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IB Biology B2.2: Organelles and compartmentalization worked solutions

Worked solutions to every practice question in B2.2: Organelles and compartmentalization of Photon Biology: Study Guide for the IB Diploma (SL & HL). Try each question, then compare your working with the full solution.

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Basic practice

Question A1

[1 mark]
Which structure is classed as an organelle in a eukaryotic cell? (A) cell wall (B) ribosome (C) cytoplasm (D) cytoskeleton
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A ribosome is a discrete subunit with a specific function, so it is an organelle (B) even though it has no membrane. The cell wall, the cytoplasm and the cytoskeleton are not counted as organelles.

Final answer

B

Question A2

[1 mark]
What is an advantage of keeping the enzymes of one pathway inside a membrane-bound compartment? (A) Enzymes and substrates are concentrated together. (B) No enzymes are needed. (C) The pathway cannot be regulated. (D) Every compartment must share one pH.
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The enzymes and their substrates are concentrated together, so the reactions run faster (A). Compartments do not remove the need for enzymes (B), they make separate regulation easier (C), and different compartments can hold different pH values (D).

Final answer

A

Question A3

[1 mark]
A white blood cell engulfs a bacterium into a vacuole. Which organelle supplies the enzymes that digest it? (A) Golgi apparatus (B) lysosome (C) rough ER (D) mitochondrion
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Lysosomes fuse with the phagocytic vacuole and release their digestive enzymes into it (B). The Golgi apparatus modifies and packages proteins, the rough ER makes proteins for export, and mitochondria make ATP.

Final answer

B

Question A4

[1 mark]
Why can a eukaryotic cell splice its mRNA before translation, while a prokaryotic cell cannot? (A) Eukaryotic ribosomes are larger. (B) The nucleus separates transcription from translation. (C) Prokaryotes have no DNA. (D) Eukaryotic mRNA is shorter.
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The nuclear envelope keeps ribosomes out of the nucleus, so the RNA can be processed before it reaches them (B). A prokaryote has no nucleus, so ribosomes meet the mRNA as it is made. Prokaryotes do have DNA (C), and the other options are not true.

Final answer

B

Question A5

[1 mark]
Cells are lysed and spun at low speed in a centrifuge. Which part of the cell is found mainly in the pellet? (A) nuclei (B) cytosol (C) free ribosomes (D) vesicles
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The nuclei are the densest and settle first, so they form the pellet (A). The cytoplasm and the smaller organelles stay in the supernatant.

Final answer

A

Question A6

HL[1 mark]
In which region of a mitochondrion is ATP synthase found? (A) outer membrane (B) inner membrane (C) matrix fluid (D) intermembrane space
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ATP synthase is embedded in the inner membrane, mainly in the cristae (B). The matrix is a fluid, the outer membrane has no ATP synthase, and the intermembrane space is the proton store.

Final answer

B

Question A7

HL[1 mark]
Where in a chloroplast are the enzymes of the Calvin cycle found? (A) thylakoid membrane (B) thylakoid space (C) outer membrane (D) stroma
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The Calvin cycle enzymes and their substrates are in the stroma (D). Photosystems are in the thylakoid membranes and protons collect in the thylakoid space.

Final answer

D

Question A8

HL[1 mark]
Which protein is made on ribosomes bound to the rough endoplasmic reticulum? (A) a glycolytic enzyme (B) actin of the cytoskeleton (C) a digestive enzyme secreted by a gland (D) a ribosomal protein kept in the cytosol
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Proteins for secretion are made on ribosomes on the rough ER (C). Proteins that stay in the cytosol, such as a glycolytic enzyme (A), are made on free ribosomes.

Final answer

C

Question A9

HL[1 mark]
What is the role of clathrin in endocytosis? (A) digests the cargo (B) moves the vesicle along fibres (C) coats and curves the membrane (D) makes the vesicle fuse with a target
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Clathrin coats the cytoplasmic face of the membrane and helps to curve it inward so a vesicle forms (C). It does not digest cargo, move the vesicle or make it fuse.

