A-Level Biology: Digestion and Absorption

Digestion links two parts of the A level biology course: the biological molecules of the first topic and the membranes and transport of the second. The large molecules in food — starch, triglycerides, proteins — have to be hydrolysed into molecules small enough to cross the epithelium of the ileum, and each group is handled by its own enzymes and its own route into the body. This material follows all three.

For carbohydrates you trace starch through amylase (from the salivary glands and pancreas) to maltose, and then the membrane-bound disaccharidases on the ileum epithelium — maltase, sucrase and lactase — that produce glucose, fructose and galactose. For lipids the key ideas are the difference between emulsification by bile salts, which is physical and increases the surface area, and hydrolysis of ester bonds by lipase; then the route of the products: micelles carrying monoglycerides and fatty acids to the epithelial cells, triglycerides reformed in the smooth endoplasmic reticulum, chylomicrons assembled in the Golgi apparatus and released by exocytosis into the lacteals. For proteins you explain how endopeptidases, exopeptidases and membrane-bound dipeptidases work together, and why cutting chains in the middle speeds up the whole process.

The absorption questions compare the mechanisms: co-transport with sodium ions for glucose, galactose and amino acids, facilitated diffusion for fructose, and diffusion of lipid-soluble products, together with the features of the ileum that keep absorption fast. The co-transport mechanism itself is treated in more depth in the membranes material, so here the focus is on which product uses which route. Lactose intolerance is used as an application: why milk causes symptoms when lactase is lacking.

The material offers a multiple-choice quiz with explained answers, a flashcard deck of enzymes, locations and products, a printable written-work sheet of eight open questions answered by hand and checked, and an oral exam in which an examiner asks one question at a time and gives feedback at the end.

The content is based on the digestion and absorption content of the DfE A level biology subject content, as taught in current exam-board specifications (for example AQA topic 3, digestion and absorption). It is practice material and does not replace your own board's specification.

• Explain digestion as the hydrolysis of large molecules into smaller, absorbable ones
• Describe the roles of amylase and the membrane-bound disaccharidases
• Distinguish emulsification by bile salts from hydrolysis by lipase, and describe micelles and chylomicrons
• Explain how endopeptidases, exopeptidases and dipeptidases work together
• Match each product of digestion to its mechanism of absorption
• Apply digestion to lactose intolerance

Practice material written by Zestly, based on the DfE A level biology subject content (exchange: digestion and absorption).