May/June 2021

The diagram is of a mitochondrion taken from an electron micrograph. Using the line X-Y, the mitochondrion’s actual length is $3000\ \text{nm}$. What magnification does the drawing of the mitochondrion have?

The microscope in cell studies

Which of the following statements about triglycerides and phospholipids is correct?

Carbohydrates and lipids

Which statement correctly describes collagen?

Proteins

For a healthy human, the average number of haemoglobin molecules in a single red blood cell is 260 million. How many $\alpha$-globin chains are present in one red blood cell in a healthy human?

Proteins

Which statements about a peptide bond are right?

Proteins

Which graph correctly displays the activation energy for a reaction when an enzyme has been added?

Mode of action of enzymes

The enzyme lactase occurs in the membranes of the epithelial cells that line the small intestine. It is made from one polypeptide, which folds to form three regions. • an active site with the free amino group on the outside of the cell • a short region within the membrane • a short region within the cell What type of amino acid would be present in each of the three regions?

Proteins

A student recorded three statements about cell signalling. Which of the statements are correct?

Fluid mosaic membranes

During which stages of mitosis do chromosomes consist of two chromatids joined by a centromere?

Chromosome behaviour in mitosis

The jellyfish $\textit{Turritopsis dohrnii}$ is said to be immortal. Provided that $T.\ dohrnii$ is not consumed by predators or affected by disease, it appears capable of living forever. There is no method for finding the biological age of an individual $T.\ dohrnii$. Which property of the cells in $T.\ dohrnii$ might account for these observations?

Replication and division of nuclei and cells

Certain chemicals, which are used to halt tumour growth, function by stopping the DNA double helix from uncoiling and splitting apart. At which stage of the cell cycle would they act?

Replication and division of nuclei and cells

A microscope is used to view a specimen of plant tissue on two occasions: first with red light of wavelength $650\ \text{nm}$, and then with green light of wavelength $510\ \text{nm}$. What changes occur in the magnification and resolution when green light is used instead of red light?

The microscope in cell studies

The four nucleotides, A, B, C and D, each contain three phosphate groups, one nitrogenous base and one pentose sugar. The table shows the features of the base and sugar parts before they are combined to make each nucleotide. Which nucleotide can bind to an adenine base during DNA replication?

The microscope in cell studies

The statements refer to the process of translation. 1. A peptide bond is formed between neighbouring amino acids. 2. Hydrogen bonds are formed between the anticodon and the codon. 3. mRNA attaches to the ribosome. 4. tRNA moves into the ribosome while carrying a particular amino acid. In what sequence does this process occur?

Proteins

The order of amino acids in one section of a polypeptide is: ... histidine - proline - aspartic acid - leucine ... A table lists possible DNA triplet codes for each amino acid. What is a correct sequence of mRNA codons for this polypeptide section?

Proteins

Which factors help water rise upward in a plant xylem vessel? 1. cohesion between water molecules due to hydrogen bonding 2. adhesion between water molecules and the cellulose walls of xylem vessels due to hydrogen bonding 3. water being removed from xylem vessels in a leaf lowers the hydrostatic pressure in the xylem

Water

The diameter of a tree trunk normally falls a little over the course of the day. Which daytime changes in environmental factors could make the diameter fall by a greater amount?

Movement into and out of cells

The diagram shows a cross-section of a stem. Which region is the phloem?

Structure of transport tissues

The diagram presents a xerophytic leaf under two conditions, P and Q. Which statements describe how the cells in layer Y differ between conditions P and Q? 1. more negative water potential in P than Q 2. more cells plasmolysed in P 3. cells less turgid in Q 4. water potential becomes zero in Q

Movement into and out of cells

Different substances, for example sucrose and amino acids, may travel in opposite directions in the phloem sieve tube elements. Which statement explains this?

Movement into and out of cells

The statements below show some events in a cardiac cycle, but they are not arranged in the correct sequence. Which statement identifies the fourth event to happen during the cardiac cycle? 1. The impulse passes through Purkyne tissue. 2. A wave of excitation spreads across the atria. 3. The atrioventricular node holds back the impulse for a fraction of a second. 4. The sinoatrial node contracts. 5. The wave of excitation moves upwards from the base of the ventricles. 6. The ventricles contract. 7. The atria contract.

