Photosynthesis: The Reaction
Recap……..
6 CO2 + 6 H20  C6H12O6 + 6 O2
http://www.youtube.com/watch?v=lqEDz2vfhp
Q&feature=related
Leaf Structure
• Most photosynthesis occurs in the palisade layer.
• Gas exchange of CO2 and O2 occurs at openings
called stomata surrounded by guard cells on the
lower leaf surface.
Palisade
Spongy
Chloroplast Structure
• Inner membrane
called the thylakoid
membrane.
• Thickened regions
called thylakoids. A
stack of thylakoids
is called a granum.
(Plural – grana)
• Stroma is a liquid
surrounding the
thylakoids.
Chloroplast Structure
• Inner membrane
called the thylakoid
membrane.
• Thickened regions
called thylakoids. A
stack of thylakoids
is called a granum.
(Plural – grana)
• Stroma is a liquid
surrounding the
thylakoids.
Pigments
• Chlorophyll A is the most important
photosynthetic pigment.
• Other pigments called antenna or
accessory pigments are also present in
the leaf.
o
o
o
Chlorophyll B
Carotenoids (orange / red)
Xanthophylls (yellow / brown)
• These pigments are embedded in the
membranes of the chloroplast in groups
called photosystems.
Photosynthesis Occurs in 2 Steps
• Light Reaction
– Light reactions are the “photo” part of photosynthesis.
Light is absorbed by pigments
– Energy from light drives the reaction
•Light Independent Reaction or Dark Reaction
•The Calvin Cycle
•the “synthesis” part of photosynthesis. Trapped energy
from the sun is converted to the chemical energy of
sugars.
•Doesn’t need direct light energy, but it does need the
high-energy products from the Light Reaction
• Two types of
photosystems
cooperate in the
light reactions
Photon
ATP
mill
Photon
Water-splitting
photosystem
NADPH-producing
photosystem
Light Reactions
• Light-dependent reactions occur on
the
thylakoid membranes.
o Light
and water are required for this
process.
o Energy storage molecules are formed.
(ATP and NADPH)
o Oxygen gas is made as a waste product.
Light Reaction
• Drawing first…….
Light Reaction
• Now words…….
STEP 1 - Light energy is absorbed by chlorophyll in
the thylakoid membranes of the chloroplast
STEP 2 - In Photosystem II the electrons become
energized or “excited”.
STEP 3 - The excited electrons are passed down a
series of molecules called the electron transport
chain.
STEP 4 - To replace the electrons that are lost, some
are stolen from water . This breaks the water
molecule apart like this:
2H2O  4H+ + O2
STEP 5 -As electrons move down the chain, their
energy is used to make ATP (a high- energy molecule)
6. STEP 6 -The electrons now go to Photosystem I.
Light energizes new electrons in PI and they
combine with NADP+ to make NADPH (another
high-energy molecule).
Reactants used during the light
reaction:
1. Water
2. Also used sunlight
The final products of the
light reaction
1. ATP
2. NADPH
3. O2
Review Light Reaction
• Light Reaction
– Where does this happen:
– Needs:
– Makes:
That’s only HALF of the Photosynthesis
Reaction……
light
H2O ---------------→ O2 + ATP + NADPH
chlorophyll
Dark Reactions
• Dark reactions (light-independent)
occur in the stroma.
• This reaction is ENZYMATIC not ELECTRONIC
• Doesn’t ACTUALLY happen in the dark….
o Carbon
dioxide is “fixed” into the sugar
glucose.
o ATP and NADPH molecules created
during the light reactions power the
production of this glucose.
Why fix CO2?
• By fixing CO2, energy is stored in organic
compounds
Dark Reaction
• 3 steps:
– Carboxylation
– Reduction
– Regeneration
*It takes 3 turns of the Calvin
Cycle to release 1 PGAL*
* 2 PGAL = 1 Glucose*
Dark Reaction
• Write it:
• Step 1: CO2 (1C) is added to RuBP (5C) making an
unstable 6C molecule that is quickly converted to 2
PGA (3C each) molecules. Catalyzed by rubisco
• Step 2: PGA (3C) is reduced to PGAL (3C) with the help of
NADPH and ATP. 2 PGA (3C)  2 PGAL (3C)
• Step 3: PGAL is converted to RuBP with the help of ATP
or “siphoned” off to create glucose (later)
Dark Reaction
• Step 4: REPEAT 2 more times
• At third turn: 1 PGAL is released. When 2
PGALs combine, glucose is created
Dark Reaction
• What we need from the Light Reaction:
– CO2
– NADPH
– ATP
Dark Reaction
• Products:
– SUGAR
AN OVERVIEW OF PHOTOSYNTHESIS
• The light reactions
convert solar
energy to chemical
energy
o
Light
o
NADP
ADP
+P
Produce ATP & NADPH
• The Calvin cycle makes
sugar from carbon
dioxide
o
Chloroplast
ATP generated by the light
reactions provides the energy
for sugar synthesis
The NADPH produced by the
light reactions provides the
electrons for the reduction of
carbon dioxide to glucose
Light
reactions
Calvin
cycle
Dark Reaction
• Build it….
Photosynthesis
• Act it….
• 1,2,3
http://www.youtube.com/watch?v=yrQzEw9x
Y5k&feature=related
• Rap
http://www.youtube.com/watch?v=pdgkuT12
e14&feature=related