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Biological Molecules & Nucleic Acids
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AQA-A-level Biology Notes for Unit 1
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Biological MoleculesIntroduction
All living things have a similar biochemical basis.
Carbohydrates: commonly used by cells as respiratory substrates, also form structural components in plasma membranes and cell walls.
Lipids: the bilayer of plasma membranes, certain hormones, and as respiratory substrates.
Proteins: form many cell structures, important as enzymes, chemical messengers, and components in blood.
Nucleic Acids: carry genetic code to produce proteins.
Water: most common component in cells
Monomers and Polymers
Monomers: smaller units from which larger molecules are made.
Polymers: many monomers joined together
Condensation Reaction: when two molecules combine to form a more complex molecule by removing water
Hydrolysis Reaction: when larger molecules are broken down into smaller ones by adding water
Examples of Monomers are amino acids, monosaccharides, nucleotides
Examples of Polymers are proteins, polysaccharides, DNA/RNA
Carbohydrates
Monosaccharides are the simplest carbohydrates (glucose, fructose, galactose)
Glucose is a hexose sugar with two isomers:
Disaccharides are formed of two monosaccharides joined together via a condensation reaction forming a glycosidic bond.
Examples:
Maltose – glucose + glucose
Sucrose – glucose + fructose
Lactose – glucose + galactose
Polysaccharides are formed of many monosaccharides joined together.
Examples:
Starch, made of amylose and amylopectin, is the main polysaccharide energy stored in plants and is composed of alpha glucose. (condensation)
Glycogen is the polysaccharide energy stored in animals and is composed of alpha glucose. (condensation)
Cellulose is a structural component in plant cell walls made of long unbranched chains of beta glucose. (condensation)
Lipids
Two groups: triglycerides and phospholipids
Triglycerides: formed by the condensation of one molecule of glycerol and three fatty acid
molecules
A condensation reaction between fatty acid and glycerol forms an ester bond (RCOOH)
R group might be saturated or unsaturated:
Saturated: no double C=C bonds and as many hydrogen atoms as possible.
Unsaturated: at least one C=C bond so fewer hydrogen atoms than possible.
Phospholipids: one molecule of glycerol,2 fatty acids, and one phosphate group.
Properties of triglycerides: key roles in respiration and energy storage due to their insolubility and high carbon: hydrogen
ratio.
Properties of phospholipids: The non-polar hydrophobic tails and the polar hydrophilic heads of phospholipids allow them
to form phospholipid bilayers.
, Proteins
Amino acids are the monomers from which proteins are made, and the general structure of an amino acid is:
There are 20 naturally occurring amino acids; the only thing that changes is the R-group.
A condensation reaction between two amino acids forms a peptide bond.
Dipeptide bonds: condensation of two amino acids.
Polypeptide bonds: condensation of many amino acids.
Polypeptides are polymers made up of many amino acids and a protein may contain one or more polypeptides
Level Description Bonds
Primary The order of amino acids in the polypeptide chain. Peptide bonds hold the primary structure.
Secondary The sequence of amino acids causes parts of a protein Hydrogen bonds hold the secondary structure.
molecule to bend into alpha helix shapes or fold into beta
pleated sheets.
Tertiary The further folding of the secondary structure forms a unique Held in place by ionic, hydrogen, and disulphide bonds.
3D shape.
Quaternary Still has a unique 3D structure but is made up of more than Held in place by ionic, hydrogen, and disulphide bonds.
one polypeptide chain.
Biochemical Tests
Sample Reagent + Steps Result
Starch Iodine/Potassium Iodide Orange to blue/black
Reducing Sugars Benedict and heat Blue to red participate (more red = higher
reducing sugar concentration)
Non-Reducing Sugars If the sample from the reducing sugars test remains blue Blue to orange/brick red
then – add acid and boil, cool the solution and add an
alkali to neutralise, add Benedict’s reagent and heat.
Proteins Biuret Blue to purple
Lipids Dissolve the sample in ethanol, add distilled water, and White emulsion
shake.
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