لخّصلي

Chapter I. The structural organization of the plant cell
1.Definition of plant cell Plant cells are the basic unit of life in organisms of the kingdom Plantae.Thallophytes: the name "Thallophyta" comes from the Greek word "Thallos" which means "undifferentiated" and "phyton" which means "plant". Thallophytes can be defined as it is a group of plants or plant-like organisms that lack differentiated stems, leaves, and roots and that were classified as a primary division of the plant kingdom (Fig. 1). Some of the important characteristics of Thallophyta are listed below; ? Their bodies are not differentiated into well-defined stems, roots, and leaves. ? They also lack cellulosic cell walls. ? They are autotrophic and store nutrition in the form of starch. ? They are the most primitive form of plants. Figure 1. Example of Thallophytes A part of the Thallophytes is autotrophic chlorophyll, this group corresponds to algae (Fig. 2a). Another part is devoid of chlorophyll and adapted to heterotrophic nutrition , in this case it is fungi (Fig.2b). Others are adapted to life in symbiosis with algae or with cyanophytes, these are lichens (Fig.2c). a B a c Figure 2. Example of some species of Thallophytes (a: Algae, b.fungi, c: Lichens) 2 2.2.2. Cormophytes Cormophytes form a group composed of higher plants that correspond to multicellular organisms and whose eukaryotic cells are united into tissues, forming in turn organs much more complex than a Thallus called the cormus, i.e., a stem with leaves and roots. (Fig.3). Cormophytes are divided into several phyla: bryophytes (non-vascular) and vascular plants ( Pteridophyta, gymnosperms and angiosperms (flowering plants) (Fig.3). Figure 3. Example of Cormophytes a. Bryophytes Bryophytes is the division of green plants, which refers to embryophytes. They are land plants, particularly non-vascular ones. There are around 20,000 different plant species in the bryophytes. Following are the characteristics of Bryophyta: ? Plants do not have a vascular system
? Autotrophic organism ? Have a real leafy stem ? They do not have a real root system (Fig.4). 4 Figure 4 : Example of Bryophyta
b. Pteridophytes The Pteridophytes are primitive vascular plants with the absence of seeds. The plant body is differentiated into true roots, stems, and leaves. Vascular tissues are present in so-called vascular cryptogams. Most of them have leaves (known as fronds), roots, and sometimes true stems or trunks, as in the tree of ferns. (Fig.5). Figure 5. Example of Pteridophytes (ferns) 5 c. Spermaphytes The spermatophytes are plants that produce seeds; hence the alternative name, seed plants. They are a subset of the embryophytes, or land plants. Seed plants (Division Spermatophyta) are the most common plants on earth today. They are divided into two major categories, which are the gymnosperms (non-flowering plants) and the angiosperms (flowering plants). c.1. Gymnosperms Gymnosperms are vascular, seed-bearing, flowerless plants, whose meaning is "naked seed" (Fig.The cell wall: A cell wall is a structural layer that lies next to the cell membrane and serves primarily to give the cell strength and protection against mechanical stress.Plastids Plastids are a double membrane diverse group of physiologically, phylogenetically, and genetically related eukaryotic organelles that play important roles in plant metabolism through 8 different processes like; Proplastids (undifferentiated plastids) may undergo differentiation into many forms, depending upon their specialized functions in the cell.If the seed has one cotyledon, they are called monocots (wheat, banana, onions), and if the seed has two cotyledons, they are called dicots (apples, mango, oranges).They are eukaryotic and autotrophic cells, which have a true nucleus along with specialized structures called organelles that carry out different functions.The classification of plants The plant kingdom, although less diverse than the animal kingdom, has more than 400.000 species if we include fungi, although they are now considered to constitute a kingdom distinct from plants and animals.The classic classification of plants is based on several cytological and morphological criteria; thus, in the plant kingdom, we distinguish: 2.1.Eukaryotic, multicellular, chlorophyll-containing, and having a cell wall, are grouped under the kingdom Plantae.Chloroplasts
?Chromoplasts
?2.2.2.2.2.1. 6).?3.3.3. 9).3.1.4. 11).

