Chapter 7: Biomolecules (Organic Molecules)

7.2. Lipids

Learning Objectives

By the end of this section, you will be able to:

  • Identify the four major families of lipids biomolecules (the nutrient lipids, phospholipids, cholesterol and its derivatives, the steroids)
  • Explain the chemistry and function of the nutrient lipids, the triglycerides (fats and oils).
  • Identify the difference between saturated and unsaturated fatty acids.
  • Identify the phospholipids forming the cell membrane: hydrophilic heads and hydrophilic tails.
  • Identify the structures and explain the functions of cholesterol and steroids.

    Lipids include a diverse group of compounds that are largely nonpolar in nature. This is because they are hydrocarbons that include mostly nonpolar carbon–carbon or carbon–hydrogen bonds. Nonpolar molecules are hydrophobic (insoluble in water). To increase solubility, they often have polar functional groups attached. 

    Lipids also serve diverse functions. Some are nutrient food sources that cells process and use to make or store energy. Other lipids are important constituent of cell membranes, and others become the building blocks of an important group of hormones, the steroids.

    Triglycerides (the nutrient lipids)

    Dietary triglycerides include the fats and oils. The two names differentiate their consistency at room temperature. Fats are solid whereas oils are liquid at room temperature. Mammals store fats in specialized cells called adipocytes, where globules of fat occupy most of the cell’s volume.

    A triglyceride molecule is formed of two main components: one glycerol and three fatty acids (Figure 7.2.1.). The three-Carbon glycerol molecule contains three hydroxyl (-OH) groups and one end of the fatty acid chain contains a carboxyl (-COOH) group. The fatty acids and glycerol are united in ester bonds formed by the dehydration synthesis reaction. The bonds are formed with the release of three water molecules.

    Fatty acids contain an even number of Carbon atoms anywhere from 4-36 with 12-18 C atoms being most common.

    The structures of glycerol, a fatty acid, and a triacylglycerol are shown. Glycerol is a chain of three carbons, with a hydroxyl (OH) group attached to each carbon. A fatty acid has an acetyl (COOH) group attached to a long carbon chain. In triacylglycerol, a fatty acid is attached to each of glycerol’s three hydroxyl groups via the carboxyl group. A water molecule is lost in the reaction so the structure of the linkage is C-O-C, with an oxygen double bonded to the second carbon.
    Figure 7.2.1: Triglyceride (or Triacylglycerol) is formed by the joining of three fatty acids to a glycerol backbone in a dehydration synthesis reaction. Three molecules of water are released in the process.

    Fatty acid structural variations

    Fatty Acids (FAs) can be 1) saturated or unsaturated; and have 2) cis- or trans- forms (Figure 7.2.2.)

    A comparison of saturated and unsaturated fatty acids is shown. Stearic acid, a saturated fatty acid, has a hydrocarbon chain seventeen residues long attached to an acetyl group. Oleic acid also has a seventeen-residue hydrocarbon chain, but a double bond exists between the eighth and ninth carbon in the chain. In cis oleic acid, the hydrogens are on the same side of the double bond. In the cis oleic acid, the 2 hydrogens on the same side cuase the chain to bend. In trans oleic acid, they are on opposite sides.
    Figure 7.2.2: Saturated fatty acids have hydrocarbon chains connected by single bonds only. Unsaturated fatty acids have one or more double bonds. Each double bond may be in a cis or trans configuration. In the cis configuration, both hydrogens are on the same side of the hydrocarbon chain. In the trans configuration, the hydrogens are on opposite sides. A cis double bond causes a kink in the chain.

    A saturated FA contains only single bonds between C atoms (Figure 7.2.2. top). Saturated FAs form long straight fatty acids that are packed tightly, making them solid at room temperature. Animal fats typically contain many saturated fatty acids. For example, the FAs stearic acid and palmitic acid are common in meat and butyric acid is common butter.

