More than 90 % of cellular cholesterol is located in the cell’s plasma membrane. What is the main role of cholesterol in the plasma membranes of mammalian cells?

A. To regulate membrane fluidity

B. To increase membrane solubility

C. To increase membrane permeability

D. To regulate membrane temperature

A

Neural pathways in living brains can now be mapped by tracking the movement of water molecules inside axons. What keeps water molecules inside axons?

A. Plasma membrane

B. Hydrogen bonding

C. Pump proteins

D. Synapse

A

How do both mitochondria and chloroplasts provide evidence for the endosymbiotic theory?

A. They have double membranes.

B. They have 80S ribosomes similar to prokaryotes.

C. They contain the same DNA as the nucleus of the cell.

D. They exist together in eukaryote cells for their mutual benefit.

A

Apparatus was set up as shown to collect data.

The graph shows the results after 47 minutes of data collection.

What causes the rates to differ?

I.   Different concentration gradients at the start

II.  Diffusion of sugar is initially greater in Y than in X

III. The systems are reaching equilibrium over time

A.  I only

B.  II only

C.  I and III only

D.  II and III only

C

Pasteur carried out a series of experiments that provided strong evidence against a widely supported theory. What was this theory?

A. Endosymbiosis

B. Spontaneous generation

C. Conservative replication of DNA

D. Evolution

B

Which structure found in eukaryotes has a single membrane?

A. Nucleus

B. Lysosome

C. Chloroplast

D. Mitochondrion

B

Which statement is evidence for the endosymbiotic theory?

A. Chloroplasts contain 70S ribosomes.

B. Protein synthesis occurs in the cytoplasm.

C. Organic molecules can be synthesised abiotically.

D. RNA is self-replicating.

A

Pasteur used swan-necked flasks and a nutrient broth to demonstrate that spontaneous generation of organisms does not occur on Earth. Some students performed a similar experiment using two swan-necked flasks, one containing broth which had been previously boiled and another containing broth which had not been boiled. 

The flasks were left in the school laboratory and observed after one week. What is the evidence against the spontaneous generation theory?

A. Microorganisms died in flask F due to high temperatures.

B. No microorganisms grew in either flask.

C. Microorganisms grew in flask G.

D. No microorganisms grew in flask F but many grew in flask G.

D

What special property of phospholipid molecules explains their ability to spontaneously assemble into a lipid bilayer?

A. They are hydrophobic.

B. They are amphipathic.

C. They are saturated.

D. They are hydrophilic

B

Dialysis membrane was set up to model digestion and absorption in the small intestine.

What is a limitation of this model?

A. There can be no active transport.

B. Maltose will pass through the membrane.

C. Lipase should be present with protein.

D. The membrane is not permeable to starch.

A

How does potassium move across the membrane of a neuron during repolarization?

A. Simple diffusion

B. Facilitated diffusion

C. Endocytosis

D. Active transport

B

The table shows concentrations of potassium ions and sodium ions inside and outside human cells.

[Source: © International Baccalaureate Organization 2019]

What explains these concentrations?

A. Potassium ions diffuse in and sodium ions diffuse out.

B. Sodium ions diffuse in and potassium ions diffuse out.

C. Active transport pumps sodium ions in and potassium ions out.

D. Active transport pumps sodium ions out and potassium ions in.

D

In an experiment on osmosis, red blood cells were immersed in a salt solution for two hours. The micrographs show the appearance of these cells before and after immersion in the salt solution.

[Source: Ed Uthman, Acanthocytes, from peripheral blood [image online] Available at:
https://en.wikipedia.org/wiki/Acanthocyte#/media/File:Acanthocytes,_Peripheral_Blood_(3884092551).jpg
This file is licensed under the Creative Commons Attribution 2.0 Generic (CC BY 2.0) https://creativecommons.org/licenses/by/2.0/ Source adapted.]

What explains the observed changes?

A. The salt solution was hypertonic and entered the red blood cells.

B. The salt solution was hypotonic and disrupted the membranes of the red blood cells.

C. The salt solution was hypertonic and water moved into it from the red blood cells.

D. The salt solution was hypotonic and mineral salts were lost from the red blood cells.

C

This question performed well in terms in terms of difficulty and in having the highest discrimination index on the paper.

