Carbohydrates and lipids
All living organisms are made of C, H, O + N molecules.
Carbohydrates
• composed of C, H, O
• divided into monosaccharides, disaccharides, polysaccharides
• monomer – one of many small molecules that combine to form a polymer, e.g. – monosaccharides, amino acids, nucleotides
• polymer – large molecule made from many similar repeating subunits, e.g. – polysaccharides, proteins, nucleic acids
• macromolecule – large molecule formed due to polymerisation of monomers, e.g. – polysaccharides, proteins (polypeptides), nucleic acids (polynucleotides)
Monosaccharides
Molecule consisting of a single sugar unit with the general formula C(H2 O)n
• dissolves in water
• main types of monosaccharides – trioses (3C), pentoses (5C), hexoses (6C)
• glucose, fructose galactose, ribose, deoxyribose
Roles of monosaccharides
1) source of energy in respiration – C–H bonds can be broken to release a lot of energy which is transferred to help make ATP from ADP
2) building blocks for larger molecules – glucose is uses to make the polysaccharides starch, glycogen, and cellulose; ribose is one of the molecules used to make RNA and ATP, deoxyribose is one of the molecules used to make DNA
Disaccharides
Sugar molecule consisting of 2 monosaccharides joined by a glycosidic bond.
• H2 O molecule is removed; the bond formed by condensation is called a glycosidic bond
Polysaccharides
A polymer whose subunits are monosaccharides joined by glycosidic bonds
• e.g., starch, glycogen, cellulose (all polymers of glucose)
• not sugars
• if glucose itself accumulated in cells, it would dissolve and make the contents of the cell too concentrated which affects its osmotic properties
• storage polysaccharides – convenient, compact, inert, insoluble
1) Starch → amylose + amylopectin
| AMYLOSE | AMYLOPECTIN |
 |  |
| made by condensation reactions between 1,4 linked ⍺–glucose molecules | also made of 1,4 linked ⍺– glucose molecules |
| long, unbranching chain | chains are shorter than amylose and branch out to sides |
| chains are curved and coil into helical structures making the final molecule more compact | branches are formed by 1-6 linkages |
2) Glycogen
• made of chains of 1–4 linked ⍺–glucose molecules with 1–6 linkages forming branches
• tend to be more branched than amylopectin molecules
• many ends due to branching aids in easy addition and removal of glucose
• compact and insoluble, doesn’t affect the water potential (Ψ)
3) Cellulose → polymer of β–glucose
• in order to form a glycosidic bond with the 4 th carbon atom where the OH group is below the ring; every other glucose is rotated 180°
• so successive glucose molecules are linked 180° to each other
• one oxygen is up and the other is down
• the molecules are still linked 1–4
• cellulose molecules become tightly cross–linked to form bundles called microfibrils
• microfibrils are held together in bundles called fibres by hydrogen bonding
• cellulose fibres have very high tensile strength – this makes it possible for a cell to withstand large pressures as a result of osmosis
• cellulose fibres, despite their strength, are freely permeable
Dipoles and hydrogen bonds
• unequal distribution of charges in a covalent bond is called a dipole
• molecules which have groups with dipoles are polar
• in water, oxygen atom gets more electrons due to it being more electronegative and therefore gets a small negative charge denoted by delta (𝛅–)
• hydrogen atoms get less electrons and therefore get small positive charges (𝛅+)
• negatively charged oxygen of one molecule is attracted to a positively charged hydrogen of another, this attraction is called a hydrogen bond
Molecules which have groups with dipoles are polar
• they’re attracted to H2 O molecules as they also have dipoles and are considered to be hydrophilic (water–loving)
• soluble in water
• e.g., glucose, amino acids, NaCl
Molecules which do not have dipoles are non–polar
• they’re not attracted to water and hydrophobic (water–hating)
• insoluble in water
• e.g., oils, cholesterol
Lipids → 3 Fatty Acids + 1 Glycerol Fatty acids
• contain the acidic group –COOH
• larger molecules in the series have long hydrocarbon tails attached to the acid which are 15–17 carbon atoms long
• of two types: saturated and unsaturated
• unsaturated fatty acids have C=C double bonds therefore don’t have maximum amount of hydrogen atoms
• form unsaturated lipids
• mostly liquid
Alcohols & Esters
• alcohols contain hydroxyl group (–OH) attached to C atom
• reaction between (fatty) acid (–COOH) and alcohol (– OH) produces an ester
• the chemical link between acid and alcohol is called an ester linkage/bond and is formed by a condensation reaction
• glyceride is an ester formed by a fatty acid combining with the alcohol glycerol (C 3 H8 O3 )
• glycerol has 3 hydroxyl groups; each one is able to undergo a condensation reaction with a fatty acid
• triglycerides are insoluble in water due to the non–polar nature of hydrocarbon tails – they don’t have uneven distribution of charges and are hydrophobic
Roles of triglycerides
• energy reserves
• insulator
• protect vital organs