Bio10 Lab 2: Cells Cells are the smallest living things and all living things are composed of cells. They are able to perform all necessary metabolic functions as well as specialized tasks such as moving, feeding and reproducing. Despite their minute size, cells are amazingly complex and often very beautiful. There are two general types of cells: Prokaryotic cells o Lack a nucleus, but they do contain DNA o Also lack other organelles, such as mitochondria, chloroplasts, etc. o Are simple unicellular organisms like bacteria. o They represent earth s oldest organisms, having evolved about 4 billion years ago Eukaryotic cells o Do contain a membrane-bound nucleus and other organelles o Most organisms are eukaryotic all protists, fungi, plants and animals are in this group. Using your text and the cell models and posters in the lab, sketch an animal cell and a plant cell on the group results sheet. Activity 1: Looking at selected eukaryotic cells Eyepieces Microscope Objective lenses Stage Coarse focusing (outer) knob Illuminator Fine focusing (inner) knob 1
Focusing the Microscope: 1. Position slide on stage; use the clips to hold the slide in place. 2. Using the 4x objective lens and the coarse focus knob, bring the specimen into focus 3. Center the specimen and switch to the 10x objective lens. Focus using the fine focus knob ** Note: the course focus knob can only be used with the 4x objective lens!** 4. Center the specimen and switch to the 40x objective lens. Focus using the fine focus knob Calculating the total magnification To determine the total magnification, multiply the magnification of the eyepiece lens (10x) by the magnification of the objective lens (4x, 10x, or 40x). Examine an animal cell (your cheek cell) under the microscope Gently scrape the inside of your mouth with a toothpick. Smear the cells on a clean slide and stain them with a drop of methylene blue (being careful not to stain your fingers). Place a coverslip on top. Discard the toothpick and slide in the biohazard container. Beginning on low power (4X), locate some cheek cells. Center them in the field of view and increase the magnification to 10X and then to 40X. You should be able to see some flat cells with a tiny, dark speck inside the cell this is the nucleus. Are we prokaryotic or eukaryotic organisms? Draw your cheek cells on the Group Results Sheet. Examine a plant cell under the microscope Make a wet-mount slide of Elodea leaves (an aquatic plant often used in fish tanks). Take just a small sample from the growing tip of the Elodea, place it in a clean drop of water and add a coverslip. Do NOT add stain. Begin on low power you know the drill. Sketch some Elodea cells on the Group Results Sheet. Activity 2: Osmosis and Water Balance In this activity, you will be looking at the diffusion of water (aka osmosis). Remember that passive transport is driven by a concentration gradient and will continue until equilibrium is reached provided the solute (e.g. salt) does not encounter any barriers it cannot pass through (like a cell membrane) Salt exists in water as sodium ions (Na + ) and chloride ions (Cl - ). Charged ions like these are unable to diffuse through most biological membranes. However, water molecules can diffuse through biological membranes so salinity imbalances are corrected by the diffusion of water across the membrane to equalize salt concentrations on both sides of the membrane. The diffusion of water across a selectively permeable membrane is known as osmosis. Consider the case of a membrane separating two solutions with different concentrations of a solute say salt (NaCl). Solutions with equal solute concentrations are said to be isotonic to each other. A solution with a higher concentration of solute is hypertonic to the other solution. The solution with a lower solute concentration is hypotonic to the other solution. 2
Note that the hypotonic solution has the lower solute concentration, but also the higher water concentration. Therefore water will diffuse across the membrane along its concentration gradient from an area of higher water concentration (the hypotonic solution) to one of lower water concentration (the hypertonic solution). Water balance in animal cells. In order to survive, marine and freshwater organisms must have a way to balance an excessive uptake or excessive loss of water. Organisms living in seawater must have a means of preventing the loss of water from their body to the highly saline environment. Freshwater organisms must deal with the opposite problem of preventing excessive amounts of water from the freshwater environment entering their more saline bodies. The control of water balance is called osmoregulation. 3
The POTATO Activity 1. In your lab groups (4 students), core out 4 equally sized pieces of potato. Keep the potato(s) that you take the cores from at your table. 2. Take 3 empty beakers and fill them with the salt water concentrations provided (0.5%, 3.5%, 7.5%). Fill them enough to cover the potato. 3. Place one piece of potato into each filled beaker. 4. Wait 10-20 minutes, and while you re waiting go to the Elodea activity (below) 5. Remove the potato cores from the solutions and try to reinsert them back into the potato that you removed them from. 6. Based on how easy it is to reinsert the core, complete the data sheet below. "Feel" of Reinsertion What do you think happened to the cells?* Is the solution hypo, iso, or hypertonic to the potato cells? Solution A (0.5%) Solution B (3.5%) Solution C (7.5%) * Are the potato cells larger? Did they shrink? Did they stay the same size? The Elodea Activity 1. Each lab group should take a single leaf of Elodea and place it on a microscope slide. Place a few drops of freshwater on the leaf and place a coverslip over it. 2. Observe it under the microscope using the 40X objective. Draw it on the Results Sheet. 3. Take a few drops from the 7.5% salt water beaker and place them at one edge of the cover slip. At the same time, place a piece of paper towel at the adjacent edge of the cover slip. This will cause the salt water to flow through the cells of the Elodea plant and get soaked up by the paper towel. 4. After 1-2 minutes, observe the Elodea cells and draw your observations on the Group Results Sheet. 5. Repeat steps 3 & 4 using regular tap water instead of the salt solution. 6. After 1-2 minutes, observe the Elodea cells and note any changes that you observe. Illustrate. 4
Lab 2 Group Results Sheet Names 1. In the space below, diagram two eukaryotic cells an animal cell and a plant cell. Using your text as well as the cell models and posters in the lab, label the following structures on your drawings. Cell membrane Mitochondria Cell wall Nucleus Chloroplast Central vacuole Animal cell Plant cell 2. Which of the structures listed above are only found in plant cells? 3. In the space below draw your cheek cells using the 40X objective. Label the cell membrane, nucleus and cytoplasm. a. What is the total magnification? TM= X 4. Sketch some Elodea cells in the space below. Label the cell wall, central vacuole and chloroplasts. a. TM= X 5
5. What do you think happened to the potato cells in each of the solutions? Did they get bigger? Shrink? Say the same? Why? Solution A (0.5%) Solution B (3.5%) Solution C (7.5%) Which solution is hypertonic? 6. What would happen to a red blood cell placed in a hypotonic solution? Why? 7. Based on your results, do you think potatoes can osmoregulate successfully? Why or why not? 8. Elodea Cell Drawings (Please note the total magnification for each image) Original wet mount After 2 drops of 7.5% salt water After 2 drops of tap water 9. Did water move into or out of the Elodea cell when the salt water solution was applied? Explain your reasoning. 10. Did water move into or out of the Elodea cell when the fresh water solution was applied? Explain your reasoning. 6