Lipids: Waterproof and Energy Rich

  • Lipids are molecules that cannot dissolve in water.
  • Cells synthesize most lipids including phospholipids, sterols, and fats.

Structure of lipids

  • All lipid molecules are hydrophobic.
  • Lipids are hydrophobic because they have several nonpolar covalent bonds.
  • Unlike other macromolecules, lipids are not polymers.
  • According to their structures, lipids are divided into many subcategories;
    • Fats and oils have a similar structure.
      • They contain fatty acids that joined the backbone of glycerol.
    • Phospholipids contain unique head groups.
    • Waxes are esters of a carboxylic acid with R groups contain long straight hydrocarbons.
    • Sterols have four fused rings. They are useful in the production of hormones.

Saturated fatty acids

  • Fatty acids are long chains of carbon and hydrogen.
  • These are components of lipids.
  • They can be unsaturated or saturated.
    • Saturated fats involve carbon atoms joined together with single bonds in fatty acids. The carbon atoms form four bonds and each contains two hydrogen bonds.
    • Unsaturated fats contain carbon atoms with double bonds. The atoms with the double bond can only join one hydrogen atom.

Forming fats and oils

  • Fats and oils are formed by attachment of three fatty acids to a glycerol backbone.
  • They are referred to as triglycerides because of the three fatty acids.
  • Glycerol is formed from three-carbon alcohol with a hydroxyl group attached to each carbon atom.
  • Fats are solids at room temperature.
  • Oils are in liquid form at room temperature.
  • Animal fats like those in meat and dairy products are saturated.
  • Fish and plant oils are unsaturated like those found in nuts.

Other types of lipids

  • Oils and fats are the most useful lipids.
  • Phospholipids and triglycerides have a very similar structure.
  • Phospholipids are formed by the condensation reaction between fatty acids and glycerol.
  • The phosphate negative charges make phospholipid head groups hydrophilic.
  • Amphipathic lipids are those with both nonpolar and polar ends.
  • Waxes are generated from long-chain alcohols and fatty acids.
    • They are long-chain hydrocarbons with several nonpolar hydrogen carbon bonds.
  • Sterols are processed from four associated carbon rings. The rings contain several carbon-hydrogen nonpolar bonds.
    • Sterols include steroid hormones, cholesterol, and vitamin D.

Roles of lipids

  • Insulation
    • Fat insulation is important in animals.
    • Fat provides a shock-absorbing layer in animal organs.
  • They are important in plasma membrane structure.
    • The plasma membrane is made up of numerous phospholipids.
    • The cell membrane contains a bilayer of phospholipids with hydrophilic heads outside the membrane while hydrophobic heads facing inwards.
  • Lipids are waterproof.
    • Plants use axes to prevent leaf damage.
    • Waterbirds use waxes to keep their feathers dry.
  • Lipids are useful in signaling.
    • Various steroid hormones like estrogen and testosterone enhance useful human developmental changes.
  • These lipids have a sugar residue.
  • They do not have phosphate groups.
  • The sugar residue can be polysaccharide, disaccharide or oligosaccharide.
  •  Glyceroglycolipids are formed from the attachment of sugar and fatty acid residues to a glycerol backbone.
  • Sphingoglycolipids are formed by attachment of fatty acids and sugar residues to a sphingosine backbone.


  • Lipoproteins are classified into two classes according to their density.
    • Low-density lipoproteins are useful in the transportation of phospholipids and cholesterol from the liver into the cells.
    • High-density lipoproteins;
      • They are used to transport cholesterol and phospholipids back into the liver from the cells.
      • High LDL and low HDL levels can be used to indicate looming atherosclerosis.


  • Eicosanoids are signaling molecules with twenty carbon atoms.
  • They are derived from polyunsaturated fatty acids like eicosapentaenoic acids or arachidonic acid.
  • The above polyunsaturated fatty acids undergo reactions that change them into different eicosanoids classes.
  • The classes include: prostaglandins, thromboxane, prostacyclin and leukotriene

