Photosynthesis

• Measuring Photosynthesis -photosynthometer

  A) Barrel of Syringe

  B) Plastic tube

  C) Board

  D) Clamp

  E) Stand

  F) Ruler

  G) Capillary tube

  H) Flared

  I) Capillary tube

  J) Water plant

  K) test tube

  L) Beaker

  M) heat

  N) Plunger

  O) syringe

  P) Thermometer

  
• Photosynthesis

  • Physiological process used by plants, algae + some bacteria
  • Convert light energy to chemical; autotrophic nutrition
  • Photosynthesising organisms are ‘photoautotrophs’; ‘producers’ at first trophic level
  • Organisms use chemical energy for autotrophic nutrition are 'chemoautotrophs'; bacteria, oxidise inorganic compounds
  • Heterotrophs: Non-photosynthetic organisms get energy by digesting complex molecules to smaller respiratory substrates
  • Equation:
  • 6CO_2 + 6H_2O -> C_6H_12O_6 + 6O_2
  • 2-stage process: Light-dependent reaction, on thylakoids + light-independent reaction, on stroma; called Calvin Cycle
  • Light independent
  • Used: Water, Light energy + NADP/ADP/Pi
  • Produced: Reduced NADP, ATP + O_2
  • Light dependent
  • Used: Reduced NADP, ATP + CO_2
  • Produced: Glucose + NADP/ADP/Pi
• Photosynthetic Pigments

  • Photosynthetic pigments coloured compounds in thylakoid membranes absorb light energy
  • Many different pigments + each absorbs distinct light wavelength range w/ absorption peaks at certain wavelengths
  • Pigments
  • Chlorophyll A
  • 2 types: P_700: in photosystem I + P_680: in photosystem II
  • Blue-green pigment; primary pigment at reaction centre
  • Absoption peaks ~420-430nm (blue-violet) + 670nm (red)
  • Chlorophyll B
  • Yellow-green accessory pigment
  • Absoption peaks 460nm, blue + 650nm, red-orange
  • Carotenoids, inc carotenes + xanthophylls
  • Red, orange, yellow or brown accessory
  • Absoption peak 450-510nm + absorb UV radiation

  Photosystems

  • Photosynthetic pigments cluster in funnel-shape embedded in thylakoid membrane
  • Each has 1 primary pigment + many accessory
  • Photosystem 1, PSI or P700 + Photosystem 2, PSII or P680

  Spectrums

  • Absorption: light absorption by pigments
  • Action: photosynthesis at dif wavelengths
• Light Dependant

  • Thylakoids, 3 processes: water split by light; photolysis, NADP reduced to NADPH + ATP; photophosphorylation
  • Light absorbed by individual chlorophylls in light-harvesting complexes
  • Transfer energy to chlorophyll a at reaction centre as resonance energy, releasing 2 electrons
  • 2 paths, depending where electrons end up:
  • Cyclic photophosphorylation uses only PSI, produce ATP if electrons pass back to same reaction centre PSI
  • Non-cyclic photophosphorylation uses PSI + PSII, produce ATP + NADPH
• Light Dependent

  • Thylakoids, 3 processes: water split by light; photolysis, NADP reduced to NADPH + ATP; photophosphorylation
  • Light absorbed by individual chlorophylls in light-harvesting complexes
  • Transfer energy to chlorophyll a at reaction centre as resonance energy, release 2 electrons

  2 paths, depending where electrons end up:

  • Cyclic photophosphorylation
  • Only PSI, when light absorbed electrons are excited
  • Accepted by electron acceptor molecule + transferred down ETC
  • Final acceptor chlorophyll so electrons are recycled
  • Electrons synthesise ATP from ADP + Pi, ATP transferred to light-independent stage
  • W/out NADPH, it does not significantly contribute to organic molecule synthesis
  • Non-cyclic photophosphorylation uses PSI + PSII, 2 processes
  • Phosphorylation: ADP + Pi -> ATP
  • Photolysis: Absorbs light energy, replace lost electrons in p680; PSII enzyme catalyses
  • 2H_2O → 4H^+ + O_2 + 4e^-
• Light independent

  • Calvin cycle at stroma, occur day or night if ATP + NADPH from light dependent
  • CO_2 in atmosphere diffuses into chloroplast envelope; to stroma
  • 5C sugar intermediate ribulose diphosphate, RuBP
  • CO_2 acceptor, carboxylated, fix catalysed by rubisco
  • Unstable 6C intermediate compound
  • 6C immediately breaks to 2 glycerate 3 phosphate, GP, molecules, 3C
  • ATP hydrolysed, for energy + NADPH, for H^+, reduce GP to triose phosphate; 3C
  • TP, some convert to AA; need nitrogen source, or fatty acids
  • TP doesn't accumulate: Most regenerate RuBP
  • Need ATP; energy + phosphate source
  • 5/6 TP molecules are recycled to make 3 RuBP
  • 1/6 convert to hexose sugar, 6C or glycerol, for lipid synthesis or to form triglycerides
  • 6 cycle turns make 1 hexose sugar
  • Hexose sugars:
  • Polymerise to cellulose, starch or pentose sugars; for nucleic acid synthesis
  • Glucose made fructose; form sucrose, sugar in phloem sieve tubes + in glycolysis
• Limiting Factor

  • Light wavelength
  • Water availability
  • Mineral ion availability
  • Light intensity
  • Inc photosynthesis + photophosphorylation rate, make more ATP + NADPH
  • More GP reduced to TP in Calvin cycle, more glucose
  • CO_2 conc
  • In atmosphere is ~0.039% by vol, but in greenhouses artificially inc by burning methane or oil-fired heaters
  • Increases photosynthesis rate as rises CO_2 rate fixate on RuBP produce GP; more TP + glucose produced
  • Temp
  • Affect photochemical reactions of light dependent
  • Calvin cycle's enzyme-catalysed reactions
  • 0-25C, photosynthesis rate double for each 10C rise (Q10 = 2)
  • Over 25C, rate levels + falls as more enzymes are denatured
• Measuring Photosynthesis

  • Photosynthesis rate measured by photosynthometer
  • Calc O_2 vol released from aquatic plant per min
  • Useful as O_2 released waste + not very water-soluble seen as bubbles
  • Measure photosynthesis rate CO_2 uptake measured, use radioactive tracer or mass inc over time
  • Calc Rate
  • Photosynthesis rate = gas vol collected per min
  • Gas Vol collected = bubble length x πr^2; cross sectional area
  • r = Capillary bore internal radius
  • Rate calc as: Bubble length x πr^2 / Time taken to collect gas bubble (min), units: mm^3min^-1
  • Limitations
  • Not all produced O_2 collected, some, dissolves to test tube water, remain in leaf air spaces
  • Escape collection in flared capillary tube part
  • Some gas collected nitrogen; in leaf air spaces + diffuse out to H_2O w/ O_2
  • Some gas collect CO_2, plant respiration product + NaHCO_3 added to release CO_2
  • Not all released used in photosynthesis + some dissolves