Chapter 7 - Photosynthesis
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Transcript of Chapter 7 - Photosynthesis
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Chapter 7 - Photosynthesis
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evolution.berkeley.edu/.../images/chicxulub.gif
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What Is Photosynthesis? The ability to capture sunlight energy and
convert it to chemical energy.
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The Photosynthetic Equation
6CO2 carbondioxide
+ 6H2O water
+ light energy sunlight
C6H12O6
glucose (sugar)
+ 6O2
oxygen
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Photosynthetic Organisms
Plants, algae, and some prokaryotes Are autotrophs (“self- feeders”)
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Photosynthesis and Cellular Respiration
Are interconnected Water, CO2, sugar, and O2 are used or
produced as byproducts in both processes
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Adaptations for Photosynthesis
Leaves
Chloroplasts
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Leaves Flattened leaf shape exposes large
surface area to catch sunlight
Epidermis upper and lower leaf surfaces
Cuticle waxy, waterproof outer surface reduces water evaporation
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Leaf Anatomy Stomata
adjustable pores allow for entry of air with CO2
Mesophyll inner cell layers that 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 Chloroplasts
bounded by a double membrane composed of inner and outer membranes
Stroma semi-fluid medium within the inner membrane
Thylakoids disk-shaped sacs found within the stroma in stacks called grana
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Location of Photosynthetic Reactions
2 sets of chemical reactions occur in the:
1. Thylakoid membranes
2. Stroma
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Light-dependent Reactions
Pigment molecules (e.g. chlorophyll) of the thylakoids capture sunlight energy
Sunlight energy is converted to the energy carrier molecules ATP and NADPH
Oxygen is released as a by-product
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Light-independent Reactions
Enzymes in stroma synthesize glucose and other organic molecules using the chemical energy stored in ATP and NADPH
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Light-Dependent Reactions
What two energy carrying molecules are used to store captured sunlight energy during light-dependent reactions?
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The Energy in Visible Light Sun radiates electromagnetic energy Photons (basic unit of light)
packets of energy with different energy levels short-wavelength photons are very energetic longer-wavelength photons have lower
energies Visible light is radiation falling between
400-750 nanometers of wavelength
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Absorption of certain wavelengths light is “trapped”
Reflection of certain wavelengths light bounces back
Transmission of certain wavelengths light passes through
Light Captured by Pigments
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Light Captured by Pigments Absorbed light drives biological processes when
it is converted to chemical energy
Pigment absorbs visible light
Common pigments:Chlorophyll a and b
absorb violet, blue, and red light but reflect green light (hence they appear green)
Carotenoids absorb blue and green light but reflect yellow, orange, or
red (hence they appear yellow-orange) Are accessory pigments
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Why Do Autumn Leaves Change Colors?
autumn-pictures.com
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Light-Dependent Reactions
Photosystems within thylakoidsAssemblies of proteins, chlorophyll, &
accessory pigments
Two Photosystems PSII (comes 1st) and PSI (comes 2nd)
Each Photosystem is associated with a chain of electron carriers
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Light-Dependent ReactionsSteps of the light reactions:
1. Accessory pigments in Photosystems absorb light and pass energy to reaction centers containing chlorophyll
2. Reaction centers receive energized electrons…
3. Energized electrons then passed down a series of electron carrier molecules (Electron Transport Chain)
4. Energy released from passed electrons used to synthesize ATP from ADP and phosphate
5. Energized electrons also used to make NADPH from (NADP+) + (H+)
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Maintaining Electron Flow
Electrons from PSII flow one-way into PS I
PSII – produces ATP
PSI – produces NADPH
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Oxygen May be used by plant or released into
atmosphere
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Light-Independent Reactions
NADPH and ATP from light-dependent rxns used to power glucose synthesis
Light not directly necessary for light-independent rxns if ATP & NADPH available
Light-independent rxns called the Calvin-Benson Cycle or C3 Cycle
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The C3 Cycle
6 CO2 molecules used to synthesize 1 glucose (C6H12O6)
CO2 is captured and linked to ribulose bisphosphate (RuBP)
ATP and NADPH from light dependent rxns used to power C3 reactions
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Relationship Between Rxns
“Photo” capture of light energy (light dependent rxns)
“Synthesis” glucose synthesis (light-independent rxns)
Light dependent rxns produce ATP and NADPH which is used to drive light-independent rxns
Depleted carriers (ADP and NADP+) return to light-dependent rxns for recharging
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The Ideal Leaf
The ideal leaf:
Large surface area to intercept sunlight
Very porous to allow for CO2 entry from air
lowcarboneconomy.com
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forestry.about.com
sbs.utexas.edu
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Leaves
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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When Stomata Are Closed
When stomata close, CO2 levels drop and O2 levels rise
Photorespiration occurs Carbon fixing enzyme combines O2 instead of CO2
with RuBP
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When Stomata Are Closed
Photorespiration:O2 is used up as CO2 is generatedNo useful cellular energy madeNo glucose producedPhotorespiration is unproductive and
wasteful
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When Stomata Are Closed
Hot, dry weather causes stomata to stay closed
O2 levels rise as CO levels fall inside leaf
Photorespiration very common under such conditions
Plants may die from lack of glucose synthesis
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weedtwister.com
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C4 Plants Reduce Photorespiration
“C4 plants” have chloroplasts in bundle sheath cells and mesophyll cells
Bundle sheath cells surround vascular bundles deep within mesophyll
C3 plants lack bundle sheath cell chloroplasts
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C4 Plants Reduce Photorespiration
C4 plants utilize the C4 pathway
Two-stage carbon fixation pathway
Takes CO2 to chloroplasts in bundle sheath cells
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Environmental Conditions C4 pathway uses up more energy than C3
pathway
C3 plants thrive where water is abundant or if light levels are low (cool, wet, and cloudy climates) Ex. : most trees, wheat, oats, rice, Kentucky
bluegrass
C4 plants thrive when light is abundant but water is scarce (deserts and hot climates) Ex. : corn, sugarcane, sorghum, crabgrass,
some thistles
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The End