When is facilitated diffusion necessary?
a. when the molecules are too large.
b. when the molecules are insoluble.
c. when the direction of movement is opposite to natural diffusion.
d. none of the above

True or false: (a) the strings of little red dots represent carbohydrates. (b) the model is called a bilayer because there are two main types of molecules present, lipids and proteins. (c) the many black lines represent amino acid tails. (d) this model shows membrane transport. (e) the blue ovals are hydrophobic. (f) the blue ovals represent phospholipid heads.