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Figure 2–69. The structural formulas 
of chlorophyll a and b.

Regardless of the functional differences between mitochondria
and chloroplasts, there is something in common regarding their general
structure: outer and inner membrane of different permeability for ions and
molecules, separated by inter-membrane space; their inner
membranes are located in the matrix and correspondingly in the stroma,
both of them rich in enzymes; they contain specific DNA, RNA and
ribosomes. Schematically that is represented in Figure 2–70.
There are also differences between mitochondria and chloroplasts. Mainly
they lie in the bioprocesses running in them. In chloroplasts the photosynthesis is
realized, which directly depends on the light (i.e. periodically) and as a
result organic compounds are formed and oxygen is released.
Briefly, it is a creative process. In mitochondria the bioprocesses do
not depend on the light, they run continuously, destroying the ready
products of photosynthesis, at that the energy of chemical bonds included in
them is released.

Photosynthesis: CO₂ + H₂O + energy → organic compounds
Respiration: Organic compounds + O₂ → CO₂ + H₂O + energy
As mitochondria (see Fig. 2–66), as well as chloroplasts possess
certain autonomy and can divide independently (Fig. 2–71). These firmly
established facts as well as the numerous data on the presence of specific
DNA, RNA and ribosomes in them were the reasons to accept the old
supposition, that they have endosymbiotic origin. This problem will be
considered in more detail in Section 2.10.

Like some other organelles, the chloroplasts possess an envelope
consisting of outer and inner membrane. The inner membrane
separates a homogeneous medium (stroma). In the stroma there are
located two kinds of thylakoid membranes — thylakoids of grans
and thylakoids of stroma. The thylakoids of grans are arranged like
coins, one on top of the other, and form chloroplast grans. The
thylakoids of stroma connect the grans one with another or do not get
in touch with them. The thylakoids of grans contain the “miraculous” molecule
chlorophyll (Fig. 2–69), proteins, lipids, carotenoids, etc. that form
peculiar antennae (light-accumulating complexes). These complexes
are fixed to the thylakoid membranes through specific protein
melecules and serve to collect the quantums of light energy, which is
transmitted to the reaction centres. The reaction centres, together with
the donors and acceptors of electrons, are the most important
structural units of the two photosystems.

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Figure 2–68. Electron micrograph of a 
giant chloroplast in unicellular alga 
Chlamydomonas (After De Robertis et 
al., 1973). 
1 — gran; 2 — pyrenoid; 3 — dictyosome; 4 — flagellum; 5 — membrane; 
6 — cell wall; 7 — nucleus; 8 — outer 
 membrane of the chloroplasts; 9 — 
vacuole.

The energy accumulated in themis used for photolysis
of water with oxygen releasing and forming NADP.H₂
and ATP, which participate in the reduction of CO₂ in organic compounds.
The released molecular oxygen (O₂) takes part in the oxidative
phosphorylation in mitochondria, i.e. in the process of respiration, in which
highly energetic molecules are synthetized.
Photosynthesis represented by the general equation

is the greatest creative process in
the living nature. It is not only a unique mechanism of synthesizing organic
compounds, but it has created the prerequisites for the appearance and
development of aerobic metabolism, that has led to increase of the variety
of living organisms. These questions were discussed in more detail in
Chapter 1 (Metabolism).

Chloroplasts

The chloroplasts (Greek: chlörós — green and plastós — formation,
creation, reproduction, modellation) are the most spread plastids in living
nature. They are obligatory organelles in all photosynthesizing cells. In the
different types of cells they vary in shape, number and size.
Also there exist other kinds of plastids — colourless (proplastids,
leucoplastids and ethioplastids) and coloured (chromoplasts). They are less
spread. The leucoplastids are colourless, with indefinite shape in most
cases. They are present in the cells of colourless parts of the plants,
epidermis, tubers, etc. They participate in the secondary synthesis of
starch, reserve proteins and lipids. The chloroplasts are coloured in yellow,
orange, red or brown. They are found in ripe fruits, leaves, algae, etc. They
contain different pigments. Phycoerythrin and phycocyanin are more known
of them. For example, the pigment lycopene from carotenoid group
determines the red colour of tomatoes.
It is known that after a complex reorganization of the inner
membranes, leucoplasts can turn into chloroplasts (turning green of potato
tubers), and chloroplasts — into chromoplasts (root fruits of carrots, the
leaves of trees in autumn, etc.). For a long time these frequently observed
facts have posed the question of the genetic relationship of the three types
of plastids, capable to change from one kind into another.
Usually chloroplasts are elliptic small bodies, 5—10 μm in length, with
a diameter of 2—3 μm. In one cell from a plant leaves one can find 15—20
or more chloroplasts, and in some algae — 1—2 giant chloroplasts of
different shape (Figs. 2–67, 2–68).

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Figure 2–67. Electron micrograph of a mesophilic cell from a pea leaf 
(Pisum sativum). Bar 0.5 μm (Courtesy of S. Doncheva and V. 
Vassileva, Institute of Plant Physiology, Sofia).
e — outer membrane; m — intermembrane space; i — inner membrane; g — 
gran; o — osmophilic globule; S — stroma; TS — thylakoids of the stroma. 

