Download Chapter IV Plant Photosynthesis

Chapter IV
Plant Photosynthesis
Main Teaching Content: Structure and physicochemical properties of photosynthetic
pigments、The process of photosynthesis、the mechanism of photosynthesis、
photorespiration、the differences of physiological characteristics in C3 and C4 plants、
Factors Affecting Photosynthesis.
priorities and difficulties: the mechanism of photosynthesis、photorespiration、the
differences of physiological characteristics in C3 and C4 plants、Factors (such as light
intensity and CO2 )Affecting Photosynthesis is the emphasis of this chapter. At the
same time, the mechanism of photosynthesis is the difficulty of this chapter.
Instructional Methods: Classroom teaching three hours, experiments three hours.
Teachers use multimedia to explain the process of photosynthesis with flash animation.
Section 1 Significance of Photosynthesis
Photosynthesis is the process that green plants absorb light energy, assimilating
carbon dioxide and water, manufacturing organic substances and the release of
oxygen.
Significance of Photosynthesis:
1. Inorganics turned into organics.
2. Photosynthesis is the conversion of light energy into chemical energy, when green
plants assimilate carbon dioxide, at the same time converting light energy into
chemical energy and storing it in the synthesized organic compounds. The utilized
energy by human, such as coal, natural gas and timber, are all formed through
photosynthesis now or in the past.
3. Maintenance relative balance between atmospheric O2 and CO2.
Section II photosynthetic organelle and photosynthetic pigments
First．Photosynthetic organelle: chloroplast
Second, The types and nature of photosynthetic pigments
Photosynthetic Pigments, that is chloroplast pigments, there are three types: The
chlorophyll, carotenoid and phycobilin. The previous two categories mainly exist in
higher plant and the phycobilin only exists in algae.
(A)The structure and physical properties of photosynthetic pigment
1. Chlorophyll There is mainly two chlorophylls in higher plant, chlorophyll a and
chlorophyll b.
They do not dissolve in water, but are soluble in organic solvents, such as ethanol,
acetone, ether, chloroform. In color, chlorophyll a is a blue-green, while chlorophyll b
is a yellow-green. According to the chemical nature, chlorophyll is a Esters of
chlorophyllin, it can occur saponification. Chlorophyllin is a two-carboxylic acid, one
of the carboxyl was esterified by Methanol, and the other was esterified by the phytol.
In the molecular of chlorophyll, there is a “head"of porphyrin ring and a "tail” of
phytol. Mg+ is in the centre of porphyrin ring and positive charge, while the nitrogen
atoms associated with it are tends to be negative charge, thus porphyrin has polarity
and is hydrophilic,it can conbine with proteins. Phytol is a diterpene, consisting of
four isoprene units, and a fat chain which fat; it determines the liposolubility of
chlorophyll. Chlorophyll does not participate in theprocess of transfering hydrogen or
hydrogen redox, but only in this manner electron transfer (that is, redox arising from
gains and losses electron) and the conjugate transfer (direct energy transfer) to
particapate in energy transfer.
2. The magnesium atom in porphyrin ring could be replaced by H +, Cu2 +, Zn2 +, H
+can easily infiltrate into chloroplast after acid treatment, and replace the magnesium
atom to form chlorophyll without magnesium atom, So that leaves were brown.
Chlorophyll without magnesium atom easy to combine with the copper atom and form
replaced chlorophyll by copper atom.therefore the colour is more stabile than before.
People often use copper acetate on the basis of this principle to treat green plant
specimens for preservation.Carotenoid：Do not dissolve in water but it is soluble in
organic solvents. The carotenoid in Chloroplast contains two pigments, that is
carotene and lutein, the former was orange yellow and the latter was yellow, which
functions is the absorption and transmission of solar energy, the protection of
chlorophyll.Phycobilin is the main photosynthetic pigments of algae, which only
exists in the red algae and cyanobacteria and often combined with the protein for
phycobiliprotein, there are main three types: phycoerythrin, phycocyanin and
allophycocyanin. Their chromophore combine with protein firmly by a covalent bond,
only by strong acid boiled, we can separate them. They are soluble in dilute salt
solution. Four pyrrole rings of Phycobilisomes form the straight chain conjugate
system, which don’t contain magnesium and phytol chain, with the functions of
collection and transmission of solar energy.
