Notes For Lipids

• Lipids 


  Lipids are a class of organic molecule composed primarily of hydrocarbon chains (fatty acids) or rings (steroids) 

  • Hydrocarbons are non-polar (they lack charged regions) and will not dissolve in polar substances like water
  • Hence lipids are considered to be hydrophobic molecules (they are water-repelling)
• Hydrophobic properties of lipids


  The hydrophobic properties of lipids have many important biological consequences

  Waxes are used to prevent water loss from leaves while birds coat their feathers with oil to render them waterproof

  Phospholipids provide a structural framework for cells by forming spontaneous membranes in aqueous solutions

  Lipids in foods help the body to absorb certain fat-soluble micronutrients, including vitamins A and D
• Transportation of Lipids and Steroid Hormones


  The hydrophobic properties of lipids make them difficult to transport around the body 

  • Digested fats are packaged within a protein coat to become water soluble lipoproteins
  • Steroid hormones are bound to carrier proteins (such as albumin) to facilitate their movement through the bloodstream
• Amphipathic Lipids and Membrane Integrity


  Certain lipids have polar components, becoming amphipathic (having both hydrophobic and hydrophilic regions)

  • Phospholipids possess a polar phosphate head, cholesterol has a polar hydroxyl group and glycolipids can have polar carbohydrate chains
  • These amphipathic molecules all have a limited capacity to interact with water, helping them to function in maintaining membrane integrity
• Amphipathic Lipids and Membrane Integrity


  Certain lipids have polar components, becoming amphipathic (having both hydrophobic and hydrophilic regions)

  • Phospholipids possess a polar phosphate head, cholesterol has a polar hydroxyl group and glycolipids can have polar carbohydrate chains
  • These amphipathic molecules all have a limited capacity to interact with water, helping them to function in maintaining membrane integrity

  
• Formation of triglycerides and phospholipids by condensation reactions


  Unlike other biological macromolecules, lipids are not polymers as they do not contain monomers (recurring subunits)

  • However, most types of lipids (excluding steroids) contain at least one fatty acid chain in their structure 
• Fatty acids


  Fatty acids can be linked to the hydroxyl group of alcohols via condensation reactions to produce an ester linkage

  • Triglycerides are composed of three fatty acid chains linked to a single glycerol molecule
  • In phospholipids, one of the fatty acid chains is replaced by a polar phosphate group

  
• Main Classes of Lipids


  Lipids can be classified in a number of different ways, according to their chemical composition, fatty acids and capacity to form soap

  • Three commonly recognised groups of lipids are simple lipids, compound lipids and derived lipids
• Simple Lipids 


  Simple lipids include esters of fatty acids and alcohol (e.g. glycerol)

  Waxes consist of a fatty acid chain linked to a long chain alcohol group and are used to create waterproof surfaces

  Triglycerides consist of three fatty acid chains linked to a glycerol molecule and are used for energy storage and insulation
• Compound Lipids


  Compound lipids include esters of fatty acids and alcohol linked to an additional group

  • Phospholipids consist of a glycerol molecule linked to two fatty acid chains and one phosphate group – they are a principle component of cell membranes
  • Glycolipids consist of a carbohydrate linked to an ester of fatty acids and alcohol – they function to facilitate cell-cell interactions
• Derived Lipids


  Derived lipids are substances produced from the hydrolysis of simple or compound lipids

  • They include signalling molecules such as steroids and prostaglandins, as well as cholesterol

  
• Similarities and differences between Fatty acids

  Fatty acids are long hydrocarbon chains that are found in most types of lipids (excluding steroids)

  • Fatty acids may differ in the length of the hydrocarbon chain, but most typically contain between 4 – 24 carbons
• Types of Fatty Acids

  Fatty acids can be classified according to the presence or absence of double bonds between the carbon atoms in the hydrocarbon chain

  • Saturated fatty acids possess straight hydrocarbon chains with no double bonds (they have the maximum possible number of H atoms)
  • Unsaturated fatty acids have double bonds – they can be either mono-unsaturated (1 double bond) or poly-unsaturated (>1 double bond)
• Unsaturated fatty acids 


  Unsaturated fatty acids may occur in either of two distinct structural configurations – cis and trans isomers

  • Cis: The hydrogen atoms attached to the carbon double bond are on the same side, causing the hydrocarbon chain to kink
  • Trans: The hydrogen atoms attached to the carbon double bond are on different sides, meaning the hydrocarbon chain remains straight

  In living organisms, unsaturated fatty acids are nearly always in the cis configuration – only ruminant animals (cows and sheep) produce trans fatty acids

  
• Fats versus Oils


  Living organism store their lipids as either fats or oils depending on the type of fatty acid involved (saturated or cis-unsaturated)

  • These fatty acids differ in the shape of their hydrocarbon chains (straight or bent)
• Fats (Saturated)


  • Saturated fatty acids have straight chains that can be more tightly packed, making them more efficient for energy storage
  • However, this tight packaging also increasing the number of intermolecular forces between the fatty acid chains, resulting in a higher melting point
  • This means it takes higher temperatures to keep them liquid and they will typically exist as fats (solid at a room temperature of 25ºC)
• Oils (Unsaturated)


