In the diagram, the fatty acid is seen attached to a glycerol molecule and a phosphate group. It is known as a phospholipid. The carboxyl end of the fatty acid is highly polar and therefore water soluble (hydrophilic meaning attracted to water). Hydrocarbon chain of the fatty acid is highly non-polar and therefore water insoluble (hydrophobic, which means scared of water).
When fatty acids interact with water, the soluble carboxyl end dissolves and forms a layer with water, while the hydrocarbon tale remains outside the water surface. This quality is important in forming the bio membrane of cells which will become clearer below.
Also, another quality to remember is when all carbon atoms of the hydrocarbon chain in the fatty acid are joined by a single bond, the compound is said to be saturated, this means that every carbon atom has hydrogen atom on both sides. In unsaturated fatty acid, one or more carbon atoms form a double bond with another carbon atom. Therefore, it will not be able to hold a hydrogen atom and therefore said to be unsaturated. As you can observe from the first diagram, there are two hydrocarbon tails, one is saturated and the other unsaturated and where the carbon atom forms a double bond, there can be seen a kink in the tail. This kink provides a fluid quality to cells that allows it more flexibility in motility and structure and therefore is healthier than saturated fats that plague processed food.
Usually fatty acids are stored in the form of triglycerides– glycerol molecule + 3 fatty-acid tails. A glycerol molecule plus a fatty-acid tail is a glyceride molecule. The above diagram shows us a diglyceride consisting of two fatty-acids linked to a glycerol molecule. Triglycerides are insoluble in water and therefore group as fat droplets in the cytoplasm of the cell. When required, they can be broken down for use as energy.
Lipids provide an important form of energy storage, since they give more than twice as much energy as carbohydrates of the same mass. Also, as previously stated, they are the major components of cell membranes. Lastly, they play important roles in cell signaling. Example: steroid hormones, such as estrogen and testosterone are made of cholesterol and used in processing food and building nerve cells, apart from other metabolic functions.
Most lipids are not soluble in water, but they do dissolve in some organic solvents.
All cells are surrounded by a layer of membrane that separates their internal environment (cytoplasm) from the external environment (exoplasm or extracellular matrix). Additionally, the organelles in cells, are compartmentalized with the help of biomembranes, with similar chemical composition as that of the plasma membrane.