How are lichens able to survive in places that most organisms are unable to live, such as on mountaintops and in a barren desert

This is because lichen is a symbiotic relationship between fungi and algae that makes them versatile. The fungi hyphae are able to produce enzymes extracellularly that dissolve the substrate and absorb the nutrient. These shallow roots are therefore able to grow on bare rocks and provide an anchor. The algae, on the other hand, are able to photosynthesize and provide carbohydrates to the fungi while the fungi protect the algae.  

This is because lichen is a symbiotic relationship between fungi and algae that makes them versatile. The fungi hyphae can produce enzymes extracellularly that dissolve the substrate and absorb the nutrient. These shallow roots can therefore grow on bare rocks and provide an anchor. The algae can photosynthesize and provide carbohydrates to the fungi while the fungi protect the algae.  

there's no big difference.

explanation:

the only little different between a rabbit and bunny is just that bunnies are much smaller than rabbits. bunnies are basically baby rabbits.