Cells & Organelles

Background Information

Generic Animal Cell
Generic Plant Cell All living things are made up of cells. Some organisms, like yeast, are only single-celled, while others, like humans, contain many cells. Cells are bounded by a plasma membrane which is so thin it is often invisible even with a light microscope. Cells of organisms such as plants have a cell wall outside the plasma membrane. The most important organelle (-elle = small) within eukaryotic cells is the nucleus. Remember that, unlike yeast, humans, and other such eukaryotes (eu- = good, well, true), bacteria, which are prokaryotes, (pro = before, in front of; karyon = nut, kernel, nucleus) do not have their DNA organized into a nucleus. When a cell is “stained,” the nucleus often takes dye well, especially in the region(s) of the nucleolus (pl. = nucleoli). The region between the nucleus and the plasma membrane is called the cytoplasm (cyto = cell), which contains a number of other kinds of organelles. Some of these are visible only with an electron microscope and/or special staining techniques, while others are easily visible with a light microscope. Various vacuoles (vacu = empty) are usually visible. Many plant cells have a large central vacuole which often takes up more space than the cytoplasm. In plants, chloroplasts (chloro = green; plasti = formed, molded) are easily seen, as well as various other plastids including leucoplasts (leuco = white) and chromoplasts (chromo = color). If present, cilia (cilium = eyelid, eyelash, small hair) and flagella (flagellum = a whip) can sometimes be seen by their shadow (or with a special stain), although not in great detail. There are special stains available to enable us to see certain of the other organelles.

The smallest cells we know of are some bacteria, the largest are bird eggs, the yolk portion of which is a single cell. The albumen (alb = white; album = the white of an egg), outer membrane, and shell are non-cellular products of the hen’s reproductive tract. The longest cells we know of are nerve cells. To reach from a human’s spinal cord to toes, a nerve cell has to be about three to four feet long — imagine a giraffe’s nerve cells.

The Cells You Will Be Viewing

Examine at 40, 100, and 400×; draw; and take notes on the following materials. Refer to the illustrations in the handout and on your own drawings, label all parts indicated in the protocol. Each drawing should be ¼ to ½ page. Do not draw circles around everything, but do watch relative space and proportion. Use your lab pen to outline drawings and fill in with color later, if desired. Label with the power of magnification to the lower right of each picture. Wash and dry your slide and coverslip between each specimen and when you are done. When you are done for the day, place your slide in the designated location to dry (do NOT return it to the box of clean, dry slides) and dispose of the coverslip in the broken-glass box. UNDER NO CIRCUMSTANCES SHOULD COVERSLIPS BE LEFT IN A SINK OR THROWN IN THE REGULAR TRASH!

Air Bubble — Not a Cell! Air bubbles are a common occurrence, and are often mistaken for something “important.” Those nice, black rings are so easy to draw, but alas, they are only air bubbles.

Cork at 40×
Cork at 100× In 1663, when Robert Hooke first saw and named cells, he was examining a section of the bark of the cork oak tree (Quercus suber, which grows in Europe), and he saw many small chambers which he called “cells” (cell = a small room).
From a piece of cork, shave off a VERY THIN slice and make a dry mount of it by just placing it under a coverslip. Your slice should be thin enough that you can almost see through it and the coverslip does not rock back and forth on top of it. Observe whether all the “cells” you see are the same size and shape or not. Since this cork was removed from its tree long ago, these cells are no longer alive. Thus, you won’t see cytoplasm or any organelles. What you will see is the cell walls that surround the spaces where the cytoplasm and organelles used to be.

Elodea leaf Elodea, also known as Elodea densa, Egeria densa, Anacharis densa or “waterweed”, is an aquatic plant in the family Hydrocharitaceae. Its leaves are only two cells thick, making it possible to easily view those cells and their organelles.
Pick an Elodea leaf. Put it in the middle of a slide with a drop of the water in which it is living, and put a coverslip on it. Note the cells that make up the midrib of the leaf and notice that the leaf is “3-D.” Locate one cell (usually ones near the edge work well) to examine more closely. Draw cells as they appear under the various powers of magnification. Label the cell wall, cytoplasm (cyto = cell), and chloroplasts (chloro = green; plasti = formed, molded) — green ovals within the cell, and the very large central vacuole (vacu = empty) which takes up almost the whole cell, or so it seems. There actually is very little cytoplasm in a thin layer between the plasma membrane and the membrane surrounding the central vacuole. You cannot see the membrane around the central vacuole, but can infer its existence from the fact that all the chloroplasts are found (and move) only around the outer edges of the cells. Try to locate the larger, oval, transparent, nucleus (if you find it — it’s usually difficult to see) within the cytoplasm. Using the fine adjustment, focus up and down to observe the central vacuole and the small surrounding layer of cytoplasm. Look for a region in that cell or another where the chloroplasts are moving, indicating that cyclosis (cyclo = a circle, wheel; -sis = the act of), or cytoplasmic streaming, is occurring (as it often does in leaf cells). You do not need to draw the whole field of view — rather, draw a representative area with a few cells. Note the general size and shape of the cells and count how many of them it takes to fill the field of view (length? width?) under both low and high powers.

