Structural organization of the plant cell

Structural organization of the plant cell

The cell is the fundamental unit of life, the building block from which all organisms are constructed. The properties of cell exhibiting the characteristics of life, define both the potential capabilities & the inherent limitations of all living organisms. The cell theory given in 1839 by German biologist M.Schleiden & Theodor Schwann was originally based on the observation that different kinds of cells resemble one another when observed microscopically &functionally. When the properties of many different cell types was examined , it was found that cells share common characteristic functions carried out by specialized subcellular structure known as organelles . 

The specialized found in plant, shoot, leaves, roots, flower and fruits are classified into three tissue system – ground tissue system ,dermal tissue system and vascular tissue system .Each tissue system carries different generalize function :the vascular tissue system transport water and solutes to long distance in the plant, the dermal tissue system provide protection and perform exchange at the surface of the plant, and the ground tissue system provide cells that carry out photosynthesis , storage and support. Each tissue system has many specialized cells, a few cell types are found in more than one tissue system. The different types of specialized plant cell are distinguish by cell shape and by properties of the cell wall and protoplast. Plant cell wall is one of the most important distinguishing feature of the different kind of specialized cell. All plant have a thin and flexible primary wall, made of the polysaccharides cellulose and other carbohydrates . Other cell types have addition to a primary wall, a thick, rigid secondary wall in which cellulose is impregnated with lignin.

The internal atmosphere of cell differs from that of its external environment. This difference is maintain throughout the life of the cell by thin surface membrane. The plasma membrane which controls the entrance & exit of molecules & ions. The capacity of the plasma membrane to act as a selective barrier between the cell & the medium is called permeability .Most plant cells, have a thick cellular wall that cover & protects the plasma membrane . Some animal cells are surrounded by a cement like layer called a cell coat, which generally plays no role in permeability but does have other important functions.

Cell requires chemical building blocks such as sugars, fatty acid, nucleotides & amino acid .Cells need energy both to drive the chemical reaction involved in building cell components. Most of the chemical reactions that take place in the cell would normally occur too slowly to maintain life as we know it. Cell requires a set of information to carry out the reactions.  

The extraneous coat of the plasma membrane of the plant cell is known as cell wall. The cell wall is a rigid and protective layer around the plasma membrane which provides the mechanical support to the cell. The surfaces of plant and bacterial cells exhibit many of these same properties, but they also exhibit a few unique features that are not shared by the cells of animals. Plant cell walls provide a supporting framework for intact plants. In addition to providing mechanical support & strength for the plant as a whole, the cell wall protects individual cells from osmotic rupture and mechanical injury .The rigid cell wall also plays a central role in determining the characteristic shapes of plant cells .Although cell walls were once viewed as relatively inert secretions of the cell they surround, more recent studies have revealed the wall to be a dynamic structure that carries out many activities. The cell wall also acts as a permeability barrier and plays a role in certain types of secretary and metabolic events.

Although cell wall provides a thick encasement for the plant cell, this barrier does not seal the cell completely from its surroundings. Plant cell wall usually contain small opening or plasmodesmeta, through which adjacent cells maintains direct contact with one another. In electron micrograph plasmodesmeta appears as a narrow channel in the cell wall that are linned by plasma membrane and often transversed by a tubular of ER. Thus the plasma membrane, cytosol and ER are all in continuity from one cell to the next. Plasmodesmeta tends to be concentrated in special areas of the cell wall called as PITS FEILDS, where the primary wall is thinner then the normal and secondary wall is absent.

The main focus of this topic is to learn about the connection between plant cells and how these connection differs from the those found in the animal cells.

