PHOTOSYNTHESIS

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PHOTOSYNTHESIS

• 6.1

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• The main form of energy from the sun is in the
  form of electromagnetic radiation
• Visible radiation (white light) used for
  photosynthesis
• Remember ROY G. BIV?

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The electromagnetic spectrum
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• A Red Object absorbs the blue and green
  wavelengths and reflects the red wavelengths

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• Why are plants green?

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• pigment a compound that absorbs light
• different pigments absorb different wavelengths
  of white light.
• chlorophyll is a pigment that absorbs red blue
  light (photons) so green is reflected or
  transmitted.
• Chlorophyll is located in the thylakoid membranes
• So, Plants are green because the green wavelength
  is reflected, not absorbed.

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2 types of chlorophyll

• Chlorophyll a involved in light reactions
• Chlorophyll b assists in capturing light energy
  accessory pigment
• Carotenoids accessory pigments captures more
  light energy
• Red, orange yellow

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The electromagnetic wavelengths and the
wavelengths that are absorbed by the chlorophyll
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• 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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Photosynthesis is -

• conversion of light energy into chemical energy
  that is stored in organic compounds
  (carbohydrates gt glucose)
• Used by autotrophs such as
• Plants
• Algae
• Some bacteria (prokaryotes)

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• glucose - energy-rich chemical produced through
  photosynthesis
• C6H12O6
• Biochemical pathway series of reactions where
  the product of one reaction is consumed in the
  next
• E.g. photosynthesis product is glucose which is
  used in cellular respiration to make ATP

(carbohydrate)
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Remember Redox Reactionreduction/oxidation

• The transfer of one or more electrons from one
  reactant to another
• Two types
• 1. Oxidation is the loss of e-
• 2. Reduction is the gain of e-

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Oxidation Reaction

• The loss of electrons from a substance or the
  gain of oxygen.

Carbon dioxide
Oxygen
Water
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Reduction Reaction

• The gain of electrons to a substance or the loss
  of oxygen.

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Photosynthesis equation

• Light energy
• 6CO2 6H2O C6H12O6 6O2
• Chlorophyll
• Reactants Carbon dioxide and water
• Products glucose and oxygen which is a
  byproduct

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• Where does photosynthesis take place?

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Plants

• Mainly occurs in the leaves
• a. stoma - pores
• b. mesophyll cells

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Mesophyll Cell of Leaf

Photosynthesis occurs in these 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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Chloroplast

• Organelle where photosynthesis takes place.

Thylakoid stacks are connected together
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Parts

• chloroplasts dbl membrane organelle that
  absorbs light energy
• Thylakoids flattened sacs contain pigment -
  chlorophyll
• Grana (pl granum) layered thylakoids (like
  pancakes)
• Stroma solution around thylakoids
• Stomata pore on underside of leaf where O2 is
  released and CO2 enters

• Stroma chloroplast

cytosol cytoplasm
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Thylakoid

Grana make up the inner membrane
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• 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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Structure of ATP

• ATP stands for adenosine triphosphate
• It is composed of the nitrogen base ADENINE, the
  pentose (5C) sugar RIBOSE, and three PHOSPHATE
  groups
• The LAST phosphate group is bonded with a HIGH
  ENERGY chemical bond
• This bond can be BROKEN to release ENERGY for
  CELLS to use

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Removing a Phosphate from ATP

• Breaking the LAST PHOSPHATE bond from ATP, will
  ---
• Release ENERGY for cells to use
• Form ADP (adenosine diphosphate)
• Produce a FREE PHOSPHATE GROUP

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High Energy Phosphate Bond
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FREE PHOSPHATE can be re-attached to ADP
reforming ATP Process called Phosphorylation
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Phosphorylation
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Photosynthesis
SUN

• 1. Light Reaction -
• Produces energy from solar power (photons) in
  the form of ATP and NADPH.
• 2. Calvin Cycle
• Also called Carbon Fixation or Carbon Cycle,
  Uses energy (ATP and NADPH) from light reaction
  to make sugar (glucose).

