Required practical activity 8: investigate the effect of light or gravity…

The idea

In this practical, students grow newly germinated seedlings under controlled conditions and measure how their shoots and roots respond to a directional stimulus. For light, seedlings can be placed in a box with a single opening so that light arrives from one side; shoots should bend towards the light, showing positive phototropism. For gravity, seedlings can be grown in a dark, upright container or fixed to a clinostat, then compared with seedlings whose orientation is changed; roots should grow downwards and shoots upwards, showing gravitropism. Whichever variable is chosen, other conditions such as temperature, water and oxygen must be kept the same so the response can be attributed to the stimulus. Results are collected over several days and compared with a control.

What earns the marks

• State a hypothesis linking the chosen stimulus, light or gravity, to the direction of shoot or root growth.
• Identify the independent variable, such as direction of light or orientation relative to gravity.
• Identify the dependent variable, such as length or direction of shoot and root growth.
• Control key variables including temperature, water volume, oxygen supply, seed type and age of seedling.
• Use a control group, for example seedlings grown with light from all directions or in a fixed upright position.
• Measure shoot and root length or angle of curvature at regular intervals and record results in a suitable table.
• Repeat the investigation or use several seedlings to improve reliability and identify anomalies.

Common mistakes and how to put them right

• Changing several conditions at once, such as light direction and temperature, so the cause of any response is unclear.

• Measuring only once at the end.

  Put it right: Growth is a gradual response, so regular measurements over several days are needed.

• Assuming roots grow towards light.

  Put it right: Roots are generally negatively phototropic and grow away from light, while shoots are positively phototropic.

Exam tips

• Write a clear prediction that names the stimulus, the plant part and the expected direction of growth.
• Describe how you would keep a named variable constant, rather than saying only that conditions were controlled.
• Present results in a table with units and use a line graph to show growth over time.