Where It Starts: Photosynthesis

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Chapter 6
Where It StartsPhotosynthesis
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Introduction

• Before photosynthesis evolved, Earths atmosphere
  had little free oxygen
• Oxygen released during photosynthesis changed the
  atmosphere
• Favored evolution of new metabolic pathways,
  including aerobic respiration

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The Role of Photosynthesis

• Sunlight is the ultimate source of energy for
  almost all life
• 1.5 x 1022 kJ falls on the earth each day
• 1 is absorbed by photosynthetic organisms,
  transformed into chemical energy
• Photosynthesis is the major biosynthetic pathway
  to convert CO2 and H2O into polysaccharides and
  major source of oxygen in the earths atmosphere
• 6CO2 6H2O ?C6H12O6 6O2 ?G0
  2870 kJ/mol
• Plant, algae, and some microorganisms catalyze
  the CO2 fixation
• 1011 tons of CO2 are fixed globally per year
• Oxygen (O2) derived from H2O is released as
  by-product of photosynthesis
• Part of energy obtained from photosynthesis or
  oxidation is trapped in ATP
• Basic principle for biosolar or biofuel cell
  design

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The Energy in Sunlight

• Sunlight waves of electromagnetic radiation or
  particles (photons)
• photons have discrete amounts of energy

Light has two aspects wavelike and particle-like
Plancks law E hn hc/l
The wave aspect of light
C 2.99 x 108 m/s Red light from neon laser l
632.8 nm or 6.328 x 10-7 m Then, n 4.73 x 1014
s-1
Quantum energy unit of photon Photons a stream
of light particles One einstein a mole of
photons
The particulate aspect of light
Plancks constant 6.626 x 10-34 J s E 6.626 x
10-34 x 4.73 x 1014 3.14 x 10-19 J 189
kJ/mol
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Electromagnetic Spectrum
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The Energy of Photons

• Photosynthesis depends primarily on light in the
  visible and near-IR region, cause transitions in
  the electronic states of organic molecules that
  drives chemical reactions
• Photons of far-UV have energies capable of
  breaking covalent bonds
• UV can penetrate only a very short distance into
  water, and is thus unavailable to photosynthetic
  organisms living in the sea
• Photons of IR radiation can do little except
  stimulate molecular vibration, perceived as heat

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Capturing Light Energy in Chemical Bonds

• Photons are too energetic for the cellular
  processes. Thus, it must be converted to organic
  molecules of lower energy before it can be used
  by living organisms.
• Pigments are molecules which absorb some
  wavelengths and reflect others
• Electrons within pigments are temporarily boosted
  to higher energy levels
• Photosynthesis begins when photons are absorbed
  by photosynthetic pigment molecules
• Pigment molecules absorb only light of particular
  wavelengths
• Photons not captured are reflected as color

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Major Photosynthetic Pigments

• There are two types of photosynthetic pigments
• Chlorophylls primary pigments
• Absorb photons of violet-blue and red
• Antenna pigments (carotenoids)
• Absorb photons of green, blue, violet Increase
  range of energy absorption
• Chlorophyll a
• Main photosynthetic pigment
• Absorbs violet and red light (appears green)
• Chlorophyll b, carotenoids, phycobilins
• Absorb additional wavelengths
• Collectively, photosynthetic pigments absorb
  almost all of wavelengths of visible light

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Exploring the Rainbow
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Engelmanns Experiment
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Absorption Spectra
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Pigments Reflect Color
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An Overview of Photosynthesis

• Photosynthesis is a process to convert light
  energy, carbon dioxide, and water into
  carbohydrates.
• Carbohydrate is a common energy storage form
• Main product of this process should be
  glyceraldehyde-3-phosphate while oxygen (O2) is
  the by-product.
• Photosynthesis can be carried out by
  cyanobacteria, algae, and all plants.
• Photosynthesis occurs in chloroplast, an
  organelle usually seen in photosynthetic
  eukaryotes.
• Photosynthesis proceeds in two stages
• Light-dependent reactions
• Light-independent reactions

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Sites of Photosynthesis Chloroplasts

