Photosynthesis
Photosynthesis is the process of converting energy from sunlight to energy
in chemical bonds.
The general equation describing photosynthesis is:
light + 6 H2O + 6 CO2
C6H12O6 + 6 O2
C6H12O6 is glucose. Sometimes you will see CH2O or (CH2O)n. These
are general formulas for glucose or any carbohydrate.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
The process of photosynthesis begins with light-absorbing pigments in
plant cells. A pigment molecule is able to absorb the energy from light
only within a narrow range of wavelengths. In order to absorb as much
of the entire bandwidth from sunlight as possible, different pigments,
capable of absorbing different wavelengths, act together to optimize
energy absorption
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
These pigments include the green chlorophyll a and chlorophyll b and the
carotenoids, which are red, orange, or yellow. When the light is absorbed
into one of these pigments, the energy from the light is incorporated into
electrons within the atoms that make up the molecule.
These energized electrons (or “excited” electrons) are unstable and
almost immediately re-emit the absorbed energy. The energy is then
reabsorbed by electrons of a nearby pigment molecule.
The process of energy absorption, followed by re-emission of energy,
continues, with the energy bouncing from one pigment molecule to
another. The process ends when the energy is absorbed by one of two
special chlorophyll a molecules, P680 and P700
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
Chlorophyll a
molecules (2)
P680
or
P700
These two chlorophyll molecules, named with numbers that represent
the wavelengths at which they absorb their maximum amounts of light
(680 and 700 nanometers), are different from other chlorophyll molecules
because of their association with various nearby pigments.
Together with these other pigments, chlorophyll P700 forms a
pigment cluster called photosystem I (PS I). Chlorophyll P680
forms photosystem II (PS II).
• During the reactions, there are two possible routes for
electron flow:
• Cyclic Photophosphorylation
• Noncyclic Photophosphorylation
and
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
Light Reactions
A) Noncyclic Photophosphorylation
Photophosphorylation is the process of making ATP from ADP and Pi
(phosphorylation) using energy derived from light (photo). Noncyclic
photophosphorylation begins with PS II and follows the steps:
1. Photosystem II.
Electrons trapped by P680 in
photosystem II are energized
by light. Two electrons are
shown “moving” up, signifying
an increase in their energy.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
2. Primary electron acceptor.
Two energized electrons are passed to a molecule called the primary
electron acceptor. This electron acceptor is called “primary” because it
is the first in a chain of electron acceptors.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
3. Electron transport chain.
Electrons pass through an electron transport chain. This chain
consists of proteins that pass electrons from one carrier protein to the
next. Some carrier proteins, like ferredoxin and cytochrome,
include nonprotein parts containing iron.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
• 4. Phosphorylation.
As the two electrons move “down” the electron transport
chain, they lose energy. The energy lost by the electrons as
they pass along the electron transport chain is used to
phosphorylate, on average, about 1.5 ATP molecules.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
5. Photosystem I.
The electron transport chain terminates with PS I (with P700). Here the
electrons are again energized by sunlight and passed to a primary
electron acceptor (different from the one associated with PS II).
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
6. NADPH.
The 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. NADPH is a coenzyme. Since the electrons have a
considerable amount of energy left, NADPH is an energy-rich molecule.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
7. Photolysis.
The two electrons that originated in PS II are now incorporated into
NADPH. The loss of these two electrons from PS II is replaced when
H2O is split into two electrons, 2 H+ and 1⁄2 O2. The process is called
photolysis and literally means decomposition (lysis) by light (photo). A
manganese-containing protein complex catalyzes the reaction. The two
electrons from H2O replace the lost electrons from PS II. One of the H+
provides the H in NADPH.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
In summary, photophosphorylation takes the energy in light and the
electrons in H2O to make the energy-rich molecules ATP and NADPH.
Because the reactions require light, they are often called the lightdependent reactions or, simply, light reactions.
The following equation informally summarizes the process:
+
H2O + ADP + Pi + NADP+ + light ATP + NADPH + O2 + H
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
B) Cyclic Photophosphorylation
A second photophosphorylation
sequence occurs when the electrons
energized in PS I are “recycled.”
In this sequence, energized electrons
from PS I join with protein carriers
and generate ATP as they pass
along the electron transport chain. In
contrast to noncyclic
photophosphorylation where
electrons become incorporated into
NADPH, electrons in cyclic
photophosphorylation return to PS I.
Here they can be energized again to
participate in cyclic or noncyclic
photophosphorylation.
