Photosynthesis

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Chapter 10

• Photosynthesis

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Photosynthetic Organisms

• All are autotrophs make organic compounds from
  inorganic compounds
• use sunlight to convert CO2 and H2O into sugar
  and release O2
• Heterotrophs make organic compounds from organic
  compounds

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Light

• Made of packets of energy called photons as well
  as wavelengths
• smallest unit of light
• fixed amount of energy per photon which is
  determined by the wavelength
• longer the wavelength, the lower the energy
  (inversely proportional)
• Pigments absorb wavelengths and this drives
  photosynthesis
• also can transmit and reflect light

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Pigments

• Organic molecules that have similar but different
  functions
• Colored because they reflect light that they
  cant absorb
• Chlorophyll is a series of small rings that
  circle a Mg2 and a hydrophobic tail to anchor it
  in the chloroplast
• absorbs violet-blue and orange-red light
  wavelengths and important in photosynthesis
• if metal ion is Fe2 then it is heme as is seen
  in hemoglobin

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Chlorophyll a

• Primary pigment, in all photosynthetic organisms
  but the bacterial ones
• Can see the wavelengths that can be absorbed but
  not green for a or b
• Chlorophyll b is bluish-green in plants, algae
  and some bacteria
• only about 50 as much as chlorophyll a

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Wavelengths of Photosynthesis

• Since the spectra are not overlapping it
  indicates that there are other pigments involved
  in photosynthesis
• Some pigments can absorb green wavelengths

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Other Pigments

• Called accessory pigments extend the useful
  wavelengths that can be used for photosynthesis
• Carotenoids are the most common, in all
  photosynthetic organisms
• red, orange and yellow pigments
• ?-carotene is most common, splits into 2
  molecules of Vitamin A, see in fall leaf color,
  tomatoes, carrots, squash, banana and avocado
• add O to carotenes to create xanthophylls which
  are red-yellow pigments in tomatoes, carrots,
  leaves, NOT as good as carotenoids in
  photosynthesis

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Photosynthesis

• 2 reactions to make the sugars
• photochemical reactions occur in the light
  light reactions
• biochemical reactions do not require light but
  the products of the light reaction to convert CO2
  to sugar
• dark reaction or Calvin cycle

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Chloroplasts

• Site of photosynthesis
• Organelle in the leaf that is bound by double
  membrane that encloses the stroma which is a
  gel-like solution that has ribosomes, DNA and
  enzymes to make sugar
• Sacs of membranes that contain the chlorophyll
  and accessory pigments called the thylakoid
  membrane unique to chloroplasts
• Stacks of thylakoids make up grana

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Leaf Structure
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Antennae Complexes

• Made up of chlorophyll a and other pigments
• Pick up the energy from the sun and move it down
  to the reaction center which is a unique and
  special chlorophyll a molecule and associated
  proteins
• Reaction center is an electron acceptor that
  participates in photosynthesis, remaining
  chlorophylls funnel the energy to it

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Photosystem

• A photosystem is made of antennae complex,
  reaction center and an acceptor molecule
• 2 types of photosystems
• one reaction center absorbs light at 700 nm and
  is P700 participates in photosystem I (PS I)
• one reaction center absorbs light at 680 nm and
  is P680 participates in photosystem II (PS II)

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Light Reactions
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Light Reactions

• Chloroplasts couple light driven flow of
  electrons between photosystems to make ATP
• making ATP from light energy is
  photophosphorylation
• Electron movement drives protons across the
  thylakoid membrane to make a gradient of H
  across the membrane which drives ATP synthesis
  thru ATP synthase
• Also generate NADPH by the light reaction using
  electron movement carries H and electrons

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Ecological Significance

• Light reaction converts light energy into
  chemical energy (ATP and NADPH) and released O2
  into the atmosphere

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Biochemical Reaction

• Convert CO2 into carbohydrate
• Includes the Calvin cycle which is a carbon
  fixation cycle to make the building block sugar
  that ultimately make all the molecules needed by
  the plant

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Calvin Cycle

• CO2 initially ends up on a molecule called 3-PGA
  or phosphoglyceric acid
• It starts out on RuBP (ribulose-1,5-bidphosphate,
  5 C sugar) to make a 6 C molecule that breaks
  down into 3-PGA
• Also called the photosynthetic carbon reduction
  cycle

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Steps of Calvin Cycle
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Details of the Calvin Cycle

• CO2 comes in the stomata, then to chloroplast
  stroma
• Enzyme responsible for C fixation is Rubisco
  most important and abundant protein on earth
• Rubisco assembly is controlled by light as are it
  activators and inhibitors
• CO2 RuBP 2 3-PGA
• ATP and NADPH made in light reaction power the
  next 2 steps
• 3-PGA is converted to PGAL glyceraldehyde 3-PO4
  a 3 C sugar or triose

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Continuation

• Some PGAL will become RuBP to renew the cycle
• Rest of PGAL becomes 1 of 2 things
• starch in the chloroplast thru additional
  reactions, can be broken down to PGAL at night to
  become sugar
• move to cytosol to be converted to sucrose
• Overall reaction
• 3 CO2 3 RUBP ? 3 RUBP triose-P
• 4 triose-P ? 1 sucrose 4 Pi
• Use sucrose to make other organic compounds

