Lecture 02 Components of plant cell

Components of Mature Cell

A typical plant cell is formed of protoplasmic (living) and non- protoplasmic contents.

protoplasmic contents

Plastids

There are several different types of plastids,

organelles unique to plants.

Three main types are:

•           Chloroplasts 

•           Chromoplasts 

•           Leucoplasts 

Mature plastids develop from small, green protoplastids in growing, embryonic plant cells.

Chloroplasts

The word chloroplast is derived from the Greek words chloros, which means green, and plast, which means form. Contain chlorophyll-bering units grana embedded in stroma. Found in tissues exposed to light. Responsible for photosynthesis.

Chromoplasts

Contain color due to carotenoid (yellow, orange, red) and xanthophyll (yellow) pigments stored in their vacuoles and oil droplets. Highly concentrated in the petals of animal-pollinated flowers and the skins of animal-dispersed fruit, function in providing color necessary to attract pollinators and seed dispersers.

Leucoplasts

Category of plastid found in plant cells. They are non-pigmented, in contrast to other plastids. Lacking pigments, leucoplasts are not green, so they are located in roots and non-photosynthetic tissues of plants. They may become specialized for storage of starch, lipid or protein and are then known as amyloplasts, elaioplasts, or proteinoplasts respectively.

Special Forms of Plastids

A. Etioplasts

Chloroplasts that have not been exposed to light. They are usually found in flowering plants (Angiosperms) grown in the dark. If a plant is kept out of light for several days, its normal chloroplasts will actually convert into etioplasts. Etioplasts lack active pigment and can technically be considered leucoplasts. High concentrations of etioplasts will cause leaves to appear yellow rather than green.

B. Statoliths

Specialised forms of amyloplast used by plants in detecting and responding to gravity. Statoliths are denser than the cytoplasm and tend to move towards the bottom of the cell, indicating which direction is 'down'. They are found mainly in root tissues.

Stromules

Plastids inside plant cells are often interconnected by a network of microscopic tubules known as stromules.

Non-Prptoplasmic contents

The ergastic substances formed as a result of metabolic activity may be waste products or stored food materials.

A. starch grains

Starch or amylum is a carbohydrate consisting of a large number of glucose units. In photosynthesis, plants use light energy to produce glucose from carbon dioxide. The glucose is stored mainly in the form of starch granules, in plastids such as chloroplasts and especially amyloplasts.

B. Crystals

      Solid waste products that are deposited in plant tissues in the form of calcium salts and silica. In some taxonomic groups the type, morphology and location of these structures is of systematic significance.

Calcium salts are mostly deposited as:

I. Calcium Oxalate Crystals

      Calcium oxalate crystals in plants are formed from endogenously synthesized oxalic acid and calcium from the soil solution in contact with plant roots. Calcium oxalate is a poisonous substance. The poisonous plant (Dieffenbachia) contains crystals and on ingestion can prevent speech and be suffocating. Kidney stone sufferers should not eat plants high in oxalates.

II. Calcium Carbonate Crystals

Deposited in the form of Cystolith ("cavity" and "stone") it is a botanical term for the inorganic concretions, usually of calcium carbonate, formed in a cellulose matrix in special cells, generally in the leaf of plants of certain families.

Meristem

   A meristem is the tissue in all plants consisting of undifferentiated cells (meristematic cells) and found in zones of the plant where growth can take place. Differentiated plant cells generally cannot divide. The term meristem was derived from the Greek word merizein, meaning to divide. Meristematic cells are analogous in function to stem cells in animals.

Characteristics of  Meristematic Cells

•           Incompletely or not at all differentiated,

•           Capable of continued cellular division (youthful).

•           Small and protoplasm fills the cell completely.

•           The vacuoles are extremely small.

•           The cytoplasm does not contain differentiated plastids, although they are present in rudimentary form (proplastids).

•           Meristematic cells are without intercellular cavities.

•           The cell wall is a very thin primary cell wall.

Classification of Meristems According to Origin (Promeristem theory)

A. Primary Meristems

•           Protoderm - Lies around the outside of the stem and develops into the epidermis.

•           Procambium - Lies just inside of the protoderm and develops into primary xylem and primary phloem. It also produces the vascular cambium, a secondary meristem.

•           Ground meristem - Develops into cortex and pith. It produces the cork cambium, another secondary meristem.

These meristems are responsible for primary growth, or an increase in length.

B. Secondary Meristems

•           Vascular Cambium - Produces secondary xylem and secondary phloem, this is a process which may continue throughout the life of the plant. This is what gives rise to wood in plants. Such plants are called arborescent. .

•           Cork Cambium - Gives rise to the periderm which replaces the epidermis.

Classification of Meristems According to Position

A.        Apical Meristems - (Shoot apex) - The source of all above-ground organs, (Root apex) - It is covered by the root cap, which protects the apical meristem from the rocks, dirt and pathogens and (Floral meristem) - When plants begin flowering, the shoot apical meristem is transformed into an inflorescence meristem which goes on to produce the floral meristem which produces the familiar sepals, petals, stamens, and carpels of the flower.

B. Lateral Meristems - They are involved in lateral growth e.g. Vascular cambium and Cork cambium.

C. Intercalary Meristems - Occur only in monocot (particularly grass) in stems at the base of nodes and leaf blades. Intercalary meristems at the nodes allow for rapid stem elongation, while those at the base of leaf blades allow damaged leaves to rapidly regrow.

Theories of Development and Differentiation of Meristems

Apical Cell Theory

Solitary apical cells occur in many bryophytes and pteridophytes formed all tissues and organs of the plant. The apical cell theory was proposed as the basis for an understanding of the method of growth and morphology in many groups. But the theory was not applicable to seed plants.

Histogen Theory

Under this theory the more or less distinct major regions of the stem and root apex were called histogens (tissue builders). This theory, in contrast to the apical cell theory, placed the origin of axis apices in a group of initials. The histogens were:

The dermatogen: a uniseriate, external layer; formed the epidermis.

The periblem: the region between plerome and dermatogen. Formed the cortex.

The plerome: a central core; formed the pith and primary vascular tissues.

Tunica and corpus Theory

The growing apex of the stem is differentiated into:

Corpus: in the middle of apical meristem, divide to form the central vascular cylinder and cortex.

Tunica: outer enveloping layer around the corpus, divide to form the epidermis and part of the cortex in some cases.

Promeristem theory

The promeristem region is differentiated into:

Protoderm: formed the epidermis in stem and piliferous layer in the root.

Procambium: formed primary phloem and primar xylem.

Ground meristem: formed the cortex and the pith.

Lecture 01 Introduction and Plant Cell Wall

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

1. 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.

2. pits

2.  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.

III. Branched Canal Pit: In thick-walled cells with small lumen (stone cells).