Photosynthesis

Photosynthesis

1. Importance

   • Autotroph - Organisms that use light energy or chemical energy & inorganic molecules to synthesise complex organic molecules
   • Heterotroph - Organisms that ingest & digest complex organic molecules, releasing the chemical potential energy stored in them
   • Respiration in plants & animals depends on products of photosynthesis
   • Oxygen and C6H12O6 produced, used in aerobic respiration
     • Produce energy and CO2 and H2O

2. Chloroplasts

   • 2-10um long
   • Surrounded by double membrane (envelope)
   • Intermembrane space
   • Outer membrane permeable to many small ions
   • Inner membrane less permeable to small ions & has transport proteins embedded in it, folded into lamellae which are stacked into grana
     • Grant increase surface area for photosynthetic pigments, electron carriers & ATP synthase enzymes for LDR
   • Intergranal lamellae between grana
   • Photosynthetic pigments - Molecules that absorb light energy, each pigment absorbs a range of wavelengths in visible region, others are reflected
   • Proteins embedded in grana hold photosystems in place
   • Stroma contains enzymes for LDR
   • Grana surrounded by stroma so products of LDR can easily pass to stroma for LIR
   • Photosynthetic Pigments
     • Chlorophylls
       • Long hydrocarbon chain & porphyrin group (similar to haem group but Mg instead of Fe)
       • Light hitting it causes pair of e- associated with Mg to become excited
       • Two forms of chlorophyll a -P680 and P700
         • Both absorb red light at slightly different wavelength
       • Found in primary pigment reaction centre
       • P680 in photosystem II
       • P700 in photosystem I
       • Chlorophyll a also absorbs blue light of wavelength 450nm
       • Chlorophyll b (blue-green) absorbs wavelengths around 500nm & 640nm
     • Accessory Pigments
       • Carotenoids reflect yellow & orange, absorb blue
       • Do not contain porphyrin group & not directly involved in LDR
       • Absorb wavelengths not well absorbed by chlorophylls & pass on energy from that light to chlorophyll a at base of photosystem
       • Carotene & Xanthophyll are main carotenoid pigments

3. Light Dependent Stage

   • Role of water
     • 2H2O ---> 4H+ + 4e- + O2
       • Some of this oxygen for respiration, but most diffuses out of leaf
       • H+ for chemiosmosis
       • e- replace those lost by oxidised chlorophyll
   • Photophosphorylation
     • Cyclic
       • Only uses PS I
       • Excited e- pass to electron acceptor & back to chlorophyll from which it was lost
       • Small amount of ATP made
       • No photolysis of water & no reduced NADP
     • Non-cyclic
       • Uses PS iI and PS I
       • 1. Light hits PS II exciting pair of e- that leave chlorophyll from primary pigment reaction center
         • 2. e- pass along chain of e- carriers & energy released as its used to synthesise ATP
           • 3. Light struck PS I & pair of e- lost
             • 4. e- along with H+ (from PS II photolysis) join NADP making reduced NADP
               • 5. e- from oxidised PS II replace e- lost from PS I
                 • 6. e- from photolysed water replace those lost by oxidised chlorophyll in PS II
                   • 7. H+ from photolysed water take part in chemiosmosis to make ATP & then captured by NADP

4. Light Independent Stage

   • Role of CO2
     • source of carbon
   • Calvin Cycle
     • 1. CO2 form air diffuses into leaf through stomata, diffuses into stroma
       • 2. In stroma combines with 5C compound, ribulose bisphosphate (RuBP) a CO2 acceptor, to carboxylate RuBP
         • Rubisco (enzyme) catalyses
         • 3. Product is two molecules of 3C compound glycerate 3-phosphate (GP), CO2 has now been fixed
           • 4. GP reduced & phosphorylated to another 3C compound, triose phosphate (TP), ATP & reduced NADP from LDR used
             • 5. Five out of every six molecules of TP recyled by phosphorylation, using ATP from LDR, to 3 molecules of RuBP
   • How products of calvin cycle are used
     • GP
       • Amino acids
       • Fatty acids
     • TP
       • Pairs form 6C sugars e.g. glucose
         • Some glucose isomerased to fructose
         • Fructose & glucose combine to form sucrose
         • Hexose sugars polymerised into other carbohydrates e.g. cellulose & starch
       • Converted into glycerol
         • Glycerol combined with fatty acids from GP to make lipids

5. Limiting Factors

   • Calvin Cycle
     • CO2
       • More CO2 leads to more carbon fixation
       • More CO2 fixation leads to more GP hence more TP and more regeneration of RuBP
       • If not enough CO2, RuBP can't be converted to GP then TP so RuBP levels go up, GP and TP levels go down
     • Light Intensity
       • More light to excite more e-
       • e- take part in photophosphorylation so more e- means more ATP and reduced NADP
         • Used in LIR to reduce GP to TP
         • ATP also makes TP into RuBP
     • Temperature
       • Little effect on LDR
       • Effect on LIR
       • As temp rises above 25 degrees, oxygenase activity of rubisco increases more than carboxylase activity increases
         • Photorespiration exceeds photosynthesis
         • ATP & reduced NADP from LDR are dissipated & wasted reducing rate of photosynthesis
         • Very high temp may denature enzymes
         • High temp means loss of water by transpiration, may lead to closure of stomata
   • Rate of Photosynthesis
     • CO2
       • Introduce more in greenhouse by burning methane
       • High levels give high rate of photosynthesis
     • Light intensity
       • Causes stomata to open, so CO2 can diffuse in
       • Trapped by chlorophyll where it excites electrons
       • Splits water molecules to produce protons
       • Rate of photosynthesis varies throughout day as levels of light change
     • Temperature
       • Enzyme-catalysed reactions in calvin cycle
       • Too high & water is lost from stomata leading to stress response to close stomata
       • Rate increases at temp increases, until 25 degrees

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