O-Level Biology · Secondary 3 and 4
Nutrition in Humans
Nutrition in humans follows food through the alimentary canal: how it is broken down physically and chemically, how the products are absorbed in the small intestine, and how they are used by the body. It builds directly on biological molecules and enzymes, and it links forward to transport, because absorbed food travels in the blood, and to respiration, because glucose is the main fuel. Questions often ask you to trace one nutrient from mouth to cell, so you need the organs, the enzymes and the products in the right order.
What the syllabus expects
- Identify the main regions of the alimentary canal and associated organs, and state their functions
- Describe physical digestion, including chewing, peristalsis and the emulsification of fats by bile
- Describe chemical digestion of starch, proteins and fats, naming the enzymes, where they act and the products
- Explain how the structure of a villus is adapted for absorption
- Describe the transport of absorbed food to the liver and the roles of the liver
- Define ingestion, digestion, absorption, assimilation and egestion
The alimentary canal and physical digestion
Nutrition has five stages: ingestion (taking food into the mouth), digestion (breaking large insoluble molecules into small soluble ones), absorption (taking the small molecules into the blood), assimilation (using them in cells) and egestion (removing undigested material as faeces). The canal runs from the mouth through the oesophagus, stomach, small intestine (duodenum and ileum) and large intestine (colon and rectum) to the anus. Physical digestion breaks food into smaller pieces without changing the molecules. Teeth chew food, increasing its surface area for enzymes. Muscles in the wall of the canal contract and relax in waves called peristalsis, pushing food along and mixing it. The stomach churns food with gastric juice. Bile, made in the liver and stored in the gall bladder, emulsifies fats into tiny droplets, increasing the surface area for lipase. Bile contains no enzymes.
Chemical digestion
In the mouth, salivary amylase begins breaking starch into maltose. In the stomach, gastric juice contains hydrochloric acid and pepsin. The acid gives a pH of about 2, kills most bacteria in food and provides the right pH for pepsin, which breaks proteins into polypeptides. The acid also denatures salivary amylase. In the duodenum, pancreatic juice and bile are added. Both are alkaline and neutralise the acid. Pancreatic juice contains pancreatic amylase (starch to maltose), trypsin (proteins to polypeptides) and lipase (fats to fatty acids and glycerol). Enzymes made by the cells lining the small intestine finish the job: maltase breaks maltose into glucose, peptidases break polypeptides into amino acids, and lipase continues digesting fats. The final products are glucose, amino acids, fatty acids and glycerol, which are small enough to be absorbed.
Absorption in the small intestine
Most absorption happens in the ileum. Its inner wall is folded and covered with millions of finger-like villi, and the cells on each villus have microvilli. Together these give a very large surface area. The wall of each villus is one cell thick, so the distance for diffusion is short. Each villus has a dense network of blood capillaries, which carries absorbed glucose and amino acids away quickly and keeps the concentration gradient steep. Glucose and amino acids enter the capillaries by diffusion and by active transport. Inside each villus is a lacteal, a lymphatic vessel, which absorbs fatty acids and glycerol after they are made into fats. The small intestine is also long, so food stays in it long enough for digestion and absorption to be completed. The large intestine then absorbs most of the remaining water.
The liver and assimilation
Blood from the small intestine travels in the hepatic portal vein to the liver, which processes the absorbed food before it reaches the rest of the body. The liver converts excess glucose into glycogen for storage and converts it back to glucose when blood glucose falls. It carries out deamination: excess amino acids cannot be stored, so the liver removes their amino group and turns it into urea, which the kidneys excrete. The rest of the molecule can be used for energy. The liver makes bile, breaks down alcohol and other toxic substances, and breaks down old red blood cells and hormones. Assimilation is the use of absorbed nutrients by cells: glucose is used in respiration, amino acids are built into new proteins for growth and repair, and fats are used in cell membranes or stored.
Worked example
Describe how starch in a piece of bread is digested and how the product is absorbed into the blood. (6 marks)
- Mouth: chewing breaks the bread into smaller pieces, increasing the surface area. Salivary amylase begins to digest starch into maltose.
- Stomach: hydrochloric acid lowers the pH to about 2, which denatures salivary amylase, so starch digestion stops here.
- Duodenum: alkaline pancreatic juice and bile neutralise the acid. Pancreatic amylase digests the remaining starch into maltose.
- Small intestine wall: maltase, made by the cells lining the small intestine, digests maltose into glucose.
- Absorption: glucose is absorbed through the one-cell-thick wall of the villi into the blood capillaries, by diffusion and active transport.
- Transport: the blood carries the glucose in the hepatic portal vein to the liver.
Answer: Starch is digested to maltose by salivary amylase and pancreatic amylase, maltose is digested to glucose by maltase in the small intestine, and glucose is absorbed into the capillaries of the villi and carried to the liver in the hepatic portal vein.
Common mistakes
- Saying bile is an enzyme or that it digests fat. Bile emulsifies fat into small droplets, which increases the surface area for lipase to act on.
- Writing that digestion of starch continues in the stomach. The acid denatures salivary amylase, so starch digestion pauses until the duodenum.
- Forgetting maltase. Amylase only takes starch to maltose. The question usually wants glucose, which needs maltase.
- Saying absorption happens mainly in the stomach or large intestine. Digested food is absorbed in the small intestine. The large intestine mainly absorbs water.
- Saying the liver stores amino acids or excess protein. Excess amino acids are deaminated and the nitrogen becomes urea.
Exam tips
- Learn a table of enzyme, where it is made, where it acts, substrate and product. Most digestion questions can be answered straight from it.
- When a question says 'trace' or 'describe the digestion of', move through the organs in order and name an enzyme and product at every stage.
- For the liver, give specific functions such as deamination, glycogen storage and bile production rather than 'cleans the blood'.
Practice questions
1. Easy: Name the enzyme that digests fats, state the products, and name two places where it acts.
Show answer
Lipase. The products are fatty acids and glycerol. It acts in the duodenum (from pancreatic juice) and in the small intestine (made by the cells lining it).
2. Medium: Explain why the hydrochloric acid in the stomach is important.
Show answer
It provides an acidic pH of about 2, which is the optimum pH for pepsin to digest proteins into polypeptides. It also kills most bacteria in food. (It denatures salivary amylase, which is why starch digestion stops in the stomach.)
3. Harder: Explain three ways in which a villus is adapted for absorption.
Show answer
Villi and microvilli give a very large surface area for absorption. The wall is one cell thick, so the diffusion distance is short. A dense capillary network carries absorbed food away quickly, keeping the concentration gradient steep. (A lacteal to absorb fats is also accepted.) Each point needs the feature and its effect.
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