Comparing Cells from Different Biological Kingdoms I. Introduction to Cells In 1655, the English scientist Robert Hooke made an observation that would change biological theory and research forever. While examining a dried section of cork tree with a crude light microscope, he observed small "chambers" and named them cells. Within a decade, researchers had determined that cells were not empty, but instead filled with a watery substance called cytoplasm. Over the next 150+ years, researchers developed what we now refer to as cell theory, which was first proposed by the German botanist Matthias Jacob Schleiden and the German physiologist Theodore Schwann in 1838 and formalized by the German researcher Rudolf Virchow in 1858. In its modern form, cell theory has four basic parts: 1. The cell is the basic structural and functional unit of life; all organisms are composed of cells. 2. All cells are produced by the division of preexisting cells. Each cell contains genetic material that is passed down during this process. 3. All basic chemical and physiological functions (such as repair, growth, movement, immunity, communication, and digestion) are carried out inside of cells. 4. The activities of cells depends on the activities of sub-cellular structures within the cell (collectively termed organelles). Thus, cell theory leads to 2 very important generalities about cells and life in general. First, cells are alive. The individual cells of your organs are just as alive as you are, even though they cannot live independently. This means cells can take in energy and building materials (such as proteins, carbohydrates and fats), and use these to make new cellular structures, repair themselves, and make new generations of cells (reproduction). Second, the characteristics and needs of an organism are in reality are the characteristics and needs of the cells that make up the organism. For example, you need water because your cells need water for a variety of cellular processes (mainly cellular respiration) and as a solvent. We breathe in oxygen so that our cells can use it for cellular respiration. We exhale carbon dioxide to get rid of this deadly waste product of cellular respiration. We eat proteins to help build the actin and myosin fibers that make up the bulk of our muscle cells. The list goes on We will spend the next two semesters in BIO 110 and 111 exploring the relationship between cell structure and function in organisms. Similarities between all cells: Cytology is the study of cells and cytologists are scientists that study cells. Cytologists have discovered that the cells of all living organisms are similar in many ways, but quite different in others. All cells contain three basic features: 1
1. A plasma membrane (or cell membrane) consisting of a phospholipid bilayer, which is a membrane that houses the cell. This membrane contains several structures that allow the cell to perform necessary tasks. For example, cell membranes possess channels that allow substances to move in and out of the cell and molecules that allow the cell to be recognized by other cells. 2. A cytoplasm containing cytosol and organelles. Cytosol is a fluid, consisting mostly of water and dissolved nutrients, wastes, ions, proteins, and other molecules. Organelles, as stated earlier, are small structures suspended in the cytosol. The organelles carry out the basic functions of the cell, including reproduction, metabolism and protein synthesis. 3. Genetic material (DNA and RNA), which carry the instructions for the production of proteins. Differences between cells: Aside from these three basic similarities of all cells, the cells of organisms from different biological kingdoms are quite different. For example, some cells are single, independent units and spend their entire existence as individual cells (these are the unicellular organisms, such as Euglena, amoebas, and bacteria). Other cells are part of multicellular organisms, and cannot survive alone. Recall that prokaryotic cells (which include bacteria) lack a nucleus and membrane-bound organelles, while eukaryotic cells (which include members of the Kingdoms Protista, Fungi, Plantae, and Animalia) contain a nucleus and membrane-bound organelles. The cells of autotrophic organisms contain an organelle called the chloroplast, which allows the cell to produce sugars using light energy in the process known as photosynthesis. The cells of some organisms are surrounded by a cell wall composed mostly of the carbohydrates. Other cells lack a cell wall but instead have a cytoskeleton, a network of long fibrous protein strands that attach to the inner surface of the plasma membrane and help them maintain shape. You will observe and document similarities and differences such as these, and many more, among various organisms during lab today! II. Objectives Upon completion of today's lab activities, you should be able to: 1. Apply techniques learned in the "Perfecting Microscope Skills" lab to observe, compare, and contrast cells from different biological kingdoms. 