Introduction to Practical Investigations
Welcome to the hands-on side of Human Biology! In this chapter, we step out of the textbook and into the lab. We will explore how scientists actually test for the "building blocks of life" (carbohydrates, proteins, and lipids) and how enzymes—those amazing biological catalysts—behave under different conditions. Understanding these practicals is vital because exam questions often ask you to describe these methods or explain how to make them "fair tests."
1. Testing for Biological Molecules
Before we can do complex experiments, we need to know what is inside our food. These qualitative tests tell us if a substance is present.
Testing for Glucose (Reducing Sugars)
The Method: Add Benedict’s solution to your food sample and heat it in a water bath set to about \(80^\circ C\).
The Result: If glucose is present, the color changes from blue to green, yellow, or brick-red (depending on how much glucose there is).
Common Mistake: Forgetting to heat the mixture. Unlike other tests, Benedict’s will not work at room temperature!
Testing for Starch
The Method: Add a few drops of iodine solution to the sample.
The Result: A change from brown/orange to blue-black indicates starch is present.
Testing for Protein
The Method: Add Biuret solution (a mixture of sodium hydroxide and copper sulfate) to the sample.
The Result: A change from blue to lilac/purple means protein is there.
Testing for Lipids (Fats and Oils)
The Method: The emulsion test. Mix the sample with ethanol, shake it, and then pour the liquid into a test tube of cold water.
The Result: If lipids are present, a cloudy white emulsion (like a milky layer) will form.
Quick Review Box:
• Glucose: Benedict's + Heat \(\rightarrow\) Brick-red
• Starch: Iodine \(\rightarrow\) Blue-black
• Protein: Biuret \(\rightarrow\) Purple
• Lipids: Ethanol \(\rightarrow\) Cloudy white
2. Investigating Vitamin C
Vitamin C is essential for healthy skin and gums (preventing scurvy). We can test for it using a blue dye called DCPIP.
Qualitative and Quantitative Content
To measure the amount of Vitamin C, we add the food juice (like orange juice) drop by drop into a fixed volume of DCPIP.
The Result: Vitamin C "decolorizes" the DCPIP, turning it from blue to colorless.
How to compare: If Juice A takes 5 drops to turn the DCPIP clear, but Juice B takes 20 drops, Juice A has a much higher concentration of Vitamin C. It is "stronger" because it did the job with fewer drops!
3. Investigating the Energy Content of Food
We can find out how much energy is in a piece of food (like a dried pasta shell or a nut) by burning it and using the heat to warm up water.
The Method
1. Measure a set volume of water into a boiling tube.
2. Record the starting temperature of the water.
3. Weigh the food sample.
4. Set the food on fire using a Bunsen burner and hold it immediately under the boiling tube.
5. Once the food is completely burnt, record the final temperature of the water.
The Calculation
You can calculate the energy released using this formula:
\(Energy\ (Joules) = mass\ of\ water\ (g) \times 4.2 \times temperature\ rise\ (^\circ C)\)
Note: The number \(4.2\) is the specific heat capacity of water (the energy needed to heat 1g of water by \(1^\circ C\)).
Why is this experiment often inaccurate?
In a simple school lab, a lot of heat escapes into the air or stays in the glass tube rather than heating the water. In professional labs, they use a "bomb calorimeter" which is insulated to prevent this heat loss.
4. Investigating Enzyme Activity
Enzymes are protein catalysts. Their shape is vital to how they work. We usually investigate how temperature or pH affects how fast they work.
The Effect of Temperature
1. Mix an enzyme (like amylase) with its substrate (starch).
2. Keep the mixture at a specific temperature using a water bath.
3. Every 30 seconds, take a drop of the mixture and add it to iodine.
4. Record how long it takes for the iodine to stop turning blue-black (this means all the starch is gone).
5. Repeat at different temperatures (e.g., \(20^\circ C, 30^\circ C, 40^\circ C, 50^\circ C\)).
What to expect:
As temperature increases, the rate increases because molecules move faster and collide more. However, above the optimum temperature (usually around \(37^\circ C\)), the rate drops to zero because the enzyme denatures (its active site changes shape and no longer fits the substrate).
The Effect of pH
This follows a similar method, but instead of changing the temperature, you use buffer solutions to keep the mixture at different pH levels (e.g., pH 4, pH 7, pH 9). Each enzyme has an optimum pH where it works fastest.
5. Immobilised Enzymes
In industry, it is expensive to keep buying new enzymes. Instead, we "immobilize" them so they can be reused and don't end up mixed into the final product.
Preparing Alginate Beads
1. Mix the enzyme (e.g., lactase) with a solution called sodium alginate.
2. Use a syringe to drop this mixture into a solution of calcium chloride.
3. Small, jelly-like beads will form. These beads have the enzyme trapped inside them.
4. You can then pour a liquid (like milk) over the beads in a column. The enzyme works on the liquid as it passes through, but the enzyme stays trapped in the beads.
Real-World Example: This is how lactose-free milk is made! The enzyme lactase in the beads breaks down the lactose sugar in the milk into glucose and galactose as the milk flows past.
Key Takeaways for Exam Success
Identify the Variables:
• Independent Variable: The thing you change (e.g., the temperature).
• Dependent Variable: The thing you measure (e.g., the time taken or the temperature rise).
• Control Variables: Things you keep the same to make it a fair test (e.g., the volume of enzyme, the concentration of substrate).
Safety First:
Always mention safety! Wear safety goggles when handling chemicals like Biuret or Benedict’s solution, and be careful with hot water baths and flames during the energy content experiment.
Note: For more details on how to design a perfect experiment or handle data, check the chapters on "Experimental design" and "Data handling."