  • Unsaturated (cis) fatty acids have kinked chains that cause them to be more loosely packed (poly-unsaturated fatty acids have multiple kinks and are consequently even more dispersed)
  • This means there are fewer intermolecular forces and less energy is required to separate the fatty acids, resulting in a lower melting point
  • Consequently, they will remain liquid at cooler temperatures and so usually exist as oils (liquid at a room temperature of 25ºC)
• Fats vs Oil Comparison

  
• Lipid Storage


  The storage of lipids as either fats or oils is primarily a consequence of an organism’s physiology and environment

  • Organisms need fatty acids that will remain liquid at their normal body temperature (to allow them to be utilized by the body)
  • However, organisms will try to produce the most tightly packed fatty acids possible in order to maximize their energy storage
• Animals


  Endotherms (warm blooded mammals) will tend to produce more saturated fats – as their higher core body temperatures can keep these fatty acids liquid

  • Conversely, ectotherms living in cold environments tend to produce cis-unsaturated oils – as saturated fats would solidify in the colder temperatures
  • This is why cold water fish oils are a rich source of poly-unsaturated fats that are essential to a human diet (omega-3 and omega-6)
• Plants


  As plants cannot control their own internal temperatures, they also predominantly produce cis-unsaturated liquid oils

  • However, tropical plants (found in warmer climates) will tend to produce comparatively more saturated fats than temperate plants (found in cooler climates)
• Energy Storage


  Triglycerides in adipose tissues are used for long-term energy storage in animals

  • Triglycerides can store roughly twice as much energy per gram as carbohydrates and do not contribute to the osmotic pressure of the cell (as they are non-polar)
  • Triglycerides are more difficult to digest (they can only be broken aerobically) and cannot be easily transported due to their hydrophobicity

  Consequently, triglycerides are more suitable for long-term energy storage whereas carbohydrates (glycogen) are more suitable for short-term energy storage

  
• Thermal Insulation


  Triglycerides have low thermal conductivity, meaning they have a limited capacity to conduct heat and are effective thermal insulators

  Mammals living in cold or aquatic environments (such as the ringed seal) will possess thick layers of subcutaneous fat to insulate their internal organs against cold exposure

  Overweight individuals tend to cool down more slowly in comparison to others. This slower cooling process can make them more vulnerable to heat stress due to the increased heat retention in their bodies.
• Structure


  Phospholipids are one of the key structural components of all cell membranes that are responsible for the formation of lipid bilayers

  • Phospholipids consist of a polar head (hydrophilic) composed of a glycerol and a phosphate molecule and two non-polar tails (hydrophobic) composed of fatty acid chains
  • Because phospholipids contain both hydrophilic (water-loving) and lipophilic (fat-loving) regions, they are classed as amphipathic
• Phospholipids 


  Phospholipids spontaneously arrange into a bilayer with the two hydrophobic tails being shielded from the surrounding polar fluids by the outward facing hydrophilic heads 

  • The phospholipid bilayer is only held together by the weak hydrophobic associations between the non-polar tails, making the bilayer both fluid and flexible 
  • Because phospholipids contain both hydrophilic (water-loving) and lipophilic (fat-loving) regions, they are classed as amphipathic

  
• Steroids


  Steroids are lipids that are composed of four fused carbon rings that are non-polar and lipophilic (fat-loving) 

  • Steroids do not resemble other types of lipids (they do not contain a fatty acid chain), but share their basic chemical properties (they are hydrophobic)
• Steroid hormones


  • A steroid that functions as a signalling molecule within the body. They are lipophilic, so they can freely diffuse across the phospholipid bilayer and bind to receptors within the target cell
  • They are hydrophobic, so they cannot be freely transported within the bloodstream and must be bound to carrier proteins (e.g. albumin)
• Steroid hormone synthesis


  Generally synthesised from cholesterol in either the adrenal gland (corticosteroids) or the gonads (sex steroids). Sex steriods Oestradiol (a type of oestrogen) and Testosterone
• The diagram shows a molecular structure. 


  The molecular structure of Lipids

  
• Type(s) of fatty acid in the diet is/are positively correlated with an increased risk of coronary heart disease 


  Saturated & Trans unsaturated
• Saturated Fats


  • Increase LDL cholesterol.
  • Contribute to plaque buildup in arteries.
  • Raise risk of heart disease and stroke.
  • Promote inflammation.
• Trans Fats


  • Increase LDL cholesterol and decrease HDL cholesterol.
  • Greatly elevate risk of heart disease.
  • Promote inflammation.
  • May contribute to diabetes through insulin resistance.
• Excess amount of stired fat tissue

  Body Mass Index : BMI. BMI = Mass(KG) / Height(m²). BMI could indicate if a person is overweight/obese/too heavy for their height;

  Overweight/obesity increases the probability of developing type II diabetes;
• Comparing and Contrasting cis-fatty acids and trans-fatty acids

  
• Endotherms


  Endotherm: An organism that maintains its body temperature through internal heat production. Example: Polar bear uses fat as insulation from the cold.