General Biology/Classification of Living Things/Eukaryotes/Protists
Out of the six kingdoms, Protista is the most diverse. This is the kingdom of organisms with strange, atypical characteristics. In essence, this kingdom is designated for organisms which do not belong in any other kingdom. The majority of protists are microscopic.
Classification of Protists 
There are three phyla of protists, based on their type of nutrition.
1. Protozoa (animal-like protists) are heterotrophs that ingest or absorb their food.
2. Algae (plant-like protists) are autotrophs they get nutrition from photosythesis.
3. Slime moulds and water moulds (fungus-like protists) are also heterotrophs, like protozoa.
As heterotrophs, protozoa scavenge materials from their surroundings. Others are predators which actively hunt or ambush small organisms such as bacteria and other protozoa for a source of nutrition. Protozoa can be parasitic as well; they may live inside larger organisms, like humans. Most protozoa live as single cells, although a few form colonies.
Protozoa are generally difficult to identify due to their varied shape. They may appear as jelly-like blobs, spherical sunbursts, or a flattened leaf. Tiny blood parasites may be only 2 μm long. On the other hand, shell-covered marine may be 5 cm or more in diameter.
Furthermore, different protozoans have their own complex life cycles. The complexity has led certain organisms to be mistakenly classified for other species.
Nevertheless, protozoa can move, and so, they are classified based on their methods of locomotion.
Characteristics of Protozoa :
- About 30,000 species known
- About 10,000 species are pathogenic, including some of the worst human diseases
- highly variable in form and life cycle
- mostly unicellular
- range in size from 0.005 mm to 50 mm
- lack cell walls
they love environment and each other..............
Algae are much simpler than protozoa. They are aquatic and contain chlorophyll. Algae can exist as a single cell or as giant seaweeds 60 m in length. Formerly, algae were classified as plants but this was incorrect as algae lack parts of true plants: leaves, stems, roots, xylem, and phloem. Since algae belong in the kingdom Protista, algae is a broad term used to denote all aquatic eukaryotes which photosynthesise; algae can differ in size and shape as well.
There are six phyla of algae: chlorophytes (green algae), phaeophytes (brown algae), rhodophytes (red algae), chrysophytes (diatoms), pyrrophytes (dinoflagellates), and euglenophytes (euglenoids).
Chlorophytes resemble plants the most. Like plants, their cell walls contain cellulose and they store food in reserve as starch. Chlorophytes can be unicellular or multicellular. Most chlorophytes use flagellae for some locomotion.
Phaeophytes are nearly all multicellular marine organisms, which are known to us as seaweeds. They have cell walls composed of cellulose and alginic acid (a substance similar to pectin). The cellulose and alignic acid help to retain water and prevent seawood from drying out when exposed to air at low tide.
Since phaeophytes live in a tidal environment, they have large, flat fronds (a large leaf) which can withstand pounding by waves. Their bases strongly anchor the algae to the rocky seabed and prevent them from being washed out to sea. Phaeophytes are usually found in areas of cold water.
Rhodophytes are typically found in warmer seawater, and are more delicate and smaller than brown algae (phaeophytes). Rhodophytes are also able to grow at deeper depths in the ocean, since red algae absorb green, violet, and blue light, the wavelengths of which penetrate the deepest below the water surface. They also have mucilaginous material to resist drying.
Chryosophytes are the most abundant unicellular algae in the oceans. They are also one of the biggest components of plankton, a free-floating collection of microorganisms, eggs, and larvae. As photosynthetic organisms, they produce a significant amount of atmospheric oxygen.
The reproduction cycle of chryosophytes is particularly interesting. Note that diatoms reproduce both asexually and sexually. Since diatoms have a rigid cell wall with an outer layer of silica (found in sand and glass), the daughter cells produced by mitosis must fit inside the original cell wall. Therefore, each generation of diatoms is smaller than the one before. The reduction in size continues until the diatoms produce sexually, producing a zygote which eventually grows to the original size as it matures.
Pyrrophytes are unicellular, photosynthetic, and mostly aquatic. They have protective coats composed of stiff cellulose. They are more easily identifiable, due to the presence of two flagellae. The longer flagellae propels the dinoflagellate, while the second shorter, flatter flagellae functions as a rudder.
Some species of pyrrophytes are zooxanthellae. Since they lack cellulose plates, they make their home in coral reefs and animals, such as sea anemones, and molluscs. In returning the favour of sheltering them, dinoflagellates provide carbohydrates to their host through photosynthesis. This is why there are nutrient-rich coral reefs in malnutritions water.
A negative aspect of pyrrophytes is that under certain conditions, species of dinoflagellates reproduce rapidly to form a harmful algal bloom (HAB), known as a red tide if dinoflagellates are the cause. Such pyrrophytes can produce toxins which may injure or kill wildlife, and additionally any consumers of contaminated wildlife.
Like pyrrophytes, euglenophytes are small unicellular freshwater organisms with two flagella. They are mainly autotrophic or heterotrophic, depending if they have a red, light-sensitive structure called an eyespot.
Slime molds & Water molds 
There are two phyla of slime moulds and one phylum of water moulds.
Oomycotes (Water moulds) 
Oomycotes are filamentous organisms which resemble fungi, in that they live as saprotrophs. Oomycotes differ from other moulds with the presence of spores and their sexual life cycle.
Myxomycotes (Plasmodial slime moulds) 
Myxomycoties are visible to the naked eye as tiny slug-like organisms which creep over decayed and dead matter. This streaming blob containing many nuclei is called a plasmodium.
Acrasiomycotes (Cellular slime moulds) and its reproductive cycle 
Acrasiomycotes exist as individual amoeboid cells with one nucleus each. When in unfavourable conditions, each acrasiomycete cell gathers together to form a pseudoplasmodium.
Reproductive Cycle: 
1. One acrasiomycete cell joins with others to form a pseudoplasmodium.
2. The pseudoplasmodium shrinks and forms a smaller plasmodium.
3. The plasmodium migrates to a suitable environment.
4. The plasmodium develops a sporangia, where original parental nuclei has divided by meiosis into haploid spores to be germinated.
5. When favourable conditions arise, the spores germinate and are carried away by animals or the wind.
6. Cycle repeats.
Protists Practice Questions 
1. Which of the following adjectives describe the major food source of protozoa?
- a) chemoautotrophic
- b) photoheterotrophic
- c) chemoheterotrophic
- d) heterotrophic
- e) A, C, D
- f) C, D
2. The protozoan Giardia lamblia can inhabit a human body's intestinal tract and cause gastroenteritis.
a) Give the abbreviated binomial name of this protozoan.
b) Would the relationship between this protozoan and human being be mutualistic, commensalistic, or parasitic?
3. Found in many products, such as Petri dishes, agar is made from mucilagnious material in seaweed. Of the six phyla of algae, which phyllum/phyla would agar be made from?
4. Which of the following adjectives describe the major food source of Euglenophytes without an eyespot?
- a) photoautotrophic
- b) photoheterotrophic
- c) chemoautotrophic
- d) chemoheterotrophic
- e) B or C
- f) C or D
5. Can coral reefs exist in nutrient-poor areas? Explain.