photosythesis Flashcards
basic equation
CO2 + H2O –> (CH2O) +O2
photosynthesis
the process by which plants utilise light energy to make ‘food’ for themselves
CH2O represents
carbohydrate
takes place
chloroplast
photosynthesis over view
-light photon is absorbed -enery used to drive an electron from water to generate NADPH -also drives protons across a membrane -these protons drive ATP synthesis -The ATP and NADPH are used in the light independent reaction to fix CO2
light dependent reaction
purpose is to produce ATP and NADPH for the calvin cycle
calvin cycle
light independent cycle–> CO2 fixation
chloroplasts
the energy of light captured by pigment molecules, called chlorophyll, in chloroplasts is used to excite electrons giving them great reducing potential
trapping light energy
-photoreceptor molecule- chlorophyll a - a substituted tetrapyrole- 4 nitrogen atoms co-ordinate a magnesium ion
structure of chlorophyll a
substituted tetrapyrol -4 nitrogen atoms coordinately bonded to a magnesium ion
photoreceptor molecule
chlorophyll a
how come chlorophyll is a good photoreceptor
has strong absorption bands int he visible region of the spectrum
what happens when light is absorbed by chlorophyll
energy from light excites an electron from its ground energy level to an excited level.
what happens to the excited electron
if a suitable electron acceptor is enaryby, the excited electron can move from the initial molecule to the acceptor molecule. -this results in a positive charge on the initial molecule and a negative on the charged acceptor molecule
photoindced charge seperation
initial molecule becomes positive and the acceptor molecule becomes negative
the site where the separation occurs i called the
special pair of chlorophylls in the reaction centre
in plants there are two kinds of membrane bound light sensitive complexes
- photosystem 1 2. photosystem 2
which comes first in the LIR chain
PS 2 (only named this because it was discovered second to PS 1)
Photosystem 1
p700 -responds to a light with wavelengths <700nm
photosystem 2
p680 -responds to light with wavelengths <680nm
the two photosystems generate a
proton gradient
the proton gradient is used to
release ATP from ATP synthase -remember H+ do not produce ATP in ATP synthase, they just release it
electrons are derived from
H2O–> photolysis
electrons derived from water are used to..
reduce NADP+
electron flow
first flow through PS2, then though the cytochrome bf (a complex homologous to Q cytochrome c oxidoreductase) , and then through PS PS 2–> cytochrome bf —> PS1
cytochrome bf
a complex homologous to Q-cytochrome c oxidoreductase
photosynthetic sequence
photosynthetic- membrane organisation
reaction that occurs at PS2
2H2O + 4hv –> 4H+ + 4e- + O2
by product of Light independent reaction
oxygen
reaction that occurs at PS1
4e-+ 2H+ + 2NADP+ + 4hv –> 2NADPH
overall LIR
2H2o + 2NADP+ + 8hv –> 2H+ + O2 + 2NADPH
how many photon need to be absorbed to make 2 nadph molecules
8
proton gradient across thylakoid mem linked to ATP synthesis- experiment
1966- Andre Jagendorf
- -thylakoid membrane were soaked in pH4 buffer for several hours
- -then rapidly submerged in pH8 buffer containing ADP and Pi
- -the pH inside the thylakoid initially remained at pH 4
- -a burst of ATP production was notes that accompanied the disappearance of pH gradient
why did a burst of ATP production accompany a disappearance of the pH gradient
due to the process of ATP production involving H+n movement through ATP synthase from one side of the thylakoid to the other
ATP synthase resembles those in mitochondria
CF1 -CF0 complex -orientation is reversed -protons flow out of the thylakoid lured -whereas in mitochondria, protons flow in to the mitochondrial matrix
diff between ATP synthase in mitochondria and chloroplast
-protons flow out of the thylakoid lime in chloroplasts -protons flow in the mitochondrial matrix -orientation is reveres
where are ATP and nADPH released into
the stromal space ready for the dark reactions (CO2 –> carbohydrate)
carbon dioxide fixation -Calvin cycle occcurs
-occurs int he chloroplast stroma
the calvin cycle requires
2NADPH and 3ATP per CO2 fixed
calvin cycle
pic
description of the CC
-CO2 enters and joins with rubisco to form a short lived intermediate. -this then becomes a three carbon molecule called 3-phosphoglycerate -Then 6ATP are oxidised to 6ADP releasing energy to form 1,3bisphosphoglycerate -this is then reduced when 6NADPH is reduced to 6NADP+ to form glyceraldehyde-3-phosphate -at this point a G3P molecule is lost to be used to create glucose and the organic compounds -then regeneration occurs where RuBP is formed–> this involves the reduction of 3ADP to form 3ATP
net production in calvin cycle
net production of 1G3P molecule requires 9ATP and NADPH
how does the LIR have enough NADP+ and ADP
ATP and NADPH used in the calvin cycle returns NADP+ and ADP to the light reaction
three parts to the CC
fixation, reduction and regeneration
CO2 is fixed by
rubisco (most abundant enzyme on earth) , which reacts CO2 with Ribulose bisphosphate to produce two molecules of 3 phosphoglycerate
3-phosphoglycerate is phosphorylated by
ATP to form 1,3, bisphosphoglyercate
1,3,bisphosphoglycerate is reduced to
glyceraldehyde-3-phosphate by NADPH
when RuBP is regenerated the byproduct is
sucrose or starch
ribulose bisphosphate carboxylase oxygenase
RUBISCO– most abundant protein int he world- 50% of the soluble protein in a lead
regulation of CO2 fixation
-CC does not occur in the dark because if they did then the necessary ATP and nADH would have to eb provided by metabolism of stood carbohydrates–> quite recycling
why does the CC not occur in the dark
would have to metabolise stored carbohydrates to provide enough ATP and NADPH–> futile recycling
key enzymes of the calvin cycle that are regulated are
-rubisco -fructose 1,6,bisphosphatase (FBPase) -sedoheptulose bisphosphatase (SBPase) these enzymes are found in the stroma
control of Rubisco, SBPase and FBPase activity
all three enzymes cycle between active forms in the light and inactive forms in the dark -pH optima=8 (for all). In the dark the pH of the storm is 7. Upon illumination, H+ are pumped out of the storm into the thylakoids causing stroll pH to increase to * -pumping of the +vely charged protons into the thylakoids during the illumination causes Mg2+ to leave the thylakoids, consequently stroll [Mg2+] increases. Mg2_ activates all three enzymes -
all three enzymes (rubisco, FBPase, SBPase) are activated by
Mg2+ - so when +vely charged protons are pumped into the thylakoid during illumination, Mg2+ leaves the thylakoids and the stroll [Mg2+] increases
pH and enzyme activity
All three enzymes have pH optima=8. In the dark the pH of the stroma is ~7. Upon illumination, H+ are pumped out of the stroma into the thylakoids causing the stromal pH to increase to ~8.
how is the active form of rubisco achieved
CO2 (not find) reacts with active site lysine of rubisco to make carbamate this is stabilised by Mg2+ to give the active form of rubisco
some CC enzymes (FPase and SBPase) can ago through..
disulphide bridge to thiol transitions due to having cis residues. The e- for reusing thioredoxin are supplied by Ps1 via ferredoxin
one molecule of triode P is produced for every
3CO2 fixed
