Amoeba is a unicellular protist with a simple-looking body that can change shape as it carries out the activities needed for life. Its single cell contains structures that allow it to move, obtain food, reproduce, exchange gases, remove wastes, and respond to changes in its surroundings.
Getting to Know Amoeba
Amoeba is a unicellular protist. Although it consists of only one cell, that cell performs the activities needed for life.
It can live in freshwater and saltwater, soil, and moist parts of animals, where some species live as parasites. It is normally found creeping along while feeding on organisms such as algae and bacteria.
One of the most noticeable features of Amoeba is its changing shape.
Unlike organisms with a definite body shape, Amoeba has no fixed shape. Under a microscope, it appears as an irregular, jelly-like mass of hyaline protoplasm.
Its outline keeps changing as small finger-like extensions develop from the cell. These extensions are called pseudopodia, or false feet.
Several species of Amoeba are known. Examples include Amoeba proteus, Entamoeba histolytica, which causes amebiasis, and Naegleria fowleri.
The changing shape of Amoeba is closely connected to the way it moves and feeds. To understand how this happens, it is useful to first look at the structure of the cell.
The Structure of Amoeba
Amoeba has three main body parts: the plasmalemma or plasma membrane, cytoplasm, and nucleus.
These parts work together to allow the cell to carry out its different life processes.
The Plasmalemma
The plasmalemma, or plasma membrane, forms the thin and elastic covering of Amoeba.
It consists of a double layer of lipid and protein molecules.
The membrane is selectively permeable, so it regulates the exchange of substances between the cell and its surroundings.
Water, oxygen, and carbon dioxide can move through the plasma membrane.
The membrane therefore does more than cover the cell. It forms an important boundary between Amoeba and its environment.
Just inside this boundary is the cytoplasm.
The Cytoplasm
The cytoplasm has two main regions, the ectoplasm and the endoplasm.
The ectoplasm is the relatively firm outer region. It is thin, non-granular, and hyaline, with conspicuous longitudinal ridges. It is considered a supporting layer.
The endoplasm lies within the ectoplasm and forms the main body mass of the cell. It is a granular, heterogeneous fluid.
Within the endoplasm are two regions with different consistencies. The outer plasmagel is relatively stiff and granular, while the inner plasmasol is more fluid. The granules in the plasmagel do not show movement.
The endoplasm also contains several organelles and structures. These include the nucleus, contractile vacuole, and food vacuole.
Each contributes to the activities that keep the cell alive.
Among these structures, the nucleus has a particularly important role.
The Nucleus
The nucleus controls the life activities of Amoeba. A cell dies when it is deprived of its nucleus.
In Amoeba proteus, there is a single conspicuous nucleus.
Its appearance can change with age. In young specimens, it appears as a biconcave disc, while in older specimens it is often folded and convoluted.
The nucleus has a firm nuclear membrane, or nuclear envelope. Inside it is a clear achromatic substance containing minute chromatin granules distributed uniformly near the surface.
The amount of nucleoplasm is small.
This type of nucleus is described as a massive or granular nucleus.
With these structures in place, Amoeba is able to carry out the activities required for life. One of the most noticeable is movement.
How Does Amoeba Move?
Amoeba moves with the help of its pseudopodia.
This type of movement is called amoeboid movement. It is the most primitive form of animal locomotion.
The movement begins when the endoplasm streams toward a particular part of the plasma membrane. A bulge forms there and gradually extends into a long pseudopodium.
The cytoplasm then flows into the developing pseudopodium. As it moves forward, it draws the plasma membrane along with it.
The result is a gradual movement of the whole cell.
A moving Amoeba has two recognizable regions. The hyaline cap forms the anterior part, while the uroid forms the posterior part of the locomotive cell.
The pseudopodia that move Amoeba also play another important role. They help the cell obtain its food.
How Does Amoeba Obtain Food?
Amoeba has a holozoic mode of nutrition, and feeding takes place by phagocytosis.
The process begins when the cell comes close to a food particle, such as an alga or bacterium.
Pseudopodia extend around the food particle and gradually engulf it. The food is enclosed within the cell and forms a food vacuole.
Enzymes are secreted into the food vacuole, where they digest the food.
The nutrients produced during digestion then diffuse into the cytoplasm. The remaining wastes are expelled from the cell.
The same temporary extensions that allow Amoeba to move therefore also help it obtain food.
But obtaining food is only one part of staying alive. Amoeba must also produce new individuals.
