The Chemistry of Life, Grade 10: Food Tests and Enzyme Experiment

Food test results in a test tube rack: blue-black for starch, brick-red for glucose, purple for protein, beside bread, potato and milk

Two practicals for Grade 10 Life Sciences, Term 1: food tests for starch, glucose, fats and protein, and an enzyme experiment with liver and hydrogen peroxide. Both are listed as essential in CAPS for this topic, along with a diet comparison further down. Each practical is set out below as a recipe you can follow in a single lesson.

Subject Life Sciences
Grade and term Grade 10, Term 1
Topic The chemistry of life
Practicals Food tests, and the effect of temperature on an enzyme
Time Food tests about 40 minutes. Enzyme experiment about 45 minutes

Practical 1: food tests

Aim

To test foods for starch, glucose, fats and protein.

What you need

Method

Mash or grind solid foods with a little water first, so each test has a liquid sample to work with. Run every test on distilled water as well. Water gives a negative result, which is what you compare the foods against.

Starch

  1. Put a little of each food in a separate hollow of the spotting plate.
  2. Add 2 drops of iodine solution to each.
  3. Note the colour.

Glucose

  1. Pour 2 ml of the food sample into a test tube.
  2. Add 2 ml of Benedict's solution.
  3. Stand the tube in the boiling water bath for 5 minutes.
  4. Lift it out with the test tube holder and note the colour.

Fat

  1. Rub a little of the food onto a piece of brown paper. Put a drop of water next to it.
  2. Leave the paper for 10 minutes so the water can dry.
  3. Hold the paper up to the light.

Protein

  1. Pour 2 ml of the food sample into a test tube.
  2. Add 2 ml of sodium hydroxide solution.
  3. Add 3 drops of copper sulphate solution and shake gently.
  4. Note the colour.

What you should see

Diagram of the four food tests showing the colour of a negative and a positive result for starch, glucose, fat and protein

Test Reagent Negative result Positive result
Starch Iodine solution Stays yellow-brown Blue-black
Glucose Benedict's solution, heated Stays blue Green, then yellow, orange or brick-red. The more glucose, the further along it goes
Fat Brown paper Paper dries and stays dull A see-through grease spot that does not dry
Protein Sodium hydroxide and copper sulphate Stays blue Purple

Milk tests positive for sugar, fat and protein. Its sugar is lactose, which turns Benedict's solution orange or red just as glucose does. Table sugar (sucrose) is the exception: it stays blue in the Benedict's test.

Safety

  • Goggles on throughout. Sodium hydroxide damages eyes.
  • Heat Benedict's tests in the water bath, never directly over a flame, and point the tube away from people.
  • Sodium hydroxide is corrosive to skin as well. Rinse any splash with plenty of water.
  • Copper sulphate and Benedict's solutions irritate skin and eyes. Never taste or swallow them.
  • Collect solutions containing copper in a waste beaker. Do not pour them down the sink.
  • Iodine stains skin and clothes. Wipe spills straight away.
  • Boiling water scalds. Keep the water bath at the back of the bench and use the test tube holder.
  • Nobody eats or drinks anything at the bench, including the food samples. Wash hands after handling raw egg white.

Practical 2: the effect of temperature on an enzyme

Aim

To find out how temperature affects the enzyme catalase in liver.

What you need

Method

Diagram of the enzyme experiment: test tubes of hydrogen peroxide in water baths at 0, 20 and 37 degrees Celsius, with the boiled liver tested at 37 degrees

  1. Label four test tubes 0 °C, 20 °C, 37 °C and Boiled, and pour 5 ml of hydrogen peroxide into each.
  2. Cut four pieces of liver, each about 1 cm on every side, and put each piece in its own empty test tube.
  3. Boil one piece of liver in water for 5 minutes. Lift it out with the forceps, put it back in its tube and let it cool.
  4. Stand each peroxide tube and its liver tube together in a water bath: 0 °C in the ice water, 20 °C at room temperature, 37 °C in one warm bath and Boiled in the other. Leave them for 10 minutes so the liver and peroxide reach the temperature of the bath. Check with the thermometer, and top up the warm baths with a little hot water to keep them at 37 °C.
  5. Tip each piece of liver into its matching peroxide tube and start the stopwatch.
  6. After 1 minute, measure the height of the foam above the liquid with the ruler.
  7. To repeat it three times in one lesson, give the same set-up to three groups and work out the average foam height for each tube. If the foam reaches the top of the tube, record the height of the tube.

The control: a fifth tube of peroxide with no liver, kept in a 37 °C bath. It shows that the peroxide does not foam on its own.

What you should see

Test tubes of liver in hydrogen peroxide with different amounts of foam, the most at body temperature and almost none with the boiled liver

Temperature Foam
0 °C, ice water A little
20 °C, room temperature More
37 °C, body temperature The most
Boiled liver, tested at 37 °C Little or none

To prove the gas is oxygen, hold a glowing splint just inside the top of the 37 °C tube. It relights.

