IB Biology: Cells and Membrane Transport — Tutor Lesson Plan (magnification, SA:V, water potential)

2026-10-28

Students can say diffusion goes from high to low concentration but cannot say why cells stay small. Surface area to volume ratio settles it.

Objectives — the student can compute magnification from a drawing, explain the size limit of a cell with surface area to volume ratio, and predict osmosis from water potential.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Units	How many micrometres in 1 mm?	1000
Area and volume	Surface area of a 2 cm cube	24 cm squared
Gradient	What is a concentration gradient?	A difference between two regions

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Membranes	Passive, active, vesicle transport	Sort three examples
8-22 min	Magnification	Image 30 mm, specimen 0.03 mm	Get 1000 times
22-36 min	Surface area to volume	2 cm cube	Get 24 to 8, ratio 3
36-50 min	Water potential	psi s -600 plus psi p 200	Get -400 kPa
50-56 min	Experimental design	Variables and a control	Write three controls
56-60 min	Exit check	Three questions marked	Finish alone

03 Teaching script (read it out)

Scene 1: Magnification: image over specimen, units first

Magnification is image size divided by actual size. An image of 30 mm for a 0.03 mm specimen gives 1000 times.
If the specimen is given in micrometres, convert to millimetres first or the answer is out by a factor of a thousand.

Student states whether higher magnification also improves resolution.

Scene 2: Surface area to volume: why a cell cannot grow forever

A cube of side 2 cm has a surface area of 24 cm squared and a volume of 8 cm cubed, a ratio of 3 to 1.
Double the side and the surface area rises four times while the volume rises eight times, so exchange cannot keep up.

Note: say surface area grows more slowly than volume, not just that the cell is too big.

Scene 3: Water potential: the more negative number is lower

psi equals psi s plus psi p. Solute potential -600 kPa with pressure potential 200 kPa gives -400 kPa.
A surrounding solution at -300 kPa has the higher water potential, so water enters the cell.

Student predicts the direction if the outside solution is -800 kPa (water leaves).

04 Question types and worked steps

Type	Example	Steps	Answer
Magnification	image 30 mm, specimen 0.03 mm	Convert units, then divide	1000 times
Ratio	2 cm cube	Surface area divided by volume	3 to 1
Water potential	psi s -600, psi p 200	Add, then compare outside	-400 kPa
Experiment	variables in an osmosis practical	Independent, dependent, controlled	Three controls named

Sanity check (teach this): Water moves from the higher water potential (less negative) to the lower (more negative). Water moving towards the higher value means the comparison is inverted.

05 Independent practice (3 questions)

Question	Answer	What to watch
Magnification, 30 mm against 0.03 mm	1000 times	Units converted
Surface area to volume for a 2 cm cube	3 to 1	Area over volume
Water potential with -600 and 200	-400 kPa	Both potentials added

06 Common mistakes and what to say

Mistake	Student writes	Say this
Mixed units	answer of a million times	Convert millimetres and micrometres first.
Ratio written upside down	1 to 3	Surface area comes first.
Osmosis direction reversed	water flows to -400	From the higher value, -300, to the lower.
Concentration only	water moves to low concentration	Use water potential; pressure potential counts.

07 Homework (paper, section and question numbers)

IB biology chapter index — Use the three questions on this page

08 Exit check (last 5 minutes)

Question	Answer	Decision
Magnification, 30 mm against 0.03 mm	1000 times	All three right: move to genetics
Surface area to volume for a 2 cm cube	3 to 1	One wrong: re-teach the ratio
Water potential with -600 and 200	-400 kPa	Two wrong: repeat scene 3

09 Tutor FAQ

Which formulas must be memorised?

Magnification and psi equals psi s plus psi p; the rest follows from reasoning.

How does IB differ from DSE?

IB weights experimental design and data analysis, so controls must be named.

How are data questions marked?

Describe the trend, explain the mechanism, then state a limitation.

Find a tutor — free

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.