Photosynthesis

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Photosynthesis

• Topic 3.8 and 8.2

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Autotrophs

• Plants are autotrophs (meaning self-feeders in
  Greek) in that they make their own food and thus
  sustain themselves without eating other organisms
  or even organic molecules.
• Chloroplasts of plant cells capture light energy
  that has traveled 150 million kilometers from the
  sun and convert it to chemical energy that is
  stored in sugar and other organic molecules.

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Producers

• Plants, algae, some prokaryotes make their own
  organic molecules and are the ultimate source of
  organic molecules for almost all other organisms.
  
• Often referred to as the producers of the
  biosphere because they produce its food supply
• All organisms that produce organic molecules from
  inorganic molecules using the energy of light are
  called photoautotrophs.

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Chloroplasts

• All green parts of a plant have chloroplasts in
  their cells and can carry out photosynthesis.
• In most plants, leaves have the most chloroplasts
  and are the major sites of photosynthesis.
• Chloroplasts are concentrated in the cells of the
  mesophyll, the green tissue in the interior of
  the leaf.
• Each mesophyll has numerous chloroplasts
• Carbon dioxide enters the leaf and oxygen exits
  via tiny pores called stomata.
• Water absorbed by the roots is delivered to the
  leaves in veins.

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Chloroplasts

• Membranes in the chloroplast form the framework
  where many of the reactions of photosynthesis
  occur, just as mitochondrial membranes do for
  cell respiration.
• Similar to mitochondria, chloroplast has an outer
  membrane and an inner membrane, with an
  intermembrane space between them.
• Inner membrane is filled with a thick fluid
  called stroma
• Stroma is where sugars are made from carbon
  dioxide and water
• Within stroma is a system of interconnected
  membranous sacs called thylakoids
• Enclose a third compartment called the thylakoid
  space
• Built into thylakoid membranes are the
  chlorophyll molecules that capture light energy.
• Membranes also house much of the machinery that
  converts light energy to chemical energy.
• In some places, thylakoids are concentrated in
  stacks called grana.

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Chloroplast
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Photosynthesis is a redox

• 6CO2 6H2O ? C6H12O6 6O2
• When water molecules are split apart, yielding
  O2, they are actually oxidized that is, they
  lose electrons along with hydrogen ions
• Meanwhile, CO2 is reduced to sugar as electrons
  and hydrogen ions are added to it.
• Overall, cell respiration harvest energy stored
  in a glucose molecule by
• oxidizing the sugar and reducing O2 to H2O,
  involving a number of energy-releasing redox
  reactions,
• with electrons losing potential energy as they
  travel down an energy hill from sugar to O2.
• Along the way, the mitochondria uses some of the
  energy to synthesize ATP.
• In contrast, photosynthesis redox reactions
  involve an uphill climb.
• As water is oxidized and CO2 is reduced,
  electrons gain energy by being boosted up an
  energy hill.
• Light energy captured by chlorophyll molecules in
  the chloroplast provides the boost for the
  electrons.
• Photosynthesis converts light energy to chemical
  energy and stores it in sugar molecules.

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Photosynthesis Overview

• Photo, from the Greek word for light, refers to
  the first stage.
• Synthesis, meaning putting together refers to
  the sugar construction in the second stage

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Photosynthesis Overview

• Occurs in two stages
• Light reactions
• Include the steps that convert light energy to
  chemical energy stored in ATP and NADPH and
  produce O2 gas as a waste product.
• Occur in thylakoid membranes
• Light energy absorbed by chlorophyll is used to
  make ATP from ADP and phophate.
• Also used to drive a transfer of electrons from
  water to NADP, an electron carrier similar to
  NAD that carries electrons in cellular
  respiration.
• NADP gets reduced to NADPH via enzymes by adding
  a pair of light-excited electrons along with an
  H
• Reaction temporarily stores energized electrons
  which originally came form water that is split
  and O2 is released.

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Photosynthesis Overview

• 2. Dark reactions, or Calvin Cycle
• Occurs in the stroma
• Does not require light directly
• Cyclic series of reactions that assembles sugar
  molecules using CO2 and the energy-containing
  products (NADPH and ATP) of the light reactions.
• Incorporation of carbon from CO2 into organic
  compounds is called carbon fixation.
• After carbon fixation, enzymes of the cycle make
  sugars by further reducing the carbon compounds.

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Light ReactionsConverting Solar Energy to
Chemical Energy

• Electromagnetic energy
• type of energy that is sunlight
• Travels in space as rhythmic waves analagous to
  those made by a pebble dropped in a puddle of
  water
• Distance between the crests of two adjacent waves
  is called a wavelength.
• In the electromagnetic spectrum, shorter
  wavelengths have more energy than longer ones.
• Visible light- the radiation your eyes can see as
  different colors, consists of wavelengths from
  about 380 nm to 750 nm

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Light ReactionsConverting Solar Energy to
Chemical Energy

• Figure 7.6B shows what happens to visible light
  in the chloroplast.
• Light absorbing molecules called pigments, built
  into the thylakoid membranes, absorb some
  wavelengths of light and reflect or transmit
  other wavelengths.
• We do not see the absorbed wavelengths their
  energy has been absorbed by pigment molecules
• We see green wavelengths when we look at plants
  that the pigments transmit and reflect.

