Lesson 1 of 8
Why do cells need ATP?
Cram notes for topic 3.1, Importance of , in WJEC A level Biology Unit 3.
is the immediate energy source in cells. of to and releases 30.6 kJ mol⁻¹. Joining and again is and needs energy from or light.
is the because it is used in all organisms and for almost all energy-requiring reactions.
- Uses: , protein synthesis, DNA replication, cell division, and activating molecules such as glucose.
- Advantages: one-step releases energy instantly, in small manageable amounts, and is easily remade.
- Energy not captured in is released as heat.
Lesson 2 of 8
How are electrons and hydrogen carried in cells?
is loss of electrons or hydrogen; is gain of electrons or hydrogen. A hydrogen atom is a plus an electron.
remove hydrogen from , which are . accept it and are : and in , in .
Lesson 3 of 8
What is the electron transport chain?BioWelsh recommended
We recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 8 marks in total. This is a revision priority, not a prediction of your exam.
The is an alternating series of and in the inner mitochondrial membrane or the membrane. Names of the proteins are not required.
We recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 8 marks in total. This is a revision priority, not a prediction of your exam.
, from and in or from light in , pass along the by reactions. They lose energy in small steps, and the pumps use this energy to move across the membrane.
The is oxygen in (forming water) and in . Without oxygen, or with , the chain stops.
Lesson 4 of 8
How is a proton gradient built up and kept?BioWelsh recommended
We recommend revising this because it has appeared regularly in past papers. Asked in 4 of 7 WJEC exam series (2017 to 2025), 10 marks in total. This is a revision priority, not a prediction of your exam.
We recommend revising this because it has appeared regularly in past papers. Asked in 4 of 7 WJEC exam series (2017 to 2025), 10 marks in total. This is a revision priority, not a prediction of your exam.
actively transport from the into the , or from the into the . This creates an : a difference in both concentration and charge.
The membranes are impermeable to except through , so the gradient is kept. More a lower pH. A difference of 3 pH units 10³ times the concentration of .
Lesson 5 of 8
How does ATP synthetase make ATP by chemiosmosis?BioWelsh recommended
We recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 13 marks in total. This is a revision priority, not a prediction of your exam.
We recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 13 marks in total. This is a revision priority, not a prediction of your exam.
is the flow of down their through , coupled to the synthesis of from and . The flow turns the rotor and stalk, which changes the shape of the head and makes .
The diagram below summarises electron transport and in a .
- Electrons from pass through three pumps; electrons from pass through two.
- Pumps move into the .
- Oxygen accepts electrons at the end and forms water.
- flow back through , making in the .
| Chemical | Effect |
|---|---|
| Stops electron flow to oxygen: no pumping, no | |
| Oligomycin | Blocks : gradient too steep, electron flow stops, builds up |
| DNP | Makes the membrane leaky to : oxygen still used, little , more heat |
Evidence: soaked at pH 4 then moved to pH 8 with and made in the dark, showing a alone can drive synthesis.
Lesson 6 of 8
How are mitochondria and chloroplasts similar in making ATP?
The diagram below compares where the is in each .
- pump from the into the .
- pump from the into the .
- In both, flow back through and is made on the low side.
| Feature | Mitochondria | Chloroplasts |
|---|---|---|
| Membrane | Inner membrane () | membrane |
| Energy source | Food, via and | Light |
| Oxygen | ||
| Name of synthesis |
In , adds to the and reducing removes from the , making the gradient steeper. gives three and gives two, because electrons pass through one fewer pump.
Lesson 7 of 8
How can dehydrogenase activity be measured?BioWelsh recommended
We recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 7 marks in total. This is a revision priority, not a prediction of your exam.
We recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 7 marks in total. This is a revision priority, not a prediction of your exam.
Artificial hydrogen acceptors are by hydrogen removed by and change colour: and turn from blue to colourless, and turns from colourless to red. The faster the colour change, the higher the activity.
