1

د. دريد كامل الطائي

Importance of Photosynthesis and the Global Role

Of Green Plants

Photosynthesis is the process through which the energy of light is absorbed by

chlorophyll molecules and converted to potential chemical energy of organic

substances, composed of carbon dioxide and water. This process includes a

large number of reactions, however, for autotrophic organisms producing

oxygen, it can be summarized by the following equation:

The Leaf as a Specialized Photosynthesis Organ Photosynthesis takes place in all plant cells that contain green pigments

(leaves, branches, young stems, sepals, unripe fruits), but the organ specialized

in fulfilling this function is the leaf, which shows some features formed during

a long process of adaptation and improvement:

a large, flat area, adapted for absorption of large amounts of solar energy and

CO2 from the atmosphere;

• an epidermis provided with stomata through which gas exchange and

transpiration occurs. Depending on the positioning of stomata in plants, they

can be divided in 2 groups: amphistomatic (with stomata present on both

sides of the leaf) and epistomatic (with stomata located only on one side of

the leaf);

the presence of organelles specialized for photosynthesis—chloroplasts;

• a bilayer structure, the assimilatory parenchyma being differentiated in

palisade parenchyma that plays the main photosynthetic role and spongy

parenchyma with a pronounced role in gas exchange. In young branches,

seeds and unripe fruits, assimilatory cells with chloroplasts are located in the

parenchymal layers under the epidermis. Intercellular spaces are very small

which causes reduced CO2 absorption from the external environment (in

comparison with green leaves);

2

د. دريد كامل الطائي

• the presence of conducting channels (phloem and xylem) which deliver

mineral compounds and water to mesophyll cells and transport the elaborated

(formed) sap with synthesized organic compounds to all plant organs.

During evolution leaves have changed generating a great diversity, determined

by the structural changes adopted for carbon assimilation (Fig. 4.2). Some

plants originating from tropical and subtropical zones (corn, sugar cane, etc.)

have leaves with a particular anatomical structure that differs from the leaves

of plants growing in temperate climate (300.000 species of plants), adapted to

carry out photosynthesis

In certain environmental conditions. The leaves of these species are well

Vascularized, the mesophyll is homogenous containing granal chloroplasts

while conducting vessels are surrounded by a compact layer of parenchymal

cells, forming a sheath of perivascular assimilatory tissue with big a granal

chloroplasts (Fig. 4.2). Perivascular sheath cells are separated from the

mesophyll and from the air of intercellular spaces by a film, resistant to carbon

dioxide diffusion.

Leaves of the plants from sunny zones, have a small area, are thick, have a

larger number of stomata and long palisade cells with chloroplasts containing

less chlorophyll, but assimilating carbon more efficiently. Another measure to

protect the cellular structures from optical radiation consists in the synthesis

of auxiliary pigments with photoprotective properties. Such substances are

anthocyanins, present in higher concentrations in young and senile plants; they

are often formed as a result of plant response to a high intensity of the visible

light, to ultraviolet radiation, to low or high temperatures and to other stress

factors. These red pigments are located in the cells of the superior epidermis

and provide an effective screening in the green region of the spectrum in which

the leaves are mostly “transparent”. On the action of UV radiation, the

synthesis of several phenolic compounds is induced. They are accumulating

3

د. دريد كامل الطائي

in the cuticle and epidermal cells, ensuring UV absorption and tissue

protection from its damaging effect.

Other compounds playing the role of photoprotection in foliar tissues

are carotenoids, which ensure, at low concentrations, a strong absorption in

the indigo-blue region of the spectrum blocking photo-destructive processes.

Their synthesis is activated before the period of vegetative pause in deciduous

trees and before drought in tropical species—a period associated with the

destruction of the photosynthetic apparatus and exposure to photo-oxidative

stress determined mostly by the high intensity of the solar light.

The Structure, Chemical Composition, Function

And Origin of Chloroplasts Chloroplasts are organelles specialized for fulfilling the photosynthetic

function and represent microstructures with the length of 5–10 μm and a

diameter of 2–3 μm, with spherical (globular), oval, discoid or ellipsoid shape.

In the majority of green plants ellipsoid chloroplasts predominate; ,developing

during the evolution of the vegetal world.

The number of chloroplasts varies from 20 to 100 per cell, depending on the

species, environmental conditions, foliar tissue. The plastids of the cell are

constantly moving, either passively with the cytoplasmic flow or actively,

requiring energy consumption and being determined by light intensity and by

other factors.

