Investigating plant responses (AQA GCSE Biology): Revision Notes
Investigating plant responses
Core practical: Light intensity experiment
Aim of the investigation
You can investigate how light intensity affects the growth of newly germinated seedlings. This helps us understand how plants respond to their environment and demonstrates important biological principles.
Understanding plant responses to environmental factors is fundamental in biology and has practical applications in agriculture, horticulture, and plant breeding.
What you need (apparatus)
- Cress seeds (or white mustard seeds work too)
- Three Petri dishes
- Cotton wool
- Water
- A ruler marked in 1mm divisions
- Bright windowsill and dark cupboard
Alternative materials: If you don't have Petri dishes, shallow containers or saucers will work. Kitchen paper can substitute for cotton wool if needed.
Method - step by step
The method involves creating different light conditions and monitoring seedling growth over time.
Setting up:
- Put some cotton wool in the bottom of three Petri dishes
- Wet the cotton wool with water
- Add about 10 seeds to each dish
- Keep the dishes warm so the seeds can germinate (start growing)
Creating different light conditions: 5. Once seeds have germinated, remove extra seedlings so each dish has the same number 6. Place one dish on a bright windowsill 7. Place one dish in a dark cupboard 8. Place one dish in dim light (you could cover it or put it in a cupboard on alternate days)
Critical for fair testing: Ensure all dishes have the same number of seedlings and are kept at the same temperature. Only light intensity should vary between the three conditions.
Taking measurements: 9. Every day for at least one week, measure how tall each seedling is 10. Record all your measurements in a table
Recording your results
Make three separate tables showing the height of each seedling each day. This gives you reliable data to compare how light affects growth.
Data collection tip: Take measurements at the same time each day for consistency. Create clear column headings including the date and light condition for each table.
Analysing your data
Once you have collected a full week of data, you need to process and interpret your results.
- Calculate the mean height of seedlings in each dish every day
- Draw a graph with:
- Day on the horizontal axis
- Mean height on the vertical axis
- Different lines for each light condition
- Compare the results - which seedlings grew tallest?
Worked Example: Calculating Mean Height
Day 5 measurements for bright light condition:
- Seedling 1: 15mm
- Seedling 2: 18mm
- Seedling 3: 16mm
- Seedling 4: 17mm
- Seedling 5: 14mm
Mean height = (15 + 18 + 16 + 17 + 14) ÷ 5 = 80 ÷ 5 = 16mm
Alternative investigation: Effects of gravity
You can also investigate how gravity affects plant growth direction, demonstrating another important plant response called gravitropism.
Simple gravity experiment
- Fold a paper towel, place it in a beaker and dampen with water
- Place a germinated bean seed between the paper towel and glass
- Let it grow until you can see both root and shoot
- Tip the beaker on its side and leave it
- Observe and record what happens to the root and shoot over the next few days
Expected Observations
What you should see: The root will grow downwards (towards gravity) and the shoot will grow upwards (away from gravity). This usually takes 2-3 days to become clearly visible.
Important tips for better results
These practical tips will help ensure your investigation produces reliable and accurate results.
Key Tips for Success:
- Use several seedlings in each condition - this makes your results more reliable
- Keep adding water to stop the cotton wool drying out
- Rest the ruler gently against each seedling when measuring - don't squash them
- Dim light conditions can be made by putting dishes in cupboards on alternate days
- Measure at the same time each day for consistency
Why these investigations matter
These experiments demonstrate how plants respond to environmental stimuli, showing two important types of plant movement responses.
Plant Movement Responses:
- Light - plants grow towards light sources (phototropism)
- Gravity - roots grow down, shoots grow up (gravitropism)
These responses help plants survive by getting the light they need for photosynthesis and anchoring properly in soil for water and nutrient uptake.
Key Points to Remember:
- Light intensity affects seedling growth - plants usually grow taller in bright light
- Multiple measurements make results more reliable than single readings
- Gravity affects growth direction - roots go down, shoots go up
- Control variables like temperature and water to make it a fair test
- Record data daily and calculate mean heights for accurate analysis