Final answer:
An onion acts as a sink when it is storing carbohydrates such as starch by converting sucrose. A sink is where the plant deposits photosynthates for growth or storage, contrary to a source, which produces these photosynthates.
Step-by-step explanation:
When an onion is converting sucrose into stored carbohydrates like starch, it is functioning as a sink. In the context of plant biology, a 'sink' refers to parts of a plant, like roots, young shoots, developing seeds, or in this case, the onion, which require inputs of photosynthates for growth or storage purposes. These sinks receive the photosynthates that are transported through the plant's phloem. On the other hand, a 'source' is an organ that produces photosynthates in the plant such as the green leaves which undergo photosynthesis.
The onion, by storing starch, is acting as a site of deposition for these carbohydrates that are synthesized in the leaves and elsewhere in the plant. As the onion is not currently photosynthesizing and is rather accumulating and storing resources for later use or for the development of the plant, it is considered a sink.