Question

Companion cells are usually seen associated with
A) Fibers
B) Tracheids
C) Vessels
D) Sieve tubes

Answer 1

Hint: Vascular plants have developed highly specialized cells to carry nutrients and developmental signals. The separation process includes the degradation of multiple organelles of the sieve element cells to assist transport. Companion cell is a type of cell found within the phloem of flowering plants. Companion cells are nucleated and have many mitochondria. Companion cells may regulate translocation.

Complete Answer:
- The companion cells were first introduced by Wilhelm in 1880 to describe the nucleate cells connected with the sieve elements in angiosperms. Companion cells have thick cytoplasm, large well-differentiated nuclei. Companion cells have been previously classified concerning the loading mechanism they use to capture nutrients.
- Ordinary companion cells use the apoplastic mechanism, where the sugars produced in mesophyll cells are exported to the cell wall space and then imported by transporters using energy generated by proton pumps. They look unspecialized.
- Ordinary companion cells are called transfer cells when they show cells in growths to better capture sugars. Companion cells use polymer trapping that is known as intermediary cells. They are larger than ordinary companion cells.
- Companion cells involved in passive loading by sugar diffusion do not have any exact name but are symmetrically branched.
- Phloem tissue connects the aerial photosynthetic organs to heterotrophic structures like roots and fruits and they distribute nutrients for adequate plant growth and fitness. In addition to transporting, it also influences plant development. To carry out these fundamental functions the sieve element cells have to become a tube in a distinctive differentiation process.
- The plasmodesmata in the cell walls linking the different sieve elements are enlarged and the desmotubule disintegrates, which results in the mature sieve plate pores that will facilitate transport. On adding together, sieve elements enucleate and lose many organelles including the rough endoplasmic reticulum, vacuole, Golgi, and cytoskeleton. Sieve elements are still alive keeping mitochondria, smooth endoplasmic reticulum, and plastids. Due to this special condition, sieve elements are physiologically supported by neighboring companion cells.

Hence, the correct option is D, ‘sieve tubes’.

Note: Companion cells are the type of cell found within the phloem of flowering plants. Each companion a cell is closely associated with a sieve element. Their function is uncertain, while it appears to control the activity of the neighboring sieve element and also to take part in loading and unloading sugar into the sieve element.