PHOTOSYNTHESIS

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PHOTOSYNTHESIS

• AP Biology
• CHAPTER 10

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- The early atmosphere lacked oxygen. From the
1st plants, it took _____ years to produce
oxygen. We now enjoy approximately _____
oxygen in the atmosphere.

• This made aerobic respiration possible.
• This formed the ozone layer (O3), which
  protects us from harmful solar radiation

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• Photosynthesis is a REDOX reaction
• Respiration is an exergonic RXN (NRG released
  from oxidation of sugar)
• Photosynthesis is an endergonic RXN (NRG needed
  to reduce CO2)
• Light NRG (boost potential energy of electrons)
• Water is split electrons are transferred to CO2
  reducing it to sugar

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• Photosynthesis has 2 stages
• LIGHT REACTION (light dependent reaction)
  convert light energy to chemical bond energy in
  ATP and NADPH
• Occurs on the thylakoids
• NADP ? NADPH
• Oxygen is a byproduct
• Generates ATP
• CALVIN CYCLE (light independent reaction) take
  carbon dioxide and REDUCE it to carbs/organic
  compounds
• Occurs in the stroma
• Carbon fixation
• Does not require light directly
• NADPH provides the reducing power
• APT provides the chemical energy

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• AUTOTROPHS self-feeders
• Photoautotrophs use light
• Chemoautotrophs- use inorganic substances such as
  sulfur or ammonia as an energy source
• producers
• Heterotrophs other feeders
• Consumers
• Chemoautotroph autotrophs that get their energy
  from chemicals
• Chloroplasts are
• Mostly in the
• mesophyll

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CHLOROPLAST
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Cross section of a leaf
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• Light can be
• Reflected (how you see things)
• Transmitted (passed through)
• Absorbed (changed from light energy to another
  form)
• Pigments substances that absorb visible light.
  They absorb different wavelengths.
• Each pigment has a characteristic absorption
  spectrum which can be determined by a
  spectrophotometer.

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PIGMENTS IN PLANTS

• Chlorophyll a molecules can participate directly
  in the light RXN accessory pigments help by
  transferring energy to chlorophyll a
• Chlorophyll b green-yellow pigment
• Carotenoids yellow, orange, and/or pink
• Anthocynanin Reds, purples and blues
• Xanthophylls - yellows

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Visible Spectrum

• Wavelength is the distance between the crests of
  electromagnetic waves.
• Visible light is detectable by the human eye
  (380-750 nm)
• Light behaves as if it consists of particles
  called photons.
• Sun radiates the full specturm of electromagnetic
  energy

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Absorption Spectrum

• chlorophyll a - "team captain"
• chlorophyll b - accessory (antenna) pigments
• Carotenoids etc - accessory (antenna) pigments

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• When chlorophyll absorbs a photon, one of its
  electrons is boosted to a higher energy state.
• Energy is captured in a chemical bond.

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LIGHT REACTION

• Occur in the thylakoid membrane
• Reduce NADP ? NADPH
• Give off O2 as a by-product
• Generate ATP (photophosphorylation)

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LIGHT DEPENDENT REACTION

• ON BOARD

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PHOTOSYSTEMS

• Pigments are assembled into photosystems in the
  thylakoid membrane (light collecting units). Each
  are composed of
• 1. antenna complex several hundred pigments each
  with different absorption spectra they absorb
  photons from a wide rage of light
• 2. reaction-center chlorophyll one of many
  chlorophyll a molecules in each complex can
  actually transfer an excited electron to start
  the light reaction. These pigments are located in
  the reaction center.
• 3. primary electron acceptor traps high energy
  e- released from the reaction center. This energy
  is stored as ATP and NADPH

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2 Photosystems (PS) in photosynthesis

• PS II comes first absorbs best at 680 (aka
  p680)
• PS I comes second absorbs best at 700 (aka
  p700)
• p680 and p700 are identical chlorophyll a
  molecules but each is associated with a different
  protein.

