{"id":96,"date":"2021-12-07T05:55:21","date_gmt":"2021-12-07T05:55:21","guid":{"rendered":"https:\/\/pressbooks.publishdot.com\/anatomyphysiology\/?post_type=chapter&p=96"},"modified":"2021-12-07T06:42:24","modified_gmt":"2021-12-07T06:42:24","slug":"2-3-lipids","status":"publish","type":"chapter","link":"https:\/\/pressbooks.publishdot.com\/anatomyphysiology\/chapter\/2-3-lipids\/","title":{"raw":"2.3 Lipids","rendered":"2.3 Lipids"},"content":{"raw":"
\r\n

Learning Objectives<\/strong><\/p>\r\n\r\n<\/header>\r\n

\r\n\r\nBy the end of this section, you will be able to:\r\n
    \r\n \t
  • Describe the four major types of lipids<\/li>\r\n \t
  • Explain the role of fats in storing energy<\/li>\r\n \t
  • Differentiate between saturated and unsaturated fatty acids<\/li>\r\n \t
  • Describe phospholipids and their role in cells<\/li>\r\n \t
  • Define the basic structure of a steroid and some steroid functions<\/li>\r\n \t
  • Explain how cholesterol helps maintain the plasma membrane\u2019s fluid nature<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
    \"Otto
    Figure\u00a02.3.1. Otter fur.<\/strong>\u00a0Hydrophobic lipids in aquatic mammals\u2019 fur, such as this river otter, protect them from the elements. (credit: Ken Bosma).<\/figcaption><\/figure>\r\nLipids<\/strong>\u00a0include a diverse group of compounds that are largely nonpolar in nature. This is because they are hydrocarbons that include mostly nonpolar carbon\u2013carbon or carbon\u2013hydrogen bonds. Non-polar molecules are hydrophobic (\u201cwater fearing\u201d), or insoluble in water. Lipids perform many different functions in a cell. Cells store energy for long-term use in the form of fats. Lipids also provide insulation from the environment for plants and animals (Figure 2.3.1). For example, they help keep aquatic birds and mammals dry when forming a protective layer over fur or feathers because of their water-repellent hydrophobic nature. Lipids are also the building blocks of many hormones and are an important constituent of all cellular membranes. Lipids include fats and oils, waxes, phospholipids\u00a0and steroids.\r\n