Cyclic photophosphorylation is
considered a primitive form of
photosynthesis but occurs
simultaneously with noncyclic
photophosphorylation.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
Calvin-Benson Cycle (Dark Reaction)
The Calvin-Benson cycle “fixes” CO2. That is, it takes chemically unreactive,
inorganic CO2 and incorporates it into an organic molecule that can be used
in biological systems. The biosynthetic pathway involves over a dozen
products. The function of the pathway is to produce a single molecule of
glucose (C6H12O6). In order to accomplish this, the Calvin-Benson cycle must
repeat six times, and use 6 CO2 molecules. Thus, all the molecules involved
have been multiplied by 6. Only the most important molecules are discussed.
C3
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
1. Carboxylation:
6 CO2 combine with 6 RuBP to produce 12 PGA.
The enzyme RuBP carboxylase, or rubisco, catalyzes the merging of CO2
and RuBP (ribulose bisphosphate). The Calvin-Benson cycle is referred to
as C3 photosynthesis because the first product formed, PGA
(phosphoglycerate), contains three carbon atoms. Other names are the
Calvin cycle and the carbon reduction cycle.
C3
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
2. Reduction:
12 ATP and 12 NADPH are used to convert 12 PGA to 12 PGAL.
The energy in the ATP and NADPH molecules is incorporated into 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.
C3
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
3. Regeneration:
6 ATP are used to convert 10 PGAL to 6 RuBP.
Regenerating the 6 RuBP originally used to combine with 6 CO2 allows the
cycle to repeat.
C3
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
4. Carbohydrate synthesis.
Note that 12 PGAL were created in step 2, but only 10 were used in step 3.
What happened to the remaining two?
These two remaining PGAL are used to build glucose, a common energystoring molecule. Other monosaccharides like fructose and maltose can
also be formed. In addition, glucose molecules can be combined to form
disaccharides like sucrose and polysaccharides like starch and cellulose.
C3
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
You should recognize that no light is directly used in the CalvinBenson cycle. Thus, these reactions are often called the lightindependent reactions or even the dark reactions. But be
careful—the all process (Photosynthesis) cannot occur in the
dark. This is because it is dependent upon the energy from ATP
and NADPH, and these two energy-rich molecules can be
created only during photophosphorylation, which can occur only
in light.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
• 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 Calvin-Benson cycle can be informally
summarized as follows:
6CO2
+
18ATP + 12NADPH + H
18ADP + 18Pi + 12NADP + 1 glucose
• Keep in mind that the reactions that occur during photosynthesis (and in
any biosynthetic pathway) do not occur spontaneously. Every product
formed in every reaction is catalyzed by an enzyme. In some reactions,
coenzymes or metal-ion cofactors may also be involved.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
Chloroplasts
Chloroplasts are the sites where both the light-dependent and lightindependent reactions of photosynthesis occur. Chloroplasts consist of an
outside phospholipid bilayer membrane enclosing a fluid called the stroma
(Figure 3-2). Suspended within the stroma are stacks of pancakelike
membranes. Individual membrane layers (the “pancakes”) are thylakoids; an
entire stack of thylakoids is a granum (plural, grana). Within the thylakoids
are the light-absorbing pigments and enzymes for the light-dependent
reactions. In the surrounding stroma are the enzymes for the Calvin-Benson
cycle. Thus, the light reactions occur on the thylakoid membranes, and the
dark reactions occur in the stroma.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
Chemiosmotic Theory
Chemiosmotic theory describes the mechanism by which ADP is phosphorylated to ATP. The process illustrated in the figure listed below.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
1. H+ ions (protons) accumulate inside thylakoids.
The light reactions of photosynthesis occur in the membranes of the
thylakoids. As photolysis occurs, H+ are created and released to the inside of
the thylakoids (the oxygen is released to the outside). Also, H+ accompany
the electrons as they pass along the electron transport chain between PS II
and PS I. These H+ come from the stroma (outside the thylakoids) and are
released to the inside of the thylakoids.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
2. A pH and electrical gradient across the thylakoid membrane is
created.
As H+ accumulate inside the thylakoid, the pH decreases. Since some of
these H+ come from outside the thylakoids (from the stroma), the H+
concentration decreases in the stroma and its pH increases. This creates a
pH gradient consisting of differences in the concentration of H+ across the
thylakoid membrane from a stroma pH 8 to a thylakoid pH 5 (a factor of
1000). Since H+ are positively charged, their accumulation on the inside of
the thylakoid creates an electric gradient (or voltage) as well.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
3. ATP synthases generate ATP.
The pH and electrical gradient represent potential energy like water behind
a dam. Similar to a dam, channel proteins, called ATP synthases, allow
the H+ to flow through the thylakoid membrane and out to the stroma. The
energy generated by the passage of the H+ (like the water through turbines
in a dam) provides the energy for the ATP synthases to phosphorylate
ADP to ATP. The passage of about three H+ is required to generate one
ATP.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
Photorespiration
Because of its critical function in catalyzing the fixation of CO2 in all
photosynthesizing plants, rubisco is the most common protein on earth.