The structure of chloroplasts. The fundamental substance of the

chloroplasts, called stroma, is limited to the exterior by a double lipoprotein

membrane (with the thickness of 10–30 nm) containing a large number of

pores with a surface area of 30–40 nm2.

The internal membrane, that has no pores, is less permeable in comparison to

the external one, but it can be passed by molecules of trioses and amino acids.

It forms folds called thylakoids (from thylakoids—“bag-shaped”) along the

longitudinal (linear) axis of the chloroplast and which either have the form of

overlayed disks (these are called granal thylakoid's) forming the structures

called grana (from granum—“granule”) or traverse the chloroplast from one

edge to another—thylakoids of the stroma

(Fig. 4.3).

4

د. دريد كامل الطائي

The

number of grana in chloroplasts and the number of thylakoids in a granum

vary within large limits. It is considered that a chloroplast contains about 40–

100 grana and the total area of the thylakoids is 500 times bigger than the

external membrane surface—a peculiarity that is considered an adaptation for

carbon assimilation by chlorophyll given the low concentration (<0.03 %) of

carbon dioxide in the atmosphere.

The chloroplasts of some superior plants, just like those of algae, have no

grana, each 2–8 lamellas being united in “packages”.

Chloroplasts can be of several types:

granal (containing granal and stromal thylakoids), characteristic for all

superior plants;

a granal, containing only stromal thylakoids—characteristic for algae and

for the perivascular sheath cells from plants with C4 photosynthesis type.

According to the results of electronic microscopy, granal and stromal

thylakoids contain a large number of lipoproteic spherical structures, called

quantasomes or photosynthetic units consisting of pigments and components

of the system of photosynthetic electron transfer (ETC) and components of the

system of ADP phosphorylation that is coupled with this transfer (Fig. 4.4).

The synthesis of the proteins and lipids (glycosyl-glycerides) of the

5

د. دريد كامل الطائي

thylakoidal membranes is regulated by both the nuclear and chloroplast

genomes. Approximately50 % of the proteins involved in the formation of

ETC complexes (40 proteins) are encoded by chloroplast DNA, the rest—by

nuclear DNA. The latter are synthesized in the cytoplasm then enter the

chloroplasts, where, by binding specific proteins, they form functionally active

protein complexes assembled and distributed in an oriented (directed to )

manner in the thylakoidal membrane. There are several types of such macro

molecular complexes, which ensure the functions of absorption and

transformation of the solar energy into that of chemical bonds (light phase of

photosynthesis):

• photosystems I (PSI) and II (PSII);

• cytochrome complex b/f;

• light-harvesting complex;

• ATPase

The coordinated activity of the nuclear and chloroplast genomes in the

synthesis of these molecular complexes is represented schematically in Fig.

4.5.

Another example of regulation of the photosynthetic apparatus by the nuclear

genome is the synthesis of the key enzyme of the dark phase (the Calvin-

Bensoncycle)—ribulose-1,5-bisphosphate carboxylase (RUBISCO). The

functionally active enzyme is composed of eight small and eight large subunits

(Fig. 4.6). The bigger subunit (54 kDa) is encoded by the chloroplast DNA,

and the smallest(14 kDa)—by the nuclear DNA, which, after being

synthesized (translated) in the cytoplasm, is transported into chloroplasts,

where the correct assembly of the enzyme takes place in the presence of

another chaperone protein of 60 kDa also encoded by the nucleus.

6

د. دريد كامل الطائي

The origin of chloroplasts. Chloroplasts represent a variety of the organelles

Specific for plant cells the plastids, formed from the so-called proplastids,

found in meristematic cells. Proplastids are colorless vesicles with a diameter

of 0.5–1.5 μm, delimited by a double lipoproteic membrane containing a

double stranded circular

7

د. دريد كامل الطائي

With cell growth proplastids increase their size as well, the internal membrane

is growing more intense, forming vesicles (thylakoids), which lie parallel

along the stroma. In the presence of light, these discs arrange in a certain

position relative to the plastid axis, suffering a differentiation into stromal and

granal thylacoids. This process is accompanied by the biosynthesis of lipids,

proteins, chlorophyll and the inclusion in the structure of the membranes. In

the dark, etioplasts form, which have an internal structure similar to a

crystalline network of protochlorophyllids (chlorophyll a precursors lacking

the phytol side chain, which can be added only in the presence of light in

angiosperms). Etiolated tissue exposure to light causes there organization of

the etioplast internal structure into a membranous structure,characteristic for

chloroplasts (Fig. 4.8).

