Prokaryotic and Eukaryotic Cells
The living world has two types of cellular organisations. In one type, the prokaryotes, there is no definite membrane bounded nucleus in contrast to the eukaryotes, which have a definite nucleus with a double membrane. The prokaryotic DNA (deoxyribonucleic acid) is naked and found as free strands in the centre of the cell-the nucleoid region. This type of organisation is commonly found in bacteria and blue-green algae (also called cyanobacteria). Blue-green algae and some bacteria, green sulfur bacteria and purple bacteria, are photosynthetic. But they lack definite chloroplasts. In fact, they have no membranous organelles except ribosomes, whereas a eukaryotic cell has a variety of organelles.
(i) Cell Wall
All bacteria and blue-green algae have cells enclosed by a rigid cell wall, like plants but unlike animals, due to which they have often been included in the plant kingdom. Cell walls are primarily protective in function but are quite different in composition and structural organisation in different organisms.(ii) Plasma Membrane
All living cells, both prokaryotic and eukaryotic, are enclosed by a differentially permeable membrane the plasma membrane. This membrane is a dynamic structure controlling the movement of substances into and out of the cell. Although all membranes are basically alike, being composed of phospholipids and protein, they differ in their finer structure because of the type of phospholipids and proteins and their interactions.
(iii) The Intracellular Membrane System.
Enclosed by the plasma membrane in a eukaryotic cell is the soluble phase, the cytosol, in which are embedded the various organelles and a profusion of membrane systems-the endoplasmic reticulum and the Golgi complex. The endoplasmic reticulum (ER) is a system of membranous channels, the form and extent of the development of which depends on the function of the cell. A cell synthesizing large amounts of protein generally has well developed endoplasmic reticulum with ribosomes attached. Such a system is called rough endoplasmic reticulum (RER) in contrast to the smooth endoplasmic reticulum (SER) which lacks the attached ribosomes.
The Golgi complex is another system of membranes inside the cell which is generally observed in the form of flattened sacs. It primarily acts as a processing and packaging centre for complex macromolecules like glycoproteins and glycolipids that may be incorporated in the plasma membrane or be exported out of the cell.
(iv) Nucleus
All eukaryotes possess a definite nucleus with a double membrane nuclear envelope interrupted by nuclear pores. In addition to the chromosomes it encloses one or more nucleoli embedded in the nucleoplasm the soluble phase of the nucleus. These nucleoli are the centres for the synthesis of ribosomal RNA (ribonucleic acid) whereas the nucleus as a whole is the control centre of all cellular activity, thereby controlling heredity.
(v) Chloroplasts
They are the centres of photosynthetic activity and, therefore, are found only in photosynthetic plant cells. Chloroplasts are bound by a double membrane, enclosing another system of membranes, the lamellae or thylakoids, and the matrix (Stroma). The photosynthetic pigments, chlorophylls and carotenoids are localised in the lamellae (thylakoids) and are responsible for the absorption of solar energy. This energy is used for synthesizing sugars which are converted into starch for storage.
(vi) Mitochondria
These are another double membrane bound organelles. Unlike chloroplasts, however, the inner mitochondrial membrane is thrown into folds to increase the surface area. The mitochondria are concerned with aerobic respiration, thereby releasing energy in the form of adenosine triphosphate (ATP) molecules and are present in all living cells except bacteria. Since they are concerned with energy production, they are often referred to as the powerhouses of the cell.
(vii) Microbodies
Microbodies generally refer to two classes of membranous organelles-the peroxisomes and the glyoxysomes which are often found closely associated with chloroplasts or the mitochondria. In a way, they are enzyme bags, since they contain a variety of oxidative enzymes acting on various substrates. The glyoxysomes also contain enzymes for fat metabolism and are abundant in germinating fatty seeds.
(viii) Ribosomes
Ribosomes are sites of protein synthesis and are present in both prokaryotic and eukaryotic cells. However, bacterial ribosomes are slightly smaller in size. Each ribosome, made up of ribosomal RNAs and proteins, consists of two sub-units which are associated in the form of a single particle at the time of protein synthesis.
(ix) Lysosome
Lysosomes are bags of hydrolysing enzymes that break down a large variety of substances which could serve as nutrients or raw materials for biosynthetic activity. They have been found occasionally to be digesting the whole cell or a part of it and, therefore, are sometimes referred to as "suicide bags."
(x) Centrioles
Centrioles are present in all animal cells and some lower plants. They are seen under the light microscope as a pair of small granules near the nucleus but the electron microscopic observation has revealed that they are cylindrical in nature, borne at right angles to each other. They are primarily concerned with the organisation of the mitotic spindle or the flagella and cilia (as basal bodies) in the flagellates.
