Lipids are molecules found throughout the cell with a wide variety of unique functions. They are hydrophobi small molecules and serve a variety of functions, including signaling, cellular structure, and energy storage. Below is a list of various lipid functions throughout the cell.
1) Membrane structure and cell organization. The most recognizable function of lipids is their use in membrane structure. Lipids are the main component of all biological membranes, including the outer plasma membrane as well as the membranes of various organelles found within eukaryotes. Their structure, an amphipathic tail with head-group that is often charged, allows them to organize tail to tail with other lipids and form a bilayer. This organization is relatively impermeable and allows for selective permeability of a variety of molecules.
2) Membrane consistency. While membranes are made of lipids, some lipids function not just to make up the bulk of the membrane but to regulate its fluidity and consistency. Cholesterol, for instance, reduces the permeability of the lipid-based membranes to sodium ions and protons. It is also critical in helping the cell "pinch off" portions of membranes for division and to form small vesicles.
3) Energy storage. Lipids are also very important for energy storage in organisms. Fat tissue, for instance, is made primarily of high energy lipids that have been sequestered from the major metabolic pathways. When broken down in fatty acid oxidation, lipids generate about 9 kcal/gram of energy. You can compare this to carbohydrates and proteins, which only produce about 4 kcal/gram. The body uses lipids to more efficiently store unused energy.
4) Cell signaling. There are a number of lipid molecules that are critical for important cell signaling pathways. Some lipids are signaling small molecules that are responsible for changes in cell growth, apoptosis and immune response. Others, like estrogen and testosterone, are important for the modulation of reproduction, organism development, blood pressure and metabolism.
5) Transport and metabolism. A few lipids, including carnitine, are instrumental in transporting fatty acid chains to the mitochondria for breakdown and metabolism. These lipids bind fatty acids directly and actually provide a transport carrier that prevents interactions with the fatty acids outside of the mitochondria.
6) Vitamins with specific roles. Vitamins, such as vitamins A, D, E and K, are fat-soluble lipids with a huge variety of uses. Vitamin A, also known as retinol, is important for vision and bone growth in higher order organisms. Vitamin D is critical for calcium absorbtion by the body. Vitamin K is an important of coagulation, which is important for repairing damage down to the body.