Instructions

In Practical 4 you will examine the affect of varying the light intensity on the rate of photosynthesis in spinach using an oxygen electrode. Light intensity is like brightness, and is measured as the rate at which light energy is delivered to a unit of surface, or energy per unit time per unit area. The rates of photosynthesis are measured by recording the amount of dissolved oxygen (DO) in the buffer solution. The following link takes you to a simple experiment that looks at the affect of light intensity on rates of photosynthesis by counting the number of bubbles being emitted from leaves. (Note that our experiment in Practical 4 is somewhat more sophisticated than the one shown here – and you won’t be counting bubbles.) Light intensity experiment (duration 2:28) Question: In regards to light intensity, the rate of photosynthesis, and oxygen, which of the following statements do you expect to observe in your experiment, and why? What will be observed: As the light intensity increases, O2 production will increase. Why: During the light dependent reactions in photosynthesis, water molecules (H20) are split into Htions, electrons, and oxygen. Thus the higher the light intensity the more H2O molecules will be split. O What will be observed: As the light intensity increases, O2 production will decrease. Why: During the light dependent reactions in photosynthesis, O2 is the final electron acceptor in the electron transport chain; thus the higher the light intensity, the faster this reaction will occur and the more O2 will be consumed. O What will be observed: As the light intensity increases, O2 production will decrease. Why: O2 is not affected by the light dependent reactions, only CO2 is affected. However, increased photosynthesis will influence cellular respiration rates. The overall affect will be that the increased rate of photosynthesis will increase the rate of cellular respiration thus O2 will be used at higher rates, measured as a decrease in O2 production. What will be observed: As the light intensity increases, O2 production will increase. Why: During the light dependent reactions in photosynthesis, carbon dioxide (CO2) is split into carbon and oxygen. Thus the higher the light intensity, the faster this reaction will occur and the more oxygen will be produced

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