May/June 2023

The electron micrograph depicts onion root cells that were prepared by means of a freeze-fracture technique. The cells were rapidly frozen and then split open mechanically. Freeze fracture separates cells along weak regions, such as membranes and the surfaces of organelles. Which statement best accounts for the appearance of the electron micrograph?

Fluid mosaic membranes

Insulin is a globular protein that functions in cell signalling. It is carried in the blood plasma from the cells that make it to its target cells. One molecule of insulin contains six sulfur-containing amino acids and consists of two polypeptide chains. Which of the following statements about insulin are correct? 1. An insulin molecule has a quaternary structure. 2. Insulin polypeptides are held together by six disulfide bonds. 3. Amino acids with hydrophobic R groups would be found in the centre of an insulin molecule.

Proteins

Which statement gives the correct explanation for why blood plasma can be kept at a constant temperature?

Water

Which part of the graph indicates the activation energy for an enzyme-catalysed reaction?

Mode of action of enzymes

The graph illustrates how an increase in substrate concentration affects the rate of an enzyme-catalysed reaction. Line P shows the outcome when the enzyme is used at its optimum pH and optimum temperature, with no inhibitor present. Line Q shows the outcome when the reaction conditions are altered. Which descriptions of changes to the reaction conditions could produce line Q if all the other conditions remained unchanged? 1. Introduce an inhibitor that binds to a site other than the active site. 2. Introduce an inhibitor with a shape similar to the substrate. 3. Introduce an inhibitor that prevents the active site of the enzyme from working. 4. Perform the reaction at a higher temperature.

Factors that affect enzyme action

Which statement concerning cell signalling is correct?

Fluid mosaic membranes

Four students, A, B, C and D, looked at plant epidermal cells after these had been kept in a concentrated sucrose solution for 30 minutes. They were told to identify the partially permeable layer and to explain how the cells looked in terms of water potential and the movement of water. Which student is correct?

Movement into and out of cells

The table shows the surface area to volume ratios for five agar blocks that have different dimensions but the same volume. These agar blocks may be used to assess diffusion efficiency, with efficiency defined by the time needed for a dye to spread to every part of the block. Which prediction can be made about how the size and dimensions of these blocks affect diffusion efficiency?

Movement into and out of cells

Mitosis is part of the cell cycle. What features describe this type of nuclear division? 1. produces daughter cells that are the same size as the parent cell 2. produces nuclei that are genetically identical 3. semi-conservative copying of DNA

Replication and division of nuclei and cells

A student examined the cells in the growing region (meristem) of an onion root and recorded the results shown. Stage and number of cells: interphase: 886 prophase: 73 metaphase: 16 anaphase: 14 telophase: 11 What percentage of the cells have chromosomes that are visible as two chromatids?

Chromosome behaviour in mitosis

Which of the following statements about messenger RNA is correct?

Proteins

Which cell structures are capable of forming vesicles?

Movement into and out of cells

Which structures take part in transcription alone?

Protein synthesis

According to the code carried by one gene, which type of molecule is produced?

Proteins

The table lists some mRNA codons that specify certain amino acids. A DNA template strand has the base sequence below. ACAGTATTATTTGCAACG What change would occur in the amino acid if the first base of the fifth DNA triplet were replaced by an A base?

Proteins

Which structures have cytoplasm that includes mitochondria and a nucleus?

Cells as the basic units of living organisms

What term is used to describe intermolecular hydrogen bonding between water molecules?

Water

The diagram depicts the contour of a xerophytic leaf that was left for 45 minutes under different conditions, P and Q. Which statements concerning the cells in layer Y of the leaf in each of conditions P and Q after 45 minutes are correct?

Movement into and out of cells

By what route does sucrose travel from chloroplasts into the phloem?

Movement into and out of cells

In what way do companion cells take part in loading sucrose into phloem sieve tube elements?

Movement into and out of cells

The photomicrograph displays blood cells as they appear under a high-power light microscope. Which row correctly identifies the various types of white blood cell?

The microscope in cell studies

Schematic diagrams of two blood vessels are shown. Which labels are right?

