AQA A-level BIOLOGY Paper 3 Verified Questions And Correct Answers Graded A

AQA A-level BIOLOGY Paper 3 Verified Questions And Correct Answers Graded A What are the useful properties of water and why ? - correct answer1) Water is a metabolite - in many metabolic reactions including condensation and hydrolysis 2) Water is a solvent - medium in which enzyme catalysed reactions take place (metabolic reactions ) and readily dissolves substances like inorganic ions ( as water is dipolar so totally surrounds the ions) , excretory products (urea/ammonia) , respiratory gases and small hydrophilic molecules ( amino acids , etc) 3) Water has a high latent heat of vaporisation (lots of energy required to break hydrogen bonds between water molecules so a lot of energy is used up when water evaporates ) so helps with temperature control as evaporation of sweat cools humans down 4) Water has a high specific heat capacity ( to separate the many hydrogen bonds requires a lot of energy) so acts as a buffer and resists temperature changes to maintain a constant internal body temperature 5) Water has large cohesive forces ( as hydrogen bonds stick together) so allows transportation of substances( water travelling in columns in xylem) and means water has a high surface tension when it comes into contact with air ( allowing pond skaters to walk on its surface) Why is water dipolar ? - correct answerAs water is made up of two hydrogen atoms and one oxygen atom , the hydrogen atoms have a slightly positive pole and the oxygen atom has a slightly negative pole making water dipolar. Hydrogen bonds - correct answerSlightly negatively charged oxygen atom is attracted to the slightly positively charged hydrogen atom of other water molecules. The attraction between opposite charges is called hydrogen bonding, which are weak bonds but strong in large numbers. This makes water a major component in cells Hydrolysis - correct answerBreaks a bond between two molecules by the addition of a molecule of water Condensation - correct answerJoins two molecules together with the formation of a bond by the removal of a molecule of water Polymer - correct answerA molecule ( carbohydrate, nucleotide or protein) made up of many monomers ( small repeating units) joined together by condensation reactions and broken down my hydrolysis Monosaccharides - correct answerAre the monomers from which larger carbohydrates are made and contain the elements C H and O. They are sweet and soluble and there are three types : fructose , glucose and galactose. Hexose sugars (glucose) have the formula C6H12O6 Glucose is the product of photosynthesis and a major substrate of respiration , draw the two structural isomers of glucose in long and short hand and state the difference between them - correct answerOn carbon 1 in beta glucose the hydroxyl group (OH) points upwards and in alpha glucose the hydroxyl group points down on carbon 1 Sucrose - correct answerGlucose and fructose Maltose - correct answerGlucose and glucose Lactose - correct answerGlucose and galactose How do monosaccharides join together - correct answerBy condensation to form a glycosidic bond between two monosaccharides and a disaccharide and a molecule of water are formed Disaccharide - correct answerTwo monosaccharides joined together by condensation forming a glycosidic bond. There are three types : maltose, sucrose and lactose. They have the chemical formula C12H22O11 Draw the hydrolysis of maltose in structural formula and short hand formula - correct answer Test for starch - correct answerAdd iodine solution to the test sample , if starch is present colour changes from yellow to a blue-black colour. Test for a reducing sugar - correct answerHeat with Benedict's reagent ( blue) and if reducing sugar is present a brick red precipitate will form Issues with the Benedict's test ? - correct answer1) It is non specific - doesn't tell which reducing sugar is present , a biosensor can be used to detect for specific sugars 2) It's qualitative so colour change is being measured so can't obtain concentration of reducing sugar but allows an estimate for how much is present in the strength of the colour 3) Is subjective How to quantify results on the concentration of unknown glucose concentration? - correct answerUse a colorimeter, as a serial dilution of known glucose concentrations can be measured producing a set of solutions with known concentrations . The Benedict's test can be performed with each of these and the colour intensity measured. A CALIBRATION CURVE can be produced from the results and used to read iff the concentration of glucose in an unknown solution. Test for a non-reducing sugar ? - correct answerHeat the sample with Benedict's reagent and if there is no colour change a reducing sugar is not present but a non-reducing sugar could be. Heat a fresh sample with dilute acid ( which will hydrolyse the non reducing sugar into its constituent monosaccharides). Neutralise with alkali and carry out the Benedict's test again If the test is positive for a non-reducing sugar it will form a brick red precipitate Polysaccharide - correct answerThree main types - starch , glycogen and cellulose which are chains of many monomers of glucose formed in condensation reactions. Starch - correct answerStarch is used for energy storage in plants and is made from condensation of alpha glucose. Amylose- compact helical chains so store lots of glucose in a small space Amylopectin - branched chains which allow many ends to be quickly hydrolysed by amylase enzymes to release glucose Insoluble and large so no osmotic effects , inert and cannot diffuse out of the cells which they are stored in . Cellulose - correct answerMajor component of plant cell walls made up of beta glucose monomer in a condensation reaction. Chains of beta glucose molecules form long straight unbranched chains. These chains run parallel to each other and hydrogen bonds form cross-linkages between chains. These many hydrogen bonds provide high tensile strength which makes the cell wall rigid and prevents osmotic lysis Hydrogen bonds also form strong fibres called microfibrils which are grouped into fibres which provide structural support. Glycogen - correct answerEnergy storage molecule in animals found in liver cells and in muscle tissue