Photosynthesis

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Photosynthesis

• Campbell 6e
• Chapter 10

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Solar Power

• Life on Earth is solar powered.
• The chloroplasts of plants capture light energy
  that has traveled 160 million kilometers from the
  sun and convert it to chemical energy in a
  process called photosynthesis.

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Overview

• The energy used by most living cells ultimately
  comes from the sun, and is captured by plants,
  algae, or bacteria via photosynthesis.
• Light dependent reactions
• capture energy from sunlight
• use energy to produce ATP and NADPH
• Calvin cycle
• formation of organic molecules

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Importance of photosynthesis

• Produces energy available to all organisms
• Generates O2
• Uses CO2 from atmosphere

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Heterotrophs depend on Autotrophs

• Almost all heterotrophs, including humans, are
  completely dependent on photoautotrophs for food
  and the oxygen we breath.
• Autotrophs produce their energy from the sun and
  inorganic molecules.
• Heterotrophs obtain energy from living organisms.

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The CO2 level in the atmosphere has increased
from 316 ppm in 1960 to 369 ppm in 2000
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The Northern Hemisphere CO2 concentration peaks
in May
Then, crops are planted, leaves come out, and
photosynthesis brings the CO2 concentration to a
minimum in September.
In September and October crops are harvested,
leaves senesce and the CO2 increases to a new
high the following May.
The annual change in CO2 concentration has been
called the breathing of the Earth. Its
magnitude is an indication of Northern Hemisphere
photosynthesis rates
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Two stages of Photosynthesisoverview
1) Light-dependent reactions - solar energy
boosts electrons in the special pigment molecule
chlorophyll to higher energy levels, which is
released bit by bit into the energy carriers ATP
and NADPH. 2) Light-independent reactions
(Calvin-Benson Cycle). These reactions do not
require light energy. Here, the bond energy in
the energy carriers (ATP, NADPH) is released and
stored in the bonds of a glucose molecule
Ribulose Biphosphate.
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Two Stages of Photosynthesis
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Leaf Structure
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Chloroplasts site of photosynthesis

• Occur mainly in the mesophyll
• cells green due to chlorophyll
• Double membrane bound organelle
• Stroma - watery space (site dark reactions)
• Lumen- space inside thylakoid disk
• Granum - stack of thylakoid disks.
• Thylakoid.

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The Chloroplast
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Thylakoids

• Thylakoid membranes house pigments for capturing
  light and the machinery to produce ATP.
• clustered together to form a photosystem
• Which acts as an antenna, gathering light energy
  harvested by multiple pigment molecules

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Inside the Chloroplast
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Thylakoids and ETS

• Chlorophyll is embedded in thylakoid membrane.
• Electron transport system in membrane harvests
  excited chlorophyll electrons, puts them to work.

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Light and Reducing Power

• Light-dependent reactions of photosynthesis use
  the energy of light to reduce NADP to NADPH and
  to manufacture ATP.
• Reducing power generated by splitting water is
  used to convert CO2 into organic matter during
  carbon fixation.

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Energy in Photons

• Energy content of a photon is inversely
  proportional to the wavelength of light.
• Highest intensity photons, at the
  short-wavelength end of the electromagnetic
  spectrum, are gamma rays.
• Ultraviolet light possesses considerably more
  energy than visible light.
• potent force in disrupting DNA

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Role of Light in Photosynthesis

• A. Photon unit of energy of light
• B. Photons absorbed can drive chemical reactions
• C. Chemical pigments absorb photons based on
  electron orbits
• D. Chlorophyll has Mg atom which is "excited" by
  photons
• Electrons are used to make ATP to "drive"
  photosynthesis
• E. Chlorophyll absorbs light in the violet-blue
  and red portions of the spectrum

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Electromagnetic Spectrum
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Absorption Spectra

• Photon absorption depends on its wavelength, and
  the chemical nature of the molecule it hits.
• Each molecule has a characteristic absorption
  spectrum.
• range and efficiency of photons the molecule is
  capable of absorbing

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Pigments

• Pigments are molecules that absorb light in the
  visible range.
• green plant photosynthesis
• carotenoids
• chlorophyll
• chlorophyll a - main pigment
• chlorophyll b - accessory pigment

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Absorption Spectra
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Chlorophyll

• Chlorophylls absorb photons by means of an
  excitation process.
• Photons excite electrons in the pigments ring
  structure, and are channeled away through
  alternating carbon-bond system.
• Wavelengths absorbed depend on the available
  energy levels to which excited electrons can be
  boosted.

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Light to Chemical Energy
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Photosystems

• Photosystem - network of pigments that channels
  excitation energy gathered by any of the
  molecules to the reaction center
• reaction center allows photon excitation to move
  away from chlorophylls and is the key conversion
  of light to chemical energy

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Photosystem Function

• Plants use two photosystems
• photosystem I and II
• generate power to reduce NADP to NADPH with
  enough left over to make ATP
• two stage process photosystem II I.
• noncyclic photophosphorylation
• ejected electrons end up in NADPH

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Reaction Centers and Photosystems

• There are two types of reaction center
  chlorophyll a molecules, each of which is
  associated with its own antennae complexes.
• P700 - this chlorophyll molecule absorbs
  maximally, light of a wavelength of 700nm. This
  complex of pigments and proteins is called
  Photosystem I.
• P680 - this chlorophyll molecule absorbs
  maximally, light at 680nm and the complex of
  pigments and proteins is called Photosystem II.

