OxfordAQA International A Level Biology 3.2.2 Digestion and Absorption Exam Questions

About OxfordAQA International A Level Biology 3.2.2 Digestion and Absorption

Digestion and absorption in Unit 2 explains how large food molecules are hydrolysed and taken into the body. You need to name the parts of the digestive system and associated glands, describe the enzymes that digest carbohydrates, lipids and proteins, and explain the role of bile salts and micelles. You also need to describe the histology of the ileum and explain absorption by diffusion, active transport and sodium-linked co-transport. It builds on biological molecules, enzymes and membrane transport from Unit 1 and links to cholera and oral rehydration. Examiners often ask about co-transport of glucose and how lipids are absorbed and re-formed as chylomicrons.

What you need to know

• Digestive system Food passes through the oesophagus, stomach, duodenum, ileum, colon and rectum, and the salivary glands and pancreas secrete enzymes. Digestion is the hydrolysis of large molecules into smaller ones, and undigested material is egested as faeces.
• Carbohydrate digestion Amylase (from salivary glands and pancreas) hydrolyses starch to maltose. Membrane-bound disaccharidases on the ileum epithelium hydrolyse disaccharides such as maltose, sucrose and lactose to monosaccharides.
• Lipid digestion Bile salts emulsify lipids into small droplets, increasing the surface area for lipase. Lipase hydrolyses triglycerides into monoglycerides and fatty acids, which form micelles with bile salts.
• Protein digestion Endopeptidases hydrolyse peptide bonds within polypeptides, exopeptidases remove terminal amino acids, and membrane-bound dipeptidases hydrolyse dipeptides into amino acids.
• Absorption Sodium ions are actively transported out of epithelial cells into the blood, keeping sodium low inside the cell. Sodium then diffuses in from the lumen with glucose or amino acids through a co-transport protein, and the glucose diffuses into the blood by facilitated diffusion.
• Lipid absorption Micelles carry monoglycerides and fatty acids to the epithelium, where they diffuse in. They are re-formed into triglycerides, packaged with proteins into chylomicrons, and enter lacteals before reaching the blood.

Common mistakes in 3.2.2 Digestion and Absorption

• Saying glucose is actively transported into epithelial cells. Glucose enters by sodium-linked co-transport; the active step is the pumping of sodium ions out into the blood.
• Saying bile salts digest lipids. Bile salts emulsify lipids, increasing surface area; lipase does the hydrolysis.
• Confusing endopeptidases and exopeptidases. Endopeptidases act within the chain, creating more ends for exopeptidases.
• Forgetting that disaccharidases are membrane-bound. They are attached to the epithelial cell membranes of the ileum.

Try three 3.2.2 Digestion and Absorption exam-style questions