The process of photosynthesis is
the process that occurs in the
chloroplasts of plant cells to
create food for the plant.
It is made up of the lightdependent reactions and the
Calvin cycle (light independent
reactions)
The resulting products of
photosynthesis are glucose,
water, and oxygen
While chloroplasts are not found in every plant cell, they are the organelle
that is required for the process of photosynthesis in plants. What types
of plant cells would you not expect to find chloroplasts in?
Roots that lie beneath the ground level, and woody stems
Talk to your partner about what the two parts
of photosynthesis are known as.
ANS: light dependent; light
independent (or Calvin Cycle)
Talk with your partner about the products of
photosynthesis.
ANS: Glucose; water; oxygen
The chloroplasts,
where
photosynthesis
occurs, are found
within the
mesophyll cells in
leaves.
There are two kinds of mesophyll cells in a typical
leaf. The ones you see in the leaf section above are
packed closely together They are in the palisade
parenchyma region. This is where most photosynthesis
occurs. The other region is called the spongy
parenchyma region. Here the cells aren't so close.
In a typical leaf, the
guard cells are open
during the day and
closed at night. This
can be a problem for
plants on hot dry days,
because too much
water is transpired, or
evaporated, through
There can be from 1 to 50 or more
the stomata. Loss of
chloroplasts in a single mesophyll cell. The water slows down
number varies with the plant species, age, photosynthesis.
and health of the cell.
Some plants close their stoma on hot dry days, but this
also slows photosynthesis, because the cells begin to run
short of carbon dioxide.
Guard cells can also have a few chloroplasts in them.
Describe to your partner the difference between
palisade parenchyma, and spongy parenchyma. What
are these two areas within the leaf known for?
ANS: Palisade are tightly packed, but spongy
have air spaces in between. These
are the areas
within plant leaves where most of the chloroplasts
are found.
What happens to plants when it gets extremely hot
and dry?
ANS: The stomata may close during the day, to
conserve water, thereby slowing transpiration,
however, depriving the plant of CO2 which is needed
for photosynthesis.
The little round flat pillow or pancake shaped things inside
chloroplasts are called thylakoids. A stack of them is called a
granum. Two or more stacks are called grana.
There can be from 2 to around 100 thylakoids in one
granum. The little tube like strands connecting thylakoids
from granum to granum are called stroma lamellae
The chlorphylls and other pigments that start the
process of photosynthesis are here, on the outer layer
of the thylakoids. Photons from sunlight hit the
pigments, electrons are "knocked" loose, and off they go
to energize the complicated process of photosynthesis.
The membrane and the
space inside it is where the
light or light-dependent
reaction takes place.
The so-called dark, or light
independent reactions, take
place in the stroma lamellae
Where are the pigments needed for
photosynthesis embedded within the
chloroplast?
ANS: The outer layer of the thylakoids,
or the thylakoid membranes.
Where does the light-dependent reaction take
place?
ANS: thylakoid membranes
Where does the light-independent reaction
take place?
ANS: stroma lamellae
Photosynthesis begins with the light-absorbing pigments in
plant cells, and, of course, photons of light (light particles)
There are many pigments to increase
potential energy absorption from
the entire bandwidth of light within
the spectrum. Each pigment absorbs
a different wavelength. They act
together to optimize energy
absorption.
• Chlorophyll A, B, and C
• Carotenoids (carotene, xanthophyll)
• Anthocyanin
When light is absorbed into one of these pigments, the energy
from the light is incorporated into electrons within the atoms
that make up the pigment molecule.
These excited electrons are highly unstable, and immediately
re-emit the absorbed energy, which is absorbed by other
electrons in nearby pigments…and that happens a bunch. The
process ends when the energy is finally absorbed by one of
two special chlorophyll “A” molecules.
• P680
680 nanometers
• P700
700 nanometers
These numbers represent
the wavelengths (in
nanometers) at which they
absorb their maximum
amounts of light.
Chlorophyll P700 forms a
pigment cluster called
photosystem I (PS I)
Chlorophyll P680 forms photosystem II (PS II)
Of what possible evolutionary advantage would
plants having multiple pigments be?
ANS: It provides a larger range of
possible energy-absorbing molecules,
increasing the efficiency of
photosynthesis.
