Biology: 3.1.3 The Nature of Lipids PDF

Biology (16 - 18) The Lipids © SSER Ltd.  What Are Lipids? Lipids are a diverse collection of substances that have a range of different functions in living systems. Lipids are compounds that serve both as structural...

Biology (16 - 18) The Lipids © SSER Ltd.  What Are Lipids? Lipids are a diverse collection of substances that have a range of different functions in living systems. Lipids are compounds that serve both as structural and nutrient substances. The lipid group includes (amongst others)...  fatty acids  glycerides (includes fats and oils)  phospholipids  steroids (sterols are one class of steroids) Lipids are soluble in nonpolar organic solvents (e.g. ethanol, ether, chloroform, acetone & benzene) and are generally insoluble in water.  Lipid Emulsion Test  The material to be tested is placed in a test tube.  Approximately 2cm3 of ethanol is added and the tube is shaken.  The ethanol layer is poured off into a second test tube.  Approximately 2cm3 of water is added to the ethanol. A milky emulsion confirms the presence of a lipid such as a fat or oil. Fatty Acids  Saturated & Unsaturated Fatty Acids Fatty acids can be saturated or unsaturated… Saturated fatty acid General formula for a saturated fatty acid. Double bond Unsaturated fatty acid: less saturated with hydrogen atoms.  Saturated Fatty Acids All the linkages between the carbon atoms in the hydrocarbon chain are single bonds. These fatty acids are therefore saturated with hydrogen atoms (no double bonds). Saturated fats consist entirely of saturated fatty acids and are the animal fats. Saturated fats are solid at room temperature.  Unsaturated Fatty Acids Unsaturated fats contain unsaturated fatty acids and thus possess a number of double bonds. In general, the greater the number of double bonds, the lower the temperature at which the lipid melts. The large number of double bonds in vegetable oils accounts for their liquid state. Glycerides (Fats & Oils)  Esters - General Structure When an alcohol reacts with an acid an ester is formed with the elimination of water (condensation reaction). In living things the acid is a carboxylic acid and the alcohol is usually glycerol. R1COOH + R2OH => R1COOR2 + H2O carboxylic acid + alcohol => ester + water The R1 and R2 stand for alkyl (hydrocarbon) groups such as CH3, C2H5, C3H7 or C4H9. Alkyl groups are alkanes with one less hydrogen atom. general structure of a carboxylic acid  Esters - General Structure If the carboxylic acid is a fatty acid and the alcohol is glycerol then the resulting esters are termed glycerides. Glycerides are one of the main classes of lipids, and make up the bulk of animal fats and vegetable oils. Esters with low molecular weight are commonly used as fragrances and found in essential oils and pheromones. Phosphoesters form the backbone of DNA molecules. The R1 and R2 can be one of many different alkyl groups and as any carboxylic acid can react with any alcohol then many different esters general structure exist in biological systems. of an ester  The Components of a Glyceride Glycerol is a 3-carbon alcohol molecule. Fatty acids are composed of hydrocarbon chains of varying length with a methyl group at one end and a carboxyl group at the other. A Fatty Acid Glycerol Carboxyl group Hydrocarbon chain Methyl group  Monoglycerides and Diglycerides Monoglycerides - a single fatty acid molecule bonds with the glycerol molecule. +H2O Condensation Reaction Diglycerides - two fatty acid molecules bond with the glycerol molecule. +2H2O Condensation Reaction  Triglycerides Triglycerides - three fatty acid molecules bond with the glycerol molecule. Triglyceride  Triglyceride Formation (Condensation Reaction) Glycerol Three fatty acid molecules Triglyceride Ester bond +3H2O Condensation Reaction  The Complete Hydrolysis of a Triglyceride Ester bond Triglyceride +3H2O Condensation Reaction Three fatty acid Glycerol molecules Triglycerides – Fats & Vegetable Oils Fats & vegetable oils consist mostly of triglycerides.  Functions of Triglycerides – Energy Storage (1) Energy storage  Fats and oils have a much higher calorific value than carbohydrates or proteins.  One gram of fat yields 38 kJ of energy on complete oxidation.  One gram of carbohydrate or protein yields 17 kJ of energy on complete oxidation.  Fats and oils oxidise more slowly than carbohydrates and a major function of these biomolecules is to act as an energy store.  Hibernating animals store extra fat.  