Lecture 13 - Photosynthesis: Light Reactions

Light Reactions

Occur on the thylakoid membranes, involves several multi-protein complexes

• Two light harvesting photosystems

• Cytochrome b6f

• ATP synthase

Only captures a small percentage of sunlight (5%) to be used for photosynthesis

• Only includes photosynthetic active radiation range of 400nm to 700nm

• Green plants absorb maximally at 680nm to 700nm

• Most sunlight is not usable, reflected, transmitted, loss as heat, or used in metabolism

Photosystems

Packed with chlorophyll molecules. The chloring ring on a chlorophyll molecule will capture photos at the light harvesting complex. This will excite the chlorophyll and it passes this energy by resonance until it eventually reaches a pair of special chlorophyll molecules at the reaction centre.

The pair of chlorophyll will then have an electron go into a higher energy state to become Cl* (reduced essentially). It will then release its energy, which can happen through multiple ways:

• Photochemistry, it loses the electorn and passes it to an electron transporter

• Fluorescence

• Heat (non-photochemical quenching)

• Convert to a damaging triplet state

There are two photosystems, PSII and PSI

PSII contain the special pair of chlorophyll P680

• Once P680 loses its electrons and become oxidised (P680+), it is a very powerful oxidant, powerful enough to srtip electrons from water

• Donates electrons to pheophytin, then to QA, then QB, then plastoquinone

PSI contain the special pair of chlorophyll P700

• Once it is reduced (P700*), it is a very strong reductant, which is required to donate electrons to NADP+ ultimately

• Donates electrons to ferredoxin then to ferredoxin NADP+ reductase

Linear Electron Flow

A linear flow of electrons from PSII, to plastoquinone, to cyt b6f, to PSI, then to NADP+

This sort of flow develops a Z-scheme for the electron

Cytochrome b6f Complex

Similar to the one in the mitochondria’s electorn transport chain. Has the role of pumping 4 protons across the membrane from the stroma into the lumen

• This helps establsih the photochemical gradient that ATP relies on

It also facilitates electron transfers and connects the two photosystems

• Accepts electrons from plastoquinone which gets it from PSII

• Donates electrons to plastocyanin which goes to PSI

• As it does this, it uses the energy from the electron to generate a proton-motive force

Photosynthesis Light Response Curve

Shows relationship between CO2 consumption and light intensity

At low light, there is a linear relationship because light acts as a limiting resource

• At even lower lights, there is negative consumption (CO2 production) due to respiration

At high light, it starts levelling off, suggesting there is a maximum output to photosynthesis

• This is because photosynthetic reaction is determined by light independent reactions, the calvin cycle can be slow (especially RuBisCO

• High light also induces damage to phosynthetic machinery

There is a light compensation point, the amount of light to balance CO2 consumption & respiration CO2 production

There are toher factors that affect photosynthetic efficiency

• Heat, drought, stressors

• Having available pools of ATP/ADP and NADPH/NADP+

Cyclic Electron Transport

Another pathway for electorns to go through. It doesn’t involve PSII at all. The electron will cycle through PSI, back to plastoquinone, through cyt b6f, then to plastocyanin, and finall back to PSI.

It’s main purpose is to help establish the proton gradient (as the normal linear electron flow actually transfer enough protons to make 3 ATP for a cycle of the calvin cycle)

• It also helps balance and alter the ratio of NADPH and ATP

• It does not make NADPH