SAMPLE DESCRIPTIVE LAB REPORT
Examination of Protozoan Cultures to Determine Cellular Structure and Motion Pattern
Protozoans are unicellular eukaryotes with either plant- or animal-like characteristics. Through careful observation, we analyzed various protozoan cultures in order to identify characteristics associated with cell structure and movement of these one-celled organisms. We found that Protists exhibit certain characteristics that allow them to be categorized into different groups, mainly determined by their locomotion patterns. Despite differences in locomotion and the varying plant-like and animal-like organelles, all protists share key characteristics and functions that allow them to feed, grow, and reproduce--processes essential for survival and common to complex organisms.
Unicellular eukaryotes belong to the kingdom Protista, and are often referred to as “protists” or “protozoans.” The name “protozoan” means “first animal,” but eukaryotes may display either plant or animal-like characteristics, or a combination of both. Although unicellular, they have a nucleus and membrane-bound organelles, making them functionally complex despite their small size. Each small protist is a self-supporting unit, carrying out all the processes for survival in just one cell. They thrive on moisture and can be found on moist soil and in fresh and marine bodies of water. There are about 30,000 known species of protozoans, commonly classified according to their movement patterns as sarcodines—moving with false feet called pseudopodia or, flagellates—moving with whip-like structures known as flagella, ciliates—moving with short hairs known as cilia, and sporozoans—with no movement. They all have varying shapes, sizes, and survival strategies. For example, some may “hunt” small particles of food such as bacteria or algae; whereas others may be parasitic, inhabiting larger organisms. Despite their differences, all protists have several characteristics in common. In addition to a nucleus or nuclei to house their genetic material, most protists have mitochondria for metabolic functions, and vacuoles for digestion and excretion. With the help of these and other cellular structures, protists may feed, grow, and reproduce.
In this lab we observed select examples of protists in order to identify their cellular structures, and determine to which group of protista they belong based on their form of movement. We also made drawings of our observations using light and dissection microscopes to practice proper microscopy skills, including making wet-mount slides and cell sizing. By observing, drawing, and classifying protista, we learned about the cell structure and movement patterns of these one-celled organisms. We also learned about the differences and similarities of various protist cells .
Since we will observe how protists move, it will be interesting to figure out patterns of locomotion. For example, what happens when the protist encounters an obstacle? Does motion change when the organism is feeding? How does motion relate to where the organism lives? What characteristics do the protists exhibit: plant, animal, or both? Do the plant/animal characteristics influence motion patterns?
Three protists were chosen for observation. See the list of protists below to choose three samples. For each of the protists, a pipette was used to extract a few drops of culture from the culture jar. The drops of culture were placed on a clean microscope slide and covered with a slide cover slip. Using a light microscope, each protist was examined at different magnifications until the best field of view was found for identifying cellular structures. The color, shape, and motion cellular structures was noted. Each of the protists was drawn and the drawings were labeled. Field-of-view, magnification, and cell size was noted on the drawings, along with the organism’s name and protist group.
Protists available for observation:
All protists that were selected had features in common, but they all moved differently. The example protists were: Euglena, Paramecium, and Amoeba. Euglena moved with a flagellum and so is classified as a flagellate (see Fig. 1). Paramecium moved with cilia and so is classified as a ciliate (see Fig. 2). Finally, Amoeba moved with a pseudopod, and so is a sarcodine (see Fig. 3). All three protists had a nucleus, as expected, but the Paramecium had two nuclei, a micronucleus and a macronucleus. The Paramecium and Amoeba both had food and contractile vacuoles, but these were lacking in the Euglena. All protists had animal-like characteristics in terms of their movements and feeding patterns. Of the three, Euglena was the only one that had chloroplasts, an organelle common in plants.
