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Cambridge IAL Biology study materials
Cambridge IAL Biology study resources

Cambridge IAL Biology practice questions

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Question 1AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement best describes the role of the fluid mosaic model in explaining membrane behaviour?

  1. It explains that phospholipid bilayers are static and do not permit lateral movement.
  2. It explains that membranes are rigid layers of protein with a fixed lipid coating.
  3. It explains that membranes consist only of a phospholipid bilayer with no associated proteins.
  4. It explains that membranes are dynamic structures in which proteins are embedded in or attached to a fluid phospholipid bilayer. Correct answer.

Explanation

The fluid mosaic model describes membranes as a phospholipid bilayer in which proteins are scattered, like tiles in a mosaic, and are able to move laterally. This dynamic arrangement accounts for membrane flexibility, selective permeability and the movement of proteins during processes such as endocytosis.

Question 2AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

A student views a specimen using a light microscope with a X10 eyepiece and a X40 objective. The measured image of a cell is 2.0 mm long. What is the actual length of the cell?

  1. 5.0 mm
  2. 0.005 mm
  3. 0.05 mm Correct answer.
  4. 0.5 mm

Explanation

Total magnification is X10 x X40 = X400. Actual size equals image size divided by magnification, so 2.0 mm divided by 400 gives 0.005 mm.

Question 3AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

In a Benedict's test, a solution changes from blue to brick-red on heating. Which conclusion is most valid?

  1. A reducing sugar is present in the solution. Correct answer.
  2. A non-reducing sugar such as sucrose is present.
  3. Protein is present in the solution.
  4. Starch is present in the solution.

Explanation

Benedict's reagent contains copper(II) ions that are reduced to copper(I) oxide by reducing sugars on heating, producing a brick-red precipitate. Sucrose is a non-reducing sugar and gives a negative result unless first hydrolysed.

Question 4AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which feature of the epithelial cells lining the ileum directly increases the surface area for absorption?

  1. The presence of microvilli on the apical surface Correct answer.
  2. The presence of a glycoprotein coat
  3. The presence of tight junctions between cells
  4. The presence of many mitochondria

Explanation

Microvilli are finger-like projections of the cell surface membrane that greatly increase the surface area available for absorption. Mitochondria, the glycoprotein coat and tight junctions serve other functions and do not directly enlarge the absorptive surface.

Question 5AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

During the cardiac cycle, which sequence correctly describes the events of one heartbeat?

  1. Diastole, ventricular systole, atrial systole
  2. Atrial systole, ventricular systole, diastole Correct answer.
  3. Ventricular systole, atrial systole, diastole
  4. Atrial systole, diastole, ventricular systole

Explanation

The cycle begins with atrial systole, which tops up the ventricles, followed by ventricular systole, which ejects blood into the arteries. This is followed by diastole, when the atria and ventricles relax and refill.

Question 6AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

In the cohesion-tension theory, which property of water is most directly responsible for maintaining a continuous column in the xylem?

  1. Its ability to act as a solvent for minerals
  2. Its high surface tension
  3. Cohesion between water molecules caused by hydrogen bonding Correct answer.
  4. Its high specific heat capacity

Explanation

Cohesion, arising from hydrogen bonding between water molecules, holds the water column together so that tension from transpiration can pull water up the xylem. Adhesion to xylem walls also helps prevent the column breaking.

Question 7AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which description of the quaternary structure of a protein is correct?

  1. The sequence of amino acids joined by peptide bonds
  2. The folding of a single polypeptide chain into a three-dimensional shape
  3. The association of two or more polypeptide chains to form a functional protein Correct answer.
  4. The coiling of a polypeptide into an alpha helix

Explanation

Quaternary structure refers to the arrangement of more than one polypeptide chain, or a polypeptide with a non-protein prosthetic group, into a functional protein. Haemoglobin, with four globin chains, is a classic example.

Question 8AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

An enzyme-catalysed reaction is investigated by adding a competitive inhibitor. Which change is expected on a graph of reaction rate against substrate concentration?

  1. The initial gradient increases while the maximum rate decreases.
  2. Both the maximum rate and the initial gradient increase.
  3. The maximum rate decreases while the initial gradient is unchanged.
  4. The maximum rate is unchanged but a higher substrate concentration is needed to reach it. Correct answer.

