PHOTOSYNTHESIS

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PHOTOSYNTHESIS
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Photosynthesis

• process by which green plants and some organisms
• seaweed, algae certain bacteria
• use light energy to convert CO2 water ?glucose
• all life on Earth, directly or indirectly,
  depends on photosynthesis as source of food,
  energy O2

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Autotrophs

• self feeders
• organisms that make their own organic matter from
  inorganic matter
• producers
• need inorganic molecules such as CO2, H2O
  minerals to make organic molecules

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Heterotrophs

• consumers
• other feeders
• depend on glucose as an energy source
• cannot produce it
• obtained by eating plants or animals that have
  eaten plants

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Carbon and Energy Flow
Heat energy
CO2 H2O
Light energy
Photosynthesis
Carbs Proteins Lipids O2
Cellular (Aerobic) Respiration (ATP Produced)
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Food Chain

• byproduct of photosynthesis is O2
• humans other animals breathe in oxygen
• used in cellular respiration

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Other Benefits of Photosynthesis

• humans also dependent on ancient products of
  photosynthesis
• fossil fuels
• natural gas, coal petroleum
• needed for modern industrial energy
• complex mix of hydrocarbons
• represent remains of organisms that relied on
  photosynthesis millions of years ago

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Photosynthesis

• plants produce more glucose than they can use
• stored as starch other carbohydrates in roots,
  stems leaves
• can draw on these reserves for extra energy or
  building materials as needed

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Sites of Photosynthesis

• leaves green stems
• in cell organelles
• chloroplasts
• concentrated in green tissue in interior of leaf
• mesophyll
• green due to presence of green pigment chlorophyll

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Chloroplasts

• each cell has 40-50 chloroplasts
• oval-shaped structures with double membrane
• inner membrane encloses compartment filled with
  stroma
• suspended in stroma are disk-shaped
  compartments-thylakoids
• arranged vertically like stack of plates
• one stack-granum (plural, grana)
• embedded in membranes of thylakoids are hundreds
  of chlorophyll molecules

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Chlorophyll

• light-trapping pigment
• other light-trapping pigments, enzymes other
  molecules needed for photosynthesis are also
  found in thylakoid membranes

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How Photosynthesis Works

• Requires
• CO2
• Water
• Sunlight
• Makes
• O2
• Glucose

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How Photosynthesis Works

• CO2 enters plant via pores- stomata in leaves
• water-absorbed by roots from soil
• membranes in chloroplasts provide sites for
  reactions of photosynthesis
• chlorophyll molecules in thylakoids capture
  energy from sunlight
• chloroplasts rearrange atoms of inorganic
  molecules into sugars other organic molecules

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Photosynthesis

• redox reaction
• 6CO2 12H2O?C6H12O6 6O2 6H2O in presence of
  light
• must be an oxidation a reduction
• water is oxidized
• loses electrons hydrogen ions
• carbon dioxide is reduced
• gains electrons hydrogens

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Photosynthesis

• relies on a flow of energy electrons initiated
  by light energy
• light energy causes electrons in chlorophyll
  pigments to boost electrons up out of their
  orbit
• hydrogens along with electrons are transferred to
  CO2?sugar
• requires that H2O is split into H O2
• O2 escapes to air
• light drives electrons from H2O to NADP which is
  oxidized? NADPH which is reduced

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Photosynthesis

• 2 stages
• light-dependent reactions
• chloroplasts trap light energy
• convert it to chemical energy
• contained in nicotinamide adenine dinucleotide
  phosphate-(NADPH) ATP
• used in second stage
• light-independent reactions
• Calvin cycle
• formerly called dark reactions
• NADPH (electron carrier) provides hydrogens to
  form glucose
• ATP provides energy

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Light Dependent Reactions

• convert light energy to chemical energy produce
  oxygen
• takes place in thylakoid membranes
• solar energy absorbed by chlorophyll?ATP NADPH

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Light Energy for Photosynthesis

