Chapter 5 Lipids - PDF

 Chemistry of lipids 0 Macromolecules which contain carbon, hydrogen and oxygen atoms. However, unlike carbohydrates lipids contain a lower proportion of oxygen 0 Non-polar and hydrophobic (insoluble in water) 0 There are two groups of lipid that you need to know: 0 Triglycerides (the ma...

 Chemistry of lipids 0 Macromolecules which contain carbon, hydrogen and oxygen atoms. However, unlike carbohydrates lipids contain a lower proportion of oxygen 0 Non-polar and hydrophobic (insoluble in water) 0 There are two groups of lipid that you need to know: 0 Triglycerides (the main component of fats and oils) 0 Phospholipids  Triglycerides 0 Are non-polar, hydrophobic molecules 0 The monomers are glycerol and fatty acids 0 Glycerol is an alcohol (an organic molecule that contains a hydroxyl group bonded to a carbon atom) 0 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 0 The shorthand chemical formula for a fatty acid is RCOOH 0 Fatty acids can vary in two ways: 0 Length of the hydrocarbon chain (R group) 0 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) 0 Unsaturated fatty acids can be mono or poly- unsaturated: 0 If H atoms are on the same side of the double bond they are cis-fatty acids and are metabolised by enzymes 0 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  Examples of different types of fatty acids with the functional groups and presence of double bonds highlighted 0 Triglycerides are formed by esterification 0 An ester bond forms when a hydroxyl (-OH) group on glycerol bonds with the carboxyl (-COOH) group of the fatty acid: 0 An H from glycerol combines with an OH from the fatty acid to make water 0 The formation of an ester bond is a condensation reaction 0 For each ester bond formed a water molecule is released 0 Three fatty acids join to one glycerol molecule to form a triglyceride 0 Therefore for one triglyceride to form, three water molecules are released Lipids with biological activities 0 Energy storage 0 The long hydrocarbon chains contain many carbon- hydrogen bonds with little oxygen (triglycerides are highly reduced) 0 So when triglycerides are oxidised during cellular respiration this causes these bonds to break releasing energy used to produce ATP 0 Triglycerides therefore store more energy per gram than carbohydrates and proteins (37kJ compared to 17kJ) 0 As triglycerides are hydrophobic they do not cause osmotic water uptake in cells so more can be stored 0 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 0 Mammals store triglycerides as oil droplets in adipose tissue to help them survive when food is scarce (e.g. hibernating bears)  Insulation 0 Triglycerides are part of the composition of the myelin sheath that surrounds nerve fibres 0 This provides insulation which increases the speed of transmission of nerve impulses 0 Triglycerides compose part of the adipose tissue layer below the skin which acts as insulation against heat loss (eg. blubber of whales)  Buoyancy 0 The low density of fat tissue increases the ability of animals to float more easily 0 The oxidation of the carbon-hydrogen bonds releases large numbers of water molecules (metabolic water) during cellular respiration 0 Desert animals retain this water if there is no liquid water to drink 0 Bird and reptile embryos in their shells also use this water  Protection 0 The adipose tissue in mammals contains stored triglycerides and this tissue helps protect organs from the risk of damage The Vital Role of Phospholipids 0 Structure 0 Phospholipids are a type of lipid, therefore they are formed from the monomer glycerol and fatty acids 0 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-) 0 As the phosphate is polar it is soluble in water (hydrophilic) 0 The fatty acid ‘tails’ are non-polar and therefore insoluble in water (hydrophobic) 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. 0 Phospholipids are amphipathic (they have both hydrophobic and hydrophilic parts) 0 As a result of having hydrophobic and hydrophilic parts phospholipid molecules form monolayers or bilayers in water  In the presence of water due to the hydrophobic and hydrophilic parts phospholipids will form monolayers or bilayers. 