The graph shows an example of an enzyme-catalysed reaction.

What does the curve labelled X represent?

A. No inhibition
B. Competitive inhibition
C. Non-competitive inhibition
D. Reversible inhibition

C

Which is a reduction reaction?

A.  ATP changing to ADP

B.  Maltose changing to glucose

C.  FAD changing to FADH₂

D.  NADPH changing to NADP

C

Where in the mitochondrion does the formation of acetyl CoA occur?

D

The discrimination index for this question was the lowest on the paper. Some of the better-prepared candidates thought that acetyl CoA is produced on the inner mitochondrial membrane, rather than in the matrix.

Which reaction does not cause a net release of energy?

A. ADP combines with inorganic phosphate to form ATP

B. ATP releases inorganic phosphate to form ADP

C. Loss of hydrogen from reduced NAD

D. Oxidation of reduced FAD

A

The image shows a portion of a cell containing a mitochondrion.

[Source: ‘TEM of a mitochondrion’ by Prof. R. Bellairs. Credit: Prof. R. Bellairs. CC BY 4.0.]

Where do glycolysis and electron transport occur?

D

The graph shows the effect of increasing substrate concentration on the rate of an enzyme-catalysed reaction.

Which type of inhibition corresponds to the labelled curves?

B

This was another relatively hard question with a low discrimination index. Surprisingly, some of the stronger candidates seem to have muddled up the effect of substrate concentration on competitive and non-competitive inhibitors.

Which process does not take place in the stroma of chloroplasts?

A. Synthesis of carbohydrates

B. Fixation of carbon

C. Reduction of NADP

D. Synthesis of ribulose bisphosphate

C

Answers were divided almost equally between the four possible answers in this question suggesting a lot of guesswork but the discrimination index shows that better-prepared candidates tended to answer correctly. Arguably none of the answers is completely correct. Shrewd candidates could eliminate answers A and D, because if D, was correct, A was also, and both could not be correct. Reduction of NADP was the expected answer but arguably it occurs in the stroma, as the reaction takes place adjacent to the outer surface of the thylakoid membrane. Even so, this was the best answer.

The diagram shows a section through a thylakoid. Electrons move from X to Y.

[Source: Courtesy Alyse Da Quynh.]

What do the letters X, Y and Z represent?

B

Which technological advance enabled Calvin to perform his lollipop experiment on the light-independent reactions of photosynthesis in 1949?

A. Methods for tracing radioactive carbon incorporated in molecules produced by the alga Chlorella

B. Development of electron microscopes enabling the molecules produced by the alga Scenedesmus to be viewed

C. Methods for changing the wavelength of light shining on the alga Scenedesmus contained in the lollipop

D. Development of X-ray diffraction techniques enabling the molecules produced by the alga Chlorella to be identified

A

What term is used for ATP synthesis coupled to electron transport and proton movement?

A.  Chemiosmosis

B.  Oxidation

C.  Glycolysis

D.  Cell respiration

A

Metabolic pathways are dependent on enzyme-catalysed reactions. Which term corresponds with the description?

D

Two reactions of the Krebs cycle are shown.

What type of reactions are isocitrate and oxalosuccinate undergoing?

D

Nearly all candidates knew that one type of reaction was decarboxylation and, from the answers offered, the other had to be either oxidation or reduction. More candidates than expected got this wrong and thought that removal of hydrogen from a Krebs cycle intermediate is reduction.

Sulfanilamide inhibits an enzyme that catalyses a reaction involving PABA, an intermediate in the synthesis of folate in bacteria. The structures of sulfanilamide and PABA are shown.

The graph shows the rate of reaction with increasing substrate concentration and with fixed low concentration of different types of inhibitors. Which line on the graph represents the effect of sulfanilamide?

D

The diagram shows a mitochondrion. Which letter indicates the structure where ATP synthase is located?

C

Which process requires oxygen in aerobic cell respiration?

A. Oxidation of triose phosphate
B. Reduction of hydrogen carriers
C. Maintaining an oxygen concentration gradient in mitochondria
D. Accepting electrons at the end of the electron transport chain

D

Which products of the light-dependent reactions are used in the Calvin cycle?

A. O₂ and hydrogen ions

B. ATP and CO₂

C. Electrons and reduced NADP

D. ATP and reduced NADP

D

The electron micrograph shows part of a plant cell. Where do the light-independent reactions of photosynthesis take place?

