Every living organism is built from cells, and every cell has DNA, watery cytoplasm and a lipid membrane. Prokaryotes are small and simple; eukaryotes divide their cytoplasm into membrane-bound compartments. Microscopes reveal this structure, and at HL the story extends to endosymbiosis and multicellularity.
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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A2.2.1 All living organisms are made of cells, so new ones can be predicted to be
2028 guide: scope reduced — Reported (unverified) that the historical development of cell theory is removed in the 2028 guide. The 2025 guidance for this statement contains no history, so nothing in this bank depends on it; q01 tests deductive prediction from cell theory and is valid under both guides. Candidates sitting May/Nov 2026 or 2027 exams still need the fuller 2025 scope.
Cell theory: every living organism consists of one or more cells.
All cells arise from pre-existing cells.
From this general theory you can deduce that an unexamined organism will be cellular.
Deduction runs from the general to the particular; induction runs the other way.
Students often call the prediction inductive. In fact it is deductive: a specific case derived from a general theory.
Students often think viruses are exceptions to cell theory. In fact viruses are not living organisms, so the theory is untouched.
A2.2.2 Using a light microscope and calculating size
Magnification = image size ÷ actual size, with both in the same units.
So actual size = image size ÷ magnification; 1 mm = 1000 µm.
An eyepiece graticule must be calibrated against a stage micrometer for each objective.
A scale bar is drawn at its stated length multiplied by the magnification.
Stains add contrast, not resolution; use coarse focus at low power only, fine focus at high power. Measuring with instruments is quantitative observation.
Students often multiply image size by magnification. In fact you divide: 30 mm ÷ 500 = 60 µm.
Students often read graticule divisions straight as micrometres. In fact each division's value changes with the objective and must be calibrated.
A2.2.3 Newer microscopy techniques and what each one gains
2028 guide: scope reduced — Basic principles and main advantages of the microscopy techniques only; detailed procedural knowledge not required. Candidates sitting May/Nov 2026 or 2027 exams still need the fuller 2025 scope.
Resolution is the smallest gap at which two points still appear separate.
Electron microscopy uses a shorter wavelength, so resolution is far higher; specimens are dead and colourless.
Freeze fracture splits a membrane between its two layers, showing embedded proteins.
Cryogenic electron microscopy freezes specimens too fast for ice crystals; no fixation or staining needed.
Fluorescent stains make one component glow; immunofluorescence uses antibodies to light up a single protein.
Students often think electron microscopes just magnify more. In fact the gain is resolution; magnifying a blurred image only enlarges it.
Students often think electron microscopes can show living cells. In fact specimens are fixed and viewed in a vacuum.
A2.2.4 What every cell has, and why
All cells have DNA as genetic material: stable, copiable, passed to daughter cells.
Cytoplasm is mainly water, the solvent in which metabolism happens.
A plasma membrane of lipids encloses the cell.
Hydrophobic tails make a barrier to water-soluble substances and control what enters and leaves.
Students often think every cell has a nucleus. In fact all cells have DNA, but only eukaryotes enclose it in a nucleus.
Students often think cytoplasm is a stiff protein jelly. In fact it is mostly water, where dissolved substances move and react.
A2.2.5 Prokaryotic cell structure
No nucleus and no membrane-bound organelles; typically 1–10 µm.
A cell wall of peptidoglycan lies outside the plasma membrane.
Naked DNA in a single loop lies in the cytoplasm: no histones attached.
Protein synthesis happens on 70S ribosomes.
The model is Gram-positive eubacteria such as Bacillus and Staphylococcus; other prokaryotes vary.
Students often think all cell walls are cellulose. In fact the bacterial wall is peptidoglycan.
Students often think "naked" means no nuclear membrane. In fact it means no histone proteins.
A2.2.6 Eukaryotic cell structure
A nucleus holds chromosomes of DNA bound to histones, inside a double membrane with pores.
80S ribosomes lie free or on rough endoplasmic reticulum; smooth ER makes lipids.
The Golgi apparatus modifies, sorts and packages proteins into vesicles, including lysosomes.
Mitochondria have a double membrane with inner cristae; they make ATP by aerobic respiration.
A cytoskeleton of microtubules and microfilaments assembles and disassembles to shape and move the cell.
Students often think the nucleus and mitochondria have single membranes. In fact both have double membranes.
Students often think the Golgi makes proteins. In fact ribosomes make them; the Golgi modifies and packages them.
A2.2.7 One cell can carry out every function of life
Homeostasis, metabolism, nutrition, movement, excretion, growth, response and reproduction.
A unicellular organism does all eight within one cell.
In Paramecium, a contractile vacuole expels water that enters by osmosis.
That is homeostasis, not excretion: the water is not a metabolic waste.
Students often think expelling water is excretion. In fact excretion removes metabolic waste; water removal is osmoregulation.
Students often think a dividing cell is only growing. In fact division makes two organisms: it is reproduction.