2. Calculate the field diameter for scanning, low, and high power 3. Estimate the approximate size of specimens in your field of view 4. Use the metric system to make basic measurements of length 5. Compare and contrast characteristics of plant and animal cells 6. Recognize various organelles of plant and animal cells observed in lab and describe their function 7. Relate the functions of cells and/or organelles to the characteristics of life 8. Explain why certain organelles (such as Golgi bodies, ribosomes, and the endoplasmic reticulum) cannot be seen with the compound microscope 9. Differentiate between: 2
a. prokaryotic and eukaryotic b. autotrophic and heterotrophic c. unicellular and multicellular d. cell wall and plasma membrane 10. identify organelles using the plant and animal cell model III. Observation of cells from different Biological Kingdoms Before we observe cells, let's go over an overview of the characteristics of the 6 Biological Kingdoms: Recall that bacteria are the only prokaryotic organisms. Taxonomists now classify all bacteria into two Domains, which are technically taxonomic groupings above the Kindgom level, the Domains Bacteria and Archaea. Protista were the first of the eukaryotic kingdoms to evolve. The chief evolutionary importance of protists is their role as a stem group for the remaining Kingdoms: Plants, Animals, and Fungi. In other words, the Kingdom Protista includes species that are similar to the ones that gave rise to fungi, plants, and animals. The major groups within the Protista include the algae and other plant-like protists, the protozoa (the "animal-like protists"), and the slime-molds (the fungi-like protists). Today, the Kingdom Protista is not considered a valid Biological Kingdom because it includes groups of unrelated lineages. Fungi are almost entirely multicellular (with yeast, Saccharomyces cerviseae, being the prominent unicellular fungus), heterotrophic (deriving their energy from another organism, whether alive or dead), with cell walls make of chitin, and usually having some cells with two nuclei (multinucleate, as opposed to the more common one, or uninucleate) per cell. Ecologically this kingdom is important (along with certain bacteria) as decomposers and recyclers of nutrients. Economically, the Fungi provide us with food (mushrooms; Bleu cheese/roquefort cheese; baking and brewing), antibiotics (the first of the "wonder drugs", Penicillin, was isolated from a fungus Penicillium), and crop parasites (molds and rusts) which do several billion dollars per year of damage. Plantae include multicellular organisms that possess cell walls (composed of cellulose) that are autotrophic. Ecologically, this kingdom is generally (along with photosynthetic bacteria and protists) termed primary producers. Economically, this kingdom is unparalleled, with agriculture providing billions of dollars to the economy as well as the foundation of civilization. Food, building materials, paper, pharmaceutical drugs, and roses, are plants or plant-derived products. Animalia consists entirely of multicelluar heterotrophs that lack cell walls that are all capable (at some point during their life history) of motility. Ecologically, this kingdom occupies the level of consumers, which can be subdivided into herbivores (eaters of plants) and carnivores (eaters of other animals). Humans, along with some other organisms, are omnivores (capable of functioning as herbivores or carnivores). Economically, animals 3
provide meat, hides, beasts of burden, pets, transportation, and even scents (as used in some perfumes). Today we will compare and contrast the structure of plant, animal, and protist cells. A) Observations of plant cells using Elodea. Elodea (a.k.a., "waterweed") is a submersed South American aquarium plant that is naturalized in ponds, streams and lakes throughout North America. Most of the Elodea cell is occupied by a water-filled, large central vacuole. The chloroplasts (the little green disks) are displaced around the periphery of the cell, just inside the cell wall. To see additional chloroplasts you must focus up and down with the fine adjustment knob of the compound microscope. The transparent nucleus may not be visible on your slide. To observe these plant cells, do the following: 1) Obtain a piece of an Elodea leaf. 2) Place it gently on a clean, dry slide. 3) Add several drops of water and cover with a coverslip. 4) Observe under the microscope at low power (10X) and draw what you see. Be sure to label your drawing. Notes: 5) Locate the cell wall. Is a nucleus visible? 6) Count and record the number of cells across the diameter of the low power field, both lengthwise and side to side. Number of cells:. 4