Fats and oils are found in the seeds and fruits of many plants where they function as an energy store for germinating seeds and young plants. Many of these oil-storing plants are important agricultural crops. Functions of Triglycerides – Energy Storage (2) Fats and oils are stored in the seeds and fruits of many plant species and extracted for commercial purposes. Oils extracted from oilseed rape, sunflower seeds, coconuts and peanuts are used for the production of margarine and cooking oil.  Functions of Triglycerides – Energy Storage (3) Many hibernating animals store additional subcutaneous fat to serve as an energy store.  Functions of Triglycerides – Insulation (1)  Fat is a useful insulator as it conducts heat fairly slowly.  Large quantities of subcutaneous fat are found in various mammals that live in cold climates and in aquatic mammals where heat conservation and buoyancy are essential. Seals store large amounts of fat under the skin as blubber (an efficient insulator) and to aid in buoyancy.  Functions of Triglycerides – Insulation (2)  Fat is a useful insulator as it conducts heat fairly slowly.  Large quantities of subcutaneous fat are found in various mammals that live in cold climates and in aquatic mammals where heat conservation and buoyancy are essential. Polar bears thrive in arctic conditions and store large amounts of fat under the skin as blubber - which is an efficient insulator.  Functions of Triglycerides – Source of Water  When fats are oxidised during respiration, a large amount of water is released.  Many desert animals store fat as a source of this metabolic water. Arabian camel Hamster Bactrian camel Desert mammals store fat - the camel stores fat in the hump - a primary use of this stored fat is to generate metabolic water. Gram for gram, fats contain more hydrogen than carbohydrates – it is the hydrogen portion of carbohydrates and fats that yields water during aerobic respiration. Phospholipids  Phospholipid Structure (1) Glycerol Fatty acid Fatty acid Phosphate group Polar end of the molecule (hydrophilic) – attracts water. Phospholipids are diglycerides and form the fabric of the cell membrane.  Phospholipid Structure (2) Hydrophobic tails OR Hydrophilic phosphate group  Phospholipid Structure (3) The phospholipid molecule has a polar phosphate – Polar phosphate containing head group and head (hydrophilic) two hydrophobic fatty acid tails. The tails vary in length and may have one or more double bonds. Fatty acid tails Each double bond creates (hydrophobic) a kink in the tail. The differences in tail length and the presence of double bonds are important for influencing the fluidity Kink due to the presence of a double bond. of the membrane.  Phospholipid Structure (4) The hydrophilic head consists of a phosphate Polar hydrophilic head group and glycerol group and is soluble in water. The hydrophobic tail consists of fatty acid tails which orientate themselves away from a watery Non-polar medium. hydrophobic tail group Two non-polar fatty acid tails are bonded to the hydrophilic head group by ester bonds. The bipolar nature of phospholipids allows these molecules to form bilayers that form a major component of cell membranes.  Membrane Structure Both Bacteria and Eukaryotes have glycerol-ester lipids. The ester bond has a central C=O functional group (carbonyl group) and two other functional groups represented by R1 and R2. R1 and R2 can be a variety of functional groups R1 can also be a hydrogen atom. In a glyceride R1 and R2 are the hydrocarbon parts of the carboxylic acid and the alcohol respectively. Ester bonds convey certain chemical and physical properties such as an ability to participate in Hydrogen bonding (hydrogen-bond acceptor) and esters are structurally flexible because some rotation of the functional groups about the ester bond is possible. Esters are volatile.  The Archaea – Unusual Membrane Structure The Archaea have membranes with glycerol-ether lipids (bacteria and eukaryotes have glycerol-ester lipids). The ether bonds are chemically more resistant than ester bonds which allows the archaea to survive extremes of temperature, acidity or alkalinity. Archaean cells have isoprene chains (with side branches) where eukaryotes and bacteria have fatty acid chains. The branching structure stops the membranes leaking at high temperatures. In archaean cells the glycerol molecule is the mirror image (optical isomer) of the glycerol found in eukaryotes and bacteria. Archaean lipid layers Archaea - phospholipid may fuse into a monolayer as shown in the diagram, e.g. F. acidophilum. This lipid monolayer Eukarya & Bacteria - phospholipid makes the membrane more rigid and better able to resist harsh environments. Steroids  Steroids - Cholesterol Steroids are a class of organic molecules that are built around a characteristic four-ringed skeleton containing 17 carbon atoms. Cholesterol is one of the most common animal steroids. Cholesterol Apart from its role as a stabilising component of cell membranes, cholesterol is also the precursor for the synthesis of many important steroid hormones.  