A2.2.8 How animal, fungal and plant cells differ
2028 guide: scope reduced — General structural differences between plant, animal and fungal cells only; detailed biochemical composition not required. Candidates sitting May/Nov 2026 or 2027 exams still need the fuller 2025 scope.
Cell wall: cellulose in plants, chitin in fungi, none in animals.
Vacuole: one large permanent sap vacuole in plants; small temporary ones in animals.
Plastids, including chloroplasts, occur only in plant cells, and only where there is light.
Centrioles, cilia and flagella occur in animal cells, not plant or fungal cells.
Students often treat fungi as plants. In fact fungal walls are chitin and fungi have no chloroplasts.
Students often think every plant cell has chloroplasts. In fact root cells have none; only cells in light do.
A2.2.9 Cells that break the one-nucleus rule
Aseptate fungal hyphae: nuclei divide without cross-walls, so many share one cytoplasm.
Skeletal muscle fibres form by fusion of cells, so each has many nuclei.
Mature red blood cells lose their nucleus, leaving room for haemoglobin.
Phloem sieve tube elements lose their nucleus but stay alive, kept by a companion cell.
Students often think every eukaryotic cell has exactly one nucleus. In fact some have many and some have none.
Students often think sieve tube elements are dead and hollow. In fact they keep cytoplasm and a membrane.
A2.2.10 Recognising cell types and organelles in micrographs
Prokaryote: small, cell wall, pale nucleoid with no membrane, no organelles.
Plant cell: cell wall, large sap vacuole, often chloroplasts.
Animal cell: no wall, no chloroplasts, sometimes microvilli.
Mitochondrion: inner membrane folded into cristae; chloroplast: stacked thylakoids, larger.
Rough ER is an extensive ribosome-studded network; the Golgi is a discrete stack with budding vesicles and no ribosomes.
Students often think any walled cell is a plant cell. In fact prokaryotes and fungi have walls too; check for a nucleus and vacuole.
Students often label any folded membranes as a mitochondrion. In fact cristae mean mitochondrion; grana mean chloroplast.
A2.2.11 Drawing and annotating cell structures from micrographs
An annotation gives the function, not just the name.
Rough ER: makes proteins for secretion. Golgi: modifies and packages them.
Mitochondrion: aerobic respiration producing ATP. Chloroplast: photosynthesis.
Secretory vesicles carry products to the membrane for exocytosis; microvilli increase surface area.
Students often think microvilli beat like cilia. In fact they are static projections that increase absorption area.
Students often think every small vesicle is a lysosome. In fact secretory vesicles carry products out; lysosomes hold digestive enzymes.
A2.2.12 Mitochondria and chloroplasts were once free-living prokaryotes HL
Endosymbiosis: a prokaryote taken in by a larger cell survived and became an organelle.
The eukaryote ancestor already had a nucleus; mitochondria came next, chloroplasts later in some lineages.
Evidence: both organelles have naked circular DNA, 70S ribosomes and replicate by division.
Both have a double membrane: inner from the prokaryote, outer from the host's vesicle.
A theory's strength lies in the range of observations it explains and predictions it supports.
Students often think the nucleus arose by endosymbiosis. In fact the ancestor already had one before mitochondria were acquired.
Students often think an unobserved ancient event makes the theory weak. In fact its wide explanatory power is what supports it.
A2.2.13 Cells specialise by expressing different genes HL
Differentiation makes cells specialised for different tissues.
Every differentiated cell keeps the whole genome.
What differs is the pattern of gene expression: which proteins are made.
Patterns are often triggered by the cell's environment, such as signals from neighbours.
Students often think differentiating cells discard unneeded genes. In fact all genes remain; only expression changes.
Students often think different cell types carry different mutations. In fact the DNA sequence is unchanged.
A2.2.14 Multicellularity evolved many times and brings two advantages HL
Multicellularity evolved repeatedly and independently in different eukaryotic lineages.
All plants and animals, and many fungi and algae, are multicellular.
Advantage one: larger body size while each cell keeps a workable surface area to volume ratio.
Advantage two: cell specialisation, with different cells doing different jobs.
Students often think multicellularity evolved once. In fact it arose separately in several lineages.
Students often think a single cell could simply grow large. In fact its surface area to volume ratio would fall too far for exchange.
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 A biologist collects an organism from a deep-sea vent that has never been examined before. Before looking at it under a microscope, she predicts that it will consist of one or more cells. Which statement best describes this reasoning?
Answer and reasoning
It is deductive: a specific prediction is derived from cell theory, and examining the organism then tests it. — Cell theory is a general theory; predicting that a particular new organism will consist of cells applies the theory to a specific case. This is deductive reasoning, and examining the organism tests the prediction, which is how a theory continues to be tested.