Protists seem to share certain characteristics even when they are classified into different groups. Their organelles are a mixture of animal and plant structures, but they all have nuclei, a feature which distinguishes Protists from other unicellular organisms. The protists’ motion was consistent with their locomotion organ: cilia, flagella, or pseudopod. This motion was very clear under the light microscope, but interactions of protists with others in the culture jar were better observed using the dissection scope. The Amoeba moves by extending part of its cell. This extruding part is the pseudopod, and allows the Amoeba to drag itself from one place to another (see Fig. 3). Its movement is slow, and changing directions is just a matter of extending a pseudopod in a new direction. Amoebas do not seem to have a particular shape, with the exception of the pseudopodia that consistently protrude from the cell. This shapeless but ever shifting quality of the Amoeba’s shape allows it to surround, engulf, and ingest its food by a process called phagocytosis.
Paramecia are smaller than Amoebas. They move with the help of microscopic hair-like structures called cilia, which act like oars to push them through the water. They swim by rotating slowly and changing directions often. If the Paramecium comes upon an obstacle, it stops, swims backwards, and then angles itself forward on a slightly different course. Cilia help the Paramecium move as well as feed. When the Paramecia feed, it does so by drawing its food into a funnel-shaped opening called the oral groove that is lined with cilia (see Fig. 2). The oral groove is like a mouth, taking food in with the help of cilia, which direct and move the food inward.
The Euglena moves rapidly, using its flagellum to propel itself through the water rather quickly, shifting directions with whip-like movements. Unlike the Amoeba and the Paramecium, the Euglena has plant-like characteristics. It is sometimes referred to as a “plant-like” protist. The organelle that gives it this plant-like quality is the chloroplast (see Fig. 1), a green organelle responsible for carrying out photosynthesis in plants. The Euglena senses light with a light-sensitive organelle called the “eyespot,” which directs the organism to a light source strong enough for photosynthesis to occur. Since it can undergo photosynthesis, Euglena is able to make its own food just like plants.
The three protists examined in this lab are examples of protists that use specialized structures for locomotion. Although the Euglena has some “plant-like” characteristics, all protists mentioned above, exhibit animal-like movements. These protists exemplify the animal-like and motile types of protozoans. As compared to other protists, the animal-like features of the protists we observed allow them to be motile. Their motility comes in handy for moving about their environment and finding food. They may be contrasted to another class of protist, the sporozoans. Sporozoans have no form of locomotion and are primarily parasitic, ingesting their food by absorption through their cell membranes. No matter what type of locomotion a protist uses, all protists must be able to carry out the metabolic functions of multicellular organisms. Based on the observations in this lab, protists are very small yet highly complex. They have all the organelles necessary for a variety of functions such as digestion, excretion, reproduction, respiration, and movement. Protists are self-supporting “one cell factories” churning out all the processes that are usually carried out by a highly-organized network of cells.
In this lab I learned about the structure and function of the smallest eukaryotic organisms, the unicellular protists. Although very tiny, these organisms are very complex, housing all the necessary life tools in one single cell. This shows that the complexity of an organism is not necessarily related to its size. I also learned to identify and classify different types of protists. I was able to observe locomotion patterns as well as other characteristic features. In doing so, I gained useful microscopy skills such as making wet mount slides, finding the proper magnification for viewing, and drawing microscope observations with all the proper labels.
WRITING A DESCRIPTIVE ESSAY
The aim of description is to make sensory details vividly present to the reader. Although it may be only in school that you are asked to write a specifically descriptive essay, description is an important element in many kinds of writing. Description embedded in an argument paper, for example, may be intended to make a position more persuasive. However, in this TIP Sheet we will discuss the descriptive essay as it is commonly assigned by instructors as an exercise in organizing sensory information and choosing vivid details.
Showing vs. telling
Sensory details are details of smell, taste, texture, and sound as well as sight. If you choose "showing" words, those that supply vivid sensory details appropriate to your subject and purpose, you will succeed in showing rather than telling. "Telling" words are usually vague or ambiguous; they can be interpreted in a variety of ways. The following first example mostly makes statements about what is lacking in the room, whereas the second example describes the sights, textures, smells, and sounds of the empty room:
The empty room smelled stale and was devoid of furniture or floor covering; the single window lacked curtains or blinds of any kind.
