Term 2 Lecture 13: Photosynthesis Flashcards
Definition
Phototrophic organisms (plants, algaes and some bacteria) use photosynthesis to produce organic substances from light, H2O and CO2. heterotrophs depend on these substances in their diet and the O2 produced as a by-product is essential for aerobic life forms
Photosynthesis:
CO2+H2O+light →glucose + O2
Respiration:
glucose + O2 →CO2+H2O
Photosystems
Plants and cyanobacteria have 2 photosystems (l and ll)
Purple photosynthetic bacteria have one known as bacteriorhodopsin.
Photosystems are membrane bound pockets of pigment
Photosystems convert light energy to chemical energy - how much energy is there in a proton?
Energy of a photon (epsilon) is
Epsilon=hv
Where h=Placks constant (6.626x10-³⁴Js)
And v= frequency of light wave (s-1)
v can be converted to wavelength:
V=c/lambda
where c= speed of light (3x10⁸ms-1)
We calculate energies for chemical reactions using units expressed on a per mole (mol-¹) basis so photons must be measured in energy per mole to be comparable.
1 mole of photons = Avogadro’s number
= 6.02x10²³
For photons of wavelength 680 nm (absorption maximum of chlorophyll a) energy in mol-¹ is:
E=NAvo x h x c/lambda Jmol-¹
=6.02x10²³x6.63x10-³⁴
x 3x10⁸/680x10-⁹Jmol-¹
= 1.76x10⁵Jmol-¹ =176kJmol-¹
This is enough energy to synthesise several (at least 3) molecules of ATP from ADP+Pi (∆G in Vivo approx 50kJmol-¹)
However conversion of light to chemical energy is not very efficient
Light reactions in chloroplasts are catalysed by enzymes on the thylakoid membrane. Dark reactions take place in the stroma
In photosynthesis 6CO2 are used to form one hexose molecule. The H for this reaction comes from H2O, molecular O2 is a by-product.
Light is required as H2O is a poor reducing agent.
Light reactions produce substances for use in the dark reaction to fix CO2
In the light reaction:
H2O→2H+ + 2e- + O2
Electrons are excited by light energy and raised to a level high enough to reduce NADP+ to NADPH + H+
The NADPH+ H+ can “fix” CO2 reductively i.e. by incorporating it into organic bonds.
Another product of the light reaction is ATP also required for CO2 fixation.
If the right amount of NADPH +H+, ATP and the required enzymes are available then CO2 fixation can also take place in darkness known as the ‘dark reaction’
The excitation of electrons to form NADPH is a complex photochemical process that involves chlorophyll - a tetrapyrrole dye containing Mg²+ and an extra phytol residue
Light reactions
Light reactions cause electrons to pass from one redox system to the next in an electron transport chain. Direction of transport is the opposite of that in the respiratory chain
> In the respiratory chain electrons from NADH and H+ pass to O2 producing water and energy
> In photosynthesis electrons are taken up from water and transferred to NADP+ with an expenditure of energy (gained from light)
Energy is taken in from light by the photosystems protein complexes containing large numbers of chlorophyll molecules and other pigments.
Another component of the transport chain is cytochrome b/f complex - an aggregate of integral membrane proteins including 2 cytochromes (b563 and f)
Plastoquinone (comparable to ubiquinone) and 2 soluble proteins, copper containing plastocyanin and ferredoxin function as mobile electron carriers.
At the end of the chain an enzyme transfers the electrons to NADP+.
Because photosystem ll and cytochrome b/f complex release protons from reduced plastaquinone into the lumen (via a Q cycle) photosynthetic electron transport establishes an electrochemical gradient across the thylakoid membrane.
Light reactions produce reactants for dark reactions.
This electrochemical gradient is used for ATP synthesis by an ATP synthase.
ATP and NADPH and H+ are needed for the dark reactions and are formed in the stroma.
Ps ll and Ps l function
Ps ll contains an oxygen evolving complex (OEC) which splits water to make O2. It requires an extremely positive redox potential of +1.2 V. The OEC contains an unusual Mn4O5Ca redox centre.
2 excitation processes are required to transfer electrons from H2O to NADP+
After excitation in ps ll E° rises from ~-1V to positive values in plastocyanin and must be increased again in Ps l
In the absence of NADP+ the electron can still be cycled to pump protons for ATP synthesis.
