AICE Biology Lab Review: Practical Skills Part 2

Every experiment needs a Control Where factor we are investigating is missing Conduct experiment without this factor and record results –If you are trying to see how long it takes an enzyme to work in breaking up sucrose by measuring how long it takes glucose solution to change color using the Benedicts test, you MUST have a control How long does it normally take to observe a color change using a Benedicts test with glucose?

Accuracy, Precision, Reliability Accuracy –How true measurement is –Depends on the calibration of instrument you are using Precision –Ability of the measuring instrument to give the same reading every time you use it –Doesn’t have to be the “true” value –Repeatable, reproducible Reliability –Degree of trust you have in the instrument –If measurements are reliable, then you expect to get same measurements in repeated experiment on that tool –Affected by ACCURACY and PRECSION and TYPE of measurements being taken –Best way to deal with poor reliability: REPEAT readings several times (multiple trials at same time)

Estimating Uncertainty Look at what the measurements are marked by –Every 1 mm Your measurement falls between the 5mm and 6mm mark –Half way between two marks would be 0.5mm (this is not marked, you eyeball half way) –You say…5.5 mm There is possible error in your measurement by 0.5 mm So your measurement is 5.5mm + 0.5mm Rule: potential error in measurement is half the value of smallest divisions on the scale you are reading from When measuring change between values, you have to add the possible errors for both measurements –You measured distance change from 5.5mm to 10.0 mm= 4.5 mm –Answer should be written: 4.5 mm + 1mm

Percent Error Sometimes you need to show degree of uncertainty as percentage Divide the degree of uncertainty by the measurement itself and then multiply by 100 Example from previous slide: –So your measurement is 5.5mm + 0.5mm –0.5mm/5.5mm x 100 = 9.1%

Displaying Data Line graphs –Use when IV and DV are both continuous Histograms –Continuous variable on x-axis and a frequency on the y- axis Bar Charts –Use when the IV is made up of a number of different, discrete categories AND DV is continuous

Line Graphs IV  x-axis DV  y-axis Each axis should be fully labeled with units (copy headings from results tables) Scales for axes should be just below (or at) your lowest reading AND go up to (or just above) your highest reading –Does not always have to start at zero Use maximum width and height for your scales…use the whole paper/grid they give you Scales should go up in regular increments (2,4,6,8 or 5, 10, 15, 20) –Good scales go by 1s, 2s, 5s, or 10s (avoid odd) PLOT EACH POINT USING A NEAT CROSS OR A DOT WITH A CIRCLE AROUND IT Draw a smooth best fit line –If you recognize a pattern (increasing or decreasing) it’s easy –If there is not a pattern, use a ruler to connect each point If you need two lines, make sure to label each clearly

Histograms Numbers on the x-axis are written ON THE LINES –Each bar includes all the values between the numbers on the lines Bars are the same width Bars are TOUCHING –Important b/c x-axis scale is continuous (different from bar chart)

Bar Charts X-axis has overall heading (Insects) Each bar has its own label (you can also make a key) The y-axis has a normal scale (same as line graph) Bars are same width Bars do NOT touch

Recording Measurements Create a data table ALWAYS Make table BEFORE you start Independent Variable should be in the FIRST column The readings/measurements you take in the next columns –Sometimes second column will be dependent variable –Sometime you will have multiple columns because you need to calculate the dependent variable Lines should be clearly drawn with a ruler and pencil Each column must be FULLY headed including the unit of measurement –Use a slash or brackets to denote the unit…be consistent length of stem/mm OR length of stem (mm) Don’t use slash to denote “per”…use negative exponents (-1) or the word “per” Velocity/ms -1 OR velocity/m per s DO NOT put units in the boxes with the actual measurements (only in headings)

Test tube # Concentration of enzyme / dm 3 Time for color change /s Rate of reaction /dm 3 s Good!!!BAD!!!!! Good!!!

More on Tables If you have to do calculations…. –Use significant figures Answers should have same amount of sig figs as measurements in calculations Keep same decimal places throughout table

Identifying Sources of Error Sometime you will have to identify experimental errors…. Experimental errors vs. mistakes –Do NOT describe “mistakes” –Mistake  something you do incorrectly Misreading a scale Not emptying your pipette completely Systematic Error –They are the same throughout your experiment –Intrinsic errors in the tools you are using Random Error –They differ throughout experiment –Fluctuating temperatures in the room –Cells take from different parts of the organism Significant Error vs. Insignificant –Think of experiment Temperature of room is important in transpiration experiment but not an osmosis experiment Enzyme experiments with water bath…. –Using electronic water bath…reliable –Bunsen burner and beaker water bath  higher chance of r andom error b/c of fluctuating room temps…

Suggesting Improvements How could you get more valid or reliable result? –Suggest controlling certain variables better in your experiment –You MUST state how you would better control the variable –Examples: Using a thermostatically controlled warm water bath instead of Bunsen burner Using a colorimeter instead of color standards DO several more repeats of experiments to get an average

Making Comparisons “Prepare the space below” usually means you need to create a table of some sort Always use a ruler Your first column should be “feature” that you are comparing Second and Third column should be the two different micrographs/organisms/diagrams you are comparing If it says OBSERVABLE differences, make sure you only list the things you actually see on the test itself (usually in a figure or diagram) Descriptions should be “positive”… if you are discussing the shape of the cell and one is smooth, you should NOT say the other is “not smooth”…you SHOULD say the other is “rough” FeatureCell ACell B Nucleus shapeRound, circularOblong, oval Nucleus sizeLarge, takes up more than half the cytoplasm Small, takes up less than a quarter of the cytoplasm Cell membrane shape/texture Smoothrough

Drawings KNOW what to draw from your slide (3 adjacent cells= 3 cells next to each other…2 guard cells, 3 phloem cells, etc…) DRAW what YOU see, not what you think it’s supposed to look like…. Use ALL the available space (no tiny, microscopic drawings) Use correct proportions –If the nucleus you are looking at is 2/3 the size of the entire cell, make sure your drawing represents this! SIMPLE and CLEAR is better Low power plan diagram  only show tissue layers…NO CELLS! –You don’t need to stay in low power on the microscope just because it’s a low power plan diagram…you may need to go to medium or high power to get an accurate visual of the shape of the tissue

Drawing Labels Use pencil Use ruler Make sure END of the label line actually TOUCHES the structure you are labeling Make sure NONE of your label lines CROSS each other write the labels horizontally (normal) Write the labels OUTSIDE the drawing itself (not on top of or inside of the drawing)