Final answer

C

Question A10

HL[1 mark]
Which molecule leaves the nucleus through the nuclear pores? (A) processed mRNA (B) chromosomal DNA (C) histones (D) splicing factors
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Processed mRNA leaves through the pores to reach ribosomes in the cytoplasm (A). Histones and splicing factors enter. Chromosomal DNA stays in the nucleus.

Final answer

A

Tutorial practice

Question B1

[3 marks]
The list gives some structures of an animal cell: ribosome, lysosome, cytoskeleton, plasma membrane, cytoplasm. (a) Define the term organelle. [1] (b) Identify the two structures in the list that are not organelles. [2]
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(a) An organelle is a discrete subunit of a cell (1) with a specific function. (b) The cytoskeleton (1) and the cytoplasm (1) are not counted as organelles. The cell wall is also not an organelle, but it is not in this list.

Final answer

(a) A discrete subunit of a cell with a specific function. (b) Cytoskeleton and cytoplasm.

Question B2

[4 marks]
A neutrophil is a white blood cell that destroys bacteria it has engulfed. (a) Outline how a phagocytic vacuole forms. [2] (b) Explain why lysosomal enzymes are enclosed in a membrane. [2]
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(a) The plasma membrane folds inwards round the bacterium (1) and pinches off to enclose it in a phagocytic vacuole (1). (b) Lysosomal enzymes would digest proteins, nucleic acids and polysaccharides of the cell if free (1); the lysosome membrane keeps them separate from the cytosol, and fusion with the vacuole concentrates them where the bacterium is (1).

Final answer

(a) The plasma membrane folds in around the bacterium and seals, so it lies in a vacuole bounded by membrane. (b) Free enzymes would digest the cell proteins, nucleic acids and polysaccharides. The membrane keeps them away from the cytosol, and in the vacuole they are concentrated at the target.

Question B3

[5 marks]
A liver homogenate was centrifuged in steps. The table gives the percentage of a mitochondrial marker enzyme and of enzyme Y in each of four fractions. Data: , fraction 1, fraction 2, fraction 3, fraction 4; Marker enzyme / %, 4, 78, 10, 8; Enzyme Y / %, 3, 12, 6, 79. (a) Identify the fraction richest in mitochondria. [1] (b) Deduce where enzyme Y mainly works, using the data. [2] (c) Explain why the 12\% of enzyme Y in fraction 2 does not settle whether Y acts in mitochondria. [2]
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(a) Fraction 2 holds 78\% of the mitochondrial marker (1). (b) 79\% of enzyme Y is in fraction 4, the supernatant that holds the cytosol (1), and only 12\% is with the mitochondria, so Y mainly works in the cytosol (1). (c) The mitochondrial fraction is not pure, so the 12\% may come from other material (1); equally a small amount of Y might act in mitochondria, so the data show an association, not proof that Y is absent from them (1).

Final answer

(a) Fraction 2. (b) Enzyme Y is mostly in fraction 4, the supernatant, so it works in the cytosol. (c) 12% of Y is in fraction 2: the fraction is not pure, so a small mitochondrial role is not excluded or shown.

Question B4

[4 marks]
In eukaryotes, mRNA is modified before it is translated. (a) State where transcription and translation occur in a eukaryote. [2] (b) State one benefit of modifying the mRNA before translation. [1] (c) Explain why a prokaryote cannot modify its mRNA before translation. [1]
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(a) Transcription occurs in the nucleus (1) and translation on ribosomes in the cytoplasm (1). (b) One from: the modifications protect the mRNA from enzymes that digest RNA; alternative splicing lets one gene give more than one mRNA (1). (c) A prokaryote has no nucleus, so ribosomes meet the mRNA while it is still being transcribed and there is no stage in which to process it (1).

Final answer

(a) Transcription in the nucleus; translation on ribosomes in the cytoplasm. (b) Protection from enzymes that digest RNA, or alternative splicing. (c) A bacterium has no nuclear envelope, so ribosomes meet the mRNA as it is made.