8

د. دريد كامل الطائي

Photosynthesis Pigments

The selective absorption of solar radiation in the visible spectrum region of

400–700 nm at levels sufficient for photosynthesis is a particular feature of

several organic compounds (pigments), which have in their structure

chromophore (colored ) group sand systems of conjugated bonds. Plastid

pigments may have a different chemical composition and are classified into

three groups: chlorophylls, carotenoids and phycobilins.

Chlorophyll pigments are present in superior plants and in algae in four forms:

chlorophyll a”, “b”, “c” and “d”. All photosynthesizing plants and all groups

of algae and cyanobacteria contain chlorophyll “a”. Chlorophyll “b” is typical

for superior plants and green algae. Brown algae contain chlorophyll “c” and

red algae —chlorophyll “d”. Photosynthesizing bacteria contain various types

of bacteriochlorophyll (Fig. 4.9). In the seeds of certain plants (pumpkin,

hemp) protochlorophyll was detected. In superior plants this chlorophyll type

is synthesized during the dark phase and on light exposure transforms into

chlorophyll.

Chlorophyll is one of the most complex organic substances, representing a

double ester of chlorophyllin with phytol (the alcohol of a higher carbohydrate

with 20 carbon atoms and a double bond) and methyl alcohol. The most

common are chlorophyll “a” and “b” (Fig. 4.10).

9

د. دريد كامل الطائي

Carotenoid pigments are liposoluble compounds present in chloroplasts and

chromoplasts. They can’t be observed in green leaves, because of the presence

of chlorophyll. In autumn, when the latter is destroyed, the foliage becomes

yellow orange due to these pigments. Bacteria and fungi also contain

carotenoids.

Approximately 400 pigments from this group were studied (Fig. 4.11).

Carotenoids are unsaturated carbohydrates with conjugated double bonds,

derived from isoprene CH2 = C(CH3)–CH–CH2. They can be acyclic

(lycopenes), mono and bicyclical (Figs. 4.12 and 4.13). Due to conjugated

double bonds carotenoids are able to perform redox reactions.

Carotenoids, like chlorophylls, are non-covalently linked with proteins and

lipids of photosynthetic membranes. Due to the physical-chemical properties,

this group of pigments fulfills a double role: that of solar energy absorption

and transfer to the chlorophyll pigments and, at the same time, it plays a

photoprotective role.

Carotenoids are synthesized in chloro-and chromoplasts by the mevalonate

cycle starting with acetyl-CoA through a series of intermediates—mevalonate,

geranylpyrophosphate, lycopene, which serve as precursors for other

carotenoids.

Phycobilin pigments enter the group of biliary ( yellow) pigments (in animals,

a representative of this group is bilirubin). They can be found in some red,

green, blue and pyrophyte algae and in a very small amount in chloroplasts of

green plants (Fig. 4.14). The following pigments from this group are known:

Phycoerythrin—C34H47N4O8, present in red, green, blue and pyrophyte

algae;

• Phycocyanin—C34H42N9O4, present in blue-green algae.

11

د. دريد كامل الطائي

Photosynthesis Energetics The most important peculiarity (property) of photosynthesis is the use of

solar energy that is the energy of electromagnetic oscillations (vibration),

characterized by a certain wavelength (distance between two consecutive

maximum points of a cycle), an oscillation frequency and a speed of

dispersion:

where

λ—wavelength, nm;

c—speed of light, 2.997 × 108 m/s;

ν—oscillation frequency, Hz.

Solar light includes radiation from:

The visible spectrum: violet (400–450 nm), indigo (400–450 nm), blue (450–

500 nm), green (500–570 nm), yellow (570–590 nm), orange (590–610 nm),

and red (610–700 nm) as well as from the invisible one, which includes:

11

د. دريد كامل الطائي

Light utilization coefficient (amount of light energy used in photosynthesis

from the total amount of energy absorbed by the leaf) is very low. Only 2–5

% of the total quantity of solar energy that reaches the leaf surface are used

in the process of photosynthesis; the rest remains unused (Fig. 4.16):

• 10–15 % is reflected from the leaf surface depending on the properties of

the cuticle (smooth/rough, glossy/matte); • 10–15 % of the energy passes

through the leaf without being absorbed, depending on the thickness of the

leaf blade;• more than 70 % of the light energy that reaches the leaf surface

is absorbed,20 % dissipates as heat, about 45 % is used in the transpiration

process as latent heat of vaporization.

