For the digestion of carbohydrate, protein and fat, enzymes such as amylase, trypsin and lipase are required. It is secreted into the duodenum through:

Correct — D, Pancreatic juice. All three enzymes named in the stem come from the same organ. The pancreas makes pancreatic juice and delivers it to the duodenum through the pancreatic duct, which joins the common bile duct just before the two open together into the gut. The juice carries pancreatic amylase, which continues the breakdown of starch into sugars; trypsin, released as inactive trypsinogen and switched on in the intestine, which cuts proteins into peptides; and pancreatic lipase, which splits emulsified fats into fatty acids and glycerol. It also carries sodium bicarbonate, which neutralises the acid chyme arriving from the stomach and gives the enzymes the alkaline medium they need. Naming all three enzymes together is the examiner's signal, since bile has none of them and neither plasma nor lymph delivers anything into the gut at all.

• (a)Bile — Bile comes from the liver, is stored in the gall bladder and reaches the duodenum by the bile duct, but it contains no digestive enzymes. It emulsifies fat into fine droplets so that lipase can work on them, and it makes the contents alkaline.
• (b)Plasma — Plasma is the liquid part of blood and stays inside the vessels. It is not secreted into the cavity of the intestine and carries no digestive enzymes there.
• (c)Lymph — Lymph moves the other way. It picks up digested fat from the lacteals of the villi and carries it away from the intestine; it delivers nothing into the gut.

Digestion in the small intestine depends on two secretions arriving at the same place. The liver sends bile, which has no enzymes but emulsifies fat and neutralises acid, and the pancreas sends pancreatic juice, which carries the enzymes for all three classes of food — amylase for carbohydrate, trypsin for protein and lipase for fat. The intestinal wall then adds its own juice with the final enzymes that complete the job.

Students lose this question by attaching enzymes to bile, which is the most common single error in the digestive system. Bile is conspicuous, it is green, it has a duct and a storage organ, and it is essential to fat digestion — yet its contribution is physical rather than chemical. Fixing the division of labour is worth more than memorising enzyme lists: bile breaks fat into droplets, lipase breaks the fat molecules. One further detail explains why trypsin is not released ready to work — it leaves the pancreas as trypsinogen and is activated by an enzyme from the intestinal lining, which keeps the pancreas from digesting itself.

• Pancreatic juice reaches the duodenum through the pancreatic duct, which joins the common bile duct before opening into the intestine.
• It contains pancreatic amylase, trypsin and pancreatic lipase, together with sodium bicarbonate to neutralise acid chyme from the stomach.
• Bile has no digestive enzymes; it emulsifies fat and provides an alkaline medium.
• Trypsin is secreted as inactive trypsinogen and activated in the intestine, a safeguard against the pancreas digesting its own tissue.
• The pancreas is a mixed gland — exocrine for pancreatic juice, endocrine for insulin and glucagon from the islets of Langerhans.
• Protein digestion begins earlier, in the stomach, where pepsin acts in an acid medium.

• Crediting bile with digestive enzymes because it is essential to fat digestion.
• Placing protein digestion entirely in the small intestine and forgetting pepsin in the stomach.
• Reading lymph as a delivery route into the gut when it is a collection route out of it.

Asked by naming three enzymes at once, which points to the single secretion that carries all three; harder versions ask which secretion lacks enzymes, or where a named enzyme is activated.

• practice — not a real PYQ

  Which one of the following secretions contains no digestive enzyme?

  • (a)Saliva
  • (b)Gastric juice
  • (c)Bile
  • (d)Pancreatic juice

  Answer(c) Bile — it emulsifies fat and makes the intestinal contents alkaline, but the enzymes come from the pancreas and the intestinal wall.

• practice — not a real PYQ

  Trypsin is secreted by the pancreas in an inactive form because

  • (a)it would otherwise digest the pancreas itself
  • (b)it needs light to become active
  • (c)it works only at the acidic pH of the stomach
  • (d)it must first combine with bile pigments

  Answer(a) it would otherwise digest the pancreas itself — trypsinogen is activated only after it reaches the intestine.