Q.The two subunits of ribosomes remain united at critical ion level of

Step 1. Recall what holds a ribosome together. A ribosome is made of two separate, rounded subunits - one small, one large - that only function as a ribosome once they are joined together as a single complete unit. That joining is not automatic; it depends on a specific ion being present at a critical level.

Step 2. Identify the ion. The chapter names this ion explicitly: Mg2+ (magnesium) is required for the structural cohesion of ribosomes. Without an adequate magnesium ion level, the two subunits do not stay associated.

Step 3. Eliminate the other options. Calcium, sodium and ferrous (iron) ions each play other, unrelated roles in cell biology (e.g. calcium in signalling and muscle contraction, sodium in membrane potentials, iron in oxygen transport and redox enzymes), but none of them is the ion the chapter identifies as holding the ribosomal subunits together.

Step 4. Confirm with the related fact about polysomes. The same magnesium dependency reappears when several ribosomes link up along a single mRNA strand to form a polysome during active protein synthesis - that linkage is also described as being held together with the help of Mg2+ ions, reinforcing magnesium's specific structural role in ribosome assembly.

Recall the specific ion the chapter names for ribosomal subunit cohesion and eliminate the other three ions, which play unrelated cellular roles.

• Guessing calcium, since it is the most commonly recalled 'signalling ion' in biology, rather than recalling the specific magnesium fact tied to ribosomes.
• Not connecting this fact to the later mention that Mg2+ also holds ribosomes together into polysomes during active protein synthesis.