Laboratory Notes for BIO 1003© 30 August 1999, John H. Wahlert & Mary Jean HollandMEASUREMENT(Click here for: Materials, Themes, Web resources) Measurement of common dimensions of organisms, such as length, width, diameter, volume, and weight, provides data that can be used to compare magnitudes. Absolute precision of measurement is never possible; the equipment used to make the measurement limits its precision. Always note the degree of precision of your measuring equipment before recording data. By the way, data is plural, datum is singular. In science the metric system of measurement is used exclusively. The textbook has a table of terms and equivalence with the English system. The most common quantities that you might measure in laboratories of this course are:
Temperature is measured in the Celsius system: 0 degrees is the freezing point and 100 degrees is the boiling point of water at sea level. 22 degrees is a comfortable room temperature; 37 degrees is normal human body temperature. Suppose you are using a caliper marked in millimeters (mm) to measure the width of a squirrel molar tooth; the caliper is accurate to the nearest tenth (0.1) mm. If the caliper reads 3.4 mm, the possible range of diameter for the squirrel tooth is from 3.35 mm to 3.45 mm. The number 3.4 is said to have two significant figures. If you add a zero in the hundredths place, 3.40 mm, the meaning is changed to three significant figures and the possible range from 3.395 mm to 3.405 mm. Recording the measurement as 3.40 mm is wrong for a caliper accurate only to tenths of a millimeter. When your data are recorded, you may present them for comparison in a table or as a graph. Your display must have a title and possibly an explanatory caption, so don't run the graph to the edge of the page. The columns in the table and the axes of a graph must be clearly labeled. The first column of a table and the x-axis (horizontal) of a graph display the independent variable, which increases or decreases by arbitrarily determined increments such as time or size ranges. The dependent variable is displayed on the y-axis and is a function of the independent variable. It could be the number of individuals of a given size range or the size of a population in a given year. The units and dimensions of the variables are always clearly marked along the two axes. Most dimensions or organisms are examples of continuous variables; these may take any numerical value within a specified range. Characters such as length and width are continuous variables because they are determined by many genes and by environmental influence on expression of the phenotype. Discontinuous variables take only discrete values and a graph will change by steps. Simple genetic systems will show such a pattern. In the following experiments you will be using an electrical balance with digital read out. The balance measures weights from 0.1 g to 200.0 g. Make sure the units are set to grams (g), and press "tare" to zero the balance before weighing objects. If the amount does not return to 0.0 g when you remove an object from the pan, tare the balance again. Experiment I. Pine cone population Weigh 20 large pine cones and record their individual weights. Does every pine cone have the same weight? What is the range of pine cone weights? Sum the weights of the cones in your sample. Calculate the mean weight of a pine cone by dividing the sum of the weights by n, the total number of pine cones weighed. Be sure to express your answer to the correct number of significant digits. Is the mean weight the same for every sample of 20 pine cones? What is the median weight? The median of a sample is the observation with an equal number of observations above and below it. In samples with an even number of observations, the value half-way between the two middle observations is used. Is the median the same value as the average? Make a bar graph or histogram of your data and of the class data. Experiment II. Broad beans Weigh 20 broad beans and record their individual weights. Keep them separated out after weighing. What is the range of the bean weights? Sum the weights. Calculate the mean weight of the beans by dividing the sum of the weights by n, the total number of beans weighed. Now place all 20 beans that you weighed individually on the pan of the balance and weigh them. Is the total weight of the beans the same as the sum of the individual weights that you calculated above? Why might there be a difference? Calculate the average weight of a bean by dividing the total weight by 20. Is the average the same as the one you calculated previously? Is the amount of variation the same or different from that of the pine cones? Experiment III. Radish seeds Can you weigh a single radish seed on this balance? What is the total weight of 100 radish seeds? Put a plastic Petri dish on the balance pan to hold them, and tare the balance before adding any seeds. What is the average weight of a radish seed in this population? Do you know the range of individual seed weights? Liquid Measurement In future laboratory exercises you will be working with solutions (in living organisms water is the solvent, dissolved molecules are the solutes). It is important that you understand how to use the available glassware for measurement of liquids. Pour water into a graduated glass cylinder that is resting on the lab bench. Fill the cylinder about half way. Look at the top of the water; the surface is curved—concave upward; this is called a meniscus. The bottom of the meniscus is tangent to the line that marks the volume. A concave meniscus forms when the cohesion between water molecules is less than the adhesive attraction between the water and glass. If you are using a plastic cylinder, to which water does not adhere, the water surface is flat.) How much water did you pour into the cylinder? Since 1 cc of water at 4°C (39°F) weighs 1 g (density = 1 gram/cc), you can use the weight and volume of water as nearly interchangeable quantities. Place a 100 ml graduated cylinder on a balance, and tare the balance. Fill the cylinder almost to the 50 ml mark, and then use a pipette or dropper to make the meniscus tangent to the 50 ml line. What is the weight of 50 ml of water? You will use this information again in the laboratory exercise on diffusion and osmosis. These experiments were designed by students and faculty for NSF Grant # DUE-9354712.
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