Why are fewer cells seen at higher power?

The field diameter shrinks in proportion, so only a small patch of the specimen is visible.

How do the nucleus and the large vacuole differ?

The nucleus holds DNA and directs the cell, while the vacuole stores water and keeps the cell turgid.

S4 Biology Cells, Paper 4: magnification, organelles and cell numbers

2026-11-02

Paper 4 pairs a 10x eyepiece with a 100x objective for magnification, real and image size, field diameter, organelle roles and cell size.

Focus　Magnification multiplies the two powers, field diameter falls as power rises, and a larger surface area to volume ratio speeds exchange.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Biology: Cells and Microscopy
Level	S4

02　Questions (10 items)

FoundationA specimen is viewed with a 10× eyepiece and a 100× objective. What is the total magnification?
FoundationAn image measures 25 mm at 1000×. What is the real length in micrometres?
FoundationWhat is the main function of the nucleus?
FoundationWhat does the large vacuole do in a mature plant cell?
IntermediateAt 50× the field diameter is 3.2 mm. What is it at 200×?
IntermediateFor a cube-shaped cell of side 4 micrometres, what is the surface area to volume ratio?
IntermediateThe total magnification is 400× and the objective is 40×. What is the eyepiece power?
ChallengeA specimen is 0.02 mm long. How long is its image at 1000×?
ChallengeCompare the surface area to volume ratios of cubes of side 2 micrometres and 6 micrometres.
Error redoError: a student says a higher power shows more cells in the field. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	1000×	Total magnification is eyepiece times objective, so 10 times 100 is 1000.
2	25 micrometres	Dividing 25 mm by 1000 gives 0.025 mm, which is 25 micrometres.
3	It stores DNA and controls the activities of the cell	The chromosomes sit in the nucleus, and the genetic instructions there direct metabolism and division.
4	It stores water and solutes and keeps the cell turgid	A water-filled vacuole presses on the wall to keep the cell firm and also stores solutes and waste.
5	0.8 mm	Four times the magnification leaves a quarter of the field, so 3.2 divided by 4 is 0.8.
6	1.5 per micrometre	Surface area is 96 and volume is 64, so the ratio is 1.5 per micrometre.
7	10×	Dividing the total magnification by the objective power gives the eyepiece power, so 400 by 40 is 10.
8	20 mm	Real length times magnification gives image length, so 0.02 times 1000 is 20.
9	Three times larger for the 2 micrometre cube	The 2 micrometre cube gives 3 and the 6 micrometre cube gives 1, so the small cell is three times as efficient.
10	The opposite: a smaller field means fewer cells are seen	A high power objective shows only a small patch of the specimen, so fewer cells fit in the field.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.