Introduction to Photosynthesis Chapter 7

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Introduction to PhotosynthesisChapter 7

• OCC BIO-114
• Dave Werner

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Animations

• Light Dark Reactions

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1.ALL LIFE REQUIRES ENERGY 2.Animals, fungi,
and most protists obtain their energy by
consuming, directly or indirectly, organic food
from their environment (heterotrophs) 3.Some
organisms (autotrophs) have the ability to
capture the energy of the sun to synthesize their
own organic food (green plants, algae) 4.THE
ULTIMATE SOURCE OF ALL ENERGY ON EARTH IS THE SUN
5.PHOTOSYNTHESIS is the link between life on
earth and the sun 6.It is a set of reactions
which convert light energy from the sun into
chemical bond energy of glucose and ATP
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Light and Pigments7.1-7.2

• The Nature of Sunlight

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The Nature of Sunlight

• light is a form of energy known as
  electromagnetic radiation
• light travels in rhythmic waves which are
  disturbances of electrical and magnetic fields

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The Nature of Sunlight

• the distance between crests of electromagnetic
  waves is called the wavelength
• the entire range of radiation is known as the
  electromagnetic spectrum

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Light Energy (Fig.7.4, 7.6)

• the narrow range from about 380 to 750nm in
  wavelength is detectable by the human eye and is
  called visible light
• the model of light as waves explains many of its
  properties, but in certain respects it behaves as
  though it consists of discrete particles
• these particles called photons act like objects
  in that each of them has a fixed quantity of
  energy
• the amount of energy is inversely related to the
  wavelength of light (shorter wavelengths have
  more energy)

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Photosynthetic Pigments

• as light meets matter, it may be reflected,
  transmitted or absorbed
• substances that absorb light are called pigments
• if a pigment is illuminated in white light, the
  color we see is the color most reflected or
  transmitted by the pigment

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Light perception

• the major pigment in leaves, chlorophyll, appears
  green because it absorbs red and blue light while
  transmitted and reflecting green
• chlorophyll is actually a family of pigments with
  similar chemical structures

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Photoexcitation of Chlorophyll

• when energy is absorbed by a molecule of pigment,
  one of the molecules electrons is elevated to
  from its ground state to a higher orbital around
  the nucleus (excited state)

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Photoexcitation of Chlorophyll

• the only photons absorbed are those whose energy
  is exactly equal to the energy difference between
  the ground state and an excited state
• the energy of the photon is converted to the
  potential energy of an electron, making the
  electron less stable
• generally, when pigments absorb light, their
  excited electrons drop back down to the ground
  state very quickly releasing their energy as heat
  and/or light (fluorescence)

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7.3 Overview of Photosynthesis Reactions

• Photosynthesis can be summarized with this
  chemical equation
• 6CO2 12H2O LIGHT ENERGY/ENZYMES --gt C6H12O6
  6O2 6H2O

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6CO2 12H2O LIGHT ENERGY -gt C6H12O6 6O2
6H2O

• The chemical change is the reverse of cellular
  respiration
• The low energy inorganic compounds (CO2 and
  water) are converted into the high potential
  organic molecule (glucose)

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The Chloroplasts Sites of Photosynthesis

• The primary function of this specialized
  organelle is to convert light energy into ATP and
  NADPH (nicotinamide adenine dinucleotide
  phosphate)
• Chloroplasts are found mainly in the cells of the
  mesophyll (about 50/cell), the green tissue on
  the interior of the leaf

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Leaf (Fig.7.6)

• Carbon dioxide enters the leaf, and oxygen exits,
  by way of microscopic pores called stomata
• The double membrane of the chloroplast regulates
  transport of materials in and out
• Chloroplasts are filled with an aqueous solution
  called the stoma which contains all the necessary
  enzymes for photosynthesis

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Chloroplast (Fig. 7.6)

• The conversion from light energy to ATP and NADPH
  occurs in the thylakoid membranes within the
  stroma
• The thylakoid membranes contain all of the
  pigments involved in the process including
  chlorophyll (green pigment) and other carotenoids
  
• The thylakoids are organized into closely packed
  stacks called grana

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Choloroplast

• Within these thylakoids and grana, light energy
  is converted into ATP and NADPH these are said
  to be LIGHT-DEPENDENT REACTIONS
• The reactions that actually convert CO2 to
  carbohydrate are LIGHT-INDEPENDENT REACTIONS or
  DARK REACTIONS

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7.4 The Light Reactions

• Must take place in the presence of light
• Steps that convert solar energy to chemical
  energy
• Light absorbed by chlorophyll drives a transfer
  of electrons from water to an acceptor named
  NADP which temporarily stores the energized
  electrons

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Light Reactions

• Water is split in the process and thus it is the
  light reactions of photosynthesis that give off
  O2 as a by-product
• The light reactions also generate ATP by powering
  the addition of a phosphate group to ADP, a
  process called photophosphorylation. This is
  done w/ the help of ATP Synthase b/c H cannot
  diffuse through the membrane.
• THE LIGHT REACTIONS PRODUCE NO SUGAR
•  

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Electron Transport (Fig.7.8)

• chloroplasts and mitochondria generate ATP by the
  same basic mechanism of chemiosmosis
• an electron transport chain embedded in the
  thylakoid membrane pumps protons across the
  membrane as electrons are passed through a series
  of carriers producing a proton-motive force
  (potential energy stored in the proton gradient)

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ETC ATP Synthesis in Chloroplasts

• ATP synthase in the membrane couples the
  diffusion of hydrogen ions down their gradient to
  the phosphorylation of ADP
• in contrast to oxidative phosphorylation in
  mitochondria, chloroplasts use light energy (not
  chemical energy in food) to drive electrons to
  the top of the transport chain
• the proton pump of the thylakoid membrane moves
  hydrogen ions from the stroma to the thylakoid
  space which functions as the H reservoir
• the membrane makes ATP in the stroma as hydrogen
  ions diffuse back down their gradient through ATP
  synthase

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Restoring PSII (Fig. 6-6)

• 2H2O ? 4H 4e- O2
• Can you explain how PSII is restored?