It is inductive: the prediction is a generalization drawn from the many organisms already examined. — A student who attaches 'inductive' to anything involving observations picks this. Induction built cell theory from many observations; using the theory to predict one new case runs the other way and is deduction.
It is unscientific: cell theory cannot be relied on until every living organism has been examined. — A student who treats a theory as a guess until every case is checked picks this. A theory's value lies in generating predictions for cases not yet examined; the prediction is tested when the organism is examined.
It is unreliable: viruses are living organisms not made of cells, so cell theory has known exceptions. — A student who counts viruses as living organisms picks this. Viruses are not cells, have no metabolism and cannot reproduce independently, so they are not classified as living organisms and are not exceptions to cell theory.
2 In a photomicrograph taken at a magnification of ×2000, a red blood cell measures 15 mm across. What is the actual width of the cell?
Answer and reasoning
30 000 mm — A student who multiplies the image size by the magnification picks this: 15 mm × 2000. That would make the real cell 30 m wide, larger than the image; actual size is image size divided by magnification.
7.5 µm — Actual size = size of image ÷ magnification = 15 mm ÷ 2000 = 0.0075 mm, which is 7.5 µm (1 mm = 1000 µm), the typical diameter of a red blood cell.
0.75 µm — A student who converts 0.0075 mm to micrometres using a factor of 100 picks this. 1 mm = 1000 µm, so 0.0075 mm = 7.5 µm; a cell 0.75 µm across would be smaller than most bacteria.
0.075 µm — A student who converts 0.0075 mm to micrometres using a factor of 10 picks this. 0.075 µm is 75 nm, about the size of a large virus; 1 mm = 1000 µm, so 0.0075 mm = 7.5 µm.
3 Ribosomes are about 25 nm across. They cannot be seen with a light microscope but are clearly visible in electron micrographs. What is the reason?
Answer and reasoning
The electron microscope magnifies many more times, so much smaller objects become visible. — A student who explains everything by magnification picks this. Magnifying a light microscope image further would only enlarge the blur; without higher resolution, objects closer together than 200 nm remain indistinguishable.
Electrons have a far shorter wavelength than light, so the resolution is much higher. — Resolution is limited by the wavelength of the radiation used. Light limits a light microscope to about 200 nm, so a 25 nm ribosome cannot be resolved; the electron beam's much shorter wavelength gives resolution below 1 nm.
Heavy-metal stains used for electron microscopy make small structures much sharper. — A student who credits stains with sharpness picks this. Stains add contrast by scattering electrons; they cannot reveal detail finer than the resolution of the microscope, which is set by wavelength.
Electron microscopes use fluorescent stains that make ribosomes glow in the dark. — A student who lumps all advanced techniques together picks this. Fluorescent stains are used in light microscopy, and they do not raise its resolution; ribosomes are resolved by the short wavelength of electrons.
4 Which description of the structure of a Bacillus cell is correct?
Answer and reasoning
The DNA is enclosed by a nuclear membrane inside a small nucleus within the cytoplasm. — A student who expects every cell to have a nucleus picks this. Prokaryotic DNA lies in the cytoplasm in a nucleoid region that is not enclosed by any membrane.
The cell wall outside the plasma membrane is built from long parallel fibres of cellulose. — A student who assumes every cell wall is a cellulose wall picks this. The cell wall of Bacillus is made of peptidoglycan; cellulose walls are found only in plant cells.
A loop of naked DNA lies in the cytoplasm, and proteins are made on 70S ribosomes. — A Gram-positive eubacterium such as Bacillus has a cell wall, a plasma membrane, cytoplasm, naked DNA in a loop lying in the nucleoid region of the cytoplasm, and 70S ribosomes on which its proteins are synthesized.
The DNA loop is wound around histone proteins to package it within the nucleoid region. — A student who thinks 'naked' refers only to the absence of a membrane picks this. Naked DNA is DNA not associated with histones; the prokaryotic loop has no histones.
5 Which statement about the structure of a eukaryotic cell is correct?
Answer and reasoning
The nucleus is bounded by a double membrane with pores, and its DNA is bound to histone proteins. — The eukaryotic nucleus is enclosed by the nuclear envelope, a double membrane perforated by pores, and contains chromosomes consisting of DNA bound to histones.
The nucleus is enclosed by a single membrane that is continuous with the plasma membrane. — A student who pictures every organelle with one outline picks this. The nuclear envelope is a double membrane with pores; it is continuous with the endoplasmic reticulum, not the plasma membrane.
The ribosomes in the cytoplasm are 70S, the same size as those found in bacteria. — A student who thinks all ribosomes are alike picks this. The ribosomes of the eukaryotic cytoplasm are 80S; 70S ribosomes are found in prokaryotes and inside mitochondria and chloroplasts.
The cytoskeleton is a rigid, permanent frame of protein rods holding each organelle in a fixed position. — A student led by the word 'skeleton' picks this. Microtubules and microfilaments are dynamic, assembling and disassembling to move organelles, change cell shape and form the spindle.
