IB DP Biology: Cell structure and microscopy (DP Biology) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

These questions cover section A2.2 (Cell structure) of the IB Biology guide, first assessment 2025, for DP Biology. Questions 1 to 11 are SL and HL. Question 12 is additional higher level and is labelled HL only. Read the study guide first if a topic is new, and use the revision notes to check recall. See also the IB DP Biology hub and the printable checklist.

Questions

1. A probe returns a sample of a previously unknown organism from deep-sea sediment.

(a) State what cell theory predicts about its structure. [1]

(b) Name two structures you would expect every one of its cells to have, and give a reason for the cytoplasm being mostly water. [3]

2. An electron micrograph of a Staphylococcus cell carries a scale bar 20 mm long, labelled 0.4 µm. The cell is 52 mm across on the print.

(a) Calculate the magnification. [2]

(b) Calculate the actual diameter of the cell in µm. [2]

3. A student calibrates an eyepiece graticule under a low-power objective. Eighty graticule divisions line up with 36 stage micrometer divisions, each of 10 µm. A cell is 12 graticule divisions long.

(a) Calculate the actual length of the cell in µm. [3]

(b) Under a high-power objective, 80 graticule divisions line up with 9 stage micrometer divisions. Calculate how many graticule divisions long the same cell now appears, and explain why the graticule must be recalibrated for each objective. [3]

4. Select the technique best suited to each aim and justify your choice.

(a) Showing proteins embedded inside a membrane. [2]

(b) Working out the shape of a protein molecule that staining or chemical fixing would damage. [2]

(c) State one limitation of conventional electron microscopy. [1]

5. Distinguish between a Staphylococcus cell and a yeast cell, using four features. [4]

6. Explain the reasons for any two of the three structures found in all cells. [4]

7. Euglena is a unicellular organism with chloroplasts, a flagellum, an eyespot and a contractile vacuole. Outline how Euglena carries out five of the eight functions of life, using these structures where you can. [5]

8. Three eukaryote cells are described.

• P has a wall containing chitin, no plastids and a vacuole of variable size.
• Q has a cellulose wall, chloroplasts and one very large central vacuole.
• R has no wall and has a pair of centrioles.

(a) Identify the type of organism each cell comes from. [3]

(b) State one role of the large vacuole in Q. [1]

(c) Q is from a flowering plant. State whether Q or R is more likely to have cilia or flagella. [1]

9. Number of nuclei can show atypical cell structure.

(a) A skeletal muscle fibre 30 mm long has about 25 nuclei per millimetre of length. Calculate the number of nuclei in the fibre and explain how one fibre comes to have so many. [3]

(b) State why a fungal hypha with no cross walls can hold many nuclei in a single cytoplasm. [1]

(c) Explain the advantage of a mature phloem sieve tube element having no nucleus. [2]

10. An electron micrograph of an intestinal cell shows four structures.

• A: an oval structure with a double membrane, the inner one folded.
• B: a stack of curved flattened sacs with small vesicles budding from the edges.
• C: flattened sacs with dark dots on the outer surface.
• D: finger-like projections of the cell surface.

(a) Identify A to D. [4]

(b) State the function of A and of B. [2]

11. A chloroplast is 45 mm long on an electron micrograph at ×9000.

(a) Calculate its actual length in µm. [2]

(b) A student draws it 22.5 mm long. Calculate the magnification of the drawing. [2]

(c) Give three annotations for a labelled drawing of the chloroplast, each including a function. [3]

(d) State two rules for the style of the drawing. [2]

12. (HL only)

(a) State three observations about mitochondria that support endosymbiosis. [3]

(b) Outline the order in which mitochondria and chloroplasts are thought to have arisen. [2]

(c) The table shows whether three genes are expressed (transcribed) in three cell types from one person.

Cell type	Gene 1	Gene 2	Gene 3
Neurone	yes	no	yes
Liver cell	no	yes	yes
Red blood cell precursor	no	no	yes

Explain what the table shows about differentiation, and suggest what gene 3 might code for. [3]

(d) State two advantages of multicellularity. [2]

Answers

1. (a) It is made of one or more cells [1]. (b) Any two of: DNA [1], a plasma membrane [1], cytoplasm. Water is a solvent, so the reactions of metabolism take place in solution [1]. [4] Examiner insight: A prediction has to be about the structure of this organism, so “it is alive” says nothing testable. Name the structures, then link water to a role to earn the reason.

2. (a) 20 mm = 20 000 µm [1]. Magnification = 20 000 ÷ 0.4 = ×50 000 [1]. (b) 52 mm ÷ 50 000 = 0.00104 mm [1] = 1.04 µm (about 1.0 µm) [1]. [4] Examiner insight: Working in mixed units is the usual way to lose marks here. Write the conversion line and give the unit with the final answer.

3. (a) 36 × 10 = 360 µm for 80 divisions [1]. One division = 360 ÷ 80 = 4.5 µm [1]. Cell = 12 × 4.5 = 54 µm [1]. (b) One division = 90 ÷ 80 = 1.125 µm [1]. 54 ÷ 1.125 = 48 divisions [1]. Each objective magnifies the specimen and the stage scale by a different amount, while the graticule stays the same, so the value of one division changes [1]. [6] Examiner insight: Calibrating first and multiplying second shows each step. A full reason mentions both the changing magnification and the fixed graticule.