Process:
Ps ll excitation from ~+0.7 V to ~-1.2V
Electrons pass from pheophytin to quinones to the bf complex (where H+ is released) to plastocyanin and on to Ps l
In Ps l excitation from E° ~+0.2V to ~1.3V occurs. Electrons pass from quinones to Fe/S compounds to ferredoxin (Fd) and finally to NADP+
The Fd can feed electrons back to the quinones of Ps ll in the absence of NADP+ in a process known as photophosphorylation.
The electron is returned to Fd via the plastaquinone pool and onto the b/f complex. This type of electron transport does not produce NADPH but does lead to the formation of an H+ gradient and therefore ATP synthesis.
Photosystem 1 structure
Has 12 different protein subunits linked to chromophores filled with chlorophyll some plastoquinones and carotenoids - so there’s a huge number of proton absorbing chemicals present.
Most chlorophyll molecules are ‘antenna pigments’ that collect light energy and conduct it to the reaction centre where an electron is excited and transferred by various steps to a ferredoxin (Fd).
Dangerous radicals can be produced during the light reaction especially singlet oxygens. Carotenoids prevent the formation of these molecules or inactivate them.
Carotenoids are the pigments that colour autumn leaves they become visible as the chlorophyll is broken down for nitrogen retrieval for the tree.
Calvin cycle
Synthesis of hexose from CO2. CO2 fixation (ie incorporation of CO2 into an organic compound) is catalysed by ribulose biphosphate carboxylase aka RUBISco the most abundant enzyme on earth.
RUBISco converts
ribulose 1,5 biphosphate+ CO2+H2O
To 2 molecules of 3-phosphoglycerate
In each cycle 6 ribulose 1,5 biphosphate
Are converted to 12 3-phosphoglycerate
Then into 12 glyceraldehyde 3 phosphate via phosphoglycerate kinase and glyceraldehyde 3 phosphate dehydrogenase.
2 of these are then converted to glucose 6 phosphate via glycogenolysis
The remaining 10 are converted back to 6 ribulose 1,5 biphosphate by phosphoribulokinase and the cycle can begin again.
In the Calvin cycle ATP is required for phosphorylation of 3-phosphoglycerate and regeneration of ribulose 1,5 biphosphate.
NADPH + H+ the second product of the light reaction is consumed in the reduction of1,3 biphosphoglycerate to glyceraldehyde 3 phosphate.
Preventing RUBISco from reacting with O2
RUBISco can react with O2 as well as CO2 creating a 2c and a 3c unit. This is highly wasteful as there’s not much that can be done with the 2c unit. So O2 must be kept away from RUBISco.
This is achieved by containing the RUBISco along with other enzymes inside carboxysomes - protein boxes made up of many subunits that partially protect from O2 exposure
The simplest photosystem: bacteriorhodopsin
A single protein found in archaeal cells.
It is purple and absorbs green light.
It uses the energy from light to pump protons across the cell membrane.
Our eyes have a similar light sensitive protein called rhodopsin in our retinas.
Rhodopsins have 7 transmembrane helixes and contain a molecule of retinal which isomerises when it absorbs a photon of light and drives a change in protein confirmation which transports H+, Richard Henderson used electron microscopy to solve the structure in 1975 receiving a Nobel prize in 2017. The proton gradient produced be bacteriorhodopsin is used to power ATP synthase as in other cells.
Light opens the channel for a proton to be pumped out, the proton then re-enters through ATP synthase.
Why do plants not use bacteriorhodopsin?
Perhaps as it is less efficient- it has only one chromophore so can only utilise a limited range of wavelengths
Rhodopsin in the eye
Rhodopsin has been coopted and used for vision. Absorption of light by retinal causes rhodopsin to change confirmation leading to activation of linked G proteins initiating a signal transduction process resulting in hydrolysis of cyclic GMP which in turn shuts a gated ion channel and generates the stimulus
Bacteriorhodopsin based photosystem is only found in archaea
It is also used as a basis for other light driven ion pumps such as chloride transport in bacteria that survive in high salt solutions
All other phototrophic systems in bacteria algae and other plants use chlorophyll or bacteriochlorophylls rather than bacteriorhodopsin to absorb light