B

What is used to reduce NADP in the light-dependent reactions of photosynthesis?

A. Conversion of ATP into ADP+P_i

B. Electrons from Photosystem I

C. Protons from the thylakoid space

D. Oxygen released by photolysis of water

B

Where are protons pumped, to allow chemiosmosis in aerobic respiration to occur?

A. From outside the mitochondrion through the double membranes

B. From carrier to carrier in the inner mitochondrial membrane

C. From the matrix of the mitochondrion to the space between the membranes

D. From the space between the membranes to the cytoplasm outside the mitochondrion

C

The graph shows the effect of substrate concentration on the rate of an enzyme-catalysed reaction with and without an inhibitor.

What do the curves J, K and L on the graph indicate?

B

The electron micrograph shows a section through part of an animal cell.

[Source: https://en.wikipedia.org/wiki/Mitochondrion#/media/File:Mitochondria,mammalian_lung_-TEM.jpg,
by Louisa Howard.]

Which process is occurring on the structures labelled X?

A. The Krebs cycle

B. The link reaction

C. Glycolysis

D. Phosphorylation of ADP

D

What does electron tomography allow mitochondria researchers to do?

A. To produce images of cristae.

B. To produce images of ATP synthase molecules.

C. To trace the movement of electrons through the electron transport chain.

D. To visualize oxidation/reduction reactions.

A

This proved to be the second most difficult question on the exam in terms of percentage success rate. There was also a poor discrimination index. For entirely understandable reasons, electron tomography was a part of the program that remained unexplored by many candidates and it was not possible to deduce the correct answer from basic biological understanding. The topic was introduced to the current IB Biology program as an exemplar of technological development leading to new research findings. Electron tomography has allowed the function of mitochondria to be studied by producing images showing the position of the cristae when production of ATP is occurring. It does not allow movement of electrons in the electron transport chain to be traced the most popular answer.

A plant is allowed to photosynthesize in an atmosphere containing radioactive ¹⁴C. Where in the plant stem would radioactive sugars be found?

[Source: sinhyu/123rf.com.]

B

This question required candidates to know that phloem transports sugars and then also either recognise phloem tissue in a micrograph or know its relative position in a vascular bundle. Of the tissues labelled, A and D were not part of the vascular bundle and C was easily recognizable as xylem. To a well-prepared candidate, the small, thin-walled cells labelled B were clearly phloem, with the characteristic pattern of larger sieve tubes and smaller companion cells just about visible. Areas of sclerenchyma and cambium in the micrograph were not labelled as they would have been more confusable with phloem. A G2 comment was made that all living tissue would eventually have received radioactive sugars in this experiment, but the candidate’s task is always to choose the best answer, and phloem would become radioactive first and reach the highest levels.

The diagram shows the structure of a chloroplast. Where is chlorophyll located?

D

Which process occurs during the light-dependent reaction of photosynthesis?

A. ATP, CO₂ and H₂O are produced.

B. CO₂ is used to produce carbohydrates.

C. ATP and O₂ are produced.

D. RuBP is phosphorylated.

C

The first enzyme in the metabolic pathway that produces isoleucine is threonine deaminase. Which graph illustrates the relationship between threonine deaminase activity and threonine concentration?

[Source: © International Baccalaureate Organization 2019]

B

The programme specifies the threonine to isoleucine pathway as the example of end-product inhibition that should be studied (8.1 of AHL). Candidates were therefore expected to know that threonine is the substrate for the enzyme threonine deaminase, but they could also have also deduced this from the name of the enzyme. It was expected that candidates would recognise the curve for the effect of increasing substrate concentration on enzyme activity and choose answer B. Two thirds of candidates successfully did this, but the discrimination index was rather low, showing that some candidates chose another answer. Perhaps they remembered that this enzyme can be affected by an end product inhibitor and therefore chose a curve where enzyme activity decreased with increased threonine concentration.

The graph shows the rate of an enzymatic reaction versus the substrate concentration, in the absence or presence of an enzyme inhibitor.

Which condition is indicated by lines Y and Z?

C

Which equation is an example of decarboxylation?

A. Pyruvate $\to$ Acetyl CoA + CO₂

B. CO₂ + H₂O H₂CO₃

C. CH₄ + 2O₂ CO₂ + 2H₂O

D.

A

Which process is common to photosynthesis and cell respiration?