    Fats and Oils<\/h2>\r\nA fat molecule consists of two main components\u2014glycerol and fatty acids. Glycerol is an organic compound (alcohol) with three carbons, five hydrogens, and three hydroxyl (OH) groups. Fatty acids have a long chain of hydrocarbons to which a carboxyl group is attached, hence the name \u201cfatty acid.\u201d The number of carbons in the fatty acid may range from 4 to 36. The most common are those containing 12\u201318 carbons. In a fat molecule, the fatty acids attach to each of the glycerol molecule\u2019s three carbons with an ester bond through an oxygen atom (Figure 2.3.2).\r\n
    \"Fatty
    Figure\u00a02.3.2.<\/strong>\u00a0Fatty acid dehydration reaction<\/strong>.\u00a0Joining three fatty acids to a glycerol backbone in a dehydration reaction forms triacylglycerol. Three water molecules release in the process.<\/figcaption><\/figure>\r\nDuring this ester bond formation, three water molecules are released. The three fatty acids in the\u00a0triacylglycerol<\/strong>\u00a0may be similar or dissimilar. We also call fats\u00a0triacylglycerols (TAGs) or\u00a0triglycerides (TGs)\u00a0because of their chemical structure. Some fatty acids have common names that specify their origin, for example, palmitic acid, a\u00a0saturated fatty acid, is derived from the palm tree. Arachidic acid (also known as eicosanoic acid) is derived from\u00a0Arachis hypogea,<\/em>\u00a0the scientific name for groundnuts or peanuts.\r\n\r\nFatty acids may be saturated or unsaturated. In a fatty acid chain, if there are only single bonds between neighbouring carbons in the hydrocarbon chain, the fatty acid is saturated.\u00a0Saturated fatty acids<\/strong>\u00a0are saturated with hydrogen. In other words, the number of hydrogen atoms attached to the carbon skeleton is maximised. Stearic acid is an example of a saturated fatty acid (Figure 2.3.3).\r\n
    \"Stearic
    Figure\u00a02.3.3.<\/strong>\u00a0Stearic acid.<\/strong>\u00a0Stearic acid is a common saturated fatty acid.<\/figcaption><\/figure>\r\nWhen the hydrocarbon chain contains a double bond, the fatty acid is\u00a0unsaturated. Oleic acid is an example of an\u00a0unsaturated fatty acid<\/strong>\u00a0(Figure 2.3.4).\r\n
    \"Oleic
    Figure\u00a02.3.4.<\/strong>\u00a0Oleic acid.<\/strong>\u00a0Oleic acid is a common unsaturated fatty acid.<\/figcaption><\/figure>\r\nMost unsaturated fats are liquid at room temperature. We call these oils. If there is one double bond in the molecule, then it is a monounsaturated fat (e.g., olive oil), and if there is more than one double bond, then it is a polyunsaturated fat (e.g., canola oil).\r\n\r\nWhen a fatty acid has no double bonds, it is a saturated fatty acid because it is not possible to add more hydrogen to the chain\u2019s carbon atoms. A fat may contain similar or different fatty acids attached to glycerol. Long straight fatty acids with single bonds generally pack tightly and are solid at room temperature. Animal fats with stearic acid and palmitic acid (common in meat) and the fat with butyric acid (common in butter) are examples of saturated fats. Mammals store fats in specialised cells, or adipocytes, where fat globules occupy most of the cell\u2019s volume. Plants store fat or oil in many seeds and use them as a source of energy during seedling development. Unsaturated fats or oils are usually of plant origin and contain\u00a0cis<\/em>\u00a0unsaturated fatty acids.\u00a0Cis<\/em>\u00a0and\u00a0trans<\/em>\u00a0indicate the configuration of the molecule around the double bond. If hydrogens are present in the same plane, it is a cis fat. If the hydrogen atoms are on two different planes, it is a\u00a0trans-fat. The\u00a0cis<\/em>\u00a0double bond causes a bend or a \u201ckink\u201d that prevents the fatty acids from packing tightly, keeping them liquid at room temperature (Figure 2.3.5). Olive oil, corn oil, canola oil, and cod liver oil are examples of unsaturated fats. Unsaturated fats help to lower blood cholesterol levels; whereas, saturated fats contribute to plaque formation in the arteries.\r\n
    \"Saturated
    Figure\u00a02.3.5.<\/strong>\u00a0Saturated fatty acids.\u00a0<\/strong>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\u00a0cis<\/em>\u00a0or\u00a0trans<\/em>\u00a0configuration. In the\u00a0cis\u00a0<\/em>configuration, both hydrogens are on the same side of the hydrocarbon chain. In the\u00a0trans<\/em>\u00a0configuration, the hydrogens are on opposite sides. A\u00a0cis<\/em>\u00a0double bond causes a kink in the chain.<\/figcaption><\/figure>\r\n

    Trans Fats<\/h3>\r\nThe food industry artificially hydrogenates oils to make them semi-solid and of a consistency desirable for many processed food products. Simply speaking, hydrogen gas is bubbled through oils to solidify them. During this hydrogenation process, double bonds of the\u00a0cis<\/em>\u2013 conformation in the hydrocarbon chain may convert to double bonds in the\u00a0trans<\/em>\u2013 conformation.\r\n\r\nMargarine, some types of peanut butter, and shortening are examples of artificially hydrogenated\u00a0trans fats<\/strong>. Recent studies have shown that an increase in trans fats in the human diet may lead to higher levels of low-density lipoproteins (LDL), or \u201cbad\u201d cholesterol, which in turn may lead to plaque deposition in the arteries, resulting in heart disease. Many fast-food restaurants have recently banned using trans fats, and food labels are required to display the trans-fat content.\r\n

