As you should recall, the cell membrane is a lipid bilayer. At body temperature, the lipids are liquid or fluid, hence the fluid mosaic nomenclature. Now this structure makes an effective barrier, because it is hydrophobic, it keeps out water and hydrophilic substances. Because the membrane is surrounded by water, this makes it difficult for lipid molecules to approach. Since we need transport across the membrane, to allow the food and nutrients in and the excrement and waste materials out, we need to modify the structure of the membrane.
Some amino acids are lipophilic and some are lipophobic.
If we construct a lipo-protein, it will float within the lipid layers. A hydrophilic, lipophobic protein, would be expelled from the lipid bath. An interesting situation is when a protein has both properties. If we alternate regions of lipophilicity with lipidphobicity, parts of the protein will be contained within the membrane and parts will be expelled. In figure 1 we have a protein with alternating regions of lipophilicity and lipidphobicity.