6 Which row correctly compares the cells of animals, fungi and plants?
Answer and reasoning
Cell wall: absent in animals, made of cellulose in fungi, made of cellulose in plants. — A student who groups fungi with plants picks this. Fungal cell walls are made of chitin, not cellulose; only plant cells have cellulose walls.
Vacuoles: absent in animals, present in fungi, present as a large sap vacuole in plants. — A student who remembers 'vacuole: plants only' picks this. Animal cells have small temporary vacuoles; the difference from the large permanent sap vacuole of plant cells is in size and function, not presence.
Centrioles: present in animals, absent in fungi, present in plants to form the spindle. — A student who thinks a spindle needs centrioles picks this. Plant cells lack centrioles and organize their spindle without them; centrioles are a feature of animal cells.
Cell wall: absent in animals, made of chitin in fungi, made of cellulose in plants. — Animal cells have no cell wall. Fungal cell walls are made of chitin and plant cell walls of cellulose; the presence and composition of the wall is one of the main differences between the three groups.
7 Which statement correctly describes the number of nuclei in an atypical eukaryotic cell?
Answer and reasoning
A mature red blood cell keeps a single nucleus so that it can control the making of haemoglobin. — A student who assumes a protein-filled cell must be making protein picks this. The haemoglobin was made before the nucleus was lost; a mature mammalian red blood cell has no nucleus.
An aseptate fungal hypha contains many nuclei within one continuous cytoplasm. — Aseptate hyphae have no cross-walls, so repeated nuclear division without division of the cytoplasm leaves many nuclei sharing a single continuous cytoplasm inside one plasma membrane.
A skeletal muscle fibre has a single, very large nucleus to control its whole length. — A student who holds to one nucleus per cell picks this. A skeletal muscle fibre forms by the fusion of many cells and so contains many nuclei spread along its length.
A phloem sieve tube element is a dead cell, so it has no nucleus and no cytoplasm. — A student who models phloem on xylem picks this. A sieve tube element is alive, with cytoplasm and a plasma membrane; it is atypical because it has lost its nucleus at maturity, and is supported by a companion cell.
8 An electron micrograph shows a cell about 3 µm long. It has a cell wall, a paler fibrous region of DNA not enclosed by any membrane, many small dense granules, and no membrane-bound organelles. How should the cell be classified?
Answer and reasoning
As a plant cell, because a cell wall is found only around the cells of plants. — A student who takes a cell wall to mean 'plant' picks this. Prokaryotes and fungi also have cell walls; a plant cell would show a nucleus, a sap vacuole and other membrane-bound organelles.
As a eukaryote, because the region containing DNA must be a nucleus, as every cell has one. — A student who believes every cell has a nucleus reads the nucleoid as a nucleus. A nucleus is bounded by a double membrane; DNA lying in the cytoplasm with no membrane is a nucleoid, a prokaryotic feature.
As a prokaryote, because its DNA lies in a nucleoid region and it has no membrane-bound organelles. — A nucleoid region of DNA without a membrane, the absence of membrane-bound organelles and the small size identify a prokaryote; the granules are 70S ribosomes and the cell wall is consistent with a bacterium.
As a eukaryote, because prokaryotic cells are too simple to contain any ribosomes. — A student who counts ribosomes as eukaryotic organelles picks this. All cells have ribosomes; the small dense granules are the 70S ribosomes of a prokaryote.
9 If mitochondria originated as free-living prokaryotes taken into a larger cell, which features would the theory of endosymbiosis predict mitochondria to have? HL
Answer and reasoning
Ribosomes identical to those of the cytoplasm, because the host cell now makes them all. — A student who thinks all ribosomes are the same picks this. Mitochondrial ribosomes are 70S, like those of prokaryotes, not the 80S ribosomes of the eukaryotic cytoplasm, which is a key piece of evidence.
Naked circular DNA and 70S ribosomes, with the ability to divide independently of the host cell. — A former prokaryote should retain prokaryotic features: naked circular DNA, 70S ribosomes and replication by dividing in two. Mitochondria show all three, which is the evidence the guide specifies for endosymbiosis.
DNA wound around histone proteins, because all DNA in the cell is packaged in that way. — A student who believes histones are universal picks this. Prokaryotic DNA is naked, and so is mitochondrial DNA; if mitochondria had histone-bound DNA it would count against a prokaryotic origin.
A double membrane in which both layers are derived from the vesicle membrane of the host cell. — A student who forgets the engulfed cell had its own membrane picks this. The inner membrane is the former prokaryote's plasma membrane; only the outer membrane came from the host vesicle.
10 A muscle cell and a neurone from the same person contain the same genome, yet they differ greatly in structure and function. What is the basis of this difference? HL
Answer and reasoning
Each cell has kept only the genes it needs and has lost the genes for the other cell type. — A student who explains permanent differentiation by loss of genes picks this. Differentiated cells keep the whole genome; the unused genes are present but not expressed.
