Pre-lab homework Lab 6: Photosynthesis & Cellular ...

Biology 101

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Pre-lab homework Lab 6: Photosynthesis & Cellular Respiration Name:

After reading over the lab answer these questions to be turned in at the beginning of the lab!

1. This week's lab covers the concepts of photosynthesis and cellular respiration. These processes are both complex chains of chemical reactions (sometimes called a metabolic pathway) that can be simplified into an overall reaction summary equation. Find the summary equations for photosynthesis and cellular respiration and write them in the space below. Photosynthesis summary reaction:

Cellular Respiration summary reaction:

2. During exercise 1 you will be testing for starch in plant leaves. Why are we testing for starch?

3. In our experiments in exercise 1 we have placed black paper on some of the leaves. Why would we put on paper that blocks all of the light?

4. When we test for carbon dioxide production in exercise 2 of the lab, what will we be counting and how will that measure CO2 production?

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Lab 6: Photosynthesis & Cellular Respiration

GOALS: After successfully completing this lab a student will be able to: ? Explain the connection between light and energy storage in plants. ? Describe the colors of light that are most effective for photosynthesis and why. ? Explain the connection between photosynthesis and cellular respiration. ? Describe the effects of some common activities on cellular respiration.

OVERVIEW: During lab this week you will perform experiments to help you understand two of the basic

processes that convert energy from one form to another in biological systems. Your investigations will start with experiments on photosynthesis. This is the process that plants use to convert the kinetic energy of light into the stored chemical energy of sugars. It can be summarized with an equation that looks like this:

6 CO2 + 6 H2O ----------> C6H12O6 + 6 O2 Light

In this lab we will investigate the role of light in controlling the amount of photosynthesis that occurs. As in any scientific experiment we will try to isolate one variable, in this case the wavelength of light, and see what effect it has on the amount of photosynthesis that a leaf undergoes.

After examining the role of light in photosynthesis this week we will investigate the process of cellular respiration. This process converts the stored chemical energy of sugar molecules (created directly or indirectly by photosynthesis) into a short term cellular storage molecule known as ATP, which is used to fuel a huge number of chemical reactions throughout the cell. This is a complicated process that in a simplified form looks like this:

C6H12O6 + 6O2 ----------> 6CO2 + 6 H2O ADP + Pi ATP

As you can see from comparing these two summary equations there are some obvious similarities between the end products of photosynthesis and the reactants (the starting chemicals) of cellular respiration. You can also see that the end products of cellular respiration are the reactants of photosynthesis. Remember that the summary reactions above simplify a complex series of reactions (both biosynthetic and degradative) so these reactions are not the exact opposite of each other but the carbon atoms that are a part of all living organisms are constantly being cycled through an ecosystem by the action of these two processes.

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Exercise 1: Photosynthesis and Light

Plant cells convert CO2 into carbohydrates in a process called photosynthesis. Recall that the equation looks like this:

6CO2 + 6 H2O ----------> C6H12O6 + 6O2 Light

To investigate this process we need to have some way to measure the amount of C6H12O6 (glucose) that is being produced by a plant. We actually will use a closely related compound, starch, which is produced by plant leaves as an energy storage product when they have excess sugars. Remember that starch is simply a polymer of many glucose molecules strung together in a very long chain. To see the starch we will use an indicator solution called Lugol's solution. This solution (it is made up of potassium and iodine) stains starch a dark purple/black color but does not stain sugars at all. So to look for regions with high rates of photosynthesis we will test for the presence of starch. Any part of the leaf that stains a dark color indicates there is starch and therefore that there has been photosynthesis happening at such a rate that excess sugars are produced.

Using this test we will examine the effects of different types of light on the ability of plants to perform photosynthesis. Hint: this could be the answer to your pre-lab homework question! A week ago filters of different colors were placed on the leaves of a few geranium plants. We used four types of filters. One is really just black construction paper and should block all of the light to the leaf underneath it. The other three are colored plastic pieces that let through only certain colors of light. The red filters let only red light through, the blue allows mostly blue light to get to the leaf, and the green filter blocks colors other than green. By using these filters we are able to alter the independent variable, in this case the color of the light, and see how it affects the dependent variable, in this case the amount of starch produced.

1. What are some other variables that you can think of that could affect the amount of photosynthesis that these plants can do?

2. Pick two of these variables (from #1) and explain how we are controlling for them.

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SETTING UP YOUR EXPERIMENT:

Because this procedure involves heating up a highly flammable liquid you need to be very careful and follow instructions carefully!

PROCEDURE FOR TESTING FOR STARCH IN LEAVES: 1. Set up a beaker of boiling water for you alcohol bath. To do this you will need to add 200 300 ml of tap water to your 600-ml beaker and place it on your hot plate. Turn the hot plate to

high. The hot plate will get hot - please be careful!

2. Now place approximately 100 ml of 80% ethanol to your 250-ml beaker and carefully set the beaker into the boiling water beaker. When the water begins to boil turn down the hot plate until the water maintains a slight boil.

3. While the alcohol is warming up remove the leaf (or leaves) assigned to your group and take them back to your desk.

4. Before you remove any of the filters sketch the leaf so that after you boil it you will know where the filters were.

5. After sketching your leaves carefully remove the filters and place the leaf in the boiling ethanol. If you put more than one leaf in the alcohol at a time make sure that you can tell the leaves apart (cutting notches in them or cutting their stems works best don't rely on the color they will all come out the same color!)

6. After the leaves are a very pale green or white color remove them and place them in a petri dish.

7. Make sure you turn off your hot plate!

8. Rinse the leaves with tap water and then add a little water so that the leaf is just covered.

9. Now add 4-5 droppers full of Lugol's iodine solution to the dish and swirl it to cover the leaf.

10. After sitting for 5 - 10 min the leaf will begin to darken where there is starch.

11. After the leaf is fully 'developed,' remove it from the stain and place it in a petri dish. You may need to add a little water to get it to spread out fully!

12. Now sketch your stained leaf next to your sketch of the leaf before it was boiled for easy comparison.

13. After your double boiler has cooled you need to make sure you dispose of the ethanol in the labeled waste jar! You can put the water down the drain.

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After completing your experiment on photosynthesis answer the following questions: 3. What is the question that we are trying to answer with this experiment?

4. Using your results what do you think is the answer to this question #3?

5. Land plants primarily use a pigment called chlorophyll to absorb light energy to power photosynthesis. Based on this experiment what color(s) of light do you think this pigment can absorb? What color(s) can't it absorb?

6. Imagine you have removed the chlorophyll from a plant's leaf what color would the pigment be (not the color of the leaf ? the color of the pigment chlorophyll!)? (Hint: this answer should agree with what you answered in #5 above!)

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