Explanation

A competitive inhibitor binds to the active site, so its effect can be overcome by increasing substrate concentration. The maximum rate is therefore unchanged, but a higher substrate concentration is required to achieve it, shifting the curve to the right.

Question 9AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement correctly describes the behaviour of chromosomes during meiosis I?

  1. Homologous chromosomes separate at anaphase I. Correct answer.
  2. Sister chromatids separate at anaphase I.
  3. Sister chromatids separate at metaphase I.
  4. Homologous chromosomes separate at anaphase II.

Explanation

In meiosis I, homologous chromosomes pair as bivalents and then separate at anaphase I, halving the chromosome number. Sister chromatids remain together until anaphase II.

Question 10AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which feature of the genetic code explains why a single base substitution may have no effect on the polypeptide produced?

  1. The code is non-overlapping.
  2. The code is degenerate. Correct answer.
  3. The code contains start and stop codons.
  4. The code is universal.

Explanation

Degeneracy means that most amino acids are coded for by more than one codon, so a substitution may still specify the same amino acid. Such a silent mutation leaves the polypeptide unchanged.

Question 11AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

In a cross between two heterozygous individuals for a single gene showing complete dominance, what is the expected phenotypic ratio in the offspring?

  1. 3:1 Correct answer.
  2. 1:2:1
  3. 9:3:3:1
  4. 1:1

Explanation

A monohybrid cross between two heterozygotes gives a genotypic ratio of 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive. Because the dominant allele masks the recessive in heterozygotes, the phenotypic ratio is 3 dominant : 1 recessive.

Question 12AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which adaptation of insect-pollinated flowers is correctly matched to its function?

  1. Large, feathery stigmas that trap pollen from the air
  2. Brightly coloured petals that attract insects Correct answer.
  3. Small, dull petals that reduce water loss
  4. Large quantities of lightweight pollen that is dispersed by wind

Explanation

Insect-pollinated flowers typically have brightly coloured petals to attract pollinators, often with scent and nectar as rewards. The other features listed are characteristic of wind-pollinated flowers.

Question 13AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

In the light-dependent stage of photosynthesis, what is the immediate role of water?

  1. It acts as the final electron acceptor to form reduced NADP.
  2. It donates electrons and protons when it is photolysed, replacing those lost by chlorophyll. Correct answer.
  3. It combines with carbon dioxide to form glucose.
  4. It accepts electrons at the end of the electron transport chain.

Explanation

Photolysis of water in the thylakoid space supplies electrons that replace those lost from chlorophyll, along with protons that contribute to the proton gradient, and oxygen is released. This keeps the electron transport chain running.

Question 14AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement about the role of the Calvin cycle in the light-independent stage is correct?

  1. It fixes carbon dioxide by combining it with ribulose bisphosphate to form two molecules of glycerate 3-phosphate. Correct answer.
  2. It produces oxygen as a by-product of carbon fixation.
  3. It uses NADPH to reduce carbon dioxide directly to glucose in a single step.
  4. It regenerates ATP using light energy directly.

Explanation

In the Calvin cycle, carbon dioxide reacts with the five-carbon compound ribulose bisphosphate, catalysed by rubisco, to give two molecules of glycerate 3-phosphate. These are then reduced to triose phosphate using ATP and reduced NADP.

Question 15AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which of the following best explains why the oxygen dissociation curve of adult haemoglobin is S-shaped rather than a straight line?

  1. The binding of the first oxygen molecule changes the shape of haemoglobin, making subsequent binding easier. Correct answer.
  2. Each haemoglobin molecule can bind only one oxygen molecule, so the curve is limited.
  3. Haemoglobin has a higher affinity for carbon dioxide than for oxygen.
  4. Oxygen molecules are transported by active transport across the red cell membrane.

Explanation

The cooperative nature of oxygen binding means that after the first oxygen molecule binds, the haemoglobin molecule changes shape, increasing its affinity for further oxygen. This produces the sigmoid curve, with a steep section where binding accelerates.

Question 16AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which comparison of the tracheal system of an insect with the mammalian breathing system is correct?

  1. Insects use lungs to ventilate their tissues, whereas mammals use gills.
  2. Both use a circulatory fluid to transport respiratory gases directly to cells.
  3. Insects deliver air directly to tissues through tracheoles, whereas mammals use lungs and blood to transport gases. Correct answer.
  4. Both rely on diffusion across a single moist body surface.