and is made from the condensation of alpha glucose. Compact so store lots of glucose in a small space Is quickly hydrolysed to release soluble glucose as many ends for amylase enzymes to attack Large and insoluble so no osmotic effects , inert and cannot diffuse out of the cells which they are stored in . Name the 4 inorganic ions and state their role - correct answerInorganic ions occur in solution in the cytoplasm and body fluids ( blood , tissue fluid) Hydrogen ions - affects pH levels, the higher the concentration of H+ ions the lower the pH( more acid ) Iron ions - important component of haemoglobin which carries oxygen around the body in the red blood cells Phosphate ions - component of DNA , ATP , RNA and phospholipids , bonds between phosphate groups store energy in ATP Sodium ions - involved in the absorption of glucose and amino acids in co transport in the small intestine Amino acids - correct answerAre the monomers from which polypeptides are made. Proteins are made up of one or more polypeptide chains. All amino acids contain the elements C H O and N and are made up of an amino group (NH2) , a carboxylic acid group (COOH) and a variable group (R) What is a conjugated protein ? - correct answerA protein with a non-protein chemical group attached Draw the structure of an amino acid - correct answer How many amino acids are there that are shared in all organisms ? - correct answer20 amino acids that differ by a different variable group. Dipeptide - correct answerTwo amino acids join to form a dipeptide and a peptide bong is formed between the amino acids by a condensation reaction. Draw the formation of a dipeptide from two amino acids? - correct answer Describe the protein structure ? - correct answerPrimary structure - sequence of amino acids in a polypeptide chain Secondary structure - folding of polypeptide chain causing hydrogen bonds to form between amino acids causing the formation of an alpha helix or a beta pleated sheet Tertiary structure - the further folding of the secondary structure which is held into shape by bonds e.g. hydrogen, ionic and disulphide. Bonds form between R groups of amino acids forming a 3D structure and is the final structure of one polypeptide chain Quaternary structure - is the final 3D structure of a protein consisting of more than one polypeptide chain What are the two types of protein ? - correct answerFibrous- form long chains running parallel to each other with cross bridges between chains producing very strong and stable proteins such as collagen and keratin. Globular - carry out metabolic functions such as enzymes, haemoglobin and antibodies Collagen - correct answerPrimary structure - unbranched polypeptide chain Secondary structure - forms alpha helix lots of amino acid glycine helps close packaging Tertiary structure - polypeptide chain is further folded Quaternary structure - 3 polypeptide chains are wound together with covalent bonds between amino acids of adjacent chains This makes collagen strong and stable and is found in tendons which join muscle to bong. Tendons need to be strong and not elastic. Test for proteins - correct answerAdd Biuret solution to a sample of solution , if protein is present a lilac colour is seen from the blue colour of Biuret solution What method separates a mixture of monosaccharides or amino acids ? - correct answerChromatography as molecules have different sizes so have different solubilities , the smaller the molecule the further it will travel up the solvent and the more soluble the molecule the further it will move up. Rf value must be calculated to allow comparison between different chromatograms run with the same solvent. How do you work out the Rf value ? - correct answerDistance from origin to solute (spot) over the distance from origin to solvent front Why are enzymes biological catalysts ? - correct answerAs they lower amount of activation energy needed to make the reaction proceed which increases the rate of the chemical reaction Enzymes - correct answerAre globular proteins in a spherical shape due to tight folding of the polypeptide chain. They catalyse metabolic reactions at cellular level( respiration) and the organism as a whole ( digestion). Enzymes can be intracellular ( inside the cell) or extracellular ( outside cell). Enzymes have a specific active site that is complementary to its substrate. Enzymes are very specific as they catalyse only one reaction. Without enzymes the temp in living cells would be too low (37 degrees) for chemical molecules to react fast enough to support life Why are enzymes specific ? - correct answerEach different enzyme has a specific tertiary structure and so a specific active site and shape. Only one substrate is complementary to the enzymes active site so combines and forms an enzyme- substrate complex. Lock and Key model - correct answerThe active site is always exactly complementary shape to the substrate so reactions are fastest in these conditions. Induced fit model - correct answerWhen the substrate bonds to the enzyme it induces a change in the tertiary structure so the active site changes shape to become complementary to the substrate How is rate measured - correct answerAmount of product or volume of substrate left over time taken. It's units are cm3s-1 What factors effect enzyme action ? - correct answerTemperature, pH , enzyme concentration and substrate concentration. How does temperature affect enzyme activity and what does the graph of rate of reaction against temperature look like ? - correct answerAt low temperatures there is less kinetic energy so enzyme molecules are inactive as they move around slowly in aqueous conditions and only collide with the substrate rarely to form ES complexes. As temperature increases enzyme and substrate molecules gain more kinetic energy so they move faster and collide more frequently so rate of reaction increases and more ES complexes form. Above 40 degrees however , rate of reaction decreases as the enzymes denature. This is because as temp increases over optimum temp , hydrogen bonds break which changes the shape of the tertiary structure and therefore the shape of the active site. So the substrate is no longer complementary and no ES complexes can form. So it has denatured as it can no longer carry out its function