Investigating dehydrogenase activity in yeast using methylene blue
Unit 3, section 1, specified practical 1 (BioWelsh entry number)Aim To find how temperature affects activity in yeast by timing how long takes to lose its colour.
Equipment
Yeast suspension in glucose solution Yeast suspension in glucose solutionSource of and .
Methylene blue : blue when , colourless when .
Water baths, thermometer, syringes, bungs and stop clock Water baths, thermometer, syringes, bungs and stop clock temperature, measure volumes, limit air and time the change.
Safety
- : irritant and allergenRiskSkin or eye irritation when using a syringe.ControlWear gloves and eye protection.
Variables
Independent variableTemperature
20 to 60 °C in water baths.
Dependent variableRate of dehydrogenase activity
1000 ÷ time for the blue colour to disappear.
Controlled variablesVolume and concentration of yeast, Glucose concentration, Volume and concentration of methylene blue, pH
5 cm³ of the same stirred suspension.
Same glucose solution for all yeast.
1 cm³ of the same solution.
pH 7 .
Control experimentBoiled and cooled yeast
Stays blue.
Variables in brief
- Independent: temperature.
- Dependent: rate, calculated from the time to .
- Controlled: yeast, glucose and dye volumes and concentrations, and pH.
- : boiled yeast, which should stay blue.
Method, step by step
- 1)
Put 5 cm³ of stirred yeast suspension into tubes and leave them, with the , in water baths at five temperatures for 10 minutes.
- 2)
Add 1 cm³ of , mix once, put in a bung and start timing.
Why?
Do not shake: oxygen re-oxidises the dye.
- 3)
Stop timing when the colour matches a yeast and water colour standard.
What you see when methylene blue is used to show dehydrogenase activity in yeast. Original BioWelsh diagram. Open full size - 4)
Use boiled yeast as a , repeat three times, and calculate rate = 1000 ÷ time.
Results and analysis
Example dataTime for to lose its colour in yeast suspension at each temperature
| Temperature / °C | Time 1 / s | Time 2 / s | Time 3 / s | time / s | Rate (1000 ÷ time) / s⁻¹ |
|---|---|---|---|---|---|
| 20 | 612 | 598 | 630 | 613.3 | 1.63 |
| 30 | 318 | 305 | 331 | 318.0 | 3.14 |
| 40 | 164 | 171 | 158 | 164.3 | 6.09 |
| 50 | 236 | 251 | 244 | 243.7 | 4.10 |
| 60 | no change in 900 s | no change in 900 s | no change in 900 s | not reached | 0 (recorded as zero) |
Rate of dehydrogenase activity in yeast at each temperature. Original BioWelsh graph.
Open full sizeWhat the results show
Rate rose from 1.63 s⁻¹ at 20 °C to 6.09 s⁻¹ at 40 °C.
It fell to 4.10 s⁻¹ at 50 °C, with no colour change at 60 °C, because the were denatured.
Worked calculations
Rate at 40 °C = 1000 ÷ 164.3 = 6.09 s⁻¹
Evaluation
The end point is subjective.
Effect on resultsTiming is less accurate.
ImprovementUse a colour standard, or with a .
Oxygen re-oxidises methylene blue.
Effect on resultsRate is underestimated.
ImprovementUse a bung and do not shake.
Model answer: describe the method
Describe how you would use to investigate the effect of temperature on activity in yeast.
Original practice response · 6 indicative marks
- Use water baths at five temperatures, for example 20 to 60 °C, checked with a thermometer.
- Add the same volume of stirred yeast suspension in glucose to tubes, keeping concentrations and pH the same, and equilibrate for 10 minutes.
- Add the same volume of , mix once, put in a bung and start timing; do not shake, so oxygen does not re-oxidise the dye.
- Time how long the blue colour takes to disappear, compared with a colour standard.
- Use boiled yeast as a , repeat three times at each temperature, and calculate rate as 1000 ÷ time.
What the answer needs to cover
- of temperatures in water baths
- Same yeast, glucose and pH
- Equilibration
- Same ; not shaken
- Time to with colour standard
- , repeats, and rate
Exam practice
Explain why the boiled yeast control stayed blue.Show answer
Boiling denatured the , so no hydrogen was removed from the and the was not .