    Omega Fatty Acids<\/h3>\r\nEssential fatty acids are those that the human body requires but does not synthesise. Consequently, they must be supplemented through ingestion via the diet.\u00a0Omega-3 fatty acids (like those in\u00a0Figure 2.3.6) fall into this category and are one of only two known for humans (the other is omega-6 fatty acid). These are polyunsaturated fatty acids and are omega-3 because a double bond connects the third carbon from the hydrocarbon chain\u2019s end to its neighbouring carbon.\r\n
    \"Fatty
    Figure\u00a02.3.6. Fatty acid omega-3.<\/strong>\u00a0Alpha-linolenic acid is an example of an omega-3 fatty acid. It has three\u00a0cis<\/em>\u00a0double bonds and, as a result, a curved shape. For clarity, the diagram does not show the carbons. Each singly bonded carbon has two hydrogens associated with it, which the diagram also does not show.<\/figcaption><\/figure>\r\nThe farthest carbon away from the carboxyl group is numbered as the omega (\u03c9<\/em>) carbon, and if the double bond is between the third and fourth carbon from that end, it is an omega-3 fatty acid. Nutritionally important because the body does not make them, omega-3 fatty acids include alpha-linoleic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), all of which are polyunsaturated. Salmon, trout, and tuna are good sources of omega-3 fatty acids. Research indicates that omega-3 fatty acids reduce the risk of sudden death from heart attacks, lower triglycerides in the blood, decrease blood pressure, and prevent thrombosis by inhibiting blood clotting. They also reduce inflammation and may help lower the risk of some cancers in animals.\r\n\r\nLike carbohydrates, fats have received considerable bad publicity. It is true that eating an excess of fried foods and other \u201cfatty\u201d foods leads to weight gain. However, fats do have important functions. Many vitamins are fat soluble, and fats serve as a long-term storage form of fatty acids: a source of energy. They also provide insulation for the body. Therefore, we should consume \u201chealthy\u201d fats in moderate amounts on a regular basis.\r\n

    <\/h2>\r\n

    Waxes<\/h2>\r\n
    \"Waxy
    Figure\u00a02.3.7.<\/strong>\u00a0Waxy leaves.\u00a0<\/strong>Lipids comprise waxy coverings on some leaves. (Credit: Roger Griffith).<\/figcaption><\/figure>\r\nWax\u00a0covers some aquatic birds\u2019 feathers and some plants\u2019 leaf surfaces. Because of waxes\u2019 hydrophobic nature, they prevent water from sticking on the surface (Figure 2.3.7). Long fatty acid chains esterified to long-chain alcohols comprise waxes.\r\n

