Osmosis is best described as the movement of which substance across a selectively permeable membrane?
- Water, from a dilute solution to a more concentrated solution
- Glucose, from high to low concentration
- Oxygen, using energy from ATP
- Salt ions, from a dilute to a concentrated solution
Osmosis is the diffusion of water across a selectively permeable membrane, from a dilute solution (more water) to a more concentrated solution (less water).
Odd one outLevel 2
22. Which one is NOT an example of passive transport?
- Oxygen diffusing into a cell
- Water moving in by osmosis
- Pumping ions against the gradient using ATPcorrect
- Carbon dioxide diffusing out of a cell
Diffusion and osmosis move substances down the gradient with no energy, but pumping ions against the gradient using ATP is active transport.
Fact or fibLevel 3
23. By osmosis, water moves from a solution with a lower (more negative) water potential to one with a higher water potential.
Answer: False
Water moves down the water potential gradient, from higher (less negative) to lower (more negative) water potential, so this statement is reversed.
Guess the numberLevel 3
24. Diffusion rate is proportional to surface area. If a surface's area is made 4 times larger and nothing else changes, how many times faster does diffusion happen?
Answer: 4 times
Because rate scales with surface area, quadrupling the area roughly quadruples the diffusion rate.

Photosynthesis

Multiple choiceLevel 1
25. Which tiny pores, mostly on the underside of a leaf, let carbon dioxide in and oxygen out?
- Stomatacorrect
- Veins
- Roots
- Seeds
Stomata are small pores in the leaf surface that allow gases to move in and out.
Build the sentenceLevel 2
26. Arrange the words to form the word equation for photosynthesis.
Answer: carbon dioxide + water → glucose + oxygen
In photosynthesis, carbon dioxide and water are combined using light energy to produce glucose and oxygen.
Choose all that applyLevel 2
27. Which of these can be a limiting factor of photosynthesis? Select all that apply.
- Light intensitycorrect
- Carbon dioxide concentrationcorrect
- Temperaturecorrect
- Oxygen concentration
Light, carbon dioxide and temperature can each limit photosynthesis; oxygen is a product, not a limiting factor.
Match the pairsLevel 2
28. Match each part of photosynthesis to its role.
Answer: Chlorophyll = Green pigment that absorbs light; Chloroplast = Structure where photosynthesis happens; Glucose = Sugar product used for energy; Oxygen = Gas released as a by-product
Chlorophyll inside chloroplasts absorbs light to help make glucose, and oxygen is given off as a by-product.
Fill the blankLevel 3
29. In the Calvin cycle, rubisco fixes carbon dioxide onto RuBP and the first stable product is two molecules of the 3-carbon compound ____.
- glycerate-3-phosphate (GP)correct
- triose phosphate (TP)
- ribulose bisphosphate (RuBP)
- oxaloacetate
Carbon fixation splits the unstable 6-carbon intermediate into two molecules of glycerate-3-phosphate (GP), the first stable 3-carbon product.
Guess the numberLevel 3
30. The Calvin cycle must fix one carbon dioxide molecule per turn. How many molecules of carbon dioxide (and so how many turns) are needed to make one 6-carbon glucose molecule?
Answer: 6 molecules of CO2
Glucose has 6 carbons, so 6 molecules of CO2 must be fixed, meaning the cycle turns 6 times per glucose.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 17 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Listen and choose
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sequence recall
Sort into groups
Spell it
Tap the pairs
True or false
Type the answer

Practise Cell Transport and Energy

130 questions across 20 steps. Start with step one and crawl at your own pace.

Play this unit

Read about Cell Transport and Energy

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

More units in Biology

See all 18 units in Biology →