What are the two “special pigment” clusters of
chlorophyll A known as?
P680: Photosystem II
P700: Photosystem I
Photophosphorylation is the process of making ATP from ADP and Pi
(inorganic phosphate)… (Phosphorylation) using energy derived from light
(photo). Noncyclic photophosphorylation begins with PS II and follows the
steps:
• Photosystem II: Electrons trapped by P680 in photosystem II are
energized by light. Two electrons move up, signifying an increase in their
energy.
• Two energized electrons
are passed to a molecule
called a primary electron
acceptor. (plastoquinone PQ)
• Electrons are then passed
through an electron
transport chain. Electrons
are passed from one carrier
protein to the next. Possible
proteins include:
• ferredoxin
• cytochrome b
Also
contain
iron
Noncyclic Photophosphorylation begins with photosystem II.
What is the maximum wavelength of light that begins this
process?
ANS: Photosystem II is P680 , so 680 nm is the
wavelength
From PS II, two electrons move up. Where do they go?
ANS: The primary electron acceptor, called
plastoquinone (PQ)
From the primary electron acceptor, what happens next?
ANS: The excited electrons move down an electrontransport chain from one carrier protein to the next.
These carrier proteins may include cytochrome b, and
ferrodoxin
• As two electrons move
“down” the electron
transport chain, they lose
energy.
Phosphorylation
• Energy lost is used to
phosphorylate, on average,
about 1.5 ATP molecules.
• ATP
• ADP
• AMP
• Photosystem I comes into play. The electron transport chain terminates
with PS I (P700). Here the electrons again become energized by sunlight,
passing to yet another electron acceptor. (different from PS II)
The energy lost in the electron transport chain
as the two electrons move down, is used to
phosphorylate how many ATP molecules?
ANS: 1.5
The electron transport chain terminates
where?
ANS: PS I; P700
Describe what happens next.
ANS: Light energy re-energizes PS I (at a
different wavelength than PS II, and passes two
electrons on to yet another electron acceptor, which
is also different than the plastoquinone.
• Two electrons pass
through a short electron
transport chain. At the end
of the chain, the two
electrons combine with
NADP+ and H+ to form
NADPH. (nicotinamide
adenine dinucleotide
phosphate)
• NADPH is a coenzyme.
Since the electrons still
have a considerable amount
of energy left, NADPH is an
energy-rich molecule.
• Photolysis occurs. Literally translated, this means
decomposition by light. Throughout the process, two electrons
have been lost. The loss of these two electrons is replaced
when H2O is split into two electrons, 2 H+ and ½ O2. The two
electrons from the H2O replace the lost electrons from PS II.
One of the H+ provides the H in NADPH.
What energy rich molecule is built in the
second electron transport chain during
noncyclic photophosphorylation?
ANS: NADPH
Describe the last process to occur in noncyclic
photophosphorylation.
ANS: Photolysis…the decomposition of
water using light. Water molecule splits, giving
off 2 electrons (which re-join photosystem
II); 2 H+ (which join with NADP to form
NADPH) and ½ O2
Photophosphorylation takes the energy in light
and the electrons in H2O to make the energyrich molecules ATP and NADPH. Because the
reactions require light, they are often called
the
light-dependent
following
What’s
left over as a reactions.
by-productThe
after
equation
informally summarizes the process:
photolysis?
ANS: Oxygen!
H2O + ADP + Pi + NADP+ + light
Reactants
ATP + NADPH + O2 + H+
Products
What two important molecules are generated
in noncyclic photophosphorylation?
ANS: ATP and NADPH
Why is water important to photosynthesis?
It is the electrons donated from the
water molecule that replenish the lost
electrons from the pigment molecules in
PS II. Also, the “H+” is donated from
water to help make the NADP energy
molecule even more energized.
In cyclic
photophosphorylation,
the energized electrons
in PS I are recycled.
They join with protein
carriers and generate
ATP as they move along
the electron transport
chain, instead of
becoming incorporated
into NADPH.
These electrons return
to PS I, so there is NO
photolysis.
There, they can be energized again to participate in cyclic, or noncyclic
photophosphorylation. CP is considered a primitive form of photosynthesis,
but occurs simultaneously with noncyclic photophosphorylation.