(B). The optical properties of photosynthetic pigments
Light is an electromagnetic wave, but it is also a moving particle flow, these
particles called photons or quantum.
1. The absorption spectra of photosynthetic pigments
After the beams through a triple prism, white light (hybrid optical) can be
divided into contious spectra of seven colors of red, orange, yellow, green, green, blue,
purple. If the chloroplast pigment solution is placed between the light source and
prism, we could see the light of some wavelength absorbed and appeare on the dark
band in this spectrum. This is the absorption spectra of chloroplast pigment.
Chlorophyll absorpt less orange light and yellow light, especially the least to the green,
therefore, the solution of chlorophyll was green. The absorption spectras of carotene
and lutein are different with chlorophyll, which maximum absorption bands are in the
blue-violet light district and they don’t absort the long wave light, such as red
light.The maximum absorption spectra of phycocyanin is at the part of orange light,
while phycoerythrin is at the part of green light and yellow light.
2. The fluorescence and phosphorescence phenomenon of photosynthetic pigments
The solution of chlorophyll under transmitted light is green, while under
reflected light is red, which is called the fluorescence phenomenon of chlorophyll.
After the chlorophyll molecules absorb light quantum, it is from the most stable, the
lowest energy state - the ground state rose to the high-energy state of instability excited state. The chlorophyll molecules have two strongest absorption area of red
and blue. If chlorophyll molecules excited by the blue light, electron transite to the
second singlet state with higher energy;while if the light is red light, electron transite
to the singlet state with lower energy. The light emited when electron from the first
singlet state return to the ground state are called on the fluorescence,while electron
from first and third singlet state in the chlorophyll molecules return to the ground state
are called the phosphorescence. The solution of chlorophyll under incident light is
green, while under reflected light is red. In chloroplast or the leaves the emited
fluorescence is very weak and it consumes little energy, normally no more than 5% of
the absorbed energy, because of most of the energy used in photosynthesis, therefore
it’s difficult to be observated by the naked eye.While pigment solution is different
with that, because of the lack of energy receptors or electronic receptors in solution,
so when give light to pigment it will launch a strong fluorescence.
Section III the mechanism and process of photosynthesis
Photosynthesis is the process of energy conversion and organic formation. In this
process first is the absorption of light energy and light energy convert into electrical
energy, and continues to create active chemical energy, finally convert into stable
chemical energy, and store in the carbohydrate.
The process of photosynthesis can be broadly divided into three steps: ① initial
reaction; ②electron transfer (water photolysis, release oxygen) and the photosynthetic
phosphorylation; ③ carbon assimilation process. The First and second steps are basic
light reaction, and the third step is dark reaction.
First, Initial reaction
Initial reaction is the process that the photosynthetic pigment molecules absorpt,
transmit and convert light energy.
According to the function, photosynthetic pigments on thylakoid membrane
could be classfied two categories: (1) reaction center pigments, a small number of
chlorophyll molecule which is in special state belong to such, it has a photochemical
activity, can capture light and converte it into electric energy (called "traps"). (2)
light-harvesting pigments, also known as antenna pigments, it do not have
photochemical activity, can absorb solar energy and transfer the absorpted light
energy to the reaction center pigments, the overwhelming majority of pigment,
including the vast majority part of chlorophyll a and total chlorophyll b, carotene,
lutein, and so belong to this category.
Light－harvesting pigments lie in the photosynthetic membrane pigment-protein
complexes, the reaction center pigment exists in reaction center. However, they are
synergies, absorbed solar energy by a number of light － harvesting pigments
molecules gathered in a reaction center pigment molecules to trigger photochemical
reaction. Generally, absorbed solar energy by about 250 ~ 300 pigment molecules
transmits to a reaction center pigment. The number of pigment molecules needed that
to complete the necessary synergies when absorb and transmit a light quantumre to
the reaction center, known as the photosynthetic units, including two parts of light－
harvesting pigments system and photosynthetic reaction center. Therefore
photosynthetic units can also be defined as: the functional units of smallest structure
that binding on the thylakoid membrane to complete the photochemical reaction.