Question B5

HL[5 marks]
Three regions of a mitochondrion are described. P is the fluid inside the inner membrane. Q is the inner membrane, which is folded. R is the narrow gap between the inner and outer membranes. (a) Identify P, Q and R. [3] (b) Explain how the folding of Q helps ATP production. [1] (c) Explain how the small size of R helps ATP production. [1]
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(a) P is the matrix (1), Q the cristae or inner membrane (1) and R the intermembrane space (1). (b) The folds give the inner membrane a large area (1) for many electron transport chains and ATP synthase molecules, so more ATP is made. (c) The small volume of R lets pumped protons raise the proton concentration rapidly (1).

Final answer

(a) P matrix, Q cristae (inner membrane), R intermembrane space. (b) Folding gives a large inner membrane area for electron carriers and ATP synthase. (c) Small volume: pumped protons raise the concentration quickly.

Question B6

HL[5 marks]
A diagram of a chloroplast shows thylakoids stacked as grana. (a) State the region where the Calvin cycle takes place. [1] (b) State where the photosystems are located. [1] (c) Explain how thylakoid structure supports the light-dependent reactions. [3]
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(a) The Calvin cycle occurs in the stroma (1). (b) The photosystems are in the thylakoid membranes (1). (c) The stacks of thylakoids give a large membrane area (1) for many photosystems, electron carriers and ATP synthase (1); protons are pumped into a very small thylakoid space so their concentration rises quickly (1).

Final answer

(a) Stroma. (b) Thylakoid membranes. (c) Large membrane area holds many photosystems, carriers and ATP synthase; the small thylakoid space lets protons build up quickly, so a gradient drives ATP synthesis.

Question B7

HL[4 marks]
A cell of the pancreas secretes digestive enzymes. It also makes enzymes of glycolysis that stay in the cell. (a) State which ribosomes make each kind of enzyme. [2] (b) Distinguish the roles of the rough ER and the Golgi apparatus in producing the secreted enzymes. [2]
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(a) Glycolytic enzymes, which stay in the cell, are made on free ribosomes (1); the digestive enzymes for secretion are made on ribosomes bound to the rough ER (1). (b) The rough ER (ribosomes) synthesises the polypeptide (1); the Golgi apparatus modifies it, sorts it and packages it into secretory vesicles (1).

Final answer

(a) Free ribosomes make the glycolytic enzymes; ribosomes on rough ER make the digestive enzymes. (b) Rough ER makes the polypeptide and passes it on; the Golgi modifies it and packages it into vesicles for secretion.

Question B8

HL[4 marks]
The nuclear envelope has pores and breaks down during cell division. (a) Describe the function of the nuclear pores. [2] (b) State what happens to the nuclear envelope during mitosis and meiosis. [2]
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(a) Pores allow selective passage of proteins into the nucleus and of RNA out (1); because only some molecules cross, they help regulate gene expression (1). (b) The envelope breaks down in prophase of mitosis and meiosis (1) and reassembles round the chromosomes when division ends (1).

Final answer

(a) Pores let proteins such as histones and splicing factors in and mRNA out; they control which molecules cross, which helps regulate gene expression. (b) The envelope breaks down in prophase of mitosis or meiosis and re-forms round the chromosomes when division ends.

Advanced practice

Question C1

HL[8 marks]
Cells were given a short pulse of a labelled amino acid. The table shows the percentage of the labelled protein in four places at four times afterwards. Data: Time / min, 0, 10, 30, 60; Rough ER, 90, 52, 12, 3; Golgi apparatus, 8, 36, 40, 9; Secretory vesicles, 2, 10, 36, 38; Outside the cell, 0, 2, 12, 50. (a) State the time at which most of the label is in the Golgi apparatus. [1] (b) Calculate the percentage decrease in the share of label in the rough ER between 0 and 30 min. [2] (c) Deduce the route taken by the labelled protein. [2] (d) Explain how the protein moves from the rough ER to the Golgi apparatus and what the Golgi does to it. [2] (e) State one limitation of this kind of evidence. [1]
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(a) The Golgi share is highest at 30 min (1). (b) \dfrac{90-12}{90}\times100=86.7\approx87\% (1 for the method, 1 for the value). (c) The share in the rough ER falls first (1); the Golgi peaks next at 30 min, the secretory vesicles later, and the outside is last at 60 min, so the route is rough ER, Golgi, secretory vesicles, release (1). (d) A transport vesicle buds from the rough ER and fuses with the Golgi (1); the Golgi modifies the protein and packages it into secretory vesicles (1). (e) The label marks one batch of protein and does not follow individual molecules; the timing is a pattern that suggests the route but does not show that each step is a direct vesicle transfer (1).

