Make your own free website on Tripod.com

Photosynthesis and Photosynthesis in Bacteria

Chlorophyll

  1. Light Harvesting/Energy Transmission
  2. Photosynthetic Electron Transport
Chlorophyl is an alpha magnesium porphyrin with extensive delocalsiation of the pi bonds enables the molecule to abosrb visible light in the 400 to 700 nm region:

Excited States (10K- but rather nice)

The excited state looses energy by slowly droping back to the ground-state slowly emitting energy.

Carotenoids

  1. Absorb light / energy transmission to the reaction centre
  2. Protect against photo-oxidative damage.

Carotenoids are polyenes:

Carotenoid type Polyenes

Carotenoids and Chlorophyll absorbance

All the pigments are held non-covalently inside proteins in special environments. This changes the properties and the absorbance values.

Several peaks arrive from one pigment due to variation in the protein environments.

The Action Spectrum

The action spectrum is rate of photosynthesis as a function of wavelength of exciting light.

Action Spectrum(20K, but gorgeous)

The Photosynthetic Antennæ

These are the "light harvesting apparatus"; they absorb incoming photons and transmit the energy to pigments in the reaction centre. The antennae surround the reaction centre in the membrane and they are in huge excess over the reaction centre membranes. Photosynthetic antennae are hydrophobic transmembrane helices, and isolating them requires detergetns.

LHCII (Light harvesting Complex II) is the most abundent of the complexes, and is the only protein to bind to chlorophyll B. It has a mass of 25000, and consists of 8 chlor a and 7 chlor b. The structure was solved by crystalisation.

Light Harvesting Proteins

There are two types: General LH Proteins (for example LHCII) and those associated either with the Reaction Centre (RC) of Photosystem I (PS I) or of Photosystem II (PS II).

Flow of Energy

The Mechanism of Energy Transfer

The photon is absorbed by the pigment, and the enregy transferred to another neighboring pigment, this is repeated many times, until the energy of a photon reaches a chlorophyll in the reaction centre protein (called ‘P’). The energy is trapped there. In PS I it is P700 and in PS II it is P680. The photon is not transferred, nor is it the electron.

It is the EXCITED STATE that is transferred.

The Mechanism Of Energy Transfer Through the Antannae

Arrangment of LH Proteins

The first excieted state has a lifetime of around 10-8 sec.
The second excited state has a lifetime of approx 10-11 seconds.

The first excited state is the most important in energy transfer between chlorophyll molecules. The proximity of the chlorophyll is important also because the energy ‘jumps’ fromone molecule to another by Förster Resonance Energy Transfer.

The dipole oscillation associated with the de-excitation as one molecule looses it's energy is coupled to a symathetic excitation in another molecule causing that to become excited. This is highly distance dependent: 1/R6.

The diraction the energy takes would be random, but directionallity is imposed by the core LH proteins. THe chlorophyll molecules in the core proteins has a lower first excited state than the chlorophyll in th LHC II proteins.

This leads to the trapping of the photon in the core antannae. Once in the core the energy is trapped. Due to the lower excited state of the Reaction Center chlorophyll (“P”) the excited state remains on P for a longer time.

The longer life of the excited state allows photochemistry to occur - The light absorbance at the cones' are different for the two photosystems:

The core LH proteins of system II absorb (relatively) short wavelength light, at <680nm.

The core LH proteins of system one ablsorb light of >680nm. Generally the long wavelength drives psI.

The plant can regulate the number of photons entering each photosystem.

Studies in Spectroscopy of Photosynthesis

Studies of Photosynthesis

The absorbance changes which occur when photosynthesis takes place are measures (this is highly complex!)...There are two changes which occur in th red end of the spectrum:

Absorbance changes are complete in less than one nanosecond. In the dark you can mimic absorbance changes by adding an oxidising agent to the suspension. the absorbance changes must only affect as small fraction of the chlorophyll present.

They take place with a high quantum efficency; 1 chlorophyll changes per photon absorbed. The absorbance changes are associated with reaction centre proteins.

You can purify a protein which does the above. ‘Special’ chlorophyll in the reaction centres (P700 & P680) is rapidly photooxidised by light. Any reaction centre can be supplied with energy, Pxxx goes into the excited state, from which it can give an electron. If P is giving off an electrion something close to P must accept it.

P goes to P*....

(For P700 the values are slightly different)

In the dark, P is in the ground state and is a poor reducing agent; the redox couple P/P* is high (Em=+0.5V).

When excited P* is a very strong reducing agent; P*700/ P+700is low (Em=-0.5V).

P700 -> p700* etc.

Since 1 electron is moved through 1.0V, the energy required is 1eV; the input energy is (1/1.5)x100. This gives an efficency of 70%!

The values are similar for P680, but P680 is an excellent oxidising agent Em~1V.

Diagram of Eh Relationships

Overall:

H2O + NADP+ --2hv--> NADPH + O2 (Unbalanced).

Accompanting these electron transport reactions Hydrogen ions are pumped across them thylakoid membrane.

Experimental

The Red-Drop

Absorbance and Action SpectrumPS I absorbs predominantly at long wavelengths (680nm), whereas PS II absorbs at <680 nm. However, short wavelength (<680nm) can go to both photosystmes - long wavelength light cannot be used to drive PS II.

In the diagram on the left (or above), the blue underlined section shows where both photosystems are active, the red underlined section shows wher only PS I is operating - hence the water is not oxidised and so the efficency drops.

The Quantum Efficency of Photosynthesis

Need 8 Quanta to produce 1 oxygen molecule, why is this?

2 H2 --> 1O2 + 8e-

2 H2 --> O2 + 4e- + 4H+

Therefore 8 quanta are required as each photosystem must be driven 4 times.

The behaviour of Cytochrome F in Elecron Transport

A- + cyt (FeIII) --> A+ + cyt (FeII)

  cyt (FeII) + B --> cyt (FeIII) + B-

C-Type Cytochrome

Experiment - Measure the oxidation states fo cytochrome f when thylakoid membranes are illuminated - measure at 550nm.

Capital DeltaA at 550nm will decrease as cytochrome f is oxidised:

Absorbance @ 550nm

Activation of PS I leads to oxidation of P700, P700+ pushes electrons out of cyt. f and into the electron transport chain.

When PS II is activated the electrons are pushed into XII and on into the electron transport chain, there is some reduction occuring.

DCMU (Dichloromethylurea) blocks electron transport between P680* and XII.

Oxygen Evolution

2 H2 --> O2 + 4e- + 4H+

The generation of P680 os a Single Electron Event.

P680 --hv--> P680+ + e- (The electron is passed into X).

Measuring oxygen production via thylakoids or algae after single flashes of light:

The duration of the flash is short enough that the process is drive through only one cycle - it is high intensity so that all the sample RC's undergo the cycle. The Oxygen yeild during the flashes requires a rapidly responding oxygen electrode.

Oxygen yeild against flash number There is a periodicity of 4 flashes to the yeild of oxygen. After each flash an oxidisng equivalent is stored in the oxygen evolving protein. After 4 such events the protein reacts with H2O to yeild O2

Arrangment of Membrane Proteins