An organelle is a discrete unit with a job; walls, cytoskeleton and cytoplasm do not count. Membranes divide the cell so enzymes are concentrated and clashing processes are kept apart. Mitochondria, chloroplasts, nucleus, ER, Golgi and vesicles each show form shaped for function.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
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Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
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B2.2.1 What counts as an organelle, and how their jobs were found
An organelle is a discrete subunit of a cell adapted to a specific function.
Nucleus, vesicles, ribosomes and the plasma membrane count; a membrane is not required.
The cell wall, cytoskeleton and cytoplasm are not organelles: none is a discrete unit.
Cell fractionation in an ultracentrifuge separates organelles by size and density for separate study.
Studying organelle function became possible only once ultracentrifuges existed: new techniques drive progress in science.
Students often think only membrane-bound structures are organelles. In fact a ribosome is one: a discrete unit with a specific job.
Students often think organelle functions were found by electron microscopy. In fact they were found by isolating each fraction and testing what it could do.
B2.2.2 Why keep the nucleus and cytoplasm apart
DNA sits in the nucleus; working ribosomes exist only in the cytoplasm.
So transcription happens in the nucleus and translation in the cytoplasm.
New mRNA can be modified before it leaves and meets a ribosome.
Prokaryotes have no nucleus, so mRNA may meet ribosomes at once, even mid-transcription.
Students often think ribosomes in the nucleus translate mRNA on the spot. In fact there are no working ribosomes there; mRNA must exit through a pore first.
Students often think all cells modify mRNA before translation. In fact prokaryotes cannot; nothing keeps their mRNA from ribosomes.
B2.2.3 Why compartments help the cytoplasm
Compartmentalisation divides the cytoplasm into membrane-bound organelles and vesicles.
It concentrates enzymes and metabolites in a small volume, so reactions run fast.
It separates incompatible processes: hydrolytic enzymes stay inside lysosomes.
Lysosomes fuse with a phagocytic vacuole and digest its contents, sparing the cell.
Students often think lysosomal enzymes could not harm the cell if released. In fact they hydrolyse proteins, lipids, carbohydrates and nucleic acids: the cell's own materials.
Students often think compartments slow metabolism because membranes get in the way. In fact concentrating reactants raises collision rates and speeds reactions.
B2.2.4 How a mitochondrion is built for ATP (HL) HL
Cristae fold the inner membrane: more area for electron transport chains and ATP synthase.
The intermembrane space is small, so few pumped protons make a steep gradient quickly.
The matrix holds Krebs cycle enzymes and substrates concentrated together, close to the inner membrane.
The double membrane creates these compartments; it is not for protection.
Students often think cristae absorb oxygen and glucose. In fact they hold the electron transport chains; area means more of them.
Students often think a bigger intermembrane space would hold more protons and make more ATP. In fact a small volume is the point: concentration rises fast.
B2.2.5 How a chloroplast is built for photosynthesis (HL) HL
Thylakoid membranes have a large total area and hold the photosystems for the light-dependent reactions.
The fluid inside each thylakoid is small in volume, so a proton gradient builds quickly.
The stroma concentrates Calvin cycle enzymes and substrates, fed by ATP and reduced NADP.
Students often place the Calvin cycle inside the thylakoids. In fact it runs in the stroma, using products made at the thylakoid membranes.
Students often think chlorophyll floats in chloroplast fluid. In fact photosystems sit in the thylakoid membranes.
B2.2.6 Why the nucleus has a double membrane with pores (HL) HL
The nuclear envelope is two bilayers; it keeps transcription apart from translation.
Nuclear pores pass through both membranes; large molecules cannot cross the bilayers otherwise.
mRNA and ribosomal subunits leave through pores; polymerases, histones and nucleotides enter.
In mitosis and meiosis the envelope breaks into vesicles, so the spindle can reach chromosomes.
Students often think DNA leaves through the pores. In fact mRNA leaves; DNA stays in the nucleus.
Students often think the nucleus pinches in two with its membrane intact. In fact the envelope breaks up and re-forms around each chromosome set.
B2.2.7 Free ribosomes and the rough ER (HL) HL
Free ribosomes float in the cytoplasm and make proteins kept inside the cell.
The rough ER is flattened sacs with bound ribosomes, continuous with the outer nuclear membrane.
