Final answer:
The exosporium, composed of proteins and glycoproteins, serves as the protective barrier on endospores.
Step-by-step explanation:
The external structure on the endospore that acts as a protective barrier is the exosporium, which is composed of proteins and glycoproteins. The exosporium is the outermost layer of the endospore, providing a tough shield against environmental stresses, thereby ensuring the endospore's resilience and longevity.
This outer layer differentiates it from other external structures mentioned, such as the glycocalyx or the capsule, which are composed of different substances and serve functions pertinent to bacterial adherence and biofilm formation rather than the specific protective function geared towards endospore survival.