Digestive System and Enzymes

Why the Digestive System Scores Easy Marks

Nutrition and digestion is one of the most frequently tested biology areas in CDS General Science, and it needs no numbers. The exam loves single-fact questions: which enzyme digests starch, where protein digestion begins, what bile does, or which part of the gut absorbs most nutrients. If your facts are crisp, these are seconds-per-mark questions.

The smartest way to learn this chapter is to picture the gut as a one-way assembly line. Food enters the mouth, moves down the food pipe, gets chemically processed in the stomach and small intestine, and the leftover waste leaves through the large intestine. At each station a specific juice and enzyme acts on a specific nutrient. Once you fix the order of stations and the enzyme at each, the objective options almost sort themselves.

This chapter also links cleanly to others you are already revising — glands and hormones, blood and circulation, and deficiency disorders — so the effort here pays off across the whole biology section.

What Digestion Actually Means

Digestion is the process of breaking down large, complex, insoluble food molecules into small, simple, soluble molecules that the body can absorb. It has two parts working together:

• Mechanical digestion — physical breakdown by chewing (teeth), churning (stomach) and segmentation, which increases the surface area for enzymes.
• Chemical digestion — enzymes and acids chemically split the bonds in carbohydrates, proteins and fats.

The three major food groups are broken down to their building blocks: carbohydrates → glucose, proteins → amino acids, and fats → fatty acids and glycerol. These small molecules are then absorbed into the blood and lymph.

The Alimentary Canal in Order

The alimentary canal (gut) is one continuous muscular tube, roughly 9 metres long in an adult, running from mouth to anus. Learn its parts in strict order:

1. Mouth (buccal cavity) → teeth, tongue, salivary glands.
2. Pharynx → common passage for food and air.
3. Oesophagus (food pipe) → carries food to the stomach by peristalsis.
4. Stomach → J-shaped muscular bag; protein digestion begins here.
5. Small intestine → duodenum, jejunum, ileum; most digestion and absorption.
6. Large intestine → caecum, colon, rectum; water absorption and faeces formation.
7. Anus → egestion of undigested waste.

Food is pushed forward by peristalsis — rhythmic waves of muscular contraction and relaxation of the gut wall. This is why you can swallow even while lying down or upside down; gravity is not required.

Mouth and Salivary Digestion

Digestion starts in the mouth. The salivary glands (parotid, submandibular and sublingual) secrete saliva, which contains the enzyme salivary amylase, also called ptyalin.

• Ptyalin acts on cooked starch and breaks it into maltose (a disaccharide).
• Saliva is slightly alkaline and also moistens food, making it easy to swallow as a soft ball called a bolus.
• Teeth carry out mechanical digestion; an adult human has 32 teeth of four types — incisors, canines, premolars and molars.

This is why a piece of bread chewed for a while begins to taste sweet — ptyalin has started converting starch into sugar in your mouth. The longer you chew, the more starch is broken down and the sweeter it feels, which is a neat real-life proof of enzyme action that examiners sometimes turn into a reasoning question.

The Stomach: Acid and Protein Digestion

The stomach wall has gastric glands that secrete gastric juice. Gastric juice has three key components, each with a clear role:

• Hydrochloric acid (HCl) — kills bacteria, makes the medium acidic, and converts inactive pepsinogen into active pepsin.
• Pepsin — the main protein-digesting enzyme; it breaks proteins into smaller peptones.
• Mucus — protects the stomach lining from being digested by its own acid.

In infants, the stomach also secretes rennin, which curdles milk protein (casein) so it can be digested. The partly digested, semi-liquid food that leaves the stomach is called chyme, and it passes into the small intestine in small amounts through a muscular valve. The stomach therefore acts as both a storage bag and a chemical mixing chamber, holding food for two to four hours while acid and pepsin do their work.

Note the contrast with the mouth: ptyalin needs a slightly alkaline, neutral medium, while pepsin needs a strongly acidic one. This is a favourite CDS trap.

