Investigating Digestion Experimentally Revision Notes | A Level AQA Biology

1. Why Investigate Digestion Experimentally?

Your digestive system breaks down enormous molecules — starch chains thousands of glucose units long, proteins folded into complex shapes — into fragments small enough to cross a single cell membrane. The question worth asking is: how do scientists actually measure that process, and how can you model it in a laboratory?

Two experimental approaches sit at the heart of this topic. First, you can directly measure how quickly a digestive enzyme works under different conditions — changing pH or adding bile salts. Second, you can use Visking tubing as a physical model of the gut wall to show which digestion products are small enough to be absorbed. Both approaches appear in AQA exam questions, so understanding the design logic behind each one is as important as knowing the results.

2. Enzyme Activity and the Variables That Affect It

Before designing any investigation, you need a clear picture of what you are measuring. A digestive enzyme catalyses a hydrolysis reaction — it uses water to break a bond in a large substrate molecule, producing smaller products. The rate of this reaction can be tracked by monitoring how quickly the substrate disappears or how quickly the product appears.

Two conditions are specified in the AQA specification as variables to investigate: pH and the presence of bile salts. These affect enzyme activity through different mechanisms, and understanding those mechanisms lets you predict results and explain anomalies — both essential skills for the higher mark bands.

2a. The Effect of pH on Enzyme Activity

Every enzyme has an optimum pH — the pH at which the active site geometry best complements the substrate. For salivary amylase, that optimum is around pH 7; for pepsin (a stomach protease), it is around pH 2. Moving away from the optimum changes the ionisation state of amino acid R-groups in and around the active site, altering the shape of the active site and reducing the rate of enzyme-substrate complex formation.

At extreme pH values, the hydrogen bonds and ionic bonds maintaining the tertiary structure of the enzyme are disrupted. The enzyme denatures — its active site is permanently distorted and it can no longer catalyse the reaction. This is irreversible, which is an important distinction from the reversible reduction in activity seen at mildly non-optimum pH values.

2b. The Effect of Bile Salts on Lipase Activity

Bile salts do not act on the enzyme itself — they act on the substrate. Lipids are hydrophobic and form large droplets in an aqueous environment. Lipase is a water-soluble enzyme that can only act at the surface of a lipid droplet, so the larger the droplet, the smaller the surface area-to-volume ratio and the fewer lipase molecules can bind simultaneously.

Bile salts are amphipathic — they have a hydrophilic end and a hydrophobic end. They surround large lipid droplets and break them into many smaller droplets, a process called emulsification. This dramatically increases the total surface area of lipid available to lipase, so the rate of lipid hydrolysis increases. Bile salts are not enzymes and do not lower activation energy; they are physical emulsifiers.

3. Designing the Investigation — pH or Bile Salts

A well-designed investigation controls every variable except the one being tested. For digestion experiments, this requires careful thought because enzymes are sensitive to temperature, pH, substrate concentration, and enzyme concentration simultaneously. Changing two variables at once makes it impossible to attribute any change in rate to a single cause.

Notice that rate is expressed as 1 ÷ time rather than time alone. This is because time and rate are inversely related — a shorter time means a faster rate. Plotting time directly would give a curve that goes the wrong way, with the lowest point at the optimum pH rather than the highest. Using 1/time converts the data into a true rate measure.

3a. Investigating Bile Salts — Adapting the Design

When investigating bile salts, the enzyme of interest is lipase and the substrate is a lipid — typically full-fat milk, which contains triglycerides. The most common indicator system uses a pH change: lipase hydrolyses triglycerides into fatty acids and glycerol, and the fatty acids lower the pH of the mixture. Adding a pH indicator (such as phenolphthalein, which is pink in alkaline conditions and colourless in acid) allows you to time how long it takes for the mixture to become acidic enough to decolour the indicator.

The independent variable is the presence or absence of bile salts (or the concentration of bile salts if you want a dose-response relationship). The dependent variable is the time taken for the phenolphthalein to decolourise, which is again converted to a rate using 1/time. Temperature, lipase concentration, milk volume, and the initial pH of the mixture must all be controlled.

3b. Reliability, Validity, and Evaluation

Reliability refers to how reproducible your results are — achieved by repeating each condition at least three times and calculating a mean. Validity refers to whether your experiment actually measures what you intend — achieved by controlling all variables except the independent variable and using an appropriate indicator for the product being measured.

