Photosynthesis in
Detail
Light Reactions and Calvin Cycle
Honors Biology
The Two Stages of
Photosynthesis: A Preview
• Photosynthesis consists of two processes
1) The Light reactions
• NEEDS LIGHT
• Light Dependent Reactions
2) The Calvin cycle
• A.k.a- Dark Reactions or Light
Independent Reactions
• DOES NOT NEED LIGHT
The Light Reactions
 Occur in the grana (& thylakoids)
 Convert solar energy to
chemical energy
 Chlorophyll absorbs solar energy
 Split water
 release oxygen gas (a by-product)
 produce ATP (using chemiosmosis)
 Forms NADPH from NADP+ (an eacceptor)
 Temporarily stores high energy e-’s
 “Electron shuttle bus”
The Calvin Cycle
 Occurs in the stroma
Forms SUGAR from carbon dioxide
Carbon fixation occurs (CO2
fixed carbon)
using ATP for energy and NADPH
for reducing power (adding e-s to
fixed carbon)
 Fixed carbon  carbohydrate
 An overview of photosynthesis
H2O
CO2
Light
NADP 
ADP
LIGHT
REACTIONS
+ P
CALVIN
CYCLE
ATP
NADPH
G3P
Chloroplast
O2
GLUCOSE
(sugar)
Starch
Photosynthesis
• Occurs in the chloroplast
• Divided into 2 sets of reactions:
- Light Dependent Reaction
- Calvin Cycle (Dark Reaction)
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Light
Reaction
• Requires Light!!
• Occurs in the thylakoid/grana (in chloroplast)
• Electron Transport Chain (ETC) makes ATP and
NADPH  sends it to the Calvin Cycle

WASTE PRODUCT: O2 Gas
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Part 1 – Light Reactions
1.
2.
3.
You need to be able to draw this
picture!
Photosystem II
ETC
Photosystem I
H2O Splits 
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What are the main steps in the light
reactions?
• Uses energy from sunlight
• Splits water into H+ and O2
• Converts ADP into ATP and NADP+ into NADPH
– Use sunlight and water
– Produces Oxygen, NADPH,
and ATP
WRITE
Light Rxn Sequence of Events…
1) Photosystem II
2) ETC
3) Photosystem I
How is light absorbed in the thylakoid?
• Photosystems absorb light
– Clusters of chlorophyll and proteins trap energy
from the sun
– Energy is transferred to electrons  makes
“excited” electrons
What are Electron Carriers?
• Compound (NADP+) that can accept a pair of
high-energy electrons and transfer them to
another molecule
• NADP+ grabs/carries 2 electrons and a H+ 
becomes NADPH
Photosystem II
• Photosystem II Steps →
1) Chlorophyll and other lightabsorbing pigments absorb
energy from the sun
•
Energy transferred to e-  high energy eleave the chlorophyll and go to the ETC
(electron transport chain)
2) H2O molecules are broken
down
–- Electrons come from H2O
–- O-, H+ and e- are separated
–For every TWO H2Os we get 4e-,
4H+, and 1 O2
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Photosystem II (cont’d)
• 3) The excited e- jump from protein to protein and
enter the ETC (electron transport chain) on the
thylakoid membrane
• As e- jump through the ETC, they lose energy
• This energy pumps/pulls H+ from the outside (stroma) to
the inside (lumen - aka - thylakoid space)
• H+ ions start to
build up inside the
thylakoid
• Next, e- move to
Photosystem I
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Photosystem I
• Photosystem I →
4) More energy is absorbed from sunlight
– Energy is added to e- from Photosystem II
– Excited e- leave the molecules
5) E- are added to NADP+ which then becomes
NADPH
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Photosystem I (cont’d)
• Now, remember…those H+ ions are building up inside the
lumen
6) H+ ions diffuse through a protein channel embedded in the
thylakoid membrane
– The protein channel is part of an enzyme called ATP Synthase
7) As the H+ ions flow through the protein channel, the enzyme
(ATP Synthase) makes ATP by adding a P group to ADP
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Light Reaction
Summary…
• Summary:
– Chemiosmosis  The process that uses the proton gradient and ATP
synthase to make ATP
– e- travels from PS II  PS I  added to NADP+ makes  NADPH
– Both ATP (via ATP Synthase) and NADPH are produced
During the light
reaction  these
then enter the
Calvin Cycle
 NADP+ is the final eacceptor in
photosynthesis
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Calvin Cycle!