Describe one way that Cyclic
photophosphorylation differes from NonCyclic photophosphorylation.
ANS: Produces only ATP, not NADPH; no
photolysis; more primitive and employed
by simple plants, Protists, and
photoautotrophic bacteria.
Where in the chloroplast exactly do these
“light-dependent” reactions occur?
ANS: Thylakoid membranes and spaces
The Calvin-Benson Cycle “fixes” CO2, or it takes the molecule
and incorporates it into an organic molecule that can be used in
biological systems. The biosynthetic pathway involves over a
dozen products, but the basic function of the cycle is to
produce a single molecule of glucose (C6H12O6). In order for
this to happen, the C-B cycle must repeat six times, and use 6
CO2 molecules.
So, while ATP and NADPH are the resulting
energy molecules from cyclic (and non-cyclic)
photophosphorylation, what molecule is the
result of the Calvin-Benson Cycle?
ANS: Glucose (C6H12O6)
In order to make glucose, what molecule must
be taken in through the stomata of the plant
leaf?
ANS: CO2
1. Carboxylation: 6 CO2
combine with 6 RuBP
(ribulose bisphosphate) to
produce 12 PGA
(phosphoglycerate).
The enzyme RuBP
carboxylase, or rubisco,
catalyzes the merging of
CO2 and RuBP.
The Calvin Cycle is known as
C3 photosynthesis because
the first product formed,
PGA, contains three carbon
atoms.
2. Reduction: 12 ATP and 12 NADPH are used to convert 12 PGA to 12
PGAL (glyceride 3-phosphate), thus making PGAL a very energy-rich
molecule. ADP, Pi, and NADP+ are released and then re-energized in
noncyclic photophosphorylation.
3. Regeneration: 6 ATP are used to convert 10 PGAL to 6 RuBP. (This
regenerates the 6 original RuBP used to combine with 6CO2, allowing the
cycle to repeat)
The Calvin-Benson Cycle is a form of
photosynthesis called C-3. Why?
ANS: Because the resulting PGA molecule
from carboxylation (the first product formed)
contains 3 carbon atoms.
Where might the ATPs used in the CalvinBenson Cycle have come from?
ANS: Cyclic and noncyclic photophosphorylation
After reduction, what happens to the NADP,
Pi, and ADP?
ANS: It goes off to be re-energized in
noncyclic photophosphorylation
4. Carbohydrate Synthesis:
Note that 12 PGAL were
created in step 2, but only 10
were used in regeration. The
other 2 are used to build
carbohydrates (glucose), an
energy storing molecule.
No light is directly used in the Calvin-Benson cycle! Because
there is no light directly involved, it is often referred to as
the light-independent reaction, or even the dark reaction
…be careful though…the energy-rich molecules of ATP and
NADPH are created only during photophosphorylation, which
can only occur in light!
What energy rich molecule is the result of
carbohydrate synthesis during the CalvinBenson Cycle?
ANS: Glucose (C6H12O6)
What are the four stages of the Calvin-Benson
Cycle?
ANS: Carboxylation; Reduction;
Regeneration; Carbohydrate Synthesis
In summary, the Calvin-Benson Cycle takes CO2
from the atmosphere and the energy in ATP
and NADPH to create a glucose molecule. Of
course, the energy in ATP and NADPH
represents energy from the sun captured
during photophosphorylation. The CalvinBenson cycle can be informally summarized as
follows:
6CO2 + 18 ATP + 12 NADPH + H+
18 ADP + 18 Pi + 12 NADP+ +1 Glucose
Remember, the biosynthesis of these molecules does not occur
spontaneously. Every product formed in every reaction is catalyzed by an
enzyme. It is for this reason that we say the Calvin Cycle is an enzymatic
cycle, while cyclic and noncyclic photophosphorylation are electric in nature
and work due to charge differential.
Why is the Calvin-Benson Cycle considered an
enzymatic reaction, while Cyclic and Non-cyclic
Photophosphorylation are electric?
ANS: C-B uses enzymes to build energy
rich molecules, while C-P and N-C-P use
charge differential to create energy-rich
molecules. (remember, flow of electrons is
electricity!)
Where EXACTLY in the chloroplast does the
“light-independent” reaction occur?
ANS: Stroma lamellae