The heart

Which row correctly identifies the components of both lymph and tissue fluid?

Movement into and out of cells

The electron micrograph depicts a structure located in the cytoplasm of an animal cell. What is this cell structure?

Cells as the basic units of living organisms

Which row gives the correct mean blood pressure in the different regions of the human circulatory system?

The circulatory system

What mechanism explains how most carbon dioxide is carried in the blood?

Movement into and out of cells

The graph displays the dissociation curves for haemoglobin at two distinct partial pressures of carbon dioxide. At which point on the graph, A, B, C, or D, is the concentration of haemoglobinic acid lowest in red blood cells?

Proteins

The table presents the partial pressure of carbon dioxide and oxygen in two blood vessels. What accounts for the difference in oxygen partial pressure between the pulmonary artery and the pulmonary vein?

Movement into and out of cells

Which graph illustrates the effect of carbon monoxide on the percentage saturation of haemoglobin with oxygen?

Proteins

Which row identifies the correct methods by which the named pathogens are transmitted?

Infectious diseases

Which disease is brought about by a eukaryote?

Infectious diseases

By what first process do bacteria acquire resistance to antibiotics?

Antibiotics

What roles do T-lymphocytes carry out?

The immune system

Which statement about the function of memory cells in long-term immunity is correct?

The immune system

Which cell structures have nucleic acid in them?

Structure of nucleic acids and replication of DNA

Which events lead to a person developing actively acquired immunity?

Antibodies and vaccination

Which of these statements about mitochondria, chloroplasts or prokaryotes are correct?

Cells as the basic units of living organisms

The extremely large ($1000\ \text{nm}$) Pandoraviruses discovered in Chile and Australia are classified as viruses because they are unable to copy their own genome and cannot produce proteins. They also possess key structural characteristics in common with other viruses. What are the key structural characteristics of viruses?

Infectious diseases

A food sample was warmed with Benedict’s solution, and the colour altered to green. A second portion of the same food was heated with dilute hydrochloric acid, then made neutral with sodium hydrogencarbonate. After that, it was warmed with Benedict’s solution, and the colour changed to red. What do these results indicate?

Testing for biological molecules

Which pair of molecules contains only macromolecules present in animal cells?

Testing for biological molecules

Homogalacturonan is a polysaccharide present in plant cell walls. The diagram illustrates a molecule of the monomer that joins together to make homogalacturonan. A student examined the structure of this monomer and compared it with the structure of the monomer used to make cellulose. Which carbon atoms in the monomer shown in the diagram have hydroxyl groups in positions that differ from those in the cellulose monomer?

Carbohydrates and lipids

Which row gives the description of the membranes around each cell structure?

Fluid mosaic membranes

The four fatty acids listed below have these formulae: caprylic acid $\text{CH}_3(\text{CH}_2)_6\text{COOH}$ lauric acid $\text{CH}_3(\text{CH}_2)_{10}\text{COOH}$ oleic acid $\text{CH}_3(\text{CH}_2)_7\text{CH}=\text{CH}(\text{CH}_2)_7\text{COOH}$ palmitoleic acid $\text{CH}_3(\text{CH}_2)_5\text{CH}=\text{CH}(\text{CH}_2)_7\text{COOH}$ Which of these fatty acids are saturated?

Carbohydrates and lipids

Which molecule has the fewest hydrogen atoms?

Water

The figure illustrates sucrose and sucralose. The enzyme sucrase hydrolyses sucrose but is unable to hydrolyse sucralose. Four students were asked to propose reasons why sucrase can hydrolyse sucrose but not sucralose. Three of the students made correct proposals. Which proposal cannot be correct?

Mode of action of enzymes

The diagram displays the energy levels involved in an enzyme-catalysed reaction. Substrate molecules X and Y join together to form product Z. Which arrow indicates the decrease in activation energy caused by the enzyme?

Mode of action of enzymes

Which functions of the cell surface membrane arise from the properties of the phospholipids?