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Light ReactionsConverting Solar Energy to
Chemical Energy

• Pigments of chloroplast
• Chlorophyll a
• Absorbs mainly blue-violet and red light
• Participates directly in the light reactions
• Looks grass-green because it reflects mainly
  green light
• Chlorophyll b
• Absorbs mainly blue and orange light and reflects
  (appears) yellow-green.
• Broadens the range of light that a plant can use
  by conveying absorbed energy to chlorophyll a,
  which then puts the energy to work in the light
  reactions
• Carotenoids
• Absorb mainly blue-green light and reflects
  yellow-orange
• Some may pass energy to chlorophyll a, as
  chlorophyll b does
• Have a protective function absorb and dissipate
  excessive light energy that would other-wise
  damange chlorophyll

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Light ReactionsConverting Solar Energy to
Chemical Energy

• The theory of light as waves explains most of
  lights properties.
• However, light also behaves as discrete packets
  of energy called photons
• A fixed quantity of light energy, and the shorter
  the wavelength, the greater the energy.
• Each type of pigment absorbs certain wavelengths
  of light because it is able to absorb the
  specific amounts of energy in those photons.

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Light ReactionsConverting Solar Energy to
Chemical Energy

• Photosystems
• Clusters of chlorophyll molecules along with
  other pigments and proteins in the thylakoid
  membrane
• Consists of a number of light-harvesting
  complexes surrounding a reaction center.
• Have chlorophyll a, chlorophyll b, and carotenoid
  pigments that function collectively as a
  light-gathering antenna.
• Pigments absorb photons and pass the energy from
  molecule to molecule until it reaches the
  reaction center.

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Light ReactionsConverting Solar Energy to
Chemical Energy

• Photosystems
• Reaction center
• A protein complex that contains a chlorophyll a
  molecule and a molecule called the primary
  electron acceptor
• Captures a light-excited electron from the
  reaction-center chlorophyll molecule and passes
  it to an electron transport chain

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Light ReactionsConverting Solar Energy to
Chemical Energy

• Photosystems
• Two types Photosystem I and Photosytem II
• Photosystem I
• Occurs second in light reactions
• Reaction center is called P700 because the
  wavelength of light it absorbs best is 700 nm
• Photosystem II
• Occurs first in light reactions
• Chlorophyll a molecule in reaction center is
  called P680 because the light it absorbs best is
  red light with a wavelength of 680nm

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Light ReactionsConverting Solar Energy to
Chemical Energy
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Light ReactionsConverting Solar Energy to
Chemical Energy

• Light Reactions
• Light energy is transformed into the chemical
  energy of ATP and NADPH
• In this process, electrons removed from water
  molecules pass from photosystem II to photosystem
  I to NADP
• Between the two photosystems, the electrons move
  down an electron transport chain and provide
  energy for ATP production.

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Light ReactionsConverting Solar Energy to
Chemical Energy

• Flow of electrons in light reactions (Figure
  7.8A)
• A pigment molecule in a light-harvesting complex
  absorbs a photon of light. The energy is passed
  to other pigment molecules and finally to the
  reaction center of Photosystem II, where it
  excites an electron of chlorophyll P680 to a
  higher energy level.
• The electron is captured by the primary electron
  acceptor.
• Water is split, and its electrons are supplied
  one by one to P680, replacign those lost to the
  primary electron acceptor. The oxygen atom
  compbines with an oxygen from another split water
  molecule to form a molecule of O2.

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Light ReactionsConverting Solar Energy to
Chemical Energy

• Flow of electrons in light reactions (Figure
  7.8A)
• 4. each photoexcited electron passes from
  photosystem II to photosystem I via an electron
  transport chain. The exergonic fall of
  electrons provides energy for the synthesis of
  ATP.
• 5. Meanwhile, light energy excites an electron of
  chlorophyll P700 in the reaction center of
  photosystem I. The primary electron acceptor
  captures the excited electron and an electron
  from the bottom of the electron transport chain
  replaces the lost electron in P700.
• 6. The excited electrons of photosystem I is
  passed through a short electron transport chain
  to NADP, reducing it to NADPH

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Light ReactionsConverting Solar Energy to
Chemical Energy

• Chemiosmosis
• Drives ATP synthesis using the potential energy
  of a concentration gradient of hydrogen ions
  across a membrane
• Gradient is created when an electron transport
  chain pumps hydrogen ions across a membrane as it
  passes electrons down the chain.