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• 3 stages of photosynthesis-

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Stages

• STAGE 1 - LIGHT REACTIONS - energy from sun is
  used to split water into H an O2
•  STAGE 2 energy is converted to chemical energy
  stored in ATP NADPH in stroma
• STAGE 3 - CALVIN CYCLE where carbon is fixed into
  glucose

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Light Reaction (Electron Flow)

• Occurs in the Thylakoid membranes
• 2 possible routes for electron flow
• Use Photosystem I and Electron Transport Chain
  (ETC) and generate ATP only
• OR use Photosystem II and Photosystem I with ETC
  and generate O2, ATP and NADPH

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Photosynthesis animation

• http//www.mhhe.com/biosci/genbio/biolink/j_explor
  ations/ch09expl.htm

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ELECTRON TRANSPORT - LIGHT REACTIONS in 5 steps
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Photosystem I and II

• Step 1 light excites e- in photosystem II
• Step 2 e- move to primary e- acceptor Step 3
  e- move along electron transport chain (etc)
• Step 4 light excites e- in photosystem I
• Step 5 e- move along 2nd (etc)
• End NADP combine H to make NADPH

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Light reaction animation

• http//www.science.smith.edu/departments/Biology/B
  io231/ltrxn.html

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Electron transport chain song

• Play the "Come On Down (The Electron Transport
  Chain)" song performed by Sam Reid. 

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Photolysis photo-chemical splitting of water
(restoring photosystem II)
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Chemiosmosis synthesis of ATP

• Powers ATP synthesis
• Takes place across the thylakoid membrane
• Uses ETC and ATP synthase
• H move down their concentration gradient forming
  ATP from ADP
• Concentration of protons is greater in thylakoid
  than stroma

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Chemiosmosis
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The Calvin Cycle

• 6.2

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Calvin Cycle -

• Biochemical pathway in photosynthesis that
  produces organic compounds using ATP NADPH
• Carbon fixation carbon atoms from CO2 are
  bonded or fixed into carbohydrates
• occurs in stroma

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Calvin Cycle

• Carbon Fixation
• C3 plants (80 of plants on earth)
• Occurs in the stroma
• Uses ATP and NADPH from light reaction as energy
• Uses CO2
• To produce glucose it takes 6 turns and uses 18
  ATP and 12 NADPH.

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Chloroplast
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Calvin Cycle (C3 fixation)
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Calvin Cycle

• Remember C3 Calvin Cycle

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Step 1 -

• CO2 diffuses fr cytosol combines with RuBP
  which splits into pair of PGA

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Step 2 -

• PGA gets phosphate gr fr ATP gets proton fr NADPH
  to become G3P
• Reaction produces ADP, NADP phosphate to be
  used again

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Step 3 -

• PGAL converts back to RuBP
• Allows Calvin cycle to continue

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Alternates

• C3 plants use Calvin cycle exclusively
• Form 3-carbon compounds
• C4 pathway evolved in hot, dry climate
• Form 4-carbon compounds
• Partially close stomata
• E.g. Corn, sugar cane, crabgrass
• CAM open stomata at night, close in day
• Grow slow, lose less water
• E.g. cactus, pineapple

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C4 Plants

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

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C4 Plants
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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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CAM Plants
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Rate of photosynthesis is effected by - light
intensity, CO2 or temperature

• High intensity or high CO2 high rate
• Growth graph levels off (plateau)
• High temp initial high rate but peaks
• Rate drops when stomata closes

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Recap

• Photosynthesis converts light energy into
  chemical energy thru series of biochemical
  pathways
• Electrons excite in photosystem II move along
  ETC to photosystem I
• electrons are replaced when water is split
• oxygen is byproduct
• ATP synthesized across thylakoid
• Calvin cycle carbon is fixed sugar produced
• 3 turns produce PGAL (PGAL keeps cycle going
• Other pathways C3, C4, CAM

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References

• www.biologyjunction.com

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