The microscope in cell studies

Four students had to link each function to the form of certain cell structures in an animal cell. The functions were given as numbers: 1 synthesis of polypeptides, 2 synthesis of lipids, 3 packaging of hydrolytic enzymes that will remain in the cell. The forms were given as letters: V membranes that enclose a central cavity, W spherical structures with no membrane, X a double membrane with pores between it, Y cylindrical structures with no membrane, Z membrane-bound sacs arranged in a flattened stack. Which student made the correct match between the numbered function and the form of the cell structure?

Cells as the basic units of living organisms

What substances are present in blood and tissue fluid?

Proteins

Which reactions would have their rate reduced by an inhibitor of carbonic anhydrase?

Factors that affect enzyme action

Which set of letters shows the correct order of events in the cardiac cycle? T atrial walls contract U the impulse is held up for a fraction of a second V the wave of excitation enters the atrioventricular node W the wave of excitation travels along the Purkyne tissue X the wave of excitation spreads out from the sinoatrial node Y ventricles contract

The heart

The diagram shows how blood pressure changes on the left side of the heart during the cardiac cycle. The valves either open or shut at the numbered points. Which row names the valves that are opening or closing at the numbered points?

The heart

An individual with no respiratory problems lies at rest for an hour. During this period, their breathing is shallow and slow, so only a small amount of air from outside the body reaches the alveoli. The person’s heart rate stays constant. Which statement is correct?

Movement into and out of cells

Which of these statements apply to every bronchiole?

The gas exchange system

The diagram illustrates several pathogens that cause disease in humans and some of the ways in which they are spread. Which pathogen and route of transmission are correct for malaria?

Infectious diseases

Several of the stages that lead to the development of a bacterial population resistant to a new antibiotic are shown below. 1 change in reproductive success of bacteria 2 increase in frequency of the resistance allele in the population 3 increase in genetic variation within the population 4 random mutation occurs in bacterial DNA What is the correct sequence of these processes?

The microscope in cell studies

If bacteria are cultured in a Petri dish with discs that contain antibiotics, zones of inhibition of bacterial growth will form. The chart presents the diameters of the zones of inhibition when a species of bacteria was incubated on five separate agar plates, each with a disc carrying a different antibiotic. Which conclusions can be drawn about the most and least effective antibiotics for this species of bacteria?

Antibiotics

What type of blood cell is unable to recognise, engulf and digest non-self particles?

The immune system

Which type of cell has a tonoplast?

Cells as the basic units of living organisms

Having malaria repeatedly leads to stronger immunity against malaria. Which kind of immunity causes this stronger immunity?

Antibodies and vaccination

Which organelles in animal or plant cells are enclosed by double membranes?

Cells as the basic units of living organisms

Some scientists believe that mitochondria originated from bacteria that moved into the cytoplasm of another cell and were able to survive there. Which structural characteristics of mitochondria provide support for this hypothesis?

Cells as the basic units of living organisms

What substance is found in all viruses, all prokaryotes and all eukaryotes?

Structure of nucleic acids and replication of DNA

The table lists a number of steps that may be used when doing Benedict’s test. Which combination of steps is needed to carry out a semi-quantitative test on a reducing sugar solution?

Testing for biological molecules

The diagrams illustrate three examples of bond types. Which bonds keep the secondary structure of proteins joined together?

Proteins

A graticule and a micrometer scale can be used to determine the size of biological structures that are observed with a microscope. Which row indicates the correct positions for putting a graticule and a micrometer scale on the microscope shown?

The microscope in cell studies

Which statement describes cellulose?

Carbohydrates and lipids

Which part of the haemoglobin structure transports oxygen?

Proteins

What is the greatest number of hydrogen bonds that can form between two individual water molecules?

Water

CYP3A4 is a key enzyme in the human digestive system, where it is required to break down a variety of toxins. The activity of CYP3A4 has been found to be lowered by compounds known as furanocoumarins. Furanocoumarins occur in some fruits, so harmful concentrations of toxins may build up in the human digestive system when fruits containing furanocoumarins are consumed. From the information given, what can be concluded about molecules of the enzyme CYP3A4?

Mode of action of enzymes

A fixed volume and concentration of substrate and enzyme were combined. Every other variable was kept unchanged. The enzyme-catalysed reaction was allowed to continue until it had finished. Which graph shows how the rate of reaction changes with time?

