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Flashcards in The mechanism of Photosynthesis Deck (14):

What are the two stages of photosynthesis

- the light-dependent stage
- the light-independent stage


What does the light dependent stage consist of

- light harvesting at photosystems
- photolysis of water
- photophosphorylation
- the formation of reduced NADP


What are the two types of photosystem

- photosystem I (PSI) - the pigment at the primary reaction centre is a type of chlorophyll a which has a peak absorption of red light at 700nm (P700)
- Photosystem II (PSII) - the pigment at the primary reaction centre is a type of chlorophyll a this has a peak absorption of red light at 680nm (P680)


What is the role of water in the light dependent stage

- in PSII there is an enzyme that in the presence of liht splits the water molecules into protons, electrons and water this is called photolysis
- some of the oxygen is used for aerobic respiration but during periods of high light intensity the rate of photosynthesis is greater than the rate of respiration so most of the oxygen will diffuse out of the stomata


write the equation for photolysis

2H2O = 4H+ + 4E- + O2


What is the role of water in plants

- the source of protons
- donates electrons to chlorophyll to replace those lost when light strikes chlorophyll
- source of the by-product oxygen
- keeps plant cells turgid enabling them to function


What is photophosphorylation

- the generation of ATP from ADP and inorganic phosphate in the presence of light


What are the two types of photophosphorylation

- non- cyclic photophosphorylation - this involves PSI and PSII, it produced ATP, oxygen and reduced NADP
- cyclic photophosphorylation - this involves only PSI and it produces ATP but in smaller quantities than are made by non-cyclic photophosphorylation
- they both involve iron containing embedded proteins in thylakoid membranes that accept and donate electrons forming an electron transport system


describe the stages of non-cyclic photophosphorylation
(The light-dependent stage of photosynthesis)

1. when a photon of light strikes PSII, energy is channeled to the primary pigment reaction centre
2. the light energy excites a pair of electrons inside the chlorophyll molecule
3. the energised electrons escape from the chlorophyll molecule and are captured by an electron carrier this has a protein with iron at its centre which is embedded in the thylakoid membrane
4. these electrons are replaced by electrons from photolysis
5. when the iron ion combines with an electron it becomes reduced and it can then donate the electron becoming reoxidised to the next electron carrier in the chain
6. as electrons are passed along a chain of electron carriers embedded in the thylakoid membrane at each step energy associated with the electrons is released
7. the energy is used to pump protons across the thylakoid membrane into thylakoid space
8. the electrons are then captured by another molecule of chlorophyll a in PSI these electrons replace those lost from PSI due to excitation by light energy
9. a protein iron sulfur complex called ferredoxin accepts the electrons from PSI and passes them to NADP in the stroma
10. as protons accumulate in the thylakoid space a protein gradient forms across the membrane
11. protons diffuse down their concentration gradient through special channels in the membrane associated with ATP synthase enzymes and as they do the flow of protons causes ADP and inorganic phosphate to join forming ATP
12. as the protons pass through the channel they are accepted along with electrons by NADP which becomes reduced, the reduction is catalysed by the enzyme NADP reductase


Describe cyclic photophosphorylation
(The light-dependent stage of photosynthesis)

- only uses PSI as light strikes a pair of electrons in the chlorophyll molecule at the reaction centre gains energy and becomes excited
- they escape the chlorophyll and pass to an electron carrier system and then pass back to PSI
- during the passage of electrons along the electron carriers a small amount of ATP is generated however no photolysis of water occurs so no protons or oxygen are produced, no reduced NADP is generated
- chloroplasts in guard cells contain only PSI, they produce only ATP which actively brings potassium ions into the cells lowering the water potential so that water follows by osmosis causing the guard cells to swell opening the stomata


What is the role of carbon dioxide in the plant

- carbon dioxide is found in organic molecules, and may be used as structures such as cell membranes, antigens, enzymes or act as energy stores
- it enters the leaf through the stomata and diffuses through the spongy mesophyll layer to the palisade layer in palisade cells through the thin cellulose cell walls and then the chloroplast envelope into the stroma
- fixation of carbon dioxide in the stroma maintains a concentration gradient that helps diffusion


Describe the Calvin cycle
( The light-independent stage)

1. carbon dioxide combines with a carbon dioxide acceptor which is a five carbon compound called ribulose biosphosphate ( RuBP), this reaction is catalysed by RuBisCO
2. RuBP by accepting the carboxyl COO- group becomes carboxylated and forms an intermediate six carbon compound that breaks down
3. the product of this reaction is two molecules of a three carbon compound GP, this carbon dioxide is now fixed
4. GP is then reduced using hydrogen ions from NADP (this is made in the light dependent stage of photosynthesis) to triose phosphate (TP), energy from ATP is also made during the light dependent stage and is used at this stage at the rate of 2 molecules of ATP for every molecule of carbon dioxide fixed at stage 3
5. in 10 of every 12 TP molecules the atoms are rearranged to regenerate 6 molecules of RuBP, this process requires phosphate groups. Chloroplasts only contain low levels of RuBP as its continually being converted to GP but is also being regenerated, the remaining two of the 12 molecules of TP are the product


Why does the Calvin cycle only run during daylight
( The light-independent stage)

- products of the light dependent stage such as ATP and reduced NADP are needed for it to run
- during the light dependent stage hydrogne ions are pumped from the stroma into the thylakoid spaces so the concentration of protons in the stroma falls and the pH raises to 8, this is the optimum for RuBisCO which is also activated bye extra ATP in the stroma
- in daylight the concentration of magnesium ions increases in stroma these attach to the active site of RuBisCO and act as cofactors which activate it
- Ferredoxin that is reduced by electrons from PSI activates enzymes that are involved in reactions of the Calvin cycle


What are the uses of triose phosphate

- syntheise organic compounds
- some glucose is converted to sucrose and some to starch and some of cellulose
- some TP is used to synthesise amino acids, fatty acids and glycerol
- rest of TP is used to regenerate the supply of RuBP ,