PHOTOSYNTHESIS

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PHOTOSYNTHESIS
Photosynthesis is the process by which plants,
algae cyanobacteria capture the energy in
sunlight and convert it into chemical
energy. Many consider photosynthesis to be the
most important chemical process on earth,
because it was not until photosynthesis began
about 2 billion years ago that oxygen began to
build up in the earths atmosphere. All oxygen
in the air we breathe has cycled through
photosynthetic organisms.

• Chapter 07

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• A. Light
• Visible light makes up only a small portion of
  the electromagnetic spectrum.

• Sunlight consists of
• 4 Ultraviolet (UV) radiation
• 44 Visible light
• 52 Infrared (IR) radiation
• Of these 3 types of radiation, we are primarily
  concerned with visible light because it provides
  the right amount of energy to power
  photosynthesis. UV radiation is too powerful,
  while infrared radiation is not powerful enough

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• Characteristics of Visible Light
• is a spectrum of colors ranging from violet to
  red
• consists of packets of energy called photons
• photons travel in waves, having a measurable
  wavelength (?)
• ? distance a photon travels during a complete
  vibration measured in nanometers (nm)
• 1 nanometer a billionth of a meter
• The wavelengths of visible light range between
  390 (violet end) and 760 nanometers (red end).

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• A photons energy is inversely related to its
  wavelength...
• ...the shorter the ?, the greater the energy it
  possesses.
• Which of the following photons possess the
  greatest amount of energy?

• Green photons ? 530nm
• Red photons ? 660nm
• Blue photons ? 450nm
• Blue photons are the most energetic because they
  have the shortest wavelengths.

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• What happens to light when it strikes an object?
• Reflected (bounces off)

• Transmitted (passes through)
• Reflected or transmitted
• wavelengths determine the
• color of the object. Leaves
• appear to be green because the pigment they
  possess reflect green wavelengths of light.
• Absorbed
• Objects that reflect all wavelengths of light
  (absorb none) are white, while objects that
  reflect none (absorb all) are black.

Only absorbed wavelengths of light function in
photosynthesis.
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• B. Photosynthetic Pigments
• Molecules that capture photon energy by absorbing
  certain wavelengths of light.
• 1. Primary pigments
• Bacteriochlorophyll - green pigment found in
  certain bacteria.
• Chlorophylls a b - bluish green pigments found
  in plants, green algae cyanobacteria.

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Chlorophyll a is the dominant pigment in plant
cells.
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• 2. Accessory Pigments
• Carotenoids - red, orange, yellow pigments found
  in plants, algae, bacteria archaea.
• Phycoerythrin - red pigment found in red algae.
• Phycocyanin - blue pigment found in red algae
  cyanobacteria.
• Each pigment absorbs a particular range of
  wavelengths.
• Only 3 accessory pigments are listed here. Table
  6.1 in text lists several others.

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• Chlorophylls absorb red (600-700nm) blue
  (400-500nm) wavelengths of light best. Since
  these are the primary photosynthetic pigments,
  photosynthesis occurs maximally under red blue
  lights (reason why grow-lamps have a purple hue).
• Accessory pigments function to capture
  wavelengths of light that chlorophylls cannot.
  They then pass that energy to the chlorphylls.
• Carotineoids absorb blue wavelengths (460-550nm)
  of light best.
• Phycoerythrin absorbs green yellow wavelengths
  of light best. Since phycoerythrins are not
  found in plants, green wavelengths of light
  contribute little to photosynthesis in plants.
• Phycocyanin absorbs yellow wavelengths of light
  best.

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• C. Chloroplasts Are usually 40-200 chloroplasts
  / cell.
• Sites of photosynthesis in plants algae.
• Concentrated in mesophyll cells of most plants.
• Note stoma (opening) in cross section of leaf.
  they function in gas exchange, allowing CO2 to
  enter O2 to exit leaf. However, stoma will
  close during hot dry conditions to conserve
  water. As we shall see later, this will impair
  photosynthesis.

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• Chloroplast structure

• Stroma - gelatinous matrix contains ribosomes,
  DNA various enzymes.
• Thylakoid - flattened membranous sac embedded
  with photosynthetic pigments.
• Double membrane surrounds stroma.
• Granum stack of thylakoids.

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• D. Photosynthesis

6CO2 12H2O ? C6H12O6 6O2 6H2O

• Occurs in two stages
• Light reactions - harvest photon energy to
  synthesize ATP NADPH.
• Carbon reactions (Calvin cycle) - use energy from
  light reactions to reduce CO2 to carbohydrate.
• Photosynthesis will be described as it occurs in
  most eukaryotic cells.

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Overview of Photosynthesis
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• 1. Light Reactions
• require light
• occur in thylakoids of chloroplasts
• involve photosystems I II (light harvesting
  systems).

Photosystems contain antenna complex that
captures photon energy passes it to a reaction
center.
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• Antenna complex contains about 300 chlorophyll
  molecules 50 accessory pigments.
• Reaction center contains a pair of reactive
  chlorophyll a molecules.
• Reaction center of photosystem I contains a pair
  of P700 chlorophyll a molecules (P stands for
  pigment they absorb light energy mostly at
  700nm).
• Reaction center of photosystem II contains a pair
  of P680 chlorophyll a molecules (they absorb
  light energy mostly at 680nm).