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A Photosystem
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Photosystems I and II
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Summary of Light Reactions

• The electrons (e-) and protons (H) that make up
  hydrogen atoms are stripped away separately from
  water molecules.
• 2H2O -gt 4e- 4H O2
• They reduce NADP to NADPH for use in the Calvin
  Cycle.
• They set up an electrochemical charge that
  provides the energy for pumping protons from the
  stroma of the chloroplast into the interior of
  the thylakoid.

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• The PROTONS also serve two functions
• They participate in the reduction of NADP to
  NADPH.
• As they flow back out from the interior of the
  thylakoid (by facilitated diffusion), passing
  down their concentration gradient), the energy
  they give up is harnessed for the conversion of
  ADP to ATP. (ADP Pi -gt ATP)
• Because it is driven by light, this process is
  called photophosphorylation.

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Light Reaction Animation

• http//www.science.smith.edu/departments/Biology/B
  io231/ltrxn.html
• NADP Reduction animation
• http//neosci.com/demos/10-1071_Photosynthesis/Pre
  sentation_15.html
• ATPase animation
• http//neosci.com/demos/10-1071_Photosynthesis/Pre
  sentation_14.html

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Calvin Cycle the C3 pathway

• CO2 combines with the phosphorylated 5-carbon
  sugar ribulose bisphosphate.
• This reaction is catalyzed by the enzyme ribulose
  bisphosphate carboxylase oxygenase (RUBISCO)(an
  enzyme which can fairly claim to be the most
  abundant protein on earth).
• The resulting 6-carbon compound breaks down into
  two molecules of 3-phosphoglyceric acid (PGA).

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• 4. The PGA molecules are further phosphorylated
  (by ATP) and are reduced (by NADPH) to form
  phosphoglyceraldehyde (PGAL).
• 5. Phosphoglyceraldehyde serves as the starting
  material for the synthesis of glucose and
  fructose.
• All the reactions of carbon fixation occur in the
  stroma of the chloroplast.

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Calvin Cycle Animation

• http//www.science.smith.edu/departments/Biology/B
  io231/calvin.html
• http//neosci.com/demos/10-1071_Photosynthesis/Pre
  sentation_6.html

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Photorespiration

• In photorespiration, O2 is incorporated into
  RuBP, which undergoes additional reactions that
  release CO2.
• decreased yields in C3 plants of photosynthesis
  on hot, dry days due to closure of stomata.

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Photorespiration Contd

• As its name suggests, RUBISCO catalyzes two
  different reactions
• adding CO2 to ribulose bisphosphate the
  carboxylase activity
• adding O2 to ribulose bisphosphate the
  oxygenase activity.
• Which one predominates depends on the relative
  concentrations of O2 and CO2 with
• high CO2, low O2 favoring the carboxylase action,
  
• high O2, low CO2 favoring the oxygenase action.

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• The light reactions of photosynthesis liberate
  oxygen and more oxygen dissolves in the cytosol
  of the cell at higher temperatures.
• Therefore,
• high light intensities and
• high temperatures (above 30C)
• favor the second reaction.

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Details of Photorespiration

• The uptake of O2 by RUBISCO forms
• the 3-carbon molecule 3-phosphoglyceric acid
  just as in the Calvin cycle
• the 2-carbon molecule glycolate.
• The glycolate enters peroxisomes where it uses O2
  to form intermediates that
• enter mitochondria where they are broken down to
  CO2.
• So this process uses O2 and liberates CO2 as
  cellular respiration does which is why it is
  called photorespiration.

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C4 Plants

• Over 8000 species of plants have developed
  adaptations which minimize the losses to
  photorespiration.
• They all use a supplementary method of CO2 uptake
  which forms a 4-carbon molecule instead of the
  two 3-carbon molecules of the Calvin cycle. Hence
  these plants are called C4 plants.

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• These C4 plants are well adapted to (and likely
  to be found in) habitats with
• high daytime temperatures
• intense sunlight.
• Some examples
• crabgrass
• corn (maize)
• sorghum

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CAM Plants

• These are also C4 plants but instead of
  segregating the C4 and C3 pathways in different
  parts of the leaf, they separate them in time
  instead.
• (CAM stands for crassulacean acid metabolism)

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At Night, CAM plants

• take in CO2 through their open stomata (they tend
  to have reduced numbers of them).
• The CO2 joins with PEP to form the 4-carbon
  oxaloacetic acid.
• This is converted to 4-carbon malic acid that
  accumulates during the night in the central
  vacuole of the cells.

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So, in the morning

• the stomata close (thus conserving moisture as
  well as reducing the inward diffusion of oxygen).
  
• The accumulated malic acid leaves the vacuole and
  is broken down to release CO2.
• The CO2 is taken up into the Calvin (C3) cycle.

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CAM Plants

• These adaptations also enable their owners to
  thrive in conditions of
• high daytime temperatures
• intense sunlight
• low soil moisture.
• Some examples of CAM plants
• cacti
• Bryophyllum
• the pineapple and all epiphytic bromelliads
• sedums

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CAM Plants
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Why produce glucose?

• one glucose molecule is smaller than one ATP
• glucose stores easier in cells
• glucose equivalent of 36 ATPs
• glucose more stable compound
• glucose can be converted to other important
  biological molecules when necessary.

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Credits

• Information taken from Campbell 6e and various
  internet sources.
• This PowerPoint is a modification of
• http//www.coe.unt.edu/ubms/documents/classnotes/F
  all2005/Chapter201020-20Photosynthesis.ppt