How Does Amoeba Move? – (Locomotion & Movement in Amoeba)
Amoeba is actually regarded as the lowest and simplest form of animal in the whole Animal Kingdom.
Their body is like a small spot of protoplasm resembling a tiny drop of jelly that has all the cell organelles to perform the movements and other mandatory activities.
Amoeba moves and feeds freely with the help of false feet or pseudopodia, which is formed as a result of the streaming flow of the cytoplasm. With the help of pseudopodia, it shows amoeboid movement which is the most primitive kind of animal movement.
Amoeba shows amoeboid movement by the formation of finger-like temporary projection from its cytoplasm and this is known as pseudopodia (false feet).
Those kinds of pseudopodia that are broad with rounded and blunted tips being composed of both ectoplasm and endoplasm or only ectoplasm are called lobopodia. These are formed due to the slow forward flow of the cytoplasm.
Amoebae are mostly aquatic and are widely distributed and commonly found on the bottom mud or on the underside of aquatic vegetation in freshwater ponds, ditches, lakes, springs, pools, and in slow-running streams as well.
So, they have a kind of body and movement style that is streamlined for its free flow in water.
What Amoeba uses to move?
Amoeba uses its false feet which are also known as the pseudopodia (singular: pseudopod) to move and locomote.
Each pseudopod is a temporary arm-like projection of the Amoeba’s cell membrane along with its cytoplasm that is developed in the direction of movement.
These pseudopodia are portions of the cytoplasm, that primarily consists of actin filaments with microtubules and intermediate filaments as well.
These are generally used for body motility and ingestion of food into the cell.
Pseudopodia or false feet can be of 4 different types: filopodia, lobopodia, rhizopodia, and axopodia. Amongst these 4 types, Lobopodia are the blunt type and is the most common type of Pseudopodia among parasitic amoebae.
Generally, for the purpose of locomotion and movement, various pseudopodia arise from the surface of the body, for example in Amoeba proteus. Or, a single pseudopod may form on the surface of the body, for example in Entamoeba histolytica.
Amoeba may put out several pseudopodia but eventually progresses its movement with one broad anterior pseudopodium known as the Lobopodium.
Amoeba with the help of its lobopodium (a type of pseudopodium or false feet) moves with an average speed of one micron per second only and that’s actually a very slow movement.
Cells that make pseudopodia are generally referred to as amoeboids and the movement with pseudopodia as an amoeboid movement. And so, amoebae are said to use amoeboid movement to move from place to place.
The mechanism of movement of pseudopodia (false feet) in Amoeba
1. Due to extracellular stimuli
The movement of Amoeba is usually done due to chemotaxis which is an extracellular stimulus to the amoeba. It is actually the movement of amoeba, in a direction corresponding to a gradient of increasing or decreasing concentration of a particular substance.
The amoeba’s cell membrane is able to sense the extracellular signaling molecules called chemoattractants (e.g. cAMP) to extend pseudopodia at the cell membrane area facing the source of these molecules.
As the pseudopodia will move forward, the chemoattractants present in the medium will bind to the G-proteins-coupled receptors of the cell membrane thus activating the Rho GTPase molecules embedded in the inner wall of the cell membrane.
These activated Rho GTPase molecules will further activate the WASp (Wiskott–Aldrich syndrome protein) which will in turn activate the Arp2/3 complex.
Arp2/3 complex is actually a seven-subunit protein complex that plays a major role in the regulation of the actin cytoskeleton of the amoeba’s cell.
Thus, when the Arp2/3 complex gets completely activated, it will now cause actin polymerization that will push the cell membrane of the amoeba to grow, forming the pseudopod.
The pseudopodium can then adhere to a surface via its adhesion proteins (e.g. integrins), and then pull the cell’s body forward via contraction of an actin-myosin complex in the pseudopod.
Rho GTPases can also activate phosphatidylinositol 3-kinase (PI3K) which recruits PIP3 to the cell membrane at the leading edge and detach the PIP3-degrading enzyme PTEN from the same area of the membrane. PIP3 then activate GTPases back via. GEFs (Guanine nucleotide exchange factors) stimulation. This causes the formation of various pseudopodia on other sides of the cell membrane in amoeba as well.
2. Due to non-extracellular stimuli
It has also been seen that, in the case, when there are no extracellular stimuli, the amoeba navigates in random directions, but it will surely keep the same direction for some time before turning.
This feature is advantageous as it allows the cells to explore large areas over the medium for colonization or searching for new extracellular stimuli.
It will simply crawl accomplishing the protrusion of cytoplasm of the cell involving the formation of pseudopodia (false-feet) and posterior uropods.
Amoeboid locomotion due to both non-extracellular and extracellular stimuli represents one of the most widespread forms of cell motility and constitutes the typical way of locomotion in a broad range of adherent and the suspended eukaryotic cell types.
