What are the Key functions of cell membranes?

Myoglobin (pictured below) is globular, oxygen-binding protein found in muscle and was the first protein whose structure was determined by x-ray crystallography. What level of protein organization does it represent? Label the levels of protein organization that can be seen in its structure.

Give a brief description of one positive feedback cycle ?

What are (the) 4:

a) Key functions of cell membranes?
b) Types of receptors used in cell-signaling
c) Factors that influence enzyme reaction rate.
d) Functional classes of proteins. (Note: Not primary, secondary, etc)
e) Factors that influence the rate of diffusion of a molecule across a membrane.