Photosynthesis Warm Up

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Photosynthesis Warm Up

• All organisms respire (by-product is CO2), while
  all plants, some bacteria and some protists also
  photosynthesize (by-product is O2)
• These autotrophic organisms harness radiant
  energy from the sun or artificial sources and
  convert it into glucose
• Heterotrophs like humans must ingest glucose in
  the form of food
• The leaf is the photosynthetic organ in plants
• Photosynthesis occurs in the mesophyll cells

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Photoautotrophs
These organisms use light energy to drive the
synthesis of organic molecules from carbon
dioxide and (in most cases) water. They feed not
only themselves, but the entire living world. (a)
On land, plants are the predominant producers
of food. In aquatic environments,
photosynthetic organisms include (b)
multicellular algae, such as this kelp (c) some
unicellular protists, such as Euglena (d) the
prokaryotes called cyanobacteria and (e) other
photosynthetic prokaryotes, such as these purple
sulfur bacteria, which produce sulfur
(spherical globules) (c, d, e LMs).
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The location of photosynthesis in a plant
Leaf cross section
Outermembrane
Granum
Stroma
Thylakoid
ThylakoidSpace
Intermembranespace
Inner membrane
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Photosynthesis Warm Up

• White light is composed of all the colors of
  light (roy g biv)
• Pigments are special chemical substances that
  selectively absorb certain wavelengths (i.e.,
  colors) of light while reflecting others
• Chlorophyll is a green pigment that absorbs red
  and blue light very well and reflects green

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The electromagnetic spectrum
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Structure of chlorophyll molecules in
chloroplasts of plants
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Which wavelengths of light are most effective in
driving photosynthesis?
RESULTS
Chlorophyll a
Chlorophyll b
Absorption of light by chloroplast pigments
Carotenoids
Wavelength of light (nm)
(a) Absorption spectra. The three curves show the
wavelengths of light best absorbed by three
types of chloroplast pigments.
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Overview of Photosynthesis

• Summary equation for photosynthesis
• 6CO2 12H2O light 6O2 C6H12O6 6H2O
• Two sets of reactions light and dark
• The light reactions require visible light (sun
  light or artificial) and occur in the granum of
  the chloroplast
• The dark reactions/Calvin cycle do not require
  light and occur in the stroma of the chloroplast

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Chloroplast structure
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Overview of Photosynthesis Light Reactions

• The light reactions require visible light (sun
  light or artificial) and occur in the granum of
  the chloroplast
• Light is absorbed by the pigment chlorophyll and
  drives the transfer of electrons from H2O to
  NADP, forming NADPH
• H2O is split during these reactions and O2 is
  released
• Photophosphorylation leads to the production of
  ATP from ADP
• The net products of the light reactions are NADPH
  (which stores electrons for later use), O2 and
  ATP
• The purpose of these reactions is to convert
  light energy into the chemical energy of ATP

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Reactions of Photosynthesis

• Photosynthesis overall summary equation
• 6CO2 12H2O light 6O2 C6H12O6 6H2O
• The light reactions summary
• 12H2OADPPiNADP light 6O2ATPNADPH
• Water is oxidized (LEO) and NADP is reduced
  (GER)
• The formation of ATP is by chemiosmosis (similar
  to the process in respiration, except here it
  occurs in the chloroplasts)

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Overview of Photosynthesis Dark Reactions

• The dark reactions/Calvin cycle do not require
  light and occur in the stroma of the chloroplast
• CO2 from the air is incorporated into usable
  organic molecules in a process called carbon
  fixation
• The fixed carbon is then used to make
  carbohydrates (e.g., glucose)
• NADPH is used to provide energy to fuel the
  process of carbon fixation, and ATP is also used
• The purpose of these reactions is to produce
  glucose
• Glucose is easily stored in plants as starch
• The glucose is then used as needed for
  respiration

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Photosynthesis Reactions

• Photosynthesis overall summary equation
• 6CO2 12H2O light 6O2 C6H12O6 6H2O
• The light reactions summary
• 12H2OADPPiNADP light 6O2ATPNADPH
• The formation of ATP is by chemiosmosis
• The dark reactions summary
• ATPNADPH6CO2 6H2O C6H12O6 NADP ADP
  Pi

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An overview of photosynthesis cooperation of the
light reactions and the Calvin cycle
H2O
Light
LIGHT REACTIONS
ATP
NADPH
Chloroplast
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The Light Reactions A Closer Look