The outer membrane separates the mitochondrion from cytoplasm,
realizes the inclusion of different substances by means of transport proteins
forming hydrophilic channels in the lipid double layer and is pervious to the
products of metabolic processes running in the matrix. It contains enzymes
performing different functions.
The inner membrane forms a number of folds in the mitochondrion
interior, which increase its total surface. It is more impermeable, compared
to the outer membrane. It contains proteins catalyzing oxidative reactions of
respiratory chain, an enzymatic complex called ATP-synthetase and
specific transport protein molecules realizing the transfer of metabolites in
the matrix and outside of it.
The distance between outer and inner membrane is called
intermembrane space. A number of enzymes are there. Mainly they
perform transport functions.
The matrix represents a mixture of different substances and high-
molecular compounds. It fills the interior of mitochondrion. In it there are
carried out most biochemical processes related with the functions of this
organelle. It contains a rich set of different enzymes, including acetyl-CoA
necessary for the inclusion in the Crebs cycle. The end products of this
complex biochemical cycle are CO₂ and H₂O, with forming NAD.H and
PAD.H₂. Carbon dioxide comes off outside, and NAD.H and PAD.H₂ serve
as main sources of electrons in their transfer along the respiratory chain
and by forming H⁺ ₂O from H and O⁻. The released energy is used in the
synthesis of ATP. In the matrix there are also located specific DNA, RNA
and ribosomes, become known as mitochondrial.
For a long time the structural organization and diverse vital important
functions of mitochondria in the cells attract the attention of investigators. The
interest in them is great also from genetic point of view. It was established that
they possess a certain autonomy and can divide independently (Fig. 2–66).

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Figure 2–66. Division of mitochondrion in a cell of rat liver (Courtesy of 
D. Friend; From Fawcett, 1981).

Noteworthy is the succession of Golgi apparatus in daughter cells.
There are no data in favour of its de novo origin. For the first time the
distribution of this organelle during mitosis has been observed by
Perroncito (1910) and has been called dictyokinesis, since in the course
of division it disintegrates into isolated dictyosomes, proved to be similar in
a great variety of objects.

Mitochondria

They have been discovered in the end of XIX century by Carl Benda
(Benda, 1897) and called chondriosomes. The term mitochondria
(Greek: mitos — thread and chondrion — granule) is introduced later.
Mitochondria present in all cells, except for bacteria and unicellular blue-green algae.
Usually mitochondria have elongated or round shape with dimensions
0.5—1.5 μm. Sometimes their number can be from dozens to hundreds,
depending on the physiological state of the cell, external influences or
various other reasons. Mitochondrion of an animal cell is represented in
Figure 2–65.

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Figure 2–65. Mitochondrion in a cell of rat pancreas (Myotis lucifugus). 
(Cpourtesy of K. Porter; From Fawcett, 1981).

Using the methods of gradient centrifugation and the high resolving
power of electron microscopy enabled to clarify a number of important
questions regarding mitochondria structure and functions. It was
established that intact mitochondria are built of four main components —
outer membrane, inner membrane, intermembrane space and matrix.

wall and is used in its building and renovating.
The membranes of Golgi complex are a connecting link between the
membranes of endoplasmic reticulum and plasmalemma.

image

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Figure 2–64. Electron micrographs of dictyosomes (Golgi Apparatus). 
A — dictyosome of a cell from the dividing zone in a root of Zea mays. 
Bar 0.1 μm (Courtesy of S. Doncheva and G. Ignatov, Institute of Plant 
Physiology, Sofia). 
c — cistern; R — regenerative or proximate pole; S — secretory or distal 
pole; v — small bubble or vesicula; er — endoplasmic reticulum.
B — two Golgi bundles in the root meristem of Ricinus communis (After 
Frey-Wissling and Mühlethaler, 1965). 

thickness. As an organelle it is very changeable. Its form and structure is
insufficiently clarified. If there are ribosomes on its surface, it is called rough
or granular, and when there are not — smooth (Fig. 2–63). The area of ER in
the cells is large and sometimes can rich about 16% of their volume.
In ER-membranes there are two types of redox systems: NADPH-reductase and
cytochrome P-450. By means of these systems there are realized a detoxication of
compounds harmful to the cell and transformation of saturated aliphatic acids into
unsaturated. In the smooth ER there are formed carbohydrates, lipids and terpenoids,
and in the granular one there are synthesized membrane proteins and enzymes necessary
for the synthesis of cell-wall polysaccharides and many other secretory proteins. Through
endoplasmic reticulum the transporat of many substances in the cell is realized. It also
participates in intercellular relations, accomplishing the connection between different cells.

image

Figure 2–63. Electron micrograph of a section of 
mammal cell, where smooth 
 and rough endoplasmic 
reticulums are seen 
(Courtesy of G. Palade; 
From Alberts et al., 1986). 