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Light ReactionsConverting Solar Energy to
Chemical Energy

• Chemiosmosis (ctd)
• Relationship between chloroplast structure and
  function in light reactions
• The two photosystems and e.t.c. are all located
  in the thylakoid membrane of a chloroplast.
• As photoexcited electrons are passed down the
  e.t.c. connecting the two photosystems, H are
  pumped across the membrane from the stroma into
  the thylakoid space. This generates a
  concentration gradient across the membrane.

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Light ReactionsConverting Solar Energy to
Chemical Energy

• Chemiosmosis (ctd)
• Similar ATP synthase complex in mitochondria
• Energy of concentration gradient drives H back
  across the membrane through ATP synthase
• ATP synthase couples the flow of H to the
  phosphorylation of ADP called photophosphorylatio
  n

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Light ReactionsConverting Solar Energy to
Chemical Energy

• Chemiosmosis (ctd)
• In photosynthesis, light energy is used to drive
  electrons to the top of the transport chain
  (whereas, cell respiration, high-energy electrons
  pass down the e.t.c. coming from oxidation of
  food molecules)
• Chloroplasts transform light energy into the
  chemical energy of ATP (whereas, mitochondria
  transfer chemical energy from food to ATP)
• In photosynthesis, the final electron acceptor is
  NADP (whereas, in cell respiration, O2 is)
• In photosynthesis, electrons are stored in at a
  high state of potential energy in NADPH (whereas,
  in cell respiration, they are at a low energy
  level in H20)

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Light ReactionsConverting Solar Energy to
Chemical Energy
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Dark Reaction (Calvin Cycle)Converting CO2 to
sugars

• Figure 7.10A Overview of Calvin Cycle
• CO2 (from air), energy from ATP and high energy
  electrons from NADPH (both generated by light
  reactions) , the Calvin Cycle constructs an
  energy-rich, three-carbon sugar,
  glyceraldehyde-3-phosphate (G3P).
• A plant cell uses G3P to make glucose and other
  organic molecules as needed.

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Dark Reaction (Calvin Cycle)Converting CO2 to
sugars

• Figure 7.10B Details of the Calvin Cycle
• Carbon fixation the enzyme rubisco attaches CO2
  to RuBP (5-C). The unstable 6-C product splits
  into two molecules called 3-PGA.
• For three CO2, six 3-PGA result
• Reduction NADPH reduces the organic acid six
  3-PGA to six molecules G3P with the assistance of
  ATP

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Dark Reaction (Calvin Cycle)Converting CO2 to
sugars

• 3. Release of one molecule of G3P
• Five G3Ps remain in the cycle, and one G3P will
  leave. Plant cells use two G3P molecules to make
  one molecule of glucose.
• 4. Regeneration of RuBP
• energy from ATP drives a series of chemical
  reactions to rearrange the atoms in the five G3P
  molecules to form three RuBP molecules. These
  can start another turn of the cycle.

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Dark Reaction (Calvin Cycle)Converting CO2 to
sugars
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Absorption spectrum

• As light meets matter, it may be reflected,
  transmitted, or absorbed.
• Pigments
• Substances that absorb visible light
• Different pigments absorb light of different
  wavelengths
• Chlorophylls absorb red and blue-violet light and
  appear green
• Carotenoids absorb blue-violet and appear orange,
  yellow, or red
• Measured with a spectrophotometer.

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Absorption spectrum

• Absorption spectrum
• Light absorption vs. the wavelength
• Absorption spectrum of different photosynthetic
  pigments provides clues to their role in
  photosynthesis, since light can only perform work
  if it is absorbed.
• Accessory pigments (chlorophyll b and
  carotenoids) absorb wavelengths of light that
  chlorophyll a cannot, pass the energy to
  chlorophyll a, broadening the spectrum that can
  effectively drive photosynthesis.

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Absorption Spectrum

• Action spectrum
• Profiles the effectiveness of different
  wavelength light in fueling photosynthesis.
• It is obtained by plotting wavelength against
  some measure of photosynthetic rate.

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Action Spectrum
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Abiotic factors impact on photosynthetic rate

• Photosynthetic rate is depended on environmental
  factors
• Amount of light available
• Level of carbon dioxide
• temperature

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Light intensity

• Up to a certain intensity, photosynthesis
  increases as more light is available to the
  chlorophyll.
• When all the chlorophyll molecules are activated
  (saturated) by the light, more light has no
  further effect.

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Light Intensity
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Temperature

• Increased temperature increases photosynthetic
  rate until an optimal temperature is reached.
• Above the optimal temperature, enzymes cannot
  function properly and photosynthesis will
  decrease.

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Temperature
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Carbon Dioxide levels

• Increased carbon dioxide levels increases
  photosynthesis, unless limited by another factor,
  then levels off.

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Carbon Dioxide
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Measuring photosynthesis

• Production of oxygen or uptake of carbon dioxide
• Increase in biomass