Its polypeptides enter the ER lumen, then travel in vesicles to the Golgi apparatus.
These are proteins for transport within the cell, membranes, or secretion.
Students often think free ribosomes make secreted proteins. In fact secreted proteins come from ribosomes bound to the rough ER.
Students often think all proteins are made on the rough ER. In fact cytoplasmic and nuclear proteins come from free ribosomes.
B2.2.8 The Golgi apparatus processes and packages (HL) HL
The Golgi apparatus is a stack of flattened sacs (cisternae).
It receives protein from the rough ER in vesicles and processes it, modifying carbohydrates.
Vesicles bud from the opposite face and carry protein to the plasma membrane for exocytosis.
Students often think the Golgi makes secreted proteins. In fact it processes and packages proteins made on the rough ER.
Students often think ER, Golgi and membrane are joined by channels. In fact vesicles carry protein between them; the organelles are separate.
B2.2.9 Vesicles and clathrin (HL) HL
A vesicle is a small sac budded from an existing membrane: plasma membrane, ER, Golgi.
It carries material and delivers it by fusing with a target membrane.
Clathrin forms a lattice on the cytoplasmic side, bending the membrane into a coated pit.
The coat is shed after budding so the vesicle can fuse with its target membrane.
Students often think the whole vesicle is pushed out of the cell. In fact its membrane merges with the plasma membrane; only contents leave.
Students often think clathrin is the receptor that recognises cargo. In fact receptors bind the cargo; clathrin only shapes the membrane from the cytoplasmic side.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 Which statement correctly applies the term organelle as it is used in IB Biology?
Answer and reasoning
The plasma membrane is an organelle, although it forms the boundary of the cytoplasm. — An organelle is a discrete subunit of a cell adapted to a specific function. The plasma membrane is such a unit, controlling exchange with the surroundings, and the guide lists it with nuclei, vesicles and ribosomes as an organelle.
Ribosomes are not organelles, because they are not bounded by a membrane. — A student who defines an organelle as a membrane-bound structure picks this. The IB definition is based on function: a ribosome is a discrete subunit adapted to protein synthesis, so it is an organelle.
The cell wall is an organelle, because it is a labelled part with a function. — A student who treats every labelled structure as an organelle picks this. The cell wall is an extracellular layer, not a discrete subunit of the cell, and the guide states it is not an organelle.
The cytoskeleton is an organelle, because it is a structure that moves organelles. — A student who groups any named internal structure with the organelles picks this. The cytoskeleton is a continuous framework running through the cytoplasm, not a discrete subunit, and is not considered an organelle.
2 In a eukaryotic cell, newly transcribed RNA is modified in the nucleus before it is translated. How does the separation of the nucleus from the cytoplasm make this possible?
Answer and reasoning
Ribosomes inside the nucleus translate the RNA while it is being modified, so the two processes run together in the same place. — A student who places translation in the nucleus picks this. There are no functional ribosomes in the nucleus; translation happens in the cytoplasm, which is precisely why modification can be completed first.
The nuclear membrane keeps the ribosomes away from the DNA, which they would otherwise translate directly into protein. — A student who skips the role of mRNA picks this. Ribosomes translate mRNA, not DNA, so the membrane is not protecting DNA from ribosomes; it is keeping the mRNA away from them until modification is complete.
Translation happens only in the cytoplasm, so the RNA cannot meet a ribosome until modified and out through a pore. — Transcription takes place in the nucleus and translation in the cytoplasm. Because the nuclear membrane separates the two, post-transcriptional modification of mRNA can happen before the mRNA meets ribosomes, which is the advantage the guide identifies.
Modification of RNA before translation happens in all kinds of cell, so the nucleus is not what makes it possible. — A student who treats the eukaryotic pathway as universal picks this. In prokaryotes there is no nucleus, mRNA may meet ribosomes immediately, and modification before translation is not possible.
3 Lysosomes contain hydrolytic enzymes at a concentration far higher than in the surrounding cytoplasm. Why is keeping these enzymes inside a lysosome an advantage to the cell?
