Algal Bead Lab 11/18 Integrated Science 3 Redwood High School Name: Per: Introduction All heterotrophs (including humans) are dependent on photosynthesis carried out by autotrophs for virtually all food/energy needs. Whether we eat plants directly or indirectly (by eating animals), it is the photosynthesis conducted in the leaves of those plants that becomes a limiting factor for our species. Recall the overall equation for photosynthesis: 6CO 2 + 6H 2 O + light energy C 6 H 12 O 6 + 6O 2 In words, this says that both carbon dioxide and water react to form glucose and oxygen. This chemical change will take place as long as light energy is present. As the plant is producing glucose through photosynthesis, at least half of this glucose is used to meet the plant s own energy needs (for growth and reproduction) through cellular respiration. If people (or other animals) eat a plant, they perform cellular respiration and use glucose that the plant has not used as an energy source for their own growth and reproduction. The overall equation for cellular respiration is written as: C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + ATP Oxygen is produced in photosynthesis as a waste product and is given off as a gas. All heterotrophs performing cellular respiration use this Oxygen and produce Carbon Dioxide as a waste product. Carbon Dioxide is used in turn by organisms performing photosynthesis. Photosynthesis is the only biological process that can capture light energy and convert it into chemical compounds (glucose) that all organisms from bacteria to humans can use to power metabolism, growth and reproduction. Cellular respiration is the process that all organisms require to convert glucose into ATP, a high energy molecule used by cells to drive energydependent reactions. The two processes are interdependent since the products of photosynthesis are the reactants of cellular respiration, and vice versa. In this lab, you will use algae beads. These are algae cells (from the algae Scenedesmus obliquus) that are encapsulated in alginate, forming beads of algae. All algae are autotrophs so each algal cell contains both a large central chloroplast where photosynthesis takes place, and smaller mitochondria where cellular respiration occurs. Although photosynthesis and cellular respiration evolved as independent processes in early prokaryotes, a look at the summary reactions (see figure) highlights their interdependence today: The products of photosynthesis oxygen and carbohydrates are the reactants for cellular respiration, and vice versa. 1
Pre-Lab Questions 1. What is autotrophy? Provide an example of an autotrophic organism. 2. What is heterotrophy? Provide an example of a heterotrophic organism. 3. In your own words (or using chemical reactions), describe how photosynthesis and cellular respiration are interdependent. 4. What type of organism would you need to use to be able to monitor both photosynthesis and cellular respiration? Why are the eukaryotic algal cells a good choice? 5. You will indirectly measure the rates of photosynthesis and cellular respiration by monitoring product generation. Considering this, what products might you monitor to determine the rate of photosynthesis? Of cellular respiration? 6. Which process (photosynthesis, cellular respiration, or both) do the algae perform when incubated in the light? In the dark? 7. Draw a ph scale. Label appropriate areas as acidic, neutral, and alkaline/basic. 2
Procedure The algae beads used in the following investigation allow you to observe both photosynthesis and cellular respiration simultaneously. The beads contain eukaryotic microalgae (Scenedesmus obliquus) encapsulated in alginate. You will incubate the algae beads in a CO 2 indicator solution that is sensitive to changes in ph caused by gaseous CO 2 dissolving in water to form carbonic acid: CO 2 + H 2 O H 2 CO 3 HCO 3 - + H + When the CO 2 indicator is at equilibrium with the atmosphere, it is dark orange. When the CO 2 levels increase, it changes to yellow, and when CO 2 levels decrease, it changes to purple. The CO 2 indicator spans the range of ph change that will be seen in the algae beads (ph 6.9 9.1), making it a convenient way to measure photosynthesis and cellular respiration. In this investigation, you will compare the rates of color change of the CO 2 indicator caused by algae beads incubated under bright light and in complete darkness. The color/ph change of the CO 2 indicator can be determined using the Indicator Color Guide. Protocol 1. Use the wash transfer pipet to remove the water from the cuvette. Discard the water into the waste container. 2. Label a new transfer pipet indicator and use it to transfer 1 ml of CO 2 indicator to each cuvette. Cap cuvettes tightly. 3. Wrap the cuvette labeled dark in aluminum foil. Place both the cuvettes labeled light and dark on their sides 15-25 cm from the lamp. Ensure that the beads distributed evenly throughout the cuvette and the clear side of the cuvette faces the light. 3
4. Collect data starting at time = 0 min. Every 5 min, thoroughly mix the CO 2 indicator in the cuvettes and determine the color. This can be done by comparing the color of the CO 2 indicator in your cuvette to the provided Indicator Color Guide. Be quick about taking this reading and immediately return the cuvettes to the experimental conditions. Record your data in Data Table 1. After Conducting Your Experiment, and before cleaning up 5. Use the transfer pipet labeled excess to remove and discard the liquid that transferred along with the beads. 6. Use the pipet labeled wash to add 1 ml of distilled water to each of the cuvettes. Let the algae beads incubate in the water for 5 minutes to allow the indicator within the bead to wash out. Data Table 1. Group Data Treatment of Cuvettes ph at Time (min) 0 5 10 15 20 25 30 35 40 45 Light Dark Record Your Data in The Class Data Tables/Spreadsheets Online (1 table/spreadsheet for LIGHT, and 1 for DARK) 4
Graph 1. Graph your class mean results. Time vs. ph. Two lines on the same graph. One line for the reaction in the light, and one line for the reaction in the dark. Post Lab Questions (to be printed double space; single sided) 1. Are your slopes positive or negative for light and dark conditions? What does this mean about the change in CO 2? 2. Under which condition did the CO 2 indicator turn more alkaline? Why? 3. How does cellular respiration impact the observed rate of photosynthesis? Is your rate of photosynthesis accurate? Why or why not? 4. Look up current ocean ph values. How do the current values compare to those from previous years? Consider what you ve just learned about algae and how the chemistry of the indicator used in the experiments you just performed works. Hypothesize why oceans are at their current ph. How is the ph of the ocean changing and why? How might this affect the organisms that live in the ocean? 5. The algae beads provide a convenient experimental system because they are uniform in size and contain roughly the same number of algal cells per bead. Why are these advantages for the experiments you performed? 6. Why is it important to keep the cuvettes at a consistent distance from the lamp as you perform this activity? 7. Scientists can measure the extent of reactions by monitoring either reactant depletion or product generation. What other substrates or products might you be able to monitor to determine the rate of reaction in this lab? 8. Photosynthesis uses CO 2 and cellular respiration produces CO 2. We call the point when the two processes are in balance when there is no net production of CO 2 the compensation point. How might you limit one of the processes in order to achieve a compensation point? 5
Further Study Experimental Organizer Complete the Experimental Organizer below for a further study you could conduct with the algal beads that could affect the rate of photosynthesis and cellular respiration. Title: Hypothesis: Independent Variable: Continuous or Discontinuous Levels of I.V. (indicate control if applicable): # of trials you will conduct for each treatment: Dependent Variable: quantitative only Quantitative Measurements (include units): Constants: 6