    <\/h2>\r\n

    Phospholipids<\/h2>\r\nPhospholipids\u00a0are major plasma membrane constituents that comprise cells\u2019 outermost layer. Like fats, they are comprised of fatty acid chains attached to a glycerol or sphingosine backbone. However, instead of three fatty acids attached as in triglycerides, there are two fatty acids forming diacylglycerol, and a modified phosphate group occupies the glycerol backbone\u2019s third carbon (Figure 2.3.8). A phosphate group alone attached to a diacylglycerol does not qualify as a phospholipid. It is phosphatidate (diacylglycerol 3-phosphate), the precursor of phospholipids. An alcohol modifies the phosphate group. Phosphatidylcholine and phosphatidylserine are two important phospholipids that are in plasma membranes.\r\n
    \"Diagram
    Figure\u00a02.3.8.<\/strong>\u00a0Phospholipid.\u00a0<\/strong>A phospholipid is a molecule with two fatty acids and a modified phosphate group attached to a glycerol backbone. Adding a charged or polar chemical group may modify the phosphate.<\/figcaption><\/figure>\r\nA phospholipid is an amphipathic molecule, meaning it has a hydrophobic and a hydrophilic part. The fatty acid chains are hydrophobic and cannot interact with water; whereas, the phosphate-containing group is hydrophilic and interacts with water (Figure 2.3.9).\r\n
    \"Phospholipid
    Figure\u00a02.3.9. Phospholipid bilayer.<\/strong>\u00a0The phospholipid bilayer is the major component of all cellular membranes. The hydrophilic head groups of the phospholipids face the aqueous solution. The hydrophobic tails are sequestered in the middle of the bilayer.<\/figcaption><\/figure>\r\nThe head is the hydrophilic part, and the tail contains the hydrophobic fatty acids. In a membrane, a bilayer of phospholipids forms the structure\u2019s matrix, phospholipids\u2019 fatty acid tails face inside, away from water; whereas, the phosphate group faces the outside, aqueous side (Figure 2.3.9).\r\n\r\nPhospholipids are responsible for the plasma membrane\u2019s dynamic nature. If a drop of phospholipids is placed in water, it spontaneously forms a structure that scientists call a micelle, where the hydrophilic phosphate heads face the outside and the fatty acids face the structure\u2019s interior.\r\n

    Steroids<\/h2>\r\nUnlike the phospholipids and fats that was discussed earlier,\u00a0steroids\u00a0have a fused ring structure. Although they do not resemble the other lipids, scientists group them with them because they are also hydrophobic and insoluble in water. All steroids have four linked carbon rings and several of them, like cholesterol, have a short tail (Figure 2.3.10). Many steroids also have the \u2013OH functional group, which puts them in the alcohol classification (sterols).\r\n
    \"Steroids\"
    Figure\u00a02.3.10.<\/strong>\u00a0Steroids<\/strong>.\u00a0Four fused hydrocarbon rings comprise steroids such as cholesterol and cortisol.<\/figcaption><\/figure>\r\nCholesterol is the most common steroid. The liver synthesises cholesterol and is the precursor to many steroid hormones such as testosterone and oestradiol, which gonads and endocrine glands secrete. It is also the precursor to Vitamin D. Cholesterol is also the precursor of bile salts, which help emulsifying fats and their subsequent absorption by cells. Although lay people often speak negatively about cholesterol, it is necessary for the body\u2019s proper functioning. Sterols (cholesterol in animal cells, phytosterol in plants) are components of the plasma membrane of cells and are found within the phospholipid bilayer.\r\n
    \r\n

    Section Review<\/strong><\/p>\r\n\r\n<\/header>\r\n

    \r\n\r\nLipids are a class of macromolecules that are nonpolar and hydrophobic in nature. Major types include fats and oils, waxes, phospholipids and steroids. Fats are a stored form of energy and are also known as triacylglycerols or triglycerides. Fats are comprised of fatty acids and either glycerol or sphingosine. Fatty acids may be unsaturated or saturated, depending on the presence or absence of double bonds in the hydrocarbon chain. If only single bonds are present, they are saturated fatty acids. Unsaturated fatty acids may have one or more double bonds in the hydrocarbon chain. Phospholipids comprise the membrane\u2019s matrix. They have a glycerol or sphingosine backbone to which two fatty acid chains and a phosphate-containing group are attached. Steroids are another class of lipids. Their basic structure has four fused carbon rings. Cholesterol is a type of steroid and is an important constituent of the plasma membrane, where it helps to maintain the membrane\u2019s fluid nature. It is also the precursor of steroid hormones such as testosterone.\r\n\r\n<\/div>\r\n<\/div>\r\n
    \r\n

    Review Questions<\/strong><\/p>\r\n\r\n<\/header>\r\n

    \r\n
    \r\n
    [h5p id=\"34\"]<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
    \r\n

