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Lipids | Biology for Grade 12 PDF Download

Introduction

  • Macromolecules which contain carbon, hydrogen and oxygen atoms. However, unlike carbohydrates lipids contain a lower proportion of oxygen
  • Non-polar and hydrophobic (insoluble in water)
  • There are two groups of lipid that you need to know:
    • Triglycerides (the main component of fats and oils)
    • Phospholipids

Triglycerides

  • Are non-polar, hydrophobic molecules
  • The monomers are glycerol and fatty acids
  • Glycerol is an alcohol (an organic molecule that contains a hydroxyl group bonded to a carbon atom)
  • Fatty acids contain a methyl group at one end of a hydrocarbon chain known as the R group (chains of hydrogens bonded to carbon atoms, typically 4 to 24 carbons long) and at the other is a carboxyl group
    • The shorthand chemical formula for a fatty acid is RCOOH
  • Fatty acids can vary in two ways:
    • Length of the hydrocarbon chain (R group)
    • The fatty acid chain (R group) may be saturated (mainly in animal fat) or unsaturated (mainly vegetable oils, although there are exceptions e.g. coconut and palm oil)
  • Unsaturated fatty acids can be mono or  poly-unsaturated:
    • If H atoms are on the same side of the double bond they are cis-fatty acids and are metabolised by enzymes
    • If H atoms are on opposite sides of the double bond they are trans-fatty acids and cannot form enzyme-substrate complexes, therefore, are not metabolised. They are linked with coronary heart disease
      Lipids | Biology for Grade 12Lipids | Biology for Grade 12
Examples of different types of fatty acids with the functional groups and presence of double bonds highlighted
  • Triglycerides are formed by esterification
  • An ester bond forms when a hydroxyl (-OH) group on glycerol bonds with the carboxyl (-COOH) group of the fatty acid:
    • An H from glycerol combines with an OH from the fatty acid to make water
    • The formation of an ester bond is a condensation reaction
    • For each ester bond formed a water molecule is released
    • Three fatty acids join to one glycerol molecule to form a triglyceride
    • Therefore for one triglyceride to form, three water molecules are released
      Lipids | Biology for Grade 12
Formation of a triglyceride from a glycerol molecule and three fatty acid molecules by the process of esterification.

Triglycerides: Structure & Function

Energy storage

  • The long hydrocarbon chains contain many carbon-hydrogen bonds with little oxygen (triglycerides are highly reduced)
    • So when triglycerides are oxidised during cellular respiration this causes these bonds to break releasing energy used to produce ATP
  • Triglycerides therefore store more energy per gram than carbohydrates and proteins (37kJ compared to 17kJ)
  • As triglycerides are hydrophobic they do not cause osmotic water uptake in cells so more can be stored
    • Plants store triglycerides, in the form of oils, in their seeds and fruits. If extracted from seeds and fruits these are generally liquid at room temperature due to the presence of double bonds which add kinks to the fatty acid chains altering their properties
    • Mammals store triglycerides as oil droplets in adipose tissue to help them survive when food is scarce (e.g. hibernating bears)
  • The oxidation of the carbon-hydrogen bonds releases large numbers of water molecules (metabolic water) during cellular respiration
    • Desert animals retain this water if there is no liquid water to drink
    • Bird and reptile embryos in their shells also use this water

Insulation

  • Triglycerides are part of the composition of the myelin sheath that surrounds nerve fibres
  • This provides insulation which increases the speed of transmission of nerve impulses
  • Triglycerides compose part of the adipose tissue layer below the skin which acts as insulation against heat loss (eg. blubber of whales)

Buoyancy

  • The low density of fat tissue increases the ability of animals to float more easily

Protection

  • The adipose tissue in mammals contains stored triglycerides and this tissue helps protect organs from the risk of damage

The Vital Role of Phospholipids

Structure

  • Phospholipids are a type of lipid, therefore they are formed from the monomer glycerol and fatty acids
  • Unlike triglycerides, there are only two fatty acids bonded to a glycerol molecule in a phospholipid as one has been replaced by a phosphate ion (PO43-)
  • As the phosphate is polar it is soluble in water (hydrophilic)
  • The fatty acid ‘tails’ are non-polar and therefore insoluble in water (hydrophobic)
    Lipids | Biology for Grade 12
Phospholipids are the major components of cell surface membranes. They have fatty acid tails that are hydrophobic and a phosphate head, that is hydrophilic, attached to a glycerol molecule.
  • Phospholipids are amphipathic (they have both hydrophobic and hydrophilic parts)
  • As a result of having hydrophobic and hydrophilic parts phospholipid molecules form monolayers or bilayers in water
    Lipids | Biology for Grade 12
In the presence of water due to the hydrophobic and hydrophilic parts phospholipids will form monolayers or bilayers.