0 Role 0 The main component (building block) of cell membranes 0 Due to the presence of hydrophobic fatty acid tails, a hydrophobic core is created when a phospholipid bilayer forms 0 This acts as a barrier to water-soluble molecules 0 The hydrophilic phosphate heads form H-bonds with water allowing the cell membrane to be used to compartmentalise 0 This enables the cells to organise specific roles into organelles helping with efficiency 0 Composition of phospholipids contributes to the fluidity of the cell membrane 0 If there are mainly saturated fatty acid tails then the membrane will be less fluid 0 If there are mainly unsaturated fatty acid tails then the membrane will be more fluid 0 Phospholipids control membrane protein orientation 0 Weak hydrophobic interactions between the phospholipids and membrane proteins hold the proteins within the membrane but still allow movement within the layer Phospholipids vs Triglycerides  Resolution and analysis of lipids 0 The emulsion test that can be carried out quickly and easily in a lab to determine if a sample contains lipids 0 This test is qualitative - it does not give a quantitative value as to how much lipid may be present in a sample 0 Lipids are nonpolar molecules that do not dissolve in water but will dissolve in organic solvents such as ethanol 0 Add ethanol to the sample to be tested, shake to mix and then add the mixture to a test tube of water 0 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 0 If no lipid is present, the solution remains clear The Emulsion test for lipids forms a milky colour. Properties of triglycerides 0 Triglycerides are mainly used as energy storage molecules 0 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 0 Triglycerides are also suitable as energy storage molecules because they are insoluble, meaning that they don’t affect the water potential inside the cell 0 Inside cells, triglycerides form insoluble droplets, with the hydrophobic (water-repelling) fatty acids on the inside and the glycerol molecules on the outside Triglycerides are suitable as energy storage molecules as they form insoluble droplets inside cells Properties of phospholipids 0 Phospholipids are another kind of lipid 0 Phospholipids are similar in structure to triglycerides 0 In phospholipids, one of the three fatty acid molecules attached to glycerol is replaced by a phosphate group 0 This phosphate group is hydrophilic (water-loving), whereas the two fatty acids are hydrophobic (like in triglycerides) 0 This makes phospholipids suitable for making up the bilayer of cell membranes, with the fatty acids facing inwards and the phosphate groups facing outwards 0 This is also useful as it means the centre of the phospholipid bilayer is hydrophobic, meaning water-soluble substances cannot easily pass through 0 This allows the cell membrane to act as a barrier, controlling what substances enter and leave the cell Phospholipids are suitable for making up cell membranes as they form a bilayer  Fat soluble 0 The fat-soluble vitamins include vitamins A, D, E, and K. 0 Fat-soluble vitamins play integral roles in a multitude of physiological processes such as vision, bone health, immune function, and coagulation.  A retinal or β-carotene Sources Yellow and orange fruits and vegetables, dark green leafy vegetables, eggs, milk, liver Function Eye and bone development, immune function Problems associated with deficiency Night blindness, epithelial changes, immune system deficiency A  D cholecalciferol Sources Dairy products, egg yolks; also synthesized in the skin from exposure to sunlight Function Aids in calcium absorption, promoting bone growth Problems associated with deficiency Rickets, bone pain, muscle weakness, increased risk of death from cardiovascular disease, cognitive D impairment, asthma in children, cancer  E tocopherols Sources Seeds, nuts, vegetable oils, avocados, wheat germ Function Antioxidant Problems associated with deficiency E Anemia  K phylloquinone Sources Dark green leafy vegetables, broccoli, Brussels sprouts, cabbage Function Blood clotting, bone health Problems associated with deficiency Hemorrhagic disease of newborn in infants; uncommon in adults K Constituents of membranes 0 The cell membranes of all organisms generally have a similar structure 0 Cell membranes contain several different types of molecules: 0 Three types of lipid: 0 Phospholipids 0 Cholesterol 0 Glycolipids (also containing carbohydrates) 0 Two types of proteins: 0 Glycoproteins (also containing carbohydrates) 0 Other proteins (eg. transport proteins) 0 Phospholipids: 0 Form a bilayer (two layers of phospholipid molecules) 0 Hydrophobic tails (fatty acid chains) point in towards the membrane interior 0 Hydrophilic heads (phosphate groups) point out towards the membrane surface 0 Individual phospholipid molecules can move around within their