DSE Biology | Photosynthesis Lesson-Prep Guide (Tutor Edition) | Light Reaction, Carbon Fixation, Limiting Factors

2026-11-01

Tutor lesson-prep guides · Secondary Biology

Students can recite the equation but answer “the plant is tired” when a curve flattens. This lesson turns a flat curve into a statement about which factor has become limiting.

Objectives — Students can: (1) write the equation and state where the products of each stage go; (2) separate net and gross photosynthesis using bubble counts; (3) read a limiting-factor curve and name the limiting factor in each section.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Balancing equations	Coefficients for photosynthesis	6 CO2 + 6 H2O gives C6H12O6 + 6 O2
Respiration	Direction of gas exchange	Takes up oxygen, releases carbon dioxide
Variables	What to hold constant when light changes	Temperature and carbon dioxide

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Equation	Reactants, products, energy source	Students write it once
10-24 min	Light reaction	Photolysis, ATP and reduced coenzyme	Draw the two stages
24-36 min	Carbon fixation	How carbon dioxide becomes sugar	Name the entry step
36-48 min	Net and gross	Adding respiration back	Get 40 and 30
48-56 min	Limiting factors	What limits each part of the curve	Read three graphs
56-60 min	Exit check	Three questions marked on the spot	Independent work

03 Teaching script (read it out)

Scene 1: Light reaction: water is split, oxygen leaves

Light is absorbed by chlorophyll and water is split: 2 H2O gives 4 H+ , 4 electrons and O2. Electrons pass down a chain to make ATP and reduce the coenzyme. Two sentences to drill: the oxygen comes from water, and the products of the light reaction are used at once in carbon fixation.

Ask the student: which raw material supplies the released oxygen (water).

Scene 2: Net equals gross minus respiration

In light the plant releases 30 bubbles per minute; in the dark it takes up oxygen at 10 per minute. Gross photosynthesis is 30 + 10 = 40; net is the 30 you can see. The compensation point is where net is zero and gross equals respiration.

Ask the student: if respiration rises to 20 per minute while the lit count stays at 30, what is gross (50).

Scene 3: Limiting factors: when the curve flattens, the bottleneck changes

Rising rate with more light means light is limiting. A flat section means light is saturated and carbon dioxide or temperature is now the bottleneck. Raising carbon dioxide lifts the plateau but does not change the initial slope. Answers must name the limiting factor explicitly.

Warn: “the rate did not change” is not the same as “nothing happened”; name the new limit.

04 Question types and worked steps

Type	Example	Steps	Answer
Equation	Complete reactants and products	Balance carbon first, then hydrogen and oxygen	6 CO2 + 6 H2O
Calculation	Gross photosynthesis	Net plus respiration	40 per minute
Graph	Plateau section	Name the new limiting factor	Carbon dioxide or temperature
Design	Show that light is needed	Cover one plant, keep all else equal	One variable only

Sanity check (teach this): Gross is always greater than or equal to net; a net value larger than gross means respiration was left out. Keep every rate in the same unit, per minute.

05 Independent practice (3 questions)

Question	Answer	What to watch
Counts of 30 in light and 10 in dark	40 per minute	Add respiration
Gross rate at the compensation point	Equals the respiration rate	Net is zero there
Effect of more carbon dioxide on the plateau	The plateau rises	The initial slope is unchanged

06 Common mistakes and what to say

Mistake	Student writes	Say this
Oxygen said to come from carbon dioxide	Answering photolysis of carbon dioxide	“Oxygen comes from water; isotope labelling proves it.”
Net reported as gross	Answering 30 as the total	“Add the respiration part back.”
Plateau explained as tiredness	No limiting factor named	“Name which factor is now the bottleneck.”
Two variables in a control	Covering the plant and cooling it	“A control changes one variable only.”

07 Homework (paper, section and question numbers)

DSE biology practice paper (plant physiology) — Section B on photosynthesis and design
Secondary biology chapter index — All chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
Gross rate from 30 lit and 10 dark	40 per minute	All three right: move to respiration
Definition of the compensation point	Net rate is zero	One wrong: reteach net and gross
Limiting factor on the plateau	Carbon dioxide or temperature	Two wrong: restart act three

09 Tutor FAQ

Must students learn every step of the Calvin cycle?

No; DSE needs only that carbon dioxide is fixed into organic matter using ATP and the reduced coenzyme.

Why is there gas exchange at night?

Respiration runs all day; in daylight photosynthesis masks it.

Which light does chlorophyll absorb?

Mainly red and blue-violet; green is largely reflected, which is why leaves look green.

Find a tutor — free Tutor Registration →

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.