Feb/March 2024

A student used a light microscope to view a blood smear on a microscope slide. An eyepiece graticule was used to measure the diameter of a white blood cell on the slide. The student noted that the white blood cell measured $5$ eyepiece graticule units across. Which further piece of information does the student need in order to find the diameter of the white blood cell in micrometres?

The microscope in cell studies

Which of the following statements about triglycerides is correct?

Carbohydrates and lipids

Many flowers make a sugary fluid known as nectar. Bees given nectar use enzyme Q to convert the nectar into honey. When a nectar sample was checked for reducing sugar with standard laboratory reagents, it was blue. When a honey sample was checked in the same way, it was orange. Which conclusion about the reaction catalysed by enzyme Q is consistent with these results?

Testing for biological molecules

The diagram illustrates the structure of a section of a peptidoglycan molecule. What type of $1,4$ linkage is present, and how many peptide bonds can be seen in this section of the molecule?

Carbohydrates and lipids

A scientist studied how two enzyme-catalysed reactions progressed in separate test-tubes, X and Y. Each reaction produced colour changes that could be detected by colorimetry. $0.5\,\text{cm}^3$ samples were removed from each test-tube at the beginning of the investigation and then at regular intervals over the following 5 minutes. Copper ions were introduced into each sample immediately after collection so that the enzymes were inactivated and the reactions could not continue. The absorbance of every sample was determined with a colorimeter. The graph presents the findings from this investigation. Which statement is consistent with the results shown in the graph?

Factors that affect enzyme action

A student carried out an investigation into the hydrolysis of lipid in high-fat milk, using the enzyme lipase. • $1\ \text{cm}^3$ of enzyme solution was added to $10\ \text{cm}^3$ of high-fat milk. • The temperature was maintained at a constant value. • The pH of the reaction mixture was measured at time $0$ minutes and then at 1-minute intervals for $20$ minutes. Which statements describe the results that would be expected for this investigation?

Mode of action of enzymes

What measurement range is correct for the width of the cell surface membrane?

Fluid mosaic membranes

In the cell surface membrane, where are the carbohydrate chains of glycoproteins and glycolipids chiefly found?

Fluid mosaic membranes

Sodium ions are able to pass into cells through the cell surface membrane. Which methods could sodium ions use to move across a cell surface membrane into a cell?

Movement into and out of cells

The diagram illustrates an artificial partially permeable membrane being used to divide a beaker containing a $5\%$ sodium chloride solution from a $10\%$ sodium chloride solution. The two parts of the beaker were marked R and S. Which row gives the correct description of, and reason for, what will happen in the part of the beaker labelled S?

Movement into and out of cells

Agar cubes may be used to show how diffusion is affected when the surface area to volume ratio changes. Three different agar cubes, prepared using a coloured indicator solution, were placed in a dilute acid which diffused into the cubes. As the acid spread through the agar cubes, the indicator solution changed colour. The cubes had volumes of $1\ \text{cm}^3$, $2\ \text{cm}^3$ and $3\ \text{cm}^3$ and were kept in the dilute acid for $10$ minutes. All the other variables were kept constant. After $10$ minutes, the agar cubes were taken out of the dilute acid and sliced in half. The cut surfaces were examined and the outcomes were recorded as diagrams. All diagrams were drawn to the same scale. The results for the $2\ \text{cm}^3$ cube are shown. Which diagrams show the results for the $1\ \text{cm}^3$ and the $3\ \text{cm}^3$ cubes?

Movement into and out of cells

Why must an electron microscope be used to observe the cristae of a mitochondrion?

The microscope in cell studies

In mammals, while sperm are being formed, a portion of each chromosome is exchanged for proteins called protamines. As a result, the DNA can be compressed much more tightly than it otherwise could be. Which component of the chromosome is replaced by protamines?

Replication and division of nuclei and cells

The diagram illustrates the mitotic cell cycle. In which phase do chromosomes condense and become visible?

Chromosome behaviour in mitosis

Which of the following statements accurately describe features of stem cells that are vital for cell replacement and tissue repair?

Replication and division of nuclei and cells

Which of the following statements are true for every nucleotide?

Structure of nucleic acids and replication of DNA

In a circular DNA molecule containing 2700 base pairs, how many phosphodiester bonds are there?

Structure of nucleic acids and replication of DNA

Which statements concerning complementary base pairing are correct?