How Does Amoeba Move: 5 Theories Explaining The Locomotion & Movement in Amoeba
How does amoeba move? How does the movement due to pseudopodia occurs? How the cytoplasmic flow is effected and the pseudopodia are formed?
Rosel von Rosenhof first observed the amoeboid movement in 1755, but during those times the reason for such a kind of movement was not known.
Since the first observation of amoeboid movement, many researchers have put forward many theories to explain these above mentioned queries.
A brief description of 5 of these theories are provided below:
1. Contraction-Hydraulic Theory
This theory was stated by Schultze in 1875.
He was on the view that the ectoplasm of amoeba undergoes contraction at the posterior end causing protoplasmic currents to flow forward, with these the pushing of the fluid-like endoplasm forward also occurs.
Thus, with the forward pushing of ectoplasm and endoplasm results in the forward growth of pseudopodium and propelling the amoeba ahead in the medium.
2. Surface Tension Theory
This theory was stated by Berthold in 1886. This theory is also supported by Rhumbler and Butshi (1898).
This theory states that pseudopodium is formed by an outflow of the protoplasm, when the surface tension at a weak point between the amoeba’s body surface and the substratum is reduced.
It states that the amoeba is fluid like and moves due to the difference between the physical characteristics of body surface and the substratum.
This theory is presently not supported at all, it’s because, it states that Amoeba has a liquid body surface, but the fact is that Amoeba’s outer body surface is actually rigid and gelatinized.
3. Rolling Movement Theory
This theory was stated by Jennings in the year 1904 with reference to Amoeba verrucosa.
This theory states that the amoeboid movement takes place due to the rolling movements of the body accompanied by the streaming movements of the protoplasm, which propels the Amoeba’s body forward in the medium.
Jennings observed in Amoeba verrucosa that a carbon particle on amoeba’s upper surface first passes forward and then turn downwards along the anterior tip, remains on the lower surface for a time as the body keeps on rolling forward, and then passes upward at the posterior end to repeat the cycle.
This theory is not universally accepted to all the species of Amoeba as it is known that a few like Amoeba verrucosa is devoid of pseudopodia.
4. Walking Movement Theory or, Contraction Theory
This theory was stated by Dellinger in the year 1906 with reference to Amoeba proteus.
This theory states that the contractile vacuole present inside the cytoplasm of the Amoeba’s cell is mainly responsible for the amoeboid movement showing a walking-like movement.
Dellinger examined an Amoeba proteus not from the top, but from the side view, and explained how the pseudopodia become attached to the substratum and then due to the contraction of the contractile vacuole, the body gets pulled forward.
Thus, the animal is pulled from in front and pushed from behind due to contractions of a contractile vacuole. In this way, the Amoeba actually walks putting one foot out, then another.
5. Sol-Gel Theory (This is the most widely-accepted theory)
This theory was first put forward by Hyman (1917) and later supported Pantin (1923-26) and mast (1925).
This theory is the most widely-accepted one, and it states the amoeboid movement due to a change in the consistency of the cytoplasm and provides the best explanation of amoeba’s movement and locomotion than any other theories.
According to this theory, the cytoplasm of Amoeba is divided into ectoplasm and endoplasm. Endoplasm is further divided into Plasmagel & Plasmasol.
According to this theory,
- First, the amoeba attaches to the substratum using its plasmalemma.
- Second, gelation of plasmasol at the anterior end of the amoeba occurs causing the further advancement of pseudopodia.
- Thirdly, solation of plasmagel at the posterior end occurs causing the receding of the pseudopodia.
- At last, contraction of plasmagel occurs at the posterior end to drive the plasmasol forward.
- The overall process is repeated in a cycle, again and again, to cause the continuous movement of Amoeba.
As the plasmasol changes into plasmagel at the anterior end, the plasmagel tube extends forwards and is converted into plasmasol at the posterior end, the plasmagel tube drives the plasmasol forwards to form a pseudopodium.
Pseudopodia are formed because plasmagel is elastic and under tension, it is pushed out where the elastic strength is the lowest.
Is Amoeba the simplest living organism?
Afterall, what do you think? Is Amoeba the simplest living organism?
The answer is, ”Big YES!“. Yes, Amoeba is the simplest living organism on earth and why they won’t, because, they have all the living mechanisms of the body like reproduction, excretion, growth, feeding, digestion, movement, etc. performed by one single cell as they are unicellular.
Amoeba is a unicellular, microscopic animalcule that simply measures about 250 to 600 microns in maximum diameter only.
They are the most popular, simple, and free-living available protozoan. And, it is regarded as the lowest form of all animals as they can perform all the essential living tasks, and yes it’s all with the help of a single cell.
The body just represents like a tiny blob of jelly and nothing else, and yet, it contains all the cellular equipments required by the organism to perform all the vital functions of life, such as movement, reproduction, excretion, respiration, etc.
And yes, it is so much of a super simple animal that it has no shape, or rather it’s shape is constantly changing.