Answer and reasoning
The enzymes are concentrated where they are needed and kept apart from the cytoplasm, whose contents they would otherwise hydrolyse. — This is the double advantage of compartmentalization the guide names: concentration of enzymes and metabolites, and separation of incompatible biochemical processes. Hydrolysis of macromolecules is incompatible with the rest of the cytoplasm.
The membrane merely stores the enzymes, since their specificity would stop them harming the cell if they escaped. — A student who reads enzyme specificity as 'acts only on its intended target' picks this. Lysosomal enzymes are specific to bond types found in every protein, lipid and nucleic acid in the cell, so their escape would be destructive.
The membrane slows hydrolysis to a safe rate by limiting how fast substrates can reach the enzymes inside. — A student who sees a compartment as a brake on metabolism picks this. Concentrating enzymes and substrates in a small volume speeds reactions up; the membrane's role is separation, not slowing.
The lysosome is where the enzymes are synthesised, so they have to be kept in the organelle that made them. — A student who slides from 'contains enzymes' to 'makes enzymes' picks this. Lysosomes have no ribosomes; their enzymes are made on the rough ER and delivered by the Golgi apparatus.
4 During aerobic cell respiration the electron transport chain pumps protons from the matrix into the intermembrane space of the mitochondrion. Why is the small volume of the intermembrane space an adaptation for ATP production? HL
Answer and reasoning
Its volume is not itself an adaptation: it is the total number of protons stored that determines the ATP yield. — A student who treats the space as a proton reservoir picks this. Chemiosmosis is driven by the concentration gradient across the inner membrane, and a small volume is what allows a steep gradient to form from few protons.
It brings oxygen entering through the outer membrane close to the cristae, which absorb it for use in respiration. — A student who explains every mitochondrial feature in terms of absorption picks this. Oxygen diffuses freely through both membranes; the intermembrane space is an adaptation for proton accumulation, not for oxygen uptake.
Few protons need to be pumped to produce a steep concentration gradient, so the gradient forms quickly. — Concentration is number of particles per unit volume. Pumping protons into a small volume raises their concentration sharply, so a steep gradient across the inner membrane is established rapidly and ATP synthase can begin working sooner.
It stops enzymes from the cytoplasm getting in and interfering with the electron transport chain. — A student who reads the double membrane as protection picks this. The outer membrane, not the size of the space, sets what enters, and the space's small volume matters because of its effect on proton concentration.
5 Which statement correctly matches an adaptation of the chloroplast to its function in photosynthesis? HL
Answer and reasoning
The thylakoid membranes provide a large surface area in which photosystems are held to absorb light. — Photosystems are embedded in the thylakoid membranes, and the large total area of these membranes allows many photosystems to be packed into the chloroplast, an adaptation the guide names explicitly.
The fluid inside the thylakoids contains the enzymes of the Calvin cycle at high concentration. — A student who places every stage of photosynthesis in the thylakoids picks this. The Calvin cycle enzymes and substrates are compartmentalized in the stroma; the thylakoid space is where protons accumulate.
The stroma holds the photosystems in solution so that light can reach them from every direction. — A student who pictures chlorophyll dissolved in fluid picks this. Photosystems are pigment-protein complexes embedded in the thylakoid membranes, not dissolved in the stroma.
The thylakoids enclose a large volume of water to act as the substrate for photolysis. — A student who reads the thylakoid as a water tank picks this. The volume of fluid inside a thylakoid is small, which lets a proton gradient build up quickly; water is available throughout the chloroplast.
6 What happens to the nuclear membrane during mitosis, and why? HL
Answer and reasoning
It pinches into two so that each daughter nucleus keeps part of the original envelope. — A student who pictures the nucleus dividing like a cell picks this. An intact envelope would keep the spindle microtubules away from the chromosomes, so the envelope must break down, not pinch.
It opens its pores wider so that the chromosomes can pass out into the cytoplasm. — A student who thinks the pores are a route for genetic material picks this. Chromosomes do not pass through pores; the envelope disassembles into vesicles and reassembles around each set of chromosomes.
It stays intact throughout, because the chromosomes can diffuse through its two bilayers. — A student who thinks anything can diffuse across a membrane picks this. Chromosomes are far too large to cross a bilayer, and it is the spindle that must reach them, which requires the envelope to break down.