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Chemiosmosis

• Happens during Light Rxns.
• Concentration gradient of protons across
  thylakoid membrane.
• Where do the protons come from?
• ATP Synthase protein that harnesses energy.
• Some Protons are used to make NADPH from NADP.
• NADPH ATP drive next set of rxns.

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7.5 Energy Flow in Photosynthesis

• in its native environment of the thylakoid
  membrane, chlorophyll is organized along with
  proteins, pigments,  and other kinds of smaller
  organic molecules into photosystems
• a photosystem has a light gathering "antenna
  complex" consisting of a few hundred chlorophyll
  a, chlorophyll b, and carotenoid molecules
• all of the antenna molecules absorb photons of
  light and the energy is transmitted from pigment
  molecule to pigment molecule until it reaches the
  reaction center

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Photosystems I and II (Fig.7.9)

• there are two photosystems in the thylakoid
  membranes, photosystem I and photosystem II
• the chlorophyll a in both photosystems is
  identical, it is their association with different
  proteins that affects their light absorbing
  properties
•  

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7.6 The Calvin Cycle (Dark Reactions)

• The "Dark Reactions" include the biochemical,
  enzyme-catalyzed reactions involved in the
  synthesis of carbohydrate from carbon dioxide
  these are collectively know as the Calvin-Benson
  cycle

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The Dark Reactions (Calvin Cycle)

• Light is not required directly for these
  reactions to occur
• These reactions incorporate CO2 from the air into
  organic material through a process known as
  carbon fixation
• The fixed carbon is then reduced to carbohydrate
  by the addition of electrons
• The reducing power is provided by NADPH and ATP
  provided by the light reactions
• Dark reactions in most plants occur during
  daylight so that the light reactions can
  regenerate NADPH and ATP
• These reactions occur in the stroma

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The Reactions In the Stroma (Fig.7.10)

• 1. CO2 combines with RuBP to form 2 molecules of
  PGA.
• 2. Each molecule of PGA is converted into a
  molecule of PGAL.
• 3. Most of the PGAL is converted back into RuBP,
  but some PGAL can be used to make a variety of
  organic compounds.
• For every three CO2 that enter the Calvin-Benson
  cycle via rubisco, a total of six molecules of
  3-phosphoglyerate (PGA) are made

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The Balance Sheet for Photosynthesis

• Each turn of the Calvin Cycle fixes one CO2
  molecule.
• It take 3 turns to produce each PGAL molecule.
• 3 turns uses nine ATP molecules and 6 NADPH
  molecules. Why nine?
• A variety of organic compounds are created amino
  acids, lipids, and carbohydrates.

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The Metabolic Fates of Glucose

• About 50 of the glucose formed is used
  immediately to meet the plants energy needs
• Excess glucose can be converted to starch within
  the stroma of the chloroplast or in specialized
  storage cells of roots, tubers, seeds, and fruits
  
• REMEMBER, plants actively metabolize glucose
  (cellular respiration) and grow in the dark and
  in the light
• The glucose may be converted to sucrose (glucose
  fructose) for transport (via the phloem cells)
  to the non-photosynthetic leaves, roots, and
  stems
• The glucose may be converted to CELLULOSE, to
  build cell walls, especially in plant cells that
  are still growing and maturing

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7.7 Alternative Pathways (Fig.6-9)

• Calvin Cyclemost common pathway for carbon
  fixation.
• Plants in harsh conditions fix carbon through
  alternate pathways and then release it to enter
  the Calvin cycle.

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C3 Plants

• Most plants are C3 plants
• C3 plants exclusively fix carbon through the
  Calvin cycle b/c of 3-carbon PGA.
• Build up of O2, which slows down sugar production.

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C4 Pathway

• Corn, sugar cane, crabgrass.
• Fix CO2 into 4-Carbon compounds C4 pathway.
• Partially close stomata during hottest part of
  the day.
• Lose about ½ as much water as C3 plants when
  producing same amount of carbs.

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CAM Pathway

• CAM plants have adapted to dry conditions by
  opening their stomata during the night and
  closing them during the day, opposite to how
  other plants behave
• when the stomata are open CO2 is incorporated
  into a variety of organic acids in a method of
  carbon fixation call crassulacean acid metabolism
  (CAM)
• the mesophyll cells of CAM plants store the
  organic acids they make during the night in their
  vacuoles until morning when the stomata close
• CO2 is released from the acids during the day for
  incorporation into the Calvin cycle
• Lose less water than C3 C4 plants.
• Desert plants - Cactus

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Rate of Photosynthesis
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Now let's revisit the summary equation for
photosynthesis note how each of the raw
materials end up in the chloroplasts so that the
whole photosynthesis deal can go down.
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Now let's do the same, except pay attention to
what happens to the products of photosynthesis.
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