Required Practical: Investigating Light Intensity and Photosynthesis

Investigating Light Intensity (Required Practical 6)

Every time you move a houseplant closer to a window, you are testing how light affects its growth.
In the lab, we investigate this using aquatic pondweed (such as Elodea or Cabomba) because the oxygen produced during photosynthesis forms visible bubbles in the water.
The independent variable is the light intensity, which is changed by moving a light source to different distances (e.g., 10 cm, 20 cm, 30 cm) from the plant.

Here is the step-by-step method to carry out the practical:

Step 1: Place a 10 cm piece of pondweed into a boiling tube. Cut the stem at an angle underwater and ensure the cut end faces upwards so bubbles can escape easily.
Step 2: Fill the boiling tube with a fixed volume of 0.2% sodium hydrogencarbonate ( $NaHCO_3$ ) solution.

Step 3: Set up an LED light source at a measured distance from the boiling tube.
Step 4: Leave the setup for an acclimatization period of 5 minutes. This allows the plant to adjust to the new light intensity until a steady stream of bubbles is produced.
Step 5: Start a stopwatch and count the number of oxygen bubbles produced in a set time (e.g., 1 minute) to measure the rate of photosynthesis.
Step 6: Repeat the count at least three times at this distance to calculate a mean, then move the lamp to the next distance and repeat the entire process.

To ensure a valid investigation, several control variables must be maintained:

Temperature: Photosynthesis is enzyme-controlled. An LED bulb is used because it emits minimal heat. If an incandescent bulb is used, a heat shield (a large beaker of water) must be placed between the lamp and the tube to absorb infrared radiation.
Carbon Dioxide ( $CO_2$ ): Kept constant by using $NaHCO_3$ solution, which provides a steady supply of $CO_2$ gas. This ensures $CO_2$ is in excess and does NOT become a limiting factor.
Pondweed: The exact same piece of plant (or the same length and species) must be used for all distances.

Improving Accuracy with a Gas Syringe

Counting fast-moving bubbles by eye can easily lead to human error.
Bubbles also vary in size, meaning a simple count does not give an exact volume of oxygen.
To improve precision and accuracy, the boiling tube can be sealed with a rubber bung and connected via a delivery tube to a gas syringe.
This allows you to collect the oxygen and measure its exact volume in $cm^3$ over a given time period.

Calculating the Rate of Photosynthesis

How exactly do we turn raw bubble counts or gas volumes into a measurable rate?
The rate is calculated by dividing the amount of oxygen produced by the time taken.
When taking multiple readings at one distance, you must identify and exclude any anomaly before calculating the mean.

Worked Example 1: Calculating a Mean Rate

A student counts bubbles at a distance of 10 cm for 1 minute. The results for three trials are 22, 24, and 5 bubbles. Calculate the mean rate of photosynthesis.

Step 1: Identify the anomaly. Trial 3 (5 bubbles) does not fit the pattern and must be excluded.
Step 2: Calculate the mean using only the valid trials.

\text{Mean} = \frac{22 + 24}{2} = \frac{46}{2} = 23 \text{ bubbles/min}

Worked Example 2: Volume Calculation (Higher Tier)

For very high precision, a capillary tube can be used to collect the oxygen. The volume is calculated using the formula for a cylinder: $V = \pi r^2 l$ . An oxygen bubble in a capillary tube with an internal diameter of $1.0 \text{ mm}$ measures $20 \text{ mm}$ in length after $5 \text{ minutes}$ . Calculate the rate in $mm^3/min$ .
Step 1: Find the radius ( $r$ ).
$r = 1.0 \text{ mm} \div 2 = 0.5 \text{ mm}$
Step 2: Calculate the cross-sectional area ( $\pi r^2$ ).
$\text{Area} = 3.14 \times (0.5)^2 = 0.785 \text{ mm}^2$
Step 3: Calculate the volume ( $V = \text{Area} \times l$ ).
$V = 0.785 \text{ mm}^2 \times 20 \text{ mm} = 15.7 \text{ mm}^3$
Step 4: Calculate the rate.
$\text{Rate} = 15.7 \text{ mm}^3 \div 5 \text{ min} = 3.14 \text{ mm}^3\text{/min}$

The Inverse Square Law (Higher Tier Only)

You might think moving a lamp twice as far away would halve the light it provides, but it actually drops by a factor of four.
The Inverse Square Law states that light intensity ( $I$ ) is inversely proportional to the square of the distance ( $d$ ) from the source.
Light intensity is measured in arbitrary units (au).