• Colour endpoint judgements are subjective — different observers may record different times for the same reaction. Using a colorimeter to measure absorbance removes this subjectivity.
• Iodine solution itself can inhibit amylase activity at high concentrations — taking small samples from the reaction tube rather than adding iodine directly to the reaction mixture avoids this.
• Buffer solutions may not perfectly maintain pH during the reaction, especially if the reaction produces acidic or basic products — checking pH with a pH meter before and after improves validity.
• Enzyme solutions may contain variable amounts of active enzyme if stored incorrectly — preparing fresh solutions and storing on ice reduces denaturation between repeats.

4. Visking Tubing — Modelling Absorption in the Small Intestine

Now that you have investigated how digestion works, the next question is: what happens to the products? In the body, small molecules produced by digestion cross the epithelium of the small intestine and enter the blood. In the laboratory, Visking tubing models this process by acting as a selectively permeable membrane.

Visking tubing is a dialysis membrane made from regenerated cellulose. Its pores are large enough to allow small molecules — glucose, amino acids, fatty acids, glycerol, water — to pass through by diffusion or osmosis, but too small to allow large molecules such as starch, proteins, or intact triglycerides to pass. This mirrors the situation in the gut, where only fully digested products are small enough to be absorbed.

The control setup is critical here. Without it, you cannot distinguish between two possible explanations for a negative iodine result in the surrounding water: either starch was hydrolysed by amylase and the products diffused out, or starch was too large to pass through the tubing at all. The control, which contains starch but no amylase, confirms that starch cannot cross the membrane regardless — so any glucose detected in the experimental setup must have come from enzymatic hydrolysis inside the tubing.

4a. Interpreting Results from the Visking Tubing Model

When you analyse results from a Visking tubing experiment, the key question is always: which molecules are present in the surrounding water, and what does that tell you about their size relative to the pore size of the membrane? A positive Benedict's test in the surrounding water confirms that reducing sugars — glucose or maltose — have passed through. A negative iodine test in the surrounding water confirms that starch has not passed through.

Worked example: Interpreting a Visking Tubing Experiment Result

A student fills Visking tubing with a mixture of starch solution and amylase. After 30 minutes in a water bath at 35°C, she tests the surrounding water. The iodine test gives no colour change. The Benedict's test gives a brick-red precipitate. Explain what these results show about digestion and absorption.

1. Iodine solution: no colour change in surrounding water — Iodine solution turns blue-black in the presence of starch. No colour change means starch is absent from the surrounding water. This could be because starch was hydrolysed inside the tubing and/or because starch molecules are too large to pass through the Visking tubing membrane.
2. Benedict's reagent: brick-red precipitate in surrounding water — A brick-red precipitate with Benedict's reagent indicates the presence of a reducing sugar — in this context, glucose or maltose produced by amylase hydrolysing starch. These small molecules have passed through the pores of the Visking tubing into the surrounding water.
3. Starch (large polymer) + amylase → maltose/glucose (small monomers/disaccharides) — Amylase inside the tubing has hydrolysed starch into smaller sugar molecules. These products are small enough to diffuse through the Visking tubing membrane, whereas intact starch molecules are not.
4. Visking tubing membrane = gut wall; surrounding water = blood — The results model what happens in the small intestine: large food molecules must be digested into small products before they can be absorbed across the gut wall into the bloodstream. The Visking tubing demonstrates this size-dependent selectivity.

Answer: The results show that amylase has hydrolysed starch into reducing sugars (glucose/maltose) inside the Visking tubing. These small molecules diffused through the selectively permeable membrane into the surrounding water, modelling absorption into the blood. Starch itself could not pass through because its molecules are too large for the pores of the Visking tubing.

5. Exam Technique — Answering Questions on Digestion Investigations

Questions on these practicals appear across a range of command words. The most common errors come from confusing what the investigation measures with what it shows, and from writing vague evaluations that do not link improvements to specific sources of error.

When a question asks you to evaluate an experimental design, structure your answer around three elements: identify the limitation, explain why it reduces validity or reliability, and state a specific improvement. Vague answers such as 'repeat the experiment more times' score poorly unless you also state why repeating improves reliability — because it allows anomalous results to be identified and a more representative mean to be calculated.

Both investigations covered in this lesson — the enzyme activity experiment and the Visking tubing model — test your ability to connect experimental design to biological principles. The rate of a digestive enzyme reaction depends on conditions such as pH and the physical state of the substrate (affected by bile salts); and the absorption of digestion products depends on molecular size relative to membrane pore size, modelled directly by Visking tubing.