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Light reaction 2 routes

• Noncyclic flow (NOT A CYCLE)
• Occurs in the thylakoid membrane
• Passes e- from water to NADP (photolysis)
• Produces ATP by noncyclic photophosphorylation
• Produces NADPH
• Produces O2

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NONCYCLIC e- flow

1. At PSII Water is split with the help of sunlight
   (which excites the e-) oxygen is given off as
   waste, e- are carried by a primary electron
   acceptor to the electron transport chain
2. The e- are passed down protein carriers (in doing
   do provides energy for chemiosmotic synthesis of
   ATP).
3. At PS I the sunlight excites the e- again! The
   e- get shot up to another primary acceptor.
4. The e- are passed down another ETC and with the
   help of NADP reducase NADP picks up 2 H and
   becomes NADPHH

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CYCLIC ELECTRON FLOW

• e- leave chlorophyll a at the reaction center
  return to the reaction center.
• Photons are absorbed by PSI (p700) releases high
  energy e- to the primary e- acceptor which passes
  them to the cycle.
• Absorption of two photons of light sends a second
  pair of e- through the cycle
• FUNCTION to produce additional ATP without the
  generation of NADPH or evolving oxygen.

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Chemiosmosis

• Coupeling of exergonic e- flow down ETC to
  endergonic ATP production by creation of an
  electrochemical proton gradient across the
  membrane.

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ETC in Mitochondria vs Chloroplast

• Mitochondria
• Transfer chem. Energy from food to ATP. High
  energy e- pass down chain are extracted by the
  oxidation of food molecules
• Inner mitochondrial membrane pumps protons from
  matrix out of the intermembrane space
• Chloroplast
• change light energy into chemical energy.
  Photosystems capture light energy to drive
  electrons to the top of the chain.
• Pumps protons from stroma into the lumen as a
  reservoir. ATP forms in the stroma where it
  drives sugar synthesis during the calvin cycle.

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Light Independent ReactionCalvin Cycle

• Occurs in the stroma
• Similar to the Krebs Cycle starting material is
  regenerated
• C enters as CO2 and leaves as carbohydrates
• ATP (chemical energy) and NADPH (reducing power)
  are energy sources
• Calvin cycle produces 3-C sugar (G-3-P)

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• 1. Carbon dioxide enters thru stomates) and
  bonds to RuBP (enzyme action)

2. ATP and NADPH (from light reaction) are
unstable and must be used quickly! They are used
to form molecules of PGAL.
PGAL regenerates RuBP for light ind. to
continue. ATP input. (PGAL is also G3P or
glyceraldehyde 3-phosphate)
PGAL is used to produce glucose, which is stable
and can be stored!
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C4 plants

• Incorporate carbon dioxide into 4-C compounds.
• Corn, sugarcane and important agricultural
  grasses
• Leaf anatomy of C4 plants spacially segregates
  the calvin cycle from the initial corporation of
  CO2 into organic compounds.

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ALTERNATIVE MECHANISIMS OF CARBON FIXATION

• C4 Plants HOT ARID CLIMATES
• Calvin cycle in most plants produces 3-PGA as the
  first intermediate  - these are called C3 plants
  because first intermediate has 3 carbons (rice,
  wheat and soybeans)
• C4 plants produce 4-C compounds initially. (ex.
  corn, sugarcane and grasses)
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• STEP 1 CO2 added to (PEP) to form oxaloacetate
  a four carbon product. In comparison to RuBP
  PEP has a higher affinity to CO2 and none for O2.
  This can fix CO2 efficiently under hot, dry
  conditions that cause the stomata to close and O2
  concentration to rise.
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• STEP 2 After CO2 fixed by the mesophyll cells
  they convert oxaloacetate to another 4-C compound
  (usually malate)
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• STEP 3 Mesophyll cells export the 4-C products
  through plasmodesmata to bundle-sheath cells.
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CAM Plants
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• CAM Plants VERY ARID CONDITIONS - NIGHT
• Plants open their stomata mostly at night and
  closes them during the day.
• Conserves water, -but doesnt allow CO2 in..
• CO2 taken in at night and incorporated into
  organic acids. Carbon fixation is called
  crassulacean acid metabolism (CAM)
• Acids stored
• Day light reaction runs as normal and acids
  release CO2 and calvin cycle runs.

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CAM Plants

• Crassulacean Acid Metabolism
• occurs mainly in Crassulacean species (and other
  succulent plants).
• The chemical reaction of the carbon dioxide
  accumulation is similar to that of C4 plants but
  here are carbon dioxide fixation and its
  assimilation not separated spatially but in time
• arid regions
• uptake of carbon dioxide during the night
• The prefixed carbon dioxide is stored in the
  vacuoles as malate, and is used during the
  daytime for photosynthesis.

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