Launch practical
About 120 minutes
- Set up the bench with the 15 items of apparatus.
- Take the readings yourself, into the results table you draw first.
- Process your results and plot the graph.
- Answer exam questions on your own results, which you mark against the mark scheme.
Runs in your browser: a 3D bench with 2D close-ups of each instrument for exact readings. Nothing to install and no account.
What you measure, and how
One run takes 42 readings. You take every one yourself, the way you would in class.
- On the 3D bench (Fig. 1) you set out the apparatus and read each instrument where it stands.
- In the 2D close-up (Fig. 2) the same instrument is drawn large, from the same run, for an exact reading. It is also how the practical runs on a device that cannot show 3D.
- In your table you write what you read, to the instrument’s resolution, and the notebook checks it against the instrument.
How to read it. The shoot beside the ruler, its 0 mark on the cotton wool. Each small division is 1 mm: read level with the top of the shoot, to the nearest mm.
What it shows
Shoots grow towards light (positive phototropism); roots grow towards gravity (positive gravitropism).
Plants respond to light and gravity by growing unevenly. Auxin, a plant hormone made in the tips of shoots and roots, is spread unevenly when light comes from one side or the plant lies on its side. In a shoot, more auxin on the shaded side makes those cells elongate more, so the shoot bends towards the light. In a root lying on its side, more auxin collects on the lower side and the upper side grows faster, so the root bends down.
Key equation for the exam: No equation: shoots grow towards light; roots grow towards gravity..
No equation to learn. Explain the bending with unequal auxin.
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Three Petri dishes with cotton wool | The seeds germinate and grow on the damp cotton wool. | — |
| White mustard seeds (untreated), one packet | The same species and age of seed in every dish. | — |
| 10 cm³ syringe and a beaker of water | The same volume of water for every dish. | 0.5 cm³ |
| Marker pen | Labels the dishes and marks the thread. | — |
| Forceps | Thin each dish to the same number of seedlings. | — |
| Light-proof box with a slit in one side | Light reaches dish A from one direction only. | — |
| Light-proof box with its slit taped over | Keeps dish B in the dark. | — |
| Two LED lamps | One shines at the slit; one lights dish C from above. | — |
| 30 cm ruler | Measures shoots and roots. | 1 mm |
| Cotton thread | Laid along a curved shoot or root, then measured against the ruler. | — |
| Two germinated broad bean seedlings | Their roots show the effect of gravity. | — |
| Pins, and two dishes of damp cotton wool | Hold each bean in place, kept damp all round. | — |
| Waterproof ink and a fine brush | Marks each root every 2 mm, to show where it grows. | — |
| Clinostat | Turns bean B slowly so gravity acts equally on all sides of its root. | — |
| Dark cupboard | Keeps light from the beans. | — |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Check the dark box is light-tight
Turn the box over a bright lamp in a dim corner and look inside. Tape over any chink of light, and tape over the slit of this box.
Why: Dish B must get no light at all. Light getting in through a gap is light from one side, and the seedlings respond to it.
Put cotton wool in three dishes
Put a layer of cotton wool in the base of three Petri dishes.
Why: The cotton wool holds water against the seeds, and the seedlings root into it.
Wet the cotton wool
Each dish needs the same volume of water now, and the same top-up each day. Damp, not flooded.
Do: Add 5 cm³ to each dish with the syringe, and 2 cm³ to each every day.
Why: Water is a control variable: a dish that dries out stops growing, whatever its light.
Sow the seeds
Spread 10 mustard seeds from the same packet over the cotton wool in each dish, not touching.
Why: The same species and age in every dish; spaced out so each seedling gets the same light.
Label the dishes
Write A, B and C on the base of the dishes with the marker pen: A light from one side, B dark, C light from all round.
Why: You must know which dish is which once they are inside the boxes.
Set up the slit box
Stand the box with the slit on the bench with its slit facing the LED lamp, 20 cm away. The lamp must shine only through the slit.
Why: Light reaches dish A from one direction only: through the slit.
Choose how dish C gets its light
Dish C needs light from all round (from above): a lamp over it, on the same bench as the other two.
Do: An LED lamp, 30 cm above dish C.
Why: Only the direction of light should differ between the dishes. The temperature must be the same.
Count the pins outSafety
Count the pins you take for the beans, and keep them in their box until you use them.
Why: Pins are sharp and easy to lose. Counting them out and back means none is left in the cotton wool or on the floor.
Take the readings
Five seedlings in each of three dishes (light from one side, dark, light from all round), measured at the start and after 4 days.
Repeats: Each dish has five seedlings, and you work out a mean increase for each dish. Seedlings vary a lot (some by a third), so a mean of several is far more reliable than one seedling, and it lets you spot one that does not fit. More seedlings, or repeating the whole investigation, would make the means more certain. Repeats do not remove a systematic error, such as a warm lamp over one dish.