Mode of action of enzymes

Which molecules in cell surface membranes are usually used for cell recognition?

Fluid mosaic membranes

Which factor can make the cell surface membrane more fluid?

Fluid mosaic membranes

The three main factors that determine the rate of diffusion across a membrane are related by the expression shown below. The rate of diffusion is proportional to $\frac{\text{surface area} \times \text{concentration difference}}{\text{thickness of membrane}}$. Which change or changes in these factors would make the rate of diffusion twice as large?

Movement into and out of cells

A student recorded how long complete diffusion of a dye into agar blocks of different sizes took while the blocks were suspended in the dye. The results are shown. What is the predicted time for complete diffusion of the dye into the agar block measuring $5\,\text{mm} \times 10\,\text{mm} \times 15\,\text{mm}$?

Movement into and out of cells

An experiment was performed to examine how the concentration of sucrose solution affected cells in a plant tissue. After 2 hours, the cylinders were taken out, dried with blotting paper and weighed again. The percentage change in mass for each cylinder was noted. The graph presents the findings of this investigation. Which row gives the explanation for the results if plant tissue cells were placed in a sucrose solution of $0.45\,\text{mol dm}^{-3}$?

Movement into and out of cells

Six organelles present in eukaryotic cells are shown. Which organelles take part in the synthesis and secretion of a glycoprotein?

Protein synthesis

The diagram presents a portion of the arrangement of a DNA molecule section and the associated histones, P and R, during prophase of mitosis. Which statement about the features labelled P, Q and R in prophase of mitosis is correct?

Chromosome behaviour in mitosis

At the beginning of the stages of the mitotic cell cycle shown, how many copies of each DNA molecule are present in a cell?

Replication and division of nuclei and cells

DNA has the feature of being a universal genetic code. What does the term universal genetic code mean?

Proteins

Which statement concerning mRNA is correct?

Proteins

The diagram illustrates a section of the translation process. What are the names of the structures marked X, Y and Z?

Proteins

During protein synthesis, DNA triplets in genes are read as amino acids or stop signals. The table lists some of these triplets. What could be the effects of one substitution mutation in a triplet that codes for tyrosine?

Proteins

Which row shows cells that have plasmodesmata?

Cells as the basic units of living organisms

Which statements concerning the apoplast and symplast pathways are correct?

Movement into and out of cells

During transpiration, from which part does water evaporate?

Movement into and out of cells

The diagram illustrates the outlines of three xerophytic leaves of the same type under three different conditions, P, R and S. Which statement about the water potential of the cells in layer Y is correct?

Movement into and out of cells

Inside which cell structures may mRNA be found?

Protein synthesis

The photomicrograph shows a transverse section through the leaf of a species of grass. This grass is specially adapted for growth in a dry habitat. Which row correctly explains how these features help the grass to grow in this habitat?

The microscope in cell studies

Which statement provides evidence for the theory that sucrose is actively loaded into companion cells?

Movement into and out of cells

Which characteristics of water are necessary for its function in the transport of blood in mammals?

Water

The diagram illustrates some of the events occurring between the plasma and the red blood cells in the circulatory system. What do the numbers 1, 2, 3 and 4 stand for?

Movement into and out of cells

These diagrams display the heart valves in cross-section from above during different points of the cardiac cycle. Which stages of the cardiac cycle are being shown?

The heart

In the human gas exchange system, where can squamous epithelial cells be found?

Movement into and out of cells

Which of the following statements about gas exchange between the air in the alveoli and the blood in the pulmonary capillaries is correct?

Movement into and out of cells

The mean thicknesses of several structures in the human respiratory system are shown. An oxygen molecule is in the alveolar air space, beside the wall of the alveolus. What is the minimum distance that the molecule must diffuse from its current position to the haemoglobin that completely fills a red blood cell in the nearest capillary? (Assume that the red blood cells are in contact with the walls of a capillary.)

Movement into and out of cells

The statements are about the disease tuberculosis (TB). Which statements give reasons why antibiotic treatment for TB needs a long time?

Replication and division of nuclei and cells

The mean sizes of some pathogens are given. One kind of air filter has been found to work in stopping any pathogens of $1\,\mu\text{m}$ or greater from entering the air system of a room. Using their size and method of transmission, which diseases would this air filter stop from entering the air system of a room?