It breaks into vesicles so that spindle microtubules can reach the chromosomes. — Spindle microtubules form in the cytoplasm. The nuclear membrane breaks into vesicles during mitosis and meiosis so that the microtubules can attach to the chromosomes and move them; the envelope re-forms from vesicles around each daughter nucleus.
7 A pancreatic cell makes the digestive enzyme trypsinogen, which it secretes, and the enzyme hexokinase, which catalyses the first step of glycolysis in its cytoplasm. Where are the two proteins synthesised? HL
Answer and reasoning
Trypsinogen on ribosomes bound to the rough endoplasmic reticulum; hexokinase on free ribosomes lying in the cytoplasm. — Proteins for secretion are made on membrane-bound ribosomes on the rough ER and enter its lumen for transport via the Golgi apparatus. Proteins retained in the cell, such as a glycolytic enzyme, are made on free ribosomes and released into the cytoplasm.
Trypsinogen on free ribosomes, which can move to the plasma membrane; hexokinase on the rough endoplasmic reticulum. — A student who reads 'free' as 'free to travel' picks this. Free ribosomes make proteins that stay in the cell; secreted proteins are made on ribosomes bound to the rough ER, which is the reverse of this option.
Both on the rough endoplasmic reticulum, from which the hexokinase is later released into the cytoplasm. — A student who thinks all protein synthesis happens on the rough ER picks this. Proteins made on the rough ER enter its lumen; a cytoplasmic enzyme is made on a free ribosome and never enters the ER.
Trypsinogen in the Golgi apparatus, which makes secreted proteins; hexokinase on free ribosomes in the cytoplasm. — A student who thinks the Golgi apparatus makes secreted proteins picks this. The Golgi apparatus has no ribosomes; it processes and packages trypsinogen made on the rough ER.
8 Pancreatic cells were supplied with radioactive amino acids for 3 minutes and then with non-radioactive amino acids. Samples of cells were taken at intervals and the location of radioactive protein was found. After 5 minutes it was mainly in the rough endoplasmic reticulum, after 20 minutes mainly in the Golgi apparatus, and after 60 minutes mainly in vesicles close to the plasma membrane. Which conclusion do these results support? HL
Answer and reasoning
Protein flows along a continuous membrane channel from the rough ER, through the Golgi apparatus, to the plasma membrane. — A student who pictures the organelles as joined picks this. The label moves between separate organelles in discrete steps and ends in vesicles; the ER and Golgi apparatus are not continuous, and vesicles carry protein between them.
Protein made on the rough ER is passed to the Golgi apparatus for processing and then packaged into secretory vesicles. — Radioactive protein appears first where it is synthesised, on the rough ER, then in the Golgi apparatus, then in vesicles that will fuse with the plasma membrane. This is the sequence of processing and secretion of protein that the guide specifies for the Golgi apparatus.
The Golgi apparatus synthesises the secreted protein, using amino acids that it receives from the rough ER. — A student who thinks the Golgi apparatus makes protein picks this. The label is already in protein in the rough ER at 5 minutes, before any appears in the Golgi apparatus, so synthesis happened on the ER.
The vesicles near the plasma membrane will be expelled whole from the cell, carrying the protein out with them. — A student who thinks exocytosis pushes the vesicle out picks this. The data show where the protein is, not how it leaves; in exocytosis the vesicle membrane fuses with the plasma membrane and only the contents are released.
9 What is the role of clathrin in the formation of a vesicle? HL
Answer and reasoning
It lines the inside of the vesicle so that it can bind and hold the cargo molecules that are being transported. — A student who reads 'coat' as a lining picks this. Clathrin is on the cytoplasmic face of the membrane, never in contact with the cargo, and is shed once the vesicle has formed.
It is the membrane receptor protein that recognises and binds the substance that is to be brought into the cell. — A student who confuses clathrin with the receptor picks this. Recognition is done by separate receptor proteins in the membrane; clathrin's role is structural.
It assembles into a lattice on the cytoplasmic side of the membrane, bending it into a pit that pinches off. — Clathrin molecules assemble into a cage-like lattice beneath the membrane, which forces the membrane to curve inwards into a coated pit; the pit pinches off as a vesicle, and the clathrin coat is then removed.
It builds a new membrane around the cargo, separate from the existing membrane, to enclose it. — A student who thinks vesicles are made from scratch picks this. A vesicle is a bud of existing membrane; clathrin shapes that membrane, it does not construct a new one.
