According the theory developed by Lynn Margulis (endosymbiotic theory, why was it adaptive for a host cell to keep an engulfed cell alive, rather than digesting it and using it as food?

The engulfed cell provided the host cell with adenosine triphosphate (ATP).

Explanation:

The engulfed cell provided the host cell with adenosine triphosphate (ATP) therefore, as long as they continued their partnership, the host cell can get free energy.

answer; both surfaces of the membrane are nonpermeable