1. What is the primary source of energy for most living things on earth?

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1 Photosynthesis Objectives: At the end of this lab you should be able to: 1. describe the relationship between photosynthesis and cellular respiration 2. describe the function of a pigment and explain their role in photosynthesis 3. explain why chlorophyll and light are needed for starch production in a leaf 4. sketch a stomata (or stoma = plural) Plants, pigments, energy and survival Interdependence is one of the characteristics of life. This characteristic is abundantly obvious in our relationship to plants. Plants capture light energy to fuel their activities. This energy is passed on, in various forms through members in the ecosystem. 1. What is the primary source of energy for most living things on earth? 2. What is the direct source of energy for human beings? Chemical reactions Today s lab brings together some concepts covered in previous labs (most notably the scientific method and ph indicators) to explore photosynthesis. Photosynthesis provides for one of the most profound examples of interdependence - the production and use of oxygen. Photosynthesis uses light energy to produce food storage molecules (sugar), producing oxygen as a by product of this biochemical process. Cellular respiration uses sugar to produce the energy needed for ATP production. In the presence of oxygen, Aerobic cellular respiration produces an abundance of ATP. All cells, and thus all organisms, use cellular respiration to produce ATP. Simple cells rely on glycolysis (a form of anaerobic cellular respiration) to make ATP. The overall chemical reaction for aerobic cellular respiration follows: Sugar + Oxygen Water + Carbon Dioxide + ATP Only cells containing the molecule chlorophyll are cable of photosynthesis The overall chemical reaction for photosynthesis follows: Water + Carbon Dioxide + Sun Sugar + Oxygen

2 Measuring Rates of Photosynthesis Define rate of photosynthesis If the rate of photosynthesis increases; what would happen to the presence of each of the following: Water Carbon dioxide Sugar Oxygen Which variable will be measured to indicate rate of photosynthesis? ph Mini Experiment When carbon dioxide mixes with water it forms carbonic acid. (Carbonic acid is the acid found in carbonated soft drinks). H CO 2 H 2 CO 3 The dye used in litmus paper is a ph indicator; it changes color when the ph changes. There are many different chemicals that can be used as a ph indicator. One such molecule is bromothymol blue. In a basic or neutral ph, bromothymol blue is blue. In an acidic ph this molecule is yellow. Based on the chemical reaction for cellular respiration, we know that animals release carbon dioxide as a byproduct of cellular respiration. If I exhale through a straw into a beaker of water; what should happen to the ph of the water? How will we gauge the rate of photosynthesis in this experiment? Photosynthesis The following experiment is designed to explore the process of photosynthesis. Procedure 1. Fill a 50 ml flask with 40 ml of water from the back sink. 2. Add a few drops of bromothymol to produce a deep-greenish blue color. 3. Use a straw to exhale bubbles slowly into the flask; stop when the water turns green-yellow. (not apple juice yellow!) 4. Divide the water in your flask equally into four test tubes. 5. Insert a 2 inch sprig of Elodea, cut edge down, into one of your test tubes. Place this test tube and a test tube with out Elodea into the 250 ml beaker. Use colored tape to label your beaker and place it in the dark. 6. Insert a 2 inch sprig of Elodea, cut edge down, into a third test tube. Place this test tube and the remaining test tube with out Elodea in the test tube rack. Fill a 1000 ml beaker with tap water and place it next to the test tube rack. 7. Place a goose neck lamp next to the beaker of water. Make sure you have positioned the lamp so that light is shining through the water directly onto the Elodea leaves.

3 Questions 1. What is the purpose of the test tubes with out Elodea? 2. What is the purpose of the test tubes in the dark? 3. What are the two independent variables in this experiment? 4. What is the dependent variable? 5. State a hypothesis regarding the ph of the water in the test tubes. Hypothesis: If the sprig of Elodea was photosynthesizing, the ph of the test tube should. The color of the water should turn from to 6. Check your results at the end of lab and record your results. 7. Look at the test tube near the light, containing Elodea. Next to the plant you can see some small bubbles. What are these bubbles and where did they come from? 8. What is your conclusion? Please rinse the Elodea and return it to the container in the window. Please rinse your glass ware. Place the test tubes upside down in the test tube rack. Thank you.

4 Starch production during photosynthesis Sugars produced by photosynthesis are often stored as starch. The presence of starch in a leaf can be used as an indirect measure of photosynthetic activity. Both light and chlorophyll are required for photosynthesis to occur and are thus required for sugar and starch production. The following experiment explores this phenomenon. What does iodine stain? I will place separate drops of water, glucose, and starch solutions on a glass slide and add a drop of iodine to each. Describe the results. Water: Glucose: Starch: Chlorophyll requirement for starch production. This experiment will be done as a demonstration. 1. Remove a leaf from one of the variegated plants. Young leaves work best for this experiment. Sketch the leaf, noting the location of the various colors. 2. Where would you predict starch has been stored in this leaf, areas that are white, red or green? Defend your answer. 3. Use forceps to place the leaf in a beaker of boiling water. Boil the leaf for 2 minutes. 4. Remove the leaf from the boiling water and place it in a beaker of boiling alcohol. Boil the leaf until it is yellow. 5. Place the leaf in a petri dish and add 10 drops of iodine. 6. Sketch the leaf, noting the location of the blue/black coloration. 7. How does the pattern of starch storage relate to the distribution of chlorophyll in this leaf? Leaf Before Staining Leaf After Staining

5 Stomata 1. Observe on the projection scope ivy leaves showing stomata. There is a diagram of these cells on page 109 of your text. 2. Sketch a few cells from this slide to illustrate the guard cells and a stoma. Label your sketch. 3. Why does the leaf need these stomata? 4. What escapes through the stomata when they are open? Leaf Under LOW Magnification Leaf under HIGH Magnification