 Second part of PS
 Occurs in the
stroma of the
chloroplast
 Can occur with or
without light
Three turns shown above
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Calvin Cycle
Summary of Steps:
• ATP and NADPH used for
energy
• CO2 is taken into the plant
• High-energy sugars are
made
• Also known as the sug ar
factory
Carbon dioxide & Calvin Cycle
• Enters cycle from the atmosphere
• Carbon from the carbon dioxide molecule is
used to make glucose (C6H12O6)
• Plants then use the glucose for:
• Food!
• Plant energy!
– Stored in bonds of the
glucose molecule
Calvin Cycle!
Main job:
• Incorporates CO2 and rearranges the
C’s to form sugar using the energy
(ATP/NADPH) from the Light Reaction
Carbon “fixing” – process of turning
CO2 into a solid compound
INPUT:
Each turn needs:
- 3 ATP
- 2 NADPH
- 1 CO2
• The CC “fixes” CO2
- Carbon enters as CO2 and
leaves as sugar
• Goal is to produce a sugar (G3P)
• Each turn “fixes” one molecule of
carbon, so one G3P takes 3 turns of
the Calvin Cycle
Three turns shown above
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The Calvin cycle has three
phases
1. Carbon fixation
2. Reduction
3. Regeneration of the CO2
acceptor (RuBP)
The Calvin Cycle Steps
CARBON FIXATION
1. CO2 enters cycle and attached to a 5carbon sugar called ribulose
biphosphate (RuBP) forming 6-C
molecule (unstable)
 Enzyme RUBISCO catalyzes reaction
2. Unstable 6-C molecule immediately breaks
down to 3 3-C molecules called 3phosphoglycerate (3-PGA)
REDUCTION
3. Each 3-phosphoglycerate (3-PGA) gets an
additional phosphate from ATP (from LIGHT
RXN)  becomes 1,3 phosphoglycerate
4. NADPH reduces 1,3 phosphoglycerate to
Glyceraldehyde-3-phosphate (G3P)
– G3P = a sugar that stores
potential energy
– Every 3 CO2  yields 6 G3P’s BUT only 1
can be counted in net gain for
carbohydrate (GLUCOSE) production
REGENERATION OF CO2
ACCEPTOR (RuBP)
5. The C- skeletons of 5 G3P
molecules are rearranged into 3
RuBP molecules
– ATP is used !!!!
The
Calvin
cycle
NOTE:
MORE ATP
is needed
than
NADPH!!
Phase 1: Carbon fixation
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
Phase 2:
Reduction
Also known as PGAL
Calvin Cycle Overview
 For 1 G3P molecule made
 9 ATP molecules are used
 6 NADPH molecules are used
 G3P (aka PGAL)= starting
material to make other organic
molecules (glucose, starch,
etc.)
Note: RuBP and Rubisco are NOT the
same thing!
Steps:
1) Enzyme Rubisco
attaches CO2 to
RuBP  creates 6
x 3-PGA
2) 6 x PGA are
reduced to = 6 x
G3P (sugar) via
ATP/NADPH
3) 1 x G3P exits as
glucose
4) 5 x G3P remain in
the cycle. ATP
rearranges the
atoms  3 x RuBP
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6CO2 + 6H2O + sunlight  C6H12O6 + 6O2
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• The rate of PS is affected by 3 factors:
– Light intensity
– Amount of CO2
– Temperature
• Why wouldn’t roots need chloroplasts?
Root cells don't have chloroplasts, because chloroplasts catch
sunlight! Since roots are underground, they are not exposed to
the sun!
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• If it is too hot out, the plant will close the stomata so that it
doesn’t lose too much water and become dehydrated
– However this eliminates the gas exchange!!
• SO  the levels of CO2 drop and the levels of O2 increase
– This results in…. PHOTORESPIRATION
• Photorespiration adds oxygen to the Calvin Cycle instead of
carbon dioxide
- This makes NO sugar or ATP
- This wastes all of the plants resources!
• Two types of plants:
1) CAM
2) C4 plants
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• Therefore, certain plants, (cacti, pineapple,
etc.) only open their stomata at NIGHT!
– This prevents them from drying out and still lets
them get CO2!
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What is wrong with this picture?
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Flower Reproductive
Leaf Structure
Chloroplast
Light Reactions
Calvin Cycle
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Exit Slip-April 9, 2014