Structural Organization of Plant Cell!
So far as the structural organization is concerned, all organisms belong to either of the two categories — the prokaryotes, including bacteria and blue-green algae, and the eukaryotes, comprising all other plants and animals. The French microbiologist Edward Chatton first used these terms in 1928.
Prokaryotes (pro: before; karyon: nucleus) lack a membrane-bound definite nucleus. The cellular contents are differentiated into two distinct regions, the cytoplasm and the nucleoid, the latter consisting of the nuclear material containing fine DNA fibrils. The prokaryotic cell size ranges from 1-10 µm.
The usual components occurring in prokaryotic cells are — the plasma membrane, ribosomes, nucleoid and the ground substance, the cytosol. Excepting the mycoplasma, a cell wall surrounding the cell membrane is usually present in all prokaryotic cells. Mitotic division is not found in prokaryotes and they divide by binary fission or other means.
In the eukaryotic (eu: true; karyon: nucleus) cell, on the other hand, the nuclear matter is bound by a double membrane called the nuclear envelope. The cell size ranges from 10-100µm.
Eukaryotic cells divide mitotically involving the, microtubules. The cytoplasm is compartmentalized by membranous systems. These two fundamentally different kinds of cellular organization have been recognized only from the early 1960s.
The eukaryotic plant cells are of many types and can be divided into two broad groups — the living cells which are responsible for performing all the metabolic activities of the plant, and the dead cells which are metabolically inactive and function either as mechanical support or for conduction of fluids throughout the plant body.
So, a living parenchymatous cell is the convenient choice for the discussion of a typical plant cell.
A typical plant cell consists of two primary components, the cell wall and the protoplast. The cell wall completely surrounds the protoplast and provides mechanical support for it.
It is composed of two layers; the outer layer is called the primary wall whilst the inner layer, nearer to the protoplast and usually composed of several sub layers, is known as the secondary wall. The cementing substance in between the primary walls of adjacent cells is known as the middle lamella.
The protoplasmic mass inside a single cell is called the protoplast. So it is the unit of protoplasm and is the metabolically active component of the cell. It consists of three parts, the cytoplasm, the nucleus and the vacuole. The fluid mass which resides immediately beneath the cell wall and surrounds the vacuole, is the cytoplasm.
The young cell contains very small vacuoles and as the cell enlarges, the vacuoles coalesce to form a large vacuole. With the increase in cell size the protoplasmic content does not increase in proportion to the vacuole size and as a result, a thin cytoplasmic layer surrounds the vacuole. This thin layer is known as the primordial utricle.
The vacuole is bound by a membrane known as the tonoplast separating the vacuolar sap or cell sap from the cytoplasm. The cytoplasm in its periphery is again bound by a membrane known as the cell membrane or plasma membrane.
The soluble portion of the cytoplasm in which the cell inclusions remain embedded, is known as the cytosol. The extensive membranous system which traverses the entire cytoplasm is called the endoplasmic reticulum (ER). It consists of a ramified network of interconnected, membrane-bound, tubules and vesicles.
The nucleus is the largest particulate inclusion, which is spherical with a diameter in the range 6-8 µm. A perforated double membrane surrounds the nucleus. This membrane is continuous with the ER membranes. The nuclear membrane surrounds a network of chromatin matter known as chromatin reticulum which remains embedded in a nuclear sap known as the karyolymph.
Also, there is a dark spherical body attached to the reticulum known as the nucleolus which is responsible for ribosome formation. The nucleus is an autonomous structure. It controls all the cell functions.
Plastids are the second largest semi-autonomous cell inclusions enclosed by a double membrane. They are confined to the plant kingdom. They may be coloured or colorless. They are typically lens-shaped with a diameter of up to 10 µm. They are present to the extent of 200-400 per cell.
Essential organelles occurring in all eukaryotic cells, are the mitochondria which are more numerous and somewhat smaller and serve as the centres of intracellular oxidation. They are generally spherical to ellipsoid, and are commonly about 1µm thick and 1-3µ long. Like plastids they are bound by a pair of membranes and are also semi-autonomous in function.
The Golgi apparatus is a component of the endomembrane system of the cell and has the property of being both particulate and membranous.
The endomembrane system of the cell consists of cisternae which are stacks of flattened vesicles, Golgi bodies or dictyosomes are closely associated with ER membrane and surrounded by tiny spherical vesicles, and Golgi apparatus. The inter-associated dictyosomes which function synchronously, form a Golgi apparatus.
The most numerous of all the subcellular particles are the ribosomes, occurring in the cytoplasm, either free or attached to ER membranes and nuclear membranes. They are also found in chloroplasts and mitochondria.
The ribosomes in the cytoplasm are slightly larger than those in the chloroplasts and mitochondria. They are more or less spherical in shape and made up of approximately equal amounts of RNA and protein and are actively involved in protein synthesis.
Micro bodies constitute another group of particulate inclusions. ‘Micro body’ is a morphological term for a distinct class of organelle. It includes peroxisomes and glyoxysomes, the former occurring in photosynthetic cells and the latter restricted to cells of the endosperm or cotyledons of fat-storing seeds. The micro bodies are roughly spherical and single-membrane structure with a diameter of 0.2 – 1.5 µm.
Another debatable particulate inclusions are the spherosomes which are restricted to the cells of lipid-storing tissue. They are also single-membrane spherical structures with a diameter of 0.4-3 µm.
All the living cells in higher plants are interconnected by thin strands of cytoplasm known as plasmodesmata. Typically they are cylindrical holes through the cell wall. Each hole is lined by a cylinder of plasma lemma, the lumen of which is filled with cytoplasm and it often contains a minute tubule, known as the desmotubule.
There are spaces around each living cell called intercellular spaces. Initially, in the meristematic region the cells remain in contact with each other in such a way that there are no intercellular spaces but during differentiation and development the primary walls separate at the corners, forming intercellular spaces.
In mature tissues these spaces become interlinked forming an Intercellular space system which ends at the stomatal openings. So through the stomatal openings the whole system gets air circulation by diffusion, ensuring an adequate supply of O2 and CO2 to the tissues for their metabolic activities.
The existence of plasmodesmata and intercellular spaces subdivides the plant body into two major compartments, known as the symplast and apoplast respectively. The former is the living part of the plant made up of the interconnected protoplasts bound by single continuous plasma-lemma.
The latter is the non-living part of the plant external to the plasma-lemma and composed of cell walls, the intercellular spaces and the dead cell lumens such as the xylem vessels. Both the compartments are utilized for the transport of materials through the plant.