How Does Amoeba Reproduce?
Amoeba reproduces asexually.
The way reproduction occurs depends on the conditions in which the cell finds itself.
When conditions are favourable, reproduction occurs by irregular binary fission. When conditions are unfavourable, Amoeba can undergo multiple fission or sporulation.
Binary Fission
Under favourable conditions, a fully grown Amoeba stops moving before division begins.
The nucleus divides first.
The cytoplasm then divides, separating the cell into two parts. The result is two daughter cells, which can grow into mature Amoeba.
Thus, one parent cell gives rise to two daughter cells.
When conditions become unfavourable, the reproductive process is different.
Multiple Fission and Sporulation
Under unfavourable conditions, Amoeba stops moving and forms a cyst around itself.
The encysted cell then divides many times. This produces many spores called pseudopodiospores, which remain contained within the cyst wall.
When conditions become favourable again, the cyst wall bursts and the spores are released.
The production of spores as a way of surviving unfavourable conditions is known as sporulation.
Reproduction in Amoeba is therefore closely connected to the conditions around the organism. The cell can produce two daughter cells when conditions are favourable and form many spores within a cyst when conditions become unfavourable.
While carrying out these activities, Amoeba also needs to remove substances that are no longer needed and control the amount of water inside the cell.
How Does Amoeba Remove Wastes and Maintain Water Balance?
Some waste materials leave Amoeba by diffusion through the plasma membrane.
These wastes include ammonia and carbon dioxide. They move from the cell into the surrounding environment.
Amoeba also has a contractile vacuole that collects and expels excess water.
This helps maintain the water and ionic balance of the cell. The process is known as osmoregulation.
The water discharged by the contractile vacuole also contains traces of metabolic wastes.
The plasma membrane and contractile vacuole therefore help Amoeba remove wastes and maintain the internal balance of the cell.
The plasma membrane has another important role. It is also the surface through which Amoeba exchanges gases.
How Does Amoeba Respire?
Amoeba exchanges dissolved gases with its surroundings through the plasma membrane by diffusion.
Oxygen diffuses into the cell, while carbon dioxide diffuses out.
Because the exchange takes place across the plasma membrane, Amoeba does not require a special respiratory organ for this process.
But survival involves more than feeding, movement, and waste removal. The organism must also respond to changes around it.
How Does Amoeba Respond to Its Environment?
Amoeba has no sense organs for perceiving stimuli. Even so, it is highly sensitive to changes in its surroundings.
When it encounters a mechanical obstacle, it gives a negative response and moves away.
It also moves away from salt, sugar, acid, and alkali to which it is unaccustomed.
Its response to light is also specific. Amoeba avoids darkness and strong light but shows a positive response toward normal light.
Temperature can affect its activity as well. Lower temperature retards its activity.
When environmental conditions become harsh, Amoeba can form a cyst.
The ability to respond in these ways allows the cell to react to changes in its environment even though it has no sense organs.
Seeing the Whole Amoeba
Amoeba may consist of only one cell, yet that cell can move, feed, reproduce, exchange gases, remove wastes, maintain its internal water balance, and respond to its surroundings.
Its structure makes all of these activities possible.
This is what makes Amoeba such a useful example when learning how a single cell can carry out the processes needed for life.
For teachers who want additional material to support this topic, the Amoeba Notes and Assessment can provide lesson material and assessment opportunities for students. The Protist Test 1 and Protist Test 2 can also be used to assess students’ understanding of Amoeba, together with Paramecium and Euglena.
Amoeba shows one way in which a single cell can be organized to support life. But other unicellular protists use very different structures to carry out similar activities.
Amoeba changes its shape and moves with pseudopodia. Paramecium, in contrast, has a more definite body shape and moves with numerous cilia covering its surface.
Next: Paramecium: Structure, Reproduction, and Other Life Processes
In the next post, we will examine Paramecium more closely, including its structure, asexual and sexual reproduction, nutrition, movement, excretion, respiration, and response to its surroundings.
Protists Resources
cool
Magnificent site. Lots of useful info here. I am sending it to some friends ans also sharing in delicious. And of course, thanks for your sweat!
I really like what you guys are usually up too. This type of clever work and exposure! Keep up the superb works guys I’ve added you guys to our blogroll.
This article was extremely interesting, especially since I was searching for thoughts on this subject last Thursday.
Good work
Thank you Patricia
This app is very good and useful
Thanks Prince, I am glad to hear that!