I \propto \frac{1}{d^2}

Worked Example 3: Applying the Inverse Square Law

Calculate the light intensity when the lamp is placed 20 cm from the pondweed.

Step 1: State the formula.
$I \approx \frac{1}{d^2}$
Step 2: Substitute the distance value.
$I = \frac{1}{20^2} = \frac{1}{400}$
Step 3: Calculate the final value.
$I = 0.0025 \text{ au}$

(Note: If the distance is tripled, e.g., from 10 cm to 30 cm, the light intensity decreases by a factor of $3^2 = 9$ .)

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Exam Tips

Common Mistake
Students often incorrectly claim sodium hydrogencarbonate 'provides water' or 'provides oxygen' — you must state that it 'provides carbon dioxide' to gain the mark.
2
In 6-mark practical questions, mentioning a 'heat shield' (like a beaker of water) to control temperature is a high-frequency mark point if an LED bulb is not used.
3
When explaining why a gas syringe is an improvement over counting bubbles, explicitly state that 'bubbles vary in size' and 'counting is prone to human error'.
Common Mistake
When applying the inverse square law, students often think doubling the distance halves the light intensity, but it actually decreases by a factor of 4 (because $2^2 = 4$ ).
5
Always exclude an anomaly before calculating a mean rate; if a bubble count is significantly lower than the rest, it could be because the pondweed stem got blocked.

Key Terms(10)

Pondweed: Aquatic plants, such as Elodea or Cabomba, commonly used in photosynthesis experiments because they produce visible oxygen bubbles.
Sodium hydrogencarbonate: A chemical compound ( $NaHCO_3$ ) dissolved in water to provide a constant supply of dissolved carbon dioxide for aquatic plants.
Acclimatization period: The time allowed for an organism to adjust to a new environmental condition before data collection begins.
Rate of photosynthesis: The speed at which a plant converts carbon dioxide and water into glucose and oxygen.
Limiting factor: An environmental variable (such as light intensity, carbon dioxide concentration, or temperature) that restricts the rate of photosynthesis when in short supply.
Precision: How close repeated measurements are to each other; this can be improved by using measuring equipment with smaller scale divisions.
Gas syringe: Laboratory equipment used to collect and accurately measure the volume of gas produced in a reaction.
Anomaly: A result that does not fit the pattern of the rest of the data, often caused by an experimental error.
Inverse Square Law: A mathematical relationship where a physical quantity (like light intensity) is inversely proportional to the square of the distance from the source.
Arbitrary units (au): Non-physical units used to represent relative values, commonly used to quantify light intensity in biology.

Previous NoteLimiting Factors, Inverse Square Law & Greenhouse Economics (HT)Next Subtopic: Glucose UtilizationGlucose Utilization

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Key Terms(10)

Pondweed: Aquatic plants, such as Elodea or Cabomba, commonly used in photosynthesis experiments because they produce visible oxygen bubbles.
Sodium hydrogencarbonate: A chemical compound ( $NaHCO_3$ ) dissolved in water to provide a constant supply of dissolved carbon dioxide for aquatic plants.
Acclimatization period: The time allowed for an organism to adjust to a new environmental condition before data collection begins.
Rate of photosynthesis: The speed at which a plant converts carbon dioxide and water into glucose and oxygen.
Limiting factor: An environmental variable (such as light intensity, carbon dioxide concentration, or temperature) that restricts the rate of photosynthesis when in short supply.
Precision: How close repeated measurements are to each other; this can be improved by using measuring equipment with smaller scale divisions.
Gas syringe: Laboratory equipment used to collect and accurately measure the volume of gas produced in a reaction.
Anomaly: A result that does not fit the pattern of the rest of the data, often caused by an experimental error.
Inverse Square Law: A mathematical relationship where a physical quantity (like light intensity) is inversely proportional to the square of the distance from the source.
Arbitrary units (au): Non-physical units used to represent relative values, commonly used to quantify light intensity in biology.

