Bodybuilders in particular need the lower body fat percentage possible in order to succeed in their career, since most of their bodies must be muscle. Professional ballet dancers need to be light and skinny to be able to perform their dance moves and to be lifted by their partners; a basketball player also needs a lower fat percentage and a higher muscle mass percentage to be able to play at his best; and a ranch hand as well needs to not be packing that much fat in order to make his job easier.
Pregnant women, however, need body fat to have healthy pregnancies. This doesn't mean that they do not need to take care of themselves, because a higher body fat than what it's needed can lead to diabetes, high blood pressure and can difficult the pregnancy.
A cell's lipid bilayer allows to small uncharged substances such as: ions, hydrophobic molecules, oxygen and carbone dioxide to travel through the membrane by simple diffusion
