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Photosynthesis

Light dependent stage:

  • -  Occurs in: thylakoids (grana)

  • -  Involved direct use of light energy

  • -  Overview:

  • -  Light harvested by photosystems

  • -  Photolysis of H2O

  • -  Photophosphorylation to produce ATP

  • -  Reduced NADP formed

  • -  In Photosystem I → chlorophyll a in form of p700, thus peak absorption is 700 nm (red light)

  • -  In Photosystem II → chlorophyll a in form of p680, thus peak absorption of 680 nm (red light)

  • -  Photolysis: Occurs in PSII as there is an enzyme which in the presence of light splits a water molecule into into protons, electrons and electrons

  • - Eq:2H2O→4H++4e-+O2

  • -  Some of the oxygen produced used in respiration (aerobic) but majority will diffuse out of

  • leaves as rate of photosynthesis can be higher than rate of respiration

  • -  The Hydrogen ions (aforementioned protons) will be used for photophosphorylation

  • -  The electrons will be donated to the chlorophyll to replace those electrons that were lost

  • when being energised by light energy

  • Photophosphorylation:

  • -  2 types: cyclic and non-cyclic phosphorylation

  • - Non-cyclic phosphorylation - acts to produce ATP, Oxygen and reduced NADP and involves both photosystem I and photosystem II

  • - The Process: Non-cyclic

  • -  Light strikes accessory pigments within PSII (P680) and carotenoids which leads

  • to electron excitation in these pigments and thus the electrons are excited into a higher energy level and fall back down releasing energy which travels through the photosystem via other pigments to the primary pigment

  • -  This causes excitation of a pair of electrons in the chlorophyll molecule

  • -  These electrons are excited and escape the chlorophyll and are captured by an

  • electron carrier which is a protein with an iron ion at its centre, it is found

  • embedded in the thylakoid membrane

  • -  The electrons lost are then replaced by the electrons of photolysis

  • -  The electron reduces the Iron ion to an Fe2+ state

  • -  The electron carrier can then donate the electron to the next electron carrier in

  • the chain becoming reoxidised in the process. This is the method of action of the

  • electron transport chain (ETC)

  • -  Electrons are passed along the ETC and with each passing on of electrons some

  • energy is released and this energy is used to pump H+ ions across the thylakoid membrane into the thylakoid space

  • -  The electrons are passed along the ETC in the thylakoid membrane until they are captured by another molecule of chlorophyll a in PSI (the e- replace the electrons lost by PSI from prior excitation)

  • -  A protein-iron-sulphur complex, ferredoxin, accepts the electrons from PSI and passes the electrons to the NADP which in the Stroma

  • -  At the same time protons have accumulated in the thylakoid space due to the pumping of H+ out by the released energy of the ETC, this creates a concentration gradient/proton gradient across the membrane

  • -  Protons diffuse down the concentration gradient through protein channels that are associated with the ATP synthase enzyme. In doing so they stimulate the bonding of ADP and inorganic phosphate joining and forming ATP

  • -  As the protons pass though the channel they then reduce NADP by bonding with NADP and electrons to form reduced NADP in the presence of the enzyme NADP reductase

  • -  The rNADP and ATP are now in the stroma for the Light Dependent stage

  • -  The Process: Cyclic photophosphorylation

  • -  Utilises only PSI

  • -  As light strikes PSI the same process occurs as in PSII and the electrons

  • are excited and escape from chlorophyll a (P700)

  • -  They are then passed through an electron carrier chain by the same

  • mechanism as above and are returned to PSI

  • -  This enables a small amount of ATP to be generated through the passing

  • through the ETC

  • -  Extra: Chloroplasts in the guard cell contain only PSI as they are only

  • required to produce ATP to actively transport potassium ions into the cell to lower WP and allow water in by osmosis to enable the stoma to open

  • Light Independent Stage:

  • -  Known as the Calvin Cycle

  • -  Requires CO2

  • -  Occurs in the Stroma

  • -  Although does not require light directly it is reliant on the products of the light dependant

  • stage and so to a degree does require light

  • -  Overview:

  • -  Carboxylation of RuBP to form GP

  • -  Reduction of GP to to TP

  • -  Combination of 2 x TP 2 out of every 12 turns to form Glucose

  • -  The Process:

  • - Carbon dioxide (brought in through the stoma and responsible for all organic

  • molecule production in a plant such as structures like cellulose cell walls or energy stores such as starch) combines with a 5 carbon compound calles Ribulose Bisphosphate (RuBP) in a reaction catalysed by RuBisCo (Ribulose bisphosphate carboxylase oxygenase)

  • -  RuBP is thus carboxylates (has a COO- group) which forms an unstable 6 carbon compound which thus breaks down immediately

  • -  This produces 3 molecules of a three carbon intermediate of Glycerate-3-phosphate) [GP or G3P] and this is the process of carbon dioxide fixation

  • -  This GP is reduced using 2 Hydrogens from the rNADP (from the light dependent stage) This also uses 2 molecules of ATP energy (also generated in Light dependent stage) [2 X rNADP & 2 x ATP for each CO2]

  • -  In 10 out of every 12 turns the TP rearrange to generate 6 molecules of RuBP which is the regeneration of the starting product enabling the cycle to continue, the remaining 2 TP molecules are the product and may form glucose or be used in other ways:

  • -  Some form glucose which can be converted to sucrose, starch or cellulose

  • -  Some TP may be used to synthesise amino acids, fatty acids or glycero Limiting Factors: Graphs

  • -  The graphs for how light intensity and carbon dioxide concentration have an affect on the levels of TP, GP, RuBP

  • -  Light Intensity:

  • -  In Low Light Levels: The rNADP and ATP reaching the Calvin Cycle will be

  • decreased and as a result there will be no H+ (or very little) to enable the GP to

  • be reduced etc.

  • -  GP cannot be reduced to TP so the levels of TP fall in low light

  • -  When TP levels fall the RubP cannot be regenerated so the relative

  • concentration of RuBP will also fall

  • -  With a fall in TP levels GP accumulates meaning GP concentration increases

  • -  Carbon Dioxide Concentration:

  • -  RuBP concentration will increase as it cannot accept CO2 to be converted to a 6

  • carbon intermediate and then GP

  • -  As a result the GP concentration will fall and as a result the TP concentration will

  • fall

  • -  Due to being unable to bond with CO2 the RuBP cannot be converted and so

  • RuBP concentration will increase

  • -  Water Stress: if not enough water is available then the roots are unable to take up as

  • much water and will not gain adequate levels to replace the water lost via transpiration as a result the cells will lose water and become plasmolysed this stimulates the roots to produce abscisic acid which is translocated to the leaves and cause stomatal closure which reduces gaseous exchange. The tissues will become flaccid and over time the leaves will wilt and photosynthesis rate is hugely reduced

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