Required practical: plant responses (phototropism/gravitropism)

What you'll learn

This required practical investigates how plants respond to directional stimuli through tropisms. You'll learn to design and conduct controlled experiments examining phototropism (plant growth response to light) and gravitropism (plant growth response to gravity), analyse results, and explain the role of auxins in these responses. Understanding this practical is essential for Paper 2 questions on plant hormones and experimental design.

Key terms and definitions

Tropism — a directional growth response by a plant to an external stimulus, where the direction of growth is determined by the direction of the stimulus

Phototropism — growth response of a plant to light; shoots typically show positive phototropism (grow towards light) whilst roots show negative phototropism (grow away from light)

Gravitropism (geotropism) — growth response of a plant to gravity; shoots show negative gravitropism (grow away from gravity, upwards) whilst roots show positive gravitropism (grow towards gravity, downwards)

Auxin — a plant hormone that controls growth and tropisms by promoting cell elongation in shoots and inhibiting elongation in roots

Independent variable — the factor you deliberately change or manipulate in an experiment (e.g., direction of light source)

Dependent variable — the factor you measure in an experiment (e.g., direction of shoot growth, angle of curvature)

Control variables — factors kept constant to ensure a fair test (e.g., temperature, water availability, type of seedling)

Control experiment — a comparison experiment where the independent variable is absent, allowing you to determine if changes are due to that variable

Core concepts

Investigating phototropism

The standard phototropism investigation examines how shoots grow towards a directional light source.

Basic method:

1. Place seedlings (commonly cress or wheat) in small pots with moist soil or cotton wool
2. Position seedlings in a controlled environment (constant temperature, away from windows)
3. Use a light box or shield to create unidirectional light from one side
4. Leave seedlings for 3-5 days, ensuring adequate water throughout
5. Observe and measure the direction and degree of curvature of shoots

Key variables:

• Independent variable: direction of light (e.g., from left, right, above, or no directional light)
• Dependent variable: direction of shoot growth, measured by angle of curvature from vertical
• Control variables: type of seedling, age of seedling, temperature, amount of water, distance from light source, light intensity, time period

Setting up controls:

You must include a control setup where seedlings receive uniform light from all directions (or are kept in darkness with one brief light exposure). This demonstrates that curvature results specifically from directional light rather than other factors.

Expected results:

Shoots grow towards the unidirectional light source, demonstrating positive phototropism. The control seedlings either grow straight upwards (uniform light) or show random growth direction (darkness). Typical curvature ranges from 20-60 degrees depending on light intensity and duration.

Investigating gravitropism

This investigation examines how roots and shoots respond to gravity when seedlings are placed horizontally.

Basic method:

1. Germinate seeds (broad beans or cress work well) on damp cotton wool in petri dishes until shoots and roots are 2-3 cm long
2. Pin or tape seedlings horizontally onto moist cotton wool in vertical petri dishes or inside plastic bottles laid on their side
3. Ensure roots and shoots are clearly visible through transparent material
4. Keep in darkness or uniform low light to minimize phototropic interference
5. Observe growth direction after 2-4 days

Key variables:

• Independent variable: orientation of seedling (horizontal vs vertical control)
• Dependent variable: direction of root and shoot growth, measured by angle from original growth axis
• Control variables: seedling type, seedling age, temperature, water availability, light exposure (kept minimal and uniform)

Setting up controls:

Include seedlings maintained in normal vertical orientation. These should continue growing with shoots upward and roots downward, demonstrating that the response in horizontal seedlings results from gravitational stimulus, not other factors.

Expected results:

When placed horizontally, shoots bend upwards (negative gravitropism) and roots bend downwards (positive gravitropism), typically within 24-48 hours. Control seedlings maintain their original growth direction. The response is usually faster and more pronounced in roots than shoots.

The role of auxin in tropisms

Understanding how auxin works is crucial for explaining your practical results.

