Feb/March 2025

On an electron micrograph, the measured length of a mitochondrion is $17.1\ \text{cm}$. The magnification of the electron micrograph is $\times 38\,000$. What is the mitochondrion’s actual length?

The microscope in cell studies

Hydroxyproline is formed when an -OH group is added onto the R-group of the amino acid proline. Hydroxyproline is a major component of collagen and helps to improve the stability of its structure. Which statement explains why adding an -OH group to proline could make collagen more stable?

Proteins

Plant cell walls gain strength from cellulose molecules that are organised into several layers. In each layer, the cellulose molecules all lie in the same direction (parallel). Which row shows the bonds that join neighbouring cellulose molecules within each layer and the arrangement of cellulose molecules in different layers?

Carbohydrates and lipids

Galactogen is a storage polysaccharide found in certain animal species. It is a branched polymer made from β-galactose monomers. Which comparison between galactogen and another polysaccharide accurately states one similarity and one difference?

Carbohydrates and lipids

Which levels of protein structure are always involved in the formation of an enzyme’s active site?

Proteins

Catalase is an enzyme which decomposes hydrogen peroxide into water and oxygen. Catalase was introduced into a hydrogen peroxide solution, and the oxygen given off was gathered in a gas syringe. The overall volume of oxygen produced from the beginning of the reaction was measured every 10 seconds for 1 minute. The results are displayed in the table. What conclusion can be drawn from these results?

Mode of action of enzymes

Succinic dehydrogenase is an enzyme that catalyses the conversion of succinate into fumarate during aerobic respiration. Malonate is a reversible inhibitor of succinic dehydrogenase. By binding to the enzyme’s active site, malonate lowers the enzyme’s activity. Malonate and succinate are unable to bind to the active site at the same time. Which statement explains the effect of malonate on the activity of succinic dehydrogenase?

Mode of action of enzymes

The diagram gives the measurements of two agar blocks. It has been drawn to scale. Both agar blocks were coloured pink using a pH indicator. Under acidic conditions, the pink pH indicator turns colourless. The two agar blocks were put into a beaker of acid at the same time. As the acid diffused into the blocks, they became colourless. What is the surface area to volume ratio of the block that turned completely colourless first?

Movement into and out of cells

The graph illustrates how the rate of facilitated diffusion of substance X across a cell surface membrane altered as the concentration of substance X rose. Every condition other than the concentration of substance X was held constant. Temperature was kept at 15 ^{\circ}C. Which statement about the rate of facilitated diffusion is correct?

Movement into and out of cells

Which molecule makes the bilayer in the cell surface membrane of a bacterial cell?

Fluid mosaic membranes

A nucleus in a body cell of a species of fruit fly contains 8 chromosomes. How many strands of DNA are found in the nucleus at the end of interphase?

Replication and division of nuclei and cells

One theory suggests that mitochondria and chloroplasts began as free-living prokaryotes. It is believed that, millions of years ago, these free-living prokaryotes entered larger cells by endocytosis and, rather than being broken down, became working organelles. Which characteristics of mitochondria and chloroplasts provide evidence for this theory?

Cells as the basic units of living organisms

The photomicrograph displays plant cells at different points in the mitotic cell cycle. Four cells are marked with numbers so that they can be identified. One of the numbered cells is in the main stage of mitosis where the spindle starts to form. Another numbered cell is in the main stage of mitosis where the spindle fibres shorten. Which row correctly identifies the two cells in these stages of mitosis?

Chromosome behaviour in mitosis

What percentage of the chromosomal DNA in a cell during $G_1$ is still present in that same cell later in the same mitotic cell cycle in prophase and in telophase?

Replication and division of nuclei and cells

The statements refer to two genes and the proteins they produce, both of which may contribute to tumour development. • The protein encoded by the $PTEN$ gene stops cells from increasing in size and dividing too quickly. • The protein encoded by the $p53$ gene stops cells from moving on through the mitotic cell cycle when those cells contain damaged DNA. Which pair of mutations in these two genes in one individual is most likely to lead to tumour formation?

Replication and division of nuclei and cells

Telomerase is an enzyme that stops telomeres from becoming shorter. It is absent from most normal cells, but it is active in an estimated 85% to 95% of human tumour cells. Which statement explains the effect of telomerase on human tumour cells?

Replication and division of nuclei and cells

Which diagram shows one of the DNA base pairs correctly?

Structure of nucleic acids and replication of DNA

The diagram shows the transcription of a gene within the nucleus of an animal cell. Which row correctly identifies P, Q and R?

Proteins

Which row correctly shows whether growing leaves function as sources or sinks for amino acids and sucrose?

Movement into and out of cells

Which row correctly pairs each description with cilia or root hairs?

Cells as the basic units of living organisms

What changes take place as amino acids are transported into phloem sieve tubes at a source?

Movement into and out of cells

The graph displays the dissociation curves for haemoglobin at two different partial pressures of carbon dioxide. Which labelled point on the oxygen dissociation curves would produce the greatest concentration of haemoglobinic acid?

Proteins

Which row states the correct function for each cell structure?

Cells as the basic units of living organisms

Which of the following statements about how carbon dioxide is transported in the blood are correct?

Movement into and out of cells

What occurs when the blood pressure in the left ventricle becomes greater than the blood pressure in the left atrium?

The heart

The scanning electron micrograph displays the inner lining of a human trachea. What is the name of the part of the electron micrograph labelled T?

The microscope in cell studies

Which rows accurately summarise the usual distribution of cartilage and smooth muscle in the different regions of the human gas exchange system?