A. Photolysis

B. The Calvin cycle

C. The Krebs cycle

D. Chemiosmosis

D

This wasn’t an exciting question but was the best discriminator on the exam, with nearly two thirds of candidates identifying chemiosmosis as a process that occurs in both photosynthesis and respiration.

The grey line in each of the graphs below represents the rate of reaction catalysed by an uninhibited enzyme as substrate concentration is increased.

Which graph shows expected results if a competitive inhibitor was added to the reaction?

[Source: © International Baccalaureate Organization 2019]

B

This reaction occurs in mitochondria.

What explains that this reaction enables energy to be converted into a usable form?

A. The oxidized NAD⁺ will transfer the energy from the C₆ compound to ATP.

B. The chemical energy stored in the C₆ compound is used to reduce NAD⁺ allowing ATP production.

C. Energy stored in the CO₂ molecule will generate an electron gradient.

D. The C₆ compound is reduced and the energy resulting from the removal of one carbon is used to oxidize NAD⁺.

B

What is reduced by Photosystem I ?

A. ADP

B. NADP

C. NAD

D. FAD

B

ATP is needed to change products of the carboxylation of ribulose bisphosphate into triose phosphate. What other substance is also needed?

A. Rubisco

B. NADP

C. NAD

D. Reduced NADP

D

The following processes occur in aerobic cell respiration.

I. Decarboxylation of pyruvate

II. Diffusion of protons through ATP synthase

III. Phosphorylation of glucose

Which is the correct sequence for these processes?

D

The diagram shows the link reaction between glycolysis and the Krebs cycle.

[Source: © International Baccalaureate Organization 2019]

Which type of reaction is occurring?

A. Pyruvate is carboxylated.

B. CO2 is oxidized.

C. NAD+ is reduced.

D. Pyruvate is phosphorylated.

C

The diagram shows some of the intermediate compounds produced during the Calvin cycle. At what stage does carboxylation take place?

C

Photolysis and carboxylation of RuBP occur during photosynthesis. Where in the chloroplast do these reactions occur?

B

A summary diagram of photosynthesis is shown. Which molecule represents ATP?

A

The diagram shows compounds in the Krebs cycle labelled as X, Y and Z.

[Source: © International Baccalaureate Organization 2019]

How many carbon atoms are there in Y and in the acetyl group of acetyl CoA?

A

This question was easy and discriminated very well.

What happens when pyruvate is converted to acetyl CoA in the link reaction?

A. Decarboxylation

B. Phosphorylation

C. Hydrolysis

D. Reduction of pyruvate

A

Some bacteria can synthesize the amino acid isoleucine from threonine, a process involving five enzymes (E₁ to E₅) and four intermediary products (P, Q, R and S). The production of isoleucine is controlled by end-product inhibition.

Which statement describes this end-product inhibition?

A. If isoleucine accumulates, it inhibits the production of P.

B. End-product inhibition causes a build-up of intermediary products.

C. Isoleucine inhibits E₅, so no more isoleucine is produced.

D. Isoleucine affects the structure of threonine.

A

The Hill reaction occurs when isolated chloroplasts are exposed to sunlight in the presence of DCPIP. DCPIP replaces NADP as the final electron acceptor for the light-dependent reactions of photosynthesis. What are products of the Hill reaction?

A. H₂O and ATP

B. ATP and CO₂

C. O₂ and ATP

D. C₆H₁₂O₆ and O₂

C

Some G2 responses mentioned that the Hill reaction does not appear in the Understandings in the guide, only in the Aims in the right-hand column in Section 8.3 (Photosynthesis). It was assumed most teachers would have mentioned the Hill reaction when covering this part of the guide, and for those who were unfamiliar with the name Hill the question included an explanation of the reaction. Statistically, the question performed well.

What molecule functions as the final electron acceptor in the mitochondrial electron transport chain?

A. Oxygen

B. ATP

C. Reduced NAD

D. Reduced FAD

A

According to the statistics, this question had the highest discrimination index with response C being the most popular incorrect response.

What are final products of photosynthesis and of aerobic respiration?

B

Which structure in the chloroplast diagram is adapted to carry out chemiosmosis?

[Source: © International Baccalaureate Organization 2019]

A

The mitochondrion in the electron micrograph shows some features that make it efficient for its function. Which labelled feature allows a rapid build-up of proton concentration for chemiosmosis?

D