Auxin distribution in phototropism:

• Light causes auxin to accumulate on the shaded side of the shoot
• Higher auxin concentration on shaded side promotes greater cell elongation
• Cells on shaded side grow longer than those on lit side
• This unequal growth causes the shoot to bend towards light

Auxin distribution in gravitropism:

• Gravity causes auxin to accumulate on the lower side of horizontally placed shoots and roots
• In shoots: higher auxin concentration on lower side promotes cell elongation, causing upward bending
• In roots: higher auxin concentration on lower side inhibits cell elongation, causing downward bending
• This explains opposite responses in shoots and roots despite same auxin distribution

Key points for exams:

• Auxin moves from cells on one side to the other (redistribution)
• Auxin has opposite effects in shoots (promotes elongation) and roots (inhibits elongation)
• The plant growth response results from differential (unequal) growth rates on opposite sides

Experimental design considerations

Understanding experimental design principles helps you evaluate and improve practical methods.

Reliability:

• Use multiple seedlings (minimum 5, ideally 10+) for each condition
• Calculate mean results to identify genuine trends
• Repeat entire experiment to check consistency
• Larger sample sizes reduce impact of genetic variation between individual plants

Validity:

• Ensure only one variable changes (independent variable)
• Keep all other variables constant
• Use appropriate control experiments
• Eliminate confounding variables (e.g., phototropism interfering with gravitropism investigation)

Sources of error:

• Seedlings at different developmental stages respond differently
• Temperature fluctuations affect growth rates
• Uneven watering creates additional variables
• Light leaks in gravitropism investigations cause phototropic interference
• Measurement difficulties when curvature is gradual
• Observer bias when interpreting growth direction

Improvements you might suggest:

• Use larger sample sizes for more reliable means
• Measure angles with protractor for quantitative data
• Photograph seedlings at regular intervals for time-series data
• Use light-proof boxes for gravitropism to completely eliminate phototropism
• Monitor and record temperature throughout investigation
• Standardize seedling age by germinating all seeds simultaneously

Recording and presenting results

How you record and present data affects marks in exam questions.

Qualitative observations:

• Initial direction: "The shoot initially grew vertically upwards"
• Change observed: "After 3 days, the shoot had curved approximately 45° towards the light source"
• Comparison: "The control shoot continued growing vertically upwards"

Quantitative measurements:

Record angle measurements in a table:

Appropriate graphs:

• Bar charts for comparing mean angles between different conditions
• Line graphs for showing change over time if measurements taken at intervals
• Always include error bars when presenting means
• Label axes with quantities and units
• Include descriptive titles

Drawing conclusions from results

Examiners expect you to link observations to biological explanations.

For phototropism:

"The shoots curved towards the unidirectional light source due to positive phototropism. Light caused auxin to redistribute to the shaded side of the shoot. The higher auxin concentration on the shaded side promoted greater cell elongation than on the lit side. This differential growth caused the shoot to bend towards the light. The control shoots, receiving uniform light, grew straight upwards as auxin was evenly distributed."

For gravitropism:

"When placed horizontally, shoots curved upwards (negative gravitropism) and roots curved downwards (positive gravitropism). Gravity caused auxin to accumulate on the lower side of both shoots and roots. In shoots, this higher auxin concentration promoted cell elongation on the lower side, causing upward bending. In roots, higher auxin concentration inhibited cell elongation on the lower side, causing downward bending. This demonstrates that auxin has opposite effects in roots and shoots."

Worked examples

Example 1: Planning an investigation

Question: A student wants to investigate how the direction of light affects the growth of cress seedlings. Describe how the student could carry out this investigation. Include details of the independent variable, dependent variable, and two control variables. [6 marks]

Answer:

Independent variable: direction of light / position of light source [1 mark]

Dependent variable: direction of shoot growth / angle of shoot curvature from vertical [1 mark]

Control variables: temperature [1 mark]; type/age of seedlings [1 mark]; volume of water [1 mark]; distance from light source / light intensity [1 mark]

Method: Plant several cress seeds in pots with soil / on damp cotton wool. Place pots in a light-proof box with a hole on one side to allow unidirectional light. Ensure temperature is constant and water seedlings with same volume daily. Leave for 3-5 days then measure angle of shoot curvature towards light using a protractor.

[Note: Students need to score 6 marks from the points above. Method detail not required if question only asks for variables, but shows good understanding.]