The microscope in cell studies

A scientist performed an experiment to separate the organelles in an animal cell according to mass. The cells were combined with a buffer solution that had the same water potential as the cells. A blender was used to break the cells open and release the organelles. The resulting mixture was filtered and then placed in a centrifuge at high speed so that the heaviest type of organelle could be separated. These organelles settled at the bottom, creating solid pellet 1. The liquid above pellet 1 was transferred to a clean centrifuge tube and spun in the centrifuge at a higher speed to separate the next heaviest type of organelle. These organelles settled at the bottom, creating solid pellet 2. This process was repeated twice more to produce pellet 3 and pellet 4, each containing one type of organelle only. What is the main function of the type of organelle obtained in pellet 2?

Cells as the basic units of living organisms

A vaccination programme is successful when it creates a level of immunity in which most individuals in a population are protected. Several factors can influence how successful a vaccination programme is. Which row correctly identifies the factors that can influence how successful a vaccination programme is?

Antibodies and vaccination

What structures are present in palisade mesophyll cells and photosynthetic prokaryotes?

Cells as the basic units of living organisms

Which polymers occur in every virus and in all prokaryotes?

Structure of nucleic acids and replication of DNA

Which sequence of steps is the most suitable way to carry out the emulsion test for lipids?

Testing for biological molecules

A student was given a carbohydrate solution. Two portions were taken from it and tests were carried out on each portion, as shown. Which statement accounts for the results?

Testing for biological molecules

Which molecules have at least two double bonds?

Carbohydrates and lipids

Which feature can be seen using a light microscope with a natural light source?

The microscope in cell studies

Cocoa butter is composed of three triglycerides. The fatty acids used to form these triglycerides are oleic acid (O), palmitic acid (P) and stearic acid (S). The triglycerides present in cocoa butter are POS, SOS and POP. The chemical structure of the triglyceride POS is shown alongside a schematic diagram of POS. Which statement is correct?

Carbohydrates and lipids

Which molecules contain no fewer than four double bonds?

Carbohydrates and lipids

The diagram represents the amino acid glutamic acid. What is the R group for glutamic acid?

Proteins

The diagram illustrates three interactions that keep protein molecules in their shape. Which row names these interactions?

Proteins

Certain animals make antimicrobial proteins that defend them against pathogens. These proteins could be used to destroy human pathogens, but when they are used as a medicine they are broken down by protein-digesting enzymes. Substituting one of the amino acids in the protein with an amino acid synthesised in the laboratory produced a modified protein that was not broken down. What might account for the fact that this modified protein was not broken down by the protein-digesting enzymes?

Proteins

A student carried out an investigation into how substrate concentration affects the rate of an enzyme-catalysed reaction. A graph was drawn to display the link between these two variables. The student was told to read values from the graph that could then be used to find the Michaelis-Menten constant, $K_m$, for this enzyme. J, K, L and M mark points taken from the graphs that the student could use to find the value of $K_m$. Which two readings does the student need to use to determine the value of $K_m$?

Factors that affect enzyme action

The final product of a metabolic pathway may function as a competitive inhibitor. This is known as end-product inhibition, and it enables a cell to regulate a metabolic pathway. The diagram illustrates a metabolic pathway in which the end-product could inhibit enzyme W. What effect would there be if, rather than enzyme W, the end-product inhibited enzyme Z?

Mode of action of enzymes

Which row indicates how cholesterol and the carbohydrate chains of glycolipids and glycoproteins are distributed in a cell surface membrane?

Fluid mosaic membranes

Which row correctly gives all the possible relationships between the concentrations of a substance when it is transported by endocytosis or exocytosis?

Movement into and out of cells

Which statement about simple diffusion is right?

Movement into and out of cells

An electron micrograph displays a structure present in the cytoplasm of an animal cell. Which biological molecules occur in this structure?

Protein synthesis

Plant cells whose cytoplasm had the same water potential were each placed in one of three sugar solutions of different concentration, 10%, 5% and 2.5%. They were left for 50 minutes and then viewed with a light microscope. Which statements are correct?

