Animal nutrition and the digestive system

What you'll learn

This revision guide covers the complete human digestive system as required for CIE IGCSE Co-ordinated Science (Double Award). You'll master the structure and function of digestive organs, understand how enzymes break down food molecules, and learn how nutrients are absorbed into the bloodstream. These notes focus exclusively on testable content, with worked examples that mirror actual exam questions.

Key terms and definitions

Ingestion — taking food and drink into the body through the mouth

Digestion — the breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed

Absorption — the movement of digested food molecules through the wall of the intestine into the blood

Assimilation — the movement of digested food molecules into cells where they are used

Egestion — the removal of undigested food from the body as faeces

Peristalsis — wave-like contractions of muscle in the alimentary canal wall that push food along

Enzymes — biological catalysts that speed up chemical reactions in living organisms without being used up

Villi — finger-like projections in the small intestine that increase surface area for absorption

Core concepts

The seven nutrient groups

The human diet must contain seven types of nutrient for health:

Carbohydrates provide energy. Sources include bread, rice, pasta, and potatoes. Made of carbon, hydrogen and oxygen. Simple sugars (glucose) and complex carbohydrates (starch) are both carbohydrates.

Proteins are needed for growth and repair of tissues. Sources include meat, fish, eggs, beans, and dairy products. Made of carbon, hydrogen, oxygen, nitrogen and sometimes sulfur. Built from amino acids joined together.

Lipids (fats and oils) provide energy and insulation. Sources include butter, oil, nuts, and oily fish. Made of carbon, hydrogen and oxygen. Consist of fatty acids and glycerol.

Vitamins are needed in small amounts for specific functions:

• Vitamin A for good vision and healthy skin
• Vitamin C for healthy skin and gums, prevents scurvy
• Vitamin D for calcium absorption and strong bones

Minerals are needed in small amounts:

• Iron for making haemoglobin in red blood cells, prevents anaemia
• Calcium for strong bones and teeth, and blood clotting

Water makes up about 70% of body mass. Needed for chemical reactions and as a solvent.

Dietary fibre (roughage) provides bulk to food and helps it move through the intestine. Not digested but prevents constipation. Found in vegetables, fruit, and wholegrain cereals.

Structure of the alimentary canal

The alimentary canal is a muscular tube running from mouth to anus, approximately 9 metres long in adults.

Mouth — teeth mechanically break down food (mechanical digestion). Saliva from salivary glands moistens food and contains amylase enzyme. The tongue shapes food into a bolus for swallowing.

Oesophagus — muscular tube connecting mouth to stomach. Peristalsis moves food down. No digestion occurs here.

Stomach — muscular bag that churns food. Gastric glands in the stomach wall secrete:

• Hydrochloric acid (pH 2) that kills bacteria and provides optimum pH for pepsin
• Pepsin enzyme that begins protein digestion
• Mucus that protects the stomach lining from acid

Food remains in the stomach for approximately 2-4 hours, becoming a liquid called chyme.

Small intestine — consists of duodenum and ileum, approximately 6 metres long. Most digestion and all absorption occurs here.

The duodenum receives:

• Bile from the liver (stored in gall bladder) which emulsifies fats and neutralises stomach acid
• Pancreatic juice from the pancreas containing protease, lipase and amylase enzymes

The ileum has walls covered with villi that absorb digested food into the bloodstream.

Large intestine (colon) — absorbs water from undigested food. Bacteria in the colon produce some vitamins.

Rectum — stores faeces temporarily.

Anus — where faeces leave the body during egestion.

Associated digestive organs

Pancreas — produces pancreatic juice containing:

• Amylase (digests starch)
• Protease (digests proteins)
• Lipase (digests lipids)
• Sodium hydrogencarbonate (neutralises stomach acid)

Liver — largest internal organ. Produces bile, which is alkaline and contains bile salts that emulsify fats (break large fat droplets into smaller ones, increasing surface area for lipase action). The liver also stores glucose as glycogen and breaks down old red blood cells.

Gall bladder — stores bile produced by the liver and releases it into the duodenum.

Enzymes in digestion

Enzymes are specific biological catalysts. Each enzyme works on only one type of substrate (the substance it acts upon).

Carbohydrase enzymes break down carbohydrates:

• Amylase: starch → maltose
• Maltase: maltose → glucose
• Found in saliva, pancreatic juice, and small intestine

Protease enzymes break down proteins:

• Pepsin in stomach: protein → polypeptides
• Proteases in pancreas and small intestine: polypeptides → amino acids

Lipase enzymes break down lipids:

• Lipase: lipids → fatty acids + glycerol
• Found in pancreatic juice and small intestine

Enzyme activity is affected by:

• Temperature: enzymes work fastest at optimum temperature (approximately 37°C in humans). Too hot and they denature (active site changes shape permanently)
• pH: each enzyme has an optimum pH. Pepsin works best at pH 2, intestinal enzymes at pH 8

Absorption in the small intestine

The ileum is highly adapted for absorption:

Villi structure:

• Thin walls (one cell thick) for short diffusion distance
• Large surface area due to millions of villi projections
• Good blood supply (network of capillaries) to maintain concentration gradient
• Lacteal (lymph vessel) in each villus absorbs fatty acids and glycerol
• Microvilli on surface of cells further increase surface area