Why it happens

Liver cells contain an enzyme called catalase, which breaks hydrogen peroxide down into water and oxygen:

2H2O2 → 2H2O + O2

The oxygen makes the foam, so more foam means the enzyme is working faster. In the cold, molecules move slowly and meet less often, so the reaction is slow, but the enzyme is not damaged. Catalase is a protein, and boiling changes the shape of its active site. Once that shape is lost, the hydrogen peroxide no longer fits, and the enzyme cannot work even when it is put back at 37 °C. This is called denaturing, and it is permanent.

Variable In this experiment
Independent (what you change) Temperature
Dependent (what you measure) Height of the foam after 1 minute
Controlled (kept the same) Size of the liver piece, volume and strength of the peroxide, time before measuring, liver from the same batch

Safety

  • Goggles on before the peroxide bottle is opened. Hydrogen peroxide can hurt the eyes.
  • Use 3% peroxide only. Stronger solutions burn skin and can foam out of the tube.
  • Handle the boiling water and the boiled liver with forceps.
  • Cut the liver on the tile, away from your fingers, and pass the scalpel handle first.
  • Treat raw liver as you would in a kitchen: no eating at the bench, wrap it and throw it away, wipe the bench and wash hands afterwards.

The chemistry of life in brief

Everything living is built from a small set of chemical compounds. Organic compounds contain carbon, hydrogen and oxygen, and some contain nitrogen and phosphorus as well. Inorganic compounds do not contain carbon and hydrogen bonded together. Water, carbon dioxide and mineral salts are examples.

The organic compounds

Compound Built from Examples and what it does
Carbohydrates Sugar units Monosaccharides, one unit: glucose, fructose. Disaccharides, two units: sucrose, maltose. Polysaccharides, many units: starch, cellulose, glycogen. Supply energy and build plant cell walls
Lipids (fats and oils) 1 glycerol and 3 fatty acids Store energy and insulate. Saturated fats are usually solid at room temperature, unsaturated fats are usually liquid oils. Too much saturated fat and cholesterol in the diet is linked to heart disease
Proteins Amino acids. Proteins contain C, H, O and N, and some also contain S, P or Fe Build and repair cells. Enzymes are proteins. Heat and the wrong pH change their shape and stop them working
Nucleic acids C, H, O, N and P DNA and RNA, which carry the instructions for the cell. DNA is found mainly in the nucleus. RNA is found in the nucleus, the cytoplasm and the ribosomes

Enzymes

Lock and key model: a substrate fits into the active site of an enzyme, is broken into products, and the enzyme is left unchanged

Enzymes are proteins that speed up chemical reactions in cells without being used up. Some break large molecules down, as in digestion, and others build large molecules up. Each one has an active site with a particular shape. Only a substrate with a matching shape fits into it, the way only one key fits a lock. This is the lock and key model.

Enzymes work best at one temperature and one pH. Biological washing powders contain enzymes that break down protein and fat stains, which is why they work well in a warm wash and poorly in very hot water.

Vitamins

Vitamin Needed for Shortage causes
A Eyes and skin Night blindness
B1 Releasing energy from food, nerves Beriberi
C Healthy gums, skin and healing Scurvy
D Taking up calcium for bones and teeth Rickets
E Protecting cell membranes Rare, but can damage nerves and muscles

Water and minerals

Water (H2O) is made of 2 hydrogen atoms and 1 oxygen atom, and it makes up about 70% of the mass of a cell. It is the solvent that reactions happen in, it carries substances around the body, and it helps control body temperature.

Mineral Needed for Shortage causes
Calcium Bones and teeth Weak bones, rickets in children
Iron Haemoglobin in red blood cells Anaemia
Iodine Making the hormone thyroxine Goitre
Sodium and potassium Nerves and muscles Cramps, weakness
Nitrogen and phosphorus (as nitrates and phosphates) Plant growth Stunted plants. Yellow leaves with too little nitrogen, purplish leaves with too little phosphorus

Animals get minerals from the food they eat. Plants absorb them from water in the soil, through their roots.

Macro-elements are needed in large amounts, for example nitrogen, phosphorus, potassium, calcium and magnesium. Micro-elements (trace elements) are needed in very small amounts, for example iron, iodine, zinc and copper.

Fertilisers and eutrophication

Soil that is farmed year after year runs short of minerals, so farmers add fertiliser. When too much is used, rain washes it into rivers and dams. The extra nitrates and phosphates make algae grow fast and cover the water. When the algae die, bacteria break them down and use up the oxygen in the water, and fish die. This is eutrophication.

Class activity: compare your diet with the RDA

  1. Each learner writes down everything they eat and drink in one day.
  2. Sort the list into carbohydrates, proteins, fats, and fruit and vegetables.
  3. Draw a pie chart showing roughly how much of the day's food falls into each group.
  4. Compare it with the recommended daily allowance (RDA) on food labels, and discuss what is too much and what is missing.

Free worksheet and marking memo

Related practicals

References