Understanding Lipids Lipids Principles of Biology Lipids are a diverse class of biological molecules with a wide range of structures and functions that all share one important property: they do not mix well with water (i.e., they are hydrophobic). Fats, phospholipids, and steroids are types of lipids that are particularly important to living cells and organisms. Principles of Biology

Lipids are nonpolar and hydrophobic. The molecular structure of a lipid consists primarily of hydrocarbons, organic molecules composed only of hydrogen and carbon atoms. Both single and double nonpolar covalent bonds hold the carbon and hydrogen atoms together in a lipid. Single and double nonpolar covalent bonds in a lipid prevent the hydrocarbon from interacting with water. A phospholipid is a type of lipid composed of two fatty acids, one glycerol and one phosphate functional group. The structure of a phospholipid is unique. The fatty acid forms the hydrophobic tail of the phospholipid, whereas the head is the hydrophilic end that interacts with water. Principles of Biology

Figure 1 Chemical structure of a phospholipid. Lipids Figure 1 Chemical structure of a phospholipid. A double layer of phospholipids is the primary component of a cell’s membrane. a) Often, phospholipids are drawn symbolically as a head with one straight tail and one bent tail. b) The structural formula of phospholipids reveals that the choline and phosphate regions contain a positive and a negative charge, respectively. These charges are attracted to the opposite charges on water molecules, and this is known as the hydrophilic head. The fatty acid tails, which have no net charges, have no affinity for water and are described as the hydrophobic tails. Principles of Biology

Phospholipids form bilayers. Phospholipids can either form a circular, single-layered micelle or two layers of phospholipids can line up together to form a bilayer. Phospholipids with short tails are more likely to form micelles, whereas those with longer tails are more likely to form bilayers. Micelles aid in transporting and absorbing complex lipids and fat-soluble vitamins such as vitamin A. Principles of Biology

Figure 2 Phospholipids can form a micelle. A single layer of phospholipids will form a circular pattern, with the hydrophilic heads interacting with water and the hydrophobic tails moving away from water. Principles of Biology

Figure 4 Formation of a phospholipid bilayer. Lipids Figure 4 Formation of a phospholipid bilayer. Phospholipids orient to keep the hydrophobic tails from the aqueous surroundings and the hydrophilic heads toward the aqueous environment. Principles of Biology

Steroids form hormones. Lipids Steroids form hormones. Steroids are composed of a carbon skeleton arranged in four fused rings and include sex hormones and cholesterol. Cholesterol is a precursor steroid for more complex steroids. Principles of Biology

Figure 5 Steroid structure. Lipids Figure 5 Steroid structure. The structure of a steroid consists of four fused carbon rings and functional groups. Cholesterol, an important precursor to other steroids and a component of the cell membrane, is shown. Principles of Biology

Steroids form hormones. Lipids Steroids form hormones. High-density lipoprotein (HDL) is one of five groups of lipoproteins that transport cholesterol in the bloodstream; low-density lipoprotein (LDL) is another. Scientists study the level of LDL versus HDL in blood to determine cholesterol levels. Principles of Biology

Fats store chemical energy. Lipids Fats store chemical energy. A fat, also called a triacylglycerol (triglyceride), is a lipid composed of smaller molecules, specifically a glycerol and three fatty acids. A fatty acid is a carboxylic acid with a hydrocarbon chain that is usually 16 to 18 carbon atoms long. Principles of Biology

Figure 6 Structure of a fatty acid. Lipids Figure 6 Structure of a fatty acid. A triacyglycerol (fat) molecule is composed of a carboxyl group and a hydrocarbon chain. Principles of Biology

Figure 7 Structure of a triacylglycerol molecule. Lipids Figure 7 Structure of a triacylglycerol molecule. A fat, or triacylglycerol molecule, is composed of one glycerol molecule and three fatty acid molecules. Principles of Biology

Fats store chemical energy. Lipids Fats store chemical energy.  A saturated fatty acid is a lipid that does not have any double bonds between the carbon atoms in the carbon skeleton. Principles of Biology

Lipids Figure 8 Saturated fats. The structure of saturated triacylglycerol molecules allows them to pack tightly together. Therefore, saturated fats are solid at room temperature. Principles of Biology

Fats store chemical energy. Lipids Fats store chemical energy. An unsaturated fatty acid is a lipid with one or more double bonds between carbon atoms. These bonds are almost always in a cis formation. Principles of Biology

Figure 9 Unsaturated fats. Lipids Figure 9 Unsaturated fats. The structure of unsaturated triacylglycerol molecules prevents them from packing tightly together. Therefore, unsaturated fats are liquid at room temperature. Principles of Biology

Fats store chemical energy. Lipids Fats store chemical energy. Scientists have developed a way to add hydrogen to an unsaturated fatty acid. The resulting transfat is an unsaturated fat with a trans double bond. Although transfats are produced in nature, most are manmade and, unfortunately, consumption leads to negative health effects. Principles of Biology