Fundamental Parts of the Plant Body
The majority of plants such as spermatophytes (phanerogams), comprising plants that produce seeds, have bodies composed of two major systems: the root system and the shoot system. The former is usually underground, and the latter above ground. To succeed and grow simultaneously in two such entirely different environments—air and soil—requires many adaptations, starting with cellular modifications into specialized kinds of tissues (groups of similar cells that are organized in a structural and functional unit) followed by development of organs (structures composed of several kinds of tissues grouped in a structural and functional unit). The acquisition of form and structure is called morphogenesis and is a highly orchestrated procedure controlled by the DNA of the plant cells but influenced as well by the environment.
The Plant Cell
The term cell is derived from the latin word cellula which means a small apartment, it is the smallest unit of structure and function of a living organism. The main components of the plant cell are cell wall, cytoplasm and nucleus.
Animal Cell Versus Plant Cell
Cell wall Absent Present
Plastids Absent Present
The Cell Wall
One of the most important distinguishing features of plant cells is the presence of a cell wall. Unlike animals, whose lack of this type of structure allows their cells more flexibility, which is necessary for locomotion.
Formation of Cell Wall
The middle lamella is laid down first, formed from the cell plate during cytokinesis, and the primary cell wall is then deposited inside the middle lamella. In some plants and cell types, after a maximum size or point in development has been reached, a secondary wall is constructed between the plant cell and primary wall. Cells with secondary cell walls are rigid.
Strata or Layers of Cell Walls
• The middle lamella, a layer rich in pectins. This outermost layer forming the interface between adjacent plant cells and glues them together.
• The primary cell wall, generally a thin, flexible and extensible layer formed while the cell is growing (cell division).
• The secondary cell wall, a thick layer formed inside the primary cell wall after the cell is fully grown (cell maturation).
Chemical Composition of Primary Cell Wall
• The main chemical components of the primary plant cell wall include cellulose (in the form of organized microfibrils), a complex carbohydrate made up of several thousand glucose molecules linked end to end.
• In addition, the cell wall contains two groups of branched polysachrides, the pectins (provides theability to resist compression) and cross-linking glycans (increase thetensile strength). Organized into a network with the cellulose microfibrils.
• In addition to these networks, a small amount of protein can befound in all plant primary cell walls. Some of this protein is thought to increase mechanical strength and part of it consists of enzymes, which initiate reactions that form, remodel, or breakdown the structural networks of the wall.
Fine Structure of Primary Cell Wall
Consists of two continueous interpenetrating systems:
• Cellulose fibrils (microscopic threads):
The chain-like molecules of cellulose are combined into bundles forming Micellae which in turn are aggregated into larger units called Microfibrils which aggregated into coarse fibrils called Macrofibrils which can be seen by light microscope.
• Microcapillary spaces: Filled with incrusting substances such as cellulose, hemi-cellulose, pectic substances, lignin and proteins. Waxes, cutin, suberin and sporopollenin are also found.
Biosynthesis of Cell Wall Materials
Pectic substances are formed in the Golgi apparatus and are transported to the cell wall in the Golgi vesicles.
Growth of Cell Wall
Two theories:
• Growth by intussusception, where new microfibrils were held to be laid down between existing microfibrils.
• Growth by apposition, where new microfibrils were laid down on top of the existing ones, forming a new layer.
Parenchyma cell is an example of a cell having primary cell wall.
Structure of Secondary Cell Wall
• Show microscopic layering. The microscopic layers are commonly known as the S1 (outer), S2 (middle) and S3 (inner).
• The S3 layer is the thinner one and may be absent altogether.
• The S1 layer normally consists of four submicroscopic lamellae, alternate ones having microfibrils in opposed helices.
• The S2 layer is the middle , consists of numerous lamellae in which the orientation of the microfibrils is at only a small angle to the long axis of the cell. The microfibrils of this very thick wall layer aggregated into macrofibrils.
Chemical Composition of Secondary Cell Wall
Deposited inside the primary cell wall as a cell matures, sometimes has a composition nearly identical to that of the earlier-developed wall (Pectin in Collenchyma). Additional substances, especially lignin (in Sclerenchyma), lignin provides structural support to plant cells and makes plant cell walls less vulnerable to attack by fungi or bacteria, as do cutin (Thick cuticle on epidermis), suberin (in Cork cells) and other waxy materials that are sometimes found in plant cell walls.
Functions of Cell Wall
The plant cell wall serves a variety of functions. Along with protecting the intracellular contents, the structure gives rigidity (mechanical support) to the plant, provides a porous medium for the circulation and distribution of water and minerals.
Special structures of cell wall
- Plasmodesmata
Plasmodesmata are narrow channels that act as intercellular cytoplasmic bridges to facilitate communication and transport of materials between plant cells. The plasmodesmata serve to connect the symplastic space in the plant and allow for intercellular movement of water, various nutrients, and other molecules. Plasmodesmata are located in narrow areas of cell walls called primary pit fields.
- pits
- Pit is a cavity in the cell wall, allowing exchange of substances between adjacent cells. In other words it is a small region of the cell wall in which the primary wall is not covered with secondary wall materials.
Pit structure
A pit consists of a pit cavity (the aperture in the secondary wall) and a pit membrane (the primary wall material adjacent to the cavity).
Types of pits
There are three main types of pits, Simple, Bordered and Branched.
I. Simple pit
A pit in which the cavity remains uniform in width or gradually becomes either wider or narrower during growth in thickness of the secondary wall, lacking a border, found in certain parenchyma cells, extraxylary fibres, and sclereids.
Types of Simple Pits
a. Simple Pit-pair
Two adjacent pits from opposing cells sharing a common pit membrane.
b. Blind pit
A pit without an adjacent complementary one in the joining wall.
II. Bordered Pit
Possess an extension of the secondary cell wall (border, arching) over part of the pit cavity.
Structure of Bordered Pits
Possess a thickening of the primary wall material, termed a torus, in the central part of the pit membrane, the remaining unthickened part being termed the margo. Bordered pits mainly occur in vessel elements, tracheids, and fibres in the xylem, but may also occur in some extraxylary sclerenchyma cells.
Types of Bordered Pits
a. Bordered pit-pair
The pairing of bordered pits from adjacent cells.
b. Half-bordered
Referring to pit-pairs in which one is bordered, and the adjacent one is simple. This is the case in pits linking parenchyma cells and tracheary cells.
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