Glossary: Nutrition and Human Digestive System

This chapter's terms in context

Chapter 9 is built on five terms describing how food is processed for use by the body: digestion, peristalsis, villus, assimilation and balanced diet. Digestion is the breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed into the bloodstream, and it happens in two complementary ways: mechanical digestion, the physical breaking of food into smaller pieces by chewing and the churning action of the stomach, which increases the surface area available for enzymes to act on; and chemical digestion, the breakdown of large molecules by specific digestive enzymes such as amylase, protease and lipase into their simpler subunits, starch into glucose, protein into amino acids, and fat into fatty acids and glycerol.

Peristalsis is the wave-like contraction and relaxation of the circular and longitudinal muscles in the wall of the alimentary canal, and it is this mechanism, not gravity, that pushes food along the oesophagus, through the stomach and intestines regardless of the body's orientation, which is why a person can swallow food while lying down or even upside down. Students frequently describe peristalsis vaguely as 'muscles moving food' without specifying that it involves the coordinated contraction of circular muscle behind the food bolus and relaxation of circular muscle ahead of it, a level of mechanical detail that separates a full-mark answer from a partial one.

Once digestion is complete, the resulting small molecules must be absorbed, mainly in the small intestine, and this is where the villus becomes essential: a villus is one of millions of tiny, finger-like projections lining the inner wall of the small intestine, and its structure is adapted for efficient absorption in several distinct ways, it has a large surface area exposed to digested food, a wall only one cell thick to keep the diffusion distance short, a dense network of blood capillaries to carry away absorbed glucose and amino acids quickly, and a lacteal (a lymph vessel) at its centre to absorb digested fats. As with the alveolus in the respiratory chapter, examiners expect several distinct structural adaptations to be named, not just one.

Assimilation is the final step, distinct from absorption: it is the process by which absorbed nutrients are used by body cells for growth, repair, energy release or other purposes once they have already crossed into the bloodstream and travelled to the cells that need them, glucose being respired to release energy, or amino acids being used to build new proteins, are both examples of assimilation, and a common exam confusion is treating absorption (nutrients entering the blood) and assimilation (nutrients being used by cells) as a single unbroken step rather than two separate, separately-marked stages.

A balanced diet is a diet that supplies the correct proportions and quantities of carbohydrates, proteins, fats, vitamins, minerals, fibre and water needed by the body, matched to a person's age, activity level, and physiological state such as pregnancy or illness, and questions on this term typically ask a candidate to identify why a stated diet is unbalanced for a stated person rather than to define the term alone. Understanding these five terms as ordered stages, digestion breaks food down, peristalsis moves it along, villi absorb it, and assimilation puts it to use, gives a clear narrative structure for answering any long-form question on human nutrition.

Paper 3 investigations in this chapter often test the digestion of starch, protein or fat under different conditions, and full marks depend on correctly identifying which enzyme and which food test applies to each substrate, iodine solution to track starch disappearance, Benedict's solution for the sugar produced, and the emulsion test for any remaining fat. A frequent scoring gap is describing 'digestion happening' without naming the specific enzyme responsible or the specific test used to detect its product, since the mark scheme typically rewards each of these details separately rather than crediting a single general statement.