The graph shows the mitotic index in the roots of lentil plants at different distances from the end of the root.

[Source: Physiologia Plantarum, Volume 105, Issue 1, January 1999, Pages 171–178, Effect of microgravity on the cell cycle
in the lentil root F. Yu, D. Driss-Ecole, J. Rembur, V. Legué, G. Perbal Wiley Online Library: https://onlinelibrary.wiley.
com/doi/abs/10.1034/j.1399-3054.1999.105125.x]

What can be deduced from the graph?

A. As the distance from the end of the root increases, more cells are undergoing mitosis.

B. At 0.5 mm from the end of the root, most of the cells are in prophase.

C. There were fewer cells observed at 1.5 mm than at 0.5 mm.

D. As the distance from the end of the root increases, the percentage of cells in interphase increases.

D

What function is performed by the part of the cell shown in the electron micrograph?

[Source: George E. Palade Electron Microscopy Slide Collection Harvey Cushing/John Hay Whitney Medical Library Yale University Library.]

A. Locomotion

B. Synthesis of proteins

C. Movement of chromosomes

D. Breakdown of cellular organelles

B

Which molecule regulates the fluidity of cell membranes?

A. Phospholipid

B. Cholesterol

C. Glycoprotein

D. Peripheral protein

B

Which cell component arose first during the formation of the earliest cells?

A. Chloroplast

B. Plasma membrane

C. Mitochondria

D. Cell wall

B

What part of the plasma membrane is fluid, allowing the movement of proteins in accordance with the fluid mosaic model?

B

In the chimpanzee (Pan troglodytes), the haploid number of chromosomes is 24. How many sister chromatids are present in the G2 phase of a somatic cell, such as a cell in the bone marrow of the chimpanzee?

A. 12

B. 24

C. 48

D. 96

D

This question proved to be too hard for most candidates, with only 15 % answering correctly lower than the expected 25 % success rate from guessing. Most candidates thought that the diploid number of chromatids are present in a cell during G2. At that stage of the cell cycle, each chromosome has been replicated so there are twice the diploid number of chromatids. Candidates may not have known the difference between chromatids and chromosomes or that replication has already happened in G2, or perhaps they did not read the question carefully enough.

What evidence from the image of Paramecium indicates whether the organism is a prokaryote or a eukaryote?

A. Compartments in the cell indicate that it is a eukaryote.

B. No nucleus indicates that the cell is a prokaryote.

C. Lack of a cell wall indicates that the cell is a eukaryote.

D. It is a unicellular organism, so it must be a prokaryote.

A

By which process do potassium ions move through potassium channels in axons?

A. Active transport

B. Exocytosis

C. Facilitated diffusion

D. Simple diffusion

C

Which features of phospholipids give them their amphipathic properties?

A. Basic phosphate groups and acidic lipids

B. Acidic phosphate groups and basic lipids

C. Hydrophobic phosphate groups and hydrophilic fatty acids

D. Hydrophilic phosphate groups and hydrophobic fatty acids

D

A G2 comment was made that the terms acidic and basic are not part of the program, but they are commonly used terms in biochemistry and in this question were only used in distractors, so candidates could identify the correct answer without being sure about them.

Which feature of the cell in the micrograph is consistent with the endosymbiotic theory?

A. X has a single membrane.

B. Y has a double membrane.

C. X contains 70S ribosomes.

D. Y contains 80S ribosomes.

C

What is evidence for the endosymbiotic theory?

A. RNA can catalyse metabolic reactions.

B. Meteorites contain organic molecules.

C. Amino acids can be synthesized from inorganic compounds.

D. Mitochondria possess their own DNA.

D

Cladograms can be created by comparing DNA or protein sequences. The cladogram on the left is based on DNA sequences and the cladogram on the right is based on comparing protein sequences.

What is the reason that cladograms based on DNA sequences are more reliable predictors of the phylogenetic relationship of species than cladograms based on protein sequences?

A. Amino acids are not as chemically stable as DNA nucleotides.

B. DNA mutates but amino acids do not.

C. Several different triplets of bases can code for the same amino acid.

D. There are 20 different amino acids but only 4 nucleotides.

C

Which statement provides evidence for endosymbiosis?

