What is Photosynthesis

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CAN YOU EXPLAIN THE
IMPORTANCE OF THE COLOR AND
SHAPE OF LEAVES?
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Photosynthesis

• What Is Photosynthesis?
• Light-Dependent Reactions How Is Light Energy
  Converted to Chemical Energy?
• Light-Independent Reactions How Is Chemical
  Energy Stored in Glucose Molecules?
• What Is the Relationship Between Light-Dependent
  and Light-Independent Reactions?
• Water, CO2, and the C4 Pathway

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The Photosynthetic Equation
6CO2 carbon dioxide
6H20 water
light energy ? sunlight
C6H12O6 glucose (sugar)
6O2 oxygen
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Cycling of Gases, Sugar, and Water in Mesophyll
Cells

• Production of carbon compounds like glucose
  (photosynthesis) is linked with energy extraction
  (in cellular respiration)
• Water, CO2, sugar, and O2 cycle between the two
  processes

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Leaf Anatomy

• Flattened leaf shape exposes large surface area
  to catch sunlight
• Upper and lower leaf surfaces of a leaf comprise
  the epidermis
• Waxy, waterproof cuticle on outer surfaces
  reduces water evaporation

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Leaf Anatomy

• Adjustable pores called stomata allow for entry
  of air with CO2
• Inner mesophyll cell layers contain majority of
  chloroplasts
• Vascular bundles (veins) supply water and
  minerals to the leaf while carrying sugars away
  from the leaf

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Anatomy of a Chloroplast

• Mesophyll cells have 40-200 chloroplasts each
• Chloroplasts are bounded by a double outer
  membrane composed of the inner and outer membranes

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Anatomy of a Chloroplast

• The stroma is the semi-fluid medium within the
  inner membrane
• Disk-shaped sacs called thylakoids found within
  the stroma in stacks called grana

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Location of Photosynthetic Reactions

• The two chemical reactions of photosynthesis are
  localized
• The conversion of sunlight energy to chemical
  energy (light-dependent reactions) occurs within
  the thylakoid membranes
• The synthesis of glucose and other molecules
  (light-independent reactions) occurs in the
  surrounding stroma

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Photosynthesis Composed of Two Groups of Reactions

• Light Dependent Reactions
• Chlorophyll and other molecules of the thylakoids
  capture sunlight energy
• Sunlight energy is converted to the energy
  carrier molecules ATP and NADPH
• Oxygen gas is releases as a by-product

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Photosynthesis Composed of Two Groups of Reactions

• Light Independent Reactions
• Enzymes in the stroma synthesize glucose and
  other organic molecules using the chemical energy
  stored in ATP and NADPH

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Cycling of Energy Molecules in Photosynthesis
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Light-Dependent Reactions How Is Light Energy
Converted to Chemical Energy?

• During Photosynthesis, Light Is First Captured by
  Pigments in Chloroplasts
• The Light-Dependent Reactions Occur Within the
  Thylakoid Membranes
• Photosystem II Generates ATP
• Photosystem I Generates NADPH
• Splitting Water Maintains the Flow of Electrons
  Through the Photosystems

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Light Dependent Reactions

• Captured sunlight energy is stored as chemical
  energy in two carrier molecules
• Adenosine triphosphate (ATP)
• Nicotinamide adenine dinucleotide phosphate
  (NADPH)

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The Energy in Visible Light

• Sunlight radiates electromagnetic energy
• Visible light is radiation falling between
  400-750 nanometers of wavelength

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The Energy in Visible Light

• Packets of energy called photons different in
  energy level
• Short-wavelength photons are very energetic
• Longer-wavelength photons have lower energies

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Visible Light Stimulates Pigment Molecules

• Action of light-capturing pigments
• Absorption of certain wavelengths (light is
  trapped)
• Reflection of certain wavelengths (light bounces
  back)
• Transmission of certain wavelengths (light passes
  through)

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Visible Light Stimulates Pigment Molecules

• Absorbed light drives biological processes when
  it is converted to chemical energy
• Common pigments found in chloroplasts include
• Chlorophyll a and b
• Accessory pigments such as carotenoids

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Chlorophylls

• Chlorophyll a and b absorb violet, blue, and red
  light but reflect green light (hence they appear
  green)

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WHY ARE THESE LEAVES GREEN?

• they contain chlorophyll a and b
• Chlorophylls absorb violet, blue, and red light
• Chlorophylls reflect green light

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Carotenoids

• absorb blue and green light but reflect yellow,
  orange, or red (hence they appear yellow-orange)
• Beta-carotene is a carotenoid that produces the
  orange color of vegetables such as carrots
• Beta-carotene is the principle source of vitamin
  A for animals and is used in capturing light in
  our eyes

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Why Autumn Leaves Turn Yellow and Red

• Both chlorophylls and carotenoids are present in
  leaves
• Chlorophyll breaks down before carotenoids in
  dying autumn leaves revealing yellow colors
• Red fall colors (anthocyanin pigments) are
  synthesized by some autumn leaves, producing red
  colors

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Light Dependent Reactions in Thylakoids

• Photosystems within thylakoids
• Photosystems are assemblies of proteins,
  chlorophyll, accessory pigments
• Two Photosystems (PSI and PSII) in thylakoids
• Each Photosystem is associated with a chain of
  electron carriers

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WHAT DO PLANTS USE THESE PIGMENTS FOR??