    Critical Thinking Questions<\/strong><\/p>\r\n\r\n<\/header>\r\n

    \r\n
    \r\n
    [h5p id=\"35\"]<\/div>\r\n<\/div>\r\n
    \r\n
    [h5p id=\"36\"]<\/div>\r\n<\/div>\r\n
    \r\n
    [h5p id=\"37\"]<\/div>\r\n<\/div>\r\n
    \r\n
    [h5p id=\"38\"]<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nClick the drop down below to review the terms learned from this chapter.\r\n
    \r\n
    [h5p id=\"39\"]<\/div>\r\n<\/div>","rendered":"
    \n
    \n

    Learning Objectives<\/strong><\/p>\n<\/header>\n

    \n

    By the end of this section, you will be able to:<\/p>\n

      \n
    • Describe the four major types of lipids<\/li>\n
    • Explain the role of fats in storing energy<\/li>\n
    • Differentiate between saturated and unsaturated fatty acids<\/li>\n
    • Describe phospholipids and their role in cells<\/li>\n
    • Define the basic structure of a steroid and some steroid functions<\/li>\n
    • Explain how cholesterol helps maintain the plasma membrane\u2019s fluid nature<\/li>\n<\/ul>\n<\/div>\n<\/div>\n
      \"Otto
      Figure\u00a02.3.1. Otter fur.<\/strong>\u00a0Hydrophobic lipids in aquatic mammals\u2019 fur, such as this river otter, protect them from the elements. (credit: Ken Bosma).<\/figcaption><\/figure>\n

      Lipids<\/strong>\u00a0include a diverse group of compounds that are largely nonpolar in nature. This is because they are hydrocarbons that include mostly nonpolar carbon\u2013carbon or carbon\u2013hydrogen bonds. Non-polar molecules are hydrophobic (\u201cwater fearing\u201d), or insoluble in water. Lipids perform many different functions in a cell. Cells store energy for long-term use in the form of fats. Lipids also provide insulation from the environment for plants and animals (Figure 2.3.1). For example, they help keep aquatic birds and mammals dry when forming a protective layer over fur or feathers because of their water-repellent hydrophobic nature. Lipids are also the building blocks of many hormones and are an important constituent of all cellular membranes. Lipids include fats and oils, waxes, phospholipids\u00a0and steroids.<\/p>\n

      Fats and Oils<\/h2>\n

      A fat molecule consists of two main components\u2014glycerol and fatty acids. Glycerol is an organic compound (alcohol) with three carbons, five hydrogens, and three hydroxyl (OH) groups. Fatty acids have a long chain of hydrocarbons to which a carboxyl group is attached, hence the name \u201cfatty acid.\u201d The number of carbons in the fatty acid may range from 4 to 36. The most common are those containing 12\u201318 carbons. In a fat molecule, the fatty acids attach to each of the glycerol molecule\u2019s three carbons with an ester bond through an oxygen atom (Figure 2.3.2).<\/p>\n

      \"Fatty
      Figure\u00a02.3.2.<\/strong>\u00a0Fatty acid dehydration reaction<\/strong>.\u00a0Joining three fatty acids to a glycerol backbone in a dehydration reaction forms triacylglycerol. Three water molecules release in the process.<\/figcaption><\/figure>\n

      During this ester bond formation, three water molecules are released. The three fatty acids in the\u00a0triacylglycerol<\/strong>\u00a0may be similar or dissimilar. We also call fats\u00a0triacylglycerols (TAGs) or\u00a0triglycerides (TGs)\u00a0because of their chemical structure. Some fatty acids have common names that specify their origin, for example, palmitic acid, a\u00a0saturated fatty acid, is derived from the palm tree. Arachidic acid (also known as eicosanoic acid) is derived from\u00a0Arachis hypogea,<\/em>\u00a0the scientific name for groundnuts or peanuts.<\/p>\n

