Organs
only exist in animals.
are less complex than the tissues which make them.
perform functions that tissues cannot carry out on their own.
are made of only one type of tissue.

Molecules like glucose and amino acids are not lipid soluble, so cannot cross the cell membrane by simple diffusion. instead, they are shuttled across by carrier proteins in the membrane from high to low concentration. does this process require energy from the cell? this is an example of what type of molecular movement?