• Chlorophylls absorb light and generate high
  energy electrons
• Carotenoids are absorb light energy and transfers
  it to chlorophylls (think of them as antennae)
• Chlorophylls and carotenoids are organized into
  photosystems

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Overview of the Light Dependent Reactions

• Accessory pigments in Photosystems absorb light
  and pass energy to reaction centers containing
  chlorophyll
• Reaction centers receive energized electrons
• Energized electrons then passed down a series of
  electron carrier molecules (Electron Transport
  Chain)

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Overview of the Light Dependent Reactions

• In Photosystem II the energy released from passed
  electrons used to synthesize ATP from ADP and
  phosphate
• In Photosystem Ithe energized electrons make
  NADPH from NADP and H

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Why do plants need water for photosynthesis?

• The two electrons that Photosystem II boosts to
  the ETS that drives ATP synthesis are generated
  by splitting water
• The H required for NADPH synthesis by the ETS
  after Photosystem I is generated by splitting
  water

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Q What left after the hydrogen and electrons
are stripped from water? A you are right, oxygen
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Time to Make Sugar.The Light-Independent
Reactions

• How Is Chemical Energy Stored in Glucose
  Molecules?
• The C3 Cycle Captures Carbon Dioxide
• Carbon Fixed During the C3 Cycle Is Used to
  Synthesize Glucose

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Light Independent Reactions

• NADPH and ATP from light-dependent reactions used
  to power glucose synthesis
• Light not directly necessary for
  light-independent reactions if ATP NADPH
  available
• Light-independent reactions called the
  Calvin-Benson Cycle or C3 Cycle

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Overview of the C3 Cycle

• 6 CO2 used to synthesize 1 glucose (C6H12O6)
• Carbon dioxide is captured and linked to
  ribulose bisphosphate (RuBP)
• ATP and NADPH from light dependent reactions used
  to power C3 reactions

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C3 cycle
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C3 Cycle Has Three Parts

• Carbon fixation (carbon capture)
• 6 Ribulose bisphosphate (RuBP) molecules combine
  with 6CO2
• Fixation step and subsequent reactions yield
  twelve 3-carbon phosphoglyceric acid (PGA)
  molecules

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C3 Cycle Has Three Parts

• Synthesis of Glyceraldehyde 3-Phosphate (G3P)
• Energy is donated by ATP and NADPH
• Phosphoglyceric acid (PGA) molecules are
  converted into glyceraldehyde 3-Phophate (G3P)
  molecules

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C3 Cycle Has Three Parts

• Regeneration of Ribulose bis-phosphate (RuBP)
• 10 of 12 G3P molecules converted into 6 RuBP
  molecules
• 2 of 12 G3P molecules used to synthesize 1
  glucose
• ATP energy used for these reactions

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Fixed Is Carbon Incorporated into Glucose

• One cycle of the C3 Cycle produce two left over
  G3P molecules
• Two G3P molecules (3 carbons each) used to form 1
  glucose (6 carbons)
• Glucose may later be broken down during cellular
  respiration or stored in chains as starch

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Summary of Photosynthesis
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Limiting Resources?

• Plants do not always have all of the water and
  carbon dioxide they need
• Plants have evolved both anatomy and biochemical
  pathways to cope with environmental conditions
• Minimizing photorespiration is a selective
  pressure in plant evolution

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Water, CO2, and the C4 Pathway

• The Ideal Leaf
• Ideal leaves have large surface area to intercept
  sunlight
• Ideal leaves are very porous to allow for CO2
  entry from air

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Water, CO2, and the C4 Pathway

• Problem Substantial leaf porosity leads to
  substantial water evaporation, causing
  dehydration stress on the plant
• Plants evolved waterproof coating and adjustable
  pores (stomata) for CO2 entry

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Photorespiration Occurs When Stomata Are Closed

• When stomata close, CO2 levels drop and O2 levels
  rise
• Carbon fixing enzyme combines O2 instead of CO2
  with RuBP (called photorespiration)

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Photorespiration is bad

• Photorespiration
• O2 is used up as CO2 is generated
• No useful cellular energy made
• No glucose produced
• Photorespiration is unproductive and wasteful

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Conditions Causing Photorespiration

• Hot, dry weather causes stomata to stay closed
• Oxygen levels rise as carbon dioxide levels fall
  inside leaf
• Photorespiration very common under such
  conditions
• Plants may die from lack of glucose synthesis

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C4 Plants Reduce Photorespiration

• C4 plants have chloroplasts in bundle sheath
  cells as well as mesophyll cells
• Bundle sheath cells surround vascular bundles
  deep within mesophyll
• C3 plants lack bundle-sheath cell chloroplasts
• C4 plants utilize the C4 pathway
• Two-stage carbon fixation pathway

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The C4 Pathway

• Outer mesophyll cells contain phosphoenolpyruvate
  (PEP) instead of RuBP
• Carbon dioxide specific enzyme links CO2 with PEP
  (unaffected by high O2)
• 4 carbon molecule then shuttled from mesophyll to
  bundle sheath cells...

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The C4 Pathway

• CO2 released in bundle sheath cells, building up
  high CO2 concentration
• CO2 in bundle sheath cells fixed by standard C3
  pathway
• 3 carbon shuttle molecule returns to mesophyll
  cells

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Where do C4 plants live?

• C4 pathway uses up more energy than C3 pathway
• C4 plants thrive when light is abundant but water
  is scare (deserts and hot climates)
• C4 plant examples corn, sugarcane, sorghum,
  crabgrass, some thistles

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C3 and C4 Plants Adapted to Different
Environmental Conditions

• C3 plants thrive where water is abundant or if
  light levels are low (cool, wet, and cloudy
  climates)
• C3 plant examples most trees, wheat, oats, rice,
  Kentucky bluegrass

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