What advantages and/or disadvantages might this have for freshwater plants?
Analysis and Discussion For cells to survive in environments whose salinity is much lower or much higher than 3%, the cells may need to pump salt in (or out) to maintain a suitable internal environment. However, molecular pumps require a lot of energy (ATP), so we would expect cells to have evolved various ways to reduce this energy expenditure.
1. Freshwater plants are surrounded by water in which the salt concentration is much lower than that of sea water. Do any of your isotonic point data suggest that freshwater plant cells might have a lower internal salt concentration than marine plant cells? What advantages and/or disadvantages might this have for freshwater plants?
2. Intertidal organisms that live at the edge of the ocean experience wide fluctuations in salinity. For example, when the tide is in, the intertidal area is covered with sea water (3% salt). As the tide goes out, tidal pools are left behind to evaporate in the sun, greatly increasing salinity in the pools. However, after a rain, freshwater running into the tidal pools can greatly reduce salinity. Do any of your data suggest that marine plant cells may have evolved ways to stablilze their internal environments in the face of the wide fluctuations in salinity found in intertidal environments? Explain.
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LAB REPORT Hand in to your discussion leader a typed lab report. You and your group members can submit identical cover pages and data tables, but your introduction and discussion must be written by you, in your own words. (1) Cover page: including the title of the experiment (in this case use “Comparing cell contents of plants from different environments” ), your name, the date, your discussion leader’s name, and the number of your discussion section. Also include the names of all your partners and their discussion section TAs. (2) Introduction: State your hypothesis about whether the concentration of solutes inside the cells of plants from different environments should be the same or different. Explain briefly why your hypothesis makes sense to you. State the prediction you generated from this hypothesis and describe (in general terms) how you tested it. You don’t need to detail the methods (because they are already in the lab guide), but you do need to define an isotonic point (especially what you consider to be its relationship to the cell’s internal solute concentration) and explain how you used isotonic points to test your prediction. (3) Results: On a separate page, summarize your data from tables 3 (onion cells) and 4 (three other kinds of cells) into one table, clearly labeled. (4) Discussion: In about 2 pages, explain what an isotonic point is and compare the isotonic points of your specimens with each other and with sea water. Does there appear to be a relationship between isotonic points and environment in which the plants are found? If not, then what cellular mechanisms (salt pumps, impermeable cell membranes, or others?) might these plant cells be using to maintain a stable internal environment despite widely differing external environments? Include some of the analysis and discussion questions raised on the previous page.