HL Paper 3
Vision is dependent on retinol (vitamin A) present in retina cells. Retinol is oxidized to the photosensitive chemical 11-cis-retinal and isomerizes to 11-trans-retinal on absorption of light.
Outline how the formation of 11-trans-retinal results in the generation of nerve signals to the brain.
Biological pigments include a variety of chemical structures with diverse functions.
The graph shows the conversion of hemoglobin to oxyhemoglobin.
Hb(aq) + 4O2(g) Hb(O2)4(aq)
The partial pressure of oxygen gas, p(O2) is proportional to its concentration.
Explain the shape of the curve at low oxygen partial pressure up to about 5 kPa.
Sketch a graph on the axes above to show the effect of decreasing pH on the binding of oxygen to hemoglobin (the Bohr Effect).
Outline the effect of decreasing pH on the oxygen saturation of hemoglobin.
Amino acids are the building blocks of proteins.
Draw the structures of the main form of glycine in buffer solutions of pH 1.0 and 6.0.
The pKa of glycine is 2.34.
Calculate the pH of a buffer system with a concentration of 1.25 × 10−3 mol dm−3 carbonic acid and 2.50 × 10−2 mol dm−3 sodium hydrogen carbonate. Use section 1 of the data booklet.
pKa (carbonic acid) = 6.36
Sketch the wedge and dash (3-D) representations of alanine enantiomers.
UV-Vis spectroscopy can be used to determine the unknown concentration of a substance in a solution.
Calculate the concentration of an unknown sample of pepsin with an absorbance of 0.725 using section 1 of the data booklet.
Cell length = 1.00 cm
Molar absorptivity (extinction coefficient) of the sample = 49650 dm3 cm−1 mol−1
A different series of pepsin samples is used to develop a calibration curve.
Estimate the concentration of an unknown sample of pepsin with an absorbance of 0.30 from the graph.
The structure of DNA (deoxyribonucleic acid) has been studied in many different ways.
State the name of the component of DNA responsible for the migration of its fragments to the positive electrode in gel electrophoresis.
In 2010, scientists claimed that they had discovered a species of bacteria capable of incorporating arsenic in place of phosphorus into the bacterial DNA. This claim has since proved controversial. Suggest one technique or evidence that might help support the claim.
Amino acids are usually identified by their common names. Use section 33 of the data booklet.
Amino acids act as buffers in solution. In aspartic acid, the side chain (R group) carboxyl has pKa = 4.0. Determine the percentage of the side chain carboxyl that will be ionized (–COO–) in a solution of aspartic acid with pH = 3.0. Use section 1 of the data booklet.
Anthocyanins are pigments that give colour to many flowers and fruits. The red colour of ripe strawberries is mainly due to the anthocyanin pigment whose structure is shown below.
Outline why this molecule absorbs visible light.
With reference to its chemical structure, outline whether this pigment is found in aqueous solution in the cells or in the lipid-based membranes.
A student investigated the ability of anthocyanins to act as pH indicators. He extracted juice from blackberries and used a UV-vis spectrophotometer to produce absorption spectra at different pH values. His results are shown below.
Deduce the colour of the juice at each pH, giving your reasoning. Use section 17 of the data booklet.
Glucokinase and hexokinase are both enzymes that catalyse the conversion of glucose to glucose-6-phosphate. The enzymes differ, however, in their affinity for the substrate, as shown in the graph below.
(i) Estimate the Km values of the two enzymes.
(ii) Suggest, with a reason, which enzyme will be more responsive to changes in the concentration of glucose in the blood.
(i) Outline what is meant by product inhibition as it applies to hexokinase.
(ii) Product inhibition of hexokinase does not affect its Km value. Using this information, deduce the type of binding site that the inhibitor attaches to.
The diverse functions of biological molecules depend on their structure and shape.
Classify vitamins A, C and D as either mainly fat- or water-soluble, using section 35 of the data booklet.
