Required practical: effect of light intensity on photosynthesis

What you'll learn

This required practical investigates how changing light intensity affects the rate of photosynthesis in aquatic plants. You'll learn the complete experimental method, how to identify and control variables, interpret graphs showing limiting factors, and apply the inverse square law. This practical is directly testable in both Paper 1 and Paper 2, with questions worth 4-6 marks typically appearing each year.

Key terms and definitions

Photosynthesis — the process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen.

Rate of photosynthesis — the speed at which photosynthesis occurs, measured by counting oxygen bubbles per minute or volume of oxygen produced per unit time.

Limiting factor — a variable that, when in short supply, restricts the rate of photosynthesis (light intensity, carbon dioxide concentration, or temperature).

Light intensity — the amount of light energy reaching a surface per unit area, measured in lux or arbitrary units.

Inverse square law — the principle that light intensity is inversely proportional to the square of the distance from the light source (light intensity ∝ 1/distance²).

Independent variable — the variable you deliberately change (distance of lamp from plant).

Dependent variable — the variable you measure (number of bubbles produced per minute or volume of oxygen).

Control variables — factors kept constant to ensure a fair test (temperature, carbon dioxide concentration, type of plant, time period).

Core concepts

The experimental setup

The standard method uses Canadian pondweed (Elodea) or Cabomba placed in a test tube or beaker of water. A lamp is positioned at different distances from the plant, and oxygen bubbles released during photosynthesis are counted.

Equipment required:

• Boiling tube or conical flask (250 cm³)
• Canadian pondweed (10 cm length, cut at an angle)
• Bench lamp with LED bulb (to minimise heating)
• Metre ruler
• Stopwatch
• Sodium hydrogencarbonate solution (0.2%)
• Thermometer
• Funnel and test tube (for oxygen collection method)

Key setup details:

• Cut the pondweed stem at an angle to increase surface area for oxygen release
• Use sodium hydrogencarbonate solution instead of tap water to provide carbon dioxide
• Position the pondweed 10 cm from the light source initially
• Allow 2-3 minutes for the plant to acclimatise before counting bubbles
• Use an LED lamp to prevent temperature increase affecting results

Method and procedure

Step-by-step method:

1. Fill the boiling tube with sodium hydrogencarbonate solution
2. Cut a 10 cm length of pondweed and place it in the tube with the cut end uppermost
3. Position the lamp 10 cm away from the tube using a metre ruler
4. Leave for 3 minutes to allow the plant to adjust to the light intensity
5. Count the number of bubbles produced in one minute
6. Repeat the count twice more and calculate a mean
7. Move the lamp to 20 cm, 30 cm, 40 cm and 50 cm away
8. Repeat steps 4-6 for each distance
9. Record all results in a table

Alternative method for higher ability: Instead of counting bubbles, invert a funnel over the pondweed with a test tube filled with water placed over the funnel stem. Measure the volume of oxygen gas collected in a fixed time period (e.g. 5 minutes). This provides more accurate, quantifiable data.

Variables in the investigation

Independent variable: Distance between lamp and plant (typically 10 cm, 20 cm, 30 cm, 40 cm, 50 cm). This changes the light intensity reaching the plant.

Dependent variable: Number of oxygen bubbles per minute OR volume of oxygen collected. This indicates the rate of photosynthesis.

Control variables and why they matter:

• Temperature — use an LED bulb rather than filament bulb; monitor with thermometer. Temperature affects enzyme activity in photosynthesis.
• Carbon dioxide concentration — use the same concentration of sodium hydrogencarbonate solution (0.2%) throughout. CO₂ is a raw material for photosynthesis.
• Type and size of plant — use the same species and similar-sized pieces. Different plants photosynthesize at different rates.
• Time period — count bubbles for the same duration each time (1 minute). Allows fair comparison.
• Volume of solution — keep constant. Affects the amount of dissolved CO₂ available.

Understanding the results and graphs

Typical pattern: As distance from the lamp increases, light intensity decreases, so the rate of photosynthesis decreases. This produces a curved graph when plotting rate against distance, but a straight line through the origin when plotting rate against 1/distance².

Graph interpretation:

When plotting rate of photosynthesis against distance:

• Negative correlation (as distance increases, rate decreases)
• Curved relationship (non-linear)
• Rate decreases more slowly at greater distances

When plotting rate of photosynthesis against light intensity (1/distance²):

• Positive correlation
• Linear relationship at low light intensities
• Curve levels off at high light intensities (plateau)
• Plateau indicates light is no longer the limiting factor

The plateau region: When the graph line becomes horizontal, increasing light intensity no longer increases the rate. Either carbon dioxide concentration or temperature has become the limiting factor. Adding more light has no effect because another factor is now restricting photosynthesis.

Applying the inverse square law

Light intensity decreases with distance according to the inverse square law:

Light intensity ∝ 1/d²

Where d = distance from light source

Practical application: If you halve the distance from 40 cm to 20 cm:

• Light intensity increases by a factor of (40/20)² = 4
• The rate of photosynthesis should also increase by approximately 4 times (if light is the limiting factor)

Common calculation: If light intensity at 10 cm = 1/10² = 1/100 = 0.01 (arbitrary units) And light intensity at 20 cm = 1/20² = 1/400 = 0.0025 (arbitrary units) The ratio is 0.01/0.0025 = 4, so light intensity at 10 cm is 4 times greater

This relationship only holds when light is the limiting factor. Once another factor limits the rate, doubling light intensity will not double the rate of photosynthesis.