10 Liver cells were broken open in a cold, isotonic, buffered solution and the homogenate was spun in an ultracentrifuge at a series of increasing speeds. Only the pellet formed at the second speed consumed oxygen and produced ATP when supplied with substrates. What does this investigation illustrate about how progress in science is made?
Answer and reasoning
Electron micrographs of the organelles in the second pellet would have revealed the same function, so the centrifuge added little that was new. — A student who thinks organelle functions were read off from images picks this. A micrograph shows structure only; oxygen consumption and ATP production could only be demonstrated by isolating the organelle and testing it, which needed the ultracentrifuge.
A technique for isolating one type of organelle made it possible to test its function directly, which could not be done before. — Differential centrifugation separated a fraction rich in mitochondria, and that fraction alone respired. Study of the function of individual organelles became possible once ultracentrifuges and cell fractionation methods had been developed: progress in science often follows a new technique.
The ATP must have come from intact cells surviving in the pellet, since organelles cannot work once removed from a cell. — A student who extends cell theory to mean that nothing smaller than a cell can carry out a process picks this. Isolated mitochondria in a suitable solution consume oxygen and make ATP; that is exactly what makes cell fractionation informative.
A single spin at the highest speed would have brought every organelle down in one pellet that could be tested just as well. — A student who thinks centrifuging merely speeds up settling picks this. A single high-speed spin would bring down every organelle together; only a series of increasing speeds separates nuclei, then mitochondria, then smaller components into different pellets. A mixed pellet could not show which organelle was responsible for the oxygen consumption.
Read the ones marked not yet in Learn, then Verify.
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8 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 A gene from a eukaryote is inserted into a bacterium and is transcribed there. In the eukaryotic cell the mRNA from this gene would be modified before translation. What happens to the mRNA in the bacterium?
Answer and reasoning
It is modified in the cytoplasm before ribosomes can bind it. — A student who assumes every cell modifies mRNA before translation picks this. A bacterium has no nucleus to hold the mRNA apart from ribosomes, so there is no stage at which modification could happen first.
It is translated inside the nucleoid before it is released. — A student who thinks translation happens where the DNA is, inside a nucleus or its equivalent, picks this. The nucleoid is not a compartment; bacterial ribosomes are in the cytoplasm and bind the mRNA as it is transcribed.
It is not translated, as bacterial ribosomes read the DNA. — A student who thinks ribosomes can translate DNA directly picks this. Ribosomes in all cells translate mRNA; in a bacterium the mRNA is translated as soon as ribosomes can attach to it.
It is translated at once, without any modification. — In prokaryotes there is no nuclear membrane separating transcription from translation, so mRNA may immediately meet ribosomes and be translated, even while it is still being transcribed. Post-transcriptional modification is not possible.
2 A phagocyte engulfs a bacterium into a phagocytic vacuole, and lysosomes then fuse with the vacuole. What is the advantage of digesting the bacterium inside this compartment rather than in the cytoplasm?
Answer and reasoning
The enzymes could safely be released anywhere in the cell, so the vacuole serves only to hold the bacterium still. — A student who believes enzyme specificity makes lysosomal enzymes harmless picks this. Released into the cytoplasm they would hydrolyse the phagocyte's own proteins and membranes; the vacuole keeps that process separate.
Hydrolytic enzymes reach a high concentration around the bacterium while being unable to digest the phagocyte's own contents. — Fusion delivers the lysosomal enzymes into the small volume of the vacuole, so they act on the bacterium at high concentration, and the vacuole membrane separates this incompatible process from the rest of the cytoplasm. Both advantages of compartmentalization apply.
The vacuole makes its own enzymes on its membrane, so the lysosomes only add more membrane to enlarge it. — A student who thinks vacuoles produce digestive enzymes picks this. A phagocytic vacuole has no enzymes of its own; it is the fusion of lysosomes that supplies them, which is the whole point of the fusion.
Digestion in the cytoplasm would be too rapid to control, so the vacuole membrane slows the enzymes' access to the bacterium inside. — A student who thinks membranes exist to slow processes down picks this. The compartment speeds digestion by concentrating enzyme and substrate together; it does not throttle it.