Make safe and clear upSafety
Switch off the lamps and the clinostat. Put the seedlings, beans and cotton wool in the bin (seal any mouldy dish in a bag first). Count the pins back into their box. Wash the Petri dishes, put the ruler, thread and boxes back, and wash your hands.
While you take readings
Thin the seedlings
After counting the seeds that germinated, use the forceps to pull out seedlings until every dish has the same number (5), spread out.
Do: Pull out seedlings so every dish has 5, spread out.
Why: The same number of seedlings in each dish. Crowded seedlings compete for light and grow taller and thinner.
Where the beans go
Put both bean dishes (A standing on its edge, B on the clinostat) in the dark cupboard for 2 days.
Why: Only gravity should affect the direction the roots grow. Light from one side would affect it too, so the seedlings are kept so that gravity is the only thing that can change the direction.
Measuring curved shoots and roots
Some shoots and roots are now curved. Measure their whole length, from the cotton wool (or the seed) to the tip.
Do: Lay a thread along the shoot, mark the tip on it, then measure the thread against the ruler.
Why: A ruler held straight beside a bent shoot measures how high it reaches, not how long it is. Say how the ruler is used, not just that it is.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Seeds treated with fungicide | Poisoning if swallowed; skin irritation | Use untreated seed, never eat seeds, and wash your hands after handling them. |
| Mould growing on damp cotton wool | Breathing in spores can cause an allergic reaction or an asthma attack | Do not open a mouldy dish: seal it in a bag and put it in the bin. |
| Sharp pins | A puncture wound to the fingers | Count the pins out and back, and push them in by the head, away from your fingers. |
| Lamps near water | Burns from a hot bulb; an electric shock with wet hands | Use LED lamps, keep the water and syringe away from the lamps and dry your hands before switching them on. |
Variables
- Independent: you change
- Direction of light
- Dependent: you measure
- Increase in height of the seedlings, and the direction they grow in mm
- Control: you keep the same
- Species and age of the seeds. One packet of mustard seed, all sown on the same day.
- Number of seedlings in each dish. Thinned to 5 in every dish.
- Volume of water. 5 cm³ in each dish at the start and 2 cm³ each day, from a syringe.
- Temperature. All three dishes on the same bench, lit by LED lamps that give out little heat.
- Time. 4 days for every dish.
Results from a specimen run
| Dish | Seeds sown | Seeds that germinated |
|---|---|---|
| A | 10 | 8 |
| B | 10 | 10 |
| C | 10 | 10 |
| Dish | Seedling | Height at the start in mm | Height after 4 days in mm | Increase in height in mm |
|---|---|---|---|---|
| A | 1 | 5 | 28 | 23 |
| A | 2 | 6 | 9 (anomalous, left out of the mean) | 3 |
| A | 3 | 4 | 29 | 25 |
| A | 4 | 4 | 26 | 22 |
| A | 5 | 4 | 30 | 26 |
| B | 1 | 6 | 40 | 34 |
| B | 2 | 6 | 42 | 36 |
| B | 3 | 4 | 36 | 32 |
| B | 4 | 3 | 31 | 28 |
| B | 5 | 3 | 33 | 30 |
| C | 1 | 5 | 23 | 18 |
| C | 2 | 5 | 26 | 21 |
| C | 3 | 6 | 35 | 29 |
| C | 4 | 4 | 21 | 17 |
| C | 5 | 5 | 31 | 26 |
| Dish | Light | Mean increase in height in mm |
|---|---|---|
| A | Light from one side | 24.0 |
| B | Dark | 32.0 |
| C | Light from all round | 22.2 |
| Bean | How it was kept | Root length at the start in mm | Root length after 2 days in mm | Increase in root length in mm | What the root did |
|---|---|---|---|---|---|
| A | Still, on its side | 37 | 48 | 11 | Bent downwards |
| B | On the clinostat | 40 | 50 | 10 | Grown straight on, in the direction it started |
| What you looked at | What you saw |
|---|---|
| Dish A (light from one side) after 2 days | Bending towards the slit |
| Ink marks on root A after 2 days | The marks near the tip have spread apart; the others are still 2 mm apart |
| Dish B (dark) beside dish C after 4 days | Dish B’s are taller, thinner and pale yellow, and they grow straight up |
Anomaly spotted: Dish A, seedling 2: height after 4 days, left out of the mean.
Processing the data
- Mean increase in height, dish A
- 24.0 mm
- Why a bar chart
- The light conditions are categories (one side, dark, all round), not numbers on a scale.
- How much more did the dark seedlings grow?
