DSE Biology — Limiting Factors in Photosynthesis Prep Guide (for tutors) | Read the plateau, find the compensation point, convert gas readings into glucose

2026-11-05

Tutor lesson-prep guides · Secondary Biology

Students can recite light, carbon dioxide and temperature, then collapse when two curves cross. This lesson teaches one reading habit: whichever line is still rising names the limiting factor.

Objectives — Students can: (1) mark the rising and plateau sections on light and carbon dioxide curves; (2) add a respiration rate of 2 micromoles per minute to net readings to recover gross photosynthesis, then locate the compensation and saturation points; (3) convert leaf area and fixation rate into 0.054 mg of glucose per hour and state three assumptions.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Photosynthesis equation	Which two raw materials does photosynthesis use, and what does it make?	Carbon dioxide and water, giving glucose and oxygen
Respiration	Do plants respire in the light?	Yes, all day and night, so net equals gross minus respiration
Enzymes	What happens to an enzyme above its optimum temperature?	It denatures, activity falls and the change is irreversible

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Prior knowledge check	Ask the three prior questions and write the equation on the board	Answer orally and copy any wrong item once
5-17 min	Act one: two plateaus	Draw the light and carbon dioxide curves side by side, marking rise and plateau	Copy both and mark where each flattens
17-30 min	Act two: net and gross	Start at net -2 and add respiration 2 point by point	Fill the gross row and find the compensation point
30-40 min	Act three: bubbles to milligrams	Show how 0.02 m2 of leaf gives 0.054 mg of glucose	Compute once and list three assumptions
40-43 min	One-sentence summary	Ask for one sentence on when light stops being limiting	Answer orally; tutor corrects only wording
43-45 min	Exit check	Hand out three items and collect them	Work alone with books closed

03 Teaching script (read it out)

Scene 1: Act one: the line that has not flattened names the limit

The light curve climbs almost linearly then levels off at the saturation point, where extra light changes nothing.
The carbon dioxide curve has the same shape at a higher plateau.
One reading rule: the factor still rising is the limiting one. Where two curves cross, the limit changes hands.
Temperature is the exception: it has a peak and then a sharp fall, because enzymes denature rather than saturate.

Ask students to repeat: an unflattened line names the limiting factor, and temperature is the exception that falls.

Scene 2: Act two: net is what is left, gross is the whole

In darkness the oxygen reading is -2 micromoles per minute, which is respiration at 2.
At 200 units of light the net rate is 5, so gross is 7; at 800 units net is 15 and gross is 17.
The light level where net is zero is the compensation point: fixation exactly matches respiration, so the plant does not grow.
The common error is treating net as gross and then claiming photosynthesis stops in the dark, when in fact gross is what falls to zero.

Ask students to repeat: net plus respiration is gross, and the compensation point is where net is zero.

Scene 3: Act three: from leaf area to milligrams

Each square metre fixes 90 micromoles of carbon dioxide an hour, so 0.02 m2 fixes 1.8 micromoles.
Six carbon dioxide molecules make one glucose, giving 0.3 micromoles of glucose an hour, and at 180 g per mole that is 0.054 mg.
Three assumptions belong in the answer: rate stays constant, no photorespiration, all carbon becomes glucose.
Even when the question does not ask, the line assuming a constant rate usually earns the mark.

Ask students to repeat: divide by six for glucose, multiply by 180 for mass, and state the assumptions.

04 Question types and worked steps

Type	Example	Steps	Answer
Curve interpretation	The light curve flattens beyond a certain intensity	Find the plateau, name the saturation point, say what limits now	Light stops limiting; carbon dioxide or temperature takes over
Compensation point	Net photosynthesis crosses zero	Find the light level where net is zero and gross equals respiration	About 50 micromoles of photons
Yield conversion	90 micromoles per m2 per hour over 0.02 m2	Multiply by area, divide by six, multiply by 180 g	0.054 mg per hour
Experimental design	Prove that light is limiting	Same plant, vary light only, control the rest, count bubbles, repeat three times	Controlled variables plus replicates

Sanity check (teach this): Three read-backs: (1) gross always exceeds net, so the reverse means respiration was forgotten; (2) at the compensation point gross equals respiration at 2, not zero; (3) the initial slope is about 0.035, roughly 3.5 percent, and a figure near 100 percent means the units slipped.

05 Independent practice (3 questions)

Question	Answer	What to watch
Respiration 3, net 9. Find gross.	12 micromoles per minute	Check whether 9 was written down directly
At the compensation point, is photosynthesis zero?	No, gross equals respiration and only the net is zero	Check whether net and gross are kept apart
Why does the temperature curve differ from the light curve?	Heat denatures enzymes, so activity collapses and does not recover	Check whether enzymes are named rather than heat alone

06 Common mistakes and what to say

Mistake	Student writes	Say this
Net read as gross	The saturated rate is 15	15 is net; gross is 17.
Compensation point read as zero photosynthesis	At the compensation point there is no photosynthesis	Gross equals respiration; only the net is zero.
Light still blamed after saturation	More light keeps raising the rate	The curve is flat, so the limit has changed hands.
The factor of six dropped	1.8 micromoles of glucose per hour	Six carbon dioxide molecules make one glucose.

07 Homework (paper, section and question numbers)

Redo this chapter — Redo the three items, writing net, gross and respiration each time
Gas exchange in humans — Compare leaf and lung exchange surfaces in three points
Biology chapter index — Pick one more chapter and draw the curve only

08 Exit check (last 5 minutes)

Question	Answer	Decision
Net minus two, respiration two: gross?	Zero	Error: answering minus two or minus four
After the curve flattens, what limits the rate?	Whichever factor is still unsaturated, carbon dioxide or temperature	Error: still answering light
How many glucose molecules from six carbon dioxide?	One	Error: answering six

09 Tutor FAQ

Which year is this for?

Mainly S5, and a quick recap for S6 metabolism.

Is 45 minutes enough for three curves?

Yes, because only the light curve is computed in full and the same reading rule covers the rest.

Is there a companion worksheet?

Three practice items with answers on the page, plus the gas exchange chapter.

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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.