11.11 Required Practicals: Hormonal Coordination

A free GCSE Biology lesson for AQA (8461) from OBA School: required practical: the effect of light or gravity on the growth of newly germinated seedlings. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Plan and carry out an investigation into the effect of light or gravity on seedling growth
• Identify the independent, dependent and control variables and assess the risks
• Record results as length measurements and careful observations, and calculate means
• Evaluate the method, identify sources of error and suggest improvements

Key words

• Independent variable: The variable you change, e.g. the direction of light.
• Dependent variable: The variable you measure, e.g. the length of the seedlings.
• Control variable: A variable kept the same so the test is fair, e.g. temperature.
• Anomalous result: A result that does not fit the pattern; it is left out of the mean.
• Germination: When a seed starts to grow, producing a root and then a shoot.

Aim and equipment

Aim: to investigate how light (or gravity) affects the growth of newly germinated seedlings. Fast-germinating seeds such as cress, mustard or radish work well.

• 3 Petri dishes with cotton wool or filter paper; Why it is used A damp surface for seeds to germinate on
• Cress seeds (same type and batch); Why it is used Germinate quickly; a fair comparison
• Measuring cylinder and water; Why it is used Add the same volume of water to each dish
• 2 cardboard boxes (one with a slit cut in the side); Why it is used Light from one direction / complete darkness
• Ruler (mm), thread; Why it is used Measure shoot lengths (thread for curved shoots)
• Lamp (optional); Why it is used A steady light source

Method

Investigating light

1. Put the same amount of cotton wool in three Petri dishes and add 10 cm³ of water to each.

2. Place 10 cress seeds, evenly spaced, in each dish. Label the dishes A, B and C.

3. Leave the dishes in the same warm place for 1–2 days until the seeds germinate.

4. Measure the starting length of each seedling with a ruler.

5. Put dish A in a box with a slit facing a light, dish B in the open with light all round, and dish C in a closed box.

6. Keep all three at the same temperature and add the same volume of water each day if needed.

7. After 5 days, measure the length of every seedling to the nearest mm and record observations: direction of growth, colour, thickness.

Investigating gravity

1. Pin or stick germinating seeds (e.g. broad bean or cress) to damp cotton wool in a Petri dish, roots pointing down. Fix the lid.

2. Stand the dish on its edge in a dark place, so light cannot affect growth.

3. After the roots have grown a little, turn the dish 90°. Observe and draw (or photograph) the roots and shoots every few hours for 1–2 days.

Variables and risk assessment

• Independent; In the light investigation Direction (or presence) of light
• Dependent; In the light investigation Length of shoots (mm) and direction of growth
• Control; In the light investigation Type and number of seeds; volume of water; temperature; time; type of cotton wool

• Seeds (some are treated with chemicals); Risk Irritation if eaten or touched; Precaution Do not eat seeds or seedlings; wash hands after the practical
• Lamp; Risk Burns; electric shock near water; Precaution Keep the lamp away from water; do not touch the hot bulb
• Mould on damp cotton wool; Risk Allergic reaction; Precaution Do not open old dishes; dispose of safely

Processing the results

Record both length measurements and careful observations. Calculate the mean change in length for each dish, leaving out any anomalous results (e.g. a seed that did not germinate).

Worked example: mean with an anomaly

Shoot lengths in the dark (mm): 39, 41, 36, 40, 7, 38, 37, 40, 37, 38
7 mm is anomalous (the seed may have been damaged), so leave it out.
Mean = (39 + 41 + 36 + 40 + 38 + 37 + 40 + 37 + 38) ÷ 9 = 346 ÷ 9 = 38.44…

Answer: 38.4 mm (1 d.p.)

• A one side; Typical observations Shoots bend towards the slit (positive phototropism); green leaves
• B all round; Typical observations Shoots grow straight up; short, sturdy and green
• C dark; Typical observations Tallest, thin and pale yellow (no chlorophyll); the seedlings use their food store to grow quickly upwards in search of light

Errors and improvements

• Curved shoots are hard to measure with a ruler; Improvement Lay a thread along the shoot, then measure the thread; or measure the angle of bending with a protractor
• Some seeds do not germinate or are damaged; Improvement Use more seeds; ignore anomalies when calculating means
• Light leaks into the “dark” box; Improvement Seal joins with tape; check the box in a bright room
• Dishes dry out at different rates; Improvement Add the same measured volume of water each day; cover the dishes

Exam tip: Repeat the experiment (or use more seeds per dish) and calculate a mean to make the results more repeatable.

Common mistakes

• Recording only lengths. The specification asks for observations too (direction, colour).
• Including an anomalous value in the mean. Identify it, give a reason, then leave it out.
• Calling light intensity the dependent variable. It is the independent variable – the one you change.
• Forgetting to keep temperature and water the same for all dishes, so the test is not fair.

Revision questions

Try these, then check your answers below each one.

Question 1. A student investigates how the direction of light affects the growth of cress seedlings. What is the independent variable? (Choose from: The direction of light, The length of the seedlings, The temperature, The volume of water.)

Answer: The direction of light

Working: The student changes the direction of light, so that is the independent variable.

Question 2. Which of these should be a control variable in the same investigation? (Choose from: The volume of water added to each dish, The direction of light, The length of the seedlings, The direction in which the shoots bend.)

Answer: The volume of water added to each dish

Working: The volume of water is kept the same for every dish; the others are the independent or dependent variables.

Question 3. Five seedlings had shoot lengths of 23, 25, 22, 26 and 24 mm. Calculate the mean length.

Answer: 24 mm

Working: (23 + 25 + 22 + 26 + 24) ÷ 5 = 120 ÷ 5 = 24 mm

Related lessons

• Previous lesson: 11.10 Using Plant Hormones
• Next lesson: 12.1 Controlling Body Temperature
• In this chapter: 11.1 Principles of Hormonal Control
• In this chapter: 11.2 The Control of Blood Glucose Levels
• In this chapter: 11.3 Treating Diabetes

Continue in the full course

This lesson is section 11.11 (ARPBH1111) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.