A. Early prokaryotes contributed to a large increase in oxygen in the atmosphere.

B. Eukaryotic mitochondria and chloroplasts have their own circular DNA.

C. Certain groups of ancient prokaryotes developed mechanisms to carry out aerobic respiration.

D. Experiments by Miller and Urey produced simple organic molecules in abiotic conditions.

B

Which statement applies to cholesterol?

A. It is hydrophobic and found on the outside of the phospholipid bilayer.

B. It is hydrophilic and found inside the phospholipid bilayer.

C. It impacts membrane fluidity.

D. It is transported in association with glucose in the blood.

C

The Davson–Danielli model of membrane structure proposed that membranes were composed of a phospholipid bilayer that lies between two layers of globular proteins, as shown in this diagram.

[Source: Cornell, B. 2016. https://ib.bioninja.com.au/standard-level/topic-1-cell-biology/13-membrane-structure/membrane-models.html]

What evidence supported this model?

A. An electron micrograph that showed two dark lines with a lighter band in between

B. Freeze-fracture electron microscopy

C. Evidence that all membranes are identical

D. The hydrophobic regions of protein would be in contact with water

A

This question was based on the second Skill in 1.3 (Membrane Structure). As part of a study of models in science and how they may be falsified, students are expected to understand that the appearance of membranes in electron micrographs led to the Davson-Danielli model, with a central layer of phospholipid coated on either side by protein. By understanding the original plausibility of this model, students should better understand that some currently accepted models may eventually be proved to be false and that challenging accepted views is an important part of the nature of science.

How is facilitated diffusion in axons similar to active transport?

A. They both require the energy of ATP.

B. They both move substances against a concentration gradient.

C. They both use sodium–potassium pumps.

D. They are both carried out by proteins embedded in the axon membrane.

D

If cells of a multicellular organism have the same genes, how can there be many different cell types in a body?

A.  Some genes but not others are expressed in each cell type.

B.  Cells lose some genes as development occurs.

C.  Genes do not determine the structure of a cell.

D.  Cells must practice division of labour in order to survive.

A

The bacterium Neisseria gonorrhoeae causes infections related to the human reproductive system. The graph shows the percentage of samples in which this bacterium showed resistance to six antibiotics over a period of ten years.

What is a possible explanation for the total percentage resistance being larger than 100% in 2010?

A. People do not take the antibiotics as prescribed.

B. More people have been sampled in that year.

C. There was an epidemic of Neisseria gonorrhoeae in that year.

D. Some bacteria are resistant to more than one antibiotic.

D

The image shows an electron micrograph of mesophyll cells.

What is the name of the structure labelled X?

A. Cytoplasm

B. Mitochondrion

C. Nucleus

D. Chloroplast

D

The electron micrograph shows a thin section through a plant mesophyll cell.

[Source: https://commons.wikimedia.org/wiki/File:Euglena_sp.jpg, by Deuterostome
https://creativecommons.org/licenses/by-sa/3.0/legalcode]

What is the magnification of the image?

A. × 75

B. × 300

C. × 3000

D. × 7500

C

Which organelle provides evidence that eukaryotic cells originated when large prokaryotes engulfed small free-living prokaryotes?

A. Chloroplast

B. Nucleoid

C. 80S ribosome

D. Vacuole

A

The cell membrane model proposed by Davson–Danielli was a phospholipid bilayer sandwiched between two layers of globular protein.  Which evidence led to the acceptance of the Singer–Nicolson model?

A. The orientation of the hydrophilic phospholipid heads towards the proteins
B. The formation of a hydrophobic region on the surface of the membrane
C. The placement of integral and peripheral proteins in the membrane
D. The interactions due to amphipathic properties of phospholipids

C

The concentrations of cyclins rise and fall in cells at certain times.

[Source: https://en.wikipedia.org/wiki/Cyclin#/media/File:Cyclin_Expression.svg]

What times are these?

A. Day and night

B. Seasons of the year

C. Stages of mitosis and interphase

D. Developmental stages in the life cycle

C

The image shows part of a mammalian cell.

[Source: Louisa Howard, Katherine Connollly - Dartmouth Electron Microscope Facility. Available at:
https://en.wikipedia.org/wiki/File:Microvilli.jpg.] 

What is the specialized function of this mammalian cell?

