can u explain the structure and function of a phloem cell?

All these can be answered through my biology textbook
Here maybe this will help you understand the phloem better?

http://en.wikipedia.org/wiki/Phloem

Umm if you could be more specific in what you were asking, I could probably answer more efficiently.

Reasons for perforated end walls and sieve plates: plant cells are surrounded by a more or less thick cell wall that in a more or less degree makes it difficult for water and nutrients to go from a cell to another one. So cell walls are thinned (perforated) in some parts. This also eases the ability of callous proteins to block the perforations when needed (usually winter time)

No nucleus: basically it's just in the way, so they get rid of it. Organelles are put along the cell membrane to easy up the flow of water an nutrients.

Companion cells: They are formed by mitosis of the sieve element and are nucleated, they have lots of relatively large plasmodesmata that connect them to the sieve element so they can exchange mRNA and otehr molecules, as the sieve element is unable to manufacture them without a nucleus

hope it helps you