Question

A scientist was investigating the factors needed to produce the green pigment chlorophyll in seedlings. Four groups of broad bean seedlings were grown from seeds under identical conditions with the exception of the presence of magnesium ions $\left(M{g}^{2+}\right)$, chloride ions $\left(C{l}^{–}\right)$ and light, which were varied as shown in the table below. A stock nutrient solution was used to administer all mineral ions necessary for healthy plant growth, except for $M{g}^{2+}$ and $C{l}^{–}$.
+ = presence
– = absence


From these results, it would be reasonable to conclude that for chlorophyll production:

• A. Chloride ions are not needed
• B. Magnesium ions are required when light is absent
• C. Either magnesium or chloride are sufficient
• D. Magnesium ions counter the effect of chloride ions

Answer 1

The correct option is A Chloride ions are not needed

Green plants have the ability to make their own food. They do this through a process called photosynthesis, which uses a green pigment called chlorophyll. Chlorophyll molecules are light trapping green pigments in plants. They are made up of a cyclic tetrapyrrole head and a long phytol tail attached to the head. One atom of magnesium is directly coordinated to the four pyrrole rings in the cyclic tetrapyrrole head.
Chlorophyll formation requires iron and magnesium along with exposure to light. Magnesium is an important structural component of chlorophyll molecules.


From the observation, the presence or absence of chloride ions does not affect the pigment on exposure to light. Hence, with respect to chlorophyll production, out of given statements option a is the most reasonable conclusion.