Fluid mosaic membranes

Which factors can be altered to influence the rate of facilitated diffusion across a cell surface membrane?

Movement into and out of cells

The diagram illustrates an experiment that uses a model cell to study the movement of substances. Which statements are correct?

Movement into and out of cells

Human chromosomes are made up of different parts. Some parts occur more often than others. Which parts are given in order from the most numerous to the least numerous in a human white blood cell?

Chromosome behaviour in mitosis

The diagram shows the cell cycle of a human cell. As the cell cycle proceeds, the number of chromatids varies. Which row gives the correct number of chromatids in M, G$_1$, and G$_2$?

Replication and division of nuclei and cells

A single skin cell was isolated and placed into growth medium in a sterile Petri dish. The Petri dish was then incubated for 16 days. Over this period, the number of skin cells increased because mitotic divisions happened repeatedly. In the nucleus of each skin cell, one chromosome has a telomere made up of many repeats of the base sequence TTAGGG. On days 4, 8, 12 and 16 of the incubation period, one cell was taken out of the Petri dish each time. For each of these cells, the total number of bases in the telomere of this chromosome was measured. The four cells had different total numbers of telomere bases for this chromosome: 5548, 5580, 5645 and 5700. What was the total number of telomere bases in the chromosome from the cell that had undergone the greatest number of mitotic divisions?

Chromosome behaviour in mitosis

In an electron micrograph, the mean width of mitochondria measures $6\ \text{mm}$. The electron micrograph has a magnification of $\times 9600$. What is the actual mean width of the mitochondria?

The microscope in cell studies

A DNA fragment that is 18 base pairs in length was examined to determine how many nucleotide bases were in each polynucleotide strand. The table shows some of the findings. How many nucleotides containing thymine were in strand 2?

The microscope in cell studies

The diagram illustrates a strand of DNA and mRNA while transcription is taking place. Which row correctly names 1, 2 and 3?

Proteins

Two students were talking about how DNA and RNA are involved in transcription and translation. Student 1 only made correct statements. Student 2 added more detail, and some of this was correct. Which extra statement made by Student 2 is correct?

Proteins

Which statements about how xylem vessel elements are adapted to their function are correct?

Structure of transport tissues

The diameter of a tree trunk normally becomes a little smaller over the course of the day. Which alterations in environmental conditions during the day could make the diameter decrease by a greater amount?

Movement into and out of cells

A student wanted to determine the rate of transpiration by using a potometer, as shown. The student was asked to calculate a transpiration value with the units $\text{mm}^3$, $\text{m}^{-2}$ and $\text{hour}^{-1}$. Which measurements would the student need to make?

Movement into and out of cells

In plants, assimilates such as sucrose are transferred from source cells into sieve tube elements. The assimilates are then moved from source to sink. What is required to move sucrose from source cells through phloem tissue to sink cells?

Movement into and out of cells

The sinoatrial node starts the cardiac cycle. In what correct sequence do the chambers of the heart contract?

The heart

The graph illustrates pressure changes in various regions of the heart during a mammalian cardiac cycle. W, X, Y and Z show the times when a valve opens or closes. Which row is the correct one?

The heart

What substance can bind to the haem group in a haemoglobin molecule?

Proteins

Which sequence gives the correct order of some stages involved in the production and secretion of an enzyme?

Protein synthesis

Which main reaction takes place in the blood capillaries at the gas-exchange surface of a human lung?

Mode of action of enzymes

Which reactions happen in blood that is flowing through active tissues?

Movement into and out of cells

Which row identifies the tissues found in the walls of the trachea and bronchus?

The gas exchange system

In some people with lung disease, the partial pressure of oxygen in the pulmonary veins is lower than the partial pressure of oxygen in the alveoli. What might account for the difference in partial pressure of oxygen?

Movement into and out of cells

How many times does a molecule of carbon dioxide need to cross a cell surface membrane as it diffuses from the plasma, through a cell in the capillary wall, into an alveolus?

Movement into and out of cells

Which disease is transmitted by a vector?

