Photosynthesis Reactions
But first…..
What do cells use for energy?
ATP!!!!




2
ATP stands for adenosine triphosphate
It is composed of the nitrogen base
ADENINE, the pentose (5C) sugar
RIBOSE, and three PHOSPHATE groups
The LAST phosphate group is bonded with
a HIGH ENERGY chemical bond
This bond can be BROKEN to release
ENERGY for CELLS to use
Removing a Phosphate from ATP
Breaking the LAST PHOSPHATE bond from
ATP, will --–
–
–
3
Release ENERGY for cells to use
Form ADP
Produce a FREE PHOSPHATE GROUP
ATP!
The negative phosphate groups are highly
unstable. Potential energy is “stored” in ATP
by forcing the 3rd phosphate group to attach.
When it falls off, ATP  ADP + P + energy.
ATP is a cell’s rechargeable battery:
ADP
ATP
Recharging ATP = Phosphorylation
6
During Photosynthesis, the energy
of electrons is used

The electrons are first excited by the sun…

Then, they can be transferred through
chemical reactions to pass on their energy.

When electrons move, this is called a redox
reaction.
Redox Reaction
The transfer of
electrons from
another
Two types:
1. Oxidation is
2. Reduction is
8
one or more
one reactant to
the loss of ethe gain of e-
Oxidation Reaction
The loss of electrons from a
substance or the gain of
Oxidation
oxygen.
6CO2 + 6H2O 
C6H12O6 + 6O2
glucose
Carbon
dioxide
9
Water
Oxygen
Reduction Reaction
The gain of electrons to a
substance or the loss of
oxygen.
Reduction
6CO2 + 6H2O  C6H12O6 + 6O2
glucose
10
The Amazing journey through
the photosynthetic reactions
The process
 Let’s
follow the molecules as they are used in the
process… starting with the source of ALL energy
Radiant Energy
The Process
 The
process of Photosynthesis involves many
steps
 These

are divided into two processes:
Light-Dependent Reactions


The cycle used to “CHARGE” the battery or (in science terms)
add the phosphate group to an ADP molecule
As with any factory, when making a product there are extra
materials known as waste, in this process, the WASTE is the
product known as Oxygen (O2) gas and is released through the
MOUTH of a leaf called the STOMA
This process sends the “ENERGY” to the next cycle
 which does NOT need light to work because it has
 the chemical energy of ATP…

5.
H2O
Reactants
1. light
SUNLIGHT
1.
.
_
4. Granum
2. Light –
Dependent
Reactions
3. Thylakoid
6. O2
Products
5.
H2 O
Reactants
1. light
SUNLIGHT
1.
.
_
4. Granum
NADP+
ADP+P
2. Light –
Dependent
Reactions
ATP
3. Thylakoid
NADPH
6. O2
Products
LIGHT REACTIONS
Light-dependent reactions
1. Photosystem II pigments, such as chlorophyll,
found on the THYLAKOIDS attract and absorb
light
2. This increases the energy of their electrons,
passing them to the electron (e- ) transport
chain, like a slide from “photo center II to photo
center I”
3. Enzymes in the thylakoid breakdown H2O
molecules (remember our equation NEEDS
water) into 2 H+ ions and 1 oxygen atom,
released as WASTE (good for us though!)
Light-dependent reactions
4. Pigments of Photosystem I reenergize the electrons which are
picked up by NADP+ (carries excited electrons like a potholder
because they are “too hot to handle”)
5. Also picks up H+ ion from the dismantled water molecule
filling up the carrier to make NADPH
6. The enzyme ATP Synthase in the thylakoid memebrane spins
like a wheel to attach the inorganic phosphates to the
"uncharged" ADP molecule to charge it up using the excited
electron's energy.
7. The "wheel" continues to rotate to deliver the charged
molecules to the next cycle
Chemiosmosis
Powers ATP synthesis
Takes place across the thylakoid
membrane
Uses ETC and ATP synthase
(enzyme)
H+ move down their concentration
gradient through channels of ATP
synthase forming ATP from ADP
20
VIDEOS
Light Reactions
https://www.youtube.com/watch?v=BK_cjd6Evcw
Hydrogen ion gradient and ATP synthase; (chemiosmosis)
3 Hydrogen ions cross the membrane to make 1 ATP
https://www.youtube.com/watch?v=3y1dO4nNaKY
The NADPH and ATP created in
the light reactions go on to drive
the Calvin Cycle (Dark Reactions)
During the Calvin Cycle, carbon dioxide is
finally assembled into glucose (or other
sugar) molecules.
This happens in the stroma of the
chloroplast
5.
H2 O
Reactants
9. CO2
1. light
SUNLIGHT
1.
8. Stroma
.
_
4. Granum
NADP+
ADP+P
7. CalvinBenson
Cycle
2. Light –
Dependent
Reactions
ATP
3. Thylakoid
NADPH
6. CO2
Products
10.
C6H12O6
The Process

