Photosynthesis

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  • Photosynthesis
    • Light-dependent Reactions
      • Photolysis - the splitting of a water molecule into 2H+, 2e- and O
        • H+ is used in chemiosmosis then accepted by NADP to form Reduced NADP
        • The oxygen released formsO2
        • Electrons replace those lost from PSII
          • 1- Light hits PSII exciting a pair of electrons which leave chlorophyll from primary pigment reaction center
      • Cyclic Photophosoporylation
        • Only uses PSI (P700)
        • No photolysis required
        • Excited electrons pass down chain of electron carriers
          • Produces extra ATP by chemiosmosis
            • ATP may be used in Calvin  cycle
            • ATP actively transports K+ into guard cells, so they fill with water
        • Produces extra ATP by chemiosmosis
          • ATP may be used in Calvin  cycle
          • ATP actively transports K+ into guard cells, so they fill with water
        • No reduced NADP is formed
      • Non-cyclic Photophosphorylation
        • 1- Light hits PSII exciting a pair of electrons which leave chlorophyll from primary pigment reaction center
        • 2- Electrons pass down chain of electron carriers forming ATP by chemiosmosis
          • These electrons replace those lost in PSI
          • In chemiosmosi H+ ions are pumped into thylakoid space by carrier proteins. they diffuse out through ATP synthase to form ATP
        • 3- Light also hit PSI causing it to loose electrons
          • These electrons join to H+ & NADP forming reduced NADP
    • Limiting Factors
      • Light Intensity
        • Light is needed to: open stomata so CO2 can enter leaves, excite electrons in photosystem, cause photolysis.
        • If only dim light is present, minimal ATP and reduced NADP is formed, so GP accumulates and TP levels fall
      • Temperature
        • Very high temperatures may denature proteins
        • Above 25 degrees, rubisco fixes more 02 than CO2 causeing more photorespiration. So ATP and reduced NADP are wasted
        • May cause water loss if high temperatures
      • CO2 Concentration
        • CO2 is needed to be fixed in RuBP
        • Reduced CO2 conc. means RuBP accumulates and less GP and TP is formed so less food synthesis occurs
      • The factor that is present at the lowest or least favourable value
    • Light -independent reactions
      • The Calvin cycle
        • 1)   CO2 diffuses into leaf through stomata, then into cells then the stroma of chloroplasts
        • 2) CO2 is fixed into the 5C compound RuBP by the rubisco enzyme forming the 3C compound GP
        • 3)  GP is then reduced to form TP using reduced NADP and ATP from the light dependent reactions
        • 4) 5 out of 6  TP molecules are recycled by phosphorylation using ATP to form 3 molecules of RuBP
      • Occurs in the stroma
      • GP from the Calvin cycle can make amino and fatty acids
      • TP can be used to make hexose sugars i.e. glucose
      • Hexose sugars can be isomerised to form otther hexose sugars, These can be used to form disaccherides which in turn can form polysaccharides
      • Photorespiration - the fixing of O2 into RuBP
        • CAM plants must conserve water, so  only open stomata at night.  CO2 is fixed and stored as malate (4C compund). CO2 is then released from malate during the day and fixed into RuBP
        • C4 plants are  efficient (don't waste reduced NADP or ATP) CO2 is fixed by the PEP enzyme to form malate  in cells near the outside of the leaf, But the CO2 is released into cells on the inside of the leaf where it is fixed into RuBP but no O2 is present
    • Chloroplasts
      • Photosystems and Pigments
        • Photosystems are located on thylakoid membrane on grana
        • Funnel shaped system
        • Primary pigment reaction center is the molecule of chlorphyl from which a pair of electrons are lost
        • Accessory pigments include Carotenoids. they absorb light wavelengths poorly absorbed by the chlorophyll. they pass energy onto chlorophyll molecules, They
        • Chlorophyll a is a primary pigment. There are two forms P680 in PSII & P700 in PSI
        • When light hits a pigment molecule it vibrates. It passes the kinetic energy through the photosystem by resonance transfer until it reaches the primary pigment reaction center where a chlorophyll molecule is reduced and looses two electrons from a magnesium ion
      • Structure
        • Has a double membrane
        • 2-10 micro meters long
        • The inner membrane is folded to form grana (made up of thylakoids) & lamellae. the membrane contains photosystems & transport proteins
        • The gap between the two membranes is filled by the stroma. The gap in the inner membrane is the thylakoid space

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