Certain seaweeds accumulate iodine in a concentration much higher than that of the surrounding ocean. How does this occur?

The process through which certain seaweeds accumulate iodine in concentrations much higher than that of the surrounding ocean is active transport. This mechanism involves the movement of ions or molecules against their concentration gradient, which requires energy, typically in the form of ATP. Seaweeds possess specialized transport proteins that facilitate this process, allowing them to actively uptake iodine from the surrounding water even when the concentration of iodine is lower outside the seaweed than inside.

In contrast, osmosis refers specifically to the movement of water across a semipermeable membrane, which is not the mechanism involved in iodine accumulation. Diffusion is the passive movement of molecules from an area of higher concentration to one of lower concentration, and while it may account for some iodine movement initially, it does not explain the accumulation in higher concentrations. Passive transport, like diffusion, does not require energy and typically facilitates movement in accordance with concentration gradients, which would not lead to the higher levels seen in seaweeds. Thus, the reliance on energy and specialized mechanisms in active transport is essential for the seaweeds to maintain their iodine levels effectively.