Photosynthesis

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Photosynthesis

• biology 1

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• Photosynthesis plays a key role in
  photo-autotrophic existence
• Photosynthesis is a redox process, occurring in
  chloroplasts, and involves two reactions
• Light dependent reaction
• Light independent reaction (Calvin Cycle)
• Pigments in chloroplasts are keyed to react to
  specific wavelengths of light
• There are different strategies to photosynthesis,
  including the C3, C4 and CAM pathways

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The importance of photosynthesis

• Life on Earth is balanced between autotrophs
  (self-feeders) and heterotrophs
  (other-feeders)
• Autotrophs synthesize complex organic molecules
  (e.g., sugar), utilizing energy from
• Light (photo-autotrophs)
• Oxidation of inorganics (chemo-autotrophs
• Autotrophs responsible for producing organic
  molecules that enter ecosystem

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Photosynthesis as a redox process

• A general equation for photosynthesis is
• 6CO2 12H2O C6H12O6 6O2 6H2O
• Indicating the net consumption of water
  simplifies the equation to
• 6CO2 6H2O C6H12O6 6O2
• The simplest form of equation is
• CO2 H2O CH2O O2
• Thus 6 repetitions of the equation produce a
  molecule of glucose

light
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Where does the oxygen come from?

• Van Niel demonstrated in chemo-autotrophs that
• CO2 2H2S CH2O H2O 2S
• Therefore, a general form for the synthesis
  equation
• CO2 2H2X CH2O H2O 2X
• Van Niel summarized that oxygen comes from water
  (later confirmed with radio-isotopes)

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Photosynthesis as a redox process

• Hydrogen is extracted from water and incorporated
  into sugar
• Electrons have higher potential energy in the
  sugar molecule
• Light is the energy source that boosts the
  potential energy of electrons as they are moved
  from water to sugar
• When water is split, electrons are transferred
  form the water to carbon dioxide, reducing it to
  sugar

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The chloroplast

• Photosynthesis occurs in chloroplasts
• Chlorophyll (and other pigments), stored in
  thylakoid membranes captures light energy - this
  is where light dependent reactions occur
• The stroma (matrix inside chloroplast) contains
  light-independent reactions, reducing CO2 to CH2O
• Thylakoids and photosynthetic cells are organized
  in grana and the mesophyll respectively to
  maximize absorption of light

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The light-dependent reaction

• Light energy is converted to chemical bond energy
  found in ATP and NADPH, occurring in thylakoid
  membranes
• NADP (nicotinamide adenine dinucleotide
  phosphate) is reduced to NADPH, temporarily
  storing energized electrons transferred from
  water, and an H
• O2 is a by-product of splitting water
• ATP is produced via photophosphorylation
• ATP and electrons are used in next stage to fix
  carbon

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The light-independent reaction

• Also known as the Calvin cycle
• Main purpose is to fix carbon (process of
  incorporating carbon into organic molecules
• NADPH provides the reducing power
• ATP provides the chemical energy

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How the light reaction works

• The nature of light
• Acts as both a particle and a waveform
• As a wave, is a type of electromagnetic energy
  visible light consists of a spectrum of
  wavelengths from 380 nm to 750 nm
• As a particle, light behaves as discrete
  particles called photons, each photon having a
  fixed amount of energy
• In photosynthesis, the most used (absorbed)
  wavelengths are blue and red. Green is
  transmitted (hence the green color of chlorophyll)

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• A substance that absorbs light is called a
  pigment
• Each pigment has a characteristic absorption
  spectrum
• In the light reaction the most important pigment
  is chlorophyll a
• However, the action spectrum for chlorophyll a
  does not match that for photosynthesis -
  therefore, other pigments are involved
• Carotenoids, e.g., xanthophyll
• Chlorophyll b
• These accessory pigments do not participate
  directly in the light reaction, but work with
  chlorophyll a to do so

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How pigments aid the light-dependent reaction

• Photo-excitation
• Absorbed wavelength photons boost one of the
  pigment molecules electrons in its lowest energy
  state (ground state) to an orbital of higher
  energy (excited state)
• The difference in energy is directly equal to the
  energy of the photon, and therefore specific to a
  specific wavelength
• Conclusion certain pigments are designed to
  absorb particular wavelengths