The apartment smelled of old cooking odors, cabbage, and mildew; our sneakers squeaked sharply against the scuffed wood floors, which reflected a haze of dusty sunlight from the one cobwebbed, gritty window.
"Showing" uses very specific details: cabbage and mildew, scuffed and dusty floors, unwashed windows. Though the writer of the second example does not actually use the word "empty," she nevertheless suggests emptiness and disuse. The suggestion of emptiness in the second example is more vivid than the statement of emptiness in the first. If you don't think the first example is vague, look at another possible interpretation of that empty room:
The sharp odor of fresh paint cut through the smell of newsprint. Four stacked cartons of inkjet printer paper sat squarely in the middle of a concrete floor, illuminated by a shaft of morning light from a sparkling chrome-framed window on the opposite wall.
Do not mistake explanation for description. Explanation is a kind of telling that interjects background material that does not contain sensory details or contribute to the overall effect–a character's motives or history, for example:
The tenants had moved out a week earlier because the house was being sold to a developer. No one had bothered to dust or clean because they assumed the apartment was going to be knocked down and replaced with single-family homes like those built just a block away.
When description devolves into explanation (telling rather than showing), it becomes boring.
Once you are ready to abandon the attempt to explain or to tell about, evaluate your subject in terms of visual, auditory, and other sensory details. Think in concrete terms. The more you are interested in and connected to the subject, the easier it will be to interest your reader, so if you describe a person, choose a person whose characteristics stand out to you. If you describe a place or a thing, choose one that is meaningful to you.
You are painting a picture that must be as clear and real as possible, so observe carefully and, preferably, in person. Note what sets this subject apart from others like it. If the subject is a person, include physical characteristics and mannerisms. Describe abstractions such as personality traits only insofar as you can observe them. For example, do not tell the reader your biology instructor is a neat, meticulous person; show your reader the instructor's "dust-free computer monitor and stacks of papers with corners precisely aligned, each stack sitting exactly three thumb-widths from the edge of the desk." How a subject interacts with others is fair game for description if you can observe the interaction. On the other hand, a subject's life history and world perspective may not be, unless you can infer them, for example, from the photos on his walls or the books on his bookshelf.
Similarly, if the subject of your description is an object or a place, you may include not only its physical appearance but also its geographic, historical, or emotional relevance-as long as you show or suggest it using sensory details, and avoid explaining.
Deciding on a purpose
Even description for description's sake should have a purpose. Is there an important overall impression you wish to convey? A central theme or general point? This is your thesis; organize your essay around it. For example, you might describe your car as your home away from home, full of snack foods, changes of clothing, old issues of the Chico News & Review, textbooks, and your favorite music. Or, you might describe your car as an immaculate, beautiful, pampered woman on whom you lavish attention and money. Just don't describe your car in cold, clinical detail, front to back (or bottom to top, or inside to outside) without having in mind the purpose, the overall impression you want to create. To achieve this impression, you should not necessarily include all details; use only those that suit your purpose.
Avoid telling a story unless it is of central importance to the description or an understanding of it. Keep background information to an absolute minimum or avoid it altogether.
Extended description that lacks organization has a confusing, surreal quality and easily loses readers' interest, so choose an organizational plan. Use whatever progression seems logical–left to right, inside to outside, top to bottom-and stick to it. For example, it does not make sense to describe a person's facial features and hair, then his sonorous voice and impressive vocabulary, and then return to details about his eyebrows and glasses.
A quote from your subject or a brief anecdote about him or her may provide an interesting introduction (or conclusion); dialogue can be a great way to add interest to a descriptive essay. In your introduction, you might be permitted to make general, abstract statements (tell about) your subject or supply background information, as long as you demonstrate these points concretely later in the body of your essay.
Use vivid nouns, verbs, and adjectives, and appropriate metaphors, similes, comparisons, and contrasts. Avoid clichés.
Like the introduction, the conclusion is another place you can get away with reflecting about your subject: Why did you write this description? What is its significance to you? To your reader? If you have achieved your purpose, your conclusion should only confirm in the reader's mind what you have already shown him by your use of selected sensory details.