When the wavelength range of 400 to 700 nm of visible light exposure to green
plants, antenna pigment molecules absorb photons and are excited, a large number of
solar energy is absorbed and transfered to the reaction center molecules through the
antenna pigments,caused photochemical reaction.
Photochemical reactions are carried out in the photosynthetic reaction center. The
reaction center is the most basic pigment-protein complex to curry out initial reaction,
which includes at least a reaction center pigment molecules, that is primary electron
donor, a primary electron acceptor, a secondary electron donor, and to maintain the
micro-environment of these electronic transmission needed proteins necessarily, it can
lead to charge separation and will convert light energy into electric energy. The
primary electron acceptor of reaction centers is the material that directly receives
electron from pigment molecules of the reaction center.The secondary electron donor
of reaction centers, refers to the substances that give electron directly to pigment
molecules of response centers. In light, initial reaction of photosynthesis is curried out
through a continuous manner; therefore, it must have the final electron donor and final
electron acceptor to participate continuously, and form the "source" and "library" of
electron. The final electron donor is water and the final electron acceptor is NADP+
in higher Plants.
Essentially photochemical reaction is caused redox reactions among the reaction
center - pigment molecules and the original electron acceptor and sub donor. After
antenna pigment molecules absorb and transmit light energy to the reaction center, the
reaction center pigment molecules (P) become excited and be excited state (P *),and
release electron to the original electronic receptor (A), at the same time left a "hole
"This has become trap. Reaction center pigment molecules are oxidized and with
positive charge (P+), and the original electronic receptor is reduced and with negative
charges (A-). After then, reaction center occurres charge separation, the pigment
molecules of reaction centers lose electron and can capture electron from the
secondary electron donor (D),Therefore reaction center pigment revert to the original
state,while The secondary electron donor was oxidized (D+). This redox reaction
occurred and completed the process that light energy converted into electrical energy.
Second, Photosynthetic electron transport and phosphorylation
Reaction center pigment excited by light and caused the charge separation, will
convert light energy into electrical energy, and the produced electron is transported
through a series of electron carrier, cause the schizolysis of water to realse oxygen and
NADP+ is reduced, and through photosynthetic phosphorylation to creat ATP, which
convert the Energy into active chemical energy.
1. Optical System
PS I photochemical reaction is the reaction of light waves, and its main feature is
the reduction of NADP+. When PS I reaction centers pigment molecule (P700) was
exicted after the absorption of light energy, the electron is transported to the various
electronic receptors, through Fd (ferredoxin), with the participation of NADP
reductase, NADP+ is reduced to be NADPH. The p in the reaction center pigment of
P700 represent pigment, the 700 refers to the maximum absorption wavelength of
pigment.
PS II photochemical reaction is the reaction short wave, and its main feature is
the photolysis of water and release oxygen. PS II reaction center pigment molecule
(P680) absorb light energy, decompose water to capture the electron to supply PS I.
2. Photosynthetic chain
Photosynthetic chain is the orbit of the electron transportion that is localized on
the photosynthetic membrane and formed by a series of the adjacent components of
electronic transmission. The electron transportion is curried out by the two optical
systems in orderly, which according to the redox potential of high or low priority of
components of electronic transmission to align, so that electron transport chain is like
the written "Z" shape.
The components of electron transport in photosynthetic chain is the
plastoquinone (PQ), complex of cytochrome (Cyt) b6 / f, ferredoxin ( Fd), and
plastocyanin (PC). People give PQ the most attention in all them, because it is the
carrier of two electron and two H+, which can transmit electron and also transfer
proton, at the same time of tranporting electron, taking H+ from outside of the
thylakoid membrane into the membrane, as results of establishing a transmembrane
proton gradient at the exterior and interior of thylakoid membrane to promote ATP
synthesis. In photosynthetic chain PSI, PSI and Cyt b6f is in the thylakoid membrane,
it is difficult to be removed, while PQ, PC and Fd can shift in the membrane or on the
membrane surface, transfering electron among the three.
3. Water photolysis and oxygen relase
Water photolysis is discovered by Hill in 1937, is also known as the Hill reaction.
