When would a cell use Active Transport? Mark all that apply. a. When the cell needs to move a compound down its concentration gradient.

b. When the cell needs to move a compound against its concentration gradient.

c. When the cell needs to move a polar compound.

d. When the cell needs to move a nonpolar compound.

e. When the cell needs to move an ion.

Option B, When the cell needs to move a compound against its concentration gradient.

Explanation:

A cell uses active transport when it has to transport substances against the concentration gradient across its membrane.

In active transport additional energy is used (generally provided by energy molecules like ATP) to push the substance across any membrane from low concentration side  to high concentration side.

Option B is correct

it can be observed without changing the identity of the object.

it depends on what substance you're talking about as well as many other factors. in quantum mechanics, the observation of particles affects the observers ability to measure certain properties. for example: you can know the velocity of a particle, but you can't know it's position; or you can know it's position, but not it's velocity. it's like trying to find out where a grain of sand is by throwing other grains of sand at it. once you find out where it is, it's been moved. it's also like trying to find an answer to the question of whether or not a tree falling in a forest makes a sound if nobody is there to hear it. paradoxically, the only way to find out is to be there and hear it.