A. Locomotion

B. Absorption

C. Reception of stimuli

D. Gas exchange

B

Which diagram(s) represent(s) processes used in asexual reproduction?

A. I only

B. I and II only

C. II only

D. I, II and III

C

The salt concentration inside an animal cell is 1.8 %. The salt concentration in the surrounding medium becomes 5 %. What will be the likely response?

A. The cell will gain water from the medium.

B. The cell will lose salt to the medium.

C. The cell will remain unchanged.

D. The cell will shrink from loss of water.

D

What provides evidence for the endosymbiotic theory?

A. Mitochondrial DNA in eukaryotic cells
B. 70S ribosomes in prokaryotic cells
C. Gene transfer from prokaryotic cells to eukaryotic cells using plasmids
D. Prokaryotic cells (Escherichia coli) in the large intestine digest proteins

A

A human organ is being prepared for transplant. In what type of solution must it be bathed?

A. A hypertonic solution

B. A hypotonic solution

C. Pure water containing no solutes

D. A solution with the same osmolarity as the organ tissue

D

Which organism has DNA located in three organelles?

A. A sponge

B. A fern

C. A flatworm

D. A bacterium

B

Which graph best represents the relationship between the concentration of chloride ions in the external environment of a cell and the rate at which the chloride ions move by facilitated diffusion into the cytoplasm of the cell?

C

It was expected that candidates would realise that the rate of facilitated diffusion would increase with the concentration of chloride ions, and that the graph would level off as the carrier proteins become saturated. Although the difficulty index indicates this was not a hard question it did not discriminate as well as expected with many of the stronger candidates opting for graph D.

Which feature(s) allow(s) transport of glucose in blood plasma?

I. It is hydrophobic.

II. It is polar.

III. Its solubility is low at 37 °C.

A. I only

B. II only

C. I and II only

D. II and III only

B

It was suggested that this may be difficult for the students, but glucose transport is included in Section 2.2 of the guide, transport of glucose in blood in relation to solubility in water.

Which process(es) occur(s) by osmosis?

I. Uptake of water by cells in the wall of the intestine

II. Loss of water from a plant cell in a hypertonic environment

III. Evaporation of water from sweat on the skin surface

A. I only

B. I and II only

C. II and III only

D. I, II and III

B

Which sequence has the cells arranged according to their ability to differentiate, starting from the least able?

A. bone marrow, neuron, embryonic, umbilical

B. neuron, bone marrow, umbilical, embryonic

C. umbilical, embryonic, bone marrow, neuron

D. embryonic, umbilical, bone marrow, neuron

B

How does mitosis produce two genetically identical nuclei?

A.  By separation of homologous chromosomes

B.  By separation of sister chromatids

C.  By division of the cytoplasm into two equal cells

D.  By division of the nuclear membrane into two equal parts

B

The diagram shows a section through a membrane. What are the modes of transport in the diagram?

[Source: © International Baccalaureate Organization 2020.]

C

The diagrams represent cells with the same concentration of dissolved substances in their cytoplasm. If all the cells were placed in the same hypertonic sucrose solution, which cell would show the greatest rate of change in the concentration of its cytoplasm?

C

This question was taken from 1.1 (Introduction of cells) and required knowledge that the rate of change in the concentration of the cytoplasm would occur in the cell with the greatest surface area to volume ratio. The question did not discriminate well; many candidates chose the cell with the biggest volume and surface area (answer D) rather than the one with the largest surface area to volume ratio which would give the greatest rate of change.

The image shows an electron micrograph of part of a cell.

[Source: Dr. Eldon Newcomb – Emeritis Professor at The University of Wisconsin – Madison.]

Which features do the two structures labelled X and Y have in common?

A. They are surrounded by a double membrane.

B. They contain 70S ribosomes.

C. They contain naked DNA.

D. They are only found in leaf cells.

A

There was criticism that the resolution of the micrograph wasn’t high enough, but it was typical for electron micrographs of plant cells and the internal structure of chloroplasts and the nucleus was clear. This skill from 1.2 of the Core was being tested: Interpretation of electron micrographs to identify organelles and deduce the function of specialized cells. The nucleus and chloroplasts both had a typical appearance, so well-prepared candidates should have been able to identify them. Answer A could then be chosen either by eliminating the incorrect answers, or by knowing that both nucleus and chloroplast are double membraned. The endosymbiotic theory for the origin of chloroplasts should have helped inform candidates about the two membranes of chloroplasts.

