Photosynthesis and cellular respiration in a plant

Question

I have to right a lab report of an experiment we did for class but im confused with the results. In the experiment we had 6 cups containing different things. In cup 1 we had a plant, a grow light, and the cup was placed in a room temp. environment. In cup 2 there was no plants, a grow light, and room temp environment. Cup 3 contained one plant, had no grow light, and was in room temp. Cup 4 had no plant, no grow light, and was in room temp. In cup 5 there was 1 plant, a grow light, was in room temp, and 15 drops of starch was added. Finally for cup 6 there was also a plant, had no glow light, room temp, and also had 15 drops of starch. The cups all had water in them, and BTB was added to the cups. All the cups started at the color green for BTB. After 24 hours Cup 1 changed to color blue, Cup 2 stayed at green, Cup 3 turned a yellowish green, Cup 4 stayed at green, Cup 5 was yellowish green, and Cup 6 changed to blue. Why was this?

Answer 1

Lizzy,

The BTB changes color based on the acidity of the water. Green means a pH of 7 or neutral (not basic or acidic). Photosynthesis creates oxygen and glucose and starch is a complex sugar made out of many glucose molecules. Cellular respiration creates carbon dioxide and water. Carbon dioxide is acidic so the water would turn toward yellow. Oxygen reacts with the hydrogen in the water molecules and produces some OH- or hydroxide ions which are basic and turn toward blue.

Cup 1 had plants and the plants were given light, photosynthesis occurred and the way you can tell is that the BTB turned blue - showing oxygen formation.

Cup 2 was your control group since there was no plant even though there was light so no reaction occurred with the BTB and stayed green.

Cup 3 had plants and no light which should means no photosynthesis can occur. Photosynthesis only can happen when light is shining on a plant. Otherwise, plants go through cellular respiration which is what animals do and us. Since the color was yellowish, that means the water was acidic showing that carbon dioxide was created.

Cup 4 seems like another control group since there is no plant and no light causing BTB to stay green/neutral.

Cup 5 had a plant and light so we know from cup 1, photosynthesis occurred. However, cup 1 turned blue or basic and this cup did not so the starch must have contributed to the change. Starch is acidic and can affect the rate of dissolving oxygen into water. I am not sure how in depth this lab is

Cup 6 had a plant but no sunlight so from cup 3 we know cellular respiration must have been produced. Since cup 3 turned yellow and this cup did not, that means that the starch caused a change in the pH of the water and affected the dissolving carbon dioxide rate.

This was answered for a high school regular or honors class level lab report not an AP or college level. I hope this gives you a good start.