Photosynthetic Process

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Photosynthetic Process
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THE SUN MAIN SOURCE OF ENERGY FOR LIFE ON EARTH
FREE ENERGY (available for work) vs. HEAT (not
available for work)
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THE BASICS OF PHOTOSYNTHESIS

• Almost all plants are photosynthetic autotrophs
  (self producing), as are some bacteria and
  prtozoas

• Autotrophs generate their own organic matter
  through photosynthesis
• Sunlight energy is transformed to energy stored
  in the form of chemical bonds

• (a) Mosses, ferns, and
• flowering plants

(c) Euglena
(d) Cyanobacteria
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Light Energy Harvested by Plants Other
Photosynthetic Autotrophs
6 CO2 6 H2O light energy ? C6H12O6 6 O2
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WHY ARE PLANTS GREEN?
Plant Cells have Green Chloroplasts
The thylakoid membrane of the chloroplast is
impregnated with photosynthetic pigments (i.e.,
chlorophylls, carotenoids).
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THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED

• Chloroplasts absorb light energy and convert it
  to chemical energy

Reflected light
Light
Absorbed light
Transmitted light
Chloroplast
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Photosynthesis occurs in chloroplasts

• In most plants, photosynthesis occurs primarily
  in the leaves, in the chloroplasts
• A chloroplast contains
• stroma, a fluid
• grana, stacks of thylakoids
• The thylakoids contain chlorophyll
• Chlorophyll is the green pigment that captures
  light for photosynthesis

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• The location and structure of chloroplasts

Chloroplast
LEAF CROSS SECTION
MESOPHYLL CELL
LEAF
Mesophyll
Intermembrane space
CHLOROPLAST
Outer membrane
Granum
Innermembrane
Stroma
Grana
Thylakoidcompartment
Stroma
Thylakoid
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Thylakoid

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Chloroplast Pigments

• Chloroplasts contain several pigments

• Chlorophyll a
• Chlorophyll b
• (Chlorophyll a (alpha) absorbs well at a
  wavelength of about 450 nm but its primary
  absorption is at 675nm in the long red
  wavelengths.Chlorophyll b (beat) absorbs most
  effectively at blue 470 but also with shorter
  peaks at 430 and 640nm)
• Carotenoids
• Xanthophyll

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Fall Colors

• During the fall, the green chlorophyll pigments
  are greatly reduced revealing the other pigments.
• Carotenoids are pigments that are either red or
  yellow.

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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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Chlorophyll a b

• Chl a has a methyl group
• Chl b has a carbonyl group

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Absorption of Chlorophyll
Absorption
wavelength
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Different pigments absorb light differently
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AN OVERVIEW OF PHOTOSYNTHESIS

• Photosynthesis is the process by which
  autotrophic organisms use light energy to make
  sugar and oxygen gas from carbon dioxide and
  water

Carbondioxide
Water
Glucose
Oxygengas
PHOTOSYNTHESIS
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AN OVERVIEW OF PHOTOSYNTHESIS

• The light reactions convert solar energy to
  chemical energy
• Produce ATP NADPH

Light
Chloroplast
NADP?
ADP P
Calvin cycle

• The Calvin cycle makes sugar from carbon dioxide
• ATP generated by the light reactions provides the
  energy for sugar synthesis
• The NADPH produced by the light reactions
  provides the electrons for the reduction of
  carbon dioxide to glucose

Light reactions
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Steps of Photosynthesis

• Light hits reaction centers of chlorophyll, found
  in chloroplasts

• Chlorophyll vibrates and causes water to break
  apart.

• Oxygen is released into air

• Hydrogen remains in chloroplast attached to
  NADPH
• THE LIGHT REACTION

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Steps of Photosynthesis

• The DARK Reactions Calvin Cycle

• CO2 from atmosphere is joined to H from water
  molecules (NADPH) to form glucose

• Glucose can be converted into other molecules
  with different flavors!

