Required Practical: Investigating Light Intensity and Photosynthesis

• 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 .

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.

• $I = \frac{1}{20^2} = \frac{1}{400}$

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