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5.2.1 Photosynthesis Reaction (non-cyclic photophosphorylation (involves…
5.2.1 Photosynthesis Reaction
Light dependant stage
photolysis of water
enzyme splits water molecules in the presence of light
2H₂O → 4H⁺ + 4e⁻ + O₂
oxygen produced is used for aerobic respiration, or diffuse out of the leaves via the stomata
roles of water:
source of protons used in photophosphorylation
donates electrons to chlorophyll to replace those lost when light strikes chlorophyll
source of oxygen
keeps plant cells turgid so they can function
photosystems
contains around ~300 molecules of chlorophyll that trap photons and pass energy to a primary pigment reaction centre, a molecule of chlorophyll a, during the light independent stage
PSII primary pigment is a type of chlorophyll a; peak absorption of red light at 680nm wavelength
system of photosynthetic pigments found in thylakoids of chloroplasts;
PSI primary pigment is a type of chlorophyll a; peak absorption of red light at 700nm wavelength
non-cyclic photophosphorylation
involves PSI and PSII
produces ATP, oxygen, and NADPH
1) photon of light strikes PSII and energy is channeled to primary pigment reaction centre
2) light energy excites pair of electrons inside chlorophyll molecule
3) excited electrons escape from chlorophyll and are captured by an electron carrier
4) electrons are replaced by electron from photolysis
5) iron ion combines with electron to become Fe²⁺, and then reoxidised to Fe³⁺ to next electron carrier in the chain
6) energy is released as electrons are passed along electron carrier chain
7) energy is used to pump protons across thylakoid membrane
8) electrons are captured by another molecule of chlorophyll a in PSI; electrons replace those lost to excitation
9) ferredoxin (protein-iron-sulfur complex) accepts electrons from PSI; passes them to NADP in stoma
10) proton gradient forms across membrane as protons accumulate in thylakoid space
11) protons diffuse down concentration gradient through special channels associated with ATP synthase enzymes; ADP and Pi join to form ATP
12) protons are accepted with electrons by NADP, which is reduced to NADPH; catalysed by enzyme NADP reductase
Photophosphorylation
production of ATP from ADP and inorganic phosphate in the presence of light
electron carriers: molecules that can accept one or more electrons then donate those electrons to another carrier; iron-containing proteins embedded in thylakoid membranes are electron carriers and form an electron transport chain or system; ferredoxin, NAD and NADP are also electron carriers
chloroplasts in guard cells contain only PSI, so only ATP is generated, which actively transports potassium ions into cell, lowering water potential so water flows in via osmosis; guard cells swell up and stomata opens
cyclic photophosphorylation
involve only PSI
produce small amount of ATP
1) photon of light strikes PSI and energy is channeled to primary pigment reaction centre
2) light energy excites pair of electrons inside chlorophyll molecule
3) excited electrons escape from chlorophyll and are captured by an electron carrier
4) electrons are passed back to PSI; small amount of ATP is generated when passing along electron carrier chain
Light independent stage
carbon dioxide fixation
uses of triose phosphate (TP)
starting material for synthesis of carbohydrates, lipids and amino acids
recycling of TP to regenerate the supply of RuBP; five molecules of TP interact to form 3 molecules of RuBP
some glucose is converted to sucrose, starch, cellulose
some TP is used tp synthesise amino acids, fatty acids, and glycerol
how products of light dependant stage are used in the light independent stage
in stroma of chloroplasts
Calvin cycle
triose phosphate (TP): a three-carbon compound, and the product of the Calvin cycle; can be used to make other larger organic molecules
ribulose biphosphate (RuBP0): a five-carbon compound present in chloroplasts; a carbon dioxide acceptor
glycerate-3-phosphate (GP): an intermediate compound in the Calvin cycle
metabolic pathway of the light-independent stage of photosynthesis, occurring in the stroma of chloroplasts where the carbon dioxide is fixed, with the products of the light-dependent stage, to make organic compounds; also occurs in many photoautotrophic bacteria
carbon dioxide combines with RuBP and is catalysed by the enzyme RuBisCo
RuBP becomes carboxylated by accepting the carboxyl (COO-) group forming an unstable intermediate six carbon compound that immediately breaks down
Two molecules of GP are formed; carbon dioxide has been fixed
GP is reduced using hydrogen from the NADPH made during the light dependent stage to TP; energy from ATP also from the LDR is used at the rate of two molecules of ATP for every molecule of carbon dioxide fixed
In 10 of every 12 TP molecules, the atoms are rearranged to generate 6 molecules of RuBP; process requires phosphate groups; the other 2 TP groups are the product. Chloroplasts contain only low levels of RuBP as it is continually being converted to GP but is also being regenerated
Sammer Sheikh