AND LIGHT PHOTOSYNTHESIS
PART 2 - PHOTOSYNTHESIS
Now that you have been exposed to the principles of light and that you know how
chlorophyll traps that light, its time to discuss the actual process of photosynthesis.
First, you need to know scientists dont know everything about photosynthesis. What
scientists do know is amazing. For our purposes, we will only brush the topic. However,
you will probably feel you have been overexposed to the process after finishing this
lecture. Just keep your eye on the concept this is the most important biochemical process
on earth and everyone needs to understand a little about it.
Autotrophic vs Heterotrophic
Scientists often categorize organisms into their forms of energy sources. Organisms that
convert carbon dioxide (an inorganic compound) into organic compounds (such as
glucose) utilizing some energy source (in our case, the sun) are called autotrophic.
Organisms that depend on other organisms for their source of organic compounds (such
as food) are called heterotrophic. Plants and some bacteria are autotrophic through the
process of photosynthesis.
Photosynthesis
There are two basic sets of reactions in photosynthesis: light dependent and light
independent. Like their name indicates, light dependent reactions require sunlight to take
place. The light independent reactions may take place in daylight or in darkness.
The light dependent reactions have two subsets of reactions: Photosystem I and
Photosystem II. The light independent reactions have one of three possible tracks:
Calvin-Benson Cycle, C-4 pathway, or the Crassulacean Acid Metabolism pathway
(CAM photosynthesis).
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LIGHT AND PHOTOSYNTHESIS
PART 2 - PHOTOSYNTHESIS
Light Dependent Reactions
Photosystem II
The light dependent reactions begin with Photosystem II (not Photosystem I).
Photosystem II is actually a cluster of chlorophyll molecules. These molecules of
chlorophyll trap sunlight in the 680 nanometer range of the visible spectrum. The cluster
of molecules is sometimes referred to as the P680 site or receptor site. As these
molecules trap sunlight, the energy is transferred to a special chlorophyll molecule called
the acceptor molecule within the site. The massive input of energy from the other
chlorophyll molecules causes two electrons to not only be bounced to a higher energy
level, but to be totally bounced out of the orbit of the molecule. These two electrons are
then trapped by another molecule within the Photosystem called Quinone.
quinone
2e-
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Quinone becomes reduced (gains two electrons) and the acceptor molecule becomes
oxidized (loses two electrons). Remember oxidation-reduction reactions go hand in
hand. As one material is oxidized, another must be reduced, in turn. Quinone passes
these highly energized electrons to cytochrome b3, one of the many cytochrome pigments
associated with photosynthesis and respiration. Cytochrome b3 becomes reduced and
Quinone is oxidized. Cytochrome b3 passes the electrons to Plastoquinone which
becomes reduced as Cytochrome b3 becomes oxidized. Plastoquinone passes the
electrons to Cytochrome f which becomes reduced. Cytochrome f finally passes the
electrons to Plastocyanin which becomes reduced.
quinone
PHOTOSYSTEM II
Cyto b3
Plastoquinone
Cyto f
2e-
Plastocyanin
sunlight
Receptor molecule
Acceptor site or P680 site
Photolysis
While these electrons are being passed from molecule to molecule, back at the
aggregation site, water is split into hydrogen atoms and the oxide ion. This splitting of
water with light is called photolysis. The hydrogen ions are used to eventually form
ATP.
quinone
Cyto b3
Plastoquinone
Cyto f
2e-
Plastocyanin
sunlight
ATP
H2O
2H+ + O
photolysis
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LIGHT AND PHOTOSYNTHESIS
PART 2 - PHOTOSYNTHESIS
Photosystem I
Photosystem I is another aggregation of chlorophyll molecules. These trap light in the
700 nanometer range. The acceptor molecule of the P700 site accepts the electrons from
Plastocyanin of Photosystem II. Sunlight energizes the P700 site and that energy is
channeled into the acceptor molecule. The two electrons (from Photosystem II) are then
shot out of their orbit to a molecule of Ferrodoxin bound which becomes reduced.
Ferrodoxin bound passes the electrons to Ferrodoxin free, (again one material is oxidized,
another reduced) and Ferrodoxin free passes them to Flavoprotein. Flavoprotein passes
the electrons to Nicotinamide Adenine Dinucleotide Phosphate (NADP) which is
reduced.