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Photosystems

• Photosystems are organizations of photosynthetic
  pigments, consisting of
• An antenna complex (responsible for inductive
  resonance, the absorption of energy associated
  with photons, and passing that energy between
  themselves
• A reaction centre chlorophyll. One molecule of
  chlorophyll a per photosystem can take that
  energy use it to push out an electron, passing it
  to
• the primary electron acceptor (the first step of
  the light reaction
• Two types of photosystem P700 (photosystem I)
  and P680 (photosystem II)

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Non-cyclic electron flow

• Excited electrons are transferred form P700 to
  the primary electron acceptor for photosystem I
• Primary electron acceptor passes excited
  electrons to NADP (goes to NADPH) via ferredoxin
• Oxidized P700 chlorophyll becomes an oxidizing
  agent (needs electrons to be replaced). These
  electrons come indirectly from photosystem II

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• Electrons ejected from P680 are trapped by the PS
  II primary electron acceptor
• These electrons are transferred to an electron
  transport chain (making ATP), eventually ending
  in P700 (PS I)
• Electron space vacated in P680 are filled
  splitting of water
• H2O O 2H 2e-

Light enzyme
chemiosmosis
to P680
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• Production of ATP via the electron transport
  chain is termed photo-phosphorylation (light
  energy used to make ADP into ATP)
• In this case, ATP production is specifically
  termed noncyclic photo-phosphorylation

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Cyclic electron flow

• Involves only PS I, P700
• In this cyclic system, ejected electrons are fed
  to the ETC to generate ATP (cyclic
  photo-phosphorylation)
• P700 acts as the ultimate acceptor of the shunted
  electrons
• No NADPH is produced, or O2
• Aim is to produce extra ATP needed for Calvin
  cycle

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The Calvin cycle

• Powered by ATP and NADPH from light-dependent
  reaction
• Carbon enters cycle as CO2 and leaves as triose
  sugar, glyceraldehyde 3-phosphate (G3P)
• Three phases
• Carbon fixation - a molecule of CO2 is attached
  to a CO2-acceptor, ribulose biphosphate (RBP)
• Unstable 6-carbon intermediate degrades into
  3-carbon molecule
• Reduction - endergonic reaction
• uses ATP (energy) and NADPH (reducing agent) to
  convert 3-phosphoglycerate to glyceraldehyde
  6-phosphate
• Regeneration of RBP (requires ATP)
• Calvin cycle needs 18 ATP and 12 NADPH to produce
  1 molecule of glucose

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Alternative mechanisms

• Photorespiration - a metabolic pathway that
  consumes O2, produces CO2, produces no ATP and
  decreases photosynthetic output
• Occurs because Rubisco (enzyme in Calvin cycle),
  can accept O2 instead of CO2
• When O2 conc. higher than CO2 conc. In leaf,
  rubisco takes O2 (e.g., when hot, stomata close,
  CO2 drops, O2 increases)

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Strategies to prevent photorespiration

• C3 plants produce 3 phosphoglycerate, the first
  stable intermediate in the Calvin cycle (eg,
  rice, soy, wheat)
• C4 plants (eg, corn, sugar, grasses) have
  different morphology. CO2 fixation occurs in
  different location
• In mesophyll cells, CO2 is added to
  phosphoenolpyruvate (PEP) to make oxaloacetate
  (4-carbons). Enzyme is PEP carboxylase, which has
  a far higher affinity for CO2 than O2
• Oxaloacetate is converted to malate (4C), and
  then moved to bundle sheath cells, where
  remaining Calvin cycle occurs

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Another strategy...

• In succulents adapted to dry, arid environments,
  stomata closed during day
• At night, stomata open and CO2 is incorporated
  into a number of organic acids (termed
  crassulacean acid metabolism CAM)
• During day, light reactions produce ATP and NADPH
  which release CO2 from organic acids
• In summary,
• C4 plants spatially separate carbon fixation from
  the Calvin cycle
• CAM plants temporally separate carbon fixation
  from the Calvin cycle