Absorption of different nutrients:

• Glucose and amino acids absorbed into blood capillaries by diffusion and active transport
• Fatty acids and glycerol absorbed into lacteals, reformed into fats
• Vitamins, minerals and water absorbed into blood
• Blood from small intestine travels to liver via hepatic portal vein

Practical tests for nutrients

These simple food tests are testable:

Benedict's test for reducing sugars (glucose):

1. Add Benedict's reagent (blue) to food solution
2. Heat in water bath (approximately 80°C)
3. Positive result: colour changes from blue → green → yellow → orange → brick red
4. More sugar present = more red precipitate

Biuret test for protein:

1. Add sodium hydroxide solution to food
2. Add copper sulfate solution
3. Positive result: colour changes from blue to purple/lilac

Ethanol test for lipids:

1. Add ethanol to food sample and shake
2. Pour mixture into water
3. Positive result: cloudy white emulsion forms

Iodine test for starch:

1. Add iodine solution (yellow-brown) to food
2. Positive result: colour changes to blue-black

Worked examples

Example 1: Enzyme specificity

Question: Explain why amylase can digest starch but not protein. [3 marks]

Model answer:

• Enzymes are specific / have a specific active site [1 mark]
• The shape of the active site is complementary to starch/substrate [1 mark]
• Protein has a different shape so does not fit the active site of amylase [1 mark]

Examiner tip: Use the lock-and-key model concept. One mark for stating specificity, one for explaining the complementary shape, one for why the wrong substrate won't work.

Example 2: Adaptation of small intestine

Question: The small intestine is adapted for absorption of digested food. Describe and explain three adaptations. [6 marks]

Model answer:

• Villi/finger-like projections increase surface area [1 mark] so more absorption can occur / faster rate of absorption [1 mark]
• Walls are one cell thick / thin [1 mark] providing short diffusion distance / faster diffusion [1 mark]
• Good blood supply / network of capillaries [1 mark] maintains concentration gradient / removes absorbed nutrients quickly [1 mark]

(Alternative answers: microvilli, large length, lacteal present)

Examiner tip: This is a "describe and explain" question worth 6 marks, so expect to give 3 features with 3 explanations. Always link structure to function.

Example 3: Digestive system pathway

Question: A student eats bread containing starch. Describe the digestion of starch from the mouth to the small intestine. [5 marks]

Model answer:

• Amylase in saliva/mouth begins digestion of starch to maltose [1 mark]
• Food passes through oesophagus by peristalsis (no digestion here) [1 mark]
• In stomach, no starch digestion occurs / acid denatures amylase [1 mark]
• In small intestine/duodenum, pancreatic amylase continues starch digestion to maltose [1 mark]
• Maltase enzyme converts maltose to glucose [1 mark]

Examiner tip: Follow the journey logically from mouth to small intestine. State the enzyme, where it's made/acts, and the substrate → product change.

Common mistakes and how to avoid them

• Don't confuse ingestion with digestion. Ingestion is simply taking food into the mouth. Digestion is the chemical and mechanical breakdown that happens afterwards.

• Don't say "amylase breaks down carbohydrates." Be specific: amylase breaks down starch (not all carbohydrates) into maltose (not glucose). Maltase then breaks maltose into glucose.

• Don't write "bile is an enzyme." Bile is not an enzyme. It emulsifies fats (physical process) but does not digest them chemically. Lipase digests fats.

• Don't confuse absorption with assimilation. Absorption is movement through the intestine wall into blood. Assimilation is when absorbed nutrients are used by body cells.

• Don't say villi are in the stomach. Villi are only found in the small intestine (ileum), not the stomach. The stomach has gastric glands in its walls.

• Don't forget that digestion involves both mechanical and chemical processes. Mechanical: teeth chewing, stomach churning. Chemical: enzyme action throughout the alimentary canal.

Exam technique for "Animal nutrition and the digestive system"

• Command words matter. "State" requires a simple fact (1 mark). "Describe" needs characteristics or how something happens (2-3 marks). "Explain" requires reasons with because/so that (2-4 marks). "Compare" needs similarities and differences between two things.

• For enzyme questions, always mention the substrate (what it acts on) and product (what it produces). Don't just name the enzyme. Example: "Amylase digests starch into maltose" scores marks; "Amylase is in saliva" alone may not.

• Draw annotations carefully on diagrams. Use a ruler for label lines. Ensure lines touch the structure being labelled, not nearby parts. Don't cross label lines.

• In longer answers about absorption, link structure to function explicitly using "so that" or "which allows." Example: "Villi have a large surface area so that more glucose can be absorbed per unit time."

Quick revision summary

Animal nutrition provides seven nutrients: carbohydrates, proteins, lipids, vitamins, minerals, water and fibre. The alimentary canal consists of mouth, oesophagus, stomach, small intestine, large intestine, rectum and anus. Mechanical digestion breaks food physically; chemical digestion uses enzymes. Amylase breaks down starch, protease breaks down proteins, lipase breaks down lipids. The liver produces bile; the pancreas produces enzymes. Villi in the small intestine provide a large surface area, thin walls and good blood supply for efficient absorption of glucose, amino acids, fatty acids and glycerol into the bloodstream.