    Unsaturated FAs can have one or more double bonds between C atoms. FAs with one double bond are monounsaturated, and FAs with two or more double bonds are polyunsaturated. The double bonds in naturally derived fatty acids (the vegetable oils) results in the structure being bent (Figure 7.2.2. middle). The kinked FA chains in a triglyceride decrease the packing between molecules resulting in the semi-solid consistency of oils.

    Each double bond may be in a cis- or trans- configuration. These two terms refer to the position of the H atoms in relation to the double bond.

    A cis- FA contains the two H atoms on the same side of the double bond between two carbon atoms (Figure 7.2.2. middle). Natural, unprocessed plant oils contain cis-bond. Humans have enzymes to breakdown these lipids and are considered to be the healthy ”fats’.

    A trans- FA contains the two H atoms on the opposite sides of the double bond (Figure 7.2.2. bottom). Humans do not have enzymes necessary to break down the lipid. If you inspect food package ingredient labels, you will most likely see ‘0 trans fats’. This refers to the absence of fatty acids in the trans-bond structure.

    History of Trans Fats

    Trans fats do not exist in nature. They are the result of food industry processing. Unsaturated vegetable oils are artificially hydrogenated under pressure to convert the liquid into a partially hydrogenated fat with a semi-solid consistency. The intent was to make a healthier but palatable substitute for butter. Margarine was an early product of this partial hydrogenation process. Unfortunately, human enzymes do not recognize the trans-FA structure. Recent data suggests that trans-fat diets may lead to an increase in levels of ‘bad cholesterol’ (low-density lipoproteins, LDLs), which in turn may lead to plaque deposition in the arteries, resulting in heart disease. Trans fats are now banned substances.

    Essential fatty acids: Omega-3 and Omega-6 polyunsaturated FAs

    Some FAs are essential FAs meaning they are required but not synthesized by the human body and must be acquired in the diet. The essential FAs are polyunsaturated (more than one double bond) and, more specifically, to be recognized by human enzymes, a double bond must be in specific locations in the FA chain.

    Chemists number C atoms according to specific rules and in the omega system, the count starts from the methyl, -CH3 functional group. Hence, an omega-3 FA is one whose first double bond is on the third C atom. Figure 7.2.3. shows an omega-FA in line structure where only the H atoms across the double bonds of the FA are shown.

    The molecular structures of alpha-linolenic acid, an omega-3 fatty acid is shown. Alpha-linolenic acid has three double bonds located eight, eleven, and fourteen residues from the acetyl group. It has a hooked shape.
    Figure 7.2.3: A cis-form polyunsaturated omega 3- fatty acid. The first double from the methyl functional group is at the 3-C position. A cis-form omega-6 fatty acid has its first double bond six C atoms from the methyl.

    Omega-3 and omega-6 fatty acids are metabolized for ATP synthesis, they form the phospholipids that make up the cell membrane, and are the building blocks for important hormones.

    Phospholipids

    Phospholipids are major constituents of the plasma membrane, the outermost layer of all living cells. Like triglycerides, they are composed of fatty acid chains attached to a backbone molecule. Instead of three fatty acids attached as in triglycerides, however, there are two fatty acids forming diacylglycerol, and the third carbon of a glycerol backbone is occupied by a modified phosphate group (Figure 7.2.4.). The phosphate group is modified by an alcohol. 
    The molecular structure of a phospholipid is shown. It consists of two fatty acids attached to the first and second carbons in glycerol, and a phosphate group attached to the third position. The phosphate group may be further modified by addition of another molecule to one of its oxygens. Two molecules that may modify the phosphate group, choline and serine, are shown. Choline consists of a two-carbon chain with a hydroxy group attached to one end and a nitrogen attached to the other. The nitrogen, in turn, has three methyl groups attached to it and has a charge of plus one. Serine consists of a two-carbon chain with a hydroxyl group attached to one end. An amino group and a carboxyl group are attached to the other end.
    Figure 7.2.4: A phospholipid is a molecule with two fatty acids and a modified phosphate group attached to a glycerol backbone. The phosphate may be modified by the addition of charged or polar chemical groups.