He added the separated chloroplast into aqueous solution with hydrogen receptor (A),
after giving light to it, water was decomposed and released oxygen.
4. Photophosphorylation
The process that chloroplast under the conditions of illumination synthesizes
ATP with inorganic phosphate (Pi) and ADP known as photo phosphorylation.
Photophosphorylation
divided
into
two
categories:
noncyclic
photophosphorylationand cyclic photophosphorylation.ATP enzyme is also called ATP
synthase, coupling factor. The structure of ATPase lies in chloroplast is similar
towhich lies in mitochondrial membrane, is a kind of bulbs, formed by two-protein
complexes: one is"CF1" complex that is hydrophilic and is prominent at the
membrane surface, and the other is the hydrophobic "CF0" complex, buried under
membrane. The catalytic site of the ATPase lies in CF1, CF1 is combined on CF0.
On the mechanism of photosynthetic phosphorylation may be explained with the
chemical infiltration theory established by British Mitchell (P. Mitchell). In Light, the
PQ transport electron downward, at the same time, taking proton in the extracellular
matrix into thylakoid membrane, this process that PQ in the thylakoid membrane
changes reciprocating in redox known as PQ shuttle. In addition, the decomposition of
water in the inner membrane also results H+ release, caused H+ concentration
increasing intramembrane, so have
inside and outside membrane potential
difference (Δ) and proton concentration difference (Δ pH), together known as proton
motive force(pmf), is the driving force of photosynthetic phosphorylation. H+ along
proton concentration gradient goes back to the membrane outside to synthesize ATP
by ATP enzyme catalyzed.
Second, Carbon dioxide (CO2) assimilation
CO2 assimilation, to abbreviate for carbon assimilation, refers to the procee that
plants use assimilation force (ATP and NADPH) forming in light reaction to change
carbon dioxide into carbohydrates. Carbon dioxide assimilation is curried out in the
matrix of chloroplast, where there are many kinds of enzymes involve in the reaction.
Carbon assimilation of higher plants is cuuried out in three ways, which is C3
pathway, C4 pathways and CAM (Sedum acid metabolism) pathway.
The diversity of plant photosynthetic carbon assimilation pathway reflects the
adaptability of plant to the diversity of ecological and environmental. But C3 pathway
is the most common and the most basic pathway of photosynthetic carbon metabolism,
at the same time, only this pathway has the capacity of synthesis of starch product,
while C4 pathways and CAM pathway it can be said that is auxiliary to C3 pathway.
Section IV photorespiration *
The process that the green cells of plants absorb oxygen and release CO2 in light
is known as photorespiration. This respiration occurred only in light, and is closely
related to photosynthesis. General living cells in light and dark can respire, which
have no special requirements for light, known as dark respiration.
Photorespiration requires the cooperation of three organells of chloroplasts,
mitochondria and peroxisome in the whole process, CO2 and O2 competite the same
active site of Rubisco, and which is the inhibitor of Plus oxygen reaction and
carboxylation reactions. The direction of Rubisco catalytic reaction is to carry out
photosynthesis or photorespiration, depending on the CO2 and O2 concentration ratio
outside world. The CO2/O2 ratio in atmosphere is very low, the activity of oxygenase
increases
inevitably. Now the photorespiration of green plants is inevitable in
atmosphere, so what is the physiological significance? So far, the main physiological
functions as follows:
The comparison of photosynthetic characteristics of C3 plants, C4 plants, C3 C4 intermediate plants and CAM plants:
According to the difference of higher plant photosynthetic carbon assimilation
pathway, plants can be classified C3 plants, C4 plants, C3 - C4 intermediate plants
and CAM plants. But the studies found that the pathway of higher plants
photosynthetic carbon assimilation changes when the plant organ, location, growth
period, and environmental conditions change. For example, sugar cane is a typical C4
plants, but its stem chloroplast only has C3 pathway; sorghum is also a typical C4
plants, but after its flowering it change into C3 pathway ; kalanchoe in short sunshine
is CAM plants, but in the long sunshine and the low temperature conditions has
turned into C3 plants. Mesembryanthemum crystallinum Linn in water stress has the
CAM pathway, while in moisture conditions, it mainly rely on C3 pathway to
photosynthesize.