3 Electron micrographs of mitochondria from two tissues were analysed. In tissue X the inner membrane had an area of about 40 µm² per µm³ of mitochondrial volume; in tissue Y the value was about 15 µm² per µm³. Which conclusion is best supported by these data? HL
Answer and reasoning
Mitochondria in tissue X absorb oxygen and glucose more quickly through their more extensive cristae. — A student who links surface area only to absorption picks this. Cristae are folds of the inner membrane and do not increase the area for uptake into the organelle; their area matters for the processes located in the inner membrane.
Mitochondria in tissue X carry out the Krebs cycle faster, since it takes place on the cristae. — A student who assigns every mitochondrial process to the cristae picks this. The Krebs cycle takes place in the matrix; the inner membrane holds the electron transport chain and ATP synthase.
Mitochondria in tissue Y store more protons, because a smaller inner membrane leaves a larger intermembrane space. — A student who thinks a bigger proton store is the aim picks this. The data say nothing about the volume of the intermembrane space, and in any case a small space is the adaptation, because it lets a steep gradient form quickly.
Mitochondria in tissue X hold more electron transport chains and ATP synthase per unit volume. — The inner membrane, folded into cristae, is where the electron transport chain and ATP synthase are located. More inner membrane area per unit volume means more of these complexes, so a higher capacity for ATP production; this is what the data directly support.
4 Both mitochondria and chloroplasts produce ATP by chemiosmosis. Which feature of the chloroplast plays the same role as the small intermembrane space of the mitochondrion? HL
Answer and reasoning
The large volume of water held inside the thylakoids, which supplies the protons released by photolysis. — A student who thinks the thylakoids store water picks this. The thylakoid space is small, not large, and that smallness is the adaptation; water is not stored there.
The small volume of fluid inside each thylakoid, in which protons accumulate to form a steep gradient quickly. — In the chloroplast, protons are moved into the thylakoid space; its small volume means their concentration rises rapidly, giving a steep gradient across the thylakoid membrane for ATP synthase, exactly as the small intermembrane space does in the mitochondrion.
The stroma, in which the photosystems pump protons into the surrounding fluid of the chloroplast. — A student who places the photosystems in the stroma picks this. Photosystems are in the thylakoid membranes, and protons are moved into the thylakoid space, not into the stroma.
The space between the two membranes of the chloroplast envelope, where protons are stored until they are needed. — A student who sees the intermembrane space as a proton store, and looks for the matching store in the chloroplast, picks this. The chloroplast's proton gradient is across the thylakoid membrane, not across the envelope.
5 A cell is treated with a substance that blocks all of its nuclear pores. Which consequence would follow? HL
Answer and reasoning
DNA could no longer leave the nucleus to be translated by the ribosomes in the cytoplasm. — A student who thinks DNA travels to the ribosomes picks this. DNA never leaves the nucleus; it is mRNA transcribed from the DNA that passes out through the pores.
Protein synthesis would carry on, as ribosomes inside the nucleus would translate the mRNA there. — A student who places translation in the nucleus picks this. There are no functional ribosomes in the nucleus, so mRNA that cannot leave cannot be translated.
mRNA could no longer leave the nucleus, so cytoplasmic ribosomes would receive no new mRNA. — The nuclear pores are the only route for mRNA and ribosomal subunits out of the nucleus and for proteins such as polymerases into it. Blocking them cuts off the supply of new mRNA to the ribosomes in the cytoplasm, which is why the double membrane needs pores.
mRNA and proteins would still cross the envelope, diffusing through its bilayers as small molecules do. — A student who assumes anything can diffuse across a membrane picks this. Large polar molecules such as RNA and proteins cannot cross phospholipid bilayers, which is precisely why the nuclear envelope must have pores.
6 A vesicle from the Golgi apparatus carries a protein to the plasma membrane for secretion. What happens to the membrane of the vesicle when the protein is released? HL
Answer and reasoning
It passes out through the plasma membrane and is lost from the cell. — A student who pictures the vesicle popping out of the cell picks this. A vesicle cannot pass through a membrane; its bilayer fuses with the plasma membrane and only the contents leave.
It is broken down into phospholipids reused to build the next vesicle. — A student who thinks vesicle membranes are built and dismantled picks this. Membrane is transferred, not destroyed: the vesicle's bilayer joins the plasma membrane, and new vesicles bud from existing membranes.