Infectious diseases

Which statement best accounts for why a measles vaccination programme using only one dose has not yet removed the disease?

Antibodies and vaccination

What has caused the rise in antibiotic resistance in bacteria?

Antibiotics

What accounts for the ability of monoclonal antibodies to target cancer cells?

Proteins

Which of the following statements accurately describe lymphocytes?

The immune system

Antibiotics work through different mechanisms. In an ideal situation, the antibiotic destroys the bacteria while leaving the infected human unharmed. One antibiotic, tetracycline, may interfere with how human mitochondria operate. What accounts for tetracycline’s effect on human mitochondria?

Antibiotics

During a measles epidemic, a country introduced measles vaccination. It was later found that children who had been vaccinated were more likely to survive childhood than children who had not been vaccinated, even in years when there were no measles epidemics. The vaccine had provided the children with some defence against other pathogenic infections. Which statement could explain this additional protection?

Proteins

A prokaryotic cell measures $5.0\ \mu\text{m}$ long. A virus particle measures $300\ \text{nm}$ long. By what factor is the prokaryotic cell bigger than the virus particle?

The microscope in cell studies

A student performs a semi-quantitative test using Benedict's solution. Which statement concerning this procedure is correct?

Testing for biological molecules

Which row shows the correct information for carbohydrates?

Carbohydrates and lipids

Which option gives the correct general formula for glycogen?

Carbohydrates and lipids

Chitin is a structural polysaccharide present in the tough outer coverings of animals such as crabs. It consists of chains of amino sugars that include NH groups. The diagram depicts two sugars in the chain of a chitin molecule. Which polysaccharide is most similar to chitin?

Carbohydrates and lipids

Which combination of measurements is accurate?

The microscope in cell studies

Which statements concerning carbohydrates and triglycerides are correct?

Carbohydrates and lipids

Which molecules include at least three double bonds?

Carbohydrates and lipids

The diagrams illustrate the structures of two amino acids. In one, there are two amino $(-\text{NH}_2)$ groups, numbered 1 and 2. In the other, there are two carboxylic $(-\text{COOH})$ groups, numbered 3 and 4. A peptide bond is made between the two amino acids. Which groups take part in forming the peptide bond?

Proteins

Which row correctly describes the structure of proteins?

Proteins

The enzyme $\beta$-galactosidase is able to catalyse the hydrolysis of four substrates that have similar structures. Each substrate produces a different $K_m$ value. Which substrate does $\beta$-galactosidase have the greatest affinity for?

Mode of action of enzymes

An investigation examined how temperature affects enzyme activity both when the enzyme is immobilised and when it is non-immobilised (free in solution). The product formed by the enzyme-catalysed reaction leads to a fall in pH. The graph presents the outcomes of the investigation. Which condition would produce the greatest yield of product?

Factors that affect enzyme action

When an indicator is combined with agar, it produces a pink colour. If this pink agar is placed in acid, it turns colourless. Pink agar blocks are cut into different sizes and then placed in acid. All the other variables are kept the same. Which block turns colourless the fastest?

Movement into and out of cells

Three identical plant cells were placed in one of three sugar solution concentrations: 10%, 5% and 2.5%. The cells were left for 50 minutes before being examined with a light microscope. Which statement is not correct?

Movement into and out of cells

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

Replication and division of nuclei and cells

Hydra are simple animals able to reproduce asexually. The photomicrograph displays an adult hydra and, attached to its side, a developing new hydra. Which processes have occurred in the two hydra?

Replication and division of nuclei and cells

The diagram illustrates an eyepiece graticule and part of a stage micrometer scale as observed at $\times 100$ magnification. Which is the correct procedure for finding the value of one eyepiece graticule unit in micrometres ($\mu\text{m}$)?

The microscope in cell studies

The diagram shows a nucleotide that contains thymine. Which of the following statements about this nucleotide are correct?

Structure of nucleic acids and replication of DNA

A number of the events that happen during transcription are shown. Before the mRNA molecule exits the nucleus, which events happen twice during transcription?