ATP, NADPH
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Rubisco

• Not a perfect enzyme fuels the loss of C via
  photorespiration
• Photosynthesis wastes much light energy and does
  poor job of fixing C
• Increased O2 around the plant can inhibit
  photosynthesis (Warburg effect) air in leaf is
  different than atmospheric air
• CO2 is only available when stomata is open and
  when there is plenty of water available
• CO2 is used up when stomata closed and release O2
  that cant leave as stomata is closed
• alters the CO2 and O2
• Rubisco can add O to RuBP because it can function
  as a carboxylase or oxigenation enzyme

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Photorespiration

• Use O2 rather than CO2
• Make phosphoglycolate (2C) and PGA no CO2 used
  in this reaction
• phosphoglycolate leaves the cycle enter
  mitochondria or peroxisome and releases the CO2
  that was previously fixed wasteful
• PGA may stay in cycle

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Photorespiration

• Occurs only in the light
• Makes no ATP and squanders large amounts of energy

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C4 Photosynthesis

• Requires 2 CO2 fixation steps during low CO2
  levels in atmosphere
• Did an experiment like with C3 plants and found
  CO2 in malic acid and not in 3 C PGA
• realized that there were different pathways for
  fixing CO2 in different plants
• Still use the Calvin cycle but only after an
  fixing CO2 in another set of reactions first

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C4 Plants

• Found malate in the thin walled mesophyll cells
  that have no Rubisco or other enzymes of the
  Calvin cycle
• Then saw PGA in the thick walled cells around the
  vascular bundles called the bundle sheath
• Kranz anatomy

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Explanation of C4 Photosynthesis

• CO2 diffuses in thru stomata and into mesophyll
  cells
• CO2 combines with 3 C compound called
  phosphoenolpyruvic acid (PEP) catalyzed by PEP
  carboxylase to make oxaloacetate (4 C) that moves
  thru plasmodesmata to the bundle sheath cells
• Bundle sheath cells split it to CO2 and 3 C
  molecule moves back to mesophyll cells and CO2
  stays to be shuttled to Calvin cycle
• C4 plants keep the CO2 high in bundle sheath
• CO2 fixed by Rubisco but since O2 is excluded
  from the bundle sheath, it is much more efficient
  at fixing CO2
• C4 plants avoid photorespiration and have a
  selective advantage over C3 plants in hot, dry
  environments

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Use of Light in C3 and C4 Plants

• C4 plants can still fix CO2 even when stomata are
  starting to close as the CO2 is in the
  intracellular spaces
• also need less water as they dont loose it while
  trying to get CO2
• Photosynthesis levels off as light intensity goes
  up and C4 can still complete photosynthesis when
  sun is really intense

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Evolution of C4 Plants

• In the hot tropics needed to adapt to the heat
  and dryness
• Many are grasses and all are angiosperms
• Found in 17 plant families which are not all C4
  plants probably evolved independently
• Many are monocots and some dicots corn,
  sorghum. sugar cane, millet, crabgrass and
  Bermuda grass
• 85 of all plants are still C3 plants cereal
  grains, peanuts, sugar beets, tobacco, spinach,
  soybeans, most trees, lawn grasses such as rye,
  fescue and Kentucky bluegrass

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Crassulacean Acid Metabolism (CAM)

• 3rd main type of photosynthesis evolved in arid
  ecosystems
• Started in the jade plants became acidic at
  night and throughout the day became more basic
• Stomata are open at night and fix CO2 into malic
  acid that is stored in vacuoles in succulent
  cells, stomata close and decrease water loss

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CAM Plants

• During the day, CO2 is liberated and fixed by the
  Calvin cycle
• CAM perform photosynthesis in same cell but
  spatially different areas (vacuole and
  chloroplast) and temporally different (night and
  day)
• C4 plants fix CO2 in different cells
• CAM open stomata at night when temperature drops
  and relative humidity is up use water more
  efficiently than C3 plants but grow more slowly
• CAM plants are more widespread than C4
• many are succulent but not all succulent are CAM
• monocots and dicots
• pineapple, cacti, maternity plant, wax plant,
  Spanish moss

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Factors That Affect Photosynthesis Rates

• CO2 affects photosynthesis plants that show
  no net CO2 fixation is said to be CO2
  compensation point increase CO2 can increase
  photosynthesis in C3 but not C4 as it decreases
  the phototranspiration

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Factors Continued

• Inability of plants to maintain enough water in
  leaves cause stomata to close no photosynthesis
  lack of water is most limiting factor
• Metabolic sinks are where sugars are stored and
  used remove sinks and photosynthesis decreases
  drastically, increase when making sinks again
• No photosynthesis at nights cellular
  respiration make CO2 and is released
• sun-plants need a lot of light and have high
  compensation point
• shade-plants grow in low lights and have low
  compensation points and low levels of
  photosynthesis
• 12 enzymes in Calvin cycle with 5 being limiting
  all 5 activated by light
• Maximum photosynthesis at noon most light

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Photosynthate

• Product of Calvin cycle photosynthate
• triose-P becomes dense granules of starch in
  chloroplast but mainly roots and stems
• small amount triose-P to make amino acids in
  chloroplasts
• triose-P moves out of chloroplast and used in
  cytosol to make sucrose that is moved in phloem
• ½ sucrose used in cellular respiration and the
  other ½ to make cellulose and cell wall
  components
• used to make 2 metabolites latex, other fuels
• use corn to make 90 ethanol to make gasohol
  (10 of gas sales)
• make 1.5 billion tons of grain for food