Explanation

In insects, air enters through spiracles and travels along tracheae to tracheoles that deliver oxygen close to the tissues, often with little reliance on the circulatory system. Mammals instead exchange gases in alveoli and rely on blood to transport oxygen and carbon dioxide.

Question 17AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

A population of 5000 individuals contains 500 showing a recessive phenotype caused by a single gene with two alleles. Assuming Hardy-Weinberg equilibrium, what is the approximate frequency of the recessive allele?

  1. 0.01
  2. 0.32 Correct answer.
  3. 0.10
  4. 0.90

Explanation

The recessive phenotype frequency is q squared = 500/5000 = 0.10. Taking the square root gives q, the recessive allele frequency, of about 0.316, which is approximately 0.32.

Question 18AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement about the role of the phloem in translocation is correct?

  1. Sucrose is loaded into sieve tube elements by active transport, lowering the water potential so water enters by osmosis. Correct answer.
  2. Water is actively pumped into sieve tubes to push sap downwards.
  3. Assimilates are transported upwards in the xylem and downwards in the phloem.
  4. Sucrose moves into sieve tubes by simple diffusion only, so no energy is required.

Explanation

Sucrose is actively loaded into the sieve tube at the source, lowering the water potential of the sieve tube contents. Water then enters by osmosis, raising hydrostatic pressure and driving mass flow towards the sink.

Question 19AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

In an experiment using a respirometer, the rate of oxygen uptake by germinating seeds is measured. Why is a control tube containing the same mass of inert material such as glass beads included?

  1. To measure the volume of carbon dioxide produced by the seeds.
  2. To allow for changes in gas volume caused by temperature and pressure fluctuations. Correct answer.
  3. To provide a source of carbon dioxide for the seeds.
  4. To increase the rate of oxygen uptake by the seeds.

Explanation

A control tube with inert material allows the investigator to correct for changes in gas volume caused by fluctuations in temperature or atmospheric pressure. Any movement in the control is subtracted from the movement in the experimental tube.

Question 20AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which feature of a root hair cell is most directly responsible for increasing the rate of water uptake from the soil?

  1. Its thin cell wall
  2. Its long, narrow extension that increases surface area Correct answer.
  3. Its large central vacuole
  4. Its numerous chloroplasts

Explanation

The long, narrow extension of a root hair cell greatly increases the surface area over which water can be absorbed by osmosis. The thin wall and large vacuole support uptake but the extension is the key surface area adaptation.

Question 21AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement correctly describes the role of ATP in the sodium-potassium pump?

  1. ATP donates phosphate groups to sodium ions so they can diffuse through the membrane.
  2. ATP is not required because the pump moves ions by simple diffusion.
  3. ATP is hydrolysed to provide energy that changes the carrier protein's shape, moving ions against their concentration gradients. Correct answer.
  4. ATP provides energy for ions to move down their concentration gradients by facilitated diffusion.

Explanation

The sodium-potassium pump is an example of active transport. Hydrolysis of ATP provides energy that causes a conformational change in the carrier protein, moving sodium and potassium ions against their concentration gradients.

Question 22AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

A mutation changes a codon in the middle of a gene from one that codes for an amino acid to a stop codon. What is the most likely consequence for the polypeptide?

  1. The polypeptide will be longer than normal because the ribosome continues past the mutation.
  2. The polypeptide will have the same length but a different amino acid at that position.
  3. The polypeptide will be shorter than normal and may be non-functional. Correct answer.
  4. The polypeptide will be unchanged because the code is degenerate.

Explanation

A nonsense mutation creates a premature stop codon, so translation ends early and the resulting polypeptide is truncated. A shortened polypeptide often loses function because its tertiary structure is altered.

Question 23AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

In an investigation of osmosis using potato tissue, cylinders are placed in sucrose solutions of different concentrations. Which observation would be expected in a solution with a water potential higher than that of the potato cells?

  1. The cylinders lose mass because water leaves the cells.
  2. The cylinders gain mass because water enters the cells by osmosis. Correct answer.
  3. The cylinders lose mass because sucrose enters the cells.
  4. The cylinders show no change in mass because the water potentials are equal.

Explanation

Water moves by osmosis from the solution with the higher water potential into the potato cells, whose water potential is lower. The cells become turgid and the mass of the cylinder increases.

