Photosynthesis Flashcards
Photosynthesis
Conversion process that transforms the energy of sunlight into chemical energy stored in organic molecules
–> Conceptually, “the opposite” of respiration
Photosynthesis General Reaction
Sunlight + CO2 + H2O = C6H12O6 + O2
Where does the O2 come from?
The splitting of water
Carbon Fixation
CO2 –> C6H12O6
Water Splitting
H2O –> O2
Types of reactions in photosynthesis:
1) Light reactions
2) Calvin cycle reactions
Light reactions
The “photo” part
–> Converts solar energy into NADPH and ATP and releases O2
–> REQUIRES LIGHT
Products of Light Reactions
1) ATP (for internal consumption)
2) NADPH (to be used in calvin cycle)
Pigments
Substances that absorb visible light
Fluorescence
Light that is reflected by a substance
Chlorophyll
A green pigment found in plants
–> Absorbs red and blue light
–> Reflects green light
2 types of chlorophyll:
1) Chlorophyll-a –> Primary light capturing pigment
–> Has a CH3 group on ring
2) Chlorophyll-b –> Accessory light capturing pigment
—> Has a CHO group on ring
Components of Chlorophyll
1) Porphyrin Ring (containing Mg center)
2) Hydrophobic tail
3) Side group (distinguishes between types of chlorophyll)
Chlorophyll hydrophobic tail function
Anchors chlorophyll to the thylakoid membrane
Porphyrin Ring Function
Light absorbing region of chlorophyll
Ground vs Excited Electron States
Ground State = Electron in normal orbital
Excited State = Electron in higher orbital
Photosystem
A reaction center complex surrounded by several light harvesting complexes
Reaction Center
A special pair (2 molecules) of chlorophyll-a
–> Same chlorophyll as other molecules but is special due to location near primary acceptor
Primary Acceptor
Molecule that receives the electrons from the reaction center and transfers them to other reactions of ATP/NADPH synthesis
Antenna Pigments
Gather light
Photoexcitation pathway
Sunlight —> The pigments (Chl. a, Chl. b, carotenoids) –> Electrons get transferred to Chl.a molecules –> Electrons are transferred to the reaction center –. electrons are transferred to the primary acceptor
Photosystem I
P700: Reaction center has chlorophyll-a molecules that absorb light at 700 nm
Photosystem II
P680: Reaction center has chlorophyll-a molecules that absorb light at 680nm
Cyclic Electron Flow
ONLY involving Photosystem I
–> Electrons originate from PS1 and go back to PS1
“replenishes itself”
ONLY produces ATP
Cyclic Electron Flow Final Electron Acceptor
PS1 itself
Non-Cyclic Electron Flow
The Z scheme
–> Involves transfer of electrons from photosystem II to photosystem I
–> Produces ATP and NADPH
Non-Cyclic Electron Flow PROCESS
1) Light excites PS2 –> Electrons go to PS2 primary acceptor
2) Electrons from PS2 go to first ETC to produce ATP
3) Water is split to replenish the electrons lost from PS2 –> Releases O2
4) Light Excites PS1 –> Electrons go to PS2 primary acceptor
5) Electrons from PS1 go through second ETC
6) Electrons are accepted by NADP+ through the NADP+ dehydrogenase enzyme to produce NADPH
7) PS1 electrons lost are replenished by ELECTRONS FROM FIRST ETC
Chloroplast ETC
1) ATP produced in STROMA
2) Protons pumped into thylakoid space
3) Proteins sit in thylakoid membrane
Cytochrome Complex
Chloroplast ETC
2 sources of proton gradient in chloroplasts:
1) Pumping H+ into thylakoid space from stroma
2) Splitting of water from light reactions
BOTH cyclic and non-cyclic flow are needed because…
The calvin cycle requires more ATP than NADPH
–> Therefore the cyclic flow provides that extra ATP needed
Calvin cycle
Anabolic cycle taking place in the stroma that utilizes ATP, NADPH, and CO2 to create simple sugars
3 main steps in the calvin cycle:
1) Carbon fixation
2) Reduction
3) RuBP Regeneration
Carbon Fixation Step
CO2 from the air is attached to the RuBP sugar through the RUBISCO enzyme
RuBP
Ribulose Bi-phosphate sugar
RUBISCO
Enzyme that attaches CO2 to RuBP
Reduction Step (calvin)
A series of reactions which:
1) Utilize NADPH as H+ source
AND
2) Utilize ATP as source for phosphate
to produce:
6 G3P molecules (3 carbon sugars)
What occurs to the 6 G3P molecules produced by calvin cycle?
1 G3P leaves the cycle to make other biomolecules
5 G3P remain in the cycle to help regenerate RuBP
Regeneration of RuBP
The remaining 5 G3P molecules are used to recreate RuBP (with the help of phosphorylation from ATP)
3 main types of plants:
1) C3
2) C4
3) CAM
Adapted version of plants (2)
1) C4
2) CAM
C4 Plants have what kind of separation?
Have SPATIAL separation
C4 plants are adapted to which envrionment?
–> Adapted to SEMI DRY conditions: stomata only half open
C4 Plants Calvin Process
1) Fix CO2 into oxaloacetate (4C) in MESOPHYLL CELLS
2) Transfer the oxaloacetate to BUNDLE SHEATH CELLS where calvin cycle takes place
C4 Examples
Corn, Cane sugar
CAM plants have what kind of separation?
Have TEMPORAL separation
CAM Plants are adapted to which environment
DRY –> Think desert like
–> Open stomata at NIGHT (when cooler)
–> Close stomata during DAY (when hotter)
CAM Plants Calvin Process
1) NIGHT: Stomata opens –> takes in CO2 –> produces oxaloacetate
2) DAY: Stomata closed –> light reactions occur –> oxaloacetate undergoes calvin cycle
CAM examples
Cacti, pineapples
