IB Biology HLPhotosynthesisPaper 1 & 2~10 min read
Photosystems
A single chlorophyll molecule hit by a single photon is not much use on its own. Plants solve this by building funnels: hundreds of pigment molecules feeding energy into one chlorophyll a at the centre.
📚 What you need to know
A photosystem is a cluster of pigment molecules and proteins embedded in the thylakoid membrane.
Most of the pigments form an antenna complex (light-harvesting complex), which absorbs photons and funnels the energy inwards.
At the centre is the reaction centre, containing a pair of chlorophyll a molecules.
Energy arriving at the reaction centre excites an electron in chlorophyll a to a higher energy level.
The excited electron is passed to an electron acceptor. This is photoactivation, and the chlorophyll is now short of an electron — it has been oxidised.
Photosystem II (PSII) absorbs best at 680 nm and is written P680. Photosystem I (PSI) absorbs best at 700 nm and is written P700.
They are numbered by the order of discovery, not the order they are used. PSII acts first.
Only PSII can replace its lost electrons by splitting water; PSI receives replacements down the chain.
Why cluster the pigments?
Photons arrive at random. If each chlorophyll molecule had to catch one itself and then act on it alone, most of the membrane would be idle most of the time.
Instead the pigments are organised so that many molecules collect energy and pass it inwards to one reaction centre. Each antenna pigment absorbs a photon, becomes briefly excited, and transfers that energy to a neighbour slightly closer to the centre. The energy migrates inwards until it reaches the chlorophyll a pair at the reaction centre.
What travels through the antenna is energy, not electrons. Only at the reaction centre does an actual electron leave the molecule.
🧠
A satellite dish
The dish is large so it can gather a weak signal over a wide area, but everything is focused onto one small receiver at the middle. The antenna complex is the dish; the reaction centre is the receiver.
What happens at the reaction centre
When enough energy arrives, an electron in one of the chlorophyll a molecules is raised to a higher energy level — it becomes excited. At that higher level the electron is held only loosely, and it is passed to a nearby electron acceptor.
This is called photoactivation. Two consequences follow, and both matter:
The electron is now outside the chlorophyll, carrying energy that came from light. It goes on to drive the electron transport chain.
The chlorophyll a has been oxidised and is missing an electron. It cannot work again until that electron is replaced.
That second point drives the whole of the next page. A photosystem that has lost an electron is dead until it gets one back — so the entire light-dependent stage is really a story about where replacement electrons come from.
The two photosystems
Photosystem II
Photosystem I
Also written
PSII, or P680
PSI, or P700
Peak absorption
680 nm
700 nm
Order of use
First in the pathway
Second
Where its replacement electrons come from
The photolysis of water
The electron transport chain from PSII
Location in the membrane
Mainly in the stacked grana
Mainly in the unstacked lamellae and edges
Where its electrons end up
Passed along the chain to PSI
Used to reduce NADP, or cycled back
Why the numbering is backwards. Photosystem I was discovered first, so it got the number 1 — but in the pathway, photosystem II acts first. Nothing in biology forces this order to make sense; it is simply history. Learn it as a quirk and move on, because examiners will absolutely test it.
The names P680 and P700 come straight from the absorption peaks: the reaction centre chlorophyll of PSII absorbs best at 680 nm, and that of PSI at 700 nm. Both are chlorophyll a; the difference in peak comes from the proteins each is bound to.
Worked examples
WE 1
Explain the structure of a photosystem
Explain how the structure of a photosystem allows a plant to make efficient use of light. (4 marks)
Point 1: many pigments
A large number of pigment molecules form an antenna complex, so light is captured over a large area of membrane.
Point 2: a range of pigments
Different accessory pigments absorb different wavelengths, so a broader part of the spectrum can be used.
Point 3: funnelling
Energy is transferred inwards from pigment to pigment to a single reaction centre, so energy from many photons is concentrated in one place.
Point 4: the outcome
An electron in chlorophyll a at the reaction centre is excited and passed to an electron acceptor, so light energy becomes chemical energy.
Wide capture, broad spectrum, concentrated into one exit pointsay that energy passes through the antenna, not electrons
WE 2
Distinguish the two photosystems
State two differences between photosystem I and photosystem II. (2 marks)
Difference 1: absorption peak
PSII absorbs maximally at 680 nm and PSI at 700 nm.
Difference 2: replacing electrons
PSII replaces its lost electrons from the photolysis of water, whereas PSI receives them from the electron transport chain.
Different peaks, and different electron sourcesthe order of use — PSII first — would also be accepted
💡 Exam tips
Use both parts of the structure by name: antenna complex and reaction centre.
The reaction centre pigment is always chlorophyll a.
Learn P680 = PSII and P700 = PSI, and that PSII acts first.
Use the term photoactivation for the excitation and release of the electron.
Say the chlorophyll is left oxidised and must have its electron replaced.
Locate photosystems in the thylakoid membrane.
⚠ Common mistakes
Saying PSI acts first. The numbering reflects discovery, not sequence.
Saying electrons pass through the antenna complex. Energy does; electrons leave only from the reaction centre.
Calling the accessory pigments useless. They widen the range of wavelengths captured.
Saying light is converted into ATP at the photosystem. Light excites an electron; ATP comes later.
Placing photosystems in the stroma. They are membrane structures.
Thinking P680 and P700 are different pigments. Both are chlorophyll a in different protein environments.
Up next: Light-Dependent Reactions. Photosystem II has just thrown away an electron and has no way to work again until it is replaced. The next page shows where that replacement comes from — and it is the source of every oxygen molecule you have ever breathed.
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