Question 24AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement about the structure of a chloroplast correctly links a feature to its function?

  1. The stroma contains chlorophyll and is the site of photolysis of water.
  2. The thylakoid membranes contain photosystems and are stacked into grana to increase the surface area for light absorption. Correct answer.
  3. The outer membrane contains enzymes for the Calvin cycle.
  4. The inner membrane is folded into cristae to increase the surface area for the light-dependent reactions.

Explanation

Thylakoid membranes house the photosystems and electron transport components of the light-dependent reactions. Their stacking into grana provides a large surface area for capturing light energy.

Question 25AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement about the significance of the surface area to volume ratio for a small organism is correct?

  1. A small surface area to volume ratio makes diffusion more efficient across the body surface.
  2. A large surface area to volume ratio means the organism must use lungs for gas exchange.
  3. A large surface area to volume ratio allows sufficient gas exchange by diffusion across the body surface. Correct answer.
  4. The surface area to volume ratio has no effect on the need for a specialised gas exchange system.

Explanation

Small organisms have a large surface area relative to their volume, so diffusion across the body surface can supply their needs without a specialised gas exchange system. As size increases, the ratio falls and specialised systems become necessary.

Question 26AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

A student views a transverse section of a dicotyledonous stem under an optical microscope. Which tissue occupies the region between the epidermis and the vascular bundles and contains loosely packed cells with large air spaces?

  1. Xylem vessels of the vascular bundle
  2. Parenchyma of the cortex Correct answer.
  3. Sclerenchyma fibres of the pericycle
  4. Phloem sieve tube elements

Explanation

The cortex lies between the epidermis and the vascular tissue and is made mainly of parenchyma cells with thin walls and intercellular air spaces that allow gaseous diffusion. Sclerenchyma and vascular tissues have thick, lignified walls or are located within the bundle itself.

Question 27AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement about the fluid mosaic model of the cell surface membrane is correct?

  1. The hydrophobic hydrocarbon tails of phospholipids face the interior of the bilayer Correct answer.
  2. Cholesterol molecules span the whole bilayer from one surface to the other
  3. Phospholipid molecules are held together mainly by covalent bonds between their tails
  4. Integral proteins are attached only to the outer hydrophilic surface of the bilayer

Explanation

In the bilayer the hydrophilic phosphate heads face the aqueous environments on each side while the hydrophobic fatty acid tails project inwards, away from water. This arrangement is stabilised by hydrophobic interactions rather than covalent bonding between tails, and cholesterol is embedded within one leaflet.

Question 28AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

A length of DNA has the base sequence 5'-TACGGATCC-3' on one strand. Which sequence represents the complementary strand written in the 5' to 3' direction?

  1. 5'-AUGCCUAGG-3'
  2. 5'-GGATCCGTA-3' Correct answer.
  3. 5'-ATGCCTAGG-3'
  4. 5'-TACGGATCC-3'

Explanation

Complementary base pairing gives A with T and G with C, and the two strands are antiparallel, so the complement of 5'-TACGGATCC-3' read in the 5' to 3' direction is 5'-GGATCCGTA-3'. Option 0 is the complement written in the wrong orientation, and uracil belongs to RNA only.

Question 29AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

During the cardiac cycle, which sequence of events correctly describes the pressure changes that close the atrioventricular valves?

  1. Atrial pressure rises above ventricular pressure during atrial systole
  2. Ventricular pressure rises above atrial pressure while the semilunar valves remain closed Correct answer.
  3. Ventricular pressure falls below aortic pressure during ventricular diastole
  4. Blood flows from the ventricles into the atria as the atria relax

Explanation

As the ventricles begin to contract, ventricular pressure rises above atrial pressure and this pressure difference pushes the atrioventricular valves shut, producing the first heart sound. At this point ventricular pressure is still below aortic pressure, so the semilunar valves have not yet opened.

Question 30AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement correctly describes a feature of the mammalian gaseous exchange system that maintains a steep diffusion gradient across the alveolar epithelium?

  1. Ciliated epithelium sweeps air towards the alveoli during inhalation
  2. Goblet cells secrete mucus that traps inhaled particles in the trachea
  3. Cartilage rings in the bronchi keep the airways open during pressure changes
  4. Ventilation replaces air in the alveoli and blood flow removes oxygen continuously Correct answer.