own monolayers by diffusion 0 Form the basic structure of the membrane (phospholipid bilayer) 0 The tails form a hydrophobic core comprising the innermost part of both the outer and inner layer of the membrane 0 Act as a barrier to most water-soluble substances (the non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane) 0 This ensures water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in 0 Can be chemically modified to act as signalling molecules by: 0 Moving within the bilayer to activate other molecules (eg. enzymes) 0 Being hydrolysed which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm 0 Cholesterol: 0 Cholesterol molecules also have hydrophobic tails and hydrophilic heads 0 Fit between phospholipid molecules and orientated the same way (head out, tail in) 0 Are absent in prokaryotes membranes 0 Cholesterol regulates the fluidity of the membrane 0 Cholesterol molecules sit in between the phospholipids, preventing them from packing too closely together when temperatures are low; this prevents membranes from freezing and fracturing. 0 Interaction between cholesterol and phospholipid tails also stabilises the cell membrane at higher temperatures by stopping the membrane from becoming too fluid 0 Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together 0 Cholesterol also contributes to the impermeabilty of the membrane to ions and increases mechanical strength and stability of membranes; without it membranes would break down and cells burst 0 Glycolipids: 0 These are lipids with carbohydrate chains attached 0 These carbohydrate chains project out into whatever fluid is surrounding the cell (they are found on the outer phospholipid monolayer) 0 Glycolipids and glycoproteins contain carbohydrate chains that exist on the surface (the periphery/extrinsically), which enables them to act as receptor molecules 0 This allows glycolipids and glycoproteins to bind with certain substances at the cell’s surface 0 There are three main receptor types: 0 signalling receptors for hormones and neurotransmitters 0 receptors involved in endocytosis 0 receptors involved in cell adhesion and stabilisation (as the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell 0 Some act as cell markers or antigens, for cell-to-cell recognition (eg. the ABO blood group antigens are glycolipids and glycoproteins that differ slightly in their carbohydrate chains)  Solute transport across membrane Term Meaning Type of transport that does not require energy to Passive transport occur A region of space over which the concentration of a Concentration gradient substance changes The quality of a membrane that allows substances Permeability to pass through it The state at which a substance is equally Equilibrium distributed throughout a space Term Meaning Type of transport that requires an input of energy Active transport to occur A region of space over which the concentration of Concentration gradient a substance changes Adenosine triphosphate, the primary energy ATP carrier in living things Types of passive transport 0 Diffusion 0 During diffusion, substances move from an area of high concentration to an area of low concentration, until the concentration becomes equal throughout a space. 0 This is also true for some substances moving into and out of cells. 0 Because the cell membrane is semipermeable, only small, uncharged substances like carbon dioxide and oxygen can easily diffuse across it. 0 Charged ions or large molecules require different kinds of transport. 0 Facilitated diffusion 0 Although gases can diffuse easily between the phospholipids of the cell membrane, many polar or charged substances (like chloride) need help from membrane proteins. Membrane proteins can be either channel proteins or carrier proteins. 0 Even though a concentration gradient may exist for these substances, their charge or polarity prevents them from crossing the hydrophobic center of the cell membrane. Substances transported through facilitated diffusion still move with the concentration gradient, but the transport proteins protect them from the hydrophobic region as they pass through. 0 Active transport 0 During active transport, substances move against the concentration gradient, from an area of low concentration to an area of high concentration. This process is “active” because it requires the use of energy (usually in the form of ATP). It is the opposite of passive transport. 0 Active transport requires assistance from carrier proteins, which change conformation when ATP hydrolysis occurs.