Membrane Characteristics

All cells are surrounded by a lipid membrane but not all membranes are made up of just lipid molecules. The bilayer is interrupted by the inclusion of membrane proteins. These proteins have hydrophobic surfaces which allow them to associate in the hydrophobic core of the membrane, also excluding them from the aqueous phases(cytoplasm). The cytoplasmic face of the membrane is meshed with cytoskeletal proteins which form chains which give the cell its shape, and acting as a motor to move cell contents through the cytoplasm.
These proteins have many functions including;

Transport Proteins

Proteins provide a means for ions and molecules to move in and out of cells. They do this through reception and recognition. Reception is when certain proteins bond to small molecules called ligands signalling their presence to the recognition proteins. These recognition proteins identify a molecule as self or non-self. This is especially useful for immune systems.

Barring diffusion

Large molecules such as glucose are unable to pass through a cell membrane and must be brought in with a specific transporter. This transporter in turn is specific to glucose and will not transport related saccharides. Ions also have trouble permeating the cell membrane without the assistance of specific transporters or channels.