Prokaryotic cells- pp. 105-107
Vacuoles- p. 115
Cell wall- p. 124
Blood cells, neurons, and muscle cells- pp. 782-783
Introduction:Every living, or once living, thing is made up of cells. Some organisms are unicellular, such as bacteria and amoeba, but most are multicellular. Cells can be divided into two broad categories based on their cellular organization: Prokaryotic and Eukaryotic. Prokaryotic cells are found only in the kingdom Monera and are comprised of blue-green algae (cyanobacteria) and bacteria and are the smallest cells. These cells lack a membrane-bound nucleus; instead their DNA consists of a single circular molecule suspended in the cytoplasm. Also located in the cytoplasm are many ribosomes which are involved in protein synthesis. Eukaryotic cells are much more complex in structure. They contain a true, membrane-bound nucleus which contains DNA. They also contain many different membrane-bound organelles, each with a specific function.
In todayís lab you will be looking at both prokaryotic and eukaryotic cells, identifying specific organelles, and comparing various cell types.
Procedures:Cyanobacteria, or blue-green algae, are the largest of the prokaryotes. These cells are unique among prokaryotes as they are photosynthetic. Chlorophyll a and other pigments necessary for photosynthesis are located in thylakoid membranes. You can often see a mucilaginous sheath, a gelatinous material that surrounds some cyanobacterial cells.
Obtain a prepared slide of Oscillatoria and Anabaena from your instructor. Using the skills you learned in your previous lab, focus your microscope on a cyanobacteria cell. Remember to begin focusing using the lowest objective before increasing magnification.
Draw what you see in the space provided below and label structures:
Make a figure legend for your drawing.
Figure 1. _______________________________________________________________
Look at the pigments in the Oscillatoria cells. How are they distributed (are they localized within each cell or evenly distributed)?
Evenly distributed throughout the cell (since these organisms do not have membrane-bound organelles)._________________________________________________________
Can you identify a heterocyst in Anabaena? What is its function (use p. 515in your textbook as a guide)?
Based on their structure, would you predict that cyanobacteria are very motile?
Bacteria are the smallest of the cells, and most are smaller than cyanobacteria. They do not photosynthesize, so they do not contain chlorophyll or other photosynthetic pigments. Many bacteria have flagella, tail-like structures, and pili, which are hair-like structures used for motility.
Obtain a prepared slide of mixed bacteria. Draw what you see and be sure to identify the three major shapes of bacteria (cocci = spheres; bacilli = rods; and spirili = spirals). Can you see any flagella (you may have to look VERY closely)?
Make a figure legend for your drawings.
Figure 2. _______________________________________________________________
What are some of the structural differences you observed between bacteria and cyanobacteria?
Bacteria- very small; some have flagella; no photosynthetic pigments.]
Cyanobacteria- larger cells; photosynthetic pigments.
Now that you are familiar with prokaryotic cells, itís time to look at eukaryotic cells. Recall that eukaryotic cells have a membrane-bound nucleus containing DNA. All eukaryotic cells have a plasma membrane surrounding the cytoplasm. Organelles can be found in the cytoplasm of the cell. The types of organelles present depends on whether the cell is a plant or an animal cell. While they have many organelles in common, plant cells have a few organelles and structures that differ from animal cells.
Begin by examining Elodea, a type of aquatic plant. Using forceps (or your fingers), remove a leaf from a sprig of Elodea . It is best to get the smaller leaves from the tip of the sprig, but other leaves will work as well. Plant cells have both a plasma membrane and a cell wall made of cellulose. In addition, there are photosynthetic pigments such as chlorophyll a located in chloroplasts. Plant cells also have central vacuoles that can make up 80% or more of a mature plant cell. These vacuoles contain a watery fluid that helps maintain pressure within the cell, while other vacuoles serve as nutrient storage and waste sites .