FDbound
PHOTOSYSTEM I
FDfree
FP
2e-
NADP+
700nm + electrons from Photosystem II
Path of Electrons
The electrons can, indeed, pass down the electron chain from Ferrodoxin bound to
NADP. However, the electrons can just as easily lose their energy and fall back to the
acceptor molecule. That energy that is lost may be converted to ATP. This process of
elevating electrons and their falling back to their original position may occur over and
over thus a cycle.
FDbound
FDfree
2e-
FP
700nm
CYCLIC PHOTOPHOSPHORYLATION
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PART 2 - PHOTOSYNTHESIS
Cyclic Photophosphorylation
The process of forming ATP by the above process in Photosystem I is called cyclic
photophosphorylation. Photo because it occurs in the presence of light.
Phosphorylation because adding a phosphate group to a compound is called
phosphorylation.
quinone
FDbound
Cyto b3
FDfree
Plastoquinone
2e
-
FP
Cyto f
2e-
700nm
Plastocyanin
sunlight
NADP+
ATP
H2O
2H+ + O
Whats Been Done
In reality, only two things have been accomplished in Photosystems II and I. ATP has
been produced (photolysis in Photosystem II and cyclic photophosphorylation in
Photosystem I). Also, NADP has been reduced.
Refer to the summary equation of photosynthesis.
sunlight
6CO2 + 12 H2O ------------------------> C6H12O6 + 6 O2 + 6 H2O
chlorophyll
glucose
The only issues addressed in the above reaction by Photosystem II and Photosystem I are
shown in red. Water was split during photolysis and oxygen generated in photolysis. No
carbon dioxide was used and no glucose was made. This occurs during the light
independent reactions of photosynthesis.
Why Cover Photosystem II First?
It was a little strange to cover Photosystem II first. The reason is Photosystem I was the
first Photosystem to evolve. Primitive plants have only Photosystem I. Higher plants on
the evolutionary scale evolved Photosystem II to boost the production in Photosystem I.
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Hydrogen to ATP
How does the chloroplast make ATP from hydrogen ions? The process is called the
chemiosmotic theory. Remember that chlorophyll is found in the thylakoids of the
chloroplasts. The thylakoids, like most membranes in cells, are composed of a
phospholipids bilayer, just like the cell membrane with proteins scattered in and about.
The materials scattered in and about not only include proteins, but may also include
Photosystem I and Photosystem II receptor sites with other pigments and molecules, as
previously discussed.
A part of the thylakoids bilayer of lipids includes the P680 site of chlorophyll molecules.
Next to that is the molecule Quinone, followed by cytochrome b3, Plastoquinone,
cytochrome f, and Plastocyanin. Next to this line of molecules is the P700 site of
chlorophyll molecules followed by Ferrodoxin bound, ferrodoxin free, flavoprotein, and
NADP. All of this is repeated over and over in the lipid bilayer of the thylakoids. In
addition, there is another molecule (a protein called ATP synthetase) found as a channel
between the lipid bilayer.
P680
Q
b3
PQ
f
PC
2H+
2H+
P700
2H+
2H+
FDb
2H+
FDf
FP
NAD+
ATPase
ATP
2H+
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LIGHT AND PHOTOSYNTHESIS
PART 2 - PHOTOSYNTHESIS
In Photosystem II, as water is split to form hydrogen ions, the hydrogen ions build up in
the lumen of the thylakoids. Soon there is a concentration gradient between hydrogen
ions inside the thylakoids and the outside of the thylakoids. Osmosis occurs with the
hydrogen ions passing through the molecule of ATP synthetase on their way to the
outside of the thylakoids. This pumping mechanism, coupled with ATP synthetase,
forms the ATP. Therefore, the pumping of hydrogen ions across a thylakoids membrane
forms ATP and is the process is called the chemiosmotic theory.
Light Independent Reactions
To finish off the equation of photosynthesis the production of glucose we must study
the light independent reactions. There are three possible paths for plants, depending upon
the species of plants:
1. Calvin-Benson Cycle temperate plants
2. C-4 Pathway tropical plants
3. CAM Photosynthesis (Crassulacean Acid Metabolism) desert plants
Calvin-Benson Cycle
The Calvin-Benson Cycle begins when carbon
dioxide (a single carbon compound) combines with
a five carbon compound called ribulose biphosphate
carboyxlase (sometimes written as diphosphate).