Example 2: Explaining results

Question: A student placed germinated seedlings horizontally in the dark. After 48 hours, the shoots had curved upwards and the roots had curved downwards. Explain these observations. [4 marks]

Answer:

The shoots show negative gravitropism and roots show positive gravitropism [1 mark]. Gravity caused auxin to accumulate on the lower side of the horizontally placed seedlings [1 mark]. In shoots, higher auxin concentration on the lower side promoted cell elongation, causing upward curvature [1 mark]. In roots, higher auxin concentration on the lower side inhibited cell elongation, causing downward curvature [1 mark].

[Alternative acceptable point: Auxin has opposite effects in shoots and roots — promotes elongation in shoots but inhibits it in roots]

Example 3: Evaluating method

Question: A student investigated phototropism using 3 seedlings exposed to light from the left side and 3 control seedlings in uniform light. After 5 days, the experimental seedlings curved 35°, 41° and 38° towards the light. The control seedlings grew straight upwards. Evaluate the student's method and suggest improvements. [4 marks]

Answer:

Strengths: The student included a control group to compare against [1 mark], and took multiple measurements to calculate a mean [1 mark].

Limitations: The sample size of 3 seedlings per group is too small to be reliable [1 mark] / to account for genetic variation / to identify anomalies [1 mark].

Improvements: Use at least 10 seedlings per group for more reliable results [1 mark]; measure angles using a protractor for accurate quantitative data [1 mark]; repeat the entire experiment to check consistency of results [1 mark]; photograph seedlings at regular intervals to track rate of response [1 mark].

[Students need 4 marks from available points. Must identify both strengths and limitations for full marks.]

Common mistakes and how to avoid them

• Confusing positive and negative tropisms: Remember positive means growing towards the stimulus (roots towards gravity, shoots towards light) and negative means growing away from stimulus (shoots away from gravity, roots away from light). The terms describe direction relative to stimulus, not whether the response is "good" or "bad"

• Failing to describe auxin's opposite effects: Many students state auxin promotes growth without specifying it promotes elongation in shoots but inhibits elongation in roots. Always specify which part of the plant you're discussing and whether auxin promotes or inhibits growth there

• Not including proper controls: A control isn't just "doing nothing" — it's maintaining all conditions the same except removing the independent variable. For phototropism, this means uniform light from all directions, not complete darkness

• Forgetting to explain differential growth: Don't just say "auxin makes the plant bend." Explain that auxin accumulates on one side, causing cells on that side to elongate more/less than the other side, creating unequal growth that produces curvature

• Inadequate sample sizes: Using only 1-2 seedlings makes results unreliable due to genetic variation and random errors. Always use multiple seedlings and calculate means in experimental design questions

• Mixing up phototropism and gravitropism variables: In phototropism practicals, minimize gravitropism by keeping seedlings upright. In gravitropism practicals, minimize phototropism by using darkness or uniform light. Identify which tropism you're investigating and control for the other

Exam technique for "Required practical: plant responses (phototropism/gravitropism)"

• Command word recognition: "Describe" requires stating what happens without explanation; "Explain" requires biological reasoning including auxin's role; "Evaluate" needs both strengths and limitations; "Suggest improvements" requires specific, practical changes with justification

• Answer structure for mechanism questions: State the tropism type → Describe stimulus and auxin distribution → Explain effect on cell elongation → Link differential growth to directional bending. This four-step approach ensures complete explanations worth full marks

• Identifying variables correctly: Independent variable is what you change (light direction, seedling orientation); dependent variable is what you measure (angle, direction); control variables are kept constant (temperature, water, seedling type). Don't confuse control variables with control experiments

• Quantitative data handling: When given measurements, calculate means correctly and comment on spread/variation. When suggesting improvements, specify using quantitative measurements (angles with protractor) rather than qualitative observations (describing bending as "slight" or "significant")

Quick revision summary

Plants respond to directional stimuli through tropisms. Phototropism investigations show shoots grow towards light due to auxin accumulating on shaded sides, promoting cell elongation and causing bending. Gravitropism investigations demonstrate shoots grow upwards and roots downwards when placed horizontally because gravity causes auxin to accumulate on lower sides—promoting elongation in shoots but inhibiting it in roots. Successful investigations require multiple seedlings, appropriate controls (uniform light for phototropism; vertical orientation for gravitropism), and constant control variables. Understanding auxin's mechanism and opposite effects in shoots versus roots is essential for explaining results.