The plant cell cytoplasm is surrounded by both cell wall and cell membrane. The specificity of transport of substances are mostly across the cell membrane, because....... .  

The transport takes place by the cell membrane because cell membrane is Semi-permeable and composed of lipids that are arranged in bilayer structure along with proteins and carbohydrate.
the lipid soluble molecules pass readily across lipid bilayer of cell membrane whereas movement of other molecules of small size is facilitated by porins, pores made up of proteins while  cell wall dose not helpes in transport of substances because it .
cell well is basically a non - living rigid structure forming outer covering over plasma membrane and it  provides shape, mechanical structure and protection to the plant cell from damages and infection.