Biology 5090/12 — May/June 2017

Walkthrough

1. Recall the function of magnesium in plants: Magnesium ($\text{Mg}^{2+}$) is an essential mineral ion. Its primary role in plant biology is as the central ion in the chlorophyll molecule. Without magnesium, a plant cannot produce chlorophyll, leading to chlorosis (yellowing of leaves).
2. Locate chlorophyll in the cell: Chlorophyll is not free-floating; it is embedded in the thylakoid membranes inside chloroplasts.
3. Interpret the diagram: The image shows a cross-section of a leaf.
   • Label A indicates the upper epidermal layer (and possibly the waxy cuticle). Epidermal cells are typically transparent and lack chloroplasts to allow light to reach the mesophyll below.
   • Label B points directly to the numerous dark, oval organelles packed inside the columnar palisade mesophyll cells. These organelles are chloroplasts.
   • Label C points to a spongy mesophyll cell. While these cells also contain chloroplasts, they are fewer in number than in the palisade layer, and the label points to the whole cell rather than a specific organelle.
   • Label D points to the xylem vessel within the vascular bundle. Xylem transports water and dissolved mineral ions (including magnesium) from the roots to the leaves, but the magnesium is eventually incorporated into chlorophyll in the mesophyll cells, not stored in high concentrations in the xylem vessels themselves.
4. Conclude: The structure with the highest concentration of magnesium is the chloroplast, because magnesium is a structural component of chlorophyll. This corresponds to label B.

Key Takeaways

• Mineral ion roles: Students must know the specific functions of key mineral ions. Magnesium = chlorophyll; Nitrate = amino acids/proteins; Iron = haemoglobin (in animals) / involved in enzyme function; Calcium = cell walls / bone (in animals).
• Leaf anatomy: The palisade mesophyll layer is packed with chloroplasts to maximise light absorption for photosynthesis. Recognising chloroplasts as the dark, oval bodies in a leaf cross-section is a standard skill.

Common Mistakes

• Confusing magnesium with other ions: Students might think of nitrogen (from nitrates) which is also needed for chlorophyll (and amino acids), but magnesium is the specific central atom. They might also choose xylem (D) thinking it transports the ions, forgetting that the ions are used up in the mesophyll cells to make chlorophyll.
• Misreading the diagram: Choosing C (spongy mesophyll cell) instead of B (chloroplast). The question asks for the structure containing the high concentration. While the cell contains the organelle, the organelle itself is the site of chlorophyll synthesis and storage, making B the most precise and correct answer.

Things to Be Careful About

• Precision in diagram labels: Ensure you are looking at exactly what the label line points to. B points to the organelle (chloroplast), not the whole cell. In 5090, if a label points to a specific organelle within a cell, that organelle is the intended answer, not the cell itself.
• Memorise ion functions: The link between magnesium and chlorophyll is a frequent 5090 testing point. Remember: $\text{Mg}^{2+}$ for chlorophyll, $\text{NO}_3^{-}$ for amino acids/proteins.