The diverse functions of biological molecules depend on their structure and shape.
Deduce the straight chain structure of deoxyribose from its ring structure drawn in section 34 of the data booklet.
The diverse functions of biological molecules depend on their structure and shape.
Draw the nitrogenous base that is paired with guanine in DNA, showing the hydrogen bonds between the bases. Use section 34 of the data booklet.
The diverse functions of biological molecules depend on their structure and shape.
Retinal is the key molecule involved in vision. Explain the roles of cis- and trans-retinal in vision and how the isomers are formed in the visual cycle.
Proteins are polymers of amino acids.
A paper chromatogram of two amino acids, A1 and A2, is obtained using a non-polar solvent.
© International Baccalaureate Organization 2020.
Determine the value of A1.
Proteins are polymers of amino acids.
The mixture is composed of glycine, , and isoleucine, . Their structures can be found in section 33 of the data booklet.
Deduce, referring to relative affinities and , the identity of A1.
Proteins are polymers of amino acids.
Glycine is one of the amino acids in the primary structure of hemoglobin.
State the type of bonding responsible for the α-helix in the secondary structure.
Proteins are polymers of amino acids.
Sketch and label two oxygen dissociation curves, one for adult hemoglobin and one for foetal hemoglobin.
Proteins are polymers of amino acids.
Explain why the affinity for oxygen of foetal hemoglobin differs from that of adult hemoglobin.
Analysis of amino acid and protein concentration is a key area of biological research.
The titration curve of aqueous glycine zwitterions with aqueous sodium hydroxide is shown from pH 6.0 to 13.0. Refer to section 33 of the data booklet.
Deduce the pH range in which glycine is an effective buffer in basic solution.
Enzymes are biological catalysts.
The data shows the effect of substrate concentration, [S], on the rate, v, of an enzyme-catalysed reaction.
Determine the value of the Michaelis constant (Km) from the data. A graph is not required.
Outline the action of a non-competitive inhibitor on the enzyme-catalysed reaction.
The sequence of nitrogenous bases in DNA determines hereditary characteristics.
Calculate the mole percentages of cytosine, guanine and thymine in a double helical DNA structure if it contains 17% adenine by mole.
Retinal, one of the many forms of vitamin A, reacts with opsin to produce rhodopsin. Refer to section 35 of the data booklet for one structure of vitamin A.
Identify the structural feature which enables rhodopsin to absorb visible light.
Outline the change that occurs in the retinal residue during the absorption of visible light.
The structures of the amino acids cysteine, glutamine and lysine are given in section 33 of the data booklet.
An aqueous buffer solution contains both the zwitterion and the anionic forms of alanine. Draw the zwitterion of alanine.
Calculate the pH of a buffer solution which contains 0.700 mol dm–3 of the zwitterion and 0.500 mol dm–3 of the anionic form of alanine.
Alanine pKa = 9.87.
DNA is a complex molecule.
Outline how its structure allows it to be negatively charged in the body.
Deduce the nucleotide sequence of a complementary strand of a fragment of DNA with the nucleotide sequence –GACGGATCA–.
A hemoglobin-oxygen saturation curve does not follow the same model as enzyme-substrate reactions.
Explain the shape of the curve from 0 to X kPa.
Explain why carbon monoxide is toxic to humans.
The graph of the rate of an enzyme-catalyzed reaction is shown below.
Determine the value of the Michaelis constant, Km, including units, from the graph.
Sketch a second graph on the same axes to show how the reaction rate varies when a competitive inhibitor is present.
Outline the significance of the value of Km.
The heme groups in cytochromes contain iron ions that are involved in the reduction of molecular oxygen.
State the half-equation for the reduction of molecular oxygen to water in acidic conditions.
Outline the change in oxidation state of the iron ions in heme groups that occurs when molecular oxygen is converted to water.
The stability of DNA is due to interactions of its hydrophilic and hydrophobic components.