4. (a) Freeze fracture [1]. A frozen membrane splits between its two lipid layers, exposing the embedded proteins for imaging [1]. (b) Cryogenic electron microscopy [1]. Rapid freezing needs no stain or chemical fixing, so the molecule stays close to its natural shape [1]. (c) Specimens are dead (or in a vacuum, or stained with heavy metals, so artefacts may appear) [1]. [5] Examiner insight: A justification only works if the technique suits the aim. State the technique first, then the property that suits the aim.

5. Four from the following, each with both cells: Staphylococcus has naked DNA in a loop; yeast has DNA bound to histones [1]. Staphylococcus has a nucleoid with no nuclear envelope; yeast has a nucleus with a double membrane and pores [1]. Staphylococcus has 70S ribosomes; yeast has 80S in its cytoplasm [1]. Staphylococcus has no membrane-bound organelles; yeast has mitochondria, ER, Golgi apparatus and others [1]. A cell wall is not a difference, as both cells have one. [4] Examiner insight: A comparison needs a statement about each cell in every point. Both cells have a wall, so offering that as a difference is a common loss.

6. Two marks for each of two structures. DNA: it stores the genetic instructions for making proteins [1], and is copied for the daughter cells at division [1]. Cytoplasm: water is a solvent [1], so metabolic reactions can occur in solution [1]. Plasma membrane (alternative to either): lipids form a barrier, so the cell keeps an internal environment different from outside. [4] Examiner insight: Each structure is worth a reason, not a restatement of its name. State what it does, then why that matters to the cell.

7. Five from: nutrition by photosynthesis in the chloroplasts [1]; movement by beating the flagellum [1]; response to stimuli, as the eyespot helps it detect light and swim towards it [1]; homeostasis, as the contractile vacuole expels excess water [1]; reproduction by dividing into two cells [1]. Also accept excretion of waste across the plasma membrane, metabolism in the cytoplasm, and growth before division. [5] Examiner insight: Each function earns credit when it is tied to a structure of this organism. Listing the eight names alone does not answer “outline how”.

8. (a) P is a fungus [1]. Q is a plant [1]. R is an animal [1]. (b) Stores substances or maintains turgor [1]. (c) R [1]. [5] Examiner insight: Identify using combinations of features. Chitin points to a fungus; cellulose with chloroplasts points to a plant.

9. (a) 30 × 25 = 750 nuclei [1]. The fibre forms when many cells fuse [1], so it contains the nuclei of all the cells that joined [1]. (b) The cytoplasm is continuous because there are no cross walls separating the nuclei [1]. (c) Without a nucleus, the cell contents leave an open channel [1], so sap can flow through easily [1]. [6] Examiner insight: In (a) the calculation and the explanation are separate marks. Say that fusion is how a single cell gets many nuclei.

10. (a) A is a mitochondrion [1]. B is the Golgi apparatus [1]. C is rough endoplasmic reticulum [1]. D is microvilli [1]. (b) A: aerobic respiration to make ATP [1]. B: modifies, sorts and packages proteins into vesicles [1]. [6] Examiner insight: Use the feature given in the description to choose the name. A function is only convincing when it is attached to the correct organelle.

11. (a) 45 mm ÷ 9000 = 0.005 mm [1] = 5 µm [1]. (b) 22.5 mm = 22 500 µm [1]. 22 500 ÷ 5 = ×4500 [1]. (c) Double membrane marks the boundary and controls what enters [1]; grana (stacks of thylakoid membranes) have a large surface area for light absorption [1]; stroma is fluid with enzymes for building sugars [1]. Also accept a starch grain that stores carbohydrate. (d) Sharp pencil with clear continuous lines and no shading [1]; ruled label lines that do not cross, plus a scale bar or magnification [1]. [9] Examiner insight: Annotation marks depend on a function attached to each label. A label showing only a name earns nothing on its own.

12. (a) 70S ribosomes, as in prokaryotes [1]. Naked circular DNA, as in prokaryotes [1]. They can replicate inside the cell [1]. (b) Mitochondria arose first, by engulfing an aerobic prokaryote, in a common ancestor [1]. In some eukaryotes chloroplasts arose later in a similar way [1]. (c) Different cell types express different genes [1], although every cell holds the same genes, since expression changes, not gene content [1]. Gene 3 is expressed in all cells, so it probably codes for something every cell needs, such as a ribosomal protein or a respiratory enzyme [1]. (d) Larger body size [1] and cell specialization [1]. [10] Examiner insight: The table shows expression only. An answer that says cells have lost genes goes beyond what the data supports.

Where marks are usually lost

• Not writing the conversion between mm and µm before dividing.
• Leaving out units or the multiplication sign on magnification.
• Naming organelles without saying what they do.
• Saying prokaryotes have no ribosomes or no DNA.
• Describing only one side in a “distinguish” question.
• Using “resolution” and “magnification” as if they mean the same thing.
• Not linking evidence to a conclusion in endosymbiosis answers (HL only).
• Saying differentiated cells have lost genes.

Next steps

• Revision notes for this unit
• Study guide for this unit
• IB DP Biology hub
• Printable checklist
• All free 10-minute diagnostics: diagnostics
• Book a free trial class: trial

Official syllabus

This resource is aligned to the IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization, section A2.2 (Cell structure).