Movement into and out of cells

Which row states some characteristics of stem cells?

Cells as the basic units of living organisms

Mitotic index is a way of measuring the proportion of cells in a section of tissue that are undergoing mitosis. It is found by using the formula shown: mitotic index = (number of cells undergoing mitosis \div total number of cells) \times 100. A scientist worked out the mitotic index for regions of onion root at different distances from the root tip. The results are shown. Which statement is correct?

Replication and division of nuclei and cells

Which molecules form ATP’s structure?

Structure of nucleic acids and replication of DNA

A DNA fragment, 19 base pairs in length, was examined to determine the number of nucleotide bases in each polynucleotide strand. Some results are shown. How many cytosines were present in strand 1?

Structure of nucleic acids and replication of DNA

Which statement correctly describes the role of DNA polymerase during semi-conservative replication of DNA?

Replication and division of nuclei and cells

The diagram illustrates translation taking place at a ribosome. What is the base sequence at S?

Proteins

Which row gives the correct explanation of how the structures of phloem sieve tube elements and xylem vessel elements are linked to their roles as transport systems?

Structure of transport tissues

Which part of a plant does the apoplast pathway use as water moves from the soil into the xylem?

Movement into and out of cells

Which statements accurately describe transport pathways in dicotyledonous plants?

Movement into and out of cells

Which cell structures are found to contain nucleic acid?

Structure of nucleic acids and replication of DNA

Plants such as the tobacco plant keep very little of the water they absorb, and the amount of water lost in transpiration is extremely large. This is because these plants can take in carbon dioxide only when the stomata are open. Plants use carbon dioxide to make glucose molecules. It is estimated that 400 molecules of water are lost for every molecule of carbon dioxide absorbed. How many water molecules are lost for a plant to make one molecule of glucose?

Movement into and out of cells

Which changes take place as sucrose moves from leaves into phloem sieve tubes for transport to a sink?

Movement into and out of cells

The graph displayed has no labels on either axis. Which row gives suitable axis labels that would account for mass flow in phloem?

Movement into and out of cells

What substance is found in the blood inside human veins?

Transport of oxygen and carbon dioxide

The photomicrograph displays two blood vessels, P and Q. These two blood vessels belong to a network that carries blood throughout the body. Which row gives the correct identities for blood vessel P and blood vessel Q?

The circulatory system

The diagram illustrates a blood capillary together with the tissue fluid that surrounds it. Tissue fluid is produced when fluid and dissolved substances from blood plasma move through very small spaces in the capillary wall. Most of this tissue fluid is then taken back into the blood inside the capillary. What pressures are required at points W, X, Y and Z for this system to work?

Movement into and out of cells

What occurs during ventricular systole in a mammalian heart?

The heart

The photomicrograph displays a section of the human gas exchange system, with one tissue marked P. Which row is right?

The microscope in cell studies

How many times does a molecule of oxygen need to cross a cell surface membrane as it diffuses from the air space inside an alveolus, through a cell in the capillary wall, before binding to a molecule of haemoglobin?

Movement into and out of cells

A student made four claims about cholera. Which claim is incorrect?

Infectious diseases

Mitochondria are believed to have evolved from prokaryotic cells that were taken in by an ancestral cell. Which feature did prokaryotes lose as they evolved into mitochondria?

Cells as the basic units of living organisms

Monoclonal antibodies are made by means of the hybridoma method. What is taken out of a mouse's spleen in order to make monoclonal antibodies?

Antibodies and vaccination

Which polymers are found in every virus, every prokaryote and every eukaryote?

Testing for biological molecules

In how many of the structures listed below would genetic material usually include telomeres? • bacterial cell • chloroplast • mitochondrion • nucleus

Chromosome behaviour in mitosis

Stages 1, 2, 3 and 4 are carried out to test for a non-reducing sugar. 1. Place $5\,\text{cm}^3$ of solution in a test-tube. 2. Add a few drops of acid. 3. Neutralise using alkali. 4. Add $6\,\text{cm}^3$ of Benedict’s solution. At what point is the solution heated or boiled?

Testing for biological molecules

Which characteristics help a cellulose molecule to function?

Carbohydrates and lipids

Which description of triglycerides is correct?

Carbohydrates and lipids