      Fatty acids may be saturated or unsaturated. In a fatty acid chain, if there are only single bonds between neighbouring carbons in the hydrocarbon chain, the fatty acid is saturated.\u00a0Saturated fatty acids<\/strong>\u00a0are saturated with hydrogen. In other words, the number of hydrogen atoms attached to the carbon skeleton is maximised. Stearic acid is an example of a saturated fatty acid (Figure 2.3.3).<\/p>\n

      \"Stearic
      Figure\u00a02.3.3.<\/strong>\u00a0Stearic acid.<\/strong>\u00a0Stearic acid is a common saturated fatty acid.<\/figcaption><\/figure>\n

      When the hydrocarbon chain contains a double bond, the fatty acid is\u00a0unsaturated. Oleic acid is an example of an\u00a0unsaturated fatty acid<\/strong>\u00a0(Figure 2.3.4).<\/p>\n

      \"Oleic
      Figure\u00a02.3.4.<\/strong>\u00a0Oleic acid.<\/strong>\u00a0Oleic acid is a common unsaturated fatty acid.<\/figcaption><\/figure>\n

      Most unsaturated fats are liquid at room temperature. We call these oils. If there is one double bond in the molecule, then it is a monounsaturated fat (e.g., olive oil), and if there is more than one double bond, then it is a polyunsaturated fat (e.g., canola oil).<\/p>\n

      When a fatty acid has no double bonds, it is a saturated fatty acid because it is not possible to add more hydrogen to the chain\u2019s carbon atoms. A fat may contain similar or different fatty acids attached to glycerol. Long straight fatty acids with single bonds generally pack tightly and are solid at room temperature. Animal fats with stearic acid and palmitic acid (common in meat) and the fat with butyric acid (common in butter) are examples of saturated fats. Mammals store fats in specialised cells, or adipocytes, where fat globules occupy most of the cell\u2019s volume. Plants store fat or oil in many seeds and use them as a source of energy during seedling development. Unsaturated fats or oils are usually of plant origin and contain\u00a0cis<\/em>\u00a0unsaturated fatty acids.\u00a0Cis<\/em>\u00a0and\u00a0trans<\/em>\u00a0indicate the configuration of the molecule around the double bond. If hydrogens are present in the same plane, it is a cis fat. If the hydrogen atoms are on two different planes, it is a\u00a0trans-fat. The\u00a0cis<\/em>\u00a0double bond causes a bend or a \u201ckink\u201d that prevents the fatty acids from packing tightly, keeping them liquid at room temperature (Figure 2.3.5). Olive oil, corn oil, canola oil, and cod liver oil are examples of unsaturated fats. Unsaturated fats help to lower blood cholesterol levels; whereas, saturated fats contribute to plaque formation in the arteries.<\/p>\n

      \"Saturated
      Figure\u00a02.3.5.<\/strong>\u00a0Saturated fatty acids.\u00a0<\/strong>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\u00a0cis<\/em>\u00a0or\u00a0trans<\/em>\u00a0configuration. In the\u00a0cis\u00a0<\/em>configuration, both hydrogens are on the same side of the hydrocarbon chain. In the\u00a0trans<\/em>\u00a0configuration, the hydrogens are on opposite sides. A\u00a0cis<\/em>\u00a0double bond causes a kink in the chain.<\/figcaption><\/figure>\n

      Trans Fats<\/h3>\n

      The food industry artificially hydrogenates oils to make them semi-solid and of a consistency desirable for many processed food products. Simply speaking, hydrogen gas is bubbled through oils to solidify them. During this hydrogenation process, double bonds of the\u00a0cis<\/em>\u2013 conformation in the hydrocarbon chain may convert to double bonds in the\u00a0trans<\/em>\u2013 conformation.<\/p>\n