Golgi Apparatus (Complex)

It is a cell organelle called after its discoverer Camilo Golgi (1898 a, b, 1899). It represents
a netlike structure built of dictyosomes (Greek: dyctyes — net), bubbles and
intercistern formations. Each dictyosome (Fig. 2–64 A, B) is built of several interconnected
cisterns. The cisterns are limited by membranes, 7—8 nm in thickness. On the one pole, called
regenerative, there proceeds a new forming of dyctyosomes from the membranes of the
smooth endoplasmic reticulum. On the other pole, called secretory, bubbles are formed
which contain substances intended to be secreted.
In dictyosomes glycoproteins andglycolipids are formed and there proceed
accumulation and “maturation” of components necessary for building the cell membranes.
The content of bubbles, whose membranes are built in plasmalemma, is transported up to the cell

Electron microscope examinations indicated that mature centrioles
consist of nine doublet and triplet microtubules connected with proteins
(mainly actin and myosin). The newly formed centrioles possess only one
microtubule. It is supposed, that every microtubule serves as a matrix for
forming doublets and triplets, typical of the mature centrioles. In their base
there is a basal small body, also called blepharoplast.

image

image

Figure 2–62. Flagellum apparatus (A) and 
basal small bodies (B) in Chlamydomonas
(After Sager, 1975).

Many investigators share the opinion, that the
centrioles and basal small bodies are similar or
identical structures and they are interchangeable. As an
illustration the unicellular green alga Chamydomonas
reinhardii is cited. It possesses two flagella with
basal small bodies in their bases (Fig. 2–62). Before
the start of mitosis the flagella disappear, and the
basal small bodies move inside the cell near to the
nucleus where they take part in forming and determining
the plane of division spindle. After the completion of
mitosis, served their purpose, they turn again into
basal small bodies from which the new flagella grow.
That suggests the idea on “organelle-phantom”, but it
needs more precise studies. It remains unclear how the migration in cells
by basal small bodies is realized when their number is more than two (there
are such cases in nature), and also which and how many of them will
participate in the formation of division spindle.

Endoplasmic Reticulum

It is discovered by Porter et al. (1945) in the cytoplasm of fibroplast in
chicken tissue cultures examined under electron microscope. Later it has
been confirmed by many other investigators. It is observed in all eukaryotic
cells, exept for mature erythrocytes which lack nuclei.
Endoplasmic reticulum (ER) or the endoplasmic net is a system of
channels, bubbles and cisterns encircled by a membrane, 5—6 nm in

Cell Centre

The cell centre (Fig. 2–61) is observed in Protozoa, different cells of
multicellular animals and some plants. For the first time it is described by
Flemming (1875) and van Beneden (1876) as polar small bodies. It is
located in the centre of cell, near to the nucleus. It consists of two tubular
small bodies (0.2—0.8 μm in length, diameter of 0.1—0.2 μm). In 1895 Th.
Boveri (1887—1907) has called them centrosomes. During the cell
division they move to the poles and determine the axis of division spindle.
After division each cell obtains a pair of centrioles.

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Figure 2–61. Cell centre (centrioles). Formation and migration of 
centrioles (asters) in round worms Ascaris (After Favard, 1961). 
A: Ch — chromosomes; ar — centriole; B: ar — centriole; mn — nuclear 
membrane, break down; er — endoplasmic bubbles.

substances are transferred. Elements of the endoplasmic reticulum are
connected with the outer membrane of nuclear envelope. In nucleoplasm
there are located parts of the chromatin, consisting of DNA, RNA and
proteins. During cell division chromatin is organized in chromosomes whose
number, as it was already mentioned, is precisely fixed and specific for each
species. The nucleoplasm of nuclei also contains numerous enzymes and
co-factors participating in the processes of phosphorylation and acetylation of
nuclear proteins, glycolytic enzymes, etc.
In the nucleus there is a nucleolus (in some cases 2 or more). It can be
seen easily under electron, as well as under light microscope. It is formed
in certain parts of DNA, called nucleolus organizer. It is considered to be
the place of ribosomal RNA (rRNA) synthesis.

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Figure 2–60. Electron micrograph of a 
prokaryotic cell of Bacillus subtilis (After 
Stent, 1974). 

The nucleus is a depot and distributor of the genetic
information of the cell. It is the place of most important genetic
processes related with its development and reproduction —
replication and transcription of DNA, as well as the synthesis of
the different kinds of RNA. Also here is the place of preparing the
start of translation. It is realized in the cytoplasm from already
synthesized information or messinger RNA (mRNA) by
means of different transport RNA (tRNA) molecules. Thus, on the
ribosomes the polypeptide chains are built. In this way, in a close
interaction with cytoplasm, the nucleus participates in realizing
the gene expression and in regulating the metabolic
processes running in the cell. In prokaryotic cells which do
not possess a clearly defined nuclei, the nuclear equivalent, i.e.
nucleoid, is located “freely” in the cytoplasm and is connected in some way
with the cell membrane (Fig. 2–60). It is considered that transcription and
protein synthesis are realized almost simultaneously in them. Here, as it was
already mentioned, the main unsolved question is the manner of uncoiling the
circular DNA-molecule during the replication. Clarifying this question will shed
more light on the structural organization of genome in prokaryotic cells.