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Redox Reaction

• The transfer of one or more electrons from one
  reactant to another.
• Two types
• 1. Oxidation
• 2. Reduction

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Oxidation Reaction

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

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Reduction Reaction

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

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• Two types of photosystems cooperate in the light
  reactions

Photon
ATP mill
Photon
Water-splitting photosystem
NADPH-producing photosystem
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1. Light Reaction (Electron Flow)

• Occurs in the Thylakoid membranes
• During the light reaction, there are two possible
  routes for electron flow.
• A. Cyclic Electron Flow
• B. Noncyclic Electron Flow

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A. Cyclic Electron Flow

• Occurs in the thylakoid membrane.
• Uses Photosystem II only
• P700 reaction center- chlorophyll a
• Uses Electron Transport Chain (ETC)
• Generates ATP only
• ADP ATP

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• Cyclic Photophosphorylation (addition of
  phosphate to ADP to make ATP.)
• Process for ATP generation associated with some
  Photosynthetic Bacteria
• Reaction Center gt 700 nm

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Plants produce O2 gas by splitting H2O

• The O2 liberated by photosynthesis is made from
  the oxygen in water (H and e-)

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In the light reactions, electron transport chains
generate ATP, NADPH, O2

• Two connected photosystems collect photons of
  light and transfer the energy to chlorophyll
  electrons
• The excited electrons are passed from the primary
  electron acceptor to electron transport chains
• Their energy ends up in ATP and NADPH

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Chemiosmosis powers ATP synthesis in the light
reactions

• The electron transport chains are arranged with
  the photosystems in the thylakoid membranes and
  pump H through that membrane
• The flow of H back through the membrane is
  harnessed by ATP synthase to make ATP
• In the stroma, the H ions combine with NADP to
  form NADPH

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Chemiosmosis
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B. Noncyclic Electron Flow

• Occurs in the thylakoid membrane
• Uses PS II and PS I
• P680 rxn center (PSII) - chlorophyll a
• P700 rxn center (PS I) - chlorophyll a
• Uses Electron Transport Chain (ETC)
• Generates O2, ATP and NADPH

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• Noncyclic Photophosphorylation
• Photosystem II regains electrons by splitting
  water, leaving O2 gas as a by-product

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B. Noncyclic Electron Flow

• ADP ? ATP
• NADP H ?? NADPH
• Oxygen comes from the splitting of H2O, not CO2
• H2O ? 1/2 O2 2H

(Reduced)
(Oxidized)
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How the Light Reactions Generate ATP and NADPH
Primary electron acceptor
NADP?
Energy to make
Primary electron acceptor
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2
Light
Electron transport chain
Light
Primary electron acceptor
Reaction- center chlorophyll
NADPH-producing photosystem
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Water-splitting photosystem
2 H? 1/2
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SummaryLight Dependent Reactions

• a. Overall input
• light energy, H2O.
• b. Overall output
• ATP, NADPH, O2.

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Light Independent Reactions aka Calvin Cycle

• Carbon from CO2 is converted to glucose
• (ATP and NADPH drive the reduction
• of CO2 to C6H12O6.)

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Light Independent Reactions aka Calvin Cycle

• CO2 is added to the 5-C sugar RuBP by the enzyme
  rubisco.
• This unstable 6-C compound splits to two
  molecules of PGA or 3-phosphoglyceric acid.
• PGA is converted to Glyceraldehyde 3-phosphate
  (G3P), two of which bond to form glucose.
• G3P is the 3-C sugar formed by three turns of the
  cycle.

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SummaryLight Independent Reactions

• a. Overall input
• CO2, ATP, NADPH.
• b. Overall output
• glucose.

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Review Photosynthesis uses light energy to make
food molecules

• A summary of the chemical processes of
  photosynthesis

Chloroplast
Light
Photosystem IIElectron transport chains
Photosystem I
CALVIN CYCLE
Stroma
Electrons
Cellular respiration
Cellulose
Starch
Other organic compounds
LIGHT REACTIONS
CALVIN CYCLE
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Photorespiration (Competing Reactions)

• Occurs under the following conditions
• Intense Light (high O2 concentrations, hot, dry,
  bright days)
• High heat (Stomatas close)
• Rubisco grabs CO2, fixing it into a
  carbohydrate in the light independent reactions.
• O2 can also react with rubisco, inhibiting its
  active site
• not good for glucose output
• wastes time and energy (occupies Rubisco)
• So Fixation of O2 instead of CO2.
• Produces no sugar molecules or no ATP.
• Photorespiration is estimated to reduce
  photosynthetic efficiency by 25

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Types of Photosynthesis

• C3
• C4
• CAM

Rubisco the worlds busiest enzyme!
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Types of Photosynthesis