This results in the formation of a six carbon
intermediate. An intermediate is simply a shortlived compound. This reaction catalyzed is by an
enzyme called ribulose biphosphate carboyxlase.
(Most enzymes end in ase.)
Naming Enyzmes
Enzymes are often named for what
they do. In the case with ribulose
biphosphate carboyxlase, the
enzyme takes ribulose biphosphate
and adds carbon dioxide to it.
Remember that adding a phosphate
is called phosphorylation. Adding
a carbon dioxide is called
carboxylation, thus the name
ribulose biphosphate carboyxlase.
ribulose biphosphate carboxylase
CO2 + ribulose biphosphate
C-C-C-C-C-C
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The six carbon intermediate breaks apart to form phosphogylceric acid (PGA). PGA, a 3
carbon compound, combines with NADPreduced and ATP to form phosphoglyceraldehyde
(PGAL) another 3 carbon compound. Some of the PGAL goes to form glucose (6
carbons) and some other goes to form additional ribulose biphosphate (5 carbons). The
ribulose biphosphate combines with another carbon dioxide and the cycle starts again.
ribulose biphosphate carboxylase
CO2 + ribulose biphosphate C-C-C-C-C-C
1C
5C
6C
NADH2
NAD+
C6H12O6
PGAL
3C
ADP
PGA
3C
ATP
You Do the Math
At first, from the above description of events, it seems a 3 carbon compound (PGAL) is
forming a 6 carbon compound (glucose) and a five carbon compound (ribulose
biphosphate). Nice trick if you can do it. Of course, you cannot. However, remember
the summary equation for photosynthesis called for 6 carbon dioxides. That would
combine with 6 ribulose biphosphates (and 6 enzymes) to for 6 six carbon intermediates
(36 carbons). That would split to form 12 PGAs and that forms 12 PGALs (36
carbons). Six of those carbons form glucose and the other 30 form 6 ribulose
biphosphates.
Corn Is A Tropical Plant
Calvin-Benson and C-4 Pathway
Strangely, corn is a
The Calvin-Benson cycle occurs in temperate plants. These are
tropical plant. At first,
plants found in the temperate zones of the world. Most of the
you think, corn is grown
continental United States falls within a temperate zone where the
in Iowa and Iowa is
winters are cold, but not too cold and the summers are hot, but
anything but tropical.
not too hot thus temperate. South Florida exhibits subtropical
Youre right on that
climate and Alaska exhibits some arctic climates. C-4 plants are
account, but the ancestor
found primarily in the tropics. The plants include palms, sugar
to corn is a plant called
cane (a grass), most grasses, bananas, and others. Whats the
Teosinte, found in Central
difference? It mostly has to do with leaf anatomy.
America and Mexico, thus
tropical.
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Structure of A Leaf
A leaf is composed of three basic parts. The part most people
recognize is called the blade or lamina. What attaches the leaf to
the stem is called a petiole. At the base of the petiole may be a
couple of leaf-like structures called stipules. If a leaf has all
three parts, it is said to be complete. Some leaves are incomplete
they are missing one or more parts. For example, some leaves
do not have stipules. Other leaves may not have petioles the
blade is attached directly to the stem as in grass. This condition
is referred to as a sessile leaf. Still other leaves may not have
blades. The cactus spines are actually the petiole and midvein
of a leaf without the blade.
Leaf Anatomy of Calvin-Benson Leaf
Think of a leaf and cut that leaf in cross section. Look down into
the leaf and imagine what you would see.
cuticle
upper epidermis
palisade layer
mesophyll
lower epidermis
vascular bundle
guard cell
Stoma*
*See insert on the right on stomata and their mechanisms.
There is an upper and lower epidermis. The upper epidermis is
often covered in a waxy cuticle. Just beneath the upper
epidermis is a group of columnar cells called the palisade layer
which is has abundant chloroplasts in the cell. Underneath the
palisade layer is a middle layer (mesophyll) that also contains
chloroplasts. The lower epidermis has a variation from the upper
epidermis. There are openings on the undersurface called
stomata. The stomata are formed by guard cells. The guard cells
also contain chloroplasts, although epidermal cells do not. The
vascular bundle is composed primarily of two types of tissue:
xylem and phloem. Xylem (in red) is tissue that is specialized
for the conduction of water (typically upward from the roots) and
phloem (in blue) is specialized tissue for the conduction of food
(typically downward from the leaves to the rest of the plant).