Replication and division of nuclei and cells

In the mitotic cell cycle, chromosomal DNA is copied. The particular positions in DNA molecules at which copying is taking place are called replication forks. A typical human chromosome contains about 150 million base pairs of DNA. Replicating the DNA of a typical human chromosome takes about 1 hour. When only one replication fork is used, the replication rate is about 50 base pairs per second. Approximately how many replication forks are needed during DNA replication in a typical human chromosome?

Replication and division of nuclei and cells

Which molecule is produced under the direct control of DNA?

Proteins

Which statement accurately describes the relationship between a companion cell and its sieve tube cell?

Structure of transport tissues

A maize seedling was cultivated in soil containing lanthanum ions that had been tagged with a chemical which fluoresces under ultraviolet light. The diagram shows what was seen when a section of root was viewed with a light microscope under ultraviolet illumination. Which conclusion about the transport of lanthanum ions in maize roots is correct?

Movement into and out of cells

Some stains can be used to identify cell structures in living cells. When a weak solution of one stain is added, the entire cell turns blue. The blue colour quickly fades from most cell structures. The cell structures that release energy remain blue. Which kind of cell structure is most likely to remain blue?

Cells as the basic units of living organisms

Which statement gives the correct description of adhesion and cohesion?

Water

What conditions are required for the mass flow of sucrose in phloem sieve tubes to occur?

Movement into and out of cells

One kind of congenital heart defect is one in which the left and right atria are not fully separated. This condition is known as an atrial septal defect (ASD). ASD commonly causes blood to pass from the left atrium to the right atrium. As a result, the blood pressure in the right atrium increases and the blood pressure in the left atrium decreases. Which row shows the other effects of ASD?

The heart

Which property of water, linked to its role in blood and tissue fluid, is described correctly?

Water

Which statement correctly describes how tissue fluid is formed?

Movement into and out of cells

Oxyhaemoglobin, carbaminohaemoglobin, haemoglobinic acid and carbonic anhydrase are present inside red blood cells. How many of these substances will have an overall fall in concentration as a red blood cell moves through capillaries in the lungs?

Proteins

Which of these structures usually contain cartilage and cilia?

The gas exchange system

Which statements about the roles of tissues in the human gas exchange system are correct?

The gas exchange system

Antibiotic-resistant strains of $\textit{Mycobacterium tuberculosis}$ are a serious issue in the treatment of TB. A new antibiotic, teixobactin, may be highly effective at destroying $M.\ \textit{tuberculosis}$, with only a small chance that the bacteria will develop resistance to teixobactin. Penicillin and related antibiotics attach to one protein, whereas teixobactin attaches to two lipids that are necessary for the bacterial cell wall to form. Teixobactin binds to parts of the two lipids that remain unchanged across many different species of bacteria. Which statements help to explain why the use of teixobactin is considered less likely to cause the evolution of antibiotic resistance than the use of many other antibiotics, such as penicillin?

Carbohydrates and lipids

Monoclonal antibodies are now used in the treatment of certain human diseases. What explains why monoclonal antibodies are suitable for this use?

Proteins

As mucus is secreted by a goblet cell, the following events occur. In what order do these events occur?

Movement into and out of cells

Measles is an infectious disease, and a virus causes it. The graph shows the annual number of measles cases in a country before and after a vaccine was introduced. What might have led to the decrease in the number of measles cases after vaccination was introduced?

The microscope in cell studies

The diagram illustrates four biological features. Which of these biological features occur in typical prokaryotes?

Cells as the basic units of living organisms

Which row indicates features present in dicotyledonous plant cells and also in typical bacterial cells? Key: $\checkmark$ = present in dicotyledonous plant cells and also in typical bacterial cells $\times$ = absent from at least one of these two cell types

Cells as the basic units of living organisms

Which flow chart that describes the test for non-reducing sugars is correct?

Testing for biological molecules

Which diagrams illustrate the release of a water molecule as a glycosidic bond forms?

Carbohydrates and lipids

Which molecules include $\text{C}=\text{O}$ bonds in their structural formulae?

Carbohydrates and lipids

Fig. 1.1 shows a diagram of part of the phospholipid bilayer in a cell surface membrane.

Transport mechanisms

Table 2.1 presents descriptions of three white blood cells. Complete Table 2.1 by giving the names of these three types of white blood cell.

The immune system

Fig. 3.1 presents a photomicrograph of part of a transverse section through the root of an iris, $\textit{Iris germanica}$. Irises are herbaceous monocotyledons. These plants contain the same transport tissues as herbaceous dicotyledons, but the arrangement of the transport tissues is different. In monocotyledons, the central region of the root is parenchyma (packing tissue).

