Lipid Structure and Function


  • as a class are characterized by insolubility in water and solubility in non-polar organic solvents.

Structural Lipids

  • Lipids are the major component of the phospholipid bilayer
    • This separates the cell interior from the surrounding
  • Each membrane components is amphipathic, which means that it has both a hydrophilic and hydrophobic region
    • For membrane lipids, the polar head is the hydrophilic regions while the fatty acid tail Is the hydrophobic region
    • This allows membrane lipids to form multiple structures in aqueous solution: liposome, micelle, phospholipid bilayer, etc.

  • Contain a polar hydrophilic head and a non-polar hydrophobic tail
    • Polar group contains a phosphate and an alcohol
    • Polar group is attached to a fatty acid tail by phosphodiester linkages
    • One or more fatty acids are attached to a backbone to form the hydrophobic tail region
  • Phospholipids can be classified by the backbone that they are attached to
    • E.g. – glycerol backbone forms phosphoglycerides or glycerophospholipids
  • All lipids have a tail that is composed of long-chain fatty acids which are varied in degrees of length and saturation
    • Length and saturation level determine how the fatty acid chain will behave
    • Saturated Fatty Acid tails: will only have single bonds since the carbon atom I considered saturated when it is bonded to four atoms
      • No pi bonds
      • These have greater Van Der Waals forces and are more stable overall
      • Solid at room temp
    • Unsaturated Fatty Acid tails: include one or more double bonds
      • Double bond introduces kinks in the fatty acid chains which makes it difficult for the chains to stack and solidify
      • Tend to be liquid at room temp

  • Are the phospholipids that contain a glycerol backbone bonded by ester linkages to two fatty acids and by a phosphodiester linkage to a highly polar head group.
  • Named according to their head group since those determine the surface properties
    • E.g. – phosphatidylcholine has a choline head group
  • Head group can of any charge (pos, neg, neutral)
  • Due to the variability in membrane surface properties, these molecules are important in cell recognition, signaling, and binding

  • Have a sphingosine or sphingoid backbone
  • Many are considered phospholipids since they contain a phosphodiester linkage. However, others contain a glycosidic linkage to sugar instead.
    • Glycolipid: any lipid that is linked to a sugar
  • Sites of biological recognition at the cell surface and can be bonded to various head groups and fatty acids.
  • Sphingolipids can be divided into four categories based on their head group
    • Ceramide: single hydrogen atom as its head group
    • Sphingomyelins: major class that is also phospholipids
      • Have either phosphatidylcholine or phosphatidylethanolamine as a head group
      • Head groups have no net charge
      • Major components in the plasma membrane of cells producing myelin (oligodendrocytes and Schwann cells)
    • Glycosphingolipids: Head groups are composed of sugar bonded by glycosidic linkages
      • Found mainly on the outer surface of the plasma membrane and can be further classified as cerebrosides or globosides
      • Cerebrosides have a single sugar
      • Globosides have two or more sugars
      • These molecules have no net charge at physiological pH
    • Gangliosides: Glycolipids that have polar head groups composed of oligosaccharides with one or more NANA molecule at the terminus
      • Negative charge
      • Role in cell interaction, recognition and signal transduction

  • Esters of long-chain fatty acids with long-chain alcohols.
  • Function as protection for both plants and animals
    • In plants, waxes are secreted as a surface coating to prevent excessive evaporation and to protect against parasite
    • In animals, waxes are secreted to prevent dehydration, as water repellent and as lubricant

Signaling Lipids

  • Lipids play an active role in cellular signaling and as coenzymes
    • They serve as coenzymes in the electron transport chain and in glycosylation reactions
  • Lipids also function as hormones and aid in our ability to see light

Terpenes and Terpenoids

  • Terpenes: Class of lipids built from isoprene (C5H8) moieties
    • The metabolic precursors to steroids and other lipid signaling molecules
    • Produced by plants and some insects
    • Strongly scented, sometimes used as a protective measure
  • Terpenes are grouped according to the number of isoprene unit’s present
    • E.g. – Monoterpenes (C10H16), Sesquiterpenes (C15H24), Diterpenes (C20H32)
    • Triterpenes (6 isoprene units) can be converted to cholesterol and various steroids
    • Tetraterpenes (8 isoprene units) that make up carotenoids
  • Terpenoids (isoprenoids): derivatives of terpenes that have undergone oxygenation or rearrangement of the carbon skeleton).
    • Can be further modified by adding an extensive variety of functional groups
    • Share similar characteristics with terpenes since they are both biological precursors and aromatic
    • Named in a similar fashion as well (e.g. diterpenoids have four isoprene units)
  • In general, both molecules are precursor molecules that feed into various Biosignaling pathways that produce important products such as steroids and Vitamin A


