Sugar and Light

By Kirstin Bittel and Rachel Hughes

Time :
Preparation Time :
Materials : / Bottle of sparkling water
Potato
Lugol’s Iodine Reagent
Geranium plants prepared on Day 1
Hot plates
1000ml and 400ml beakers
80% ethyl alcohol
Photosynthesis overhead

Abstract

Carbon dioxide and water make sugar? Students connect starch to sugar as the storage form of energy and find that no starch is produced in the plant without light. Students perform the old favorite of looking at starch deposition in geranium leaves using Lugol’s Iodine reagent.

Objectives

Students will be able to:

  1. Identify light as necessary to the process of photosynthesis
  2. Recognize sugar as a product of photosynthesis and starch as the method by which the plant stores energy.
  3. Use Lugol’s Iodine Reagent as an indicator for starch.

National Science Education Standards

Plant cells contain chloroplasts, the site of photosynthesis. Plants and many microorganisms use solar energy to combine molecules of carbon dioxide and water into complex, energy-rich organic compounds and release oxygen to the environment. This process of photosynthesis provides a vital connection between the sun and the energy needs of living systems.

The energy for life primarily derives from the sun. Plants capture energy by absorbing light and using it to form strong (covalent) chemical bonds between the atoms of carbon-containing (organic) molecules. These molecules can be used to assemble larger molecules with biological activity (including proteins, DNA, sugars, and fats). In addition, the energy stored in bonds between the atoms (chemical energy) can be used as sources of energy for life processes.

Teacher Background

- a comprehensive review

- a comprehensive review

- more information

Related URLs

animations of this exercise with a twist

animations of this exercise with a twist

Activity

Remind students to check on their cotton seeds.

1. Yesterday students explored common misconceptions that challenge their ability to understand photosynthesis. Today, continue to challenge their understanding by presenting them with a bottle of sparkling water. Yesterday they were told that carbon dioxide and water produced sugars and here they have water with bubbles, which is actually carbon dioxide. Based on what they have learned over the past few days, mixing these two substances should produce sugar, right? Share with the students that you have drunk a little and it wasn’t sweet at all. You’re thinking that maybe you should mix it a little, perhaps shake the bottle up. What do they think? What do they predict will happen? Will it be sweet then? If not, why not? If students do not recognize the need for another form of energy to progress the reaction this is the time to lead them through the use of light as a source of energy. They are going to be exploring the connection between light and the production of sugar in plants today.

2. Present students with the geranium plants whose leaves you partially covered up on Day 1 of ‘Jolly Green Thumb’. Explain to the students that you placed the foil or paper on the leaves two days ago. Given the connection between the photosynthesis reaction and light, what do students think has happened to the part of the leaf that has not been receiving light? Have students make predictions in their notebooks? What might they expect to happen if an area of a leaf is covered up? Why?

3. Explain that while sugar is a product of photosynthesis, it is stored in the leaf as starch. Share with the students that starch can be detected using Lugol’s Iodine Reagent. Demonstrate the detection of starch using a slice of potato, something that the students identify as being “starchy.”

4. Give each group a leaf that has been partially covered for the past two days and one from the same plant that was not covered. Students should sketch each leaf in their notebooks, illustrating where on the leaf the foil or paper is positioned.

5. Provide students with the following information (also attached as the student protocol sheet)

Safety:

  1. Ethyl alcohol is very flammable and should not be in direct contact with the hot plate.
  2. Do not leave the ethyl alcohol unattended.
  3. Everyone should be wearing safety goggles.
  1. Set up a boiling water bath: Put a 1000ml beaker on the hot plate, which should be off, and fill with 300-400ml of water. Carefully place a 400ml beaker containing 200ml of 80% ethyl alcohol inside the 1000ml beaker. (This will function like a double boiler.) Turn on the hot plate and bring the alcohol to a gentle boil. Once the alcohol begins to boil, turn down the hotplate so the alcohol maintains a gentle boil.
  2. Remove the paper clip and paper or foil from the leaves. Do you notice any difference between the area that was covered and the area which was not covered ? Using forceps drop both leaves into the boiling alcohol. What do you notice?
  3. As the leaves whiten remove them from the boiling alcohol using the forceps and place in separate dishes. Turn off the hot plate. Using distilled water, rinse the leaves off and add just enough water to cover the leaves.
  4. Drop by drop add Lugol’s Iodine reagent to the water until the water is a very pale amber.
  5. After 5 minutes note the coloration pattern on each leaf. Sketch the leaves and the patterns of coloration into your notebooks.

6.Based upon their prior knowledge and this experience, students write what they think the relationship between sunlight and starch is.

7.Have students take out their photosynthesis jigsaw and assemble the sugar. Present the molecular structure of starch, what do they notice? Are there any similarities between starch and glucose? Students should recognize that there is some similarity. Explain that starch is actually hundreds of glucose molecules joined up.

8. Why might the plant store starch rather than sugar? Ask students for possible ideas. Then, demonstrate the solubility of sugar and the lack of solubility of starch. A couple of teaspoons of each in separate beakers should be sufficient to demonstrate their solubility. Ask students why it might be better to store energy in an insoluble form. Keeping its store of energy in an insoluble form means a plant can keep this store in a discrete place within the cell. If the energy were stored as sugar, a soluble format, it would impact the concentration of dissolved solutes in the cytoplasm, which would impact water movement.

9. Using the overhead with the basic photosynthesis equation written on it, add light to the equation. Walk students through the equation. If there is no light, then there is no photosynthesis, no sugar is produced and converted to starch.

10. Students should write a summary describing the relationship between photosynthesis, light, sugar and starch. Within that summary they should also describe why the sugar is converted to starch, possibly using an analogy.

Embedded Assessment

Summary describing relationship between photosynthesis, light, sugar and starch