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 K224 / K225, 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
In practical questions, the results table earns marks before the biology does. Draw the table with a ruler, with a border and lines between the columns. Put the independent variable, the factor you change (such as temperature), in the first column. Put each repeat of the dependent variable, the factor you measure (such as time), in its own column, then a column for the mean.
Each heading has a quantity and a unit, written "Temperature / degrees C" or "Time / s". Put the unit in the heading only, never in the boxes. Give every reading in a column to the same number of decimal places. A stopwatch read to the nearest second gives whole numbers, so write "43", not "43.0". Give the mean to the same number of decimal places.
Sometimes a reaction does not finish in the time you allow. If starch is still there after 300 s, write "> 300". Do not make up a number, and do not use it in a mean.
A control set-up shows that the factor you are testing caused the change. For amylase, boil some amylase, cool it, and set it up with starch just like the other tubes. Boiling denatures the enzyme. If the starch is still there at the end, then in the other tubes it was the active enzyme that made the starch disappear.
Every source of error needs its effect. "Human error" scores nothing. For example, the blue-black colour fades slowly, so you may record the end point too late, making the time too long. Compare each drop with a spot of iodine and water. Another example: if the solutions are mixed before they reach the water-bath temperature, the time will be too long. Leave them in the water bath for 5 minutes first.
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?