Potato Tissue
Potato Nucleus Between Stained Starch
Potato Starch — Note Concentric Rings
Potato Starch with Iodine From a potato, take a small, VERY THIN slice (you should be able to see through it). Make a wet mount of your slice, examine, and draw. Note the cell walls and the leucoplasts (leuco = white — the membranes delineating the leucoplasts are not visible) containing starch grains. Since some cells were cut open, there will be a lot of loose starch grains. Generally, the nucleus is obscured by the starch grains, but if you are very lucky, you may see one, especially if you stain the cell. Examine single starch grains and note the concentric layers (the light has to be just right).
Iodine, or more correctly, triiodide ions (a solution of which is brownish-orange in color), reacts with starch to form a purple complex, and as many people know, potatoes contain starch. Thus, the starch grains can be stained/highlighted by the addition of iodine to the slide. Remove your slide from the microscope and drop a tiny drop of iodine to one side of the coverslip. The iodine will be pulled under the coverslip, but if necessary, a Kimwipe or paper towel may be touched to the opposite side of the coverslip to pull the iodine underneath the coverslip. Examine and draw you potato slice now. Note in your drawing either by words or colored pencils what colors things are now (especially make note of what color the starch grains are now). Optionally, try staining a potato slice with methylene blue to attempt to see the nucleus.

Buccal Smear at 100×
Buccal Smear at 400× By carefully following directions, make a buccal smear slide as follows.

1. Use your fingernail to gently scrape the lining of your cheek (the oral mucosa), as though you were “scratching an itch” on the inside of your cheek.
2. Spread this material over 0.5 cm² in the center of a clean slide — make a buccal smear (bucca = cheek). Remember which side of your slide contains your specimen!
3. Allow it to air dry.
4. Fix the slide: when it is dry, pass it through the flame of a Bunsen burner three times — it is not necessary to bake, scorch, brown, or cook the specimen, just pass it through the flame, right-side up (it should NOT be too hot to hold). Note: this is not designed to dry out a wet specimen, either, merely help it to stick to the slide better (like three-day old spaghetti dried onto a plate). This warming will fix the cells to the slide so the stain won’t wash them off (if you are careful).
5. Place the slide, smear-side up, on a paper towel and put drops of methylene blue onto the smear to cover it. Let the stain sit on the slide for exactly one minute (time it), then (over the sink) tip the slide and allow the excess stain to run off.
6. Rinse GENTLY with tap water from one of the SQUIRT BOTTLES so labeled (NOT under the faucet), shake off the excess water, and allow the slide to air dry right-side up (you may use a Kimwipe to dry the bottom and edges of the slide, but do not attempt to blot the area where the smear is or you will wipe it off).
7. You do NOT need and should NOT use a coverslip with this slide.

Examine and draw your cells. Look for small, usually somewhat oval or round cells alone or in small groups. If grouped, note how this affects their shape, making them more hexagonal. Focus up and down with the fine adjustment to see if you can observe any thickness to the cells. The nucleus should show up as a darker blue oval or round region near the center of each cell. Tiny, darkly-stained objects which adhere to the cell membrane are bacteria which are commonly found in the mouth. The cytoplasm will be a pale blue. Optionally, if someone has a lot of interesting bacteria, your instructor may set up a microscope to view them with the oil immersion lens at 1000×. Note how flat or rounded these cheek cells appear to be — can you relate this to their function as a lining layer of cells? What is the ratio of the diameter of the nucleus to the overall diameter of these cells? Again, how many cells does it take to span the field of view?

Onion at 100×
Onion at 400×
Peeling Onion Epidermis Each layer of an onion consists of a thick, fleshy layer sandwiched between two shiny, transparent, membranous, epidermal layers. Peel a small piece of the transparent epidermis from a layer of an onion (NOTE: YOU DO NOT WANT the whole, thick, fleshy part, just the transparent “skin” layer). Place it on a slide with a drop of water, then put the coverslip on. Drop a small drop of methylene blue at one edge of the coverslip (the dye should be pulled under the coverslip). Examine and draw the cells. If you have the correct epidermal layer, you should be seeing long, thin cells. If you’re seeing mostly round-ish or square-ish “spongy-looking” cells, you have too much of the middle, fleshy layer of the onion, and not the epidermal layer, so make another slide, and try to get just the shiny, “skin” layer. Pick several cells to examine in more detail and draw. Focus up and down with the fine adjustment to see the third dimension of the cells. Label the thin cell walls between cells, the nucleus, which should have stained dark blue and which may contain one or more even darker blue nucleoli (sing. = nucleolus), the central vacuole which, again, takes up most of the cell (this vacuole contains watery “sap” and is separated from the rest of the cell by a membrane that cannot be seen without special stain), and the scant cytoplasm which may be found especially near the edges and corners of the cell, as well as occasional streaks across the cell — you may be able to see cytoplasmic streaming, as evidenced by movement of the various organelles within the “strands” of cytoplasm. As a reminder, you may have to adjust the iris diaphragm or light level to get optimal contrast. Note how many cells it takes to fill the field of view lengthwise and widthwise.