Final answer

(a) 30 min. (b) 87%. (c) Rough ER, then Golgi, then secretory vesicles, then outside: each peak is later. (d) A vesicle carries the protein from the rough ER to the Golgi; the Golgi modifies and packages it. (e) The label follows a batch, not single molecules; it does not show that vesicles move directly.

Question C2

HL[8 marks]
Mitochondria were measured in a flight muscle cell and in a skin cell. Data: , flight muscle, skin; Inner membrane area / \mu\mathrm{m}^2, 36, 12; Matrix volume / \mu\mathrm{m}^3, 0.9, 1.2. (a) Calculate the inner membrane area per unit of matrix volume for each cell. [2] (b) Explain why the flight muscle mitochondrion is likely to make ATP faster. [3] (c) State why the answer to (b) is a prediction and not a measurement. [1] (d) Describe the structure in a chloroplast that plays the same part as the cristae, and say why it does. [2]
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(a) Muscle: \dfrac{36}{0.9}=40\,\mu\mathrm{m}^{-1} (1); skin: \dfrac{12}{1.2}=10\,\mu\mathrm{m}^{-1} (1). (b) The muscle mitochondrion has four times as much inner membrane for the same volume (1); the cristae carry electron transport chains and ATP synthase (1), so more of them fit and the muscle mitochondrion can probably make ATP faster (1). (c) The data give structure only, and the rate of ATP production was not measured (1). (d) In a chloroplast the thylakoid membranes play the part of the cristae (1): a large folded membrane area holding photosystems, carriers and ATP synthase (1).

Final answer

(a) 40 and 10 per micrometre. (b) The muscle mitochondrion has four times the inner membrane area for each unit of matrix volume, so more electron transport chains and ATP synthase fit; it can probably make ATP faster. (c) The rate of ATP production was not measured. (d) Thylakoid membranes also provide a large area for photosystems and ATP synthase.

Question C3

[6 marks]
Eukaryotic cells are divided into compartments by membranes. Explain the advantages of compartmentalization, using named examples.
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One mark for each point, maximum 6: a membrane-bound compartment concentrates the enzymes and substrates of a process (1), so reactions are faster (1); different processes that would interfere are kept apart (1); a compartment can have its own conditions such as pH (1); lysosome: the membrane keeps digestive enzymes away from the proteins and nucleic acids of the cytosol (1) and they act inside a phagocytic vacuole (1); mitochondrial matrix: the Krebs cycle enzymes and substrates are close together (1); nucleus: transcription is separated from translation, so mRNA is processed first (1). Naming an organelle without saying what the membrane allows earns no mark.

Final answer

Six points from: concentration of enzymes and substrates; separation of incompatible processes; lysosome membrane keeps digestive enzymes from cytosol; phagocytic vacuole; matrix holds Krebs enzymes; nuclear envelope separates transcription from translation so mRNA is processed; each compartment can hold its own conditions.

Question C4

HL[6 marks]
A drug blocks the assembly of clathrin coats. Neurons were stimulated five times and the neurotransmitter released was recorded as a percentage of the first release. Data: Stimulus train, 1, 2, 3, 4, 5; Control, 100, 98, 97, 99, 96; Drug, 100, 81, 60, 39, 22. (a) Describe the trend in release in the drug-treated neurons. [1] (b) Calculate the percentage decrease in release by train 5 in the drug-treated neurons. [1] (c) Explain the trend, using the role of clathrin. [3] (d) Predict the effect of the drug on the area of the presynaptic plasma membrane. [1]
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(a) With the drug the release falls steadily at each train, from 100\% to 22\%, while the control stays close to 100\% (1). (b) 100-22=78\% decrease (1). (c) Clathrin coats curve the membrane to form vesicles by endocytosis (1); these recycle the vesicle membrane through endosomes into new synaptic vesicles (1); without coats fewer vesicles are replaced and refilled, so less neurotransmitter is released at each train (1). (d) Vesicles still fuse and add membrane, but it is not recovered, so the area of the plasma membrane of the terminal increases (1).

