Science and Plants for Schools



Name…………………………. Date………………...

Light intensity and the rate of photosynthesis

Student Worksheet

Introduction

Follow the instructions on the students’ sheet “Light intensity and the rate of photosynthesis” and complete this worksheet to show evidence of the work you have done.

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What did you do?

What organism/species did you investigate? Also record the room temperature and the number of days between making the algal balls and the experiment as well as anything else about when and where you did the experiment that you think might influence the results.

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Were your algal balls spherical and a consistent size? If so did you have to modify anything to ensure this? If not how might this have affected your results?

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Why did you have to thoroughly mix the algae and the sodium alginate? What effect would an uneven mix have had on your experiment?

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What happens to the sodium alginate and algae mix when it drops into the calcium chloride? Explain how this happens.

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What are the advantages and disadvantages of immobilising the algae in this experiment?

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Why is it necessary to rinse the algal balls with distilled water?

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Why is the hydrogen carbonate indicator necessary?

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Why is it useful to use a colorimeter to get a numerical value for colour in this experiment?

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How was the colorimeter set up, what settings were used and what values were recorded? Explain.

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What other environmental factors (as well as light intensity) might vary with distance from the light source? How could you check whether they do vary and how could you control them?

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Why do you need a control bottle containing only indicator solution?

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Why are the lids kept on the bijou bottles until you are ready to use the colorimeter?

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Why is it important to end the experiment by switching the light source off, the laboratory ceiling lights on and removing the box covering the lid in the dark before starting to record the final colour of the solution in each of the lids?

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Why is it important to measure the absorbance of the indicator solution relatively quickly?

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Your results

Record your results in a suitable table using the column suggestions in the instructions on the student sheet.

Plot a graph of Absorbance at 550nm against light intensity and draw a line of best fit through the data (attach the graph below).

Describe the shape of your line of best fit in as much detail as possible.

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Is there a light intensity above which there is no (or very little) increase in absorbance? If so what is this value?

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Record the light intensity at which the line of best fit crosses the x-axis.

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Draw a straight line of best fit through the section of data that crosses the x-axis. Record the light intensity at which this straight line of best fit crosses the x-axis.

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Comment on the two values you have obtained for where the lines of best fit cross the x-axis.

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Calculate the equation for the straight line of best fit you have drawn (in the form y=mx+c) and use it to calculate where this line intercepts the x-axis. (show your working)

How does this value compare with the value you got when reading off the intercept point from your line?

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Input your data used to plot the graph into a graph drawing package on your computer and use it to draw a graph. See if you can get the software to draw a trend line – there are various options for the general shape of the trend line and each is effectively a model of your data. Is there an appropriate option for your data?

Attach a print out from your graph plotting package with this worksheet and comment on what you did below.

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Use a subset of your data (the same subset that you used to draw your straight line of best fit) to plot another graph in your graph plotting package of the section of data that crosses the x-axis. Add a straight trend line to this graph and get your graph plotting package to show you the equation for this line. Solve the equation of the line for y=0 to identify the point where the line intercepts the x-axis (the light compensation point).

Attach a print out from your graph plotting package with this worksheet and show your working for the rest of the task below.

Compare your hand calculated equation and intercept point with the Excel derived one.

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Your computer generated straight trend line is a simple computer model of your data. What assumptions does this modelling make? What are the advantages and disadvantages of modelling data in this way? Is a straight line the best model for these data? Explain.

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If the experiment was left for much too long you might expect to see that all of the bottles contained either yellow or dark purple liquid but no bottles with an intermediate colour except for the control. Why would this occur and what precaution does it suggest you should take with when to finish your experiment? If this did happen what would now not be possible to work out and what conclusions would you still be able to draw?

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If the experiment was stopped too quickly how might this affect the shape of the graph and how would this affect the confidence you have in your identification of key points in the graph (the light intensities at which the line of best fit levels off and where it crosses the x-axis)?

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The biology of what you see

Write a simplified word equation and balanced symbol equation for photosynthesis.

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Explain the purpose of photosynthesis and respiration in organisms, as well as the relationship between the two processes.

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Explain what is happening in the solution when you get a positive Absorbance value.

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Explain what is happening in the solution when you get a negative absorbance value.

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Explain what is happening to the rate of photosynthesis and why this is occurring as light intensity increases in the part of your graph that has a relatively steep positive correlation.

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What is a limiting factor (or rate limiting factor)? Give 4 examples of potential limiting factors for the rate of photosynthesis.

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What is limiting the rate of photosynthesis at the light intensities where your graph shows a steep positive correlation between light intensity and Absorbance?

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What is a “light saturation point”?

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If there is a light intensity above which there is no (or very little) increase in Absorbance, explain why this happens. If not, do you think that if light intensity was increased further and further above what you managed in your experiment that Absorbance would continue to increase and increase? Explain your thoughts.

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What could be limiting the rate of photosynthesis at light intensities where a graph shows no increase in Absorbance with an increase in light intensity?

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What is the name of the point at which your line of best fit crosses the x-axis, what is happening at this light intensity, and what is its significance for the photosynthesising organism?

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Scientific curiosity: implications of the biology for horticulture and agriculture, further experiments and exploring the use of smart phones for data collection

Use your own paper to record your thoughts on some of the areas below. They will get you stretching your thinking about how this biology could be applied in industry and what other experiments would be interesting to conduct.

Implications of the biology for horticulture and agriculture

a) Design a series of investigations to advise about the suitability of different plant species for use as pot plants in a particular, relatively dimly lit office space. Explain what you would be looking for and the biology behind it.

b) Design a series of investigations to advise a commercial horticulture business about whether they would benefit from investing in brighter lighting in their environmentally controlled greenhouses. Explain what you would be looking for

c) and the biology behind it.

Further experiments into factors affecting the rate of photosynthesis

a) Design a series of experiments, and explain what you would be looking for, to identify what the limiting factor for the rate of photosynthesis is, in this experiment, when light intensity is no longer limiting (don’t worry too much about how you would change or measure different factors just say what you would change or measure).

Exploring the use of smart phones for data collection

New tools for data collection are always being designed and people are always looking for cheaper and more readily available tools. There are also initiatives to get more people involved in conducting good science and pooling their findings. Smart phones provide a good opportunity to empower large scale data collection by members of the public, however their ability to measure reliably and accurately needs to be analysed first. If proven to be suitable measuring tools, smart phones may also be good data collection tools for places in the world where it is too difficult or too costly to obtain standard scientific measuring equipment.

There would be real value in designing (and conducting) a study into the accuracy and reliability of the use of a smart phone as a light meter.

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