Back to Journals

Journal 10

Photosynthesis, Light, and Life
To solve the problem below, you must write a clear and convincing justification for accepting one of the possible answer choices as the best or most appropriate and for rejecting each of the other choices as less appropriate. Your reasons for rejecting choices are just as important as your reasons for choosing the best answer.
A fully illuminated plant was placed in an atmosphere containing CO2 composed of radioactive carbon. Then the atmosphere surrounding the plant was exchanged for air containing no radioactive carbon, and the plant was placed in total darkness. When the atmosphere surrounding the plant was sampled for radioactive carbon dioxide at various times after the plant was placed in total darkness, the following data were obtained.
Time (hr.) 1 2 3 4
CPM (counts/min.) 40 86 126 159
Which one of the following is the most likely interpretation?
1
 Plants cannot respire in the dark.
2
 Respiration was occuring in the dark.
3
 Respiration, but not anerobic glucose breakdownm occurs in the dark.
4
 Fermentation and glucose breakdown were occuring in the dark.
5
 The interpretation provided in choices 2 and 4 are equally valid.
Prior to 1930, it was generally thought that in the overall equation for photosynthesis
CO2 + H2O + Light Energy ---> (CH2O) + O2
light energy seperated the carbon and oxygen in CO2. The carbon from CO2 then combined with water to form carbohydrate (CH2O) and the oxygen from CO2 was released as oxygen gas. Which of the following would provide evidence contradicting this view?
1
 Expose a lighted plant to CO2 composed of radioactive carbon.
2
 Increase the illumination of the plant.
3
 Increase the amount of water vapor in the surrounding air.
4
 Measure the total amount of carbohydrates and oxygen produced by the plant over a given period of time.
5
 Demonstrate that oxygen evolves from plant cells when no CO2 is present.
A biologist predicted that if part of a leaf is shaded and other parts of the leaf are fully illuminated then equal amounts of starch will subsequently be found in the shaded and illuminated parts of the leaf. On which of the following is this biologist most likely basing his prediction?
1
 Chlorophyll is present in both the shaded and illuminated parts of the leaf.
2
 Photosynthesis occurs at an accelerated rate in the shaded parts of the leaf.
3
 Carbon dioxide and water can enter cells in both the shaded and illuminated parts of the leaf.
4
 Shading part of the leaf increases the rate at which photosynthesis occurs in the illuminated part of the leaf.
5
 Starch can move to different parts of the leaf.