Required Practical: Investigating Light Intensity and Photosynthesis

Investigating Light Intensity (Required Practical 6)

Every time you move a houseplant closer to a window, you are testing how light affects its growth.
In the lab, we investigate this using aquatic pondweed (such as Elodea or Cabomba) because the oxygen produced during photosynthesis forms visible bubbles in the water.
The independent variable is the light intensity, which is changed by moving a light source to different distances (e.g., 10 cm, 20 cm, 30 cm) from the plant.

Here is the step-by-step method to carry out the practical:

Step 1: Place a 10 cm piece of pondweed into a boiling tube. Cut the stem at an angle underwater and ensure the cut end faces upwards so bubbles can escape easily.
Step 2: Fill the boiling tube with a fixed volume of 0.2% sodium hydrogencarbonate ( $NaHCO_3$ ) solution.

Step 3: Set up an LED light source at a measured distance from the boiling tube.
Step 4: Leave the setup for an acclimatization period of 5 minutes. This allows the plant to adjust to the new light intensity until a steady stream of bubbles is produced.
Step 5: Start a stopwatch and count the number of oxygen bubbles produced in a set time (e.g., 1 minute) to measure the rate of photosynthesis.
Step 6: Repeat the count at least three times at this distance to calculate a mean, then move the lamp to the next distance and repeat the entire process.

To ensure a valid investigation, several control variables must be maintained:

Temperature: Photosynthesis is enzyme-controlled. An LED bulb is used because it emits minimal heat. If an incandescent bulb is used, a heat shield (a large beaker of water) must be placed between the lamp and the tube to absorb infrared radiation.
Carbon Dioxide ( $CO_2$ ): Kept constant by using $NaHCO_3$ solution, which provides a steady supply of $CO_2$ gas. This ensures $CO_2$ is in excess and does NOT become a limiting factor.
Pondweed: The exact same piece of plant (or the same length and species) must be used for all distances.

Improving Accuracy with a Gas Syringe

Counting fast-moving bubbles by eye can easily lead to human error.
Bubbles also vary in size, meaning a simple count does not give an exact volume of oxygen.
To improve precision and accuracy, the boiling tube can be sealed with a rubber bung and connected via a delivery tube to a gas syringe.
This allows you to collect the oxygen and measure its exact volume in $cm^3$ over a given time period.

Calculating the Rate of Photosynthesis

How exactly do we turn raw bubble counts or gas volumes into a measurable rate?
The rate is calculated by dividing the amount of oxygen produced by the time taken.
When taking multiple readings at one distance, you must identify and exclude any anomaly before calculating the mean.

Worked Example 1: Calculating a Mean Rate

A student counts bubbles at a distance of 10 cm for 1 minute. The results for three trials are 22, 24, and 5 bubbles. Calculate the mean rate of photosynthesis.

Step 1: Identify the anomaly. Trial 3 (5 bubbles) does not fit the pattern and must be excluded.
Step 2: Calculate the mean using only the valid trials.

\text{Mean} = \frac{22 + 24}{2} = \frac{46}{2} = 23 \text{ bubbles/min}

Worked Example 2: Volume Calculation (Higher Tier)

For very high precision, a capillary tube can be used to collect the oxygen. The volume is calculated using the formula for a cylinder: $V = \pi r^2 l$ . An oxygen bubble in a capillary tube with an internal diameter of $1.0 \text{ mm}$ measures $20 \text{ mm}$ in length after $5 \text{ minutes}$ . Calculate the rate in $mm^3/min$ .
Step 1: Find the radius ( $r$ ).
$r = 1.0 \text{ mm} \div 2 = 0.5 \text{ mm}$
Step 2: Calculate the cross-sectional area ( $\pi r^2$ ).
$\text{Area} = 3.14 \times (0.5)^2 = 0.785 \text{ mm}^2$
Step 3: Calculate the volume ( $V = \text{Area} \times l$ ).
$V = 0.785 \text{ mm}^2 \times 20 \text{ mm} = 15.7 \text{ mm}^3$
Step 4: Calculate the rate.
$\text{Rate} = 15.7 \text{ mm}^3 \div 5 \text{ min} = 3.14 \text{ mm}^3\text{/min}$

The Inverse Square Law (Higher Tier Only)

You might think moving a lamp twice as far away would halve the light it provides, but it actually drops by a factor of four.
The Inverse Square Law states that light intensity ( $I$ ) is inversely proportional to the square of the distance ( $d$ ) from the source.
Light intensity is measured in arbitrary units (au).