This cycle is known as the Calvin-Benson Cycle
(aka Light-Independent Reactions or DARK
cycle)

This is the process that uses the charged battery, or chemical
energy in the form of ATP, to run its “machinery” to create its
product

The product that it makes is carbohydrates, which are either
used by the plant for its FOOD , to regenerate the cycle, or
stored as STARCH where it can be passed on to other
organisms, known as consumers, to “eat” so that the
molecules are passed on giving that next organism energy

This process occurs in the STROMA of the chloroplasts (think
stroma=syrup=sugar!)
VIDEO link
https://www.youtube.com/watch?v=E_XQR800AgM
Calvin Cycle (C3 fixation)
(36C)
6C-C-C-C-C-C
(6C)
6CO2
(unstable)
6C-C-C
(30C)
6C-C-C-C-C
RuBP
C3
(30C)
glucose
6ATP
6ATP
6NADPH
6NADPH
6C-C-C
6ATP
6C-C-C 12PGA
(36C)
(36C)
6C-C-C 12G3P
(6C)
C-C-C-C-C-C
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Glucose
Calvin
Cycle
Calvin Cycle
Remember: C3 = Calvin Cycle
C3
Glucose
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The Process
 IT DOES NOT REQUIRE LIGHT, so it is sometimes called the
Light-Independent or Dark Reactions
Steps for Light-independent Reactions:
1. 6 Carbon dioxide molecules (CO2) enter the cycle
from the atmosphere and combine with 6 5carbon molecules =
12 3-carbon molecules
(36 total carbon atoms)
Starting 5-carbon molecules are called Ribulose-1,5bisphosphate
Ribulose-1,5-bisphosphate carboxylase/oxygenase,
commonly known by the abbreviation RuBisCO, is an
enzyme involved in the first major step of carbon fixation,
The Process
2. The 12 3-Carbon molecules are then converted
to higher energy forms from high-energy
electrons of ATP and NADPH.
3.2 of the 12 3-Carbon molecules leave the cycle.
(Plant cells use these sugars for food, growth,
and development.)
The Process
4. The 10 remaining 3-Carbon molecules are
converted into 6 5-Carbon sugars. These
molecules combine with 6 new carbon dioxide
molecules to begin the next cycle.
The Calvin Cycle uses 6 molecules of
carbon dioxide to produce a single 6-carbon
molecule.
The Process
As the Calvin cycle continues, it works
steadily to remove carbon dioxide from the
atmosphere and turn it into usable sugars
These sugars are used by the plant
(autotrophs) and the consumers
(heterotrophs) who eat them and absorb
their nutrients.
The Reactions of Photosynthesis
Factors Affecting Photosynthesis:
Shortages of water can decrease the process of
photosynthesis
Ex. Desert plants
Temperature must be between 0 C and 35 C
Enzymes that aid on the reactions only function properly in this range
Intensity of Light
Increasing the intensity of light increases the rate of photosynthesis
ONLY to a certain point and then it levels off
The level can vary from plant to plant
Not all organism do
light reactions the
same way
Many photosynthetic
bacteria and algae
just make ATP from
light
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Cyclic Electron Flow
Occurs in the thylakoid membrane.
Uses Photosystem I only
P700 reaction center- chlorophyll a
Uses Electron Transport Chain (ETC)
Generates ATP only
ADP + P
ATP
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Cyclic Electron Flow
Primary
Electron
Acceptor
SUN
e-
e-
ePhotons
P700
e-
ATP
produced
by ETC
Accessory
Pigments
Photosystem I
Pigments absorb light energy & excite e- of
Chlorophyll a to produce ATP
36
Some
Sometimes, due to the lack
of water available, plants
need to do photosynthesis
with stomata close = lack of
no C02
This is called
Photorespiration
Photorespiration
Occurs on hot, dry, bright days
Stomates close
Fixation of O2 instead of CO2
Produces 2-C molecules instead of
3-C sugar molecules
Produces no sugar molecules or no
ATP
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Photorespiration
Because of photorespiration, plants
have special adaptations to limit the
effect of photorespiration:
1. C4 plants
2. CAM plants
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C4 Plants
Hot, moist environments
15% of plants (grasses,
corn, sugarcane)
Photosynthesis occurs in 2
places
Light reaction - mesophyll
cells
Calvin cycle - bundle
sheath cells
41
C4 Plants: 4 carbon sugars are
made to store and donate CO2
Malate-4C sugar
C-C-C-C
Malate
C-C-C-C
Transported
CO2
CO2
C3
Vascul
Tissu
glucose
C-C-C
PEP
ATP
Mesophyll Cell
C-C-C
Pyruvic Acid
Bundle Sheath Cell
42
CAM Plants
Hot, dry environments
5% of plants (cactus and ice plants)
Stomates closed during day
– conserve water
Stomates open during the night
– CO2 stored in 4C sugar
Light reaction - occurs during the day
Calvin Cycle - occurs when CO2 is
present
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CAM Plants
Night (Stomates Open)