      Margarine, some types of peanut butter, and shortening are examples of artificially hydrogenated\u00a0trans fats<\/strong>. Recent studies have shown that an increase in trans fats in the human diet may lead to higher levels of low-density lipoproteins (LDL), or \u201cbad\u201d cholesterol, which in turn may lead to plaque deposition in the arteries, resulting in heart disease. Many fast-food restaurants have recently banned using trans fats, and food labels are required to display the trans-fat content.<\/p>\n

      Omega Fatty Acids<\/h3>\n

      Essential fatty acids are those that the human body requires but does not synthesise. Consequently, they must be supplemented through ingestion via the diet.\u00a0Omega-3 fatty acids (like those in\u00a0Figure 2.3.6) fall into this category and are one of only two known for humans (the other is omega-6 fatty acid). These are polyunsaturated fatty acids and are omega-3 because a double bond connects the third carbon from the hydrocarbon chain\u2019s end to its neighbouring carbon.<\/p>\n

      \"Fatty
      Figure\u00a02.3.6. Fatty acid omega-3.<\/strong>\u00a0Alpha-linolenic acid is an example of an omega-3 fatty acid. It has three\u00a0cis<\/em>\u00a0double bonds and, as a result, a curved shape. For clarity, the diagram does not show the carbons. Each singly bonded carbon has two hydrogens associated with it, which the diagram also does not show.<\/figcaption><\/figure>\n

      The farthest carbon away from the carboxyl group is numbered as the omega (\u03c9<\/em>) carbon, and if the double bond is between the third and fourth carbon from that end, it is an omega-3 fatty acid. Nutritionally important because the body does not make them, omega-3 fatty acids include alpha-linoleic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), all of which are polyunsaturated. Salmon, trout, and tuna are good sources of omega-3 fatty acids. Research indicates that omega-3 fatty acids reduce the risk of sudden death from heart attacks, lower triglycerides in the blood, decrease blood pressure, and prevent thrombosis by inhibiting blood clotting. They also reduce inflammation and may help lower the risk of some cancers in animals.<\/p>\n

      Like carbohydrates, fats have received considerable bad publicity. It is true that eating an excess of fried foods and other \u201cfatty\u201d foods leads to weight gain. However, fats do have important functions. Many vitamins are fat soluble, and fats serve as a long-term storage form of fatty acids: a source of energy. They also provide insulation for the body. Therefore, we should consume \u201chealthy\u201d fats in moderate amounts on a regular basis.<\/p>\n

      <\/h2>\n

      Waxes<\/h2>\n
      \"Waxy
      Figure\u00a02.3.7.<\/strong>\u00a0Waxy leaves.\u00a0<\/strong>Lipids comprise waxy coverings on some leaves. (Credit: Roger Griffith).<\/figcaption><\/figure>\n

      Wax\u00a0covers some aquatic birds\u2019 feathers and some plants\u2019 leaf surfaces. Because of waxes\u2019 hydrophobic nature, they prevent water from sticking on the surface (Figure 2.3.7). Long fatty acid chains esterified to long-chain alcohols comprise waxes.<\/p>\n

      <\/h2>\n

      Phospholipids<\/h2>\n

      Phospholipids\u00a0are major plasma membrane constituents that comprise cells\u2019 outermost layer. Like fats, they are comprised of fatty acid chains attached to a glycerol or sphingosine backbone. However, instead of three fatty acids attached as in triglycerides, there are two fatty acids forming diacylglycerol, and a modified phosphate group occupies the glycerol backbone\u2019s third carbon (Figure 2.3.8). A phosphate group alone attached to a diacylglycerol does not qualify as a phospholipid. It is phosphatidate (diacylglycerol 3-phosphate), the precursor of phospholipids. An alcohol modifies the phosphate group. Phosphatidylcholine and phosphatidylserine are two important phospholipids that are in plasma membranes.<\/p>\n

