Lesson 8 of 8 / Biological molecules
Results tables, controls and errors
How do you show that the enzyme, and nothing else, caused the change?
I can draw a results table to the marked rules, set up a control with its reason, and state a source of error with its effect on the result.
I can investigate and explain temperature and pH effects on enzyme reactions.Syllabus
Syllabus K327 / K328, Topic 3(e). Investigate and explain temperature and pH effects on enzyme reactions.
About 7 min
Make a guess. You are not marked.
In an amylase experiment you set up a second tube with boiled, cooled amylase and starch. What is that tube for?
Show the answer
To show the change needs active enzyme.
Boiled amylase is denatured. If starch stays in that tube, the change in the test tube was caused by active enzyme.
The key idea
A marked results table puts the independent variable first, the unit only in each heading, and every reading to the same number of decimal places. A boiled-enzyme control shows that the change needs active enzyme. Every source of error needs its effect: which way it pushes the reading.
A control set-up is identical to the test set-up except that the factor being tested is removed or made inactive. A source of error is a feature of the method that makes a reading differ from its true value.
Explore the idea
Would this table earn the marks?
| Temp | Time | Time | Time |
|---|---|---|---|
| 20 | 182 s | 176 | 179.0 |
| 30 | 94 | 90 s | 98 |
| 40 | 41 | 45 | 43.0 |
| 50 | 66 | 71 | 64 |
| 60 | 300 | 300 | 300 |
Find five problems before you switch to the marked layout: look at the headings, the units, the decimal places, the missing column and the 60 degrees C row.
Control: the only difference between the two tubes is that the control enzyme was boiled, which denatures it. Starch remains in the control tube, so active amylase, not heat or time, made the starch disappear in the test tube.
Original practice values, not measured data. The two tubes look the same at the start; only the iodine spots at the end show which one had active enzyme.
Explanation
A practical question usually gives marks for the table before it gives marks for the biology. Rule the table with a border and lines between columns. Put the independent variable in the first column. The independent variable is the factor you change on purpose, such as temperature. Put each repeat of the dependent variable in its own column, then a mean column. The dependent variable is the factor you measure, such as the time for starch to disappear.
Each heading has a quantity and a unit, written "Temperature / degrees C" or "Time for starch to disappear / s". The unit goes in the heading only, never in each cell. Every reading in a column has the same number of decimal places, matching the instrument. A stopwatch read to the nearest second gives whole numbers, so "43" is right and "43.0" or "43 s" in a cell is wrong. Give a mean to the same number of decimal places as the readings.
Some results have no end point in the time you allow. If starch is still present after 300 s, record "> 300" rather than inventing a number, and do not use it to calculate a mean or a rate. Then the reader knows the reaction was too slow to measure.
A control set-up shows that the change you measured is caused by the factor you are testing. For an amylase experiment, boil a sample of the amylase for a few minutes, cool it to the test temperature, and run it with starch exactly like the test tubes. Boiling denatures the enzyme: its active site loses its 3D shape. If the iodine still turns blue-black at the end, starch was not digested, so the disappearance of starch in the test tubes was caused by active enzyme, not by heat or by starch breaking down on its own. Water in place of the enzyme can be a second control.
Sources of error must come with their effect. "Human error" scores nothing. Judging the end point by eye is a common one: the blue-black colour fades gradually, so you may record the end point late, making the time too long and the calculated rate too low. Compare each spot with a reference spot of iodine and water. A second is temperature: if the enzyme and starch are mixed before both reach the water-bath temperature, the reaction starts at a lower temperature, so the time is too long. Leave both solutions in the water bath for 5 minutes before mixing. A third is the sampling interval: sampling every 30 s means the true end point could be up to 30 s earlier than recorded, so sample every 10 s.
Step by step
- 1
Lay out the table
Independent variable first, then repeats, then mean. Rule the lines.
- 2
Write the headings
Quantity / unit in each heading. No units in the cells. Same decimal places down each column.
- 3
Add a control
Boil and cool the enzyme, or replace it with water, and keep everything else the same. State what the control shows.
- 4
Name an error and its effect
Say what goes wrong, whether the reading becomes too high or too low, and how to improve it.
Worked example
Fix the end-point problem
A student times how long amylase takes to digest starch at 40 degrees C. She mixes room-temperature solutions and then puts the tube in the 40 degrees C water bath. State the source of error, its effect on the time and an improvement.
One way to explain it
The mixture starts below 40 degrees C, so for the first part of the experiment the enzyme works at a lower temperature. The recorded time is too long, so the rate is underestimated. Leave the starch and the amylase in separate tubes in the 40 degrees C water bath for 5 minutes before mixing.
Why this answer works
- Name the exact problem, not "human error".
- Give the direction of the effect on the reading and on the rate.
- The improvement fixes that particular problem.
Watch out for this
A student says: "A control is a set-up where you keep the variables constant." What is wrong with this?
Show the answer
Keeping variables constant makes the test fair. A control set-up is a separate set-up with the tested factor removed or made inactive, such as boiled enzyme. It shows that the result needs that factor.
Check your understanding
Why does an amylase experiment include a tube of boiled and cooled amylase with starch?
Show the answer
To show that starch disappears only when active enzyme is present.
Boiling denatures the enzyme. If starch remains in that tube, the change in the test tubes was caused by active amylase.
Quick recall
How do you show that the enzyme, and nothing else, caused the change?