Role

  • The main component (building block) of cell membranes
  • Due to the presence of hydrophobic fatty acid tails, a hydrophobic core is created when a phospholipid bilayer forms
    • This acts as a barrier to water-soluble molecules
  • The hydrophilic phosphate heads form H-bonds with water allowing the cell membrane to be used to compartmentalise
    • This enables the cells to organise specific roles into organelles helping with efficiency
  • Composition of phospholipids contributes to the fluidity of the cell membrane
    • If there are mainly saturated fatty acid tails then the membrane will be less fluid
    • If there are mainly unsaturated fatty acid tails then the membrane will be more fluid
  • Phospholipids control membrane protein orientation
    • Weak hydrophobic interactions between the phospholipids and membrane proteins hold the proteins within the membrane but still allow movement within the layer

Phospholipids v Triglycerides

Lipids | Biology for Grade 12

Biochemical Tests: Lipids

  • The emulsion test that can be carried out quickly and easily in a lab to determine if a sample contains lipids
  • This test is qualitative - it does not give a quantitative value as to how much lipid may be present in a sample
  • Lipids are nonpolar molecules that do not dissolve in water but will dissolve in organic solvents such as ethanol
  • Add ethanol to the sample to be tested, shake to mix and then add the mixture to a test tube of water
  • If lipids are present, a milky emulsion will form (the solution appears ‘cloudy’); the more lipid present, the more obvious the milky colour of the solution
  • If no lipid is present, the solution remains clear
    Lipids | Biology for Grade 12
The Emulsion test for lipids forms a milky colour.

Lipid Diagrams & Properties

  • Triglycerides are a form of lipid
  • They are made up of one molecule of glycerol with three fatty acids attached to it
  • These fatty acids have long hydrocarbon ‘tails’
  • Fatty acids occur in two forms:
    • saturated fatty acids
    • unsaturated fatty acids
  • The difference between these fatty acid types is found in their hydrocarbon tails

Saturated fatty acids

  • In saturated fatty acids, the bonds between the carbon atoms in the hydrocarbon tail are all single bonds
  • The fatty acid is said to be ‘saturated’ with hydrogen
    • This means that each carbon atom in the hydrocarbon tail (except for the final carbon atom) is bonded to two hydrogen atoms
      Lipids | Biology for Grade 12
An example of a saturated fatty acid

Unsaturated fatty acids

  • In unsaturated fatty acids, the bonds between the carbon atoms in the hydrocarbon tail are not all single bonds
    • There is at least one carbon-carbon double bond (in some unsaturated fatty acids there are many carbon-carbon double bonds)
    • These double bonds cause the hydrocarbon tail of unsaturated fatty acids to kink (bend slightly), meaning they are not as straight as saturated fatty acids
  • The fatty acid is said to be ‘unsaturated’ because the hydrocarbon tail does not contain the maximum number of hydrogen atoms possible
    • This is because each carbon atom in a carbon-carbon double bond can only bond to one hydrogen atom (instead of two)
      Lipids | Biology for Grade 12

Properties of triglycerides

  • Triglycerides are mainly used as energy storage molecules
  • This is because the long hydrocarbon tails of the fatty acids in triglycerides contain large amounts of chemical energy, which can be released when the fatty acids are broken down
  • Triglycerides are also suitable as energy storage molecules because they are insoluble, meaning that they don’t affect the water potential inside the cell
  • Inside cells, triglycerides form insoluble droplets, with the hydrophobic (water-repelling) fatty acids on the inside and the glycerol molecules on the outside
    Lipids | Biology for Grade 12

Triglycerides are suitable as energy storage molecules as they form insoluble droplets inside cells

Properties of phospholipids

  • Phospholipids are another kind of lipid
  • Phospholipids are similar in structure to triglycerides
  • In phospholipids, one of the three fatty acid molecules attached to glycerol is replaced by a phosphate group
  • This phosphate group is hydrophilic (water-loving), whereas the two fatty acids are hydrophobic (like in triglycerides)
  • This makes phospholipids suitable for making up the bilayer of cell membranes, with the fatty acids facing inwards and the phosphate groups facing outwards
  • This is also useful as it means the centre of the phospholipid bilayer is hydrophobic, meaning water-soluble substances cannot easily pass through
  • This allows the cell membrane to act as a barrier, controlling what substances enter and leave the cell
    Lipids | Biology for Grade 12
Phospholipids are suitable for making up cell membranes as they form a bilayer
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