Kingdom Protista consists of many unrelated groups, as a result their external most membrane may be a cell membrane, cell walls or pellicle as their outer covering.
Protists are composed of varied types of unicellular, eukaryotic organisms that are grouped together under a kingdom because of their un-relatedness to all the other kingdoms under the domain, Eukarya.
Do protists have a cell wall or cell membrane?
Protists can be animal like, plant like or fungus like. Depending on their mode of nutrition, they can have different type of external most coverings.
Animal like protists: They usually lack a cell wall but do possess cell membranes. Some examples are Amoeba, Paramecium, Plasmodium, Trichomonas and Trypanosoma.
Fungus like protists: Not all fungus like protists possess a cell wall. In slime moulds, cellulose like cell wall is present only during specific stages of life cycle. Cell wall of water moulds are composed of cellulose and glycans. Their cell wall composition is the main point of difference between moulds and fungi.
Plant like protists: Plant like protists such as algae, dinoflagellates, diatoms and charophytes, may possess cell walls composed of cellulose, polysaccharides, calcium carbonate, silica and other proteins.
Which protists has a cell wall?
In general, plant and fungi like protists possess cell wall composed of different types of repeating units.
Cell walls of diatoms are composed of silica. The monomers of silicic acid undergo polymerisation to produce silica polymers which get secreted out to form the cell wall. As, a diatom is composed of two different cell halves such that one fits within another, their cell walls are also referred to as frustules or tests. They may possess another layer over the call wall, composed of pectin or other organic compounds.
They are green algae or sea weeds that grow huge. Their cell wall, composed of D-xylose and pectin, tends to extend into the cytoplasm of the cells.
Usually all the species of Volvox live in colonies which act as a single unit. These cells form a ball like hollow structures that float on the water surface. It is so because their cell walls form intercellular bridges between the cells. Also, the colony is covered within the gelatinous extracellular matrix which is composed of glycoproteins.
These are filamentous algae that are formed of microscopic unicellular organism. They possess two layers of cell water, of which the inner cell wall is composed of cellulose while the outer one is composed of pectin. These filamentous colonies are also covered within a layer of mucilage.
Also known as brown algae, they have cell wall composed of hemicellulose, glucuronic acid, mannuronic acid and rhamnose. The presence of rhamnose and the uronic acids play a major role in preventing the charophytes from desiccation, even though they are aquatic organisms, by forming a mucilage layer around the cells.
Why do Protists have cell walls?
As, not all protists contain a cell wall, the ones that do possess it, do so because they strongly resemble with the plants or fungal cells which possess cell walls too. Autotrophic organisms do not need to move around in search of prey. Instead, they can lead a sedentary lifestyle and photosynthesise under the sunlight to obtain energy.
Leading a sedentary lifestyle requires for more protection from the pathogens, for a stronger hold onto the surface to which the organism is attached and for a rigid structure that could withstand harsh changes in environment. Hence, the cell wall came to exist and evolved according during the course of evolution.
Protists cell wall characteristics
- Protistic cell walls can be composed of diverse polymers as it is constituted of diverse unrelated organisms.
- Cell walls provide a barrier to the cell against different molecules and pathogens as it acts as the first line of defence against them.
- They are responsible for preventing the cells from wear and tear, desiccation and other environmental stress.
- They are also responsible for maintaining the size and the shape of the cells.
- Cell walls provide turgidity to the cells and thus prevent them from bursting out or from dying in response to the change in the osmolarity.
- They also allow some colonial organisms to act as a single unit.
Is Protists cell wall permeable?
Major function of a cell wall in any organism is to provide an extra barrier to the cell and regulate the passage of molecules in and out of the cell. So, cell walls of protists are permeable in nature, although the method by which the materials pass in and out of the cell depends upon the type of the cell wall present. For example, in diatoms, the slits and pores present in the cell wall are involved in the producing mucilage and disposal of waste materials.
Do Protists cells have a chloroplast?
Photosynthetic protists possess chloroplast and show autotrophic mode of nutrition but some genus can also show mixotrophic mode nutrition.
Plant like protists such as algae, euglenoids, diatoms and crysophytes, generally possess chloroplasts, although the type of pigment present in it may vary from one species to another.
On the other hand, organisms that are animal like or fungi like protists, do not usually possess chloroplast as they generally show heterotrophic modes of nutrition.
Do all Protists have a cell wall?
Plant- and fungi-like protists may possess cell walls that are similar to that of the plants, structurally and functionally. But the animal like protists do not usually possess a cell wall. Some of the plant like protists lack cell wall as well and genus of fungi like protists possess a cell wall only for a short duration in their life cycle.
Which protists have no cell wall?
Animal like protists such as Amoeba proteus, Paramecium spp., Plasmodium, Trichomonas and Trypanosoma do not possess cell wall just like how the animal cells are devoid of it as well. They can be broadly classified into amoeboid, flagellates, ciliates and sporozoans. Although all plant like protists possess a cell wall, there are some exceptions.
They are photosynthetic green algae that lack a cell wall but possess a pellicle. Pellicle is a proteinaceous structure which acts like an elastic membrane that covers the whole body of the protozoan cells including Euglena. It is present below the cell membrane and surrounds the cytoplasm, giving the cells a specific shape and helping in mobility. Hence, Euglena is a plant like protist due to the presence of the chloroplast while the presence of pellicle in place of cell wall makes it similar to heterotrophic protozoans or unicellular eukaryotes.
Why do protists not have cell walls?
Absences of the cell walls in some protists is due to their evolutionary history and their phylogeny. Especially in protistic cells resembling an animal cell, the cell walls are absent. During the course of evolution, plants and other organisms resembling plants required to have a rigid structure in order to face the surrounding environment as they have a sedentary lifestyle.
On the other hand, animals have evolved to have a mobile lifestyle where they are adapted to either swim, crawl, hop, walk, run and fly in different environmental conditions. Higher animals can even show more than one type of movement. As a result, animal like protistic cells lack a cell wall. They do not need a rigid cell wall for the kind of life they live. For example, amoeba moves with the help of pseudopodia which would otherwise have been impossible to achieve if they had a rigid cell wall covering the whole cell.
Kingdom protista is composed of diverse categories of unicellular organism that may or may not possess cell walls.