Explanation

A steep gradient is maintained because breathing brings fresh air with a high oxygen concentration into the alveoli while the blood supply carries oxygen away, keeping the alveolar concentration high and the blood concentration low. Cartilage, mucus and cilia relate to support and defence rather than to maintaining the concentration gradient.

Question 31AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

A plant cell with a water potential of -400 kPa is placed in a solution with a water potential of -250 kPa. Which statement describes the net movement of water and the reason for it?

  1. Water enters the cell because the pressure potential of the solution is greater than that of the cell
  2. Water leaves the cell because the cell has the more negative water potential Correct answer.
  3. No net movement occurs because the two water potentials differ by less than 200 kPa
  4. Water enters the cell because the solution has a higher solute potential than the cell

Explanation

Water moves by osmosis from a region of higher water potential to a region of lower water potential. The cell at -400 kPa has the lower (more negative) water potential, so water moves from the solution into the cell, meaning water leaves the solution and enters the cell.

Question 32AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement about the role of the loop of Henle in the mammalian kidney is correct?

  1. The ascending limb is permeable to water and allows it to leave by osmosis
  2. The descending limb actively transports sodium and chloride ions into the medulla
  3. The loop generates a region of low water potential in the medulla that aids water reabsorption Correct answer.
  4. The loop reabsorbs glucose and amino acids from the filtrate by active transport

Explanation

The loop of Henle creates a sodium chloride gradient in the medulla, lowering the water potential of the tissue fluid so that water can be withdrawn from the collecting duct by osmosis. The ascending limb actively pumps out ions and is relatively impermeable to water, while the descending limb allows water to leave.

Question 33AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

In a genetic cross, a male fruit fly with grey body and normal wings is crossed with a female that has black body and vestigial wings. All the male offspring have grey bodies and normal wings, while all the female offspring have grey bodies and vestigial wings. Which conclusion is best supported?

  1. Body colour is sex-linked while wing type is autosomal
  2. Both genes are autosomal and independently assorting
  3. The genes for body colour and wing type are sex-linked and show linkage Correct answer.
  4. The genes are on the same autosome and show complete linkage

Explanation

The difference between male and female offspring indicates sex linkage, and the fact that body colour and wing type are inherited together in each sex indicates the two genes are linked on the X chromosome. Independent assortment or autosomal inheritance would not produce sex-specific combinations of the two characters.

Question 34AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement about the process of transcription in a eukaryotic cell is correct?

  1. The mRNA produced is an exact copy of the template strand with uracil replacing thymine
  2. Transcription of a gene occurs on both DNA strands simultaneously in opposite directions
  3. Hydrogen bonds between the DNA strands are broken by DNA helicase before RNA polymerase binds Correct answer.
  4. RNA polymerase adds ribonucleotides to the growing strand using the coding strand as a template

Explanation

Before transcription the DNA double helix must be unwound, and hydrogen bonds between complementary bases are broken so that the template strand is exposed. RNA polymerase then builds an RNA strand complementary to the template strand, which is not an exact copy of it.

Question 35AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

A respirometer is used to measure the rate of oxygen uptake by small invertebrates at different temperatures. Which precaution is most important for obtaining valid comparative results?

  1. Using organisms of different species in each temperature trial
  2. Allowing the carbon dioxide produced to accumulate in the chamber
  3. Keeping the mass of the organisms and the volume of the apparatus the same in each trial Correct answer.
  4. Recording the reading only once after a long equilibration period

Explanation

To compare the effect of temperature fairly, all other variables that could affect oxygen uptake, such as the mass of the organisms and the volume of the chamber, must be controlled. Using different species or letting carbon dioxide accumulate would confound the results, and repeated readings are needed to calculate a reliable rate.

Question 36AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement correctly describes the role of the Golgi apparatus in a cell that secretes digestive enzymes?

  1. It produces ATP that provides energy for exocytosis of the enzymes
  2. It synthesises the polypeptide chain and folds it into its tertiary structure
  3. It modifies proteins by adding carbohydrate groups and packages them into vesicles Correct answer.
  4. It contains hydrolytic enzymes that digest worn-out organelles

Explanation

The Golgi apparatus modifies proteins, for example by glycosylation, and packages them into secretory vesicles that fuse with the cell surface membrane. Polypeptide synthesis occurs on ribosomes and ATP production occurs in mitochondria.