- denatured
- No hydrogen passed to
Lesson 8 of 8
How do the ideas in this topic fit together?
in six steps
- 1) enter the chain
- 2) Electrons lose energy along the
- 3) Pumps move across the membrane
- 4) A builds up
- 5) diffuse back through
- 6) + →
- is produced; energy is released.
- Pumps move , not electrons.
- is in the ; is in the .
- Shaking lets oxygen re-oxidise .
Worksheets
Written by BioWelsh in the style of the WJEC A level Biology examination. Not an official WJEC paper.
Check your understanding.
31 questions. Practise in a focused view, with your answers kept while this page is open.
Related learning
- Nucleic acids
The structure of and its role as an energy are covered in Unit 1.
- Cell structure & organisation
You need the structure of and .
- Membranes & transport
carry out , and the membrane must be impermeable to .
- Enzymes
Temperature affects activity in the same way as other .
- Photosynthesis
uses the and .
- Respiration
and from , the and the supply electrons for .
- Experimental skills
Timing colour changes, calculating rates and evaluating methods are practical skills tested in Unit 5.
Specification & sources
- https://www.wjec.co.uk/media/mxbh1zaf/wjec-gce-biology-spec-from-2015-e-101025.pdf
- https://openstax.org/books/biology-2e/pages/6-4-atp-adenosine-triphosphate
- https://openstax.org/books/biology-2e/pages/7-1-energy-in-living-systems
- https://openstax.org/books/biology-2e/pages/7-4-oxidative-phosphorylation
- https://openstax.org/books/biology-2e/pages/8-2-the-light-dependent-reaction-of-photosynthesis
- WJEC GCE A Level Biology Unit 3 mark schemes, Summer 2018 (Q4 and Q6), 2019 (Q4), 2023 (Q1(c) and Q3), 2024 (Q1(b) and Q2) and 2025 (Q1 and Q2): marking guidance and examiner reports used for depth and exam wording
- WJEC GCE A Level Biology Unit 5 mark schemes, Summer 2018 (variant 1, Q1(c)) and 2023 (variant 2, Q1(b)): practical risk and evaluation guidance
- Drawings and graphs are original BioWelsh images and are not copied or adapted from any published resource. The other images are used under the licences below. Captions already include credits. Show this list at the bottom of the topic page.
- u3.atp__photo-atp-synthase-model.jpg · Zhou A, Rohou A, Schep DG, Bason JV, Montgomery MG · CC0 · https://commons.wikimedia.org/wiki/File:Bos_taurus_mitochondrial_ATP_synthase_state_1a_5ARA.png
- u3.atp__photo-mitochondrion-diagram.jpg · Kelvinsong · CC0 · https://commons.wikimedia.org/wiki/File:Mitochondrion_(standalone_version)-en.svg
- u3.atp__photo-chloroplast-diagram.jpg · Kelvinsong · CC0 · https://commons.wikimedia.org/wiki/File:Chloroplast_(standalone_version)-en.svg
- u3.atp__photo-mitochondrion-tem.jpg · Louisa Howard · Public domain · https://commons.wikimedia.org/wiki/File:Mitochondria,_mammalian_lung_-_TEM.jpg
- u3.atp__photo-mitochondrion-tem-b.jpg · Louisa Howard · Public domain · https://commons.wikimedia.org/wiki/File:Mitochondria_-_TEM.jpg
- u3.atp__photo-alga-tem.jpg · Dartmouth Electron Microscope Facility, Dartmouth College · Public domain · https://commons.wikimedia.org/wiki/File:Chlamydomonas_TEM_05.jpg
- All other images · BioWelsh (original illustrations and graphs) · Owned by BioWelsh · Not applicable
- The two diagrams were rendered from SVG to JPG and all images were resized for the web; no other changes were made.
- Licence link: CC0 https://creativecommons.org/publicdomain/zero/1.0/