Saturated Fatty AcidsFatty acids with only single bonds between neighboring carbons. They are saturated with hydrogen, maximizing the number of hydrogen atoms attached to the carbon skeleton. Stearic acid is an example of a saturated fatty acid.
Unsaturated Fatty AcidsFatty acids with double bonds in the carbon chain. They can be in a cis or trans configuration. In the cis configuration, hydrogen atoms are on the same side, causing a bend in the chain and a liquid/oil consistency at room temperature. In the trans configuration, hydrogen atoms are on opposite sides, resulting in a straight chain and a solid/fat consistency at room temperature.
Trans FatsOils that have been artificially hydrogenated to make them semi-solid for processed food products. Hydrogenation converts cis double bonds to trans double bonds. Examples include margarine, some types of peanut butter, and shortening. High trans fat intake may lead to increased levels of “bad” cholesterol (LDL) and contribute to heart disease. Many fast food restaurants have banned trans fats, and food labels must display trans fat content.
Omega Fatty AcidsEssential fatty acids required by the body but not synthesized. Omega-3 and omega-6 fatty acids are the only essential ones known for humans. Omega-3 fatty acids have a double bond on the third carbon from the end, and examples include alpha-linoleic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). Omega-3s have various health benefits, such as reducing heart attack risk and inflammation.
TriacylglycerolsFat molecules composed of glycerol and fatty acids. Three fatty acids are attached to the glycerol backbone through ester bonds. They are the main storage form of fats in mammals and are obtained from the diet. Unsaturated fats/oils are usually of plant or fish origin, while saturated fats are typically found in animal products.
PhospholipidsMajor constituents of the plasma membrane in animal cells. They consist of a glycerol backbone, two fatty acids, and a modified phosphate group. Phospholipids have a hydrophilic head (phosphate group) and hydrophobic tails (fatty acids). They form a bilayer in the cell membrane, with the hydrophobic tails facing inward and the hydrophilic heads facing outward.
SteroidsLipids with a fused ring structure. They are hydrophobic and insoluble in water. Steroids have four linked carbon rings and often include the -OH functional group, making them classified as alcohols (sterols). Cholesterol is the most common steroid and serves as the precursor for steroid hormones, Vitamin D, and bile salts. It is also a component of the plasma membrane in animal cells.
Trans fatTrans fat is required to be listed on food labels since 2006. It occurs naturally in some foods, but 80% of trans fat in the diet comes from processed foods. In some cases, a food containing up to 0.5g of trans fat can be labeled as “trans fat-free.”
Major sources of trans fatCakes, cookies, crackers, pies, bread, animal products, margarine, fried potatoes, potato chips, corn chips, popcorn, household shortening, breakfast cereal, and candy are major sources of trans fat in the diet.
Safe levels of trans fatThere is no safe level of trans fat identified. However, it is recommended to consume no more than 1% of total calories from trans fat.
CDC impact of avoiding trans fatAvoiding artificial trans fat completely could prevent up to 20,000 heart attacks and 7,000 coronary disease deaths in the US every year, according to the CDC.
Phospholipids and SterolsPhospholipids and sterols make up 5% of the lipids in the diet.
Lecithin structureLecithin is a phospholipid that has 2 fatty acids attached to a glycerol molecule, with a phosphate group at the 3rd position and a molecule of choline.
Benefits of phospholipidsPhospholipids act as emulsifiers, with one end being hydrophobic (fat-soluble) and the other end being hydrophilic (water-soluble), allowing them to promote the mixing of oils and fats in watery solutions. Foods such as eggs, liver, soybeans, wheat germ, and peanuts are rich in phospholipids.
Roles of phospholipids in the bodyPhospholipids are important for cell membranes (e.g., lecithin), allowing fat-soluble vitamins and hormones to pass in and out of cells easily. They also act as emulsifiers in the body, helping to keep fats suspended in blood and body fluids. Supplementation of phospholipids is not required.
Sterols and CholesterolSterols are found in animal products, and the most significant sterol is cholesterol. Plant sterols, also known as phytosterols, are structurally similar to cholesterol and interfere with cholesterol absorption. A diet high in plant sterols can lower blood cholesterol levels.
TLC for lowering cholesterolTLC (Therapeutic Lifestyle Changes) includes recommendations for lowering cholesterol, such as consuming low saturated fat, low cholesterol, low sodium, 10-20g of soluble fiber, and 2g of plant sterols and stanols.
Functions of CholesterolCholesterol serves as the starting material for bile acids, sex hormones, adrenal hormones, vitamin D, and cell membranes.
Cholesterol manufacturingCholesterol manufactured inside the body is known as endogenous cholesterol.
Cholesterol from consumptionCholesterol consumed from outside the body is known as exogenous cholesterol.
CCK and bile releaseFat in the small intestine triggers the release of the hormone CCK, which signals the gallbladder to release bile.
Bile acids and emulsificationBile acids act as emulsifiers, aiding in the digestion and absorption of fats. They can be reabsorbed or excreted in the body, with fiber reducing their reabsorption and lowering.

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