Potential Final Exam Questions

1. How might global warming change the three types of photosynthesis we studied?

C3 photosynthesis. The light and dark reactions happen at spatially and temporally similar locations. An increase in CO2 is likely to increase photosynthesis as long as other materials do not become limiting.
C4 photosynthesis. The light and dark reactions happen in spatially different locations. Thus the CO2 can be sequestered with less regard to water loss through the open stomata than C3. This implies the potential for greater photosynthesis levels, but again the plant would be restricted by some limiting input.
CAM photosynthesis. The light and dark reactions are temporally separate allowing the plant to be freer from the temperature of the day and the correlated water loss from open stomata collecting CO2. However, since CO2 is converted and stored as malic acid during the evening there is an inherent limit to how much malic acid the plant can store. It would seem that this is the current limiting step in CAM photosynthesis, so unless malic acid storage also changes with increased CO2 levels, these plants probably won’t see much of a change in photosynthetic rate.

2. What broad changes in soil composition might occur because of climate change?

The freezing period that soils in temperate zones experience allows the A horizon to create the B horizon, or later the E horizon through weathering. The temperature change stops the decomposition process in the O layer and allows the soil to weather. This creation of more soil layers and complexity adds nutrients to the soil that are otherwise locked in the organic layer. Changing how often and/or where freezing occur could significantly impact the soil weathering process. Less layered soils will likely dominate areas that once had an extended period of dormancy.

3. Describe the dangers of melting sea ice for both the planet’s temperature and the ocean’s currents.

As seas ice melts, this not only creates a positive feedback loop of warming and decreasing albedo, but it also decreases the salinity of the water. The decreasing albedo of the earth means less light and heat is being reflected out into space, which leads to a warmer earth, further promoting melting sea ice. The decreasing salinity that results has the potential to alter the circulation of the deep ocean currents. As water moves towards the poles it cools and becomes more dense (water evaporates but leaves the salt behind and as it cools it can hold less solutes). The warmer and less salty water will take longer to sink, slowing the overall process of circulation. This slowing or stopping of circulation has the potential to effect the weather of the planet—indeed this change has been a suspected cause for the little ice age of the 1650’s.

4. How have humans changed the global nitrogen cycle?

The nitrogen cycle has changed significantly in the amount of nitrogen cycling throughout the planet. Prior to the green revolution when massive amounts of manufactured fertilizer were not available throughout the world, the amount of nitrogen plants had access to had to be shared with the soil bacteria and balanced with the amount in the air. After the industrial revolution an increasing number of NOx gasses were added to the atmosphere. After the green revolution an increasing amount of nitrogen in the form of fertilizer was being consistently added to the land. These additions of nitrogen has changed the overall amount of available nitrogen, so far mostly to our benefit through increased crop yields.

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