Question
Sodium alginate beads were prepared containing a suspension of Chlorella algae. These beads were placed in nine small jars containing hydrogencarbonate indicator solution. Hydrogencarbonate indicator is commonly used to detect changes in pH.
The jars were placed at different distances from a lamp, which was the only light source. The distances ranged from 40 to 130 centimetres. Jars were placed one at a time to avoid the jars from shading each other. Colour changes were observed.
a. State the name of the process which occurs in the chloroplasts of Chlorella, that uses CO2. [1]
b.i. The colours of the indicator in the jars ranged from yellow to purple after five hours.
Predict the distribution of indicator colours in jars 1 to 9. [1]
b.ii. Explain the scientific reasoning for your hypothesis. [3]
c. Suggest one other factor that has to be kept constant in all the jars during the experiment. [1]
▶️Answer/Explanation
a.
Answer: Photosynthesis (or light-independent reaction of photosynthesis).
b.i.
Answer: Jars closer to the lamp (e.g., Jar 1) will have the indicator colour closer to purple, while jars further away (e.g., Jar 9) will have the indicator colour closer to yellow.
b.ii.
Explanation:
- High light intensity near the lamp increases the rate of photosynthesis in Chlorella algae.
- Photosynthesis consumes CO₂, reducing its concentration in the solution.
- As CO₂ is removed, the pH of the solution increases (becomes more alkaline), causing the hydrogencarbonate indicator to turn purple.
- Jars further from the light receive less light, so photosynthesis is reduced.
- Respiration, which produces CO₂, may dominate, increasing CO₂ concentration and lowering the pH, turning the indicator yellow (more acidic).
c.
Answer:
One factor to keep constant is temperature (or volume of indicator, number of beads, size of beads, or density of Chlorella).
————————————————————————————————————————Markscheme————————————————————————————————
photosynthesis/light independent reaction (of photosynthesis)
the jars closer to the light had more purple colours
OR
the jar the furthest from the light was yellow
OR
purple to yellow
Allow for answers indicating colours for each jar within this range.
a. high light (intensity) increases photosynthesis;
b. photosynthesis consumes CO2;
c. (more photosynthesis/less CO2) increases pH/decreases acidity;
d. less light means more respiration (than photosynthesis);
e. respiration produces CO2 AND lowers pH/increases acidity;
Allow inverse for all answers.
Do not accept reference to colour only instead of pH.
temperature/volume of indicator/identical jars/number of beads/size of beads / density of Chlorella / other reasonable answer
Do not accept light/pH/humidity.