Yeast, hydrogen peroxide and a balloon. Change the substrate, watch the balloon, then ask what it really measured.
Type your results into the tables. The page works out radius, volume, mean, standard deviation and standard error, and draws the graphs. Tap or hover any pink-underlined number to see the equation and working. The thinking is your job.
Fill in the white columns as you work. Calculate the shaded columns using the formulas in the dashed boxes.
Your answers save in this browser. To print, open the page in your browser and use File → Print.
1
The big idea
Cells make hydrogen peroxide as a toxic by-product. The enzyme breaks it down very fast.
2 H₂O₂ → 2 H₂O + O₂
The is H₂O₂. Catalase is the enzyme. It binds the substrate at its , and is not used up.
Two different questions: How much oxygen was made in total (the )? How fast was it made (the )? Keep these apart all lesson.
Safety: goggles on the whole time. H₂O₂ irritates eyes. Stand flasks on a tray because they can foam over.
2
Predict
P2What shape will each graph be?
Try: I think … because more substrate means … . The rate will … because the active sites …
3
Method
You will need (per group)
5 × 250 mL conical flasks
5 balloons
Small funnel
5 g dried yeast (5 × 1 g)
H₂O₂ at 0, 0.5, 1, 2 and 3%
25 mL measuring cylinder
String and 30 cm ruler
Stopwatch, goggles, tray
Press Play to watch the method, or step through it at your own pace.
3
Your group’s results
Method reminder: 20 mL of H₂O₂ in each flask, 1 g yeast in each balloon, tip all at once. Rank at 30 s. Measure circumference at 5 min round the widest part.
Practice numbers: made up Clear these before you type your own results.
If a balloon did not inflate at all, enter 0 for its circumference.
Radius: r = C ÷ 2π · Volume of a sphere: V = ⁴⁄₃ π r³
Graph: volume of O₂ at 5 minutes
4
Class results
Estimated O₂ volume at 5 minutes (cm³) for every group. The pink columns calculate themselves. Tap any value to see how it was worked out.
Practice numbers: made up Replace with your class’s real results.
Mean: x̄ = Σx ÷ n · Standard deviation: s = √[ Σ(x − x̄)² ÷ (n − 1) ] · Standard error: SE = s ÷ √n · Error bars: x̄ ± 2SE
Graph: mean O₂ volume with ±2SE error bars
5
Why does AP use standard error?
Standard deviation and standard error answer different questions.
Standard deviation (SD)
How spread out are the individual results?
It describes the data. Collecting more groups does not make it smaller. It just gets a better estimate of the real spread.
Standard error (SE)
How sure are we about the mean?
SE = SD ÷ √n. More groups → smaller SE. It describes how precise the mean is.
You want to compare means: did 2% really make more O₂ than 1%? That is a question about the means, so AP uses error bars of ±2SE. That range is about a 95% confidence interval for the mean.
The rule of thumb: if the ±2SE error bars of two groups do not overlap, the difference is probably real (statistically significant). If they overlap, you cannot claim a difference.
Try it: drag to add more groups
SD = cm³ (stays put) · SE = cm³ (shrinks)
6
Analyse
Enter class data to see a summary.
A1Which concentration gave the largest mean volume of O₂?
A2Do the ±2SE error bars for 1% and 2% overlap?
A3After about 5 minutes the fizzing stops. What has run out?
Try: The final size measures how much … there was, not how … catalase worked, because …
A6Compare with the most O₂ possible.
Enter class data for the 3% flask.
7
Making it quantitative
Mr Bradford says
Balloons are brilliant for seeing a reaction. They are terrible for measuring one. Let’s work out why, and what scientists would do instead.
Think about: shape · stretchiness · gas escaping when you attach it · reading the string
M2We measured the widest part and treated the balloon as a sphere. Will the volume be too high or too low?
M3Which method would measure the rate?
M4With a proper rate method, what shape would rate against H₂O₂ concentration be?
8
Conclude
Step 1: build the science
As H₂O₂ concentration increased, the total volume of O₂ .
The reaction stopped because the ran out.
So the final balloon size measures .
To compare rates, we must measure .
Step 2: which statements does your evidence support?
Tick only the ones your data and the science actually back up. Watch out for claims that go too far.
Step 3: write your conclusion (Claim, Evidence, Reasoning)
Try: At 3%, the mean volume was … ± … cm³, compared with … at 1%.
9
Evaluate
Don’t just name a problem. Say what it did to your results and exactly how to fix it.
Weakness in method
Effect on results
Specific improvement
Ideas to think about: gas lost while attaching · balloon shape · yeast clumping · string reading · starting all flasks together
✓
Before you hand in
Biology by Bradford · AP Biology catalase lab · Topics 3.2 and 3.3
Biology by Bradford
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