How does the structure of the plasma membrane allow materials to move across it in both directions?

Answer 1

The structure of the plasma membrane consists of the phospholipid bilayer, which gives it a fluid structure, and not rigid, allowing the materials ease to pass through. Next, there are also integral proteins which can be thought of as 'tunnels', allowing the materials to come in and out. Then, there are peripheral proteins which also aid in getting the materials inside an out. On the cell membrane, there are receptors which recognize outside materials, and if they are really big, the cell engulfs these molecules through endocytosis [pinocytosis, and phagocytosis, which are basically for liquid molecules, and solid molecules]. This takes us back to the fluidity of the cell membrane. That's roughly how the cell membrane allows for the entering and exiting materials.

Answer 2

The membrane is a phospholipid bilayer. This means it is made up of a double-layer of molecules. The polar phosphate heads face outwards and the lipid tails point towards each other in the middle of the membrane. This means that the surface of the membrane is the same on the inside and outside of the cell.