DSE Biology — Photosynthesis and Limiting Factors: Lesson Prep Guide (Tutor Edition)

2026-11-03

Tutor lesson-prep guides · Secondary Biology

Students can recite the equation but answer every data table with “more light, faster photosynthesis”. Here one set of bubble counts forces them to compute rates, percentages and the plateau, then decide which factor is actually limiting.

Objectives — Students will be able to: (1) write 6CO2 + 6H2O -> C6H12O6 + 6O2 and check both sides equal 372 g using molar masses; (2) compute mean rates from triplicate bubble counts of 45.0, 29.0, 16.0 and 8.0 per minute at 10, 20, 30 and 40 cm, and show that 40 cm reaches only 17.8 per cent of 10 cm; (3) use the inverse-square law to explain that light intensity differs 16-fold between 10 and 40 cm while the rate does not, so carbon dioxide or temperature has become limiting.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
The equation	How many moles of each reactant and product?	Six of carbon dioxide and water give one glucose and six oxygen
Diffusion	Which way do gases move?	From higher to lower concentration
Enzymes	What happens above the optimum temperature?	The enzyme denatures and the rate collapses
Variables	What must a fair test hold constant?	Everything except the independent variable

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0–8 min	Check the equation	Write it and have students total both sides with molar masses	Both sides equal 372 g
8–20 min	Read the table	Give triplicate counts and demand means first	Means of 45.0, 29.0, 16.0, 8.0
20–32 min	Distance and intensity	Use 1/d squared and compare 10 cm with 40 cm	A 16-fold difference
32–44 min	Find the plateau	Ask why a 16-fold intensity gap gives only a 5.6-fold rate gap	Name carbon dioxide or temperature as the new limit
44–52 min	Question drill	One calculation, one curve, one design, one application item	Work independently for eight minutes
52–60 min	Exit check	Mark three written items on the spot	Complete independently

03 Teaching script (read it out)

Scene 1: Turn the equation into something countable

6CO2 + 6H2O -> C6H12O6 + 6O2: the left side is 6 x 44 + 6 x 18 = 264 + 108 = 372 g, the right side is 180 + 6 x 32 = 180 + 192 = 372 g. Conservation of mass is a checking tool for coefficients, not a slogan. Because oxygen moles equal carbon dioxide moles fixed, bubble counts can stand in for rate.

Ask students: why can counting oxygen bubbles measure photosynthesis?

Scene 2: Averages first, comparisons second

Three runs at each distance: 44, 46, 45 at 10 cm gives 45.0; 28, 30, 29 at 20 cm gives 29.0; 15, 17, 16 at 30 cm gives 16.0; 8, 9, 7 at 40 cm gives 8.0. A fresh cut stem and a warmed lamp matter. The 40 cm rate is 17.8 per cent of the 10 cm rate, a factor of 5.6.

Ask students: why three repeats rather than the single best run?

Scene 3: A plateau means the limiting factor changed

Intensity follows 1/d squared, so 10 cm and 40 cm differ by 16 times, yet the rate differs by only 5.6 times: at long distance light is limiting; shortening the distance further stops helping, so carbon dioxide or temperature takes over. The temperature table gives 12 at 15 C, 45 at 25 C, 38 at 35 C and 2 at 45 C: 45 C is denaturation, not enthusiasm. 48 cm3 of oxygen in 5 minutes is 9.6 cm3 per minute and 576.0 cm3 per hour.

Ask students: when the curve flattens, which variable must you change to identify the limit?

04 Question types and worked steps

Type	Example	Steps	Answer
Data calculation	Average triplicate counts and compare rates	Total, divide by three, state units, compare like with like	10 cm 45.0, 40 cm 8.0, which is 17.8 per cent
Curve explanation	Explain why the light-rate curve flattens	Identify the rising part, the plateau, and the new limit	On the plateau carbon dioxide or temperature limits; light no longer does
Experimental design	Prove carbon dioxide limits the plateau	Change only carbon dioxide, hold distance and temperature, add a control	Adding hydrogencarbonate at fixed distance raises the rate
Application	Why control light, carbon dioxide and temperature in a greenhouse	Find the first shortfall, correct it, warn about overheating	Above the optimum near 25 C the rate falls

Sanity check (teach this): Sanity check: use 1/d squared, so 10 cm against 40 cm is 16 times, never four; percentages must come from means; temperature claims need the same stem and distance; the count of 2 at 45 C is denaturation, not random error.

05 Independent practice (3 questions)

Question	Answer	What to watch
Counts of 44, 46 and 45: what is the mean?	45.0 per minute	Check the total is divided by three, not the middle value taken
What percentage of the 10 cm rate survives at 40 cm?	17.8 per cent	Check means are divided and multiplied by 100
48 cm3 of oxygen in five minutes: rate per hour?	9.6 per minute and 576.0 per hour	Check division by five happens before multiplying by 60

06 Common mistakes and what to say

Mistake	Student writes	Say this
Assuming brighter is always faster	Double the light doubles the rate	Past the plateau the bottleneck is carbon dioxide or temperature.
Concluding from one run	I once saw 50, so 10 cm gives 50	Triplicates suppress random error; never pick the prettiest number.
Treating distance and intensity as linear	Four times the distance, a quarter of the light	Intensity falls with the square, so four times the distance is one sixteenth.
Reading heat as activity	45 C is fastest because it is hottest	Enzymes denature; the optimum here is about 25 to 35 C.

07 Homework (paper, section and question numbers)

Secondary Biology chapter index — Read the index and copy the equation and molar masses once
Cellular respiration (S4 chapter) — Redo practice question 2 there with the equation reversed
Enzymes and factors (S4 chapter) — Use that temperature curve to explain the count of 2 at 45 C

08 Exit check (last 5 minutes)

Question	Answer	Decision
What is the total mass on each side of the equation?	372 g	If only 180 g is given: reteach molar mass
How many times stronger is the light at 10 cm than at 40 cm?	16 times	If four times: reteach 1/d squared
What does a flat curve mean?	Light is no longer limiting; carbon dioxide or temperature is	If “the plant is tired”: reteach limiting factors

09 Tutor FAQ

Which year level is this for?

S4; S5 revision of metabolism and experimental design can reuse the data.

Is 60 minutes enough?

Enough for three acts and one drill; repair molar mass in five minutes and move the application item to homework.

Is there practice attached?

The homework section links to the site chapter pages, where answers appear under independent practice.

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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.