Small Intestine: Where Digestion Is Completed

The small intestine is where digestion of all three nutrients is finished. It receives two crucial secretions plus its own intestinal juice:

• Bile from the liver (stored in the gall bladder).
• Pancreatic juice from the pancreas.
• Intestinal (succus entericus) juice from the intestinal wall.

Pancreatic juice enzymes

• Pancreatic amylase — digests remaining starch → maltose.
• Trypsin — digests proteins and peptones → peptides/amino acids.
• Lipase — digests emulsified fats → fatty acids and glycerol.

The medium in the small intestine is alkaline, which is why these enzymes work best here. The intestinal juice supplies final enzymes such as maltase, sucrase, lactase and peptidases that convert the remaining sugars and peptides into glucose and amino acids.

The Special Role of Bile

Bile is a yellowish-green fluid produced by the liver and stored in the gall bladder. Two facts about bile are tested almost every year:

• Bile contains no enzymes. It does not chemically digest food.
• Bile emulsifies fats — it breaks large fat droplets into tiny droplets, hugely increasing surface area so that lipase can act fast.

Bile is also alkaline: it neutralises the acidic chyme arriving from the stomach, creating the alkaline medium that intestinal and pancreatic enzymes need. Bile salts assist this emulsification, while bile pigments (bilirubin) give faeces and urine their colour. Because bile prepares fat for digestion and sets up the right pH, a person with a poorly functioning liver or a blocked bile duct struggles to digest fatty meals — a clinical clue that the CDS paper occasionally uses to test whether you really understand bile’s job.

Absorption and the Role of Villi

Once food is broken into simple molecules, it must enter the blood. This absorption happens mainly in the small intestine, whose inner wall is lined with millions of tiny finger-like projections called villi.

• Villi enormously increase the surface area for absorption.
• Each villus has a network of blood capillaries and a lymph vessel called a lacteal.
• Glucose and amino acids pass into the blood capillaries; fatty acids and glycerol enter the lacteals (lymph).

The absorbed nutrients are carried to the liver and then around the body for energy, growth and repair — a process called assimilation. In the large intestine, water and some salts are reabsorbed, and the remaining undigested matter forms semi-solid faeces, finally egested through the anus. If too little water is reabsorbed the stools become watery, which is what happens in diarrhoea; if the food stays too long and excess water is removed, constipation results. This neat link between large-intestine function and common health conditions is a favourite way for examiners to test whether you have understood absorption rather than just memorised it.

Enzyme Cheat-Sheet to Memorise

If you remember nothing else, lock this enzyme map into memory — it answers the majority of CDS digestion questions:

• Salivary amylase (ptyalin) — mouth — starch → maltose.
• Pepsin — stomach — proteins → peptones (acidic medium).
• Rennin — stomach (infants) — curdles milk casein.
• Trypsin — small intestine — proteins/peptones → amino acids.
• Pancreatic amylase — small intestine — starch → maltose.
• Lipase — small intestine — fats → fatty acids + glycerol.
• Bile (no enzyme) — small intestine — emulsifies fat.

Alongside the enzymes, fix a few frequently confused facts that decide tricky options. Protein digestion begins in the stomach, never the mouth. Bile is produced by the liver but stored in the gall bladder, and it contains no enzyme. The liver is the largest gland in the human body, while the pancreas is the mixed gland that doubles as an endocrine organ. Candidates lose marks every year by swapping the acidic and alkaline media: ptyalin and the intestinal enzymes prefer an alkaline or neutral medium, whereas pepsin demands a strongly acidic one. Keep these contrasts side by side in your notes so a cleverly worded distractor cannot catch you.

Worked Example: Tracing a Mouthful of Food

Let us trace what happens to a slice of bread spread with butter, which contains both starch and fat.

So starch digestion is started in the mouth and completed in the small intestine, while fat is digested only in the small intestine after bile prepares it. This stepwise tracing is exactly how CDS frames its tougher application questions.

Previous-Year Style Question

Notice how the question rewards the three-fact habit — enzyme, site and medium. A candidate who learned only “pepsin digests protein” might still pick a wrong option if the distractor swaps the medium.

Quick Revision

Revise this recap the night before the exam and you will handle almost every digestion question the CDS paper throws at you.