  • Metabolic derivatives of terpenes and very different from other lipids
  • Characterized by four cycloalkane rings fused together: 3 cyclohexane and 1 cyclopentane
    • Functionality of the steroid is determined by oxidation of the rings as well as their functional groups
    • Large number of carbons and hydrogens in steroids make them non-polar
  • Steroid Hormones: are steroids that act as hormones (secreted by endocrine glands)
    • Potent biological signals that require gene expression and metabolism
    • E.g. – testosterone, estrogens, cortisol and aldosterone
  • Cholesterol: major component of the phospholipid bilayer and is responsible for mediating membrane fluidity
    • An amphipathic molecule: interactions with both the hydrophobic tails and hydrophilic tails of phospholipids allows cholesterol to maintain relatively constant fluidity in cell membranes
      • At low temps, keeps cell membrane form solidifying
      • At high temps, hold the membrane intact and prevents it from becoming too permeable
    • Also is a precursor to many molecules such as steroid hormones, bile acids or vitamin D


  • 20-carbon molecules that are unsaturated carboxylic acids
    • Derived from arachidonic acid and contain one five-carbon ring
  • Act as paracrine or autocrine signaling molecules
  • Main biological function is to regulate the synthesis of cyclic adenosine monophosphate (cAMP), which is a ubiquitous intracellular messenger
    • cAMP mediates the action of many hormones which means that prostaglandins is the root of effects on smooth muscle function, sleep-wake cycle, body temperature
  • Nonsteroidal anti-inflammatory drugs inhibit the enzyme COX, which aids in the production of prostaglandins.

Fat-Soluble Vitamins

  • Vitamin: essential nutrient that cannot be synthesized by the body
  • Lipid-Soluble Vitamins: can accumulate in stored fats while water-soluble vitamins are excreted through urine.

Vitamin A (carotene)
  • Unsaturated hydrocarbon that is important in vision, growth, development and immune function
  • Retinal: aldehyde form of Vitamin A that is component of the light-sensing molecule system in the human eye.
  • Retinol: storage form of Vitamin A
    • Can be oxidized to retinoic acid: a hormone that regulates gene expression

Vitamin D (cholecalciferol)
  • Consumed or formed in a UV-light driven reaction in the skin
  • In liver and kidneys, Vit D is converted to calcitriol which is the biologically active form of vitamin D.
    • Increases calcium and phosphate uptake in the intestines which promotes bone production
  • Rickets: lack of vitamin D which results in underdeveloped, curved long bones.

Vitamin E
  • Characterizes a group of lipids called tocopherols and tocotrienols
  • Substituted aromatic ring with a long isoprenoid side chain that are hydrophobic.
  • Tocopherols are antioxidants that destroy free radicals with their aromatic ring
    • Prevents oxidative damage

Vitamin K
  • Group of compounds including phylloquinone (K1) and menaquinones (K2)
  • Vital to the post-translation modifications required to form prothrombin – clotting factor in blood.
  • Also required to introduce calcium binding sites on several proteins

Energy Storage

  • Triacylglycerols are a class of lipids used for energy storage
  • The body preferred method of storing energy for the long term:
    • Carbon atoms of fatty acids are more reduced than those of sugars which results in yielding twice the amount of energy per gram when the triacylglycerol is oxidized.
    • Triacylglycerols are hydrophobic which means that they do not draw in water and do not require hydration for stability which subsequently decreases their weight
    • Can also serve as insulation

  • Composed of three fatty acids bonded by ester linkages to glycerol. The three fatty acids are usually not the same
  • Compounds are non-polar and hydrophobic which contributes to their insolubility in water
    • Non-polar due to polar hydroxyl groups of glycerol and polar carboxylates of fatty acids bonding together
  • Deposits are observed as oily droplets in the cytosol which serve as depots of metabolic fuel
  • Adipocytes: special cells in animals that store large amounts of fat and are found primarily under the skin, around mammary glands, and in the abdominal cavity.
  • In plants, deposits can be found in seeds as oils
  • Triacylglycerols travel bi-directionally between the liver and adipose tissue
  • Characteristics are determined by the saturation level of the fatty acids that make them up

Free Fatty Acids and Saponification

  • Free fatty acids are unesterified fatty acids with a free carboxylate group
  • Circulate in the blood by being non-covalently bonded to serum albumin
  • Saponification: the ester hydrolysis of triacylglycerols by using a strong base.
    • Lye: traditional base that is used (NaOH or KOH)
    • Results in the basic cleavage of the fatty acid which leaves the sodium salt of the fatty acid and glycerol
  • Soaps act as Surfactants: lowers the surface tension at the surface of a liquid
    • Mechanism is through the formation of micelles (tiny aggregates of soap with the hydrophobic tail turned inwards and the hydrophilic head turned outward. So these micelles can dissolve in aqueous solution
    • Non-polar compounds are then able to dissolve in the hydrophobic interior of the water-soluble micelle
    • This allows cleaning agents to dissolve both water-soluble and water-insoluble messes and wash them away together.
  • Micelles are also important in the body for absorption of fat-soluble vitamins and complicated lipids
    • Fatty acids and bile salts from the gall bladder form micelles that increase the surface area available for lipolytic enzymes

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