Each cell's fate was fixed at fertilization by its genes alone, with no influence from its surroundings. — A student who thinks genes act independently of the environment picks this. Since every cell has the same genes, it is signals from each cell's surroundings that trigger the different patterns of expression.
Mutations that accumulated in each cell type during development altered the DNA sequence differently. — A student who knows only mutation as a way of changing a cell picks this. The DNA sequence is the same in both cells; what differs is which genes are switched on, which is controlled without changing the sequence.
Different sets of genes are expressed in each cell, often triggered by its environment. — Differentiation is based on different patterns of gene expression: the same genome is present, but each cell type expresses a different set of genes, and these patterns are often triggered by changes in the environment of the cell, such as signals from neighbouring cells.
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15 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 With the ×40 objective lens in place, 40 divisions of an eyepiece graticule line up exactly with 100 µm on a stage micrometer. Using the same objective, a cell spans 44 graticule divisions. What is the actual length of the cell in millimetres?
Answer and reasoning
0.044 mm — A student who assumes each graticule division is 1 µm, whatever the objective, takes the cell as 44 µm and converts that to 0.044 mm. The graticule has no fixed value; the stage micrometer shows each division is 2.5 µm here.
0.0176 mm — A student who applies the calibration the wrong way round, dividing 44 by 2.5 instead of multiplying by it, gets 17.6 µm and picks this. The calibration gives the actual length of one division, so the number of divisions is multiplied by it: 44 × 2.5 µm = 110 µm = 0.11 mm.
0.11 mm — Calibration: 100 µm ÷ 40 divisions = 2.5 µm per division with this objective. The cell spans 44 divisions, so its length is 44 × 2.5 µm = 110 µm, which is 0.11 mm (1 mm = 1000 µm).
1.1 mm — A student who obtains 110 µm and converts to millimetres using a factor of 100 picks this. 1 mm = 1000 µm, so 110 µm ÷ 1000 = 0.11 mm; a cell 1.1 mm long would be visible to the naked eye.
2 A student makes a drawing of a cell at a magnification of ×1250 and wants to add a scale bar representing 10 µm. How long should the scale bar be drawn?
Answer and reasoning
0.008 µm — A student who divides by the magnification, as when finding an actual size, picks this: 10 µm ÷ 1250 = 0.008 µm. The scale bar is a magnified length, so the actual 10 µm is multiplied by 1250, not divided.
125 mm — A student who calculates 12 500 µm and converts to millimetres using a factor of 100 picks this. 1 mm = 1000 µm, so 12 500 µm ÷ 1000 = 12.5 mm, not 125 mm.
10 mm — A student who reads the label's number as the drawn length picks this. The bar must be drawn at the actual length multiplied by the magnification: 10 µm × 1250 = 12 500 µm = 12.5 mm; a 10 mm bar would represent only 8 µm at ×1250.
12.5 mm — The drawn length of a scale bar is the actual length it represents multiplied by the magnification: 10 µm × 1250 = 12 500 µm = 12.5 mm.
3 A student focuses a stained temporary mount at low power, then rotates the high-power objective lens into place. The image is now slightly blurred. What should the student do?
Answer and reasoning
Turn only the fine adjustment until the image is sharp. — At high power the objective is close to the slide and the depth of focus is very small, so the fine adjustment is the only focusing control used; it moves the lens by tiny amounts to sharpen the image without risk to the slide or lens.
Turn the coarse adjustment, since it changes the focus most quickly. — A student who uses the coarse adjustment for any blurred image picks this. At high power the coarse adjustment overshoots the focus and can drive the objective into the slide; it is used only at low power.
Add more stain to the mount so the structures show up clearly. — A student who credits the stain with sharpness picks this. Stain adds contrast between structures; it has no effect on focus, which is set by the distance between lens and specimen.
Switch to an even higher-power objective lens to enlarge the details. — A student who thinks more magnification makes an image clearer picks this. Magnifying a blurred image gives a larger blurred image; the image must first be focused with the fine adjustment.
4 In freeze fracture, a rapidly frozen cell is split with a blade, the exposed surface is coated with metal, and the replica is examined by electron microscopy. Replicas of plasma membranes show a smooth face scattered with small bumps. What does this show?
Answer and reasoning
The fracture cut straight across the cell, and the bumps are organelles seen in cross-section. — A student who pictures fracturing as cutting a section picks this. The fracture follows the weakest plane, inside membranes, rather than crossing them; the replica shows a membrane face, not a section.
Ice crystals grew inside the membrane as it froze, and the bumps are the crystals left in the replica. — A student who assumes freezing always forms ice crystals picks this. The cell is frozen so rapidly that no crystals form, which is what preserves the membrane structure for fracturing.