Outline the interactions of the phosphate groups in DNA with water and with surrounding proteins (histones).
Insulin was the first protein to be sequenced. It was determined that the end of one chain had the primary structure Phe–Val–Asn–Gln.
Describe how DNA determines the primary structure of a protein such as insulin.
An enzyme catalyses the conversion of succinate to fumarate ions in a cell, as part of the process of respiration.
The rate of the reaction was monitored and the following graph was plotted.
Determine the value of the Michaelis constant, Km, by annotating the graph.
The malonate ion acts as an inhibitor for the enzyme.
Suggest, on the molecular level, how the malonate ion is able to inhibit the enzyme.
Draw a curve on the graph above showing the effect of the presence of the malonate ion inhibitor on the rate of reaction.
An inhibitor reduces the rate, V, of an enzyme-catalysed reaction.
Explain with reference to the binding site on the enzyme how a non-competitive inhibitor lowers the value of Vmax.
Outline the significance of the value of the Michaelis constant, Km.
Hemoglobin contains an iron ion that can bind to oxygen as part of the process of respiration.
Hemoglobin’s oxygen dissociation curve is shown at a given temperature. Sketch the curve on the graph at a higher temperature.
Outline two differences between normal hemoglobin and foetal hemoglobin.
Lactose, found in milk and dairy products, is a disaccharide formed from two different monosaccharides. The structure of lactose is shown with numbered carbon atoms.
Starch and cellulose are both formed from glucose and have similar structures.
Name the type of link between the two monosaccharide residues.
Outline how the two monomer structures, galactose and glucose, differ.
Outline the difference between their structures.
Outline why cellulose is an essential part of human diet.
The transport of oxygen around the body is essential for life.
A graph showing saturation of oxygen against partial pressure of oxygen is shown.
Explain the shape of the graph from 0 to 50 % saturation.
Explain why carbon monoxide is very toxic and how it may be possible to treat carbon monoxide poisoning.
The kinetics of an enzyme-catalysed reaction are studied in the absence and presence of an inhibitor. The graph represents the initial rate as a function of substrate concentration.
Identify the type of inhibition shown in the graph.
Determine the value of and in the absence and presence of the inhibitor.
Outline the significance of the value of the Michaelis constant, .
Genetic information is stored in DNA.
State the feature of DNA that determines the primary structure of proteins synthesised by a cell.
Suggest one concern about the use of genetically modified, GM, food.
Aspartame is formed from the two amino acids aspartic acid (Asp) and phenylalanine (Phe).
Chromatography is used in the analysis of proteins in the food and pharmaceutical industry.
Draw the structure of the dipeptide Asp–Phe using section 33 of the data booklet.
Describe, using another method, how a mixture of four amino acids, alanine, arginine, glutamic acid and glycine, could be separated when placed in a buffer solution of pH 6.0.
Suggest why alanine and glycine separate slightly at pH 6.5.
Calculate the ratio of [A−] : [HA] in a buffer of pH 6.0 given that pKa for the acid is 4.83, using section 1 of the data booklet.
Changes in physiology can impact living creatures.
The graph shows the change in oxygen partial pressure in blood, measured at different pH values.
Explain the effect of changing pH on the percentage saturation of hemoglobin at a given partial pressure of oxygen.
Explain the biomagnification of the pesticide DDT.
Vitamins are organic compounds essential in small amounts.
State the name of one functional group common to all three vitamins shown in section 35 of the data booklet.
Aspartame is a derivative of a dipeptide formed between two amino acids, phenylalanine (Phe) and aspartic acid (Asp).
Draw a circle around the functional group formed between the amino acids and state its name.
Name:
A mixture of phenylalanine and aspartic acid is separated by gel electrophoresis with a buffer of pH = 5.5.
Deduce their relative positions after electrophoresis, annotating them on the diagram. Use section 33 of the data booklet.
Aspartic acid is obtained synthetically as a racemic mixture. Draw the three‑dimensional shape of each isomer showing their spatial relationship to each other. Use section 33 of the data booklet.
