Occurs naturally on earth
Hypothalamus and pituitary glands are two endocrine glands that are made of a mixture of nervous and epithelial tissues. The endocrine system is made up with a series of glands located around the body. These glands includes the pituitary gland, thyroid glands, parathyroid glands, thymus, pancreas adrenal glands among others. Each of the glands produces hormones, which have a specific function in the body.
D. Polarity and size.
<span>The size, polarity, and charge of a substance will determine whether or not the substance can cross the cell membrane by diffusion. The cholesterol was an example of a lipid, and is highly soluble in the nonpolar environment of the lipid bilayer. You saw, in the animation above, the cholesterol freely passing into the hydrophobic environment of the membrane. Cholesterol distributes freely in the membrane and then some fraction will dissolve in the aqueous environment of the cytoplasm. Water, on the other hand, while polar, is small enough to cross the membrane at a slow rate. Note that specialized transport proteins in certain cell membranes can provide a channel for the water, greatly increasing its rate of crossing the membrane. The lipid bilayer is much less permeable to the ion, because of its charge and larger size. As a general rule, charged molecules are much less permeable to the lipid bilayer.</span>