- 44 %
- Labelled drawing of a seedling
- Labelled drawing, magnification × 2.5
- What height can tell you
- The results compare elongation. Height alone does not measure how much food the seedlings made.
| Quantity | Specimen run | Accepted |
|---|---|---|
| Mean increase in dish A (yours: careful technique; accepted: the same seedlings, careful technique) | 24.0 mm | 23.8 mm |
| Mean increase in dish B (yours: careful technique; accepted: the same seedlings, careful technique) | 32.0 mm | 31.7 mm |
| Mean increase in dish C (yours: careful technique; accepted: the same seedlings, careful technique) | 22.2 mm | 22.0 mm |
Across 120 simulated careful groups, mean increase in height, dish A came out between 21.6 and 33.0 mm (median 27.6). That is the simulation's model, not a promise of what a real class gets.
Common mistakes and the fix
Mistakes to check in this practical, and what to do instead. In a write-up or an exam answer, the mark goes to naming the step and saying what it does to the readings.
Method: Use it as it is
A pin-prick of light gets in at one corner. Dish B’s seedlings all lean 15–35° towards it and are not as tall or as yellow as true dark-grown seedlings, so dish B is no longer a fair “dark” control (a systematic error).
The fix: Check the dark box is light-tight. Dish B must get no light at all. Light getting in through a gap is light from one side, and the seedlings respond to it.
Method: Wet each dish under the tap, and add a splash when it looks dry
One dish gets less water and dries out after about 2 days. Its seedlings then grow at about a third of their rate, so that dish’s mean increase is too small (a systematic error in that dish).
The fix: Wet the cotton wool. Water is a control variable: a dish that dries out stops growing, whatever its light.
Method: The old filament lamp, close above dish C so it gets plenty of light
A filament lamp close above warms dish C by 5–8 °C. Its seedlings grow 20–30 % faster, so dish C’s increase is too big and the comparison is not fair (a systematic error). A heat shield, or a lamp that stays cool, keeps the temperature the same.
The fix: Choose how dish C gets its light. Only the direction of light should differ between the dishes. The temperature must be the same.
Dish A: seeds that germinated: Typed 2 seeds
The fix: That is how many did not germinate. Count the seeds that did: those with a root or shoot showing.
Dish A after 2 days: how the shoots are growing: Recorded “Bending away from the slit”
The fix: Look again at which side of the box the slit is on (the bright line), and which way the tips point.
Exam answer: Leaving out: Phototropism (allow positive phototropism) (do not accept negative phototropism; do not accept phototrophic)
The fix: The name for a growth response to light is phototropism (positive phototropism for a shoot growing towards it).
Exam answer: Leaving out: Volume (amount) of water
The fix: Water is one: the same volume in every dish.
Worked exam-style questions
Written for Study Hack in the style of exam questions, on the specimen run above. They are not taken from any exam board's papers. Try each one before you open the worked answer.
- Question 1Naming the response · Knowledge[1 mark]
A student investigated the effect of light on mustard seedlings. Dish A was in a box with a slit facing a lamp (light from one side), dish B in the same box with the slit taped over (dark), and dish C in the open under a lamp (light from all round). The student measured the height of five seedlings in each dish at the start and after 4 days.
Your dish A shoots bent about 53° towards the slit.
Name the response of the shoots in dish A to light from one side.
Worked answer and how the marks are given
A full-mark answer
(Positive) phototropism.
Mark scheme
- Phototropism (allow positive phototropism) (do not accept negative phototropism; do not accept phototrophic)
Where marks are usually lost
- Missing: Phototropism (allow positive phototropism) (do not accept negative phototropism; do not accept phototrophic). The name for a growth response to light is phototropism (positive phototropism for a shoot growing towards it).
- Question 2Mean without the anomaly · Applying knowledge[2 marks]
Look again at Table 1.
Calculate the mean increase in height of the seedlings.
Do not use the anomalous result. Give your answer to 1 decimal place.
Mean increase in height = mm
Worked answer and how the marks are given
A full-mark answer
mean = (23 + 25 + 22 + 26) ÷ 4
= 96 ÷ 4
= 24.0 mmMark scheme
- (23 + 25 + 22 + 26) ÷ 4 (the anomaly left out) (method)
- 24.0 (mm)
- Allow answers from 23.95 to 24.05 mm (the marker's tolerance; correct rounding of the student's own working is also accepted).
- Allow the unit written as millimetres, millimeters, millimetre, mm.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 19.8 mm (no marks). You included the anomalous seedling. Leave seedling 2 out: 96 ÷ 4 = 24.0 mm.
- Answer 19.2 mm (gains 1 mark). You left the anomaly out of the total but still divided by 5. Divide by the number of results you added (4): 24.0 mm.
- Answer 96.0 mm (gains 1 mark). That is the total. Divide by 4: 24.0 mm.