However, it is not a particularly efficient molecule. In addition to its CO2fixing capabilities, it is also able to fix oxygen.
The biosynthetic pathway that leads to the fixation of oxygen is called
photorespiration.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
Photorespiration leads to two problems.:
• The first problem is that the CO2-fixing efficiency is
reduced because, instead of fixing only CO2, rubisco fixes
some O2 as well.
• The second problem is that the products formed when O2
is combined with RuBP do not lead to the production of
useful, energy-rich molecules like glucose. Instead,
specialized cellular organelles, the peroxisomes, are found
near chloroplasts, where they function to break down
photorespiration products. Thus, considerable effort is
made by plants to rid the cell of the products of
photorespiration.
• Since the early atmosphere in which primitive plants
originated contained very little oxygen, it is hypothesized
that the early evolution of rubisco was not influenced by its
O2-fixing handicap.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
C4 Photosynthesis
Improving upon photosynthetic
efficiency, some plants have evolved a
special “add-on” feature to C3
photosynthesis.
When CO2 enters the leaf, it is
absorbed by the usual
photosynthesizing cells, the mesophyll
cells. Instead of being fixed by rubisco
into PGA, the CO2 combines with PEP
(phosphoenolpyruvate) to form OAA
(oxaloacetate or oxaloacetic acid).
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
C4 Photosynthesis pathway
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
bundle sheath
mesophyll
The fixing enzyme is PEP carboxylase. OAA, the first product of this
pathway, has 4 carbon atoms, thus the name C4 photosynthesis. OAA
is then converted to malate, and the malate is shuttled to specialized
cells within the leaf, the bundle sheath cells.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
Here malate is converted to pyruvate and CO2. The pyruvate is
then shuttled back to the mesophyll cells where one ATP (broken
down to AMP, instead of ADP) is required to convert the pyruvate
back to PEP. Then the process repeats. The overall effect is to
move CO2 from mesophyll cells to the bundle sheath cells.
When malate delivers CO2 to a bundle sheath cell, rubisco begins the
Calvin-Benson cycle (C3 photosynthesis). Because little oxygen is
present, rubisco can fix CO2 without competition from O2. Thus, little
hotorespiration takes place, and photosynthesis is more efficient.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
• The purpose for moving CO2 to bundle
sheath cells is to increase the efficiency of
photosynthesis.
• The bundle sheath cells surround the leaf
veins and are themselves surrounded by
densely packed mesophyll cells. Since
bundle sheath cells rarely make contact with
an intercellular space, very little oxygen
reaches them.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
In order for photosynthesis to occur, the stomata must be open to allow CO2
to enter. However, when the stomata are open, H2O can escape. The higher
rate of photosynthesis among C4 plants allows them to reduce the time that
the stomata are open, thereby, reducing H2O loss. Thus, C4 plants are
found in hot, dry climates, where they possess an adaptive advantage over
C3 plants.
This advantage apparently compensates for the additional energy
requirement (1 ATP to AMP) for C4 photosynthesis.
C4 photosynthesis occurs in about a dozen plant families. Sugarcane and
crab grass are two examples.
Dr. Abboud ElKichaoui
crab grass
Sugarcane
Islamic University- Biology and
Biotechnology Department.
General Biology
CAM Photosynthesis
Another “add-on” feature to C3 photosynthesis is crassulacean acid
metabolism (CAM). The physiology of this pathway is almost identical to
C4 photosynthesis, with the changes that follow:
1. PEP carboxylase still fixes CO2 to OAA, as in C4. Instead of malate,
however, OAA is converted to malic acid. (This is a minor difference,
since malate is merely the ionized form of malic acid).
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
• 2. Malic acid is shuttled to the vacuole of the cell (not moved out of
the cell to bundle sheath cells as in regular C4).
• 3. Stomata are open at night. During the night, PEP carboxylase is
active and malic acid accumulates in the cell’s vacuole.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
• 4. Stomata are closed during the day (the reverse of other plants).
At this time, malic acid is shuttled out of the vacuole and
converted back to OAA (requiring 1 ATP to ADP), releasing CO2.
•
The CO2 is now fixed by rubisco, and the Calvin-Benson cycle
proceeds.
• The advantage of CAM is that photosynthesis can proceed during
the day while the stomata are closed, greatly reducing H2O loss.
• As a result, CAM provides an adaptation for plants that grow in
hot, dry environments with cool nights (such as deserts).
• The name crassulacean acid metabolism
comes from the early discovery of CAM in
the succulent plants of the family
Crassulaceae and the discovery of the
accumulation of malic acid in vacuoles
during the night.
•In addition to the Crassulaceae, CAM is
found among plants in over a dozen
different families, including cacti.
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology
Dr. Abboud ElKichaoui
Islamic University- Biology and
Biotechnology Department.
General Biology