Day (Stomates Closed)
Vacuole
CO2
C-C-C-C
Malate
C-C-C-C
Malate
C-C-C-C
Malate
CO2
C3
C-C-C
PEP
ATP
C-C-C
Pyruvic acid
glucose
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Which reaction is this?
CO2 + H2O + Light  C6H12O6 + O2
Which reaction is this?
Most of any
plant’s mass is
actually coming
from….
CO2 !
Full equations?
“Photo-” (light reactions):
H2O + ADP + Pi + NADP+  O2 + ATP + NADPH + (H+)
“-synthesis” (Calvin Cycle):
NADPH + (H+) + ATP + CO2  C6H12O6 + ADP + Pi + NADP+
Why make sugar, if plants can
transfer sun energy to ATP?
ATP is too unstable but excellent for quick reactions
and immediate energy
Glucose & starch are much more ‘shelf stable’…
good for storage and as building material
What is “plant food”?
NOT organic, not food!
It contains nitrogen and
phosphorous that is
helpful for plant cells to
build AMP/ ADP/ATP,
chlorophyll molecules,
and NADP+ in new cells
Supplements poor
soil
How does a tree
transport
chemicals for
photosynthesis?
Stomata = leaf “nostrils”
Xylem = tubes that carry water and
ions like phosphates and nitrate from
roots upward (using H-bonds to help!)
Phloem = “food” tubes that distribute
freshly made glucose from leaves to the
rest of the plant.
Energy Flow
Sun
 Plant, algae, and bacteria
Photosynthesis makes
carbs, fat, protein
 Cellular respiration
Food breaks down to
make ATP available
 Death and decay
-returns nutrients to soil
and air
Next topic… what kinds of foods are autotrophs ‘passing on’ to us?
Photosynthesis
Review questions
Identify these parts of ATP:
What is the most important aspect of the
light reactions?
Where do the light reactions take place?
light reactions of photosynthesis supply the
Calvin cycle with
What happens during electron transport?
What is the overall reaction for the light
reactions?
Do the dark reactions have to happen in the
dark?
Where does the energy for the Calvin Cycle
come from?
What is the overall reaction of the dark
reactions?
CO2 + ATP + NADPH + (H+)
 C6H12O6 + ADP + NADP+
Mr. Orend’s more technical
explanations
(for honors & absentees)
Light Reactions - purpose is to convert solar
energy into usable chemical energy
How?
1) light strikes chlorophylls on surface of thylakoid membranes within the chloroplast of a plant cell.
2) This rips water molecules in half... the oxygen immediately diffuses out the cell and ultimately the leaf...
3) The hydrogens will be saved for building sugar later, but in the meantime, you have them just "sitting
around", so we can use them to...
4) build up a hydrogen proton gradient INSIDE the thylakoid. As more and more hydrogen protons (H+) get
shoved in their "against their will", the pressure (and potential energy) increases greatly. When the inside of the
thylakoid is stuffed full....
5) An enzyme gate called ATP Synthase opens up in the surface of the thylakoid. All the H+ rush out excitedly
(ready to get out of that crowded "room"), which turns the ATP Synthase mill. This mill turns once for every H+
that goes through. This catalyzes the formation of ATP from ADP and P. (This ATP is the "chemical energy"
made from solar energy and is used to power enymzes of the Calvin Cycle).
6) Because H+ is only HALF of a hydrogen atom, the electrons need to be transported to the dark reactions,
too. Here's where NADP+ comes in. It (being a positively charged molecule) is able to grab onto the loose
electrons from water's Hydrogen. When it is "babysitting" or "chaperoning" electrons, it is called NADPH. *I
remember this by looking at the + or the H at the end of NADP. If it's +, then it doesn't have the max
electrons. If you add an electron to a proton, it gives you "H" (hydrogen). NADP+ knows where to pick up the
high-energy electron by just hanging out at the end of the electron transport chain of the thylakoid membranes.
OVERALL
H2O + NADP+ + ADP + (P) --> O2 (waste) + NADPH + ATP + (H+)
Light-Independent rxns/ Dark rxns/ Calvin Cycle –
purpose is to build the sugar
How?
3 CO2s can enter the cycle at a time, aided by attaching to an enzyme called Rubisco
many many steps happen, but just generally understand that ATP energy is used to break and rework
bonds in CO2 and the original hydrogens from the light reactions. The electrons carried by NADPH
and the protons (H+) need to join up to make complete hydrogens.
a half-sugar called G3P (glyceraldehyde - 3-phosphate) pops out with each turn of the cycle
Rubisco recruits the second group of 3 CO2s so that the second half of the sugar can be made.
OVERALL - dark reactions = NADPH + ATP + (H+) + CO2 --> C6H12O6 + ADP + (P) + NADP+
What NADPH looks like (do NOT need to memorize, just an FYI)
Melvin Calvin, 1950 University of California Berkeley