Investigating limiting factors of photosynthesis

In a nutshell

The impact of light intensity on the rate of photosynthesis can be investigated by measuring the amount of oxygen an aquatic plant produces in a set amount of time. Light intensity can be changed by moving a lamp closer or further away from the plant. The more oxygen that is produced, the greater the rate of photosynthesis.

Equations

This is the word and symbol equation for photosynthesis. It is important you recognise this equation as you are investigating the rate at which photosynthesis is happening.

Word equation	symbol equation
$carbon \space dioxide + water \rightarrow glucose + oxygen$	$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

Equipment list

The following equipment can be used to investigate the impact of light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis.

Equipment	Use
Beaker	To hold the water and pondweed in.
Sodium hydrogen carbonate solution	To release carbon dioxide into the solution and ensure the carbon dioxide concentration is not a limiting factor of photosynthesis.
Filter funnel	To place on top of the pondweed and push the oxygen bubbles up to the boiling tube.
Boiling tube	To be placed on top of the funnel and trap the oxygen that is produced.
Freshly cut piece of pondweed ( $10\ cm$ )	This is the plant that will be photosynthesising.
Light source	This will be providing the light energy for photosynthesis.
Meter ruler	To measure the distance between the light source and the plant.
Stop watch	To measure the time.
Thermometer	To ensure the temperature remains constant.

Experiment 1: Investigate the effect of light intensity on the rate of photosynthesis

Experimental variables

Every experiment will have an independent, dependent and constant variable. The independent variable is the variable that you change. The dependent variable is the variable which depends on the independent variable, therefore it is the one you measure. The control variable is the variable that is kept the same during the experiments.

Independent variable	$Distance\space between\space the\space light\space source\space and\space the\space pondweed.$
Dependent variable	$Amount\space of\space oxygen\space produced\space (measured\space as\space the\space number\space of\space bubbles\space produced).$
Control variables	$Temperature,\space length\space of\space the\space plant,\space carbon\space dioxide\space concentration\space\\ and\space the\space amount\space of\space water\space in\space the\space beaker.$

Safety precautions

When carrying out experiments it is very important to consider the safety precautions so nobody gets hurt.

Hazard	Risk	Safety measure
Lamp	The lamp may get hot and burn you.	Don't touch the bulb of the lamp. Move the lamp by the base.

Method

Biology; Biology practicals; KS4 Year 10; Investigating limiting factors of photosynthesis

1.	Set up the equipment as shown in the diagram, ensuring the lamp is a set distance away from the beaker containing the pondweed. This may be around $10\ cm$ .
2.	Switch the light on.
3.	In a table, record the number of bubbles produced in one minute.
4.	Repeat this three times.
5.	Repeat the experiment with increasing distances of the plant from the light source. Examples $20\ cm$ , $30\ cm$ , $40\ cm$ and $50\ cm$ .

Analysis

This is how you would use your data to form conclusions. At this stage you should have three values for the number of bubbles for each distance you have chosen.

1.

Using the data you collected in the experiment, calculate the mean number of bubbles for the original lamp position.

$\text{mean} = \frac{\text{bubbles produced in repeat 1 + bubbles produced in repeat 2 + bubbles produced in repeat 3}}{3}$

2.

Calculate the mean number of bubbles for each distance.

3.

Use this data to plot a graph. The dependent variable, number of bubbles produced, is plotted on the $y$ -axis and the independent variable, the distance between the light source and the pondweed, is plotted on the $x$ -axis.

Conclusion

Your results should show a negative correlation between the number of bubbles produced and the distance of the pondweed from the lamp. This means that as the distance between the lamp and the pondweed increases, the number of bubbles decreases.

This increased distance represents a decrease in light intensity. Therefore, the results of the experiment should show that as light intensity decreases, so does the amount of oxygen being produced. This reflects a decrease in the rate of photosynthesis as less oxygen is being produced.

