AHL: Photosynthesis extensions
IB Biology HLΒ· Theme B: Form and Function, AHL PhotosynthesisΒ· 25 min read
1. Photosystem Structure and Functionβ β β βββ± 8 min
Photosystems are large protein-pigment complexes embedded in the thylakoid membrane of chloroplasts, responsible for capturing light energy to power the light-dependent reactions of photosynthesis. Each photosystem has an antenna complex of light-harvesting pigments surrounding a central reaction center.
Reaction Center
A special pair of chlorophyll a molecules that accepts excited energy from antenna pigments and donates high-energy electrons to the electron transport chain
Example:
Photosystem II has reaction center P680 that absorbs 680 nm light; Photosystem I has P700 that absorbs 700 nm light
Compare the roles of Photosystem II and Photosystem I in light-dependent reactions
- 1
- Photosystem II (PSII) functions first in electron flow, despite being named after PSI by discovery order. It splits water via photolysis to replace electrons lost from P680, releasing oxygen as a byproduct.
- 2
- Excited electrons from PSII move along an electron transport chain (ETC), releasing energy that pumps H+ into the thylakoid lumen to build an electrochemical gradient.
- 3
- Photosystem I (PSI) receives electrons from the PSII ETC, then re-excites them to a higher energy level for reduction of NADP+ to NADPH.
Exam tip:
Exam questions often test matching reaction centers to photosystems: remember P680 = PSII, P700 = PSI (6 comes before 7).
2. Cyclic vs Non-Cyclic Photophosphorylationβ β β β ββ± 7 min
Photophosphorylation is the process of generating ATP from ADP and inorganic phosphate using light energy and chemiosmosis. There are two distinct pathways that operate in chloroplasts, with different products and functions.
Key differences between the two pathways are outlined below:
Non-cyclic photophosphorylation
Electrons flow linearly from PSII β ETC β PSI β NADP+, and do not return to the starting photosystem
+ Pros: Produces both ATP and NADPH required for the Calvin cycle
β Cons: Produces less ATP than the Calvin cycle requires, needs constant input from water photolysis
Cyclic photophosphorylation
Electrons excited from PSI P700 return to PSI via a short ETC, and are never passed to NADP+
+ Pros: Produces extra ATP to meet the Calvin cycle's higher ATP demand, runs when NADPH levels are high
β Cons: Does not produce NADPH or oxygen, cannot sustain the Calvin cycle alone
Test your understanding below:
Which product is only made by non-cyclic photophosphorylation, not cyclic?
ATP
NADPH
Proton gradient
Triose phosphate
Reveal answer
1 βCorrect! Cyclic photophosphorylation does not pass electrons to NADP+, so no NADPH is generated.
Exam tip:
The Calvin cycle uses 3 ATP and 2 NADPH per CO2 fixed: cyclic photophosphorylation makes up the ATP deficit.
3. Chemiosmosis in Chloroplastsβ β β βββ± 5 min
Chemiosmosis is the shared mechanism for ATP production in both photosynthesis and aerobic respiration, relying on an electrochemical gradient of hydrogen ions across a membrane.
Outline the steps of chemiosmosis that produce ATP in chloroplasts
- 1
- Energy released from electrons moving down the ETC is used to actively pump H+ ions from the stroma into the thylakoid lumen.
- 2
- This creates a higher concentration of H+ in the lumen than the stroma, forming an electrochemical gradient called proton motive force.
- 3
- H+ can only diffuse down their gradient through the transmembrane enzyme ATP synthase.
- 4
- The flow of H+ through ATP synthase provides the energy to phosphorylate ADP to ATP:
- 5
4. Photorespiration and RuBisCO Activityβ β β β ββ± 5 min
RuBisCO (ribulose bisphosphate carboxylase-oxygenase) catalyzes the fixation of CO2 to RuBP, the first step of the Calvin cycle. However, RuBisCO can also bind oxygen to RuBP, leading to the process of photorespiration.
Photorespiration
A wasteful process where RuBisCO binds oxygen instead of CO2, reducing net sugar production and consuming ATP and NADPH
Example:
Photorespiration rates increase at higher temperatures, as RuBisCO's affinity for oxygen rises faster than for CO2
Explain why photorespiration reduces photosynthetic efficiency
- 1
- When RuBisCO binds O2 instead of CO2, only one 3-carbon GP molecule is produced instead of two from CO2 fixation.
- 2
- The 2-carbon product of oxygenation is broken down in a series of reactions that consume ATP and NADPH, and release CO2.
- 3
- No net sugar is produced, and energy from the light reactions is wasted, leading to a 20-50% reduction in efficiency in C3 plants.
5. Common Pitfalls
Wrong move:
Stating Photosystem I comes before Photosystem II in electron flow
Why:
Photosystems are numbered by discovery order, not functional order
Correct move:
Photosystem II (P680) functions first, splits water, and feeds electrons to Photosystem I (P700)
Wrong move:
Claiming cyclic photophosphorylation produces NADPH
Why:
Electrons are recycled back to Photosystem I and never passed to NADP+
Correct move:
Cyclic photophosphorylation only produces ATP, to meet the extra ATP demand of the Calvin cycle
Wrong move:
Stating photolysis of water occurs in Photosystem I
Why:
Photolysis is only needed to replace electrons lost from Photosystem II's reaction center
Correct move:
Photolysis of water occurs in Photosystem II, producing oxygen as a byproduct
Wrong move:
Claiming H+ is pumped into the stroma for chemiosmosis
Why:
The gradient is built with high H+ concentration inside the thylakoid
Correct move:
H+ is pumped into the thylakoid lumen, diffuses out to the stroma through ATP synthase to make ATP
Wrong move:
Confusing photorespiration with aerobic cellular respiration
Why:
Photorespiration does not produce ATP, unlike aerobic respiration
Correct move:
Photorespiration is wasteful, consumes energy, occurs in chloroplasts, and reduces photosynthetic output
6. Quick Reference Cheatsheet
Feature | Photosystem II | Photosystem I | Non-cyclic | Cyclic |
|---|---|---|---|---|
Reaction center | P680 | P700 | Both PSII + PSI | PSI only |
Products | O2, excited electrons | Excited electrons to NADP+ | ATP, NADPH, O2 | Only ATP |
Photolysis required? | Yes | No | Yes | No |
Electrons recycled? | No | No | No | Yes to PSI |
7. Frequently Asked
Is this topic required for IB Biology SL?
No, this is an AHL (additional higher level) topic that is only assessed in HL exams.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 Β· 1
Compare cyclic/non-cyclic photophosphorylation
- 2023 Β· 2
Explain photorespiration in C3 plants
- 2021 Β· 1
Role of photosystems in light reactions
Going deeper
What's Next
This AHL topic builds on core photosynthesis content and provides the foundation for understanding how photosynthetic efficiency is regulated, and how plants have adapted to different environments to reduce the impact of photorespiration. The concepts of chemiosmosis explored here also connect directly to oxidative phosphorylation in aerobic respiration, so revisiting that topic will help reinforce your understanding of this shared mechanism. Understanding photosynthesis extensions is also critical for analyzing data on limiting factors of photosynthesis, which is a common practical and extended response question in IB exams.