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Photosynthesis
Bio 391 – Ch4
How Exactly is Sunlight captured and converted into Food?
What are autotrophs?
Obtains energy from nonliving sources
Two typesPhotoautotrophsPhotoautotrophs
Photosynthesis
Sun energy converts CO2 into sugars
Enzymes convert sugars into amino acids and other needed compounds
ChemoautotrophsChemoautotrophsSpecialized bacteria
No sunlight – use energy of inorganic substances (Fe, S, etc.)
Electromagnetic SpectrumWide range of energy types – travels in waves – energy is defined by their wavelength
λ = wavelength = distance between two adjacent wave crests or wave troughs
Visible LightVery small section of the electromagnetic spectrum
ROYGBIV
ChloroplastsStructureStructure
ThylakoidsHighly folded inner membrane
surface area
Holds pigments
GranumStack of thylakoid membranes
StromaLiquid between thylakoid and outer membrane of chloroplast
Have their own DNA & RNA
Chlorophyll & Accessory Pigments
Pigments = light absorbing molecules
Found on the thylakoid membrane
ChlorophyllChlorophyllTwo types – “a” and “b”
Absorbs violet-blue and orange-red colors
~ 350-500 nm & 650-700 nm
Reflects green plants have green color
AcceAccessoryssory Pigm Pigmentsents Absorb other colors of light and transfer Σ to chlorophyll-a
Most noticeable in the fall monthsEX: carotenoids
Absorption Spectrums of Pigments
Photosynthesis Simplified
Can be broken down into two steps:Light ReactionsLight Reactions
Pigments in thylakoids absorb lightLight converted into chemical energy
Calvin CycleCalvin Cycle (a.k.a. “Dark Reactions”)Chemical energy from light reactions used to make 3 carbon sugars from CO2
Used to make more complex sugars or other biochemical molecules
Overall ReactionOverall Reaction6CO2 + 6H2O C6H12O6 + 6O2
Light Dependent Reactions
Broken into Photosystem II and Photosystem I
Reactants: light, water
Use: ADP and Pi to make ATP
NADP+ to make NADPH (similar to NAD+/NADH)
Happens on the thylakoid membrane
Light Dependent Reactions
Photosystem IILight hits the chlorophyll molecules and excites them – releasing two high energy electrons
Electrons are used to create a H+ gradient across the thylakoid membrane
This gradient drives the formation of ATP (similar process to the ETC in respiration)
Photophosphorylation
Light Dependent Reactions
Photosystem ILight hits the chlorophyll molecules and excites them – releasing two high energy electrons
Electrons from Photosystem II replace the electrons that leave chlorophyll molecule
Electrons are captured by NADP+ to make NADPH
Light Dependent Reactions
ATP and NADPH are used in the light independent reactions
How are electrons from Photosystem II replaced?
Water is splitO2 – waste product – released by the plant
Electrons – go into chlorophyll to replace lost e’s
H+ - used to make gradient to help make ATP
LIGHT DEPENDENT REACTIONS
Cyclic v. Noncyclic Photophosphorylation
Cyclic – photosystem I only – electrons are recycled (use no NAPDH)Chemiosmosis – process of using proton movement to join ADP and Pi
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter39/cyclic_and_noncyclic_photophosphorylation.html
Simple vs. Complex Autotrophs
Generates ATP but not NADPH. Why?
Light Independent Reactions
Also called the Calvin Cycle
Reactants: ATP, NADPH, and CO2
Use: ATP to make ADP and Pi
NADPH to make NADP+
Sugars are created
Happens in the stroma
Calvin Cycle
Keys to understanding….It’s all about rearrangement of carbon atomsIt’s all about rearrangement of carbon atoms
CO2 enters cycle by attaching to RuBP RuBP is a 5-carbon molecule
Similar to Acetyl CoA entering Krebs cycle
Creates 2 PGAPGA is a 3-carbon molecule
PGA turns into PGAL PGAL is a PGA molecule that has been energized by the ATP and NADPH
Calvin Cycle Summary
Each turn fixes 1 CO2 to a RuBP
RubiscoRubiscoEnzyme that catalyzes CO2 fixation
Activated by light thus Calvin cycle requires some level of light to occur
Can bind O2 if present
3 turns = 1 PGAL
“C3 plants” – those that fix 3 CO2 into 1 PGAL
Calvin Cycle Summary
PGAL
Light Reactionhttp://vcell.ndsu.edu/animations/photosynthesis/movie.ht
m
Calvin-Benson Cyclehttp://www.youtube.com/watch?v=mHU27qYJNU0
Photosynthesis
includes
of
take place intakes place in uses
to produce to produce
use
Light-dependentreactions
Calvin cycle
Thylakoidmembranes Stroma NADPHATPEnergy from
sunlight
ATP NADPH O2 Chloroplasts High-energysugars
Section 8-3
Concept Map
Factors Effecting the Rate of
Photosynthesis
Light IntensityMore light = higher rate
Reaches saturation pointEnzymes of light reaction going as fast as possible
Higher than saturation point PS declines
Chlorophyll accumulates light faster than it can transfer it to ETS
Extra energy goes to oxygen producing OH- when reaction w/H2O
OH- or H2O2 damages chloroplasts
PHOTOINHIBITION
CO2 Concentration
• Similar to light intensity
• Hits a saturation point
• Does not decline after saturation
Temperature
• Optimal temperature range
– If too high…• Proteins denature
– If too low…• Molecular movement is
slower
• High Temps = cause stomata to close
– Prevents water loss– Increases
photorespiration– C4 and CAM adaptations
A metabolic pathway in plants that consumes oxygen, produces carbon dioxide, generates no ATP, and reduces photosynthesis
O2 Concentration / Photorespiration
• REMEMBER Rubisco binds CO2 and O2 equally as well
– Molecular shapes are similar
• Halves productivity of PGA– Carbon fixation = 2 PGA– Photorespiration = 1 PGA
• Glycolate = toxic to plant
• Benefits of photorespiration?– Occurs when stomata close
• Dry and hot• Evolutionary of C4 and CAM plants
– Still makes some CO2 and thus some sugars
C3 vs C4 vs CAMhttp://wc.pima.edu/~bfiero/tucsonecology/
plants/plants_photosynthesis.htm
Leaf Anatomy – C3 vs. C4
C3 plants•CO2 pulled through stomata and immediately goes to mesophyll cells to complete photosynthesis•Called C3 because it makes PGA (3-Carbon molecule)•Stomata open during day•Efficient in cool and moist envir.
C4 plants•CO2 pulled through stomata and immediately goes to mesophyll cells then to the bundle sheath cells to complete photosynthesis•Called C4 because it makes a 4-carbon molecule first (using PEP carboxylase•Stomata open during day•Efficient in higher temps and higher light intensity
Reducing Photorespriation:CAM plants
CAM plants•Crassulacean Acid Metabolism•CO2 pulled through stomata and stored as an acid. During the day, stomata close, CO2 is released, then the cell goes through the Calvin cycle•Stomata open during night
• Close during the day to prevent water loss
•Efficient in extremely hot and dry environments
Photosynthesis Song 1: The Light Reactions SongPhotosynthesis Song 2: The Calvin Cycle