IB Biology — Photosynthesis and Limiting Factors: Tutor Prep Guide | Light reactions, Calvin cycle, rate maths

2026-11-09

Students recite that the light reactions make ATP and NADPH, then blame light for every plateau. One set of rate data links the two stages to the graph.

Objectives — Students can: (1) say where the light reaction products are used; (2) compute a photosynthetic rate from volume and time, then a percentage change; (3) explain a plateau with a limiting factor.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Rate	Oxygen volume 4.8 cm3 collected in 4 minutes	1.2 cm3 per minute
Percentage change	From 1.2 to 1.8	50 per cent
Absorption peaks	Two wavelengths chlorophyll absorbs best	430 nm and 662 nm

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Two stages	Ask where the oxygen comes from	Say the thylakoid hosts the light reactions
8-22 min	Light reactions	Photolysis and chemiosmosis	Draw ATP and NADPH leaving
22-36 min	Calvin cycle	RuBP and rubisco	Say which step uses ATP
36-46 min	Rate maths	4.8 over 4 and 7.2 over 4	Get 1.2 and 1.8
46-56 min	Limiting factor	What a plateau means	Name temperature or carbon dioxide
56-60 min	Exit check	Collect three items	Work alone

03 Teaching script (read it out)

Scene 1: Light reactions: water is split, protons drive ATP synthesis

Photosystem II absorbs near 680 nm, and photolysis of water releases electrons, protons and oxygen, so all the oxygen comes from water.
Electrons pass down the chain, protons are pumped into the thylakoid space, and their return through ATP synthase makes ATP, while photosystem I hands electrons to NADP plus to form NADPH.

Ask them to repeat: the three words for the oxygen source (water, photolysis, photosystem II).

Scene 2: Calvin cycle: ATP and NADPH are spent on reduction, not on fixation

In the stroma, carbon dioxide joins RuBP under rubisco to give two molecules of GP, which are reduced to TP using ATP and NADPH, and part of the TP regenerates RuBP.
Make them say it: stop the light reactions and the cycle stops, because ATP and NADPH dry up.

Watch for: fixation needs no ATP; reduction does.

Scene 3: Limiting factors: the plateau names a different variable

Raising light intensity lifts the oxygen rate from 1.2 to 1.8 cm3 per minute, a rise of 50 per cent.
Push light higher and the rate stops rising, so light is no longer limiting and temperature or carbon dioxide takes over.
Gross photosynthesis is 24 and respiration 6, so net is 18, and the compensation point is where net returns to zero.

Have them judge: raise temperature from 25 to 40 degrees and the curve falls, because enzymes denature.

04 Question types and worked steps

Type	Example	Steps	Answer
Rate	4.8 cm3 in 4 minutes	Volume divided by time	1.2 cm3 per minute
Percentage change	1.2 rising to 1.8	Difference over original, times 100	50 per cent
Gross from net	Net 18, respiration 6	Gross equals net plus respiration	24
Limiting factor	The plateau section	Compare the variables one at a time	A second variable is limiting

Sanity check (teach this): Net photosynthesis is always below gross, because respiration is subtracted; a rate must carry a per-minute unit; a percentage change above 100 per cent means the denominator was wrong.

05 Independent practice (3 questions)

Question	Answer	What to watch
Rate of the first run	1.2 cm3 per minute	Did they divide volume by time
Percentage change after doubling light	50 per cent	The denominator is the original 1.2
Net photosynthesis	18	Was respiration subtracted

06 Common mistakes and what to say

Mistake	Student writes	Say this
Inverting the rate	Writes 4 divided by 4.8	“Rate is volume divided by time, never time divided by volume.”
Absolute difference as a percentage	Answers 0.6	“Percentage change divides by the original value, then multiplies by 100.”
Treating net as gross	Writes gross 18	“18 is net. Add the respiration 6 back to get gross.”
Calling the plateau light-limited	Still names light	“The plateau means light is sufficient, so another variable limits.”

07 Homework (paper, section and question numbers)

IB Biology chapter index — Use the three items on this page

08 Exit check (last 5 minutes)

Question	Answer	Decision
Photosynthetic rate	1.2 cm3 per minute	All three right, move to respiration
Percentage change	50 per cent	One wrong, reteach the denominator
Net photosynthesis	18	Two wrong, return to act three

09 Tutor FAQ

Must every step of the chain be named?

Photosystem II, the proton gradient and ATP synthase are enough.

Do limiting factor graphs always appear?

Almost every year, often asking which section is limited by which variable.

Should photosynthesis and respiration be taught together?

Yes, the net relationship is a standard comparison question.

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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.