AP BIOLOGY

Detailed explanation-1: -C) The rate of photosynthesis decreased in the dark, thus the elodea used more CO2. The elodea and snails continued to consume CO2 through cellular respiration, lowering the level of CO2 in the tube, causing the solution to become more basic (yellow).

Detailed explanation-2: -In the dark, an elodea plant will turn a bromothymol blue solution yellow as it produces carbon dioxide as a product of respiration. An elodea plant exposed to light will turn yellow bromothymol blue back to the blue color as it uses carbon dioxide.

Detailed explanation-3: -carbon dioxide is absorbed from water by the plant in presence of light. The absorption of Co2 from water by the elodea in light is revealed through the change in color of the bromothymol blue from blue to green.

Detailed explanation-4: -Plants excrete more carbon dioxide at night compared to during the day because they cannot do photosynthesis at night. Photosynthesis requires sunlight. So, during the day plants take in carbon dioxide and water and use sunlight to do photosynthesis to make oxygen and glucose.

Detailed explanation-5: -The excitation of electrons from P680 is dependent on light energy. If electrons are not removed from P680 they do not have to be replaced by the splitting of water. As a result, the production of oxygen gas resulting from the splitting of water would not occur in the dark.