Lipids are a very diverse group of fatty, water insoluble compounds that function mainly as stored nutrients, components of cell membranes, and hormones. Lipids are the only category of macromolecules or biomolecules that are not polymers because they are not formed from repeating monomers. Like carbohydrates they are made up of carbon (C), hydrogen (H), and oxygen (O), but contain less oxygen than carbohydrates do. Lipids contain a simple 3-carbon molecule called glycerol plus long molecules that resemble tails known as fatty acids.
Lipids are defined by their solubility. They are molecules that are insoluble in polar solvents (such as water), but will dissolve in non-polar solvents. Therefore lipids are characterized as non-polar. Fats, oils, waxes and the steroids are all lipids. Most lipids that come from animal sources are such as lard and butter are fats, while most plant lipids are oils. Lipids function as energy storage molecules, as insulation and protection for internal organs, as lubricants and as hormones.
One type of lipid, the phospholipids are the major structural elements of membranes of cells. Phospholipids also contain a phosphate group. Polar molecules are hydrophilic (water soluble), therefore, the phosphate head of the phospholipid is water soluble. The fatty acid tails, however, remain nonpolar and hydrophobic (insoluble in water). The diagram below shows a phospholipid.
The membranes that surround cells form from a double layer of phospholipids known as a bilayer. This is essentially a fat sandwich, with the fatty acids forming the filling and the phosphate heads forming the "bread". Since cells have much water in them and are surrounded by a watery medium, the phospholipid bilayer separates these two watery areas. The middle region of the membrane will be hydrophobic and the outside and inside will be hydrophilic.
When a phospholipid is placed in water it usually forms a ball called a micelle. This happens because the hydrophobic parts of the phospholipid try to avoid the water and cluster together in the middle of the droplet. The polar phosphate heads, however, arrange themselves so that they are exposed to the positive hydrogen regions of water molecules. The micelle is the result.
Triglycerides (also known as Triacylglycerols)
One type of lipid or fat is made up of fatty acids and glycerol molecules in a 3:1 ratio. The bonding of three fatty acids to one glycerol molecule creates a triglyceride. Triglycerides are the most important form of lipid in the body, and more than 90% of our lipids are in this form. Triglyceride levels in the blood are also important predictors of artery disease.
The structural form of a Triglyceride is shown below:
Steroids: Ringed structures
Steroids are lipid-related molecules whose structure includes 4 linked carbon rings. Cholesterol is the precursor or basis for several important steroid hormones in the human body. It is also a component found in animal cell membranes.
Eicosanoids: Locally Acting Signal Molecules
The eicosanoids (eikosi, twenty) are modified 20-carbon fatty acids that are found in animals. These molecules all contain a complete or partial carbon ring at one end with 2 long carbon "tails" extending out from it. The most important eicosanoids to study in physiology are thromboxanes, leukotrienes, and prostaglandins. They are regulators of various physiological functions in the body.
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Not all macromolecules (biomolecules) are simple pure protein, pure carbohydrate, or pure lipid. Conjugated molecules are molecules that are composed of of more than 1 type or kind of molcule. For example, lipoproteins are protein moleclues combined with lipids. Lipoproteins are found in cell membranes and they mainly function in transporting hydrophobic molecules such as cholesterol, in the blood.
Proteins combine with carbohydrates to form glycoproteins, and lipids combine with carbohydrates to form glycolipids. Both glycoproteins and glycolipids are important components of cell membranes.