The fracture ran between the two phospholipid layers, and the bumps are proteins embedded within the bilayer. — The weakest plane in a frozen membrane is the hydrophobic interior of the bilayer, so the fracture splits the membrane into its two layers. Proteins embedded in the bilayer are left standing as bumps, which was evidence for the fluid mosaic model.
The fracture exposed the outer surface, and the bumps are the protein coat covering the bilayer. — A student who pictures the membrane as a lipid layer coated with protein picks this. The fracture exposes the interior of the bilayer, and the particles are proteins embedded within it, not a coat on its surface.
5 A researcher wants to see where one particular protein is located inside cultured cells. Which approach will show this?
Answer and reasoning
A general stain such as methylene blue, which binds to proteins so that every location of the protein is coloured. — A student who thinks all stains work like methylene blue picks this. A general stain colours many components of the cell at once and cannot distinguish one protein from all the others.
A higher-power objective lens, since enough magnification will show which structures contain the protein. — A student who expects magnification to reveal anything picks this. Magnification does not distinguish one protein from another, and resolution, not magnification, limits what a light microscope can show.
Transmission electron microscopy of the living cells, since its resolution is high enough to show single protein molecules. — A student who thinks electron microscopes view living cells picks this. Specimens must be dead, fixed and in a vacuum, and even at high resolution one unlabelled protein cannot be told from another.
Immunofluorescence: antibodies that bind only that protein carry a fluorescent marker seen with a light microscope. — Antibodies bind specifically to one protein. Linking them to a fluorescent marker makes that protein, and only that protein, glow under a fluorescence microscope, revealing where it is in the cell.
6 What is the main advantage of cryogenic electron microscopy over conventional electron microscopy?
Answer and reasoning
Rapid freezing keeps unfixed, unstained molecules near their natural state, so structures are resolved in near-atomic detail. — Plunge-freezing solidifies the water without ice crystals, so no fixation or staining is needed and molecules keep their natural shape. Combining images of many identical molecules gives three-dimensional structures of proteins at near-atomic resolution.
Heavy-metal stains bind more firmly at very low temperature, so the contrast between structures in the specimen is much greater. — A student who assumes cryogenic specimens still need staining picks this. The point of the technique is that no stain or fixative is used, so molecules are seen as they are rather than as a stain outlines them.
Freezing makes the cell brittle, so it can be fractured to reveal the interior of its membranes in section. — A student who confuses cryogenic electron microscopy with freeze fracture picks this. Fracturing frozen cells to expose membrane interiors is freeze fracture; cryogenic electron microscopy images the frozen specimen directly.
The very low temperature prevents the electron beam from killing the cells, so living cells can be observed in action. — A student who thinks electron microscopy can be made to work on living cells picks this. A vitrified specimen at very low temperature in a vacuum is not alive; the advantage is structural preservation, not life.
7 Every living cell has DNA as genetic material, a cytoplasm composed mainly of water, and a plasma membrane composed of lipids. Which statement gives a correct reason for one of these features?
Answer and reasoning
The cytoplasm is mainly water so that it forms a stiff gel that holds every organelle in a fixed position. — A student who pictures cytoplasm as a solid jelly picks this. Cytoplasm is mainly water because water is the solvent for metabolism; organelles move within it and are positioned by the cytoskeleton.
The membrane is made of lipids because their hydrophobic tails form a barrier to the water-soluble contents of the cell. — Phospholipids arrange themselves into a bilayer with their hydrophobic tails inward. The hydrophobic interior blocks the passage of water-soluble substances, so the membrane keeps the aqueous cytoplasm separate from the surroundings and controls exchange.
The membrane is made of lipids because lipids form a rigid layer that gives the cell its strength and shape. — A student who gives the membrane the role of a wall picks this. The phospholipid bilayer is fluid and flexible; it is a barrier, not a support. Strength and shape come from a cell wall, where present, or the cytoskeleton.
DNA is the genetic material because it is protected inside a nucleus in every kind of living cell. — A student who thinks every cell has a nucleus picks this. Prokaryotes keep their DNA in the cytoplasm without a nucleus; DNA is the genetic material because it stores information stably and can be replicated.
8 A unicellular organism is found to have a cell wall, a plasma membrane and ribosomes, but no nucleus. Its DNA is a single circular molecule, not associated with histones, lying in the cytoplasm. A student concludes that the organism is a prokaryote. Which evaluation of this conclusion is correct?
Answer and reasoning
It is wrong: the presence of a cell wall means the organism must be either a plant or a fungus. — A student who treats a cell wall as a eukaryotic feature picks this. Prokaryotes also have cell walls, so the wall does not rule out a prokaryote; the naked DNA without a nucleus is decisive.
It is wrong: prokaryotes are too simple to have ribosomes, so this must be a eukaryote. — A student who thinks ribosomes are eukaryotic organelles picks this. Every living cell has ribosomes; prokaryotes have 70S ribosomes and make all their proteins on them.