Cholesterol in the Phospholipid Bilayer Cholesterol molecules are located between the tails of the phospholipid molecules where they serve to stabilise the membrane. Many hormones, alkaloids and vitamins are steroids. phospholipid bilayer of cell membrane cholesterol stabilising the membrane  Steroids - Sex Hormones Testosterone Progesterone The steroid sex hormones testosterone, progesterone and oestrogen are synthesised from cholesterol.  More Lipids (Extension Work) The lipid group includes more classes of molecules in addition to those already mentioned – each has very different chemical and physical properties.  Glycolipids & Membrane Structure Glycolipids are an integral part of cell membranes and form a significant part of the structure of the myelin sheath surrounding nerve cells. Glycolipids and glycoproteins form part of the external structure of the membrane. carbohydrate carbohydrate glycoprotein group glycolipid group bimolecular phospholipid layer extrinsic intrinsic cholesterol stabilising protein protein the membrane Glycolipids play a part in communication between cells and cell to cell recognition. Cholesterol molecules are positioned within the bilayer close to the fatty acid chains; these molecules partially immobilise these chains and help to stabilise the membrane.  Waxes (1) Waxes are lipids with waterproof properties. They provide a protective covering for leaves and stems to restrict water loss and they coat the feathers and fur of many birds and mammals. Waxes also serve in construction of the honeycombs of a beehive. Wax coats the exoskeleton of arthropods such as insects and crustaceans. Waxes also protect the feathers of birds and the fur of mammals, e.g. lanolin. Wax forms the cuticle of leaves – conveying a waterproof layer which resists water loss.  Waxes (2) Wax forms the structure of the honeycomb in a beehive. beeswax candle Wax forms the cuticle of leaves – conveying a waterproof layer which resists water loss. Waxes consist of long alkyl chains. Natural waxes often contain esters of carboxylic acids and long chain alcohols. Plant waxes contain ester bonds less frequently than animal waxes.  Terpenes & Plant Resin Terpenes are lipids found in many plants (especially conifers) and some animal groups (e.g. insects). Terpenes are the major components of resin and thereby protect the plants that produce them by deterring herbivores and by attracting predators and parasites of herbivores. resin from a conifer amber is fossilized plant resin  Lipids - Summary (1)  Fats & oils are made up of carbon, hydrogen and oxygen atoms.  The building blocks (monomers) of fats and oils are glycerol & fatty acid molecules.  Fats & oils are triglycerides.  Three fatty acid molecules bond to each glycerol molecule by condensation reactions.  The bonds formed from these condensation reactions are called ester bonds.  Fats & oils are chemically similar but physically different.  Fats are solid at room temperature whereas oils are liquid.  Fats are detected by the alcohol emulsion test.  Monoglycerides & diglycerides also form when glycerol and fatty acids bond by condensation reactions.  Lipids - Summary (2)  Monoglycerides form when only one fatty acid bonds with a glycerol molecule.  Diglycerides form when two fatty acids bond with a glycerol molecule.  Phospholipids are diglycerides.  Phospholipids form when two fatty acids and a phosphate group bond to a glycerol molecule.  The phosphate end of the molecule is hydrophilic (water - loving) and the two fatty acids tails are hydrophobic (water - hating).  Phospholipids are a major structural component of cell membranes.  Steroids (includes sterols), waxes and terpenes are also classed as lipids and fulfil a diversity of functions in the plant and animal kingdoms. End Show Copyright © SSER Ltd. and its licensors. 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