    A phospholipid is an amphipathic molecule, having both hydrophobic and hydrophilic regions. The fatty acid chains are hydrophobic and cannot interact with polar water, whereas the phosphate-containing group is hydrophilic and interacts with water (Figure 7.2.5.).

    An illustration of a phospholipids bilayer is shown. The phospholipids bilayer consists of two layers of phospholipids. The hydrophobic tails of the phospholipids face one another while the hydrophilic head groups face outward.
    Figure 7.2.5: The phospholipid bilayer is the major component of all cellular membranes.

    The head is the hydrophilic part, and the tail contains the hydrophobic fatty acids. In a cell 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 (Figure 7.2.5.).

    Phospholipids are responsible for the dynamic nature of the cell plasma membrane. If a drop of phospholipid is placed in water, it spontaneously forms a structure known as a micelle in which the hydrophilic phosphate heads face the outside and the fatty acids face the interior of this structure.

    Fats are amphipathic molecules, meaning they have both polar and nonpolar regions. The long hydrocarbon tail is hydrophobic (nonpolar) and the glycerol regions is hydrophilic (polar). When in water, fats will arrange themselves into a ball called a micelle so that the hydrophilic “heads” are on the outer surface facing the polar covalent water, and the hydrophobic “tails” are on the inside away from the surrounding water (Figure 7.2.6.).

    An illustration shows a circular arrangement of hydrocarbons with the hydrophilic head at the outer circumference, and the hydrophobic tails all pointing toward the centre of the circle. This arrangement allows the micelle to remain in suspension in an aqueous medium. A few water molecules are shown surrounding the micelle, unable to pass through the membrane.
    Figure 7.2.6: Micelle formation

    Cholesterol and Steroids

    Unlike the phospholipids and triglycerides, cholesterol and its derivates, the steroids, have a fused ring structure. Although they do not resemble the other lipids, they are grouped with them because they are also hydrophobic and insoluble in polar covalent water. All steroids have four linked carbon rings and several of them, like cholesterol, have a short tail (Figure 7.2.7.). Many steroids also have the –OH functional group, which puts them in the alcohol classification (sterols).

    The structures of cholesterol and cortisol are shown. Each of these molecules is composed of three six-carbon rings fused to a five-carbon ring. Cholesterol has a branched hydrocarbon attached to the five-carbon ring, and a hydroxyl group attached to the terminal six-carbon ring. Cortisol has a two-carbon chain modified with a double-bonded oxygen, a hydroxyl group attached to the five-carbon ring, and an oxygen double-bonded to the terminal six-carbon ring.
    Figure 7.2.7: Steroids such as cholesterol and cortisol are composed of four fused hydrocarbon rings.

    Cholesterol is the most common steroid. Cholesterol is mainly synthesized in the liver. Although dietary intake of cholesterol is often spoken of in negative terms by lay people, the steroid plays diverse roles and is necessary for proper functioning of the body:

    1. It is the precursor to many steroid hormones: gonadal hormones (testosterone and estradiol), adrenal gland hormones (cortisol, aldosterone).
    2. It is also the precursor to Vitamin D.
    3. Cholesterol is also the precursor of bile salts, which help in the emulsification of fats and their subsequent absorption by cells.
    4. It is a component of the plasma membrane of animal cells and is found within the phospholipid bilayer. Being the outermost structure in animal cells, the plasma membrane is responsible for the transport of materials and cellular recognition and it is involved in cell-to-cell communication.

    License and attributions:

    • Biology for AP Courses, 2018, Zedalis, J. et al. License: CC BY 4.0. Located at https://openstax.org/books/biology-ap-courses/pages/3-3-lipids
    • Biology, Second edition, 2018, Clark, M.A. et al. License: CC BY 4.0. Located at https://openstax.org/books/biology-2e/pages/3-3-lipids

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    BIO130: Introduction to Physiology Copyright © 2024 by Dinor Dhanabala; Sandra Fraley; and Gordon Lake is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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