3.3.7Inquiry Process
Photosystems — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 122 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Structure of a Photosystem
Photosystems group pigments so that light energy is captured and passed to a reaction centre.
Location
- Location Photosystems are protein-pigment complexes embedded in the thylakoid membranes of chloroplasts and of cyanobacteria.
Pigment molecules
- Pigment molecules Each contains many chlorophyll molecules and accessory pigments such as carotene and xanthophylls that act as a light-harvesting antenna.
Reaction centre
- Reaction centre At the centre is a special chlorophyll a molecule to which absorbed energy is passed, and from which electrons are donated.
Two types
- Two types Photosystem I has the reaction centre P700 and photosystem II has P680, named after the wavelengths of light they absorb best.
Section 2 of 5How a Photosystem Works
1
Pigments in the antenna absorb photons of different wavelengths.
2
The energy is passed from molecule to molecule until it reaches the chlorophyll of the reaction centre.
3
An electron in the reaction centre is excited to a higher energy level, so the chlorophyll is photoactivated.
4
The excited electron is passed to an electron acceptor, so the chlorophyll is oxidised and the acceptor is reduced.
5
The electron then moves along the electron transport chain, and the reaction centre is supplied with replacement electrons.
Section 3 of 5Why Many Pigments?
Why Many Pigments?
Wider absorption
- Wider absorption Different pigments absorb different wavelengths, so together they capture much more of the available light than one pigment could.
Controlled excitation
- Controlled excitation The ordered arrangement allows electrons to be excited and passed on in a controlled way.
Proteins
- Proteins Light-harvesting complex proteins include enzymes such as ATP synthase and electron carriers that form the electron transport chain.
Order of events
- Order of events Photosystem II is activated first in the light-dependent reactions, even though photosystem I was discovered first.
| Feature | Photosystem II | Photosystem I |
|---|---|---|
| Reaction centre | P680 | P700 |
| Position in sequence | First | Second |
| Electron source | Water (photolysis) | Electrons from photosystem II chain |
Section 4 of 5Photosystems in the Light-Dependent Reactions
Photosystems in the Light-Dependent Reactions
Starting point
- Starting point Photosystem II absorbs light first and its excited electrons enter the electron transport chain,
- while water supplies replacement electrons.
Second excitation
- Second excitation Photosystem I absorbs light a second time and re-energises the electrons, so that they can reduce NADP⁺ to NADPH.
Electron carriers
- Electron carriers Mobile and fixed carriers in the thylakoid membrane link the two photosystems and transfer the electrons between them.
Position
- Position The arrangement in the membrane keeps the photosystems, carriers and ATP synthase close together, which makes the transfer efficient.
Section 5 of 5Exam Checklist & Tips
3.3.7 – Key Points for the Exam
- Photosystem A group of pigments and proteins in the thylakoid membrane that captures light.
- Reaction centre The part of a photosystem where electrons are excited and donated.
- Accessory pigment A pigment that absorbs light and passes the energy on to the reaction centre.
- Photoactivation Excitation of electrons in chlorophyll by light.
Common Mistakes
- Thinking photosystem I comes before photosystem II in the pathway; photosystem II acts first.
- Saying that every pigment molecule passes out electrons; only the reaction centre does this.
- Placing photosystems in the stroma instead of the thylakoid membrane.
- Writing that the pigments make ATP directly.
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