HKDSE Biology | Cells and the Microscope Tutor Prep Guide | Magnification, organelles and slide technique

2026-11-01

Tutor lesson-prep guides · Secondary Biology

Students can draw a cell but add the two lens powers when asked for magnification, and they cannot say why a slide must be thin. This lesson separates three skills: what each organelle does, how magnification multiplies, and why the specimen must be thin and stained.

Objectives — Students can: (1) match major organelles to their functions; (2) compute magnification as image size divided by actual size; (3) explain that total magnification is eyepiece times objective and how the field of view changes; (4) justify thin specimens and staining.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Unit conversion	How many micrometres in a millimetre	1000
Plant versus animal cells	The clearest difference	Cell wall and chloroplasts
Microscope parts	Which part controls light	Diaphragm and condenser

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-12 min	Organelle match-up	One verb per organelle	Mitochondrion releases energy
12-26 min	Plant versus animal cells	Tabulate three differences	Wall, chloroplast, large vacuole
26-42 min	Magnification	Image over actual, units first	40 mm / 0.05 mm = x800
42-54 min	Field of view and slides	Higher power, smaller field	20 cells across 1400 micrometres
54-60 min	Observation steps and exit check	Thin, stained, low power first	Air bubble versus cell

03 Teaching script (read it out)

Scene 1: Act 1: one verb per organelle

Mitochondrion releases energy; chloroplast carries out photosynthesis; ribosome synthesises protein; cell membrane controls what enters and leaves; nucleus stores genetic material; large vacuole stores water and maintains turgor. Bind each organelle to a single verb so students cannot write long sentences with no content.
Ask: which organelle do both cell types have, but plant cells usually larger? (The vacuole.)

Task: build a two-column organelle-and-verb table, close the notes and recite it.

Scene 2: Act 2: magnification multiplies, never adds

Magnification equals image size divided by actual size. An image of 40 mm of a 0.05 mm object gives x800. Total microscope magnification is eyepiece times objective, so 10x with 40x gives x400. Units must match first: a millimetre is 1000 micrometres, and this is where most marks are lost.
Ask: going from 100x to 400x, by what fraction does the field area shrink? (About one sixteenth.)

Task: an image of 6 mm of a 15 micrometre object, find the magnification (400x).

Scene 3: Act 3: thin, stained, low power first

A specimen must be thin so light passes through and cells do not overlap. A 1400 micrometre field holds 1400 / 70 = 20 cells of 70 micrometres. Staining with methylene blue adds contrast to colourless cells, but too much stain kills and obscures them. Always locate the target at low power before switching up, otherwise the narrow high-power field makes searching hopeless.
Ask: why label the magnification on a drawing? (Without it the actual size cannot be judged.)

Task: list the five steps from sampling to high-power observation and name the riskiest one.

04 Question types and worked steps

Type	Example	Steps	Answer
Organelle function	Given a function	One organelle, one verb	Mitochondrion releases energy
Compute magnification	Image and actual size	Convert units, then divide	x800
Field of view	Field diameter and cell size	Divide diameters	20 cells
Experimental design	Why thin, why stain	Light transmission, contrast	Thin slide plus methylene blue

Sanity check (teach this): Magnification is a pure number: x800 carries no unit. 10x times 40x is 400; an answer of 50 means the powers were added. Higher magnification means a smaller, dimmer field, so adjust the diaphragm rather than the coarse focus. 1400 / 70 = 20, and a count must be a whole number of cells. Plant cells always have a wall and animal cells never do; chloroplasts appear only in green parts.

05 Independent practice (3 questions)

Question	Answer	What to watch
Image 40 mm, actual 0.05 mm	x800	Convert to the same unit
10x eyepiece with 40x objective	x400	Multiply, not add
Cells of 70 micrometres across a 1400 micrometre field	20	Divide diameters

06 Common mistakes and what to say

Mistake	Student writes	Say this
Adding lens powers	Answers 10x + 40x = 50x	Say: “Powers multiply: 10 times 40 is 400, not 50.”
Mixing units	Divides 40 by 0.05 with one side in centimetres	Say: “Convert everything first; 1 mm is 1000 micrometres, do not skip it.”
Coarse focus at high power	Turns the coarse knob at 400x	Say: “At high power use only the fine focus, or you will crack the slide.”
Drawing an air bubble as a cell	Submits a plain circle	Say: “A bubble has a thick dark rim and empty inside; a cell has a membrane and a nucleus.”

07 Homework (paper, section and question numbers)

Secondary Biology chapter index — Cells set, questions 1 and 3
Prep guide directory — Other chapters in this subject
HKDSE Biology practice papers — Cells and microscope questions 1-5

08 Exit check (last 5 minutes)

Question	Answer	Decision
Total magnification of 10x with 40x	400	All three right: move to practical questions
Image 40 mm, actual 0.05 mm	800	One wrong: reteach units
Three structures unique to plant cells	Cell wall, chloroplast, large vacuole	Two or more wrong: restart at Act 1

09 Tutor FAQ

Why is the field dimmer at high power?

The same light is spread over a larger magnified image, so brightness falls and the diaphragm or lamp must compensate.

Do all plant cells have chloroplasts?

No. Root cells receive no light and usually lack chloroplasts, though they still have a wall and a large vacuole.

Why methylene blue rather than red ink?

Methylene blue binds nucleic acids and gives clear contrast between nucleus and cytoplasm, while ink particles are large enough to hide structure.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.