12

د. دريد كامل الطائي

Photosynthesis Mechanism

According to the modern theory regarding the molecular mechanism of

photosynthesis, this process is a chain of successive redox-reactions, which

requires sunlight at early stages (Robin Hill phase), while subsequent steps can

occur in the dark (F.F. Blackman phase) (Table 4.3).

13

د. دريد كامل الطائي

In the light phase of photosynthesis absorption of light occurs by chlorophyll

molecules “a” with the participation of auxiliary pigments (chlorophyll “b”,

carotenoids, phycobilins) and transformation of solar energy into ATP and

NADPH +H+. All these processes are carried out in photochemically active

chloroplasts membranes, and represent a complex system of photophysical,

photochemical and chemical reactions.

In the dark phase of photosynthesis carbon fixation by the primary acceptor

(ribulose-1,5-diphosphate) happens, involving enzymes located in the

chloroplast stroma and with energy consumption in the form of ATP and

NADPH+H+ which are the final products of the light phase.

Light Phase of Photosynthesis The processes occurring during the light phase of photosynthesis can be

related to:

(1) Absorption of carbon dioxide; (2) Absorption of solar energy and its

transformation into chemical energy.

(1) Absorption of carbon dioxide from the external environment happens

through the open osteole ( stoma) (photoactive physiological reaction).

Carbon dioxide enters the sub substomatal cavity (sinus) , from where it

diffuses through the free intercellular spaces to directly contact the cellulose

membranes of palisade assimilatory parenchyma, situated on the upper side of

the leaf blade, or the cells of the spongy parenchyma from the inferior side

In the envelopes of assimilatory cells are continuously irrigated with water

absorbed from the soil, the CO2 from the air that circulates in the intercellular

spaces, possessing a high hydro solubility, dissolves and forms carbonic acid

(H2CO3), which dissociates in H+, HCO3

_, CO3

2_. In the ionic form carbon

dioxide enters the cytoplasm and reaches chloroplasts.

Consequently or so , it results that the first condition of photosynthesis is the

degree of osteole opening and the presence of a sufficient amount of water in

foliar tissues. At night, when stomata are closed (photoactive closure) as well

as in drought conditions (hydro active closure), when the cellular membranes

of the leaf mesophyll cells are dry, photosynthesis is blocked and plant growth

stagnates.

Absorption of solar energy and its transformation into

chemical energy happens via several successive stages:

• solar energy absorption and excitation energy migration to the system of

Pigments;

• oxidation of the reaction center and stabilization of the separated charges;

• electron transfer through the electron transport chain (ETC);

14

د. دريد كامل الطائي

• water photo oxidation and molecular oxygen elimination;

• Conjugation of electron transport with proton transfer and the synthesis of

ATP.

These processes are carried out in granal and stromal thylakoids with the

participation of different molecules that make up two specific structures in

superior plants—photosystem I (PS I) and photosystem II (PS II), which

differ in their protein components, pigments and optical properties. Each

photosystem is formed of a reaction center conjugated with electron donors

and acceptors together with the “antenna” pigments

Absorption of solar energy and excitation energy migration in system of

pigments.

The primary processes of the light phase consist in light capturing in the

form of photons by antenna-pigments. The intensity of this photophysical

process is proportional to the number of absorbed photons, that’s why the

light necessary for photosynthesis can be expressed by the number of quanta

per molecule (Einstein).

Pigment molecules, absorbing the energy of light quanta, enter an electron

excitation phase (the electron jumps to a higher energetic level). Electrons

from the molecule have certain energy values, which can be associated with

energetic levels

15

د. دريد كامل الطائي

Oxidation of the reaction center and stabilization of separated charges.

The first reactions following light absorption and chlorophyll excitation in

the reaction centers are processes involving electron transfer between

different macromolecular entities. The sequence of events occurring in the

reaction center is similar in all photosynthesizing systems. The first

photochemical reaction in the reaction center is the fast transfer of electrons

from the photoactive pigment (P*) to the primary acceptor (I), for instance

bacteriopheophytin (BPP) in case of bacterial photosynthesis, the monomeric

form of chlorophyll (A0) or pheophytin (Phe) for photosystem I and

photosystem II, respectively

Finally, this process yields a reductant I− (donor of electrons) and a strong

oxidant P+ (acceptor of electrons):

This marks the second important stage of solar energy transformation within

the process of photosynthesis (charge separation in the reaction center).

The subsequent electron transfer happens outside the center of reaction,

through an electron transport chain (ETC), which unites both reaction centers

by means of cytochrome b6-f. The positive charge of the reaction center,

formed during its oxidation, is neutralized by acceptance of an electron,

returning to its fundamental reduced state.