Question 37AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which feature of a xerophytic plant is an adaptation that reduces water loss by transpiration?

  1. Large, numerous stomata on the upper leaf surface
  2. Sunken stomata located in pits on the leaf surface Correct answer.
  3. A thin waxy cuticle covering the leaf epidermis
  4. Broad leaves with a high surface area to volume ratio

Explanation

Sunken stomata create a humid microclimate in the pit, reducing the water potential gradient between the leaf air spaces and the atmosphere and so reducing transpiration. A thin cuticle, many exposed stomata and a large leaf area would all increase water loss.

Question 38AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement about the structure of a cellulose microfibril in a plant cell wall is correct?

  1. It contains alternating alpha and beta glucose units joined by 1,6 glycosidic bonds
  2. It consists of beta glucose monomers joined by alpha 1,4 glycosidic bonds
  3. It is formed from unbranched chains of beta glucose held together by hydrogen bonds Correct answer.
  4. It is a branched polymer of beta glucose with many side chains

Explanation

Cellulose is made of long unbranched chains of beta glucose joined by beta 1,4 glycosidic bonds, and adjacent chains are cross-linked by hydrogen bonds to form strong microfibrils. Branching and alpha glucose are characteristic of starch and glycogen instead.

Question 39AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

An enzyme is immobilised on a solid support and used in a continuous industrial process. Which advantage is most directly a consequence of immobilising the enzyme?

  1. The enzyme can be recovered and reused, reducing costs Correct answer.
  2. The enzyme is no longer affected by changes in pH or temperature
  3. The enzyme's active site becomes more flexible, increasing its activity
  4. The enzyme can pass through the product stream and be purified with it

Explanation

Immobilised enzymes are held in place so they can be separated from the product and reused many times, which lowers the cost per unit of product. Immobilisation may actually reduce flexibility and activity, and it does not make the enzyme immune to pH or temperature effects.

Question 40AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement about the relationship between the structure of starch and its function as a storage carbohydrate is correct?

  1. Starch contains beta glucose and forms strong structural fibres
  2. Starch is soluble and increases the water potential of the cell
  3. Starch is highly branched and therefore rapidly hydrolysed to release glucose
  4. Starch is insoluble and osmotically inactive, so it can be stored in large amounts in cells Correct answer.

Explanation

Starch is a compact, insoluble polysaccharide that does not affect the water potential of the cell, so large quantities can be stored without drawing water in by osmosis. Its branched amylopectin structure allows rapid hydrolysis when glucose is needed, but that is a separate feature from its storage suitability.

Question 41AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement about the behaviour of chromosomes during meiosis II is correct?

  1. Sister chromatids separate and move to opposite poles at anaphase II Correct answer.
  2. Crossing over between non-sister chromatids occurs during prophase II
  3. Homologous chromosomes pair and form bivalents at metaphase II
  4. The chromosome number is halved from diploid to haploid during meiosis II

Explanation

Meiosis II resembles mitosis: sister chromatids separate at anaphase II and move to opposite poles. Pairing of homologous chromosomes, crossing over and the reduction in chromosome number all occur in meiosis I.

Question 42AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement about the importance of maintaining biodiversity in an ecosystem is correct?

  1. Genetic diversity within a species has no effect on its ability to adapt to environmental change
  2. Ecosystems with few species recover more quickly from disturbance than diverse ecosystems
  3. Biodiversity is measured only by the number of individuals of the most common species
  4. High species diversity increases the stability of an ecosystem because of more complex food webs Correct answer.

Explanation

Greater species diversity generally means more numerous and more complex interactions such as food webs, which can buffer the ecosystem against disturbance. Genetic diversity provides the variation on which natural selection acts, so it is important for adaptation.

Question 43AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

A student uses a colorimeter to measure the absorbance of a solution containing a coloured product of an enzyme reaction at one-minute intervals. Which improvement would most increase the reliability of the calculated initial rate?

  1. Reading the colorimeter against a blank of distilled water once at the start
  2. Increasing the substrate concentration in every trial
  3. Taking repeat readings and plotting a mean value at each time point Correct answer.
  4. Using a single reading at the end of the reaction only

Explanation

Reliability is improved by repeating measurements and using means, which reduces the effect of random error. A single end-point reading does not allow a rate to be determined, and changing substrate concentration alters the variable being investigated.

