Converts water and carbon dioxide from the environment into organic food molecules and oxygen gas

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PHOTOSYNTHESIS
Uses energy from sunlight
Converts water and carbon dioxide from the
environment into organic food molecules and
oxygen gas
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Photosynthetic organisms many kinds,
terrestrial and aquatic
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Grasslands
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Oceans
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Freshwaters
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Two Sets of Reactions
1. Light-dependent (light) Reactions
- thylakoid membranes - chlorophyll absorbs
sunlight - makes energy molecules ATP -
Makes O2 gas - reduces electron carrier NADP
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2. Light-independent reactions

• Calvin cycle
• (dark reactions)
• Uses energy molecules from light reactions
• Fixes carbon dioxide
• Changes inorganic CO2 into a compound plants can
  use
• Oxidizes NADPH
• Makes glucose

6 CO2 6 H2O ? C6H12O6 O2
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Visible light drives the light reactions
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Sunlight is white light, containing all colors
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• Colors of light
• Depends on wavelength (l)
• Shorter wavelength
• blue-violet end of spectrum
• ? higher energy
• Longer wavelength
• red-orange end of spectrum
• ? lower energy

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Visible light small part of Electromagnetic
spectrum travels as a wave ---- behaves as a
particle (photon)
Shorter wavelength Higher energy
Shorter wavelength Higher energy
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Absorption of Light Energy
Plants absorb blue and red light best
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Photosynthetic pigments

• Plants have multiple pigments to absorb as much
  sun energy as possible
• Chlorophyll a is the primary pigment starts the
  chain of reactions
• Chlorophyll b, carotenes, xanthophylls and others
  are accessory pigments.
• They absorb wavelengths that chlorophyll a cannot
  absorb ? use more of sunlight

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Chromatography
Separates a liquid mixture by solubility
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Colors of light absorbed by a chloroplast
Colors NOT absorbed are reflected or
transmitted -- the colors we SEE
Absorbed light energy is transferred to
electrons in pigment -- energized electrons
Chlorophyll absorbs mostly from the red and blue
ends of the spectrum - reflects green.
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Parts of a chloroplast
Thylakoid membranes - have chlorophyll -
absorb sunlight - site for 1st set of
reactions
Granum stack of thylakoid sacs Stroma fluid
surrounding thylakoids - site for 2nd set of
reactions
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Parts of a Leaf
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What is a Photosystem?
A cluster of pigments and other molecules in
thylakoid membranes

• Capture light energy
• Energized electrons begin reaction chains that
  make ATP and NADPH

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What are electron carriers?

• Electron carriers are compounds that accept
  high-energy electrons from one molecule in a
  chain of reactions and transfer them to another
  molecule in a later reaction.
• a. Also usually accept hydrogen ions (H)
• b. Transferring electrons and H transfers
  energy
• In photosynthesis, NADP accepts electrons and
  H, becoming NADPH
• Helps change sunlight to chemical energy

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Light reactions are a series of oxidation and
reduction reactions

• Oxidation losing energy
• Lose an electron (alone or on atoms)
• Lose a hydrogen atom (H e-)
• Lose a phosphate group (PO4)
• Be broken apart (decomposed, hydrolyzed)
• Reduction gain energy
• Gain electrons (alone or on atoms)

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What is NADP?

• NADP accepts electrons and H ions
• NADP H ? NADPH
• H comes from water in light reactions
• Gives H to CO2 in dark reactions
• Electron energy helps power reactions
• Coenzyme helps in a reaction but is not the
  catalyst

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The Light Reactions

• 1. Photosystems absorb sunlight
• excited electrons from chlorophyll transfer to
  Electron Transport Chains
• Make ATP and NADPH

• 2. Light splits a water molecule
• H2O ? 2 H 2 e-
  O
• H reduce NADP Oxygen ? O2
• Electrons -- replace electrons lost from
  chlorophyll

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Light reactions
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Electron Transport Chain
Electrons pass from one acceptor molecule to the
next in a series (chain) of oxidation-reduction
reactions Energy from electrons pumps H
across membrane ? proton gradient in
thylakoid space
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Chemiosmosis and ATP Synthase

• ATP Synthase an enzyme in thylakoid membrane
• Electron transport chain generates H gradient
• H ions diffuse through ATP synthase
• activate synthase enzyme
• Phosphorylates ADP
• (ADP P)
• Makes ATP

ADP P ? ATP
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Chemiosmosis
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Summary of Light reactions

• 1. Capture light energy, make ATP
• Split water (H2O) into 2 H O 2e-
• Electrons replace those lost from chlorophyll
• O makes oxygen gas
• 3. Add H and e- to NADP ? NADPH
• 1) Later they become part of glucose molecule

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Light Reactions
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Calvin Cycle Dark reactions, or
Light-Independent Reactions

• In stroma of chloroplast
• Uses ATP made in light reactions
• Fixes CO2 from air
• Adds H ions from water
• Adds electrons from ETC
• Makes GLUCOSE