I \propto \frac{1}{d^2}

Worked Example 3: Applying the Inverse Square Law

Calculate the light intensity when the lamp is placed 20 cm from the pondweed.

Step 1: State the formula.
$I \approx \frac{1}{d^2}$
Step 2: Substitute the distance value.
$I = \frac{1}{20^2} = \frac{1}{400}$
Step 3: Calculate the final value.
$I = 0.0025 \text{ au}$

(Note: If the distance is tripled, e.g., from 10 cm to 30 cm, the light intensity decreases by a factor of $3^2 = 9$ .)

Sign up to continue reading

Get full access to revision notes, key terms, and exam tips for every subtopic.

Exam Tips

Common Mistake
Students often incorrectly claim sodium hydrogencarbonate 'provides water' or 'provides oxygen' — you must state that it 'provides carbon dioxide' to gain the mark.
2
In 6-mark practical questions, mentioning a 'heat shield' (like a beaker of water) to control temperature is a high-frequency mark point if an LED bulb is not used.
3
When explaining why a gas syringe is an improvement over counting bubbles, explicitly state that 'bubbles vary in size' and 'counting is prone to human error'.
Common Mistake
When applying the inverse square law, students often think doubling the distance halves the light intensity, but it actually decreases by a factor of 4 (because $2^2 = 4$ ).
5
Always exclude an anomaly before calculating a mean rate; if a bubble count is significantly lower than the rest, it could be because the pondweed stem got blocked.

Key Terms(10)

Pondweed: Aquatic plants, such as Elodea or Cabomba, commonly used in photosynthesis experiments because they produce visible oxygen bubbles.
Sodium hydrogencarbonate: A chemical compound ( $NaHCO_3$ ) dissolved in water to provide a constant supply of dissolved carbon dioxide for aquatic plants.
Acclimatization period: The time allowed for an organism to adjust to a new environmental condition before data collection begins.
Rate of photosynthesis: The speed at which a plant converts carbon dioxide and water into glucose and oxygen.
Limiting factor: An environmental variable (such as light intensity, carbon dioxide concentration, or temperature) that restricts the rate of photosynthesis when in short supply.
Precision: How close repeated measurements are to each other; this can be improved by using measuring equipment with smaller scale divisions.
Gas syringe: Laboratory equipment used to collect and accurately measure the volume of gas produced in a reaction.
Anomaly: A result that does not fit the pattern of the rest of the data, often caused by an experimental error.
Inverse Square Law: A mathematical relationship where a physical quantity (like light intensity) is inversely proportional to the square of the distance from the source.
Arbitrary units (au): Non-physical units used to represent relative values, commonly used to quantify light intensity in biology.

Previous NoteLimiting Factors, Inverse Square Law & Greenhouse Economics (HT)Next Subtopic: Glucose UtilizationGlucose Utilization

Back to Rate of Photosynthesis

Put your knowledge into practice — try past paper questions for Biology

Key Terms(10)

Pondweed: Aquatic plants, such as Elodea or Cabomba, commonly used in photosynthesis experiments because they produce visible oxygen bubbles.
Sodium hydrogencarbonate: A chemical compound ( $NaHCO_3$ ) dissolved in water to provide a constant supply of dissolved carbon dioxide for aquatic plants.
Acclimatization period: The time allowed for an organism to adjust to a new environmental condition before data collection begins.
Rate of photosynthesis: The speed at which a plant converts carbon dioxide and water into glucose and oxygen.
Limiting factor: An environmental variable (such as light intensity, carbon dioxide concentration, or temperature) that restricts the rate of photosynthesis when in short supply.
Precision: How close repeated measurements are to each other; this can be improved by using measuring equipment with smaller scale divisions.
Gas syringe: Laboratory equipment used to collect and accurately measure the volume of gas produced in a reaction.
Anomaly: A result that does not fit the pattern of the rest of the data, often caused by an experimental error.
Inverse Square Law: A mathematical relationship where a physical quantity (like light intensity) is inversely proportional to the square of the distance from the source.
Arbitrary units (au): Non-physical units used to represent relative values, commonly used to quantify light intensity in biology.