It is wrong: every cell has a nucleus, so the DNA must lie in a nucleus that was overlooked. — A student who believes a nucleus is universal picks this. Prokaryotes have no nucleus; their DNA lies in the cytoplasm, exactly as described, so nothing was overlooked.
It is justified: naked circular DNA in the cytoplasm with no nucleus is a prokaryotic feature. — DNA in a single naked loop lying in the cytoplasm, with no nucleus, is characteristic of prokaryotes. The cell wall and ribosomes are consistent with this but do not decide it, since eukaryotes may have both.
9 Which statement correctly describes a membrane-bound organelle of a eukaryotic cell?
Answer and reasoning
The Golgi apparatus is a stack of flattened sacs studded with ribosomes that synthesize proteins. — A student who merges the Golgi apparatus with rough endoplasmic reticulum picks this. The Golgi apparatus has no ribosomes; it modifies and packages proteins that were synthesized on the rough endoplasmic reticulum.
Lysosomes are vesicles bounded by a single membrane that contain hydrolytic enzymes. — Lysosomes are one of the variety of vesicles in eukaryotic cells: single-membrane sacs containing hydrolytic enzymes, kept separate from the rest of the cytoplasm, that digest material taken in and worn-out organelles.
Mitochondria are bounded by a single membrane folded inward to form cristae. — A student who gives every organelle one membrane picks this. A mitochondrion has a double membrane; it is the inner of the two that is folded into cristae.
Secretory vesicles are lysosomes under another name, digesting worn-out organelles. — A student who treats every small vesicle as a lysosome picks this. Secretory vesicles carry products from the Golgi apparatus to the plasma membrane for release by exocytosis; lysosomes are a different kind of vesicle that digests material inside the cell.
10 Paramecium is a unicellular organism that lives in fresh water. Which statement correctly identifies one of its functions of life?
Answer and reasoning
Growth: dividing by binary fission, so that the single cell becomes two separate cells. — A student who sees division as the organism getting bigger picks this. In a unicellular organism the cell is the organism, so division produces two organisms: that is reproduction. Growth is the increase in cell size between divisions.
Excretion: a process not carried out, because removing metabolic waste requires organs such as kidneys. — A student who ties excretion to organs picks this. A single cell must carry out every function of life; the waste products of its metabolism, such as carbon dioxide, leave by diffusion across the plasma membrane.
Homeostasis: expelling water that enters by osmosis, keeping the water content of the cytoplasm within limits. — Water enters continuously by osmosis; the contractile vacuole collects it and expels it, keeping the internal water content within limits. The water is not a product of metabolism, so this is homeostasis rather than excretion.
Metabolism: releasing energy from food by respiration, without building up any new molecules. — A student who uses the everyday meaning of metabolism picks this. Metabolism is all the enzyme-catalysed reactions of the cell, including the synthesis of proteins and other molecules as well as respiration.
11 A cell taken from a multicellular organism has a cell wall, a nucleus, no chloroplasts and no centrioles. A student concludes that it must be a fungal cell. Is this conclusion justified?
Answer and reasoning
No: a plant root cell also fits every feature listed, and the composition of the wall would decide between them. — Plant cells lack centrioles, and plant cells that are not exposed to light have no chloroplasts, so a root cell matches every feature listed. A chitin wall would indicate a fungus and a cellulose wall a plant.
No: fungal and plant cell walls are both made of cellulose, so the composition of the wall cannot help to decide either. — A student who thinks fungi have cellulose walls picks this. Fungal walls are made of chitin and plant walls of cellulose, so the composition of the wall is exactly what distinguishes them.
Yes: every plant cell contains chloroplasts, so the absence of chloroplasts rules out a plant cell. — A student who takes the leaf cell as the model for all plant cells picks this. Root cells and many other plant cells have no chloroplasts, so their absence does not rule out a plant.
Yes: plant cells need centrioles to form a spindle, so a walled cell without any centrioles cannot be a plant cell. — A student who believes a spindle requires centrioles picks this. Plant cells have no centrioles and still form a spindle, so the absence of centrioles is consistent with a plant cell.
12 In an electron micrograph of an animal cell, a structure appears as a discrete stack of curved, flattened membrane sacs with small vesicles budding from its edges and no ribosomes attached. Which structure is it?
Answer and reasoning
Smooth endoplasmic reticulum, where lipids are made — A student who identifies any membrane sacs as endoplasmic reticulum and then decides by the absence of ribosomes picks this. Smooth endoplasmic reticulum is a network of tubules, not a discrete curved stack with budding vesicles.
Mitochondrion, where aerobic cell respiration is carried out — A student who labels any structure with internal membranes as a mitochondrion picks this. A mitochondrion is enclosed by a double membrane with the inner membrane folded into cristae; it is not an open stack of sacs shedding vesicles.
