Q.Answer briefly:

1. Why are villi present in the intestine and not in the stomach? Villi are finger-like projections of the mucosal lining that greatly increase the internal surface area available for absorption (further multiplied by the microvilli/"brush border" on each villus cell). The small intestine is the principal site of absorption of digested end-products — glucose, amino acids, fatty acids, glycerol, vitamins, minerals — so its lining is folded into villi to maximise uptake. The stomach's job is storage, churning and partial (protein) digestion, not absorption (it absorbs only small amounts of water, alcohol and certain drugs), so it has no functional need for villi and its lining instead has gastric pits/glands for secretion.
2. How does pepsinogen change into its active form? Pepsinogen is the inactive zymogen secreted by the chief (peptic) cells of the gastric glands. It is converted into active pepsin by the hydrochloric acid secreted by the oxyntic (parietal) cells of the stomach, which alters its conformation to expose the active site. Once a small amount of pepsin is formed, it can itself activate further pepsinogen molecules — a self-sustaining process called autocatalysis.
3. What are the basic layers of the wall of the alimentary canal? From outermost to innermost:

• Serosa — thin outermost membranous covering.
• Muscularis — smooth muscle, usually arranged as an outer longitudinal layer and an inner circular layer, responsible for peristalsis.
• Sub-mucosa — a layer of loose connective tissue containing blood vessels, lymph vessels and nerves.
• Mucosa — the innermost, glandular layer lining the lumen; it may be folded, and forms villi in the small intestine.

(d) How does bile help in the digestion of fats?

Bile, secreted by the liver and stored/concentrated in the gallbladder, contains bile salts (but no digestive enzymes). Bile salts act as emulsifying agents, breaking large fat globules into much smaller droplets (emulsification). This dramatically increases the total surface area of fat exposed, allowing pancreatic and intestinal lipases to hydrolyse the fat far more efficiently than they could act on large, undispersed globules. Bile also helps neutralise the acidic chyme entering the duodenum from the stomach and assists in the absorption of fatty acids and fat-soluble vitamins.

For each sub-part, recall the specific structural or functional fact directly: villi ↔ absorption-surface reasoning; pepsinogen activation ↔ HCl plus autocatalysis; gut wall layering ↔ serosa/muscularis/sub-mucosa/mucosa in order; bile's fat role ↔ emulsification (a physical, not enzymatic, action).

• Believing bile itself chemically digests fat — it only emulsifies fat; the actual hydrolysis is done by lipases.
• Listing the gut-wall layers in the wrong order or omitting the sub-mucosa.
• Saying pepsin is secreted directly, rather than being activated from inactive pepsinogen.