Which part of alimentary canal receives bile from the liver ?

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Correct — D, Small intestine. NCERT's Class 10 chapter on Life Processes puts it in one sentence: 'The small intestine is the site of the complete digestion of carbohydrates, proteins and fats. It receives the secretions of the liver and pancreas for this purpose.' More precisely, bile made in the liver is concentrated and stored in the gall bladder and delivered through the common bile duct into the duodenum, the first and shortest part of the small intestine, where the pancreatic duct opens alongside it. Bile does two distinct jobs there, and both explain why the duodenum and not any other part of the gut is the delivery point. First, it neutralises: NCERT notes that 'the food coming from the stomach is acidic and has to be made alkaline for the pancreatic enzymes to act. Bile juice from the liver accomplishes this in addition to acting on fats.' Pancreatic enzymes such as trypsin and lipase work in an alkaline medium, so the acid chyme leaving the stomach must be neutralised the moment it arrives, not later. Second, it emulsifies: 'Fats are present in the intestine in the form of large globules which makes it difficult for enzymes to act on them. Bile salts break them down into smaller globules increasing the efficiency of enzyme action' — NCERT's own comparison is with the action of soap on dirt. Note also what bile is not: it contains no digestive enzyme. It is a secretion of bile salts, bile pigments, cholesterol and water that prepares fat for lipase rather than digesting it. Since the whole point of the delivery is to condition food for enzymes that act in the small intestine, any part of the canal upstream or downstream of that would be useless — and the three wrong options are precisely one upstream, one upstream, and one downstream.

Why the other options are wrong — (a) The food pipe, and no more than that. NCERT describes it simply as the passage by which food is taken from the mouth to the stomach, moved along by the peristaltic contractions that occur all along the gut. No gland empties into it and no digestion of consequence happens there, so bile arriving in the oesophagus would meet food that has not yet been acidified, let alone reduced to chyme.  (b) The most tempting wrong answer, because the stomach is where fatty food feels heaviest and where a lot of digestion is happening. But its secretions come from the gastric glands in its own wall — hydrochloric acid, the protein-digesting enzyme pepsin, and protective mucus — and its medium is deliberately acidic, which is the opposite of what bile is delivered to create. Bile entering the stomach would work against pepsin, not with it.  (c) Downstream of the point that matters. By the time material reaches the large intestine, digestion and absorption in the small intestine are already over; NCERT's account of the large intestine is that its walls absorb more water from the unabsorbed food, after which the residue is removed via the anus. Delivering a digestive aid there would be too late to act on anything.

Concept: The alimentary canal is one continuous tube — mouth, oesophagus, stomach, small intestine, large intestine, anus — and the digestive glands are separate organs that empty their secretions into it at fixed points. That distinction is what most questions on this topic test. The salivary glands empty into the mouth and supply ptyalin, which begins starch digestion. The gastric glands lie in the stomach wall itself and supply hydrochloric acid, pepsin and mucus, working in an acid medium. The liver and the pancreas are outside the canal and both deliver into the duodenum: the liver's bile, stored in the gall bladder, neutralises the acid and emulsifies fat, while pancreatic juice supplies trypsin for protein and lipase for emulsified fat. The intestinal glands in the wall of the small intestine then complete the job, converting proteins to amino acids, complex carbohydrates to glucose, and fats to fatty acids and glycerol. Absorption follows in the same organ, across the finger-like villi that multiply the surface area and pass the products into a rich network of blood vessels. Only water absorption and the formation of residue are left for the large intestine.

State PCS 2024 Q116 — Science & Technology PYQ with Answer

State PCS 2024 Q116 — Science & Technology PYQ with Answer