Rough endoplasmic reticulum, where secreted proteins are made — A student who takes 'rough' to mean the folded texture of the membranes picks this. Rough endoplasmic reticulum is identified by attached ribosomes, which are absent here; a ribosome-free stack of curved sacs with budding vesicles is the Golgi apparatus.
Golgi apparatus, where proteins are modified and packaged — The Golgi apparatus is a stack of curved cisternae, separate from the nuclear envelope, with no ribosomes on its surface and vesicles budding from its edges as proteins are packaged for transport.
13 A student draws a cell from an electron micrograph of a gland that secretes enzymes, and must annotate each structure with its function. Which annotation is correct?
Answer and reasoning
Rough endoplasmic reticulum — synthesizes proteins that are destined for secretion from the cell. — Ribosomes attached to the rough endoplasmic reticulum synthesize proteins for secretion, which pass into the reticulum and are carried in vesicles to the Golgi apparatus, then to the plasma membrane in secretory vesicles.
Golgi apparatus — synthesizes the enzymes that the cell will later secrete by exocytosis. — A student who gives the Golgi apparatus the job of protein synthesis picks this. Proteins are synthesized only on ribosomes; the Golgi apparatus modifies and packages them into vesicles.
Microvilli — beat rhythmically to move the secreted enzymes along the surface of the cell. — A student who confuses microvilli with cilia picks this. Microvilli are static folds of the plasma membrane that increase surface area; they do not beat.
Secretory vesicles — break down worn-out organelles inside the cell using the hydrolytic enzymes they hold. — A student who treats all small vesicles as lysosomes picks this. Secretory vesicles carry the cell's products to the plasma membrane for release by exocytosis; digestion of organelles is the role of lysosomes.
14 Evidence suggests that all eukaryotes evolved from a common unicellular ancestor that had a nucleus, that mitochondria then evolved by endosymbiosis, and that in some eukaryotes chloroplasts subsequently also had an endosymbiotic origin. Which conclusion is consistent with this evidence and with the nature of scientific theories? HL
Answer and reasoning
The earliest eukaryotes were photosynthetic, and lineages such as animals and fungi have since lost their chloroplasts. — A student who assumes producers came first at every level picks this. Chloroplasts arose after mitochondria and only in some lineages; there is no evidence that animals or fungi ever possessed them.
The nucleus itself arose when an early cell engulfed a prokaryote, in the same way as mitochondria arose. — A student who extends endosymbiosis to every double-membraned structure picks this. The common ancestor already had a nucleus before mitochondria were acquired, and the nucleus lacks the prokaryotic features seen in mitochondria.
The ancestors of plants already had mitochondria when a photosynthetic prokaryote was taken in as a chloroplast. — Mitochondria were acquired before the eukaryote lineages diverged, and chloroplasts were acquired later in only some lineages, so the cell that took in the future chloroplast already contained mitochondria. This explains why plants have both organelles and animals only mitochondria.
Because the engulfing events were not observed, endosymbiosis remains a weak theory with little supporting evidence. — A student who takes 'theory' to mean speculation picks this. A theory's strength comes from the observations it explains and the predictions it supports, and endosymbiosis accounts for a wide range of observations about mitochondria and chloroplasts.
15 Which statement about the evolution of multicellularity is correct? HL
Answer and reasoning
Multicellularity evolved independently in several eukaryotic lineages, including plants, animals and fungi. — Multicellularity has evolved repeatedly: the closest relatives of plants, of animals and of fungi are unicellular, and several unrelated groups of algae are also multicellular. All plants and animals, and many fungi and eukaryotic algae, are multicellular.
Multicellularity evolved only once, so all multicellular organisms descend from a single multicellular ancestor. — A student who models multicellularity on features that arose once, such as mitochondria, picks this. Plants, animals, fungi and several algal groups each became multicellular separately from unicellular ancestors.
Unicellular eukaryotes are transitional forms that will become multicellular given enough time. — A student who pictures evolution as a ladder of progress picks this. Evolution has no goal; unicellular eukaryotes are abundant and successful, and most eukaryotic lineages have remained unicellular.
Larger body size could be reached equally well by one cell growing larger, so specialization is the only real advantage. — A student who imagines a body scaled up as a single cell picks this. A cell's surface area to volume ratio falls as it grows, limiting its size; multicellularity allows a large body made of small cells, as well as specialization.
That was your twenty minutes. Real practice on A2.2 is past-paper questions marked against the mark scheme.
What the exam asks of A2.2
Paper 1A asks you to identify a cell type or organelle from a micrograph or description, or to pick the correct magnification calculation. Paper 1B may give a micrograph with a scale bar and ask you to calculate actual size, or to justify an identification. Paper 2 uses *draw* and *annotate* for a prokaryote or an organelle, *outline* for the functions of life, and *compare* for animal, fungal and plant cells or for prokaryotes and eukaryotes. At HL, expect *explain* on the evidence for endosymbiosis and *discuss* on why a theory is accepted; state the observations and the reasoning.
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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