Question

questions 2 and 3 refer to the following

figure 1. effect of light intensity (μmol⋅m⁻²⋅s⁻¹) on the ph of a solution containing algae
students investigated the effect of light intensity on the rates of photosynthesis and cellular respiration in algae. they placed equal numbers of algal cells in test tubes containing a solution at ph 7.0. the test tubes were divided into treatment groups, and each treatment group was exposed to a different light intensity. the ph in the tubes was measured every 10 minutes and used as an indicator of co₂ concentration because co₂ produces an acidic solution when dissolved in water (figure 1).

3 mark for review
scientists claim that changes in the rate of photosynthesis will be reflected by changes in ph. which of the following best supports their claim?
a algae performing photosynthesis remove co₂ from the solution, increasing ph
b algae performing photosynthesis release co₂ into the solution, decreasing ph
c algae performing photosynthesis remove o₂ from the solution, decreasing ph
d algae performing photosynthesis release o₂ into the solution, increasing ph

Explanation:

Photosynthesis uses CO₂, reducing dissolved CO₂. CO₂ in water forms carbonic acid (H₂CO₃ ⇌ H⁺ + HCO₃⁻), so less CO₂ lowers H⁺ concentration, increasing pH. Option D correctly links photosynthesis (O₂ release is a byproduct) to pH increase, supporting the scientists’ claim. Options A (CO₂ removal decreasing pH is wrong), B (CO₂ release decreasing pH contradicts), C (O₂ removal decreasing pH is irrelevant) are incorrect.

Answer:

D. Algae performing photosynthesis release O₂ into the solution, increasing pH