This bilayer is stable in water because the hydrophilic and hydrophobic parts are suspended in environments with similar solubility properties. These bilayers are the primary structural framework of cellular membranes. The various steroids differ in many ways, including the position and the number of double bonds in the rings, and the different side groups which are attached to the rings. The sterols, the most abundant steroids, have a hydroxuul group that is linked to one end and a complex, nonpolar carbon chain at the opposite end of the ring structure. Cholesterol is a very important part of the plasma membrane in animal cells because it loosens the packing of membrane phospholipids.
Some steroids are very important as the hormones in an animal.
The sex hormones in males and females are steroid as are the hormones of the adrenal cortex, which regulate cell growth. A glycolipid has one or several carbohydrate groups linked to its polar segments. These carbohydrates may be very simple or extremely complex, having more than 30 different sugars attached. Animal plasma membranes contain glycolipids in varying amounts, depending on the animal species. They also appear in the thylakoid membranes of the chloroplast.