• sun energy is radiation
• electromagnetic energy
• travels as waves
• distance between 2 waves- wavelength
• light contains many colors
• each has defined range of wavelengths measured in
  nanometers
• range of wavelengths is electromagnetic spectrum
• part can be seen by humans
• visible light

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Pigments

• light absorbing molecules
• built into thylakoid membranes
• absorb some wavelengths reflect others
• plants appear green because chlorophyll-does not
  absorb green light
• reflected back.
• as light is absorbed?energy is absorbed
• chloroplasts contain several kinds of pigments
• different pigments absorb different wavelengths
  of light
• red blue wavelengths are most effective in
  photosynthesis
• other pigments are accessory pigments
• absorb different wavelengths
• enhance light-absorbing capacity of a leaf by
  capturing a broader spectrum of blue red
  wavelengths along with yellow and orange
  wavelengths

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Pigment Color Maximum Absoption

• Violet   400 - 420 nm
• Indigo   420 - 440 nm
• Blue   440 - 490 nm
• Green   490 - 570 nm
• Yellow   570 - 585 nm
• Orange   585 - 620 nm
• Red   620 - 780 nm

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Chlorophylls

• Chlorophyll A
• absorbs blue-violet red light
• reflects green
• participates in light reactions
• Chlorophyll B
• absorbs blue orange light
• reflects yellow-green
• does not directly participate in light reactions
• broadens range of light plant can use by sending
  its absorbed energy to chlorophyll A

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Carotenoids

• yellow-orange pigments
• absorb blue-green wavelengths
• reflect yellow-orange
• pass absorbed energy to chlorophyll A
• have protective function
• absorb dissipate excessive light energy that
  would damage chlorophylls

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Light Energy

• light behaves as discrete packages of energy
  called photons
• fixed quantity of energy
• shorter wavelengths have greater energy
• violet light has 2X as much energy as red

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Light Energy

• when pigment absorbs a photon
• pigments electrons gains energy
• electrons are excited
• unstable
• electrons do not stay in unstable state
• fall back to original orbits
• as electrons fall back to ground state heat is
  released
• absorbed energy is passed to neighboring
  molecules

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Photosynthesis

• Pigments
• Absorb light
• Excites electrons
• Energy passed to sites in the cell
• Energy used to make glucose

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Photosystems

• chlorophyll other pigments are found clustered
  next to one another in a photosystem
• energy passes rapidly from one chlorophyll
  pigment molecule to another

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Photosystems

• two photosystems participate in light reactions
• photosystem I II
• each has a specific chlorophyll at reaction
  center
• photosystem II
• chlorophyll P680
• photosystem I
• chlorophyll P700
• named for type of light they absorb best
• P700 absorbs light in far red region of
  electromagnetic spectrum

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Reaction Center

• when photon strikes one pigment molecule
• energy jumps from pigment to pigment until
  arrives at reaction center
• electron acceptor traps a light excited electron
  from reaction center chlorophyll
• passes it to electron transport chain which uses
  energy to make ATP NADPH

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Reaction Center
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Light Reactions

• during process of making ATP NADPH
• electrons are removed from molecules of water
• passed from photosystem II to photosystem I to
  NADP

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Photosystem II

• water is split
• oxygen atom combines with oxygen from another
  split water forming molecular oxygen-O2
• each excited electron passes from photosystem II
  to photosystem I via electron transport chain

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Photosystem I

• primary electron acceptor captures an excited
  electron
• excited electrons are passed through a short
  electron transport chain to NADP reducing it to
  NADPH
• NADP is final electron acceptor
• electrons are stored in a high state of potential
  energy in NADPH molecule
• NADPH, ATP and O2 are products of light
  reactions

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ATP Formation-Chemiosmosis

• uses potential energy of hydrogen ion
  concentration gradient across membrane
• gradient forms when electron transport chain
  pumps hydrogen ions across thylakoid membrane as
  it passes electrons down chain that connects two
  photosystems

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ATP Formation-Chemiosmosis

• ATP synthase (enzyme) uses energy stored by H
  gradient to make ATP
• ATP is produced from ADP Pi when hydrogen ions
  pass out of thylakoid through ATP synthase
• photophosphorylation

