Photosynthesis

• structure of a chloroplast

  disc shaped

  double membrane

  thylakoids -> flattened disc sacks stacked on top of each other to form grana

  intergranal lamellae -> tubular extensions that join thylakoids in adjacent grana

  stroma -> fluid filled matrix

  starch grain -> stores photosynthetic products
• light dependent reaction location

  thylakoids
• light independent reaction location

  stroma
• photosynthetic pigments

  absorb a range of wavelengths of light to maximise photosynthesis
• primary pigment

  chlorophyll a
• accessory pigments

  chlorophyl b

  carotenoids ( carotene and xanthophylls )
• processes in non-cyclic only

  reduction of NADP

  photolysis of water
• photoionisation

  energy from photons of light are absorbed by photosynthetic pigments in photosystem 2

  this excites the electrons by raising them to a higher energy level

  causing them to be released from the pigments
• cyclic phosphorylation electron transport chain

  the higher energy electrons are received by an electron acceptor and are passed down a series of carrier proteins embedded in the thylakoid membrane and undergo a series of redox reactions which releases the energy needed to convert ADP to ATP
• chemiosmosis theory in cyclic phosphorylation

  H+ are pumped into the thylakoid space from the stroma

  they move down their concentration gradient via a transmembrane channel protein known as ATP synthase which catalyses the condensation reaction of ADP into ATP
• non-cyclic phosphorylation

  photosystems 1 and 2

  excited electrons enter the ETC to produce ATP from ADP and Pi

  water is then photolysed to release electrons to replace the ones removed from photosystem 2

  NADP acts as the final electron acceptor and is reduced to NADPH using the protons dissociated from the water molecule
• purpose of cyclic phosphorylation

  to produce additional ATP to meet the surplus energy demands of cells
• photolysis of water

  2 H2O -> 4 H+ + 4 e- + O2
• reduction of NADP

  NADP + 2H+ + 2 e- -> NADPH

  catalysed by dehydrogenase enzymes

  in the stroma of chloroplasts
• light independent vs light dependent

  LID:

  CO2 as an input

  triose phosphate as an output

  LD:

  light and water as an input

  ATP O2 and reduced NADPH as an output
• three stages in the light independent reaction

  carbon fixation

  reduction

  regeneration
• carbon fixation

  reaction between carbon dioxide and ribulose bisphosphate

  catalysed by Ribulose Bisphosphate Carboxylase (RuBisCo)

  produces two glycerate 3-phosphate (GP)
• reduction

  both glycerate 3-phosphate are reduced to two triose phosphate

  requires 2 ATP and 2 NADPH

  forms 2 ADP and 2 NADP
• role of triose phosphate in the light independent reaction

  raw material: some leaves the calvin cycle to produce organic substances such as amino acids and monosaccharides

  regeneration: Ribulose Phosphate is converted into Ribulose Bisphosphate using energy from ATP
• light independent reaction break down

  CO2 + RuBP -> 2 GP

  using RuBisCo

  2 GP -> 2 TP

  using 2 ATP and 2 NADPH

  2 TP -> RuP -> RuBP

  using ATP
• what is TP also known as

  GALP
• number of carbon atoms in GP TP and RuBP

  (in order in question)

  3

  3

  5
• limiting factor

  a factor that determines the maximum rate of reaction
• limiting factors of photosynthesis

  light intensity (LD stage)

  CO2 levels (LID stage)

  temperature (reduction of GP to TP)

  magnesium (chlorophyll)
• light intensity

  slower light dependent reaction

  less ATP and NADPH produced to convert GP into TP

  GP levels rise as they are not being converted into TP

  TP and RUBP fall as they are not being made and regenerated
• carbon dioxide levels

  rate of RuBP into GP slows down

  levels of RuBP increases

  levels of GP and TP decrease
• principle of paper chromatography

  molecules in a mixture are separated by their relative attraction to the mobile phase compared to the stationary phase
• assuming 8.5 × 1018 Bq are generated by each 14C atom in the molecule, how many new TP molecules are produced after 30 seconds in the dark?

  find difference between light and dark reactions

  ( 10.1 - 5.5 = 4.6 )

  4.6 x 10 27 divided by 8.5 x 10 18 = 541176470.6

  as TP contains 3 carbon atoms you have to divide by 3

  541176470.6 divided by 3 = 1.8 x 10 8

  
• role of accessory pigments

  pass energy to a reaction center

  where primary pigments are then oxidised to release electrons into the electron transport chain
• why do aquatic photosynthetic organisms need to have pigments different from terrestrial photosynthetic organisms

  absorb shorter wavelengths as some wavelengths of light won't reach their depth
• compensation point


  rate of photosynthesis and respiration are equal