evolution.berkeley.edu/.../images/chicxulub.gif the ability to capture sunlight energy and convert...
TRANSCRIPT
Chapter 7 - Photosynthesis
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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
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
Adaptations for Photosynthesis
Leaves
Chloroplasts
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
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
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
Fig. 7-5b, p. 111
two outer membranes of chloroplast
stroma
part of thylakoid membrane system:
thylakoid compartment, cutaway view
B Chloroplast structure. No matter how highly folded, its thylakoid membrane system forms a single, continuous compartment in the stroma.
Location of Photosynthetic Reactions
2 sets of chemical reactions occur in the:
1. Thylakoid membranes
2. Stroma
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
Fig. 7-5c, p. 111
sunlight O2 H2O CO2
CHLOROPLAST
light-dependent reactions
NADPH, ATP
NADP+, ADP
light-independent
reactions
sugars
CYTOPLASM
C In chloroplasts, ATP and NADPH form in the light-dependent stage of photosynthesis, which occurs at the thylakoid membrane. The second stage, which produces sugars and other carbohydrates, proceeds in the stroma.
Light-independent Reactions
Enzymes in stroma synthesize glucose and other organic molecules using the chemical energy stored in ATP and NADPH
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
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
Light Captured by Pigments
Absorbed light drives biological processes when it is converted to chemical energy
Pigments absorb 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
Electromagnetic Spectrum of Radiant Energy
Why Do Autumn Leaves Change Colors?
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Why Autumn Leaves Turn Color
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
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
The Thylakoid Membrane
Light-Dependent Reactions
Steps 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+)
Fig. 7-8, p. 113
to second stage of reactionsThe Light-Dependent Reactions of Photosynthesis
ATP synthase
light energylight energy NADPH ATP
ADP + Pi
photosystem IIelectron transfer chain photosystem I
thylakoid compartment
stroma
A Light energy drives electrons out of photosystem II.
C Electrons from photosystem II enter an electron transfer chain.
E Light energy drives electrons out of photosystem I, which accepts replacement electrons from electron transfer chains.
G Hydrogen ions in the thylakoid compartment are propelled through the interior of ATP synthases by their gradient across the thylakoid membrane.B Photosystem II pulls
replacement electrons from water molecules, which dissociate into oxygen and hydrogen ions (photolysis). The oxygen leaves the cell as O2.
D Energy lost by the electrons as they move through the chain causes H+ to be pumped from the stroma into the thylakoid compartment. An H+ gradient forms across the membrane.
F Electrons from photosystem I move through a second electron transfer chain, then combine with NADP+ and H+. NADPH forms.
H H+ flow causes the ATP synthases to attach phosphate to ADP, so ATP forms in the stroma.
NADP+
Maintaining Electron Flow
Electrons from PSII flow one-way into PS I
PSII – produces ATP
PSI – produces NADPH
Oxygen
May be used by plant or released into atmosphere
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
The C3 Cycle
6 CO2 molecules used to synthesize 1 glucose (C6H12O6)
CO2 is captured and linked to a sugar called ribulose bisphosphate (RuBP)
ATP and NADPH from light dependent rxns used to power C3 reactions
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
The Ideal Leaf
The ideal leaf:
Large surface area to intercept sunlight
Very porous to allow for CO2 entry from air
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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
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
When Stomata Are Closed
Photorespiration:O2 is used up as CO2 is generatedNo useful cellular energy madeNo glucose producedPhotorespiration is unproductive and
wasteful
When Stomata Are Closed
Hot, dry weather causes stomata to stay closed
O2 levels rise as CO2 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 and mesophyll cells
Bundle sheath cells surround vascular bundles deep within mesophyll
C3 plants lack bundle sheath cell chloroplasts
C4 Plants Reduce Photorespiration
C4 plants utilize the C4 pathway
Two-stage carbon fixation pathway
Takes CO2 to chloroplasts in bundle sheath cells
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
The End