• Certain plants have developed ways to limit the
  amount of photorespiration
• C3 Pathway
• C4 Pathway
• CAM (Crassulacean Acid Metabolism) Pathway
• Both convert CO2 into a 4 carbon intermediate ?
  C4 Photosynthesis

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C3 Photosynthesis  C3 plants

• Called C3 because the CO2 is first incorporated
  into a 3-carbon compound.
• Stomata are open during the day.
• RUBISCO, the enzyme involved in photosynthesis,
  is also the enzyme involved in the uptake of CO2.
• Photosynthesis takes place throughout the leaf.
• Adaptive Value more efficient than C4 and CAM
  plants under cool and moist conditions and under
  normal light because requires less machinery
  (fewer enzymes and no specialized anatomy)..
• Most plants are C3.

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C4 Photosynthesis  C4 plants

• Called C4 because the CO2 is first incorporated
  into a 4-carbon compound.
• Stomata are open during the day.
• Uses PEP Carboxylase for the enzyme involved in
  the uptake of CO2. This enzyme allows CO2 to be
  taken into the plant very quickly, and then it
  "delivers" the CO2 directly to RUBISCO for
  photsynthesis.
• Photosynthesis takes place in inner cells
  (requires special anatomy called Kranz Anatomy)

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C4 Photosynthesis  C4 plants

• Adaptive Value
• Photosynthesizes faster than C3 plants under high
  light intensity and high temperatures because the
  CO2 is delivered directly to RUBISCO, not
  allowing it to grab oxygen and undergo
  photorespiration.
• Has better Water Use Efficiency because PEP
  Carboxylase brings in CO2 faster and so does not
  need to keep stomata open as much (less water
  lost by transpiration) for the same amount of
  CO2 gain for photosynthesis.
• C4 plants include several thousand species in at
  least 19 plant families. Example fourwing
  saltbush pictured here, corn, and many of our
  summer annual plants.

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CAM Photosynthesis  CAM plants. CAM stands for
Crassulacean Acid Metabolism

• Called CAM after the plant family in which it was
  first found (Crassulaceae) and because the CO2 is
  stored in the form of an acid before use in
  photosynthesis.
• Stomata open at night (when evaporation rates are
  usually lower) and are usually closed during the
  day. The CO2 is converted to an acid and stored
  during the night. During the day, the acid is
  broken down and the CO2 is released to RUBISCO
  for photosynthesis

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CAM Photosynthesis  CAM plants.

• Adaptive Value
• Better Water Use Efficiency than C3 plants under
  arid conditions due to opening stomata at night
  when transpiration rates are lower (no sunlight,
  lower temperatures, lower wind speeds, etc.).
• May CAM-idle. When conditions are extremely arid,
  CAM plants can just leave their stomata closed
  night and day. Oxygen given off in photosynthesis
  is used for respiration and CO2 given off in
  respiration is used for photosynthesis. This is a
  little like a perpetual energy machine, but there
  are costs associated with running the machinery
  for respiration and photosynthesis so the plant
  cannot CAM-idle forever. But CAM-idling does
  allow the plant to survive dry spells, and it
  allows the plant to recover very quickly when
  water is available again (unlike plants that drop
  their leaves and twigs and go dormant during dry
  spells).
• CAM plants include many succulents such as
   cactuses and agaves and also some orchids and
  bromeliads

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Leaf Anatomy

• In C3 plants (those that do C3 photosynthesis),
  all processes occur in the mesophyll cells.

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C4 Pathway

• In C4 plants photosynthesis occurs in both the
  mesophyll and the bundle sheath cells.

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C4 Pathway

• CO2 is fixed into a 4-carbon intermediate
• Has an extra enzyme PEP Carboxylase
  (Phosphoenolpyruvate carboxylase) that initially
  traps CO2 instead of Rubisco makes a 4 carbon
  intermediate

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C4 Pathway

• The 4 carbon intermediate is smuggled into the
  bundle sheath cell
• The bundle sheath cell is not very permeable to
  CO2
• CO2 is released from the 4C malate ? goes through
  the Calvin Cycle

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How does the C4 Pathway limit photorespiration?

