The Digestive System and Food Tests

The digestive system is an organ system: a group of organs working together to break food down and absorb the useful molecules into the blood.

The food you eat is made of large insoluble molecules — starch, proteins and lipids — that are too big to pass through the wall of the gut into your bloodstream. Digestion breaks these into small soluble molecules that can be absorbed.

Food travels along the alimentary canal (the gut) in a fixed order. Each part has a job:

Three organs sit alongside the canal and release substances into it rather than passing food through:

• Salivary glands — release amylase into the mouth.
• Pancreas — releases amylase, protease and lipase into the small intestine.
• Liver — makes bile (stored in the gall bladder), which flows into the small intestine.

The digestive system is the spec's named example of an organ system: several organs cooperating to carry out one function — digesting and absorbing food.

Trace what happens to a meal as it moves along the gut. Each region adds something or absorbs something.

Organ	What happens there
Mouth	Teeth chew food (mechanical breakdown); salivary glands release amylase to start digesting starch
Oesophagus	Muscular tube; squeezes the food ball down to the stomach
Stomach	Muscular bag; churns food and releases protease (pepsin) and hydrochloric acid (acidic conditions, about pH 2)
Small intestine	Receives enzymes from the pancreas and its own wall, plus bile from the liver; digestion is completed and products are absorbed into the blood
Large intestine	Water is absorbed from the remaining material, forming faeces
Rectum/anus	Stores and removes faeces (egestion)

The hydrochloric acid in the stomach gives the optimum pH for stomach protease and helps kill bacteria in food.

Enzymes are biological catalysts that break large molecules into small soluble ones; their detailed action (active site, pH and temperature effects) is covered in the separate enzymes lesson.

Three groups of digestive enzymes break down the three food groups. Each is made in specific organs and produces specific small molecules.

Enzyme	Where produced	Substrate (breaks down)	Products
Amylase (a carbohydrase)	Salivary glands, pancreas	Starch	Simple sugars (e.g. maltose, glucose)
Protease	Stomach, pancreas, small intestine	Proteins	Amino acids
Lipase	Pancreas, small intestine	Lipids (fats and oils)	Fatty acids and glycerol

As word equations (no symbol equations are needed for this spec):

• starch → simple sugars (glucose)
• protein → amino acids
• lipid → fatty acids + glycerol

Learn enzyme, site of production and products together as a set. A common exam question gives you one and asks for the other two.

The body reuses the products: amino acids build new proteins, fatty acids and glycerol build new lipids, and glucose is used in respiration to release energy or stored as glycogen.

Bile is not an enzyme — it does not break chemical bonds. It is a greenish alkaline fluid that supports lipid digestion in two ways.

• Made in the liver, stored in the gall bladder, released into the small intestine.
• Neutralises stomach acid: food leaving the stomach is acidic. Bile is alkaline, so it raises the pH to give the optimum pH for the enzymes in the small intestine (which work best in slightly alkaline conditions, unlike stomach protease).
• Emulsifies lipids: it breaks large fat droplets into many tiny droplets. This increases the surface area of the fat exposed to lipase.

Together, the alkaline pH and the larger surface area increase the rate of fat breakdown by lipase.

Bile emulsifies fat — it does not digest it. Lipase does the chemical digestion; bile just speeds it up by increasing surface area and providing the right pH.

A bigger surface area means more lipid is in contact with lipase at any moment, so more fatty acids and glycerol are produced per second.

Required practical 4 uses qualitative reagents to test a food sample for sugars, starch, protein and lipids. "Qualitative" means the result is a colour change (present or not), not a measured amount.

Prepare the food first: grind the sample, mix with distilled water, and filter to get a clear solution to test.

Nutrient	Reagent	Method	Positive result (colour change)
Reducing sugar	Benedict's solution	Add to sample, heat in a water bath (~75 °C) for a few minutes	Blue → green → yellow → orange → brick-red
Starch	Iodine solution	Add a few drops at room temperature	Orange-brown → blue-black
Protein	Biuret reagent	Add to sample, no heating needed	Blue → purple/lilac
Lipid	Ethanol (emulsion test)	Mix sample with ethanol, then add to water	Clear → cloudy white emulsion

Safety and method points to remember:

• Benedict's test needs heat — use a water bath, not a naked flame, because ethanol and some samples are flammable. Wear eye protection.
• A negative Benedict's result stays blue; a negative iodine result stays orange-brown; a negative Biuret result stays blue.
• The deeper the Benedict's colour (yellow → orange → brick-red), the more reducing sugar is present, so it gives a rough indication of concentration.

The lipid emulsion test in AQA 8461 uses ethanol then water, giving a cloudy white emulsion if lipid is present. There is no Sudan III test required for this spec.

Walk-through 1 — predict the colour for a slice of white bread.

White bread is rich in starch and contains some protein (gluten), with little free sugar and little fat. Predict and justify each test:

• Iodine: turns blue-black — bread is high in starch (positive).
• Benedict's: stays blue, or only turns slightly green — little free reducing sugar (negative or weakly positive).
• Biuret: turns purple/lilac — bread contains protein (positive).
• Emulsion test: stays mostly clear — little lipid (negative).

If you first chew the bread and leave it before testing, salivary amylase begins breaking starch into sugars — so iodine goes weaker and Benedict's gets stronger. That single observation demonstrates enzyme action.

Walk-through 2 — trace a starch molecule from plate to bloodstream.

1. Mouth: salivary amylase starts breaking starch into smaller sugars.
2. Stomach: little starch digestion here — the acidic pH stops amylase working; food is churned and proteins start being digested.
3. Small intestine: pancreatic amylase completes the breakdown of starch into glucose; bile has neutralised the acid to give the optimum pH.
4. Small intestine wall: glucose is small and soluble, so it is absorbed into the blood and carried to cells.
5. Cells: glucose is used in respiration to release energy, or stored as glycogen.

Absorption of the products of digestion happens in the small intestine; the large intestine absorbs water, not nutrients. Mixing these up is a frequent error.

1. Saying bile digests or breaks down fat

Bile emulsifies fat into small droplets and neutralises acid. Lipase carries out the chemical digestion. State both roles of bile (emulsification and neutralising acid) and link them to increasing the rate of lipase action.

2. Confusing where products versus water are absorbed

The small intestine absorbs the small soluble products of digestion (glucose, amino acids, fatty acids, glycerol). The large intestine absorbs water. Name the correct organ for each.

3. Getting the food-test colour changes the wrong way round

Iodine starts orange-brown and goes blue-black with starch. Biuret goes purple/lilac with protein. Benedict's goes blue to brick-red with reducing sugar on heating. State the starting colour and the positive colour, and remember Benedict's needs heating.

4. Forgetting that lipase produces two products

Lipids break down into fatty acids and glycerol — name both. Proteins give amino acids; starch gives simple sugars (glucose).

5. Calling bile an enzyme

Bile contains no enzyme and breaks no chemical bonds. Describe it as an alkaline fluid that emulsifies lipids and neutralises stomach acid.

6. Naming the wrong enzyme source

Amylase is made in the salivary glands and pancreas; protease in the stomach, pancreas and small intestine; lipase in the pancreas and small intestine. Match each enzyme to its production sites rather than guessing a single organ.