It stays joined to the Golgi apparatus as a channel for more protein. — A student who imagines continuous channels between organelles picks this. A vesicle is a separate, detached compartment; it buds from the Golgi apparatus and later fuses with the plasma membrane.
It fuses with the plasma membrane and becomes part of it. — In exocytosis the vesicle membrane fuses with the plasma membrane, releasing the contents outside the cell. The vesicle's phospholipids and proteins are added to the plasma membrane, which is also how the membrane of a growing cell is enlarged.
7 The enzymes and substrates of the Krebs cycle are held together in the mitochondrial matrix rather than being spread through the cytoplasm. Why is this compartmentalization an adaptation for the production of ATP? HL
Answer and reasoning
The matrix holds the Krebs cycle enzymes pressed tightly against the cristae, so that they can pump protons directly into the intermembrane space. — A student who assigns every mitochondrial process to the cristae picks this. The Krebs cycle is a series of reactions in solution in the matrix; proton pumping is done by the electron transport chain in the inner membrane, not by Krebs cycle enzymes.
Enzymes and substrates are concentrated in a small volume, so reactions run rapidly, and reduced NAD is formed close to the inner membrane. — Compartmentalizing the Krebs cycle in the matrix keeps enzymes and metabolites at high concentration, increasing reaction rate, and places the source of reduced NAD next to the inner membrane where it is oxidized. This is the third adaptation named in the guide for B2.2.4.
Being enclosed by two membranes protects the Krebs cycle enzymes from being damaged by substances in the rest of the cytoplasm. — A student who reads the double membrane as protection picks this. The membranes create compartments in which concentration gradients and concentrated enzyme-substrate mixtures can be maintained; protection is not the adaptation.
The inner membrane slows the entry of substrates into the matrix, so that the Krebs cycle runs at a safe and controlled rate. — A student who thinks compartments exist to slow metabolism picks this. Concentrating enzymes and substrates together speeds the cycle; transport proteins in the inner membrane supply substrates rather than throttling them.
8 In a chloroplast the enzymes of the Calvin cycle are located in the stroma, while ATP and reduced NADP are produced by the thylakoid membranes. Why is locating the Calvin cycle in the stroma an adaptation for photosynthesis? HL
Answer and reasoning
The Calvin cycle actually takes place inside the thylakoids themselves, and the stroma serves only to store the carbohydrate that the cycle produces. — A student who places every stage of photosynthesis in the thylakoids picks this. The light-dependent reactions are in the thylakoid membranes; the Calvin cycle is a series of enzyme-catalysed reactions in solution in the stroma.
The stroma holds the photosystems in solution, so the Calvin cycle enzymes must be located there too in order to receive light energy directly. — A student who pictures chlorophyll dissolved in fluid picks this. Photosystems are embedded in the thylakoid membranes; the Calvin cycle does not use light directly but the ATP and reduced NADP made by the thylakoids.
Enzymes and substrates are concentrated together in the stroma, next to the thylakoids that supply ATP and reduced NADP, so fixation is rapid. — Compartmentalization of the Calvin cycle enzymes and substrates in the stroma keeps them at high concentration and next to the thylakoid membranes that supply ATP and reduced NADP. This is the third adaptation named in the guide for B2.2.5.
The stroma provides the large volume of water that the thylakoid membranes need for the photolysis of water in the light. — A student who thinks the chloroplast's compartments are water stores picks this. Water is abundant throughout the chloroplast; the stroma's role in this adaptation is to concentrate the Calvin cycle enzymes and substrates.
That was your twenty minutes. Real practice on B2.2 is past-paper questions marked against the mark scheme.
What the exam asks of B2.2
Paper 1A asks whether a named structure is an organelle, or which organelle matches a described function or adaptation. Paper 1B may give electron micrographs or cell fractionation data and ask you to identify fractions or explain the order in which they pellet. Paper 2 uses *outline* for compartmentalisation advantages and *explain* at HL for how mitochondrial or chloroplast structure suits its function: name the feature, then the process it serves. Expect *describe* for the route of a secreted protein from ribosome to exocytosis, and a nature-of-science question on how ultracentrifugation opened organelle study.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
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