Chapter I. The structural organization of the plant cell

1.Definition of plant cell
Plant cells are the basic unit of life in organisms of the kingdom Plantae. They are eukaryotic
and autotrophic cells, which have a true nucleus along with specialized structures called
organelles that carry out different functions. Plant cells have special organelles called
chloroplasts, which create sugars via photosynthesis. They also have a cell wall that provides
structural support.

2. The classification of plants
The plant kingdom, although less diverse than the animal kingdom, has more than 400.000
species if we include fungi, although they are now considered to constitute a kingdom distinct
from plants and animals. Plants are subdivided into two approaches, one classic and the other
phylogenetic. The classic classification of plants is based on several cytological and
morphological criteria; thus, in the plant kingdom, we distinguish:
2.1. Prokaryotic cells are unicellular living beings that reproduce by simple cell
multiplication.

2.2. Eukaryotic, multicellular, chlorophyll-containing, and having a cell wall, are grouped
under the kingdom Plantae. It is popularly known as the plant kingdom. We distinguish two
depending on the structural organization of the plant: the presence of a Thallus or a Cormus,
and therefore we distinguish: Thallophytes and Cormophytes.
2.2.1. Thallophytes: the name "Thallophyta" comes from the Greek word “Thallos” which
means "undifferentiated" and “phyton” which means "plant". Thallophytes can be defined as
it is a group of plants or plant-like organisms that lack differentiated stems, leaves, and roots
and that were classified as a primary division of the plant kingdom (Fig. 1). Some of the
important characteristics of Thallophyta are listed below;
 Their bodies are not differentiated into well-defined stems, roots, and leaves.
 They also lack cellulosic cell walls.
 They are autotrophic and store nutrition in the form of starch.

 They are the most primitive form of plants.
Figure 1. Example of Thallophytes
A part of the Thallophytes is autotrophic chlorophyll, this group corresponds to algae (Fig.
2a). Another part is devoid of chlorophyll and adapted to heterotrophic nutrition , in this case
it is fungi (Fig.2b). Others are adapted to life in symbiosis with algae or with cyanophytes,
these are lichens (Fig.2c).
a
B
a
c
Figure 2. Example of some species of Thallophytes (a: Algae, b.fungi, c: Lichens)
2
2.2.2. Cormophytes
Cormophytes form a group composed of higher plants that correspond to multicellular
organisms and whose eukaryotic cells are united into tissues, forming in turn organs much
more complex than a Thallus called the cormus, i.e., a stem with leaves and roots. (Fig.3).

Cormophytes are divided into several phyla: bryophytes (non-vascular) and vascular plants (
Pteridophyta, gymnosperms and angiosperms (flowering plants) (Fig.3).
Figure 3. Example of Cormophytes
a. Bryophytes
Bryophytes is the division of green plants, which refers to embryophytes. They are land
plants, particularly non-vascular ones. There are around 20,000 different plant species in the
bryophytes. Following are the characteristics of Bryophyta:
 Plants do not have a vascular system

 Autotrophic organism
 Have a real leafy stem
 They do not have a real root system (Fig.4).
4
Figure 4 : Example of Bryophyta

b. Pteridophytes
The Pteridophytes are primitive vascular plants with the absence of seeds. The plant body is
differentiated into true roots, stems, and leaves. Vascular tissues are present in so-called
vascular cryptogams. Most of them have leaves (known as fronds), roots, and sometimes true
stems or trunks, as in the tree of ferns. (Fig.5).
Figure 5. Example of Pteridophytes (ferns)
5
c. Spermaphytes
The spermatophytes are plants that produce seeds; hence the alternative name, seed plants.
They are a subset of the embryophytes, or land plants. Seed plants (Division Spermatophyta)
are the most common plants on earth today. They are divided into two major categories,
which are the gymnosperms (non-flowering plants) and the angiosperms (flowering plants).
c.1. Gymnosperms
Gymnosperms are vascular, seed-bearing, flowerless plants, whose meaning is “naked seed”
(Fig. 6). Examples include pine trees and palms. The characteristics of gymnosperms are:
 Cone-bearing, vascular plants
 Do not produce flowers, but do produce seeds.
 Do not require wet conditions for reproduction.
Figure 6 . Example of Gymnosperms