• Bundle sheath cells are far from the surface
  less O2 access
• PEP Carboxylase doesnt have an affinity for O2 ?
  allows plant to collect a lot of CO2 and
  concentrate it in the bundle sheath cells (where
  Rubisco is)

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CAM Pathway

• Fix CO2 at night and store as a 4 carbon molecule
  
• Keep stomates closed during day to prevent water
  loss
• Same general process as C4 Pathway

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How does the CAM Pathway limit photorespiration?

• Collects CO2 at night so that it can be more
  concentrated during the day
• Plant can still do the calvin cycle during the
  day without losing water

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CAM Plants
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Summary of C4 Photosynthesis

• C4 Pathway
• Separates by space (different locations)
• CAM Pathway
• Separates reactions by time (night versus day)

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Bio cell uses photosynthesis to generate
electricity

• Bio cell inserted into a cactus and a graph
  showing the intensity of the electric current
  generated as a function of light that fell on the
  plant (in black, glucose, and red, O2).Scientists
  at the research institute CNRS, France, changed
  the chemical energy generated by
  photosynthesis of a plant into electrical energy.
•  The research demonstrates a new route
  for artificial photosynthesis , a promising area
  of research that aims to develop a strategy for
  conversion of sunlight into electricity even more
  efficient and more environmentally friendly than
  solar cells .

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Artificial photosynthesis

• Artificial photosynthesis is a research field
  that attempts to replicate the natural process
  of photosynthesis, converting sunlight, water,
  andcarbon dioxide into carbohydrates and oxygen.
  Sometimes, splitting water into hydrogen and oxyge
  n by using sunlight energy is also referred to
  as artificial photosynthesis.

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Photoelectrochemical cell

• Research is being done into finding catalysts that
  can convert water, carbon dioxide, and sunlight
  to carbohydrates. For the first type of
  catalysts, nature usually uses the oxygen
  evolving complex. Having studied this complex,
  researchers have made catalysts such as blue
  dimer to mimic its function, but these catalysts
  were very inefficient. Another catalyst was
  engineered by Paul Kögerler, which uses four
  ruthenium atoms.
• The carbohydrate-converting catalysts used in
  nature are the hydrogenases. Catalysts invented
  by engineers to mimic the hydrogenasesinclude a
  catalyst by Cédric Tard,3 the rhodium atom
  catalyst from MIT,4 and the cobalt catalyst
  from MIT. Dr. Nocera of MIT is receiving funding
  from the Air Force Office of Scientific Research
  to help conduct the necessary experiments to push
  forward in catalyst research.

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Dye-sensitized solar cell

• Possibly the most exciting technological
  development in nanotechnology is a photovoltaic
  cell that uses photosynthesis to generate
  electricity. The first solar photovoltaic chip
  was made using ground-up spinach tissue by
  scientist Shuguang Zhang at MIT. He was building
  on work by a group of researchers who had earlier
  figured out how to harness energy from a plant.
  That group was able to extract electrical current
  using a plants photosynthesis for a period of
  three weeks. Zhangs chip converted approximately
  12 of the light energy absorbed to electrical
  current. This compares to the 24 efficiency of
  silicon power cells. In the future, it is hoped
  that by adding layers of chips, efficiency will
  be increased to 20. size of this photosynthetic
  solar chip is Ten to twenty nanometers or, small
  enough to fit about a hundred of them in the
  width of a human hair. Would result in
  lightweight computers and other electronic
  devices, not to mention more environmentally
  friendly.

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Electricity Generation by Photosynthetic Biomass
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Advantages

• Dye-sensitized cells can be made at one-fifth of
  the price of silicon cells.
• The solar energy can be immediately converted and
  stored, unlike in PV cells, for example, which
  need to convert the energy and then store it into
  a battery (both operations implying energy
  losses). Furthermore, hydrogen as well as
  carbon-based storage options are quite
  environmentally friendly.
• Renewable, carbon-neutral source of energy, which
  can be used for transportation or homes. Also the
  CO2 emissions that have been distributed from
  fossil fuels will begin to diminish because of
  the photosynthetic properties of the reactions.

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Disadvantages

• Artificial photosynthesis cells (currently) last
  no longer than a few years (unlike PV and
  passive solar panels, for example, which last
  twenty years or longer).
• The cost for alteration right now is not
  advantageous enough to compete with fossil
  fuels and natural gas as a viable source of
  mainstream energy.