Lipids play several roles in our bodies.
The iodine number is the maximum mass of iodine that reacts with 100 g of an unsaturated compound.
Determine the iodine number of stearidonic acid, C17H27COOH.
State two functions of lipids in the body.
Outline one effect of increased levels of low-density lipoproteins in the blood.
Enzymes are mainly globular proteins.
Describe the interaction responsible for the secondary structure of a protein.
Explain the action of an enzyme and state one of its limitations.
Contrast the actions of non-competitive and competitive inhibitors of an enzyme and state their effects on the maximum rate of reaction, Vmax, and the Michaelis–Menten constant, Km.
Carotenoids are pigments found in leaves.
The absorption spectrum of β-carotene is shown below.
Explain its colour in terms of its absorption bands. Use section 17 of the data booklet.
The absorption spectrum of chlorophyll a is shown below.
Suggest how the combination of chlorophyll a and carotenoids is beneficial for photosynthesis.
Alcohol dehydrogenase (ADH) catalyses the oxidation of methanol. The products of oxidation, methanal and methanoic acid, are toxic.
A Michaelis–Menten plot for an enzyme-catalysed reaction is shown.
Sketch a curve to show the effect of a competitive inhibitor.
Suggest, based on the Michaelis–Menten plot, how a competitive inhibitor such as ethanol reduces the toxicity of methanol.
Enzymatic activity is studied in buffered aqueous solutions.
Calculate the ratio in which 0.1 mol dm−3 NaH2PO4 (aq) and 0.1 mol dm−3 Na2HPO4 (aq) should be mixed to obtain a buffer with pH = 6.10. Use section 1 of the data booklet.
pKa (NaH2PO4) = 7.20
The main fatty acid composition of cocoa butter and coconut oil is detailed below.
The melting points of cocoa butter and coconut oil are 34 °C and 25 °C respectively.
Explain this in terms of their saturated fatty acid composition.
Fats contain triglycerides that are esters of glycerol and fatty acids. Deduce an equation for the acid hydrolysis of the following triglyceride.
The addition of partially hydrogenated cocoa butter to chocolate increases its melting point and the content of trans-fatty acids (trans-fats).
Outline one effect of trans-fatty acids on health.
Aspirin can be obtained from salicylic acid.
Additional information can be obtained from the 1H NMR spectrum of aspirin.
Unreacted salicylic acid may be present as an impurity in aspirin and can be detected in the infrared (IR) spectrum.
Name the functional group and identify the absorption band that diff erentiates salicylic acid from aspirin. Use section 26 of the data booklet.
Name:
Absorption band:
Deduce the protons responsible for signals X and Y by marking them on the structure of aspirin in (a). Use section 27 of the data booklet.
Identify the splitting pattern of signals X and Y.
X:
Y:
Vitamins can be water-soluble or fat-soluble.
Retinal is the key molecule involved in vision. Explain the roles of cis and trans-retinal in vision and how the isomers are formed in the visual cycle.
Starch is a natural polymer of glucose.
Draw the structure of the repeating unit of starch and state the type of linkage formed between these units.
Type of linkage:
Formulate the equation for the complete hydrolysis of a starch molecule, (C6H10O5)n.
Calculate the energy released, in kJ g−1, when 3.49 g of starch are completely combusted in a calorimeter, increasing the temperature of 975 g of water from 21.0 °C to 36.0 °C. Use section 1 of the data booklet.
Explain how the inclusion of starch in plastics makes them biodegradable.
Polymers of glucose include starch and cellulose.
Outline why cellulose fibres are strong.
Proteins have structural or enzyme functions.
Oil spills are a major environmental problem.
Some proteins form an α-helix. State the name of another secondary protein structure.
Compare and contrast the bonding responsible for the two secondary structures.
One similarity:
One difference:
Explain why an increase in temperature reduces the rate of an enzyme-catalyzed reaction.
