In this article we will discuss about the morphology of amoeba.
Shape and Size:
Amoeba is a unicellular microscopic animal about 0.25 to 0.6 mm in diameter. Under microscope, it appears to be transparent, irregular and colorless mass of protoplasm which constantly changes in shape by giving out finger-like processes called pseudopodia. The animal is without any firm cell-wall but the thin delicate outer membrane is called the plasmalemma. Just beneath this is non-granular ectoplasm and inner granular endoplasm. However, there is no line of demarcation between ectoplasm and endoplasm.
Plasma Membrane or Plasmalemma:
There is very thin, delicate plasma membrane or cell membrane being from 1/4 micron to 2 microns thick. It is composed of a double layer of lipid and protein molecules. This layer allows water and some small solute molecules to pass freely in both directions while the longer particles are not allowed to pass through. It is sufficiently firm, but elastic.
Hence it neither allows the protoplasm to flow away, nor hinders body growth and formation of pseudopodia. It is extremely regenerative, if injured, it regenerates fast enough to prevent any loss of protoplasm. It is also capable of invaginating into the body, if drop of fluid is taken in it is called pinocytosis and if solid particle, then phagocytosis.
Pseudopodia (Gr. pseudos, false; podo, foot) are temporary finger-like blunt processes of the cell body. The pseudopodia of A. proteus are fairly large and tubular with rounded tips. Such pseudopodia are known as lobopodia and are composed by ectoplasma as well as endoplasma.
The pseudopodia move by pressure flow mechanism and help the animal in locomotion and food ingestion. In actively moving amoebae new pseudopodia can be seen being formed at forwarding direction, while the older ones are in the process of withdrawal at hinder end, marking it posterior end for the time being which is called uroid.
The cytoplasm of amoeba consists, as in an animal cell, several types of formed structures suspended in a continuous fluid phase, called cytoplasmic matrix, hyaloplasm or cytosol. The letter is differentiated into an outer thin cortical layer, called ectoplasm, and an inner medullary mass, called endoplasm.
Ectoplasm lies immediately beneath the plasmalemma. It is thin, dear and transparent. It is rigid, contractile and under tension. It is thickened into a hyaline cap at the advancing end of the tips of pseudopodia.
The ectoplasm has a number of conspicuous longitudinal ridges. It is considered as a supporting layer and lends form to the cell body. This is itself differentiated into an inner stiff layer of granular, viscous and jelly-like cytoplasm called plasma gel.
Endoplasm—forms the main body mass completely surrounded by the ectoplasm. It is granular, heterogenous fluid containing bi-pyramidal crystals forming the plasmasol, streaming movements, called cyelosis, occur all the time in it. According to Mast, the endoplasm occurs in two colloidal states.
The peripherial viscid or gel state is known as plasmagel and the central flowing or sol state is termed as plasmasol. The plasmagel forms a tube through which flows the plasmasol. The two colloidal states of endoplasm are interchangeable. The plasmasel is highly granular fluid having various inclusions and it shows streaming movements. Besides granules, endoplasm contains a number of inclusions such as nucleus, contractile vacuole, food vacuoles, mitochondria, Golgi complex, fat globules and plate-like crystals.
The nucleus in Amoeba proteus is practically always found in the plasmasol. Usually it is somewhere between the middle and the posterior end, but sometimes it may be seen in the very tip of an actively extending pseudopod. Now and then it becomes attached to the inner surface of the plasmagel and remains stationary for a time, while the posterior end moves forward.
It is set free due to the liquefaction of the plasmagel when it reaches the posterior end or is separated off by the action of the streaming plasmasol. Thus it is being continuously carried back and forth, occupies no fixed position. It differs in shape in different species. It is biconcave, biconvex, oval discoid. If it is removed by micro-dissection, the protoplasm may live without it for some time, but shall remain incapable of carrying out all it vital activities.
It has a firm nuclear membrane and contains a clear aeromatic substance with minute chromatin granules distributed uniformly near the surface. The nucleoplasm is small in quantity. Such a nucleus is called massive or granular nucleus. The surface below the nuclear membrane is a honey comb-like lattice.
This consists of a layer of hexagonal membranous tubes extending into the nucleus from the inner nuclear membrane where each tube surrounds a nuclear pore. The nucleoplasm contains few nucleoli and about 50 chromosomes with coiled DNA.
A single contractile vacuole, generally seen in ectoplasm near trailing end of body even under light microscope, is the most obvious cytoplasmic inclusion in fresh-water amoebae. It appears like a bubble of clear, watery fluid, enclosed within a delicate and elastic condensation membrane resembling plasmalemma.
Mitochondria are often seen aggregated around it. Its function is osmoregulation- excess quantity of water regularly collects into it from cytoplasm and is periodically discharged out by it. Obviously, it is a pulsatory organelle, rhythmically dilating and contracting at short intervals. As it grows, it comes to the peripheral plasmagel and is left at the posterior end as the endoplasm flows onwards.
At the posterior end, it bursts by contraction of its wall and the contents are forced out. The mitochondria surrounding the vacuole provide energy for the formation and working of the vacuole. The vacuole removes CO2 and waste products from the animal. It is not only excretory and respiratory in nature but the important function is that it acts as hydrostatic organ as it constantly removes water which the animal absorbs and thus regulates the osmotic pressure and harmonizes the tension between the protoplasm and the surrounding water.
A number of food vacuoles are scattered in endoplasm. Each food vacuole contains a morsel of food under digestion. These are formed when Amoeba engulfs food with a drop of water. Digestion of food takes place inside these vacuoles. These are carried about by the movement of the endoplasm and finally disappear with the egestion of non-digestible food from the holy.
These are perfectly transparent, colourless and non-contractile, spherical spaces filled with water. Electron microscopy does not reveal the two colloidal phases, sol and gel, in the endoplasm. The nucleus of A. proteus shows structure like a honeycomb lattice. Nuclear membrane is shown as a double membrane intercepted by pores.
The nucleoplasm contains a few nucleoli and a large number of chromosomes (about 500) with helically coiled DNA. Various organelles, characteristic of an animal cell, are also revealed. The endoplasmic reticulum forms a network of tubules as well as vesicles. The ribosomes occur scattered as well as on some endoplasmic vesicles.
The Golgi bodies appear as groups of sac-like tubules. The mitochondria are more or less oval with tubular cristae. The lysosomes occur scattered as minute membrane-bound spherical bodies. Endoplasm abounds in reserve food material in the form of plate-like or bi-pyramidal crystals.