Question

Mix the watery environment of Elodea with a pH indicator called phenol red. In the presence of an acid, phenol red will appear yellow. In the presence of a strong base, phenol red will appear purple. Intermediate colors correspond with intermediate pH values. Recall that plants use CO₂ for photosynthesis; the concentration of CO₂ will determine the pH of a solution. Plants also produce CO₂ as a byproduct of cellular respiration.

1. Will a solution with a high concentration of CO₂ be acidic or basic? Why? (search on the internet)
2. As Elodea uses CO₂ during photosynthesis, will the solution become more acidic or basic? If the solution is also mixed with phenol red, what color should it become over time?
3. Will the concentration of CO₂ increase or decrease for Elodea in darkness? What color will the phenol red indicator show over time?
4. Which colors of visible light are used by chlorophyll for photosynthesis? Which are not?
5. Write a hypothesis for how the intensity of light would affect the photosynthetic rate in Elodea.

Answer 1

• Will a solution with a high concentration of CO₂ be acidic or basic? Why?

Answer - Acidic. The increase of CO₂ concentration in water will form carbonic acid in the solution resulting the solution acidic in nature. The reaction follows-

CO₂(aq) + H₂O → H₂CO₃(aq) carbonic acid

This H₂CO₃ formed easily dissociates into H+ ions causing the formtion of acidic solution.

H₂CO₃(aq)→ H+(aq) + HCO₃-(aq) , HCO₃-(aq) → H+(aq) + CO₃–2(aq)

All reactions are reversible.

• As Elodea uses CO₂ during photosynthesis, will the solution become more acidic or basic? If the solution is also mixed with phenol red, what color should it become over time?

Eloda gives out CO₂ during respiration and takes CO₂ during photosynthesis. But the quantity of CO₂ used for photosynthesis per unit time is much greater (10-30 times greater) than the amount generated by respiration. So that will reduce the amount of CO₂ in solution resulting more basic solution. If the solution is also mixed with phenol red the colour changes to purple indicating basic nature over time.

• Will the concentration of CO₂ increase or decrease for Elodea in darkness? What color will the phenol red indicator show over time?

In darkness, the photosynthesis will stop but respiration will continue as it is independent of light. So the CO₂ intake due to photosynthesis will stop and release of CO₂ will increase. This will increase the acidic nature of solution turning Phenol red into yellow colour over time.

• Which colors of visible light are used by chlorophyll for photosynthesis? Which are not?

The best wavelengths of visible light for photosynthesis fall within the blue range (425–450 nm) and red range (600–700 nm)

Chlorophylls do not absorb wavelengths of green and yellow 500nm to 600nm.

• Write a hypothesis for how the intensity of light would affect the photosynthetic rate in Elodea.

As the intensity of light increases, the rate of photosynthesis should increase thereby increasing the emission of O₂ and decreasing the emission of CO₂ . Thus the solution will not turn into acidic nature at high intensity light hypothetically.