5.25 Investigating plant growth responses
6 exam questions worth 15 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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A student puts a dish of cress seedlings in a box with a slit in one side.
Give two control variables the student should keep the same for every dish.
Mark scheme
Any two from: type of seed / number of seeds / volume of water / temperature / time / light intensity (1 each)
Model answer
The type of seed and the volume of water added to each dish.
Explain why the student also grows a dish of seedlings with light from all around.
Mark scheme
- it is a control (1)
- to show that the bending is caused by light from one side / for comparison (1)
Model answer
It is a control. Comparing with it shows that the bending is caused by light coming from one side, not by something else.
The table shows the mean height of seedlings after 5 days.
| Dish | Day 0 | Day 5 |
|---|---|---|
| Light from one side | 5 | 31 |
| Darkness | 5 | 52 |
(a) Calculate the increase in mean height for the seedlings grown in darkness. (b) Suggest why the seedlings in darkness grew taller.
Mark scheme
(a) 52 - 5 = 47 (mm) (1) (b) - (in the dark) cells elongate more / faster (1)
- (so the seedling grows tall quickly) to reach light (for photosynthesis) (1)
- allow: uses stored food (in the seed) for growth in length
Model answer
(a) 52 - 5 = 47 mm. (b) In the dark, the cells of the shoots elongate more, using food stored in the seed, so the seedlings grow tall quickly; in nature this would help them reach light for photosynthesis.
Describe how the student could measure the length of a curved shoot accurately.
Mark scheme
- lay a piece of thread / string along the shoot (1)
- mark the length and measure the thread with a ruler (1)
Model answer
Lay a thread along the curve of the shoot, mark where it ends, then straighten the thread and measure it with a ruler.
Give three rules for making a good biological drawing of a seedling.
Mark scheme
Any three from: use a sharp pencil / clear continuous lines / no shading / draw only what you see / label lines drawn with a ruler / label lines do not cross / include a title / include a scale or magnification / draw large enough (1 each)
Model answer
Use a sharp pencil with clear lines and no shading, draw label lines with a ruler so they do not cross, and give the drawing a title and a scale.
Bean seeds are pinned in a Petri dish with their roots pointing in different directions. The dish is stood on its edge in the dark.
Predict what happens to the roots and explain why the dish is kept in the dark.
Mark scheme
- all roots grow / bend downwards (1)
- because of (positive) gravitropism / auxin collects on the lower side and slows growth there (1)
- kept in the dark so light does not affect the direction of growth / only gravity is tested (1)
Model answer
All the roots bend and grow downwards, because roots show positive gravitropism. The dish is kept in the dark so that light cannot affect the direction of growth; only gravity is being tested.
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