Final answer

(a) Release with the drug falls steadily from 100 to 22%. (b) 78%. (c) Clathrin-coated vesicles recover the membrane and recycle synaptic vesicles; without coats fewer vesicles are refilled, so each train has less to release. (d) The plasma membrane area at the terminal increases.

Connected questions

Question X1

Connected questions · HL[6 marks]
B2.2 + C1.2 + B2.1. A drug makes the inner mitochondrial membrane leak protons. (a) Explain why ATP production falls. [2] (b) Predict and explain the effect on the rate of oxygen consumption. [2] (c) Identify the compartment where protons are normally concentrated and explain why it suits this job. [2]
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(a) Protons leak back across the inner membrane without passing through ATP synthase (1), so the gradient falls and less ATP is made by chemiosmosis (1). (b) The electron transport chain keeps pumping protons to rebuild the gradient (1), so more oxygen, the final electron acceptor, is used (1). (c) Protons are normally concentrated in the intermembrane space (1) because its small volume lets the concentration rise quickly (1).

Final answer

(a) The proton gradient across the inner membrane is lost, so ATP synthase has little driving force and less ATP is made. (b) The electron transport chain runs faster to rebuild the gradient, so oxygen use rises. (c) The intermembrane space; its small volume lets pumped protons raise the concentration quickly.

Question X2

Connected questions · HL[6 marks]
B2.2 + D1.2 + D1.3. A gland cell makes a hormone that is secreted. (a) Outline how the mRNA for the hormone is made and how the protein reaches the rough ER. [4] (b) A mutation changes one codon near the start of the gene to a stop codon. Predict the effect on the hormone and on secretion. [2]
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(a) The gene is transcribed in the nucleus (1); the mRNA is processed (1) and leaves through a nuclear pore (1); a ribosome on the rough ER translates it so the polypeptide enters the ER (1). (b) A premature stop codon ends translation early, so the polypeptide is short and cannot function (1) and little or no hormone is secreted (1).

Final answer

(a) Transcription in the nucleus; mRNA processed; leaves through a pore; translated on ribosomes at the rough ER. (b) A stop codon early gives a very short polypeptide with no function; little or none is secreted. (c) The DNA is the template and is never exported, so it stays protected.

Question X3

Connected questions · HL[6 marks]
B2.2 + A2.2 + C1.3. Mitochondria and chloroplasts both have a double membrane, their own DNA and ribosomes. (a) Give two features that support the hypothesis that these organelles arose from prokaryotes. [2] (b) Explain how the compartment around the thylakoids helps the Calvin cycle. [2] (c) Identify where protons are pumped in a chloroplast and explain why the compartment suits this job. [2]
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(a) Two from: they contain their own DNA (1); they contain their own ribosomes of a bacterial kind (1); they have two membranes, as expected if a prokaryote was engulfed into a vesicle (1). (b) The stroma holds the enzymes and substrates of the Calvin cycle in one place, so the reactions run faster (1) and the sugar is made next to the ATP and NADPH from the thylakoids (1). (c) Protons are pumped into the thylakoid space (1), whose small volume makes the concentration rise quickly and drives ATP synthase (1).

Final answer

(a) They have their own DNA and ribosomes and a double membrane, like a bacterium that was engulfed. (b) The stroma holds the enzymes and substrates of the Calvin cycle together. (c) The thylakoid space; its very small volume lets pumped protons build up quickly, so a gradient drives ATP synthase.

From the book

Photon Biology: Study Guide for the IB Diploma (SL & HL)

Organelles and compartmentalization from the core ideas to full exam questions, with question types, worked examples and graded practice.

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