      \"Diagram
      Figure\u00a02.3.8.<\/strong>\u00a0Phospholipid.\u00a0<\/strong>A phospholipid is a molecule with two fatty acids and a modified phosphate group attached to a glycerol backbone. Adding a charged or polar chemical group may modify the phosphate.<\/figcaption><\/figure>\n

      A phospholipid is an amphipathic molecule, meaning it has a hydrophobic and a hydrophilic part. The fatty acid chains are hydrophobic and cannot interact with water; whereas, the phosphate-containing group is hydrophilic and interacts with water (Figure 2.3.9).<\/p>\n

      \"Phospholipid
      Figure\u00a02.3.9. Phospholipid bilayer.<\/strong>\u00a0The phospholipid bilayer is the major component of all cellular membranes. The hydrophilic head groups of the phospholipids face the aqueous solution. The hydrophobic tails are sequestered in the middle of the bilayer.<\/figcaption><\/figure>\n

      The head is the hydrophilic part, and the tail contains the hydrophobic fatty acids. In a membrane, a bilayer of phospholipids forms the structure\u2019s matrix, phospholipids\u2019 fatty acid tails face inside, away from water; whereas, the phosphate group faces the outside, aqueous side (Figure 2.3.9).<\/p>\n

      Phospholipids are responsible for the plasma membrane\u2019s dynamic nature. If a drop of phospholipids is placed in water, it spontaneously forms a structure that scientists call a micelle, where the hydrophilic phosphate heads face the outside and the fatty acids face the structure\u2019s interior.<\/p>\n

      Steroids<\/h2>\n

      Unlike the phospholipids and fats that was discussed earlier,\u00a0steroids\u00a0have a fused ring structure. Although they do not resemble the other lipids, scientists group them with them because they are also hydrophobic and insoluble in water. All steroids have four linked carbon rings and several of them, like cholesterol, have a short tail (Figure 2.3.10). Many steroids also have the \u2013OH functional group, which puts them in the alcohol classification (sterols).<\/p>\n

      \"Steroids\"
      Figure\u00a02.3.10.<\/strong>\u00a0Steroids<\/strong>.\u00a0Four fused hydrocarbon rings comprise steroids such as cholesterol and cortisol.<\/figcaption><\/figure>\n

      Cholesterol is the most common steroid. The liver synthesises cholesterol and is the precursor to many steroid hormones such as testosterone and oestradiol, which gonads and endocrine glands secrete. It is also the precursor to Vitamin D. Cholesterol is also the precursor of bile salts, which help emulsifying fats and their subsequent absorption by cells. Although lay people often speak negatively about cholesterol, it is necessary for the body\u2019s proper functioning. Sterols (cholesterol in animal cells, phytosterol in plants) are components of the plasma membrane of cells and are found within the phospholipid bilayer.<\/p>\n

      \n
      \n

      Section Review<\/strong><\/p>\n<\/header>\n

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      Lipids are a class of macromolecules that are nonpolar and hydrophobic in nature. Major types include fats and oils, waxes, phospholipids and steroids. Fats are a stored form of energy and are also known as triacylglycerols or triglycerides. Fats are comprised of fatty acids and either glycerol or sphingosine. Fatty acids may be unsaturated or saturated, depending on the presence or absence of double bonds in the hydrocarbon chain. If only single bonds are present, they are saturated fatty acids. Unsaturated fatty acids may have one or more double bonds in the hydrocarbon chain. Phospholipids comprise the membrane\u2019s matrix. They have a glycerol or sphingosine backbone to which two fatty acid chains and a phosphate-containing group are attached. Steroids are another class of lipids. Their basic structure has four fused carbon rings. Cholesterol is a type of steroid and is an important constituent of the plasma membrane, where it helps to maintain the membrane\u2019s fluid nature. It is also the precursor of steroid hormones such as testosterone.<\/p>\n<\/div>\n<\/div>\n

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      Review Questions<\/strong><\/p>\n<\/header>\n

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