The non-polar tails are excluded from the matrix of water hydrogen bonds, and so tend to form a hydrophobic phase . These lipid bilayer membranes are crucial to cellular function. They separate the compartments of the cell, providing semi-permeable barriers. We will return to the properties of lipid membranes in a few weeks. Storage lipids Lipids also have important roles as energy and carbon storage molecules.
Triglycerides are particularly prominent as animal storage molecules.
On a gram basis the caloric content of lipids is twice as high as carbohydrates, so motile animals get more calories per stored gram by synthesizing fats rather than accumulating starch. Plants are generally non-motile and so generally need not convert their carbohydrate reserves to fat. Interesting exceptions are oil rich seeds and some floating planktonic algae which store lipids, where weight is a concern. Proteins have a central role in cell biology. There are structural proteins, transport proteins, and protein catalysts called enzymes.