The Protist Way of Life
Unicellular eukaryotes make up most of the protist phylum. They are found almost anywhere there is water including within larger organisms (as parasites or symbionts). There is considerable variation in their anatomy, ecology, size and appearance.
Paramecium:
Paramecia are found in stagnant water feeding on the bacteria decomposing dead vegetation which has fallen in. Very fine hairs called cilia line the gullet and as these hairs move they suck in water and any bacteria floating in the water. This food is surrounded by a food vacuole, enzymes are poured in and the food is digested.
This organism has a fairly stiff outer covering called a pellicle, very like the cell wall of a plant cell. Cilia also cover this surface and move continually, causing the Paramecium to spiral rapidly through the water. These hairs can even beat in the opposite direction so that the organism reverses to avoid obstacles.
Oxygen enters through the pellicle and carbon dioxide leaves the same way. Wastes leave out of the anal pore. As Paramecium live in fresh water, water tends to flood in and if not removed the organism would burst. To prevent this, two contractile vacuoles gather the water and squirt it to the outside.
Paramecium can reproduce by splitting in two, producing identical offspring. Two Paramecium can also fuse together and swap small nuclei to reproduce sexually.
Chlamydomonas:
Chlamydomonas is found in still, stagnant water where it is often eaten by mosquito larvae. In oxidation ponds at sewage works, large numbers provide oxygen for the decomposers (at times there are so many the water can go cloudy green).
The firm cellulose cell wall keeps this alga in an oval shape. At the front ends are 2 flagella, which act like oars to propel the organism along. Contractile vacuoles collect water to expel out of the body of the organism. Most of the cytoplasm of this cell is taken up by the cup-shaped green chloroplast, which even covers the nucleus a lot of the time. Near the top of the chloroplast is an eye spot, which is clear like a lens and coloured orange.
Chlamydomonas is a plant so it makes food by photosynthesis. The CO2 is absorbed directly across the cell wall from the water in which the organism lives. The chloroplast uses light energy to make sugars from the CO2 and H2O and then the pyrenoid turns the sugars into starch. It is, however, an unusual plant because it can move from place to place. It is very similar in structure to the male sex cells of some more complicated algae such as Neptune’s Necklace.
Euglena:
These organisms are found in both fresh and salt water, and in damp soil, especially in areas with a high concentration of organic nitrogen such as farmyard puddles.
Euglena photosynthesises in the light with the aid of large chloroplasts, but in the dark it can feed on soluble organic nutrients, taken in by the gullet. A contractile vacuole maintains water balance.
Euglena swim towards the light which they detect with an orange eye spot containing a light-sensitive pigment, carotene. A large flagellum is used as a whip to pull the organism through the water and the pellicle can also contract and relax like a worm to provide another form of locomotion.
In favourable conditions Euglena reproduces by binary fission. In times of drought cysts form and can survive until conditions improve.
Amoeba:
These organisms inhabit slow-moving freshwater, especially mud at the bottom of ponds. Some live in the soil, one lives in human intestines and causes dysentery, and another is found on the surface of teeth and gums and is harmless.
Amoeba are most unusual because of the way they move. While most one-celled organisms stay pretty much the same shape, Amoeba shape changes from minute to minute, like a moving blob of granular, colourless jelly. Amoeba moves by pushing out projections of the cytoplasm, called pseudopodia (“false feet”), rather like a rubber tube which is continually being turned inside out. Amoeba tend to move away from light and carbon dioxide and towards oxygen and food.
The cytoplasm has a clear outer region (the ectoplasm) and a granular inner area (endoplasm) and the whole organism is surrounded by a cell membrane. The nucleus is large and central. Other substances in the cytoplasm may be droplets of fat, starch or wastes.
Amoeba are the predators of the microscopic world, preying upon other one-celled organisms like Paramecium & Chlamydomonas. To take in this food, the Amoeba’s body surrounds it, forming a food vacuole. Enzymes are poured in and the organism is digested.
Amoeba gets its oxygen from the water in which it lives - it just comes in through the cell membrane. Likewise, carbon dioxide exits the same way. As Amoeba live in fresh water they also have a contractile vacuole to deal with inflooding water.
Amoeba mostly reproduce by splitting in half. In extreme environments, Amoeba can survive as cysts, which hatch into live Amoeba when conditions improve.
Protist Way of Life
1. List the four organisms in order of size (smallest first).
2. Some of these organisms are autotrophic and some are
heterotrophic. [Check in your textbook if you don’t remember
what these mean. It would be a good idea to write these
definitions down as you do need to know them]. In a table
compare the nutrition and movement of these 4 protists (you
are expected to use these words!).
3. Describe the structure and explain the function of each of
these:-
A. Cilia
B. Flagella
C. Contractile vacuole
D. Pseudopod
E. Eye spot
F. Oral groove
G. Anal pore
4. Explain why an autotroph might need an eye spot.
5. Explain why it is difficult to classify some of these organisms as animals or plants.
6. Give a good labelled diagram of one of these (only label the
structures, not their functions). Don’t forget to give a title.