PHOTOSYNTHESIS

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PHOTOSYNTHESIS
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Autotrouphs
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Photosynthesis

• Anabolic (small molecules combined)
• Endergonic (stores energy)
• Carbon dioxide (CO2) requiring process that uses
  light energy (photons) and water (H2O) to produce
  organic macromolecules (glucose).
• 6CO2 6H2O ? C6H12O6 6O2

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Question

• Where does photosynthesis take place?

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Plants

• Autotrophs produce their own food (glucose)
• Process called photosynthesis
• Mainly occurs in the leaves
• a. stoma - pores
• b. mesophyll cells

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Stomata (stoma)

• Pores in a plants cuticle through which water
  vapor and gases (CO2 O2) are exchanged between
  the plant and the atmosphere.

Stoma
Found on the underside of leaves
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Question

• Why are plants green?

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Chlorophyll Molecules

• Located in the thylakoid membranes
• Chlorophyll have Mg in the center
• Chlorophyll pigments harvest energy (photons) by
  absorbing certain wavelengths (blue-420 nm and
  red-660 nm are most important)
• Plants are green because the green wavelength is
  reflected, not absorbed.

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Wavelength of Light (nm)

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Absorption of Light by Chlorophyll
Chlorophyll absorbs blue-violet red light best
Absorption
wavelength
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Question

• During the fall, what causes the leaves to change
  colors?

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Fall Colors

• In addition to the chlorophyll pigments, there
  are other pigments present
• During the fall, the green chlorophyll pigments
  are greatly reduced revealing the other pigments
• Carotenoids are pigments that are either red,
  orange, or yellow

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Redox Reaction

• The transfer of one or more electrons from one
  reactant to another. This allows atoms to be
  rearranged into new molecules.
• Two types
• 1. Oxidation is the loss of e-
• 2. Reduction is the gain of e-

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Question

• What do cells use for energy?

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Energy for Life on Earth

• Sunlight is the ULTIMATE energy for all life on
  Earth
• Plants store energy in the chemical bonds of
  sugars
• Chemical energy is released as ATP during
  cellular respiration

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Parts of Photosynthesis
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Photosynthesis Overview
Reactants
Products
3 carbon sugar (G3P)
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Two Parts of Photosynthesis

• Two reactions make up photosynthesis
• 1.Light Reaction or Light Dependent Reaction -
• Produces energy from solar power (photons) in
  the form of ATP and NADPH.

SUN
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Two Parts of Photosynthesis

• 2. Calvin Cycle or Light
  Independent Reaction
• Also called Carbon Fixation or C3 Fixation
• Uses energy (ATP and NADPH) from light reaction
  to make sugar (glucose).

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Energy Carriers

• Nicotinamide Adenine Dinucleotide Phosphate
  (NADP)
• NADP Oxidized Form (Empty, needs e-)
• Picks Up 2 high-energy electrons and H from the
  Light Reaction to form NADPH (Full)
• NADPH formed in the light reactions carries high
  energy electrons to be passed on to another
  molecule in the calvin cycle.

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NADPH
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Light Reactions
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STEP 1 Photosystem II

1. Sun Light hits pigments
2. excites 2 electrons.
3. Electrons jump from pigment to pigment
4. Electrons reach P680 (reaction center a
   specialized chlorophyll)
5. Passed to primary electron acceptor (Protein)

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STEP 2 Replacement of Electrons

• Water is split
• 2. 2 electrons (from H) replace those lost in PS
  II
• 2. H builds a gradient (High concentration) in
  Thylakoid space
• 3. Oxygen gas is released out

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STEP 3 Electron Transport Chain

• Electrons are passed down a series of electron
  acceptor proteins
• As electrons are passed down energy is lost
  with each transfer
• Energy is used by to create H gradient in
  thlylakoid for production of ATP

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STEP 4 - Photosystem I

• SAME AS PHOTOSYSTEM II
• Light excites 2 electrons in pigment
• Electrons are passed until reach reaction center
  (P700)
• Electrons are sent to second primary electron
  acceptor.
• Electrons from first ETC fill missing space.

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STEP 5 ETC

1. Electrons are passed from Primary acceptor to
   NADP reductase
2. Energy from small ETC is used to add electrons to
   NADP to form NADPH and H (electron carrier)

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ATP SYNTHESIS

• Enzyme in thylakoid membrane called ATP Synthase
• As H ions passed through thylakoid membrane,
  enzyme attaches a free phosphate group to ADP
• Forms ATP in stroma to be used in CALVIN CYCLE

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Chemiosmosis

• Powers ATP synthesis
• Takes place across the thylakoid membrane
• Builds H (hydrogen ion) gradient (high
  concentration in thylakoid space)
• Uses ETC and ATP synthase (enzyme)
• H move down their concentration gradient through
  channels of ATP synthase forming ATP from ADP

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Chemiosmosis
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Calvin Cycle

• Light Independent Reaction
• Uses CO2 RuBP (5 C) and an enzyme called
  RuBISco to carry out Carbon Fixation
• C3 plants (80 of plants on earth) product is a
  3 carbon sugar called G3P
• Occurs in the stroma of chloroplasts
• A series of steps modify the molecule RuBP until
  final product is produced (biochemical pathway)
• 1 G3P is the product for sugar production 5 G3P
  are rearranged to form the starting molecule 3
  RuBP
• Uses ATP for energy and NADPH for reduction from
  light reaction
• To produce glucose it takes 6 turns and uses 18
  ATP and 12 NADPH.

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Carbon dioxide added 1 at a time! 3 turns to make
1 G3P 6 turns to make 1 glucose.
RuBISco control enzyme
RuBP
ATP used ADP and Pi released
ATP and NADPHrelease ADP, Pi, and NADP
molecules rearranged and split to form 6 (3C)
G3P
2 G3P make glucose It can make any monomer!
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Photorespiration

• Occurs on hot, dry, bright days
• Stomates close
• Fixation of O2 instead of CO2
• Produces 2-C molecules instead of 3-C sugar
  molecules
• Produces no sugar molecules or no ATP

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Photorespiration

• Because of photorespiration, plants have special
  adaptations to limit the effect of
  photorespiration
• 1. C4 plants
• 2. CAM plants

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C4 Plants

• Hot, moist environments
• 15 of plants (grasses, corn, sugarcane)
• Photosynthesis occurs in 2 places to prevent
  oxygen build up.
• Light reaction - mesophyll cells
• Calvin cycle - bundle sheath cells

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CAM Plants

• Hot, dry environments
• 5 of plants (cactus and ice plants)
• Stomates closed during day
• Stomates open during the night
• Light reaction - occurs during the day
• Calvin Cycle - occurs when CO2 is present

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Question

• Why do CAM plants close their stomata during the
  day?

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• Cam plants close their stomata in the hottest
  part of the day to conserve water