Biomolecules - Lipids

LIPIDS LIPIDS Lipids are a class of non-polar organic molecules that serve a variety of func1ons within cells. Unlike other classes of organic molecules, they are not composed of recurring monomers – although they may possess discrete subunits. Lipids are commonly composed of hydrocarbons arranged into either chains (fa$y acids) or fused rings (steroids). All lipids possess hydrophobic proper1es, meaning they will have low solubility in aqueous solvents (but will dissolve in non-polar solvents). This makes a lipid an effec1ve water-repellent. TYPES OF LIPIDS Lipids can be categorised into three groups based on their chemical composi1on and physical proper1es: • Simple lipids include esters of faBy acids and alcohol (waxes and triglycerides are common examples) • Compound lipids are simple lipids that have been linked to an addi1onal group (such as phospholipids) • Derived lipids consist of hydrocarbon rings produced from the hydrolysis of simple lipids (e.g. steroids) SIMPLE: TRIGLYCERIDE DERIVED: STEROID COMPOUND: PHOSPHOLIPID ESTER LINKAGES Simple and compound lipids contain faBy acids within their structure, which are covalently aBached to an alcohol (such as glycerol) via an ester bond. This linkage involves a condensaEon reacEon (water released). EtOH H O Alcohol H O O H H H H C C C C C H H H H Fa(y Acid H EtOH Water O + O H H H H C C C C C H H H H Simple Lipid H FATTY ACIDS Lipids will possess different proper1es according to the type of faBy acid they possess. Triglycerides can differ in their physical state (solid fats or liquid oils), while phospholipids will possess different levels of fluidity. FaBy acids are primarily classified according to the presence or absence of double bonds between the carbon atoms in the chain. • Saturated faBy acids possess no double bonds (saturated with H) • Unsaturated faBy acids have double bonds (either one or many) Saturated faBy acids will form straight (linear) hydrocarbon chains, while unsaturated faBy acids will have a kinked chain. Unsaturated faBy acids may be either monounsaturated (has one double bond) or polyunsaturated (mul1ple double bonds). Polyunsaturated faBy acids will possess an even greater kink in their hydrocarbon chain. SATURATED FATTY ACID MONO 1 POLY 1 2 UNSATURATED FATTY ACIDS FATS AND OILS Triglycerides can be stored by organisms as either solid fats or liquid oils depending on the type of faBy acid they possess. As saturated faBy acids have straight chains, they can be packed more 1ghtly together, which means more intermolecular forces can form between the chains. This increases the mel1ng temperature of the triglyceride, meaning triglycerides with saturated faBy acids will exist as solid fat at room temperature. Unsaturated faBy acids have kinked chains and are packed more loosely, with fewer intermolecular forces between the chains. This results in a lower mel1ng point, so they exist as liquid oil at room temperatures. The storage of triglycerides as fats or oils is determined by an organism’s physiology and environment. FaBy acids need to be as a liquid within 1ssues to allow them to be u1lised by cells, but should also be packed as 1ghtly as possible to maximise storage. Warm-blooded mammals (endotherms) will tend to produce more saturated fa$y acids – as their higher core body temperatures can keep these faBy acids liquid. However, cold-blood animals (or those living in colder climates) will tend to produce a rela1vely higher propor1on of unsaturated faBy acids – as saturated fats would likely solidify. This is why cold-water fish are an excellent source of polyunsaturated fats that are essen1al to human diets (omega-3 and omega-6 oils). Plants cannot control their internal temperatures and so predominantly produce unsaturated fa$y acids. However, plants found in warmer climates (tropical plants) will produce more saturated faBy acids than temperate plants. COMPARISON: FATS VERSUS OILS FATS OILS Found in animals (endotherms) Found in plants (and fish oils) Saturated faBy acids Unsaturated faBy acids Chains are straight and 1ghtly packed Chains are kinked and loosely packed Solid at room temperatures Liquid at room temperatures Higher mel1ng point than oils Lower mel1ng point than fats Dietary Fats: Meats Cheese Milk Eggs Dietary Oils: Canola Oil Avocados Nuts FaBy fish TRANS FATS Unsaturated faBy acids can exist in two isomeric forms. Most unsaturated faBy acids produced by organisms are cis-isomers, whereby the hydrogen atoms aBached to the carbon double bond are on the same side (causing a kink). However, unsaturated faBy acids can also be trans-isomers, when the hydrogen atoms aBached to the carbon double bond are on different sides. This will not cause the hydrocarbon chain to kink, meaning that the trans-faBy acids have similar proper1es to saturated fats. These trans-fats are not typically produced by living organisms (except ruminants) and are instead produced by industrial cooking prac1ces to improve the shelf life. cis Trans LIPID FUNCTIONS The different types of lipids serve different func1ons within cells, due to the differences in their proper1es: 1. Triglycerides func1on as a long-term energy storage molecule and also are effec1ve thermal insulators 2. Phospholipids play a structural role in cells by forming a primary component of all cellular membranes 3. Steroids may func1on as signalling molecules by binding to intracellular receptors (e.g. sex hormones) 1. TRIGLYCERIDES In animals, triglycerides are stored within a subcutaneous layer of 1ssue known as adipose Essue. This 1ssue layer func1ons as both a means for thermal insula1on and a source of energy (long-term). Fats have a very low level of thermal conduc1vity and so have a very limited capacity to conduct heat. Consequently, they are very effec1ve insulators. Many marine mammals will possess an especially thick layer of adipose 1ssue (blubber) in order to reduce the amount of heat lost to the water and also to increase its buoyancy. Triglycerides can also be u1lised as an energy storage op1on instead of carbohydrates. These lipids have key proper1es that make them beBer at storing the energy long term: • Solubility: Triglycerides are non-polar and insoluble in water (harder to transport) • Osmosis: Triglycerides will create less osmo1c pressure in cells (beBer for storage) • DigesEon: Triglycerides are harder to digest than carbohydrates (less accessible) • ATP ProducEon: Triglycerides will produce roughly twice as much energy per gram 2. PHOSPHOLIPIDS Phospholipids are the primary component of cell membranes because they are amphipathic (have hydrophilic and hydrophobic regions). Phospholipids consist of a phosphate group connected to two faBy acid chains by a central glycerol molecule. As a phosphate group is polar, it forms a head region that orientates away from the faBy acid tails. In cells, phospholipids form bilayers, with phosphate heads facing out into the aqueous solu1ons, whilst the faBy acid tails face in to hold the bilayer together (via hydrophobic associa1ons). Lipid Bilayer 3. STEROIDS Steroids are derived lipids composed of four fused carbon rings. Because they are non-polar, they can pass through the phospholipid bilayer and will commonly be u1lised as signalling molecules (hormones). Some examples of steroids include the sex hormones – oestradiol (released by ovaries) and testosterone (testes).