In the diagram, which part of the membrane structure does the molecule below form?

A

In the diagram, which structure is an intrinsic or integral protein?

B

What is a role of cholesterol in animal cells?

A. It increases body fat.

B. It controls membrane fluidity.

C. It lines the inner wall of capillaries.

D. It is a constituent of bile.

B

What is/are required for facilitated diffusion?

I. A concentration gradient

II. ATP

III. A channel protein

A. I only

B. I and III only

C. II and III only

D. I, II and III

B

Students examined micrographs and counted cells in the different stages of mitosis as well as those cells with no visible chromosomes. The table shows their results.

What is the mitotic index?

A. 0.2

B. 0.4

C. 0.6

D. 0.7

B

Which living structure is an exception to the cell theory?

A. Striated muscle fibres

B. A single-celled alga carrying out all of the functions of life

C. The artificial synthesis of the organic molecule urea

D. A multicellular organism with cells undertaking specialized roles

A

Which feature of striated muscle cells allows them to be considered as a possible exception to the cell theory?

A. They are found in multicellular organisms.
B. They contain more than one nucleus.
C. They are specialized for movement.
D. They do not carry out mitosis.

B

Which processes are involved in the development of cancer?

I. Mutations occur in oncogenes.

II. Oncogenes prevent cancer.

III. Oncogenes affect cell cycle regulatory proteins.

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

B

This question was too easy, as answer II was too obviously wrong. This question did not discriminate well because most candidates had it right. Candidates did not seem to be put off by the fact that in I it said that mutations occur in oncogenes instead of saying that oncogenes are the result of a mutation in protooncogenes.

Which characteristic of stem cells makes them useful for treating Stargardt’s disease?

A. They can differentiate into retinal cells.
B. They are readily available from especially created embryos.
C. They transport white blood cells to the eyes.
D. They divide by binary fission so provide sufficient cells.

A

The images of the radiolarian, a single-celled marine organism, were produced using a light microscope (left) and a scanning electron microscope (right).

[Source: Munir,S.;Sun,J.;Morton, S.L. The First Record and Classification of Planktonic Radiolarian
(Phylum Retaria) and Phaeodarian (Phylum Cercozoa) in the Eastern Indian Ocean. Biology 2021, 10, 202.
https://doi.org/10.3390/biology10030202 Copyright: © 2021 by the authors. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).]

What is a reason for the difference in quality of these images?

A. Light cannot pass through the specimen.

B. Higher magnification can be achieved with the electron microscope.

C. The resolution of the electron microscope is higher.

D. Samples are stained with methylene blue when viewed with the light microscope.

C

The majority of students answered this correctly. The most common mistake was to answer that a higher magnification can be achieved with the electron microscope. This could not be determined from the images as both were at the same scale.

The graph shows the survival probabilities for current smokers and for those who never smoked among women 30 to 80 years of age.

What can be deduced from this graph?

A. There is a correlation between smoking and cancer.

B. Smoking reduces life expectancy.

C. Smoking causes cancer.

D. 70 % of smokers survive to 80 years old.

B

Which organelle is involved in generating vesicles destined for the cell membrane?

A. Golgi apparatus

B. Smooth endoplasmic reticulum

C. Rough endoplasmic reticulum

D. Lysosome

A

The micrograph of a section through a plant stem shows at least ten different types of cells.

[Source: Joan Carles Juarez / Shutterstock.com.]

What explains the differences between these cells?

A. Only one gene is expressed in each cell type.

B. Different genes are expressed in each cell type.

C. Only useful genes remain in the DNA of each cell type.

D. Changes in the DNA sequence take place when these cells develop.

B

It was commented that the use of plant material was an unnecessary distractor but that did not show in the student responses with over 70 % of students answering correctly and the question discriminating well.

The diagram represents the nucleus of a cell 2 n = 8 in late prophase of mitosis.

Which diagram represents a cell from the same species in anaphase II of meiosis?

C

Which graph represents the change in cell surface area to volume ratio with increasing cell diameter?