• Light-dependent reactions occur at a much-folded
  thylakoid membrane
• Forms a single, continuous compartment inside the
  stroma (chloroplasts semifluid interior)
• Light-independent reactions occur in the stroma

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Sites of Photosynthesis
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Sites of Photosynthesis
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Photosynthesis light reactions dark reactions

• Photosynthesis proceeds in two stages
• Light-dependent reactions
• Light-independent reactions
• Light reactions capture light energy and convert
  it to chemical energy to carry out the
  photochemical oxidation of H2O, results in the
  reduction of NADP to from NADPH, and
  phosphorylating ADP to produce ATP with evolution
  of O2, within or on the thylakoid membrane
• Dark reactions (occur both in the dark and
  light) use NADPH and ATP to drive the endergonic
  process of hexose sugar formation from CO2 in a
  series of reactions, in the stroma

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Visual Summary of Photosynthesis
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Light-Dependent Reactions

• Two types of photosystems
• In thylakoid membrane
• Light-harvesting complexes
• Absorb light energy and pass it to photosystems
  which then release electrons
• Electrons enter light-dependent reactions

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Noncyclic Photophosphorylation

• Electrons released from photosystem II flow
  through an electron transfer chain
• At end of chain, they enter photosystem I
• Photon energy causes photosystem I to release
  electrons, which end up in NADPH
• Photosystem II replaces lost electrons by pulling
  them from water (photolysis)

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Cyclic Photophosphorylation

• Electrons released from photosystem I enter an
  electron transfer chain, then cycle back to
  photosystem I
• NADPH does not form, oxygen is not released
• ATP Formation
• In both pathways, electron flow through electron
  transfer chains causes H to accumulate in the
  thylakoid compartment
• A hydrogen ion gradient builds up across the
  thylakoid membrane
• H flows back across the membrane through ATP
  synthases
• Results in formation of ATP in the stroma

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Energy Flow in Light-Dependent Reactions
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Light Independent ReactionsThe Sugar Factory

• Light-independent reactions proceed in the stroma
  
• Carbon fixation Enzyme rubisco attaches carbon
  from CO2 to RuBP to start the CalvinBenson cycle
  
• CalvinBenson Cycle
• Cyclic pathway makes phosphorylated glucose
• Uses energy from ATP, carbon and oxygen from CO2,
  and hydrogen and electrons from NADPH
• Reactions use glucose to form photosynthetic
  products (sucrose, starch, cellulose)
• Six turns of CalvinBenson cycle fix six carbons
  required to build a glucose molecule from CO2

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Light-Independent Reactions

• Second stage is the synthesis part of
  photosynthesis
• Enzymes speed assembly of sugars from carbon and
  oxygen atoms, both from carbon dioxide
• Reactions use ATP and NADPH that form in the
  first stage of photosynthesis
• ATP delivers energy, and NADPH delivers electrons
  and hydrogens to the reaction sites
• Details of the reactions vary among organisms

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Adaptations Different Carbon-Fixing Pathways

• Environments differ
• Plants have different details of sugar production
  in light-independent reactions
• On dry days, plants conserve water by closing
  their stomata
• O2 from photosynthesis cannot escape
• C3 plants
• High O2 level Rubisco attaches to O2 instead of
  CO2 to RuBP Photorespiration reduces efficiency
  of sugar production

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Plant Adaptations to Environment

• C4 plants
• Carbon fixation occurs twice
• First reactions release CO2 near rubisco, limit
  photorespiration when stomata are closed

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Plant Adaptations to Environment

• CAM plants
• Some C4 plants separate processes using time
• Crassulacean Acid Metabolism
• CAM plants open their stomata at night
• CO2 enters and is converted to malate
• Stomata close during the day to conserve water
• Malate broken down into CO2 to drive Calvin cycle

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A Burning Concern

• Photoautotrophs remove CO2 from atmosphere
  metabolic activity of organisms puts it back
• Human activities disrupt the carbon cycle
• Add more CO2 to the atmosphere than
  photoautotrophs can remove
• Imbalance contributes to global warming
• Fossil Fuel Emisssions