• Photons of light are absorbed by certain groups
  of pigment molecules in the thylakoid membrane
  of chloroplasts
• These groups of pigment molecules are called
  photosystems
• There are two photosystems important to
  photosynthesis Photosystem I and Photosystem II
• Photosystems have chlorophyll and accessory
  pigments (carotenoids), which allows them to
  gather light effectively
• When chlorophyll absorbs light energy in the form
  of photons, one of the chlorophylls electrons is
  raised to a higher energy orbital, and is then in
  an excited state
• This electron is unstable, and can be used in an
  electron transport chain to provide chemical
  energy for reactions

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The Light Reactions Continued

• The light reactions begin with Photosystem II
  (PSII), which has chlorophyll called P680 that
  specifically absorbs light in the 680nm
  wavelength range
• When light is absorbed by P680, an electron
  becomes excited, is released from P680, and is
  captured by another molecule, the primary
  electron acceptor of Photosystem II (PSII)
• P680 has lost an electron (LEO) and needs a
  replacement
• An enzyme splits a water molecule into two
  hydrogen ions, two electrons and an oxygen atom
• The electrons go to P680, and the oxygen atom
  combines with another oxygen to form O2, a
  by-product of photosynthesis

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The Light Reactions Continued

• The original excited electron from P680 passes
  from the primary electron acceptor of PSII to PSI
  through an electron transport chain
• The energy from the transfer of electrons down
  the electron transport chain (ETC) in the
  thylakoid membrane is used for ADP Pi gt ATP
• This process is called non-cyclic electron flow,
  and is similar to chemiosmosis
• This ATP will be used later in the Calvin cycle
  to form glucose
• The electrons leave PSII and continue being
  transferred and end up in the chlorophyll of PSI
  (this chlorophyll is called P700 because it
  absorbs in 700nm wavelength)

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The Light Reactions Continued

• PSI needs an electron because light energy is
  also hitting the P700 chlorophyll and is causing
  an electron to become excited and then lost to
  the primary electron acceptor of PSI
• The primary electron acceptor of PSI passes the
  excited electrons along another electron
  transport chain
• The electrons are received by NADP, which gains
  them and is reduced to NADPH, the second of the
  two important light-reaction products
• The NADPH and ATP formed by the light reactions
  are used by the Calvin cycle to make carbohydrate

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Comparison of chemiosmosis in mitochondria and
chloroplasts
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The Calvin Cycle/Dark Reactions/Light-Independent
Reactions

• No light is necessary for these reactions
• In the course of the Calvin cycle, CO2 is
  converted into a 3-carbon carbohydrate called
  glyceraldehyde-3-phosphate (G3P)
• ATP and NADPH are both consumed to fuel the
  conversion
• Since a 3-carbon molecule (G3P) is being
  synthesized from a 1-carbon molecule (CO2), the
  cycle must go through three rotations and
  fix/convert three molecules of CO2
• The G3P is later converted into glucose (a
  6-carbon molecule) and other carbohydrates

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A review of photosynthesis
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Alternate Pathways for Hot, Arid Climates

• Many plants living in hot, dry climates use C4
  fixation instead of C3 (the first carbon compound
  formed in the Calvin cycle has 4 carbons rather
  than 3)
• C3 plants have one type of photosynthetic cell
  called the mesophyll cells
• C4 plants have two kinds of photosynthetic cells
  bundle sheath and mesophyll cells
• The bundle sheath cells are grouped around the
  leafs veins and the mesophyll cells are
  dispersed throughout the leaf
• The last steps of the Calvin cycle occurs in the
  bundle sheath cells, which are close to the veins
  of the plant
• This spatial separation enhances carbohydrate
  production

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Alternate Pathways for Hot, Arid Climates

• CAM photosynthesis is another adaptation
• These plants keep their stomata closed during the
  day to prevent excess water loss
• Of course, this also prevents gas exchange
• At night, the stomata open and CO2 is taken in
• Cells convert the CO2 into various organic
  molecules and store them in vacuoles
• In the morning when the stomata close, the plant
  cells retrieve the stored CO2 and use it for
  photosynthesis
• This temporal separation limits dehydration and
  enhances carbohydrate formation

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C4 leaf anatomy and the C4 pathway
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C4 and CAM photosynthesis compared
CO2
Organic acids release CO2 to Calvin cycle
Organic acids release CO2 to Calvin cycle