MEMBRANE & WALLS

Cell maintains a selective barrier called the plasma membrane, a structure measuring 7-8 nm in thickness that constitute the outer membrane boundary of all living cells .By regulating the passage of material into & out of cell, the plasma membrane ensures that optimum conditions for living processes prevail within the cell interior .Plasma membrane also plays an important role in cell -to-cell communication, transmitting signals from the cell exterior to the cell interior. In plants a rigid cell wall is found directly outside the plasma membrane .The rigidity and of the cell wall provides the shape of the cell it encloses and protect the cell from adverse environmental condition . In addition to the presence of plasma membrane at the outer cell surface, membranes are also employed to partition the cell interior into multiple compartments which is exclusively found in animal and plant cells. An elaborated system of interconnected membrane channels and vesicles known as Endoplasmic reticulum (ER) plays an important role in transporting newly synthesised proteins to various destinations within the cell. Closely associated with ER is the Golgi complex, a stack of membranes involved in processing newly synthesised proteins and packaging them into membrane vesicles for storage and secretion. Several other membrane bound organelles also occurs in plants. Lysosomes are small membrane and enclosed structure that serve a digestive functions to breaking down foreign materials & intracellular constituents that are no longer needed by the cell. Peroxisomes carry out certain kind of oxidation reactions and vacuoles, which are especially prominent in plant cells, a large vesicle that functions   as storage compartments & in maintaining water balance. In addition to the preceding membrane -enclosed organelles, we will see shortly that membranes surround the genetic material and energy transforming organelles of plant cell

NUCLEUS & RIBOSOME

Cells utilize genetic information to guide the synthesis of most of the cell's component. This genetic information is stored in the nucleus surrounded by a double membrane envelope. 

The nucleus of plant cell is largely occupied by a mass of intertwined chromatin fibres, which contains the DNA molecules which stored most of cell genetic information .A small portion of nuclear DNA is localized in a small spherical structure known as nucleolus, which contains DNA information involved in the formation of ribosomes. 

A fluid like material called nucleoplasma fills the space around the chromatin fibres & nucleoli condense into compact structure known as chromosomes. The outer boundary of the nucleus is formed by two concentric membrane that together forms the nuclear envelope. Connections are observed between the outer membrane of the nuclear envelope & ER. 

The most distinctive structural feature of nuclear envelope is the presence of numerous nuclear pores .Nuclear pores help to regulate the flow of the material between nucleus & cytoplasm. Genetic information and encoded in the cells DNA guides the synthesis of specific protein molecule. 

This process of protein occurs on small cytoplasmic granules known as Ribosomes. Prokaryotic ribosomes are slightly smaller than eukaryotic. Eukaryotic ribosomes are found free in the cytoplasm and attached to the membrane of ER

  

MITOCHONDRIA & CHLOROPLASTS

Metabolic reactions occurs virtually everywhere in the cell, but those that are most central to the flow of energy are localised predominantly in the cytoplasm. Reactions involved in the initial breakdown of energy rich nutrients occurs in the cytosol, which is the fluid like portion of the cytoplasm that surrounds the cytoplasmic organelle. Once this initial process is complete, a second set of reactions occurs in which the most of the energy contained in the nutrients is released and used to drive the formation of energy rich molecule ATP. Mitochondria are large and enclosed by two membranes. The inner of the two membranes is folded into a series of CRISTAE that project into the internal cavity, or matrix, of the mitochondria. They are the site of many chemical reactions, the matrix also contains DNA and ribosomes. In addition to obtain energy from the breakdown of energy rich nutrients, plant cells are capable of trapping energy from sunlight & converting into chemical energy. Photosynthesis takes place in especialized organelle called CHLOROPLAST. Although chloroplast are usually longer than mitochondria, the structure of the two organelles exhibits some fundamental similarities. Like mitochondria, chloroplast are enclosed by two membranes surrounding an internal compartment, in this place designated the stroma. Within stroma are the thylakoid membrane, which contain chlorophyll and other light absorbing pigments. Like the matrix space of mitochondria, the stroma of chloroplast contain both DNA and ribosomes. 

 

 

CYTOSKELETAL FILAMENTS

Plant cells have developed an elaborate network of cytoplasmic filaments, such network of the filaments are called cytoskeleton. It consist of three distinct components-

(i)Microtubules- it is the largest filaments, which is rigid hollow structures measuring 25nm in diameter and contain tubulin protein. They are used in the construction of the mitotic spindle that moves chromosomes during cell division. 

(ii) Actin filaments or Microfilaments- generates movements within the cytoplasm. They measures 6nm in diameter and are constructed from actin. Actin filaments generate cytoplasmic streaming.                 

(iii) Intermediate filament- It is found between that of actin filaments and microtubules. Its protein composition varies, depending on the cell type. The role of intermediate filaments in motility is yet too established, but they play important role in structural support and in anchoring.