What is the free surface / edge of the epithelial cells referred to as?

There is an exposed edge of epithelial tissue known as the apical surface

Explanation: The attachment to connective tissue is known as the basement membrane and is composed of proteins.

explanation:

b,bacteria convert nitrates

into nitrogen gas.

d,plants and animals take in

and use nitrogen compounds.

e,bacteria break down waste    

products and return nitrogen  

to the soil.

a,bacteria convert atmospheric

nitrogen to ammonia.

c,bacteria convert ammonia

to nitrites and nitrates.

answer; both surfaces of the membrane are nonpermeable