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You Are The Sunshine in My Life

In this lab you will use a photosynthetic organism to investigate the requirementsof an anabolic reaction like photosynthesis. The major site of photosynthesis in

any plant is its leaves. Carbon dioxide enters the leaf’s surface through tiny holescalled  stomates. Water enters a plant from the roots. Special organelles in leaf 

cells absorb energy from the sun using primary pigments called chlorophyll. The

energy from captured light manipulates water and carbon dioxide to produce

 biomolecules like glucose. What happens if we prevent the leaves from obtainingany of these substances? Think about this as you complete the following lab.

Procedure:

1. Obtain a Petri dish and label each half either “light” or “dark”.

2. Obtain a half of a leaf from a plant that has grown in the absence of light energy, and

another that has received plenty of light. (in other words share a leaf from each conditions

with another group) Observe any outward differences between the two.

3. Immerse each leaf in boiling water for about one minute (be sure to keep tabs on

which leaf is which). Remove your leaves from the boiling water with forceps.

4. Immerse each leaf in a hot ethyl alcohol water bath. This will remove all the pigments

(chlorophyll and accessories) from the leaves.

5. When most of the pigment has been removed (the leaves will be very light green,

almost white), use forceps to remove them from the hot alcohol.

6. Prevent each leaf from becoming brittle by dipping them back into the hot water used

in Step 3 for a few seconds.

7. Place each leaf in the properly labeled Petri dish and cover each leaf with a coupledrops of iodine solution. Allow the leaf to sit for a few minutes.

8. Gently rinse each leaf of the iodine solution by dipping them into a beaker of cleanwater.

9. Compare the staining pattern and colors of each leaf and record in words and drawings

what you observe.

10. Be aware that iodine is used to detect the presence of the polymer starch. Do you

remember what the monomers of starch are and why that is relevant.

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Observations of leaves before treatment

Observations of leaves after treatment

Questions:

1. When the interactions of two or more substances result in the formation of different

substances it is called a chemical reaction. These reactions are expressed in the form of 

an equation. Write the overall equation for photosynthesis using both words and

symbols.

2. What can you conclude about the amount of starch that was found in the plant grown

in the dark and light. How does this connect to photosynthesis?

3. What do you think would eventually happen to the plants grown in the dark? Bespecific.

4. Does this test provide evidence that photosynthesis is anabolic or catabolic? Exogonicor endogonic? Explain.

5. Using your own words, describe what happens in the light cycle of photosynthesis.

6. Using your own words, describe what happens in the light independent reactions

(Calvin cycle) of photosynthesis.

7. Assume two humans are provided with the same food, air, and water, but one person

lives in an area with little sunlight while the other lives in a sunny climate. Would their 

growth rates be significantly affected by how much light they get?