Photosynthesis

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Photosynthesis

• Chapter 7

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Photosynthesis Overview

• Energy for all life on Earth ultimately comes
  from photosynthesis.
• 6CO2 12H2O C6H12O6 6H2O 6O2
• Oxygenic photosynthesis is carried out by
• cyanobacteria, 7 groups of algae, all land plants
  

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Photosynthesis Overview

• Photosynthesis takes place in 3 stages
• Capturing energy from sunlight
• Using the energy to make ATP and reduce NADP to
  NADPH
• (nicotinamide adenine dinucleotide phosphate)
• Using the ATP and NADPH to synthesize organic
  molecules (glucose) from CO2

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Photosynthesis Overview

• Photosynthesis is divided into
• light-dependent reactions
• -capture energy from sunlight
• -make ATP and reduce NADP to NADPH
• carbon fixation reactions (light-independent
  reactions)
• -use ATP and NADPH to synthesize organic
  molecules from CO2

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Photosynthesis Overview

• Photosynthesis takes place in chloroplasts.
• thylakoid membrane internal membrane arranged
  in flattened sacs
• contain chlorophyll and other pigments
• Organized into photosystems
• Capture light and transfer energy (to pigment
  molecules)
• grana stacks of thylakoid membranes
• stroma semiliquid substance surrounding
  thylakoid membranes (houses the enzymes to make
  organic molecules)

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Photosynthesis Overview

• Photosynthesis takes place in the green portions
  of plants
• Leaf of flowering plant contains mesophyll tissue
• Cells containing chloroplasts
• Specialized to carry on photosynthesis
• CO2 enters leaf through stomata
• Diffuses into chloroplasts in mesophyll cells
• In stroma, CO2 fixed to C6H12O6 (sugar)
• Energy supplied by light

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Discovery of Photosynthesis

• The work of many scientists led to the discovery
  of how photosynthesis works.
• Jan Baptista van Helmont (1580-1644)
• Joseph Priestly (1733-1804)
• Jan Ingen-Housz (1730-1799)
• F. F. Blackman (1866-1947)

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Discovery of Photosynthesis

• C. B. van Niel, 1930s
• -proposed a general formula
• CO2H2A light energy CH2O H2O 2A
• where H2A is the electron donor
• -van Niel identified water as the source of the
  O2 released from photosynthesis
• -Robin Hill (in the 1950s) confirmed van Niels
  proposal that energy from the light reactions
  fuels carbon fixation (making glucose from CO2)

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Pigments

• photon a particle of light
• -acts as a discrete bundle of energy
• -energy content of a photon is inversely
  proportional to the wavelength of the light
• photoelectric effect removal of an electron from
  a molecule by light
• -occurs when photons transfer energy to electrons

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Electromagnetic Spectrum
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Pigments

• Pigments molecules that absorb visible light
• Each pigment has a characteristic absorption
  spectrum, the range and efficiency of photons it
  is capable of absorbing.

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Pigments

• chlorophyll a primary pigment in plants and
  cyanobacteria
• -absorbs violet-blue and red light
• chlorophyll b secondary pigment absorbing light
  wavelengths that chlorophyll a does not absorb

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Pigments

• A graph of percent of light absorbed at each
  wavelength is a compounds absorption spectrum.
• Action spectrum
• Oxygen production and therefore photosynthetic
  activity is measured for plants under each
  specific wavelength when plotted on a graph,
  this gives an action spectrum for a compound.
• The action spectrum for chlorophyll resembles its
  absorption spectrum, thus indicating that
  chlorophyll contributes to photosynthesis.

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Pigments

• accessory pigments secondary pigments absorbing
  light wavelengths other than those absorbed by
  chlorophyll a
• -increase the range of light wavelengths that can
  be used in photosynthesis
• -include chlorophyll b, carotenoids,
  phycobiloproteins
• -carotenoids also act as antioxidants

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Photosystem Organization

• A photosystem consists of
• 1. an antenna complex (light harvesting complex)
  of hundreds of accessory pigment molecules that
  gather photons and feeds energy to reaaction
  center
• 2. a reaction center of one or more chlorophyll a
  molecules pass electrons out of photosystem
  (photochemical reactions)
• In summary, energy of electrons is transferred
  through the antenna complex to the reaction
  center.

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Photosystem Organization

• At the reaction center (transmembrane protein
  complex), the energy from the antenna complex is
  transferred to chlorophyll a.
• This energy causes an electron from chlorophyll
  to become excited.
• The excited electron is transferred from
  chlorophyll a to an electron acceptor.
• Water donates an electron to chlorophyll a to
  replace the excited electron.

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Converting light to chemical energy
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Photosynthesis Overview
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• Light Reactions
• Two electron pathways operate in the thylakoid
  membrane the noncyclic pathway and the cyclic
  pathway.
• Both pathways produce ATP only the noncyclic
  pathway also produces NADPH.
• ATP production during photosynthesis is called
  photophosphorylation therefore these pathways
  are also known as cyclic and noncyclic
  photophosphorylation.