Yeast, unstained
Yeast with Methylene Blue Place one drop of yeast solution on the center of the slide and add a coverslip. Examine your slide under 40, 100, and 400× (ALWAYS START AT 40× = 4× objective). Draw what you see at each power. Remember to make your drawings large enough. Carefully focus up and down with the fine adjustment to observe the fact that these cells are three-dimensional (adjustment of the iris diaphragm and rheostat may help you to see this better. Yeast cells should be fairly oval in shape. How much size variation can you see? Do you see any cells with reproductive buds attached? Can you see any of the organelles within the cells? Yeast cells do have a thin cell wall and clear cytoplasm. The nucleus cannot be seen unless special staining techniques are used. After observing the cells unstained, add a small drop of methylene blue by removing the slide from the microscope and dropping a drop of methylene blue next to one edge of the coverslip. Again, examine under each power and draw what you see. What difference(s) does the methylene blue make in the “visibility” of the yeast cells or their organelles? Note any other observations (for example, have all of the cells taken up the dye equally?).

Additional, Optional Cells to Examine

Also look at these slides if they are available and there’s time.

Tomato Pulp at 100×
Tomato Cell with Nucleus From a tomato (or red pepper), take a bit of the red “pulp” or tissue beneath the skin (NOT the skin itself). Gently spread it out a bit on your slide and make a wet mount. You should not have a big, red blob on the slide, because you won’t be able to see anything that way. Rather, the smear should be thin enough that you can just barely see it. Examine under low power and draw, then under high power and draw. Note whether cells that are still attached to each other and cells that have come free from the rest are the same shape — does being in contact with other cells influence the shape of a cell? Locate the cell walls. Notice the small, rust-colored chromoplasts (chromo = color) which give the tomato its color and are located in the cytoplasm (of which, once again, there is very little). The majority of the cell is a central vacuole and the cytoplasm will appear as thin streaks of grayish or speckled matter. Optionally, stain with methylene blue by putting a drop at the edge of the coverslip and if needed, drawing it through by touching the other side with a Kimwipe. DON’T GET METHYLENE BLUE ON THE MICROSCOPE!!!

Broccoli Epidermis Tear a broccoli, kale, or geranium leaf “sideways” so that a portion of the lower (clear) epidermis (epi = upon, over; derm = skin) is exposed. Cut this off with a razor blade or scalpel (or tear off with your fingernails) and make a wet mount of it. Observe and draw. Label epidermal cells (clear and irregular in shape), their cell walls, and the smaller, oval areas that resemble cat-eyes which consist of a pair of crescent-shaped guard cells surrounding a small opening, the stomate (stoma = mouth). The stomates are used for exchanging CO₂ and O₂ with the outer air. The guard cells control the size (opening and closing) of the stomate, closing it in dry weather to conserve water. Notice the green chloroplasts in the guard cells. On your slide, you may also see a few larger, rounded, green (due to chlorophyll) mesophyll cells (meso = middle; phyll = leaf), some of the cells from the middle of the leaf that have come off with your epidermal layer.

Moses-in-the-Boat crystas at 40×
Moses-in-the-Boat crystals at 100× Moses-in-the-Boat (Rhoeo spathacea) in plant family Commelinaceae gets its common name from the fact that the clusters of flowers are borne in a “boat-shaped” pair of specially-modified leaves. These flowers have three, tiny, white petals, and like several other related plants, the petals contain crystals of calcium oxalate (similar to the oxalic acid that gives the plant Oxalis its taste and name). Other parts of the plant occasionally contain these crystals, too, but typically not as many as in the flower petals. These crystals, which look like pick-up sticks or knitting needles, are primarily an “excretion” product of the plant and are stored in the central vacuole.
If enough flowers are available, make a wet mount of a petal of a Moses-in-the-Boat flower petal. If not enough are available for everyone to make mounts, the instructor should set one up as a class demonstration. The cells of these flower petals contain long, slender crystals of calcium oxalate (oxa = sharp, acute, acid) in their central vacuole. Also, notice the cell walls. Due to the fact that some cells were torn open when the petal was removed from the flower, you may also see bunches of crystals that have “escaped” and are out by themselves. Draw and label what it looks like.

Other materials and/or prepared slides may be available, and if so, examine and draw. If available and time allows, examine a drop of pond water to see what “lives” there (draw). Alternately, your instructor may decide to show you how to find and view eyelash mites.

Other Things to Include in Your Notebook

Make sure you have all of the following in your lab notebook:

• all handout pages (in notebook or separate protocol book)
• all notes you take during the introductory mini-lecture
• all notes and data you gather as you perform the experiment
• drawings (yours! — at whatever power works best for that tissue) of each tissue examined, to scale (proportions relative to each other are about right) with all visible organelles and all powers of magnification labeled (cork bark, buccal smear, Elodea leaf, potato pulp, onion epidermis, tomato pulp, kale leaf, Moses-in-the-Boat flower, etc.)
• answers to all discussion questions, a summary/conclusion in your own words, and any suggestions you may have
• any returned, graded pop quiz