Stomata
The word stoma meaning
opening. The plural is stomata.
Higher plants typically have
stomata on the lower surfaces of
their leaves (although you may
also find them on upper
surfaces, unless it is an aquatic
plant with floating leaves).
These openings allow the
passage of three gases in and
out of the leaf: carbon dioxide,
oxygen, and water vapor.
The openings are formed by
cells called guard cells. Guard
cells have a large, single
chloroplast in each cell. Based
on the condition of the
environment around the guard
cell, the guard cells either swell
or shrink based on hypertonic or
hypotonic environments.
The question is: do guard cells
open or close in a hypertonic
environment? The answer is
they close. The guard cells
open (and form the stoma)
when they are exposed to
hypotonic environments. Think
of two long, slender balloons
full of air. Place the balloons
between two solid surfaces so
they line up like two hotdogs
(parallel) and their ends are
blocked by something. Think
of those balloons if you
continue to add air. Since the
ends are blocked, the balloons
have to expand and the only
way they can do it is to bow
outward from the middle, thus
creating a gap between the
balloons. Thats the same way
stomata work.
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Leaf Anatomy of C-4 Leaf
bundle sheath
There are two major differences between a Calvin-Benson leaf and a C-4 leaf. The first
should be obvious in that there is no palisade layer in a C-4 leaf. In addition, the C-4 leaf
has another set of cells surrounding the vascular bundle called the bundle sheath.
C-4 Pathway
In the C-4 pathway, carbon dioxide combines with a three carbon compound called
phosphoenolpyruvate (PEP). This forms a 4-carbon compound called oxaloacetate
(OAA). The reaction to form OAA from PEP and carbon dioxide is catalyzed by
phosphoenolpyruvate carboyxlase (PEP carboyxlase). OAA breaks apart to form two
compounds: malate and aspartate. These two compound diffuse from the mesophyll cells
to the bundle sheath. In the bundle sheath, they recombine to form OAA. OAA, in the
bundle sheath, then splits to form PEP and carbon dioxide. The carbon dioxide then
feeds into the Calvin-Benson cycle in the bundle sheath to make glucose. The PEP
diffuses from the bundle sheath back to the mesophyll to begin again the process of
forming OAA.
PEP carboylase
MESOPHYLL
CO2 + PEP
1C
3C
OAA
4C
malate
BUNDLE SHEATH
Ribulose biphosphate CO2
aspartate
malate
aspartate
OAA
4C
Calvin-Benson
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At first, it seems mother nature has gone a little crazy. Why go through the C-4 pathway
in the mesophyll if all you are going to do is go through Calvin-Benson in the bundle
sheath? Mother nature doesnt do anything capriciously.
Oxygen Inhibition of Ribulose Biphosphate Carboxylase
Review the material on the Calvin-Benson. Ribulose biphosphate carboyxlase, the
enzyme important in Calvin-Benson, is inhibited by high levels of oxygen. In the tropics,
you find plenty of water, plenty of carbon dioxide, and plenty of sunlight for the light
dependent part of photosynthesis which makes oxygen. This oxygen would shut down
the light independent reaction of Calvin Benson and the plant would starve as it produced
oxygen. Instead, in the tropics, plants run the C-4 pathway in the mesophyll. PEP
carboyxlase is not inhibited by high levels of oxygen, so it runs quite nicely in the tropics.
The products of C-4 are then sent to the bundle sheath which has much less surface area
than the rest of the leaf. Oxygen inhibition is not a problem there since oxygen, even
though abundant, cannot get access to the ribulose biphosphate carboyxlase in the bundle
sheath. In essence, the C-4 pathway is a means of getting around oxygen inhibition.
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Crassulacean Acid Metabolism (CAM Photosynthesis)
CAM photosynthesis occurs in a group of plants collectively known as succulents.
Succulents are plants with fleshy stems and/or leaves, such as cacti, Aloe vera, and
stonecrops. These are plants of dry climates or deserts. Their chief problem is water loss
by transpiration through the stomata on the leaves. As a consequence, desert plants do
not open their stomata during the day the time of greatest potential water loss. Instead,
they open them only at night with threat of evaporation and transpiration is less and they
then run a modified Calvin-Benson cycle to produce glucose.
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