Photosynthesis

Critical Thinking Activity

Objectives:

• Use the resources on the world wide web to investigate questions about how photosynthesis works and how plants in different environmental conditions have modified the process of photosynthesis to their advantage.
• Explore some ideas of the role of photosynthesis in the ecological balance on Earth.  

Approach:

• I have selected four topics about which I have been curious to find out more. For each topic I will pose a question that extends beyond the information presented in the text. I will offer you some URLs where you can investigate additional information on each topic. You can use CUT and PASTE to bring information, including graphics, back to this web page to include in your answers in addition to the writing that you do. You do not have to do the questions in order. As is common on the web, there is always more that you can find to investigate beyond the suggestions I am providing for you.

Dr. Paul Tabor

OK, here are the questions:

1.  You are most likely aware that there are light-dependent, energy yielding reactions and light-independent, biomass synthesizing reactions in photosynthesis.  You can take a look at a review of the light reactions with some great diagrams to help you with question #2.  This question is concerned with the light-independent reactions.  The review of the light independent or dark reactions points out that the synthesis of organic carbon from CO₂  is not very efficient and high temperatures and arid conditions make it even harder for plants to synthesize organic carbon.  But some plants called C4 plants have modified the dark reactions to become more efficient in these conditions.  What is the advantage C4 plants have?   Orchids are C4 plants that have a further modification for control of gas exchange in their leaves.  Just for fun here is a site that has photo collections of orchids.

2.  Some herbicides that are in use today are very specific for plant life and that suggests the "active ingredient" targets some metabolic process found only in plants.  Wouldn't the capture of photon energy and process of conversion to chemical energy in photosynthesis be such a plant-specific process?  Compounds that inhibit specific reactions in photosynthesis are useful to help us understand the process.  The herbicide Diuron is used in experiments to study the Hill Reaction. What is the specific action of the herbicide Diuron?  Can you think of conditions that would inhibit this herbicide?  

3. In an ecological perspective, the productivity of an ecosystem is driven by photosynthesis.  The major biomes on Earth (e.g., forests, grasslands, tundra, the oceans, etc.)  have different rates of primary productivity (the production of biomass due to photosynthesis). Which biome on earth has the highest productivity?  Which biome on earth produces the greatest total amount of biomass from photosynthetic carbon fixation?  

4. CO₂ is necessary for photosynthesis.  Question #1 was concerned with how plants try to maximize the use of CO₂ that is available.  However, over the Earth the level of CO₂ in the atmosphere is rising.  How will that affect the rate of growth of agricultural plants and trees?  Will all plants have the same response to higher levels of CO₂?  Will plants adapt to higher levels of CO₂?  Is it feasible to think that plants can be used to offset the increase in CO₂ levels?  Some scientists wonder if increased levels of CO₂ will cause a loss of diversity of plants.  From Question #1 you have seen that C3 and C4 plants process CO₂ differently.  Will one type of plant be less competitive and be more likely to become endangered if CO₂ levels are higher?