Using Light Energy to Make Organic Molecules

Free Response

Why is the third stage of the Calvin cycle called the regeneration stage?


Because RuBP, the molecule needed at the start of the cycle, is regenerated from G3P.

Which part of the light-independent reactions would be affected if a cell could not produce the enzyme RuBisCO?


None of the cycle could take place, because RuBisCO is essential in fixing carbon dioxide. Specifically, RuBisCO catalyzes the reaction between carbon dioxide and RuBP at the start of the cycle.

Why does it take three turns of the Calvin cycle to produce G3P, the initial product of photosynthesis?


Because G3P has three carbon atoms, and each turn of the cycle takes in one carbon atom in the form of carbon dioxide.

Imagine a sealed terrarium containing a plant and a beetle. How does each organism provide resources for the other? Could each organism survive if it was the only living thing in the terrarium? Why or why not?


An energy cycle between a plant and a beetle would be as follows:

  1. Plant consumes carbon dioxide and releases oxygen as a byproduct of photosynthesis
  2. Beetle consumes oxygen and releases carbon dioxide to create chemical energy during aerobic respiration
  3. Plant takes up carbon dioxide from the air
  4. Repeat cycle

The plant would also provide a carbon-based food source for the beetle.

  1. The beetle is a heterotroph, and would not survive without the plant because it would deplete all the oxygen within the terrarium.
  2. The plant is an autotroph and could survive without the beetle, but it would be unlikely to grow. Through photosynthesis, the plant can make and store its own energy in carbon-based molecules, and produce oxygen. The oxygen can then be used to power aerobic respiration in the plant, which releases carbon dioxide. However, since the plant essentially continues to reuse its own resources cycling between carbon- and oxygen-consuming pathways, its growth would be limited.

Compare the flow of energy with the flow of nutrients in a closed, sunny ecosystem consisting of a giraffe and a tree.


In the defined ecosystem, energy would radiate from the Sun, and be absorbed by the chlorophyll in the leaves of the tree. Photosynthesis would occur in the leaves, transforming the light energy into stored chemical energy in the covalent bonds of carbon molecules. The giraffe would eat the leaves of the tree, and digest the carbon molecules to release energy.

In the same ecosystem, nutrients would cycle between the tree and the giraffe. The giraffe would consume oxygen and release carbon dioxide as its cells perform aerobic respiration to create chemical energy. The tree will consume the released carbon dioxide during photosynthesis to create its own stored chemical energy, and release oxygen as a by-product.