Organisation: Exam Practice

This final lesson in the Organisation unit brings together everything you have learned and focuses on applying your knowledge to exam-style questions. It also covers the Required Practical: Food Tests — one of the most important practicals for this topic. You must know the reagents, methods and expected results for testing food samples for the presence of sugars, starch, protein and lipids.

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Required Practical: Food Tests

AQA requires you to know how to test for the presence of four types of biological molecule in food samples. These are the Required Practical food tests that you must be able to describe in detail.

Overview of All Four Food Tests

Food Molecule	Reagent Used	Method	Positive Result	Negative Result
Reducing sugars (e.g. glucose)	Benedict's reagent	Add Benedict's reagent to the food sample and heat in a water bath at approximately 75-80 degrees C for 5 minutes	Colour change from blue to green, yellow, orange or brick red (depending on sugar concentration)	Stays blue
Starch	Iodine solution	Add a few drops of iodine solution to the food sample	Colour change from brown/yellow to blue-black	Stays brown/yellow
Protein	Biuret reagent (sodium hydroxide + copper sulfate)	Add Biuret reagent to the food sample and mix gently (no heating required)	Colour change from blue to purple/lilac	Stays blue
Lipids (fats and oils)	Sudan III stain	Add Sudan III stain to the food sample mixed with water and shake	A red/orange layer forms on the surface of the liquid	No red layer; the solution remains in one uniform colour

graph TD
    A[Food Sample] --> B{Test for reducing sugars}
    A --> C{Test for starch}
    A --> D{Test for protein}
    A --> E{Test for lipids}
    B -->|Benedict's reagent + heat| B1[Blue to brick red = positive]
    C -->|Iodine solution| C1[Brown to blue-black = positive]
    D -->|Biuret reagent| D1[Blue to purple = positive]
    E -->|Sudan III stain| E1[Red/orange layer = positive]

Exam Tip: You MUST know all four food tests — the reagent, the method and the colour change for both a positive and negative result. This is one of the most commonly examined Required Practicals. A top tip: only Benedict's test requires heating. The other three tests are carried out at room temperature.

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Benedict's Test for Reducing Sugars — In Detail

Benedict's reagent is a blue solution containing copper sulfate. When heated with a reducing sugar (such as glucose, maltose or fructose), the copper ions in Benedict's reagent are reduced, causing the colour to change.

Step-by-Step Method

1. Place the food sample in a test tube
2. If the sample is solid, first grind it up and dissolve/mix it in water
3. Add approximately 2 cm cubed of Benedict's reagent to the test tube
4. Place the test tube in a water bath at 75-80 degrees C for 5 minutes
5. Observe the colour change

Interpreting Results

The colour of the solution after heating indicates the concentration of reducing sugar:

Colour	Sugar Concentration
Blue	No reducing sugar present (negative)
Green	Very low concentration
Yellow	Low concentration
Orange	Moderate concentration
Brick red	High concentration

This is a semi-quantitative test — it gives an indication of how much sugar is present, not just whether it is there or not.

Exam Tip: In the exam, always state that Benedict's test requires heating in a water bath (not a Bunsen burner flame directly, for safety reasons). If asked about safety, mention using a water bath to control temperature, wearing eye protection and handling hot test tubes with tongs.

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Iodine Test for Starch — In Detail

Step-by-Step Method

1. Place the food sample on a white tile or in a test tube
2. Add a few drops of iodine solution (iodine dissolved in potassium iodide solution)
3. Observe the colour change immediately

Interpreting Results

Result	Interpretation
Colour changes from brown/yellow to blue-black	Starch IS present (positive)
Colour remains brown/yellow	Starch is NOT present (negative)

This test is extremely quick and does not require heating.

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Biuret Test for Protein — In Detail

Biuret reagent is a combination of sodium hydroxide solution (NaOH) and copper sulfate solution (CuSO4). When proteins are present, the peptide bonds in the protein react with the copper ions, producing a purple/lilac colour.

Step-by-Step Method

1. Place the food sample in a test tube
2. If solid, dissolve or mix in water
3. Add approximately 2 cm cubed of sodium hydroxide solution and mix
4. Add a few drops of dilute copper sulfate solution and mix gently
5. Observe the colour change (no heating required)

Interpreting Results

Result	Interpretation
Colour changes from blue to purple/lilac	Protein IS present (positive)
Colour remains blue	Protein is NOT present (negative)

Exam Tip: Biuret reagent is made of two separate chemicals (NaOH and CuSO4) that are added in sequence. Some versions of the test use a pre-mixed Biuret solution. Either way, the key colour change is blue to purple. No heating is required.

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Sudan III Test for Lipids — In Detail

Step-by-Step Method

1. Place the food sample in a test tube
2. Add a few cm cubed of water and mix (or crush solid food first)
3. Add a few drops of Sudan III stain
4. Shake the test tube gently
5. Allow the mixture to settle and observe

Interpreting Results

Result	Interpretation
A red/orange layer forms on the surface	Lipids ARE present (positive) — the Sudan III stain has dissolved in the fat layer floating on the water
No red/orange layer	Lipids are NOT present (negative) — the mixture remains uniform

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Exam Technique: Extended Response Questions

The Organisation topic frequently features 6-mark extended response questions. Here are the most common types and how to approach them:

Common 6-Mark Questions

Topic	What the Question Might Ask
Digestive system	Describe the journey of food through the digestive system, explaining what happens in each organ
Enzymes	Explain how enzymes work, including the lock and key model, and describe the effect of temperature or pH on enzyme activity
Heart and circulation	Describe the pathway of blood through the double circulatory system, naming all chambers and vessels
Blood and blood vessels	Compare the structure and function of arteries, veins and capillaries
CHD treatments	Evaluate treatments for coronary heart disease (stents, statins, transplants)
Plant transport	Explain how water is transported from the roots to the leaves, including the role of transpiration
Food tests	Describe how you would test a food sample for a named nutrient, including reagent, method and results

Tips for 6-Mark Questions

1. Plan your answer briefly before you start writing — jot down the key points
2. Use scientific terminology throughout (e.g. enzyme-substrate complex, active site, denatured, oxyhaemoglobin)
3. Write in a logical order — if describing a process, follow the correct sequence
4. Link your points — show how one thing leads to another (cause and effect)
5. Include specific examples where possible (e.g. name enzymes, name blood vessels)
6. If the question says "evaluate," give advantages AND disadvantages

Exam Tip: In 6-mark questions, marks are often awarded for the quality of your written communication as well as the science. This means you should write in full sentences, use paragraphs, and organise your answer clearly. Bullet points can be useful for lists, but extended answers should be mostly in prose.

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Key Equations and Values to Remember

Item	Value or Equation
Cardiac output	Heart rate (bpm) x Stroke volume (ml) = Cardiac output (ml/min)
Optimum temperature for human enzymes	Approximately 37 degrees C
Optimum pH for pepsin	pH 2 (acidic)
Optimum pH for salivary amylase	pH 7 (neutral)
Optimum pH for pancreatic enzymes	pH 7-8 (slightly alkaline)
Number of alveoli (per lung)	Approximately 300 million
Total alveolar surface area	Approximately 70 square metres
Length of small intestine	Approximately 6-7 metres
Resting heart rate	60-80 bpm
Resting stroke volume	Approximately 70 ml

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Common Misconceptions to Avoid