Lipids
A triglyceride consists of glycerol attached to three fatty acids. A typical phospholipid has a polar phosphate-containing head and two largely nonpolar tails, making it amphipathic. In water, phospholipids tend to organize so heads contact water and tails are sheltered from it. Saturated fatty acids lack carbon–carbon double bonds. Cis double bonds create bends that impede tight packing, so a membrane with more cis-unsaturated tails generally remains more fluid at low temperature, all else equal. Steroids have a fused-ring framework and illustrate why lipid does not mean one repeated molecular structure.
Two model membranes have equally long tails, but one has more cis double bonds. At the same cool temperature, the more unsaturated membrane generally has greater fluidity because its tails pack less tightly.
A phospholipid head is not hydrophobic. Also, “unsaturated” does not mean fewer fatty acid tails; it refers to carbon–carbon double bonds.