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How is sugar made?
CO2 H ? PGAL (3 carbon
sugar) 2 PGAL ? 1 glucose Carbon fixing CO2 gas
made into an organic compound Energy needed to
make glucose comes from ATP made in the light
reactions
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Calvin Cycle
in stroma
Carbon is fixed
RuBP (5-C) ribulose bisphosphate
Citric Acid (6-C)
2 PGA (3-C)
ATP and NADPH are oxidized
Remaining PGAL ? RuBP
2 PGAL (G3P) ? 1 glucose
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LE 7-5
Photosynthesis uses light energy to make food
molecules
CO2
Chloroplast
H2O
Light
Fixes carbon (CO2 becomes part of organic
molecule)
NADP
Absorbs light energy
ADP
P
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LIGHT REACTIONS
CALVIN CYCLE (in stroma)
(in thylakoids)
Makes energy molecules ATP, NADPH
ATP
Uses energy molecules made in light
Electrons
NADPH
Makes sugar
Light splits water -makes O2
Starch
Lipids
O2
Sugar
proteins
cellulose
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Factors Affecting Photosynthesis

• Light bright sun, more energy
• a. Long days (summer), more light absorbed
• b. Wavelength cannot absorb green light
• 2. Temperature warm, but not too hot
• a. Hot days stomata close to save water
• 3. Water soil must be moist
• Water comes up through xylem in veins
• Exits through open stomata
• Water low? stomata close

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C 3 plants in hot weather?

• C-3 carbon fixed into 3-carbon compound
• In hot, dry weather, C-3 plants
• leaf openings close to save water
• No more CO2 comes in
• Photorespiration
• Intermediate products used for energy by plant
• No glucose made

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Guard cells open/close stomata
Close when CO2 or water is low in plant - saves
energy
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C-4 Plants
C-4 plants fix and store CO2 in a 4-carbon
compound while stomates are open - use stored
carbon when stomates close - in hot weather,
can still make sugar
Corn Sugar Cane
crabgrass
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C-4 Pathway
Stores carbon as a 4-carbon compound in special
cells around veins called bundle sheath cells
When stomates close, stored carbon is changed
back to CO2 for Calvin cycle
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CAM Plants
Succulents cacti
pineapples
Fix CO2 during the night, when it is cool enough
for open stomates - Do photosynthesis during the
day, using the stored carbon
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Parasitic plants
Supplement nutrition by taking from other
organisms

Dodder Plant Pure parasite cannot
photosynthesize - Special roots tap into veins of
host plant
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Mistletoe supplements photosynthesistaps into
host cell veins drains sap
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Carnivorous Plants
Eat insects to get nitrogen Live where
decomposition is slow
Pitcher plant
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Pitcher plant
Sweet nectar at bottom of tube - Insects stuck in
nectar, digested
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Sundew eats insects
Sugary dew attracts insects

• Insects get stuck
• Leaves enclose and digest
• Absorb nutrients from insect

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Venus Fly Trap
Hinged leaves with trigger hairs
Insect walking on leaves touches hairs - Leaves
close, insect digested
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Indian Pipe
Cannot photosynthesize
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Carotenes in nature
Where can we see carotenes, xanthophylls, and
other pigments besides in autumn leaves?
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Photosynthesis reduces atmospheric CO2

• Excess greenhouse gases
• - from human activity
• -burning fossil fuels
• - deforestation
• - increases global warming
• - photosynthesis removes CO2 and helps moderate
  warming

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Greenhouse - Traps heat indoors
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TALKING ABOUT SCIENCE
7.14 Mario Molina talks about earths protective
ozone layer

• Nobel Prize winner Mario Molina has studied how
  pollutants are affecting Earth's ozone layer
• Solar radiation converts O2 high in the
  atmosphere to ozone (O3)
• The ozone layer shields organisms on Earth's
  surface from damaging UV radiation
• CFCs have caused dangerous thinning of the ozone
  layer
• International restrictions on CFC use are
  allowing a slow recovery

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LE 7-14b
The ozone layer
Southern tip of South America
Antarctica
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Pond Mud and its Bacteria (Winogradsky Column)

Anaerobic Phototrophs - purple bacteria -
green sulfur bacteria
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• TRANSPORT IN A LEAF
• Water enters a leaf through the veins xylem
  carries water up from the soil. Water then
  diffuses from the veins into the mesophyll cells.
• Carbon dioxide diffuses into the leaf through
  small openings on the surface called stomates
• Phloem carries the newly synthesized glucose to
  other parts of the plant where it can be used for
  energy or stored.

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The Light Reactions of Photosynthesis one (of
two) electron transport chains
Photosystem cluster of chlorophyll, other
pigments, proteins and other molecules that all
work together to harvest light energy and package
it in ATP and NADPH

• Chlorophyll absorbs sunlight
• Energized electrons in chlorophyll go to E.T.C
• ETC makes ATP (ADP P)
• Water is split electrons from water replace the
  electrons lost from chlorophyll
• Another chlorophyll also absorbs sunlight
• This pathway makes NADPH (NADP H)

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ATP synthesis As in cell respiration, electron
transport chain powers production of ATP by
chemiosmosis through enzyme ATP synthase
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The Dark (Light-Independent) Reactions of
Photosynthesis
Calvin Cycle

• Uses energy molecules made in light reactions
• ATP, NADPH
• a 5-carbon compound in the stroma is used in the
  reactions and then recycled
• CO2 combines with hydrogen (from NADPH) to make
  3-carbon PGAL
• 2 PGAL combine to make one glucose

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Overview of Photosynthesis