Make a wet mount by placing a drop of water on your microscope slide, placing the leaf face up on the drop of water, and placing a coverslip on top of the leaf. If it looks too dry, add a drop of water at the edge of the cover slip; the water will begin to diffuse and your specimen will not dry out. If your slide is wet on the bottom, carefully dry it off before placing it on the stage of your microscope. Begin focusing using the low power objective. Each of the small, rectangular looking shapes is a cell. You will be observing this slide again, so keep it for future reference.
Draw what you see and label the cell wall, chloroplasts, and vacuole (if evident).
Create a figure legend.
Figure 3. ________________________________________________________________________
What are the functions of the cell wall?
__Structural support, maintains shape of mature plant and individual cells. ______________________________________________________________________
What is the function of the vacuole?
____Storage of water soluble compounds such as pigments and waste products. Also maintains turgor pressure.
In order to see structures of plant cells more clearly, you will look at onion cells. Using a stain to dye specific structures will allow you to see more detail in the cells.
Cut a small piece of onion. Snap it backward and peel the thin membrane from the rest of the leaf (it looks something like plastic wrap). This is the epidermis of the onion. Prepare a wet mount in the same manner as above and examine the cells of the onion epidermis.
Draw what you see in the space below.
Are there organelles in Elodea that are not found in your onion skin? If so, what are they and why would you not expect them in the onion?
Chloroplasts. Onions grow underground (at least the bulb part) and do not undergo photosynthesis.________________________________________________________________________
Remove your onion slide and add a drop of neutral red to the edge of your coverslip. Neutral red will collect in the cytoplasm but does not stain the cell wall. If it does not begin to diffuse, place the tip of a paper towel on the opposite side of the coverslip to draw it through. It will take 5-6 minutes for the stain to work. Focus your microscope again (from lowest to highest magnification) and observe the cells.
Can you see a difference between the stained and the unstained cells? If so, describe these differences.
____Staining with neutral red allows the nucleus, and maybe the cell wall, to become visible.____________________________________________________________________
Try different stains to see which one makes the organelles easier to see. Are there differences in what structures you can see using different stains? If so, what are they?
Having looked at many of the structures and organelles of plant cells, you can now examine specific structures more closely. Begin with the cell wall. When you examined the leaf of Elodea and epidermis of the onion, you saw many regularly shaped cells that were connected. That is because the cell wall is comprised of an outer layer called the primary cell wall and a substance that holds the cell walls of two adjacent cells called the middle lamella. Adjacent cells are further connected via the plasmodesmata, which consists of minute strands of cytoplasm that extend through the cell walls of adjoining cells.
Take another look at your Elodea slide and look closely at the cell walls under high magnification. Can you see a faint line between cells? That is the middle lamella.
Now get a prepared slide ofplasmosesmata in persimmon (Diospyros) endosperm and notice the very thick cell walls. Again locate the middle lamella (the faint line between cells). You should also see some darkened lines that are perpendicular to the middle lamella that connect the protoplasts of adjacent cells; these lines are the plasmodesmata.
Why do you think you can see the plasmodesmata in the persimmon but not in the Elodea ?
_The persimmon has much thicker cell walls which better outline the plasmodesmata and makes them easier to see.
Plants, through the process of photosynthesis, produce and store food. Plastids are organelles where this occurs. Chloroplasts are a type of plastid, but others are amyloplasts and chromoplasts. Amyloplasts store starch and chromoplasts contain various colored pigments.
Carefully slice a very thin piece of potato tuber with a razor blade. Make a wet mount. If the coverslip will not sit properly, your slice is too thick. Place the slide on your microscope stage and begin focusing. Try to find the clam-shaped structures in the cell; these are the amyloplasts. Add a drop of iodine to the edge of the coverslip. Remember that amyloplasts are the site of starch storage. Iodine reacts with starch, turning it a blue-black or violet color.
How much of the potato cell is stained?
The sites of starch storage stain dark purple. The majority of the potato cell is stained.
Why are potatoes such a good food product?