[Source: © International Baccalaureate Organization 2019]

B

Contrary to the trend of the first question being easy, this proved to be the most difficult in the paper with fewer than 30 % of candidates answering it correctly. Most missed that the graph showed the ratio of surface area to volume, not simply surface area, or perhaps they did not understand that this ratio gets smaller as cell size increases, choosing instead answers showing it getting larger. A and B both showed the ratio getting smaller, but A could be rejected because the line intercepted both x- and y-axes, which isn’t possible if we assume the origin of the axes is zero. A more time-consuming method of identifying B as the correct answer was to do a few sample calculations with cubes of different diameter. For example, the surface area/volume ratios of cubes of diameter 1, 2 and 3 are 6, 3 and 2 respectively, so equal increases in diameter give decreasing decreases in ratio. The line on the graph is therefore curved as in B. Given the significant consequences to living organisms of surface area to volume ratios, this is an area of the programme that deserves more emphasis, if it has previously been neglected.

The table shows the number of cells in various stages of the cell cycle in four samples of ovarian tissue from different patients. Which tissue sample A, B, C or D has the highest mitotic index?

C

The images are microscopic views of two similar cells.

What is a reason for the differences between the two micrographs?

A.  The lower image has a higher magnification.

B.  The lower image has greater resolution.

C.  A nucleus can only be seen in the upper image.

D.  The upper image is an electron micrograph.

B

A tissue sample was examined under the microscope in order to determine a mitotic index. The number of cells in each stage of the cell cycle was determined and the data were entered into a table.

What is the mitotic index?

A. 0.125

B. 0.25

C. 0.75

D. 1.00

B

The following electron micrograph shows part of a palisade mesophyll cell. Which of the labelled structures controls the exchange of substances to and from the cell?

C

What feature of both striated muscle and aseptate fungal hyphae is different from typical cell structure?

A. They have multiple nuclei within a structural unit.

B. They have a cell wall that is not made of cellulose.

C. They have plasmids.

D. They have an absence of cholesterol in the plasma membrane.

A

The image shows cells in the different stages of mitosis. Which cell is in telophase?

[Source: © The Trustees of Indiana University]

D

The image shows a micrograph of a cell.

[Source: © 2014, https://commons.wikimedia.org/wiki/File:Mitosis_(261_13)_Pressed;_root_meristem_of_onion_(cells_
in_prophase,_metaphase,_anaphase,_telophase).jpg by Doc. RNDr. Josef Reischig, CSc.]

What explains the appearance of the cell in the micrograph?

A. The cell is dying.

B. The DNA is replicating.

C. The cell is in metaphase.

D. The cell is in telophase.

C

Candidates also found this question easy, but it was a very good discriminator, this means that good candidates answered it well while weaker candidates did not. It therefore proved to be an effective question.

When compared to other body cells, which characteristic of stem cells is the most important for therapeutic uses?

A. Less differentiation

B. Less excretion

C. Lower rate of reproduction

D. Lower rate of metabolism

A

The giant alga Acetabularia has a feature that suggests it is an exception to the cell theory. What feature is this?

A. It lacks a nucleus.

B. It lacks a cell wall.

C. It has only one mitochondrion.

D. It lacks subdivision into separate cells.

D

The images show a sequence of changes in an organism.

[Source: Copyright (2007) National Academy of Sciences, U.S.A. 'A comparative analysis of frog early development'.
Eugenia M. del Pino, Michael Venegas-Ferrín, Andrés Romero-Carvajal, Paola Montenegro-Larrea, Natalia
Sáenz-Ponce, Iván M. Moya, Ingrid Alarcón, Norihiro Sudou, Shinji Yamamoto, and Masanori Taira,
PNAS July 17, 2007 104 (29) 11882–11888; https://doi.org/10.1073/pnas.0705092104]

What is the change and which process is necessary for it to occur?

B

This question was also criticised by some teachers, because the programme does not specify that students should view microscope images of embryos. A principal aim in IB Biology exams is to test whether candidates can apply their knowledge and understanding in a wide range of contexts. There was no expectation that images of embryos had already been seen. This question had the highest percentage of candidate answering correctly and the lowest discrimination index of any in the exam. We can be pleased that there was widespread understanding that embryo development was shown and that it involves cell differentiation. However, because so many candidates got the question right, it did not help much in determining which the strongest candidates were. To do that a success rate of a half to two thirds is better.

