Remember that each line in these diagrams of chemical structures represents a covalent bond. Many steroids also have the –OH functional group, which puts them in the alcohol classification (sterols). All steroids have four linked carbon rings and several of them, like cholesterol, have a short tail (Figure 6). Although they do not resemble the other lipids, they are grouped with them because they are also hydrophobic and insoluble in water. Unlike the phospholipids and fats discussed earlier, steroids have a fused ring structure. It is a single layer of phospholipids that form spontaneously. Figure 5 A micelle may be the very early precursor of a cell. If a drop of phospholipids is placed in water, it spontaneously forms a structure known as a micelle, where the hydrophilic phosphate heads face the outside and the fatty acids face the interior of this structure (Figure 5). Phospholipids are responsible for the dynamic nature of the plasma membrane. The hydrophobic tails are sequestered in the middle of the bilayer. The hydrophilic head groups of the phospholipids face the aqueous solution. Figure 4 The phospholipid bilayer is the major component of all cellular membranes. This forms a hydrophobic layer on the inside of the bilayer, where the tails are located. In a membrane, a bilayer of phospholipids forms the matrix of the structure, the fatty acid tails of phospholipids face inside, away from water, whereas the phosphate group faces the outside, aqueous side. The fatty acid chains are hydrophobic and cannot interact with water, whereas the phosphate-containing group is hydrophilic and interacts with water (Figure 4). The head is the hydrophilic part, and the tail contains the hydrophobic fatty acids. Both choline and serine attach to the phosphate group at the position labeled R via the hydroxyl group indicated in green.Ī phospholipid is an amphipathicmolecule, meaning it has a hydrophobic and a hydrophilic part. Two chemical groups that may modify the phosphate, choline and serine, are shown here. The phosphate may be modified by the addition of charged or polar chemical groups. Figure 3 A phospholipid is a molecule with two fatty acids and a modified phosphate group attached to a glycerol backbone. Phosphatidylcholine and phosphatidylserine are two important phospholipids that are found in plasma membranes. Instead of three fatty acids attached as in triglycerides, however, there are two fatty acids forming diacylglycerol, and the third carbon of the glycerol backbone is occupied by a modified phosphate group (Figure 3). Like fats, they are composed of fatty acid chains covalently bonded to a glycerol or sphingosine backbone. Phospholipids are major constituents of the plasma membrane, the outermost layer of animal cells. Figure 2 Waxy coverings on some leaves are made of lipids. Waxes are made up of long fatty acid chains covalently bonded to long-chain alcohols. Because of the hydrophobic nature of waxes, they prevent water from sticking on the surface (Figure 2). Wax covers the feathers of some aquatic birds and the leaf surfaces of some plants. Three molecules of water are released in the process. Figure 1 Triacylglycerol is formed by the joining of three fatty acids to a glycerol backbone in a dehydration reaction (remember this removes a water molecule and forms a covalent bond). In a fat molecule, the fatty acids are attached to each of the three carbons of the glycerol molecule with a covalent bond. Fatty acids have a long chain of hydrocarbons to which a carboxyl group is attached, hence the name “fatty acid.” The number of carbons in the fatty acid may range from 4 to 36 most common are those containing 12–18 carbons. Glycerol is an organic compound (an alcohol) that contains three carbons, five hydrogens, and three hydroxyl (OH) groups (Figure 1). A fat molecule consists of two main components-glycerol and fatty acids.
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