Question 44AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement about the effect of a non-competitive inhibitor on an enzyme-catalysed reaction is correct?

  1. It increases the affinity of the enzyme for its substrate
  2. It permanently denatures the enzyme so that no reaction can occur
  3. It binds away from the active site and decreases the maximum rate of reaction Correct answer.
  4. It binds to the active site and its effect is reduced by increasing substrate concentration

Explanation

A non-competitive inhibitor binds to a site other than the active site, changing the shape of the enzyme so that it cannot catalyse the reaction as effectively, which lowers the maximum rate. Increasing substrate concentration does not overcome this inhibition because the substrate and inhibitor do not compete.

Question 45AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement about the role of the vascular bundle in a dicotyledonous leaf is correct?

  1. Xylem in the midrib supplies water to the mesophyll for transpiration and photosynthesis Correct answer.
  2. Phloem in the midrib carries water upwards to the leaf from the roots
  3. Xylem in the midrib carries sucrose away from the leaf to the roots
  4. The vascular bundle is surrounded by a single layer of cells with no role in transport

Explanation

Xylem tissue in the leaf veins delivers water to the mesophyll cells, where it is used in photosynthesis and lost by transpiration. Phloem, not xylem, transports sucrose away from the leaf, and the bundle sheath cells around the vein are involved in loading and support.

Question 46AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

In an experiment on membrane permeability, beetroot cylinders are placed in water at different temperatures and the intensity of the red pigment released is measured. Which explanation for the results is most valid?

  1. Higher temperatures cause the pigment to be actively transported out of the cells
  2. The red pigment is produced by the cells in response to heat stress
  3. Higher temperatures increase the kinetic energy of phospholipids, disrupting the membrane and allowing pigment to leak out Correct answer.
  4. Lower temperatures increase membrane fluidity and cause more pigment to be released

Explanation

As temperature rises, phospholipids and proteins gain kinetic energy, the membrane becomes more fluid and proteins may denature, so the membrane loses its integrity and the pigment leaks out. The pigment is released passively from damaged cells, not actively transported.

Question 47AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement correctly describes how a vaccine produces immunity against a pathogen?

  1. It kills all pathogens already present in the body at the time of vaccination
  2. It provides passive immunity that lasts for many years without further exposure
  3. It contains antibodies that neutralise the pathogen before it can infect cells
  4. It stimulates memory cells so that a rapid secondary response occurs on later exposure Correct answer.

Explanation

A vaccine introduces antigens that trigger a primary immune response, generating memory B and T cells, so that a subsequent infection produces a faster and greater secondary response. Vaccines do not contain antibodies and do not act on pathogens already present.

Question 48AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

A student measures the diameter of a plant cell using an eyepiece graticule. The cell occupies 40 eyepiece divisions, and the graticule is calibrated so that 1 division equals 2.5 micrometres. What is the diameter of the cell?

  1. 160 micrometres
  2. 100 micrometres Correct answer.
  3. 16 micrometres
  4. 0.1 micrometres

Explanation

The diameter equals the number of divisions multiplied by the value of one division, so 40 times 2.5 micrometres gives 100 micrometres. Dividing rather than multiplying, or misplacing the decimal point, produces the other values.

Question 49AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement about the significance of the genetic code being degenerate is correct?

  1. Each amino acid is coded for by only one codon
  2. All codons code for an amino acid, with no stop codons
  3. A single amino acid can be coded for by more than one codon, reducing the effect of some mutations Correct answer.
  4. The code is different in different species of organism

Explanation

Degeneracy means that most amino acids are specified by several different codons, so a substitution in the third base often still codes for the same amino acid and the polypeptide is unchanged. The code is also universal and includes stop codons that do not code for amino acids.

Question 50AS Level content (main theoretical concepts, current applications of biology, advanced practical skills)

Which statement about the role of the stroma in a chloroplast is correct?

  1. It forms a barrier that prevents carbon dioxide entering the chloroplast
  2. It contains the enzymes needed for the light-independent reactions Correct answer.
  3. It is the site of ATP synthesis by photophosphorylation
  4. It contains stacks of thylakoids where the light-dependent reactions occur

Explanation

The stroma is the fluid-filled matrix of the chloroplast and contains the enzymes such as rubisco that catalyse the light-independent reactions of the Calvin cycle. Thylakoid membranes, stacked into grana, are the site of the light-dependent reactions.