Edexcel IGCSE Biology Problem-solving and Application

A student uses this method to investigate the effect of fertiliser on the growth of plant seedlings.
- set up two trays with an equal mass of compost in each tray
- plant 100 seeds, equally spaced, in each tray
- place the trays under the same lamp until the seeds start to germinate
- water each tray every day with the same volume of water
- add fertiliser to one of the trays every day
- remove five seedlings from each tray every four days for a period of 20 days
- dry these seedlings in an oven and find their mass

The table shows the student's results.

The student dries the seedlings in an oven to find their dry mass.

Suggest why it is important to use dry mass in this investigation.

A teacher uses this method to estimate the speed of a nerve impulse.
- students stand in a circle and hold hands
- student A in the circle starts a timer and at the same time squeezes the hand of student B on his left
- when student B feels his hand being squeezed, he immediately squeezes the hand of the student on his left
- this process continues around the circle of students until student A feels his hand being squeezed and he stops the timer

Explain what other measurements the teacher would need to make in order to calculate the speed of a nerve impulse.