Photosynthesis Questions

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Bring on the tough stuff

1. Scientists on another planet have made an exciting discovery about a new photosynthetic plant. The plant appears black instead of green. What does the plant’s color tell you about its photosynthesis?
    
2. Jan Van Helmont hypothesized that in order for a tree to grow and gain mass, it must find food somewhere else. He carefully weighed a tree and the soil that he wanted to plant the tree in. Time passed, and the tree grew. After five years, he reweighed the tree, which had grown quite large, and the soil. To his surprise, the soil weighed basically the same amount, suggesting to Jan that the plant couldn’t be gaining mass from the soil. If you were Jan, what would you do next to try to figure out what the plant was consuming for food?


3. Photosynthesis is often described in two steps: the light reactions and the dark reactions. Given what you have learned about photosynthesis and the ways that plants have adapted to optimize it, why is dividing photosynthesis into these categories misleading?


4. Are plants more important to people, or are people more important to plants? Explain.


5. Why can’t we humans perform photosynthesis?


6. Describe the major differences and similarities between C₃, C₄, and CAM plants.
     
7. Why is the spatial separation of photosynthesis within the chloroplast important?

Photosynthesis Answers

1. Answer: On Earth, the pigment in a plant's leaves reflects the light that isn't utilized by the plant. If this plant on another planet appears black, that must be because it is able to harness the entire spectrum of visible colors in light for energy.
    
2. Answer: Jan suspected that the tree was gaining mass from the water he was adding. In order to confirm this hypothesis, he could use isotonically labeled water (or use any other molecule that you thought the plant was using). If the plant was incorporating any part of this chemical into its mass, you could then look for that isotopic label in the tissues of the plant.
    
3. Answer: It is misleading because, although the activation of the photosystems is light-dependent, the Calvin cycle can run in the light; it is light-independent, meaning that it does not require light to occur.
    
4. Answer: We could argue that plants are more important to people because we subsist on plants. That is, we consume them and are higher up in the food chain. However, many plants have been domesticated by people and require people to be able to actually disseminate their seeds; corn is one example. Therefore, in the case of domesticated plants, people are just as important to their survival as we are to theirs.
    
5. Answer: Humans lack chloroplasts, which are the places where all the major proteins and pathways required for photosynthesis occur. Instead of being autotrophs ourselves, we eat the carbon of others and are therefore heterotrophs.
    
6. Answer: C3, C4, and CAM plants all perform photosynthesis and generate their own carbohydrates from CO₂. C3 plants do this through a 3-carbon intermediate (PGA, or phosphoglycerate) while C4 plants do this through a 4-carbon intermediate (malate). C3 plants have adapted to reduce the amount of photorespiration that occurs, which is basically RuBisCOs malfunction, by isolating RuBisCO from oxygen and shuttling CO₂ to RuBisCO via the C4 intermediate.  CAM plants have adapted to dry climates by only opening their stomata at night to minimize water loss, as well as storing their CO₂ as malate, just like in C4 plants.
    
7. Answer: Spatial separation of photosynthesis in the chloroplast allows for oxidative phosphorylation, or the generation of ATP through the proton gradient, to occur. Additionally, the light-independent reactions, which are oxygen sensitive, must be kept separate from the light-dependent reactions, which generate oxygen.