Photosynthesis
Bio 391 – Ch4
How Exactly is Sunlight captured
and converted into Food?
What are autotrophs?
Obtains energy from nonliving sources
Two types
Photoautotrophs
Photosynthesis
Sun energy converts CO2 into sugars
Enzymes convert sugars into amino acids and
other needed compounds
Chemoautotrophs
Specialized bacteria
No sunlight – use energy of inorganic substances
(Fe, S, etc.)
Electromagnetic Spectrum
Wide 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 Light
Very small section of the electromagnetic spectrum
ROYGBIV
Rainbows are
separated white light
* white reflects all light
* black absorbs all light
* color seen is that color
reflected
Structure
Thylakoids
Highly folded inner
membrane
surface area
Holds pigments
Granum
Stack of thylakoid
membranes
Stroma
Liquid between
thylakoid and outer
membrane of
chloroplast
Have their own DNA & RNA
Chloroplasts
Chlorophyll & Accessory Pigments
Pigments = light absorbing molecules
Found on the thylakoid membrane
Chlorophyll
Two types – “a” and “b”
Absorbs violet-blue and orange-red colors
~ 350-500 nm & 650-700 nm
Reflects green  plants have green color
Accessory Pigments
Absorb other colors of light and transfer Σ to
chlorophyll-a
Most noticeable in the fall months
EX: carotenoids
Absorption Spectrums of Pigments
Photosynthesis Simplified
Can be broken down into two steps:
Light Reactions
Pigments in thylakoids absorb light
Light converted into chemical energy
Calvin 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 Reaction
6CO2 + 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 II
Light 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 I
Light 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 split
O2 – 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.mcgrawhill.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
atoms
CO2 enters cycle by attaching to RuBP
RuBP is a 5-carbon molecule
Similar to Acetyl CoA entering Krebs cycle
Creates 2 PGA
PGA 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
Rubisco
Enzyme 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 Reaction
http://vcell.ndsu.edu/animations/photosynthesis/movie.htm
Calvin-Benson Cycle
http://www.youtube.com/watch?v=mHU27qYJNU0
Concept Map
Section 8-3
Photosynthesis
includes
Lightdependent
reactions
Calvin cycle
take place in
Energy from
sunlight
Thylakoid
membranes
to produce
ATP
takes place in
use
NADPH
Stroma
of
O2
Chloroplasts
uses
ATP
NADPH
to produce
High-energy
sugars
Factors Effecting
the Rate of
Photosynthesis
Light Intensity
More light = higher rate
Reaches saturation point
Enzymes 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 C and CAM plants
4
Still makes some CO2 and thus
some sugars
C3 vs C4 vs CAM
http://wc.pima.edu/~bfiero/tucsonecology/plan
ts/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 4carbon 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 Song
Photosynthesis Song 2: The
Calvin Cycle