c.2. Angiosperms
The angiosperms (angios =covered, sperm = seed) produce fruits and seeds. They all produce
flowers containing sexual reproduction structures. There are currently 235.000 known living
flowering plant species. Angiosperms are divided into two categories based on the number of
6
cotyledons in a seed. If the seed has one cotyledon, they are called monocots (wheat, banana,
onions), and if the seed has two cotyledons, they are called dicots (apples, mango, oranges).
3. The characteristics of the plant cell
Most organelles are common to both animal and plant cells. However, plant cells also have
features that animal cells do not have: a cell wall
, a large central vacuole
, and plastids such
as chloroplasts
. Plants have very different lifestyles from animals, and these differences are
apparent when you examine the structure of the plant cell. Plants make their own food in a
process called photosynthesis. They take in carbon dioxide (CO2) and water (H2O) and
convert them into sugars (Fig.7):
Figure 7. Schematic representation of a plant cell
3.1. The cell wall: A cell wall is a structural layer that lies next to the cell membrane and
serves primarily to give the cell strength and protection against mechanical stress. Its primary
role is to offer the cell rigidity, durability, and resistance against stress from mechanical
forces. It composed of 90% carbohydrates (pectin, hemicellulose and cellulose) and 10%
proteins. Plant cell wall may have up to three strata or layers (Fig.8):
7
The primary cell wall:
It is often a thin, extensible layer generated as the cell grows.
The secondary cell wall:
The middle lamella:
After the cell has fully developed, a thick layer called the secondary
cell wall is created inside the primary cell wall. It is not present in every type of cell.

A stratum high in pectin is called the middle lamella. This top layer
creates the connection and interaction between neighboring plant cells.
Figure 8. The different parts of Cell wall
3.2. The vacuole
Most mature plant cells have a central vacuole that occupies more than 30 % of the cell's
volume. The central vacuole can occupy as much as 90% of the volume of certain cells.

The central vacuole is surrounded by a membrane called the tonoplast. It has many functions.

Aside from storage, the main role of the vacuole is to maintain turgor pressure against the cell
wall.

The central vacuole contains large amounts of a liquid called cell sap, which differs in
composition to the cell cytosol. Cell sap is a mixture of water, enzymes, ions, salts, and other
substances.
3.3. Plastids
Plastids are a double membrane diverse group of physiologically, phylogenetically, and
genetically related eukaryotic organelles that play important roles in plant metabolism through
8
different processes like; Proplastids (undifferentiated plastids) may undergo differentiation
into many forms, depending upon their specialized functions in the cell. Plastids may develop
as following variants:
 Chloroplasts

 Chromoplasts

 Leucoplasts
 Amyloplasts
3.3.1.Chloroplasts
: are the organelle of photosynthesis. They capture light energy from the
sun and use it with water and carbon dioxide to make food (sugar) for the plant (Fig. 9).
Figure 9. Example of chloroplast of plant cell

3.1.2.Chromoplasts
: include all plastids containing the pigment carotene in their structure. It
is this carotene that gives its yellow, red or orange color to flowers, ripe fruits and leaves in
the fall (Fig.10). Chromoplasts are usually found in plant cells exposed to light. However,
some cells not exposed to light may also contain carotene (the carrot in the soil).
Figure 10. Chromoplasts
9
3.1.3. Leucoplasts
: These are the non-pigmented organelles which are colourless.

Leucoplasts are usually found in most of the non-photosynthetic parts of the plant like roots.
They act as a storage sheds for starches, lipids, and proteins depending on the need of the
plants.
3.1.4. Amyloplasts:
are plastids specialized in starch storage. They are presented in particular
in the cells of reserve organs, such as the hypertrophied underground stems (tubers) of
potatoes (Fig. 11).
Figure 11. Example of Amyloplasts