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Light Reactions of Photosynthesis
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• Both photosystems and electron transport chains
  are embedded in thylokoid membranes.
• Light reactions begin with photosystem II.
• 1. Light strikes PSII, exciting 2 electrons,
  which are passed to an electron acceptor.
  Electrons lost from PSII must be replaced -
  replacement electrons are obtained by splitting
  water (O2 is released as a byproduct of the light
  reactions).
• 2. Electrons (from PSII) flow down ETC, providing
  energy for production of ATP by chemiosmotic
  phosphorylation.
• 3. Light strikes PSI, exciting 2 electrons, which
  are passed to an acceptor molecule. Electrons
  reaching bottom of ETC are passed to PSI as
  replacement electrons.
• 4. Excited electrons flow down a 2nd ETC,
  providing energy for production of NADPH.
• Note electrons released when water was split
  eventually end up in NADPH!!!!

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• ATP Production by Chemiosmotic Phosphorylation

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• ATP is produced as electrons from PSII flow down
  the electron transport chain toward PSI.
• 1. As electrons flow down the chain, energy is
  released. Energy is used to pump H (protons)
  from the stroma into the thylakoid space,
  creating a proton gradient.
• 2. Protons within the thylakoid space flow back
  into the stroma through channels called ATP
  synthases.
• 3. As protons flow through the ATP synthase, ADP
  is phosphorylated, forming ATP.
• The coupling of ATP formation to energy release
  from a proton gradient is called chemiosmotic
  phosphorylation.

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• 2. Carbon Reactions (Calvin cycle C3 cycle)
  
• Called Calvin cycle in honor of the American
  biochemist Melvin Calvin.
• Called C3 cycle because CO2 is fixed as a 3C
  compound (PGA).
• do NOT require light (occur in both darkness
  light as long as ATP NADPH are available)
• occur in stroma of chloroplasts
• require ATP NADPH (from light reactions), and
  CO2
• NADPH is often the limiting factor of carbon
  reactions, because cells have only 1 mechanism
  for its production (light reactions of
  photosynthesis). NADPH cannot be made at night,
  so when it runs out, carbon reactions cease.
• Not likely that ATP will be a limiting factor of
  carbon reactions because cells have other
  mechanisms for making ATP.

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Calvin Cycle
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• 1. Carbon fixation
• The enzyme rubisco fixes CO2 attaches CO2 to
  the 5-carbon sugar, ribulose biphosphate (RuBP).
  The resulting 6C compound is unstable
  immediately splits to form two 3C molecules
  (PGA).
• Rubisco is one of the most important abundant
  enzymes in the world.
• 2. PGAL synthesis
• The energy in ATP NADPH is used to convert PGA
  PGAL (phosphoglyceraldehyde).
• PGAL is the direct carbohydrate product of the
  carbon reactions.
• 3. PGAL molecules are siphoned off combined to
  form glucose, sucrose, starch other organic
  molecules.
• 4. Regeneration of RuBP
• Some of the PGAL is rearranged to regenerate
  RuBP. essential step in perpetuating the cycle

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• Plants that use only the Calvin cycle to fix
  carbon are called C3 plants.
• Ex. cereals, peanuts, tobacco, spinach, sugar
  beets, soybeans, most trees lawn grasses.
• 85 of all plant species are C3 plants.

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• E. Photorespiration
• Process that counters photosynthesis.
• Occurs when stomata close under hot, dry
  conditions
• O2 levels in plant increase
• CO2 levels in plant decrease
• Under these conditions, rubisco fixes O2 (rather
  than CO2).
• Thus, PGAL is NOT produced.
• Stoma close on hot dry days (to conserve water).
  Thus, CO2 is steadily being depleted, while O2 is
  steadily increasing inside the plant.
• Photorespiration severely hampers photosynthesis
  in C3 plants.

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• F. C4 and CAM Photosynthesis
• Called C4 photosynthesis because carbon dioxide
  is fixed as a 4C compound malic acid before it
  enters the Calvin cycle.
• C4 plants include sugarcane, corn, millet
  sorghum.
• About 0.4 of plant species are C4 plants.
• Adaptations that allow certain plants to conserve
  water and reduce photorespiration at higher
  temperatures.
• 1. C4 Photosynthesis
• C4 plants reduce photorespiration by physically
  separating the light reactions and Calvin cycle.

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Leaf anatomy of a C4 plant

• C4 Photosynthesis
• Light reactions occur in chloroplasts of
  mesophyll cells.
• Calvin cycle occurs in chloroplasts of bundle
  sheath cells.

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• C4 plants have an additional biochemical pathway
  that allows them to fix CO2 even when levels
  within the plant fall very low.
• CO2 is fixed initially in mesophyll cells using
  the enzyme PEP carboxylase. PEP carboxylase has
  a high affinity for CO2.
• PEP carboxylase converts CO2 into a 4C compound,
  malic acid.
• Mesophyll cells actively pump malic acid into
  bundle sheath cells. CO2 is released enters
  Calvin cycle (fixed by rubisco). This adaptation
  keeps CO2 levels high in bundle sheath cells, so
  rubisco functions optimally (photorespiration
  does not occur).
• Note C4 plants dominate in hot, dry
  environments because they have a distinct
  advantage over C3 plants (able to inhibit water
  loss reduce photorespiration).
• However, C4 plants are not as abundant in other
  habitats because they are at an energetic
  disadvantage. They must use use energy to pump
  malic acid into bundle sheath cells.

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• 2. CAM Photosynthesis
• CAM plants reduce photorespiration by acquiring
  CO2 at night.

CAM plants include cacti, pineapple, Spanish
moss, orchids, some ferns the wax
plant. About 10 of plant species are CAM
plants.

• Night
• mesophyll cells fix CO2 as malic acid
• malic acid is stored in vacuoles.
• Day
• malic acid releases CO2 which enters Calvin cycle.

Malic acid