Evaluation and improving the method

Sources of error:

• Bubble counting is subjective — bubbles vary in size; difficult to count rapid bubbles accurately. Improvement: collect oxygen over water in a measuring cylinder or use an oxygen sensor.
• Temperature fluctuations — lamp heats the water over time. Improvement: use LED lamp; place heat shield between lamp and plant; allow water to cool between readings.
• Light from windows — background light affects results. Improvement: conduct experiment in a darkened room or use a cardboard box around apparatus.
• Plant damage — pondweed photosynthesis rate decreases over time. Improvement: use fresh pondweed for each set of repeats; start with furthest distance first.
• Anomalous results — air bubbles trapped in plant stem released irregularly. Improvement: gently shake plant before starting; take multiple readings and calculate mean.

Validity considerations: The experiment measures oxygen production, which is an indirect measure of photosynthesis rate. We assume all oxygen produced comes from photosynthesis, but plants also respire. At very low light intensities, respiration rate may exceed photosynthesis rate.

Worked examples

Example 1: Describing the method (4 marks)

Question: Describe how you would investigate the effect of light intensity on the rate of photosynthesis using pondweed.

Mark scheme answer:

• Place pondweed in a test tube containing sodium hydrogencarbonate solution (1 mark)
• Position a lamp at a measured distance from the pondweed, such as 10 cm (1 mark)
• Count the number of bubbles produced in one minute / measure volume of oxygen collected (1 mark)
• Repeat at different distances from the lamp / change the independent variable (1 mark)

Examiner note: Must specify what solution to use, what to measure, and that distance changes. Simply saying "count bubbles" without a time frame loses marks.

Example 2: Graph interpretation (3 marks)

Question: A student obtained these results:

Explain why the number of bubbles decreased as distance increased.

Mark scheme answer:

• As distance increases, light intensity decreases (1 mark)
• Less light energy available for photosynthesis (1 mark)
• So rate of photosynthesis decreases / less oxygen produced (1 mark)

Examiner note: Must link distance → light intensity → photosynthesis rate. Don't just describe the pattern; explain the biological cause.

Example 3: Inverse square law calculation (3 marks)

Question: A lamp is placed 20 cm from pondweed and produces 32 bubbles per minute. The lamp is moved to 40 cm away. Use the inverse square law to predict the number of bubbles per minute at 40 cm.

Mark scheme answer:

• Light intensity at 40 cm = (20/40)² = 1/4 of the intensity at 20 cm (1 mark)
• Bubbles per minute = 32 ÷ 4 (1 mark)
• = 8 bubbles per minute (1 mark)

Alternative acceptable answer: (1/20²)/(1/40²) = 400/100 = 4, so 32/4 = 8 bubbles per minute

Common mistakes and how to avoid them

• Confusing distance and light intensity — students often plot rate against distance instead of 1/distance². Remember: light intensity = 1/distance², not just 1/distance.

• Ignoring control variables — stating only one or two control variables in 4-mark questions. Always mention temperature, CO₂ concentration, type of plant, and time period for full marks.

• Inaccurate inverse square law calculations — forgetting to square the distance. If distance doubles, light intensity becomes ¼ (not ½) of the original.

• Misidentifying limiting factors — claiming light is limiting when the graph has plateaued. When rate stops increasing despite more light, another factor (CO₂ or temperature) is limiting.

• Vague method descriptions — writing "put the plant in water" instead of specifying sodium hydrogencarbonate solution. Precision matters for method marks.

• Not calculating means — reporting a single reading instead of repeating counts and calculating a mean. Always take at least three readings and calculate the average to improve reliability.

Exam technique for "Required practical: effect of light intensity on photosynthesis"

• Command word "describe" (method questions) — state the steps in sequence: what you use, what you change, what you measure, how you ensure fair test. Typically 4-6 marks available; one mark per distinct point.

• Command word "explain" (results questions) — must give reasons using biological knowledge. Link the change in independent variable → effect on photosynthesis → change in dependent variable. Use "because," "therefore," or "so" to show causal relationships.

• Graph questions — when asked to "draw a conclusion," describe the trend AND provide numbers from the graph as evidence. When asked to "explain the shape," identify any plateau and state which factor becomes limiting.

• Calculation questions — show your working for inverse square law calculations. Even if final answer is wrong, method marks are available. Remember to include units (bubbles per minute, cm³/min) in your final answer.

Quick revision summary

This practical investigates how light intensity affects photosynthesis rate by measuring oxygen production from pondweed at different lamp distances. Key points: use sodium hydrogencarbonate for CO₂; control temperature with LED lamps; count bubbles or collect oxygen; light intensity follows inverse square law (1/d²); graphs plateau when another factor becomes limiting; calculate means from repeats for reliability. Common errors include confusing distance with light intensity, incomplete control variable lists, and incorrect inverse square calculations.