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Light ReactionsThe Noncyclic Electron Pathway

• Takes place in thylakoid membrane
• Uses two photosystems, PS-I and PS-II (consists
  of pigment complexes)
• PS II captures light energy
• Causes an electron to be ejected from the
  reaction center (chlorophyll a)
• Electron travels down electron transport chain to
  PS I
• Replaced with an electron from water
• causes H to concentrate in thylakoid chambers
• causes ATP production
• PS I captures light energy (electrons and H)
• Transferred permanently to a molecule of NADP
• Causes NADPH production

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Light ReactionsNoncyclic Electron Pathway
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Light ReactionsThe Cyclic Electron Pathway

• Uses only photosystem I (PS-I)
• Begins when PS I complex absorbs solar energy
• Electron ejected from reaction center
• Travels down electron transport chain
• Causes H to concentrate in thylakoid chambers
• Which causes ATP production
• Electron returns to PS-I (cyclic)
• Pathway only results in ATP production

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Light ReactionsCyclic Electron Pathway
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• The Organization of the Thylakoid Membrane
• PS II consists of a pigment complex and
  electron-acceptor molecules it oxidizes H2O and
  produces O2.
• The electron transport system consists of
  cytochrome complexes and transports electrons and
  pumps H ions into the thylakoid space.
• PS I has a pigment complex and electron-acceptor
  molecules it is associated with an enzyme that
  reduces NADP to NADPH.
• ATP synthase complex has an H channel and ATP
  synthase it produces ATP.

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ATP Production

• Thylakoid space acts as a reservoir for hydrogen
  ions (H)
• Each time water is oxidized, two H remain in the
  thylakoid space
• Electrons yield energy
• Used to pump H across thylakoid membrane
• Move H from stroma into the thylakoid space
• Flow of H back across thylakoid membrane
• Energizes ATP synthase
• Enzymatically produces ATP from ADP P
• This method of producing ATP is called
  chemiosmosis

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Calvin Cycle ReactionsCarbon Dioxide Fixation

• CO2 is attached to 5-carbon RuBP molecule
• Result in a 6-carbon molecule
• This splits into two 3-carbon molecules (3PG)
• Reaction accelerated by RuBP Carboxylase
  (Rubisco)
• CO2 now fixed because it is part of a
  carbohydrate

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Calvin Cycle ReactionsCarbon Dioxide Reduction

• 3PG reduced to BPG
• BPG then reduced to G3P
• Utilizes NADPH and some ATP produced in light
  reactions

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Calvin Cycle ReactionsRegeneration of RuBP

• RuBP used in CO2 fixation must be replaced
• Every three turns of Calvin Cycle,
• Five G3P (a 3-carbon molecule) used To remake
  three RuBP (a 5-carbon molecule)

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The Calvin Cycle Fixation of CO2
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Importance of Calvin Cycle

• G3P (glyceraldehyde-3-phosphate) can be converted
  to many other molecules
• The hydrocarbon skeleton of G3P can form
• Fatty acids and glycerol to make plant oils
• Glucose phosphate (simple sugar)
• Fructose (which with glucose sucrose)
• Starch and cellulose
• Amino acids

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Other Types of Photosynthesis C4 Photosynthesis
and CAM Photosynthesis
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Most plants are C3 plants

• In C3 plants, the Calvin cycle fixes CO2
  directly the first molecule following CO2
  fixation is 3PG.
• In hot weather, stomata close to save water CO2
  concentration decreases in leaves O2 increases.
• O2 combines with RuBP instead of CO2
• This is called photorespiration since oxygen is
  taken up and CO2 is produced this produces less
  3PG.

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C4 Photosynthesis

• In a C3 plant, mesophyll cells contain
  well-formed chloroplasts, arranged in parallel
  layers.
• In C4 plants, bundle sheath cells as well as the
  mesophyll cells contain chloroplasts.
• In C4 leaf, mesophyll cells are arranged
  concentrically around the bundle sheath cells.

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C4 Photosynthesis

• Remember C3 plants use RuBP carboxylase to fix
  CO2 to RuBP in mesophyll the first detected
  molecule is 3PG.
• C4 plants use the enzyme PEP carboxylase
  (PEPCase) to fix CO2 to PEP (phosphoenolpyruvate,
  a C3 molecule) the end product is oxaloacetate
  (a C4 molecule).
• In C4 plants, CO2 is taken up in mesophyll cells
  and malate, a reduced form of oxaloacetate, is
  pumped into the bundle-sheath cells here CO2
  enters Calvin cycle.
• In hot, dry climates, net photosynthetic rate of
  C4 plants (e.g., corn) is 23 times that of C3
  plants.
• Photorespiration does not occur in C4 leaves
  because PEP does not combine with O2 even when
  stomata are closed, CO2 is delivered to the
  Calvin cycle in bundle sheath cells.
• C4 plants have advantage over C3 plants in hot
  and dry weather because photorespiration does not
  occur e.g., bluegrass (C3) dominates lawns in
  early summer, whereas crabgrass (C4) takes over
  in the hot midsummer.

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CAM Photosynthesis

• CAM (crassulacean-acid metabolism) plants form a
  C4 molecule at night when stomata can open
  without loss of water found in many succulent
  desert plants including the family Crassulaceae.
• At night, CAM plants use PEPCase to fix CO2 by
  forming C4 molecule stored in large vacuoles in
  mesophyll.
• C4 formed at night is broken down to CO2 during
  the day and enters the Calvin cycle which now has
  NADPH and ATP available to it from the
  light-dependent reactions.
• CAM plants open stomata only at night, allowing
  CO2 to enter photosynthesizing tissues during
  the day, stomata are closed to conserve water but
  now CO2 cannot enter photosynthesizing tissues.
• Photosynthesis in a CAM plant is minimal, due to
  limited amount of CO2 fixed at night but this
  does allow CAM plants to live under stressful
  conditions.