Protein synthesis

The table lists the DNA triplet codes for some amino acids. The base sequence on the DNA template strand that codes for part of a polypeptide is given below. $\text{CCA TTC ACG GCG TTA GCA}$ During DNA replication, two mutations take place in this sequence. Which mutated DNA would produce a polypeptide with one amino acid different?

Proteins

One gene determines the amino-acid sequence of a single polypeptide. Haemoglobin is composed of two $$-globins and two $$-globins. How many genes are required to code for one haemoglobin molecule?

Proteins

Which characteristics of water molecules are important in the upward movement of water through the xylem?

Water

The graph illustrates the rate at which a plant absorbs water and the rate at which it loses water over a single 24-hour period. The plant was growing in natural conditions. What can be concluded from the graph?

Movement into and out of cells

Which statements describe why a stem is cut beneath water and linked to a potometer beneath water?

Movement into and out of cells

Sucrose is transported from a phloem sieve tube element into an actively dividing shoot tip. Which pair of changes to water potential and to the volume of liquid in the phloem sieve tube element is correct?

Movement into and out of cells

Sucrose is transferred from companion cells into phloem sieve tubes. Which sequence of statements correctly describes this mechanism?

Movement into and out of cells

Which structures are present in arteries?

The circulatory system

A prokaryotic cell with a diameter of $1\,\mu\text{m}$ is enlarged $50\,000$ times in an electron micrograph. What diameter does it appear to have in the electron micrograph?

The microscope in cell studies

Which diagram accurately shows the direction in which blood flows through the heart?

The heart

Which row correctly shows the pulmonary artery?

The microscope in cell studies

What substance is present in all blood vessels, lymph, and tissue fluid?

Water

At high altitudes, the oxygen content of the air can be only a third of the amount at sea level. As a person makes a gradual climb up a mountain, the body slowly adapts to the high altitude. What is increased during this time of adjustment?

Transport of oxygen and carbon dioxide

Which line correctly indicates the features found in terminal bronchioles?

The gas exchange system

The photomicrograph displays a section of lung tissue. Which structures can be seen in this photomicrograph?

The microscope in cell studies

The symptoms of two diseases are given. Which row correctly identifies diseases 1 and 2?

Infectious diseases

Disease spread can be lowered in several ways: • antibiotic therapy for sufferers • vaccination for non-sufferers • more living space per person Which disease’s spread can be reduced by all three of these methods?

Infectious diseases

What characteristic is shared by the pathogens that cause HIV/AIDS, malaria and TB?

Infectious diseases

The diagram illustrates the immune response after a virus infection. Which row correctly identifies the cells labelled E, F, G and H?

The immune system

Which cell structures have DNA in them?

Cells as the basic units of living organisms

Monoclonal antibodies are employed to detect whether the hormone HCG is present in the urine of a human female in the early stages of pregnancy. Which statements explain how the monoclonal antibodies used in this test are produced?

Proteins

Four students were set the task of matching each function to the appearance of certain cell structures in an animal cell. The functions were given as numbers: 1 mRNA passes through to the ribosome; 2 synthesis of polypeptides; 3 synthesis of lipids. The appearances were given as letters: V membranes that enclose an inner cavity; W non-membrane bound, spherical structures; X a double membrane containing pores; Y non-membrane bound, cylindrical structures; Z membrane-bound sacs arranged as a flattened stack. Which student made the correct match between the numbered function and the appearance of the cell structure?

Cells as the basic units of living organisms

Which row gives the correct structures found in eukaryotic cells?

Cells as the basic units of living organisms

Four solutions were examined for the presence of four different biological molecules. The appearance of the solutions after each test is shown in the table. Which solutions contained molecules with ester bonds?

Carbohydrates and lipids

The diagrams show two monosaccharides that share the same molecular formula $(\text{C}_6\text{H}_{12}\text{O}_6)$. As illustrated, each may occur in either an alpha $(\alpha)$ or beta $(\beta)$ form. The diagram also indicates a lactose molecule produced by condensation between glucose and galactose. Which molecules have condensed to make lactose?

Carbohydrates and lipids

A covalent bond of this kind between two monomers is illustrated. Which molecules have this covalent bond?

Proteins