2.3 - Carbohydrates and lipids

Question 1

Carbohydrates

Answer 1

• Monosaccharide monomers are linked together by condensation reactions to form disaccharides and polysaccharide polymers.
• Glucose, fructose and ribose are all examples of monosaccharides
• Monosaccharides can be linked together to create bigger molecules
• Monosaccharides are single sugar units
• Disaccharides consist of two monosaccharides linked together. (maltose = two linked glucose molecules)
• Polysaccharides consist of many monosaccharides linked together (starch, glycogen) made by linking glucose molecules together
• When monosaccharides combine this is called condensation. This involves the loss of an OH from one molecule and a H from another molecule which create H2O. Condensation involves the combination of subunits and creates water.
• Linking monosaccharides together to form disaccharides and polysaccharides is an anabolic process and energy is used (ATP). ATP is supplied to monosaccharides and is used when condensation occur.

Question 2

Lipids

Answer 2

• Triglycerides are formed by condensation from three fatty acids and one glycerol
• Lipids are insoluble in water
• Triglycerides are a principal form of lipids
• In humans’ lipids are fat in adipose tissue and oil in sunflower seeds
• Fats are solid at room temperature but liquid at body temperature
• Oils are liquids at both temperatures
• Three water molecules are produced in the condensation reaction of triglycerides
• The link between fatty acid and glycerol is an ester bond
• Ester bonds are formed when acid reacts with the OH group in an alcohol, in this case the reaction is between the COOH group on a fatty acid and an OH on the glycerol
• Triglycerides are used as energy stores, the energy stored by them can be released by aerobic respiration. Also useful as heat insulators (blubber)

Question 3

Fatty acids

Answer 3

• Fatty acids can be saturated, unsaturated or polyunsaturated
• Fatty acid structure: chain of carbon atoms, hydrogen atoms linked to them by single covalent bonds. This is a hydrocarbon chain. At one end of the chain is the acid part of the molecule, this is a carboxyl group represented as COOH.
• The length of the hydrocarbon chain is variable but most fatty acids used by living organisms have between 14 and 20 carbon atoms. The bonding between the carbon atoms can also vary in length. In some fatty acids all the carbon atoms are lined by single covalent bonds, in other fatty acids there are one or more positions in the chain where carbon atoms are linked by double covalent bonds.
• If a carbon atom is linked to adjacent carbons in the chain by single bonds it can also bond to two hydrogen atoms.
• If a carbon is linked by a double bond to an adjacent carbon in the chain it can only bond to one other hydrogen atom.
• A fatty acid with single bonds between all of its carbon atoms therefore contains as much hydrogen as it possibly could and is called a saturated fatty acid.
• Fatty acids that have one or more double-bonds are unsaturated because they contain less hydrogen than they could.
• If there is one double bond, the fatty acid is monounsaturated, and if it has more than one double bond it is polyunsaturated.

Question 4

Unsaturated fatty acids

Answer 4

• Unsaturated fatty acids can be cis or trans isomers
• Hydrogen atoms are nearly always on the same side of the two carbon atoms that are double bonded – these are cis-fatty acids
• Trans-fatty acids have hydrogens on the opposite sides
• In cis fatty acids there is a bend in the hydrocarbon chain at the double bond, this makes triglycerides containing cis-unsaturated fatty acids less good at packing together in regular arrays than saturated fatty acids, this lowers the melting point. Triglycerides with cis-unsaturated fatty acids are therefore usually liquid at room temperature (oils)
• Trans-fatty acids do not have a bend in the hydrocarbon chain at the double bond, so they have a higher melting point and are solid at room temperature.
• Trans-fatty acids are produced artificially by partial hydrogenation of vegetable or fish oils. This is done to produce solid fats for use in margarine and some other processed foods.