State and explain how a competitive inhibitor affects the maximum rate, Vmax, of an enzyme-catalyzed reaction.
Suggest two reasons why oil decomposes faster at the surface of the ocean than at greater depth.
Oil spills can be treated with an enzyme mixture to speed up decomposition.
Outline one factor to be considered when assessing the greenness of an enzyme mixture.
DNA is a biopolymer made up of nucleotides. List two components of a nucleotide.
Amino acids contain both acidic and basic functional groups.
An aqueous solution of glutamine contains 0.600 mol dm−3 of the zwitterion and 0.300 mol dm−3 of the anionic form.
pKa1 = 2.2 and pKa2 = 9.1
Outline which pKa value should be used when calculating the pH of the solution, giving your reason.
Calculate the pH of the glutamine solution using section 1 of the data booklet.
Describe what is meant by the genetic code and how it relates to protein synthesis.
Hemoglobin contains heme groups with the porphyrin ring bound to an iron(II) ion.
A hemoglobin’s oxygen dissociation curve is shown.
Outline why the complex formed between Fe2+ and oxygen is red. Refer to the diagram above and section 17 of the data booklet.
Explain the shape of the curve.
Sketch another line to show the effect of an increase in body temperature on the oxygen saturation of hemoglobin.
Phosphatidylcholine is an example of a phospholipid found in lecithin.
Phosphatidylcholine may be formed from propane-1,2,3-triol, two lauric acid molecules, phosphoric acid and the choline cation.
Deduce the structural formula of phosphatidylcholine.
Identify the type of reaction in (a).
Lecithin is a major component of cell membranes. Describe the structure of a cell membrane.
Lecithin aids the body’s absorption of vitamin E.
Suggest why vitamin E is fat-soluble.
Phospholipids are also found in lipoprotein structures.
Describe one effect of increased levels of low-density lipoprotein (LDL) on health.
Sucrose is a disaccharide.
State the name of the functional group forming part of the ring structure of each monosaccharide unit.
Classify, giving your reason, the hexose (six-membered) ring of sucrose as an α or β isomer.
Sketch the cyclic structures of the two monosaccharides which combine to form sucrose.
DNA, deoxyribonucleic acid, is made up of nucleotides.
List two components of nucleotides.
Explain how the double-helical structure of DNA is stabilized once formed.
The Michaelis–Menten equation describes the kinetics of enzyme-catalysed reactions.
Outline the significance of the Michaelis constant Km.
Compare the effects of competitive and non-competitive inhibitors.
Describe the function of chlorophyll in photosynthesis.
Compare and contrast the structures of starch and cellulose.
One similarity:
One difference:
Explain why maltose, C12H22O11, is soluble in water.
Stearic acid (Mr = 284.47) and oleic acid (Mr = 282.46) have the same number of carbon atoms. The structures of both lipids are shown in section 34 of the data booklet.
The iodine number is the number of grams of iodine which reacts with 100 g of fat. Calculate the iodine number of oleic acid.
The chemical change in stored fats causes rancidity characterized by an unpleasant smell or taste.
Compare hydrolytic and oxidative rancidity.
State one similarity and one difference in composition between phospholipids and triglycerides.
Similarity:
Difference:
New genetically modified organisms (GMO), especially plants, are continually being developed in research laboratories.
Outline what is meant by genetically modified organisms.
Outline one benefit of the use of these products.
Anthocyanins are naturally occurring plant pigments. Depending on the solution pH, they can exist as quinoidal bases or flavylium cations as shown in section 35 of the data booklet.
Outline why anthocyanins are coloured.
Explain why the blue colour of a quinoidal base changes to the red colour of a flavylium cation as pH decreases.
Explain how the structure of vitamin A is important to vision using section 35 of the data booklet.
Ascorbic acid and retinol are two important vitamins.
Explain why ascorbic acid is soluble in water and retinol is not. Use section 35 of the data booklet.