Biology by Bradford

AP Biology · Lab 2 · Cellular respiration

Which sugars can yeast use?

Same yeast, same dough, five different sugars. Which ones does yeast actually turn into gas?

Name Group

Type your results into the tables. The page works out the means, standard deviation, standard error and 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.

The big idea

Yeast respires sugar without oxygen (). The CO₂ it makes gets trapped as bubbles, so the dough rises.

C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂

Only a can enter . A bigger sugar must first be split by an enzyme. No enzyme, no fuel.

Substrate	Type	Built from	Enzyme needed to split it
Glucose	Monosaccharide	–	None
Sucrose	Disaccharide	Glucose + fructose	Invertase (sucrase)
Lactose	Disaccharide	Glucose + galactose	Lactase
Starch	Polysaccharide	Hundreds of glucose units, branched	Amylase
Control	No sugar added (just the flour)

Mr Bradford says

Starch is a storage molecule. It is great for packing away energy, but the cell has to unpack it before it can be used. Keep that in mind when you predict.

Predict

P1Rank the five doughs from most rise (1) to least rise (5).

P2Explain your ranking using the idea of enzymes.

Try: I think … will rise most because … . I think … will rise least because yeast does not have …

P3Sucrose and lactose are both disaccharides. Will they give the same result?

Method

You will need (per group)

4 g dried yeast
75 mL warm water (35–40 °C)
Plain flour (about 100 g)
1 g each: glucose, sucrose, lactose, starch
5 small beakers + 100 mL beaker
5 × 10 mL measuring cylinders
Water bath at 35 °C, thermometer
Spatulas, marker, stopwatch

Press Play to watch the method, or step through it at your own pace.

Your group’s results

Method reminder: fill each tube to the 3.0 mL mark, stand in the 35 °C water bath, read the top of the dough every 5 minutes.

Change in volume: Δ = final volume − starting volume

Water bath temperature (°C)

Graph: dough volume over time

I can read the title and both axis labels with units
Each substrate has its own line and marker shape in the key
I can see which line is steepest (the fastest rate)

Send my changes to the class table as group

Class results

Change in volume after 30 minutes (mL) for every group. The pink columns calculate themselves. Tap any value to see how it was worked out.

Mean: x̄ = Σx ÷ n · Standard deviation: s = √[ Σ(x − x̄)² ÷ (n − 1) ] · Standard error: SE = s ÷ √n · Error bars: x̄ ± 2SE

Graph: mean change in volume with ±2SE error bars

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.

In this lab you want to compare means: is glucose really better than sucrose? 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

Number of groups, n = 6

SD = mL (stays put) · SE = mL (shrinks)

Analyse

Enter class data to see a summary.

A1Which substrate gave the largest mean rise?

A2Do the ±2SE error bars for glucose and sucrose overlap?

So can you say glucose is better than sucrose? Explain.

A3Is starch clearly different from the control?

A4Yeast has the enzyme invertase but not lactase. Use this to explain your sucrose and lactose results.

Try: Sucrose rose by … mL because … . Lactose only rose by … mL because …

A5The control dough still rose a little. Where did that sugar come from?

A6Baker’s yeast cannot digest starch, but flour contains its own amylase. Who is breaking down the starch in your dough? Why does starch rise only a little more than the control?

A7Why is starch a good storage molecule but a slow fuel?

Think about: branched · compact · insoluble · must be hydrolysed first

Conclude

Step 1: build the science

Yeast produced the most CO₂ with . Sucrose works because yeast has , which splits it into glucose and fructose. Lactose rose about the same as the control because yeast . Starch must be before yeast can use it, so it is a slow fuel.

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.

Glucose and sucrose gave the largest mean rise.
Yeast can always use any sugar if you give it enough time.
If the ±2SE bars for glucose and sucrose overlap, we cannot say one is really better.
Our results prove that starch is useless as food for all organisms.
Lactose gave little rise because yeast lacks the enzyme to split it.
Some of the control’s rise came from sugars already in the flour.

Step 3: write your conclusion (Claim, Evidence, Reasoning)

ClaimAnswer the question: which sugars can yeast use?

EvidenceQuote class means with ±2SE. Say which error bars overlap.

Try: Glucose had a mean rise of … ± … mL, compared with …

ReasoningExplain using enzymes, monosaccharides and glycolysis.

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: dough consistency · air pockets · domed surface · water bath temperature · reading time

E1Your reading uncertainty is about ±0.1 mL, but class results varied much more. What causes the extra variation?

E2Suggest a method that measures CO₂ more directly than dough height.

✓

Before you hand in

My table has my own results, not the practice numbers
I have quoted means with ±2SE in my evidence
I said whether error bars overlap, and what that means
My reasoning uses the words enzyme, monosaccharide and glycolysis
Each weakness has an effect and a specific improvement

Biology by Bradford · AP Biology yeast respiration lab · Topics 1.5 and 3.6