The function of the electron donor is performed by cytochrome “c” in

bacteria, by plastocyanine in PSI and by the tyrosine radical—a component

of the water dissociation system in the PS II.

Electron transfer mechanism. The electron is transferred from the reaction

center at long intermolecular distances from one side of the membrane to

another at high speed. For example, the electron is transferred from P* to

QA, at a distance of about 50 Å in 150 ps ( picosecond ) (Fig. 4.23).

16

د. دريد كامل الطائي

An important role in electron transfer is played by the protein medium

between the redox cofactors (with transport function), which represents not

only a physical support for the carriers, but also actively influences this

process. Two theories are currently available explaining the mechanism of

electron transport through ETC.

1- Electron transfer mediated by proteins is based on the so-called electron

tunneling effect—a quantum mechanics phenomenon. Due to its oscillatory

nature, the electron literally “slips” under energetic barriers, tunneling from

one transporter to another (with a probability which decreases exponentially

with the increase of the height of the barrier). It is considered that during

tunneling, the electron loses part of the energy, which passes into oscillations

of the light atomic groups of the proteins.

2- Electron transfer through the ETC in photosynthesis or in respiration is

sometimes presented as a ball that moves downwards on a ladder where each

step represents the energy level of the transporter. While moving to a new step,

part of the electron energy can be converted into heat or stored in the form of

ATP. Every time when the ball falls down on the corresponding step it rotates

it in the direction that will facilitate ball movement to the next step. This

process is so fast that the possibility of backward movement is much smaller

than that of forward transport, which provides for the efficiency and

irreversibility of the transport.

Transfer of electrons through the electron transport chain (ETC).

transport chains contain a wide variety of carriers. Some carriers transport a

single electron, while others transport an electron and a proton, or even more

electrons and/or protons. But, an important feature of theirs is the fact that

they are located in membranous structures of the cell—in chloroplast and

mitochondrial membranes. Among the compounds that participate in

processes of electron transfer one could distinguish:

cytochromes (proteins whose prosthetic groups are represented by the

heme group). The active redox component is represented by the iron atom,

that can exist as both Fe(II) and Fe(III) (Fe3++ē → Fe2+).

Proteins with iron and sulfur. The iron atoms in these proteins are in tight

association with groups of sulfur containing atoms, which are normally

found in the cysteine residues of the protein chain. Iron and sulfur clusters

contain an equal number of sulfur and iron atoms and their common

combinations are 4Fe+4S, 3Fe3+S → and 2Fe+2S;

Flavoproteins, which are widely spread among the redox enzymes, are

characterized by the presence of some strong non-covalent flavin-nucleotide

bonds. The structure of the isoalloxazine ring undergoes the transfer of

17

د. دريد كامل الطائي

two ions of hydrogen (2ē + 2H+);

Copper proteids, the paramagnetic ion of copper (Cu2+), is part of the

structure of the active center.

Chinones, which have methyl, methoxy, amino—or hidroxyl substituents,

that significantly affect their electrochemical properties (especially their

potential for oxidoreduction). Chinones in water solutions undergo a

reduction reaction (2ē + 2H+).

These compounds form macromolecular protein complexes located in the

thylakoid membrane (cytochrome b6-f, of water photo-oxidation; electron

transporter ferredoxin—NADP—reductase), which, together with the mobile

transporters (plastoquinone, plastocyanin and ferredoxin molecules), transfer

electrons, ensuring conjugated functioning of the reaction centers

18

د. دريد كامل الطائي

Photo-oxidation of water and elimination of molecular oxygen. The

photosynthetic oxidation of water is performed by the macromolecular

complex PS II, which includes three basic structural and functional parts:

the complex of antenna-pigments, located on proteins with a molecular

massranging from 25,000 to 47,000;

• the reaction center with all its basic components located on a complex

formed by 2 proteins with a molecular mass of about 32,000, called D1 and

D2 which are located across the photosynthetic membrane (Fig. 4.26);

19

د. دريد كامل الطائي

The resulting electrons are taken by the manganese atoms which, thus, reduce.

The protons are accumulating in the lumen of the thylakoides while oxygen

diffuses in the cytoplasm, where it is dissolved in water, passes into the

intercellular spaces in the form of gas and is eliminated into the external

environment through the open stomatal pores. The tyrosine radical (Tyr Z)—

the aromatic amino acid from the D1 protein (161st from the N-terminus)

which plays the role of intermediary transporter

21

د. دريد كامل الطائي