_Potatoes contain high amounts of starch (and protein and vitamin C) that are beneficial to humans.
Now thinly slice a carrot (again, as thin as possible), and make a wet mount. Carrots have chromoplasts containing carotenoid pigments. Focus your specimen and look for irregularly shaped, orange-colored organelles. Those are the chromoplasts. How are they different from the amyloplasts you saw in potatoes?
____Chromoplasts are scattered throughout the carrrot and are responsible for pigment storage. The also have a different shape than the amyloplasts.
Obtain a root cross section and observe the different types of cells in the root. Notice that there are many different cell types, each with its own distinct function. Moving inward, toward the center of the root (and using Fig. 35.16 in your textbook as a guide), identify the following: epidermal cells; cortex cells; xylem cells; and phloem cells.
Make a sketch in the space below.
Notice that the cortex cells are full of starch grains (stained purple). What might be the function of the cortex?
Also notice that xylem cells are entirely hollow. If you were to observe a root in longitudinal section, you would see that xylem cells are shaped like elongated cylinders (similar to small straws). What might be the function of xylem cells?
_____xylem cells are involved in water conduction through out the plant
Now that youíve looked at different types of plant cells, you can compare what youíve seen to animal cells. Recall that animal cells differ from plant cells in that they lack a cell wall, they do not have a central vacuole and they do not contain chloroplasts. However, they share many of the other organelles with plants. The first cells you will examine are human epithelial cells.
Place a drop of water on a microscope slide. Using the broad end of a toothpick, gently scrape the inside of your cheek. Swirl the toothpick in the drop of water on your slide and carefully add a coverslip, trying to avoid bubbles from being trapped. Place the slide on your microscope and focus. Describe what you see. Be sure to place your toothpick in Lysol when finished.
Now place a drop of methylene blue to the edge of your coverslip and draw it through by placing the corner of a piece of paper towel on the opposite side. It only takes a few seconds for the stain to work. What do you see? Did the stain help you visualize any organelles? If so, what are they?
_The stain allowed you to see the nucleus better.
Paramecium are single-celled eukaryotic organisms. They are commonly found in pond water.
Obtain Paramecium from the container at the bench by placing a drop on a microscope slide. Add a coverslip and observe it with your microscope. Describe what you see.
Based on structures seen, what mode of locomotion do you think the Paramecium uses?
Can you identify a contractile vacuole? What is the function of this vacuole (see p. 115 in your textbook)?
________pumps excess water out of the cell
Now observe living Amoeba. How does this one-celled organism move? Do you think this cell is enclosed by a cell wall?
_the amoeba moves by pseudopodia- does not likely have a cell wall.
Now look at prepared slides of neurons, muscle tissue and blood cells. Do all of these cells have a nucleus? If not, what can you conclude about the need for a nucleus in order for a cell to be functional at maturity?
_Blood cells do not have a nucleus at maturity. Therefore, a nucleus is not necessary for the proper functioning of all MATURE cells. _______________________________________________________________________
Based on your observations of the neurons, blood cells, and muscle cells, how does the structure of each cell type reflect its function? Explain in terms of what the cell does, where it may be located in the human body, and its turn over rate (time it takes to replace the cells) in the body compared to cheek epithelial cells.
Neurons- these are fairly long cells with numerous, branching ends. They are well-designed for making many connections with neighboring neurons. They also are not replaced, even if injured (hence permanent paralysis that can occur from injury). However, the organelles within neurons can be replaced.
Blood cells- these cells don't really need a nucleus at maturity (they simply need to be able to carry oxygen). Since they are so small, they have a large surface area to volume ratio (to carry a lot of oxygen in the membranes) and can also fit through tiny capillaries. They are replaced on a regular basis (produced in bone marrow).
Muscle cells- these cells are found as dense tissue and overlap each other extensively. Many muscle cells are also interconnected to allow the tissue to contract as a single unit. Once mature, they aren't well suited for cell division and thus the cells are not replaced on a regular basis.