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Chemiosmosis
H
H
pH 7
pH 8
Chemiosmosis
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Substrate-level Phosphorylation
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Calvin Cycle

• light independent reactions
• depend on light indirectly to obtain inputs for
  cycle-ATP NADPH
• takes place in stroma of chloroplast
• each step controlled by different enzyme
• cycle of reactions
• makes sugar from CO2 energy
• ATP provides chemical energy
• NADPH provides high energy electrons for
  reduction of CO2 to sugar

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Steps of Calvin Cycle

• starting material-ribulose bisphosphate (RuBP)
• first step-carbon fixation
• rubisco (an enzyme) attaches CO2 to RuBP
• Next-reduction reaction takes place
• NADPH reduces 3-phosphoglyceric acid (3-PGA) to
  glyceraldehye 3-phosphate (G3P) with assistance
  of ATP
• to do this cycle uses carbons from 3 CO2
  molecules
• to complete cycle must regenerate beginning
  component-RuBP
• for every 3 molecules of CO2 fixed, one G3P
  molecule leaves cycle as product of cycle
• remaining 5 G3P molecules are rearranged using
  ATP to make 3 RuBP molecules

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Calvin Cycle

• regenerated RuBP is used to start Calvin cycle
  again
• process occurs repeatedly in each chloroplast as
  long as CO2, ATP NADPH are available
• thousands of glucose molecules are produced
• used by plants to produce energy in aerobic
  respiration
• used as structural materials
• stored

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Photosynthesis Variations

• plants vary in the way they produce glucose and
  when

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C3 Plants

• use CO2 directly from air
• first organic compound produced is a 3 carbon
  compound 3-PGA
• reduce rate of photosynthesis in dry weather
• CO2 enters plants through pores in leaves
• on hot days stomata in leaves close partially to
  prevent escape of water
• with pores slightly open, adequate amounts of CO2
  cannot enter leaf
• Calvin cycle comes to a halt
• no sugar is made
• in this situation rubisco adds O2 to RuBP
• 2-carbon product of this reaction is broken down
  by plant cells to CO2 H20
• Photorespiration
• provides neither sugar nor ATP

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C4 Plants

• have special adaptations allowing them to save
  water without shutting down photosynthesis
• corn, sugar cane crabgrass
• evolved in hot, dry environments
• when hot dry stomata are closed
• saves water
• sugar is made via another route
• developed way to keep CO2 flowing without
  capturing it directly from air

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C4 Plants

• have enzymes that incorporate carbon from CO2
  into 4-C compound
• enzyme has an intense desire for CO2
• can obtain it from air spaces even when levels
  are very low
• 4-C compound acts as a shuttle
• transfers CO2 to nearby cells -bundle-sheath
  cells
• found in vast quantities around veins of leaves
• CO2 levels in these cells remain high enough for
  Calvin cycle to produce sugar

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CAM Plants

• pineapple, some cacti succulent plants
• conserve water by opening stomata letting CO2
  in at night
• CO2 is fixed into a 4-C compound
• saves CO2 at night releases it in the day
• photosynthesis can take place without CO2 needing
  to be admitted during the day when conditions are
  hot and dry

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Environmental Consequences of Photosynthesis

• CO2 makes up 0.03 of air
• provides plants with CO2 to make sugars
• important in climates
• retains heat from sun that would otherwise
  radiate from Earth
• warms the Earth
• greenhouse effect

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Global Warming

• CO2 traps heat?warms air
• maintains average temperature on Earth about 10oC
  warmer than without it.
• Earth may be in danger of overheating because of
  this greenhouse effect
• CO2 in air is increasing because of
  industrialization
• when oil, gas and coal are burned CO2 is released
• levels in atmosphere have increased 30 since
  1850
• increasing concentrations have been linked to
  global warming
• slow steady rise in surface temperature of
  Earth
• could have dire consequences for all life forms
  on Earth