Biology; Biology practicals; KS4 Year 10; Investigating limiting factors of photosynthesis

Evaluation

After completing the experiment, you must comment on the quality of your data. You should also think about how your method could be improved.

A common source of error is incorrectly measuring the distance between the lamp and the pondweed. You should use a ruler and be careful with your measurements. The pondweed may have a different number of leaves and this could impact the rate of photosynthesis even if they are cut to the same length.

Another common source of error is not changing the water between repeats. Temperature also impacts the rate of photosynthesis, therefore it is a control variable in this experiment. As the bulb may heat the water slightly, this should be changed between repeats.

Experiment 2: Investigate the effect of carbon dioxide concentration on the rate of photosynthesis

The experiment above can be adapted to investigate the effect of carbon dioxide concentration on the rate of photosynthesis.

Experimental variables

Independent variable	$Amount\space of\space sodium\space hydrogen\space carbonate\space solution.$
Dependent variable	$Amount\space of\space oxygen\space produced\space (measured\space as\space the\space number\space of\space bubbles\space produced).$
Control variables	$Temperature,\space length\space of\space the\space plant,\space light\space intensity\space and\space the\space amount\newline of water\space in\space the\space beaker.$

Method

You should repeat experiment one but instead of changing the position of the lamp, you should change the amount of sodium hydrogen carbonate in the beaker. In one repeat you should use $0.5\ g$ of sodium hydrogen carbonate solution. In a second repeat you should use $1g$ sodium hydrogen carbonate solution. You can increase the amount of sodium hydrogen carbonate by $0.5\ g$ each time. In a final repeat, you should use distilled water with no sodium hydrogen carbonate.

Analysis

Repeat the same analysis for experiment one, except the $x$ -axis of your graph should be the concentration of sodium hydrogen carbonate as this is the new independent variable.

Conclusion

Your results should show a positive correlation between the number of bubbles produced and the concentration of sodium hydrogen carbonate solution. This means that as the concentration of sodium hydrogen carbonate increases, the rate of photosynthesis also increases as more oxygen is being produced.

The sodium hydrogen carbonate increases the concentration of carbon dioxide in the water. This means that as the amount of sodium hydrogen carbonate increases, so does the amount of carbon dioxide available for photosynthesis.

Biology; Biology practicals; KS4 Year 10; Investigating limiting factors of photosynthesis

Evaluation

You should repeat the same process as in experiment one to evaluate your data.

Experiment 3: Investigate the effect of temperature on the rate of photosynthesis

Experiment one can be adapted to investigate the effect of temperature on the rate of photosynthesis.

Experimental variables

Independent variable	$Temperature\space of\space the\space sodium\space hydrogen\space carbonate\space solution.$
Dependent variable	$Amount\space of\space oxygen\space produced\space (measured\space as\space the\space number\space of\space bubbles\space produced).$
Control variables	$Length\space of\space the\space plant,\space light\space intensity,\space sodium\space hydrogen\space carbonate\space concentration\space \\and\space the\space amount\space of\space water\space in\space the\space beaker.$

Method

You should repeat experiment one but instead of changing the position of the lamp, you should change the temperature of the sodium hydrogen carbonate solution in the beaker. In one repeat you could use sodium hydrogen carbonate solution with a temperature of $20\ \degree C$ . For each repeat you could increase the temperature in $5\ \degree C$ increments.

Analysis

Repeat the same analysis for experiment one, except the $x$ -axis of your graph should be the temperature of the sodium hydrogen carbonate solution as this is the new independent variable.

Conclusion

Your results should show a positive correlation between the number of bubbles produced and the temperature of the sodium hydrogen carbonate solution. This means that as the temperature of sodium hydrogen carbonate increases, the rate of photosynthesis also increases as the enzymes involved in photosynthesis will work quicker. However, at higher temperatures, the enzymes may denature so the rate of photosynthesis will drop.

Biology; Biology practicals; KS4 Year 10; Investigating limiting factors of photosynthesis

Evaluation

You should repeat the same process as in experiment one to evaluate your data.