Why isn't energy required for the passenger molecule to be carried across the cell membrane by the carrier molecules?

Energy is not required for the passenger molecule to be carried across the cell membrane by the carrier molecule  simply because they are moving down a concentration gradient, that is to say movement is from an area of high concentration to an area of low concentration.  

When particle move from areas of high concentration to areas of low concentration, they do not need energy. For example this is how the aroma of cooking spreads from the kitchen (higher concentration)  to all the other rooms in the house.

african americans demonstrated their newly found freedom by relocating often in search of relatives whom they had been separated from during slavery. others established schools to educate freed people and they built institutions such as churches and political organizations to advocate for their rights.

answer by yourhope:

how do models compare to real-world objects?

d. most models represent a real-world object in a simpler way.

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