- Question 3Planning: the direction of light · Knowledge[6 marks]
A student wants to investigate the effect of the direction of light on the growth of seedlings.
The student has: pots of seedlings, a cardboard box with a slit in one side, a lamp, a ruler, some thread and a measuring cylinder.
Plan an investigation to find the effect of the direction of light on the growth of seedlings.
Worked answer and how the marks are given
A full-mark answer
1. Use two groups of seedlings of the same species and age, with the same number of seedlings (for example 10) in each pot.
2. Measure and record the height of each seedling with a ruler.
3. Put one pot in a box with a slit in one side, facing a lamp, so the light comes from one direction.
4. Put the other pot in the dark (or in light from all round) as a control.
5. Keep the temperature and the volume of water the same for both, and leave them for the same time (3 days).
6. Measure the height of each seedling again, laying a thread along any curved shoot and measuring the thread with the ruler.
7. Record which way the shoots bend, and calculate the mean increase in height for each pot.Mark scheme
Indicative content:
- Seedlings of the same species and age; the same number in each pot
- Measure the height of the seedlings at the start
- Put one pot in a box with a slit / hole so light comes from one direction
- A control pot in the dark, or in light from all round (or on a clinostat)
- Keep the temperature, volume of water and soil the same
- Leave them for a suitable time (hours to days), the same for both
- Measure the height again at the end
- Measure a curved shoot with a thread (or straighten it) against the ruler
- Record the direction the shoots grow (bend), or measure the angle with a protractor
- Calculate the mean increase in height for each pot
Levels of response Level Marks What the answer does 3 5–6 A coherent method that would give valid results: all the important steps are there, in a sensible order. 2 3–4 A method that might not give valid results: most of the important steps are there, but some are missing or out of order. 1 1–2 A method that would not give valid results: some relevant steps are given, but they are not linked into a workable method. In Study Hack a six-mark answer is not given an automatic level: you tick the points your answer makes, then choose the level that fits it.
- No relevant content: 0 marks.
Evaluate
| Source of error | Type | Effect | Improvement |
|---|---|---|---|
| Measuring bent shoots with a ruler held upright | Systematic | A bent shoot reaches less high than its length, so dish A’s heights and increase are too small. | Lay a thread along each curved shoot, then measure the thread against the ruler. |
| One dish drying out (water added by eye) | Systematic | The seedlings in the dry dish stop growing, so that dish’s mean increase is too small. | Add the same volume of water to every dish with a syringe (5 cm³), and top each up by the same volume every day. |
| A warm lamp over dish C | Systematic | Dish C is warmer, so its seedlings grow faster: temperature has changed as well as light. | Use an LED lamp (or a heat shield) and keep all three dishes at the same temperature. |
| Light leaking into the dark box | Systematic | Dish B’s seedlings get light from one side and lean towards it, so dish B is not a fair dark control. | Check the box over a lamp first and tape every gap. |
| Different numbers of seedlings in each dish | Systematic | Crowded seedlings compete for light and grow taller, so the dishes are not a fair comparison. | Thin every dish to the same number of seedlings (5). |
| Differences between seedlings | Random | Seeds germinate at different times and grow at different rates, so seedlings in one dish differ by up to a third. | Measure more seedlings in each dish (or repeat the investigation) and use the mean. |
| Only five seedlings in each dish | Random | A mean of five is still affected by one unusual seedling, and one anomaly leaves only four. | Use more seedlings in each dish, for example ten. |
| Reading a small, curved shoot to the nearest millimetre | Random | Each height can be up to 1 mm out, which is a large fraction of a 5 mm seedling at the start. | Measure every seedling the same way (with the thread), and measure over a longer time so the increases are bigger. |
| Beans left in the light | Systematic | Light from one side also changes the direction roots grow, so the clinostat root does not grow straight. | Keep both bean dishes in a dark cupboard. |
Repeatable. Your five seedlings in one dish, measured the same way by you, give similar increases, and doing it again gives a similar mean.
Reproducible. Another group, with different seedlings (cress instead of mustard) or a different box, also finds the shoots bend towards the slit and the dark seedlings grow tallest.
Other exam boards
How this practical lines up with other GCSE boards' required or core practicals. Codes only; check your own specification for the exact activity.
| Board | Practical | Match | What is different |
|---|---|---|---|
| AQA | Required practical 8 | This page | — |
| Edexcel | — | Not on its list | — |
Print it
- Blank write-up: the nine-step lab book with the correct table headings, ready to print for a real lesson.
- Specimen lab book: this page's run written up in full, with its exam answers ticked against the mark scheme.
- Printable method card: the practical on one index card. Practise with the cards in Study Hack.
- Practical workbook: a full simulated data set to process on paper, with exam questions and answers.
- Teacher and technician pack: apparatus lists, technician notes and marking points.