7) Using the number you recorded in #6, calculate and record the length and width of an Elodea cell (in micrometers and millimeters) in the space below. 8) Now observe the Elodea cell at high power (40x) and draw what you see. Be sure to label your drawing. Notes: 9) How many cell layers thick is this leaf? 10) How were you able to determine #9? HINT - remember what we learned in the "Perfecting Microscope Skills" lab on depth of focus. 11) Based on your observations, are the chloroplasts evenly distributed throughout the cell? YES or NO (Circle one) 12) How are the chloroplasts distributed? 13) Switch back to a lower magnification and look to see if any cells show any internal movement. You should be looking for an obvious flowing movement of the chloroplasts around the periphery of the cell. This movement is called cyclosis or cytoplasmic streaming. 5
14) Locate the nucleus (if you haven't already). Where is it located? Why is it found in that location? 15) Locate the central vacuole (if you haven't already). Where is it located? How does its location affect the location of other organelles within the cell? Where are the other organelles located? Why are they found in that location? 16) Roughly how much of the area within each cell, on average, does the central vacuole occupy? 17) Based on your observations from #15 and 16, why are leafy vegetables, such as lettuce and cabbage, low in calories? 18) Observe the cell wall. Is it located INSIDE or OUTSIDE (Circle one) of the plasma membrane? B) Observation of animal cells using human epithelial cells (FYI - epithelial cells cover the body's surface and line its cavities). To observe these animal cells, do the following: 1) Obtain a sterile toothpick (your instructor will let you know where to get these) 2) Gently scrape the inside of your cheek with the toothpick. 3) Place the scrapings on a clean, dry slide. 4) Add a drop of methylene blue stain and cover with a coverslip (be careful with this stain - it will stain your clothes!) 5) Observe under the microscope, using the directions for focusing given above. Start with the scanning power objective to find some cells, then observe under both low and high power. 6) Locate the cell membrane, the cytoplasm, and the nucleus. 7) Make a drawing of what you see. Be sure to label the drawing with what it is, and what power you were using when you made your drawing. 6
8) Estimate and record the number of cells across the diameter of the high power field, both lengthwise and side to side. Number of cells:. 9) Using the number you recorded in #8, calculate and record the length and width of an cheek cell in micrometers and millimeters. 10) Record some obvious differences between the human cheek cells and the plant cells that you observed: 11) Record any similarities between the two types of cells. 7
C) Observation of unicellular protists. 1) make wet mounts or use prepared slides to observe the following unicellular protists: Euglena and Paramecium. 2) Sketch each specimen in the circles below, making notes of any visible characteristics you observed on each organism. Notes: Notes: 8
3) In what ways are Euglena similar to plant cells? To animal cells? 4) In what ways are Euglena different than plant cells? Than animal cells? 5) In what ways are Paramecium similar to plant cells? To animal cells? 6) In what ways are Paramecium different than plant cells? Than animal cells? IV. Questions Upon completion of this lab exercise, answer the following questions: 1) Why was it helpful today for your microscope to be parfocal? 2) List five things did before storing your microscope today. 3) What characteristics did all of the cells which you observed have in common? Hint: Consider things such as appearance (size, color, shape, organelles, etc.), behaviors, if any (movements, responses to stimuli, growth, etc.), etc. 9
4) In what respects do animal cells (e.g., human mouth epithelium) differ from plant cells (Elodea leaf)? Hint: You may wish to check your text to learn what characteristics do distinguish Animalia from Plantae. 5) Were any of the protists you observed today more similar to plants than animals or vice versa? If so, which do you think were "plant-like" protists and which were "animal-like" protists? 6) What does your answer to #5 above tell you about the organisms within the Kingdom Protista? 7) Do you think that most plant and animal cells are similar to those we observed today? Hint: The animal cells observed were from tissues at the surface (epithelium) and the plant cells were from leaves. Would you expect cells of wood, roots, muscle, bone, blood, and nerve to be constructed in any particular way to allow them to perform specialized functions? Explain. 10