Cellular membranes contain a multi-layered sheet of lipids and protein molecules that is hydrophilic on both surfaces and extremely hydrophobic at the core. Embedded proteins, then, need to have hydrophilic surface regions surrounding a hydrophobic membrane-spanning segment.
Alpha helices can contain long hydrophobic regions because their main-chain amino and carboxy groups are hydrogen-bonded. Beta sheets, in contrast, have edges with exposed, main-chain, polar amino and carboxy groups.