Structure of transport tissues

Table 4.1 gives a 12-nucleotide sequence in the template strand of a short DNA segment, together with the matching primary transcript and the four amino acids encoded by the sequence. The table is not complete.

Protein synthesis

The agent that causes cholera is a prokaryote.

Infectious diseases

Fig. 6.1 shows a simplified representation of a section through the human immunodeficiency virus (HIV) particle that causes HIV/AIDS. The diagram depicts the virus particle just before it attaches to the cell surface membrane of a T-helper cell at a receptor protein known as CD4. A second protein (coreceptor) called CCR5 is also required for the virus particle to enter and then infect the T-helper cell.

The immune system

Fig. 1.1 presents a diagram of a nephron.

Homeostasis in mammals

Striated muscle is made up of myofibrils. The myofibrils contain several structural proteins, including troponin, tropomyosin, actin and myosin.

Control and coordination in mammals

Gregor Mendel, the scientist, examined how stem length varied in the pea plant, Pisum sativum. He published the findings of this study of the trait (characteristic) in 1866. Fig. 2.1 contains a diagram of a pea plant. Mendel noted that the pea plants he cultivated had either tall stems or dwarf (short) stems. In his investigation, he used crosses between pea plants showing these two phenotypes.

Passage of information from parents to offspring

Cystic fibrosis is an autosomal recessive genetic disease. Individuals with cystic fibrosis have a homozygous recessive genotype.

Passage of information from parents to offspring

Holstein Friesian cattle are a cattle breed that dairy farmers in many countries use because their cows give a high milk yield. In Holstein Friesian cattle, milk yield is influenced by heat stress. Heat stress happens when homeostatic mechanisms are insufficient to keep body temperature at normal levels. Air temperature is one factor that adds to heat stress. Fig. 4.1 shows Holstein Friesian cattle. Fig. 4.2 presents the mean daily air temperature in Central Europe and the mean monthly milk yield per cow of Holstein Friesian cattle in Central Europe.

Natural and artificial selection

Fill in the paragraphs below with the most suitable word or words.

Evolution

A respirometer is apparatus used to measure the respiration rate of living tissue, such as germinating peas. A simple respirometer is shown in Fig. 6.1. A student investigated how temperature affects the respiration rate of germinating peas. The student assembled the respirometer as in Fig. 6.1 and put it in a water-bath at $10^\circ\text{C}$. After five minutes, the student used the syringe to move the coloured liquid on the right-hand side of the U-shaped tube until it was level with $0\ \text{cm}$ on the ruler. The timer was started immediately. As the germinating peas used up oxygen, the coloured liquid in the U-shaped tube moved. The student measured how far the coloured liquid moved after 20 minutes. The experiment was repeated at $20^\circ\text{C}$, $30^\circ\text{C}$, $40^\circ\text{C}$ and $50^\circ\text{C}$.

Respiration

The light-dependent stage of photosynthesis takes place in chloroplasts. During this stage, electrons are released from chlorophyll $a$ molecules and passed on to electron acceptors. If a redox indicator such as DCPIP is added to a suspension of illuminated chloroplasts, electrons are transferred to DCPIP, causing its colour to change from blue to colourless. A student investigated how the wavelength of light (colour of light) affected the rate of photosynthesis.

Photosynthesis as an energy transfer process

Around $2 \times 10^9$ people worldwide are presently infected with the bacterial disease tuberculosis (TB), which is caused by Mycobacterium tuberculosis. Early diagnosis matters because treatment can then start without delay. APOPO is a non-profit organisation that has trained African giant pouched rats, Cricetomys gambianus, to use their sense of smell to find M. tuberculosis. They do this by sniffing a sample of thick mucus from the lungs of people who may have TB. The African giant pouched rats can detect the presence of M. tuberculosis with an accuracy of 87-93%. Fig. 8.1 shows an African giant pouched rat.

Classification

An experiment was done to find out how different blue-light intensities affect the percentage germination of barley seeds. The barley seeds were exposed to blue light for seven days. All other variables were kept the same. The findings are given in Table 9.1. The influence of blue light on the concentration of abscisic acid (ABA) was also examined. ABA concentration was recorded at intervals across seven days in barley seeds exposed to blue light with an intensity of 57 arbitrary units. The findings are given in Table 9.2. For comparison, in the dark the concentration of ABA in barley seeds decreased from $100\ \text{au}$ at the start (day 0) to $45\ \text{au}$ on day 1 and then stayed unchanged from day 1 to day 7.

Control and coordination in plants