Photosynthetic pigment extraction

(f) define the term photosynthetic pigment;(g) explain the importance of photosynthetic

pigments in photosynthesis;

What colour should plants growing below the surface of water be??

• Light intensity varies hugely with depth.• Why does the intensity get lower as water depth increases?

Light absorption by chlorophylls

• All chlorophyll is associated with proteins to form light-harvesting complexes in the thylakoid membrane

• There is no free chlorophyll

Structure of LHCII trimer

Kühlbrandt et al. (1994) Liu et al. (2004)

LHCII trimers in grana stack

Light is absorbed by individual chlorophylls in the light-

harvesting complexes

Energy is transferred from one pigment to another via Resonance Energy Transfer

This transfer funnels the energy to a reaction centre where

electron transfer starts

Energy transfer in light-harvesting complexes

Analysis...

Rf value = distance pigment travelled distance solvent travelled

• Carotene (yellow/orange) 0.98• Xanthophylls (yellow) 0.15 or 0.28• Chlorophyll a (yellow/bright green) 0.59• Chlorophyll b (olive or blue/green or yellow/green) 0.42• Pheophytin (olive green) 0.81• Anthocyanin (red/pink/purple/blue) 0.32-0.62

• Photon of light strikes accessory pigment• Light energy is passed down to the reaction centre• 2 electrons are excited and released from the Mg atom

PSI = chlorophyll a P700nmPSII = chlorophyll b P680nm

Low resolution structures of photosystem II

• electron microscopy• membrane preparations• single particles - negative stain

arrangement in thylakoid membrane

Photosystem II - at 3.5Å resolution