The microscope in cell studies

An oxygen molecule diffuses from the air in an alveolus to haemoglobin inside a red blood cell. If the molecule moves across cellular layers by going through cells, not between them, what is the smallest number of phospholipid layers through which the oxygen molecule has to pass?

Movement into and out of cells

Which row indicates the kind of pathogen responsible for cholera and how it is transmitted?

Infectious diseases

Which statement about tuberculosis (TB) is incorrect?

Infectious diseases

More malaria cases are now being recorded in Europe. Other diseases, once limited to tropical countries and spread in the same manner as malaria, have also reached parts of Europe. Tropical countries have a higher mean temperature and humidity than Europe. What could account for the rise in the number of cases of these diseases in Europe?

Infectious diseases

The diagrams illustrate three different bonds. Which of these bonds are present in an antibody molecule?

Proteins

Some monoclonal antibodies can be used to treat certain cancers. Trastuzumab is one example, and it can be used for treating cancer-related tumours. Which statements might help to explain why monoclonal antibodies such as trastuzumab are suitable for this purpose?

Proteins

Which cell structures are found in typical prokaryotic cells?

Cells as the basic units of living organisms

The HIV virus causes disease by infecting and destroying T-helper cells. As a result, AIDS develops and the body is unable to mount an effective immune response. Which parts of the immune system are made less effectively when AIDS develops?

Antibodies and vaccination

X-ray analysis of fossilised cells located in rocks in central India that date from $1.6$ billion years ago has shown several characteristics. Which two characteristics, considered together, give the strongest support for the idea that the cells are plant cells?

Cells as the basic units of living organisms

The electron micrograph displays an organelle present in some cells of many multicellular organisms. Which row indicates the structures that would be expected in cells containing this organelle?

Cells as the basic units of living organisms

What may happen in a hydrolysis reaction?

Carbohydrates and lipids

Sugars that are in a ring form can also exist in a linear form. Which sugar molecules might be shown by the linear structure in the diagram?

Carbohydrates and lipids

Which statement concerning triglycerides is correct?

Carbohydrates and lipids

Smilax china is a herbaceous plant. Fig. 1.1 displays part of a transverse section through a root of S. china, with root hair cells shown.

Transport mechanisms

Fig. 2.1 shows an incomplete structural diagram of an $\alpha$-glucose molecule. Complete Fig. 2.1 so that it shows the structure of an $\alpha$-glucose molecule.

Protein synthesis

Fig. $3.1$ and Fig. $3.2$ show transverse sections of the human heart at different points in the cardiac cycle. The cuts pass through the heart at a height just above the valves.

The heart

Vaccination programmes are commonly used to limit the spread of infectious diseases.

Antibodies and vaccination

DNA is found in both eukaryotic cells and prokaryotic cells.

Structure of nucleic acids and replication of DNA

Fig. 6.1 presents photomicrographs of separate cells from the onion root tip, $\textit{Allium sp.}$, taken at different points in the mitotic cell cycle.

Structure of nucleic acids and replication of DNA

Different respiratory substrates can possess different energy values, so they release different amounts of energy when respired.

Respiration

Fig. 10.1 illustrates chemoreceptor cells inside a taste bud. Two of the chemoreceptor cells have formed synapses with dendrites of sensory neurones.

The roles of genes in determining the phenotype

The orca, Orcinus orca, has the broadest range of any aquatic mammal and occurs in almost every sea and ocean. Orca are social mammals and usually live in groups, although group size varies. Several distinct forms of orca exist, and these are classified as the same species. However, there is evidence that sympatric speciation is taking place. In the Northeast Atlantic Ocean there are two distinct forms of orca: Type 1 and Type 2. Type 1 orca feed mainly on fish, whereas Type 2 orca feed mainly on aquatic mammals such as seals. Fig. 2.2 shows where Type 1 orca and Type 2 orca have been seen in the Northeast Atlantic Ocean. Orca are not restricted to only these regions, and some groups travel very long distances.

Evolution

Gentamicin is an antibiotic that is used to treat serious bacterial infections in children.

Principles of genetic technology

The $HFE$ gene specifies the $HFE$ protein, which helps regulate how much iron the body absorbs. Iron is an essential mineral that can be obtained only from food in the diet. A mutation of the $HFE$ gene called C282Y causes hereditary haemochromatosis, which is an autosomal recessive disease. The mutant allele codes for a non-functioning $HFE$ protein. Individuals who are homozygous for the mutant allele make no functioning $HFE$ protein, so an excess of iron is absorbed by the body. The build-up of iron in body organs over many years can lead to organ damage. People who are heterozygous for the $HFE$ gene do not have hereditary haemochromatosis. They absorb more iron from their diet than people without the mutation, but this usually has no health effects.

Passage of information from parents to offspring

Fig. 5.1 shows a photomicrograph of one plant cell at a stage in meiosis.

Passage of information from parents to offspring

Fig. 6.1 presents part of the control mechanism responsible for regulating blood glucose concentration.

Homeostasis in mammals

The Sumatran tiger, $Panthera\ tigris\ sumatrae$, is listed as critically endangered on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species™. Fig. 7.1 shows a Sumatran tiger.

Conservation

In the link reaction, pyruvate is turned into a two-carbon acetyl group. This acetyl group is then passed to coenzyme A, forming acetyl coenzyme A (acetyl‑CoA). State the terms used for the two chemical changes that happen in the link reaction as pyruvate is converted into an acetyl group.

Respiration

Leaves of tobacco plants, Nicotiana sp., contain chlorophyll a and chlorophyll b in their chloroplasts.

Investigation of limiting factors