Cisplatin is an anti-cancer drug that prevents tumour cells from dividing by mitosis as it inhibits cell processes at stage S of interphase. How does cisplatin prevent cancer cells from dividing?

A. It inhibits the replication of DNA.

B. It inhibits the growth of the spindle fibres.

C. It prevents the breakdown of the nuclear membrane.

D. It prevents the condensation of chromosomes.

A

How many chromosomes are there in a cell during anaphase of mitosis, if the diploid number of the cell is 20?

A. 10

B. 20

C. 40

D. 80

C

Some candidates were caught out by this question, either because their understanding of mitosis had weaknesses or because they did not think carefully enough. In anaphase, the two sister chromatids that have made up each chromosome in prophase and metaphase have separated. Guidance in sub-topic 1.6 of the program indicates that they should be referred to as chromosomes as soon as separation has occurred, rather than as chromatids. In anaphase there are therefore double the diploid number of chromosomes in a cell. Cytokinesis will subsequently halve the number, so the daughter cells are diploid.

What is the name of the cell component labelled Y?

A. Golgi apparatus

B. Nucleus

C. Cytoplasm

D. Vacuole

B

The discrimination index for this question was fairly low, suggesting that some of the stronger candidates chose the wrong answer. The commonest mistake was to think that the nucleus was a vacuole, but the grainy appearance with darker lumps of heterochromatin was very characteristic of nuclei.

The electron micrograph shows part of a cell. Which organelle is the site of aerobic respiration?

C

A cell contains chloroplasts, plasma membrane and 80S ribosomes. What type of cell could it be?

A. Bryophyte

B. Lymphocyte

C. Prokaryote

D. Neuron

A

Which structures are found only in prokaryotic cells?

A. Ribosomes

B. Pili

C. Cell walls

D. Flagella

B

This was not an easy first question. In the SL Bio guide section 1.2 the guidance states that pili and flagella should be shown on diagrams of prokaryotic cells. There is a note that cell walls can be found on both but does not mention that flagella can also be found on eukaryotic cells. An example where flagella can be found is in sperm cell tails. It is obvious from the answers that candidates did not know this, as many answered question D instead of B.

In the micrograph, which letter points to a cell in anaphase?

[Source: Berkshire Community College Bioscience Image Library,
https://commons.wikimedia.org/wiki/File:Mitotic_Stages_in_Apical_Meristem_of_Allium_Root_Tip_(36762516673).jpg,
licensed under Creative Commons CC0 1.0 Universal Public Domain Dedication]

B

Where could genes be located in a prokaryotic cell?

B

Which mitotic phase is labelled X in the micrograph of an onion (Allium cepa) root tip?

A. Prophase
B. Metaphase
C. Anaphase
D. Telophase

B

What occurs during meiosis but not mitosis?

A. Spindles are formed from microtubules.

B. Chromosome number is conserved.

C. Homologous chromosomes pair up.

D. Centromeres split.

C

What do chloroplasts and mitochondria have in common?

A. Both are found in the cells of Filicinophyta.

B. Both contain grana.

C. Both occur in all eukaryotic cells.

D. Both are found in a Paramecium.

A

When during the cell cycle does DNA replication take place?

C

In mammals, mature red blood cells are specialized in that they lack nuclei, mitochondria or ribosomes. Which statement applies to red blood cells?

A. No chemical reactions take place within their cytoplasm.

B. They cannot produce new enzymes.

C. Materials cannot enter red blood cells.

D. Materials cannot exit red blood cells.

B

Which function is accomplished by structures X and Y in the Paramecium?

B

The image shows an electron micrograph of a cell.

[Source: Junqueira’s Basic Histology, 15th edition, by Anthony L. Mescher, McGraw-Hill publisher.]

Which organelles correspond to the labels in the electron micrograph of this cell?

C

The magnification of the micrograph is 2000×.

[Source: https://upload.wikimedia.org/wikipedia/commons/6/6c/HeLa_cells_stained_with_Hoechst_33258.jpg
by TenOfAllTrades.]

What is the maximum diameter of the nucleus in the cell labelled X?

A. 10 μm

B. 10 nm

C. 20 μm

D. 20 nm

A

Many students found it difficult to calculate the maximum diameter of the nucleus without a calculator.