Digestive System | ICSE Class 9 Biology Notes

This note covers the ICSE Class 9 Biology topic Digestive System: the organs of the alimentary canal, the digestive glands and their secretions, the enzymes and what each one digests, and how digested food is absorbed and assimilated, with a brief look at the teeth and the dental formula because digestion begins in the mouth.

What is digestion, and why is it needed?

The food we eat is made of large, complex molecules such as starch, proteins and fats. These cannot pass through the wall of the gut into the blood. Digestion is the process by which these complex food substances are broken down into simple, soluble substances that can be absorbed.

Digestion has two parts:

• Mechanical (physical) digestion: food is crushed by the teeth, mixed by the tongue and churned by the muscular walls of the stomach, giving small particles with a larger surface for enzymes to act on.
• Chemical digestion: biological catalysts called enzymes break the large molecules into small ones.

The whole process of using food has five steps: ingestion (taking food in), digestion, absorption (passing digested food into the blood and lymph), assimilation (using the absorbed food in cells) and egestion (removing undigested waste).

What are the parts of the human digestive system?

The digestive system consists of the alimentary canal and the digestive glands. The alimentary canal is a long, coiled tube running from the mouth to the anus. Its regions are specialised for different functions:

1. Mouth (buccal cavity): teeth, tongue and saliva.
2. Pharynx: the common passage for food and air at the back of the mouth.
3. Oesophagus (food pipe): carries food to the stomach.
4. Stomach: a J-shaped muscular bag on the left side of the abdomen, below the diaphragm.
5. Small intestine: the longest part, made of the duodenum, jejunum and ileum.
6. Large intestine: the caecum with the vermiform appendix, the colon and the rectum.
7. Anus: the opening through which undigested waste leaves the body, controlled by the anal sphincter.

The digestive glands are the salivary glands, the gastric glands in the stomach wall, the liver, the pancreas and the intestinal glands. The liver and pancreas lie outside the canal and pour their secretions into it through ducts.

The wall of the canal has muscles that contract rhythmically to push food forward. These wave-like contractions are called peristalsis, and they occur all along the gut.

An outline of the human body showing the mouth (buccal cavity) and tongue at the top, the oesophagus running down through the chest past the diaphragm to the stomach on the body's left side (the right-hand side of the picture).

Below the diaphragm, on the body's right (the left of the picture), are the liver and the gall bladder (which stores bile), joined to the intestine by the bile duct, with the pancreas below the stomach.

The coiled small intestine fills the middle of the abdomen, framed by the large intestine (colon), with the appendix at the body's lower right (the left of the picture) and the anus at the bottom.

See Fig. 5.6 in your NCERT textbook

What happens to food in the mouth?

Digestion begins in the mouth. The teeth cut, tear and grind the food into small pieces (mastication). The muscular tongue mixes the food with saliva, helps in tasting, and rolls the chewed food into a soft ball called a bolus, which it pushes back for swallowing.

Salivary glands and saliva

There are three pairs of salivary glands: the parotid glands in front of and below the ears, the submandibular (sub-maxillary) glands below the lower jaw, and the sublingual glands below the tongue. They secrete saliva, which:

• contains the enzyme salivary amylase (ptyalin), which begins the digestion of starch into the sugar maltose;
• contains mucus, which moistens and lubricates the food for easy swallowing;
• contains lysozyme, which kills some bacteria;
• keeps the mouth moist and helps in tasting and speaking.

Teeth and the dental formula

Humans have four types of teeth, each with a different function: incisors for biting and cutting, canines for tearing, and premolars and molars for grinding and crushing.

The number of each type in one half of the upper and lower jaw is written as the dental formula. For an adult it is:

I 2/2, C 1/1, PM 2/2, M 3/3 = 8 on each side of each jaw, so 8 × 4 = 32 teeth.

Each tooth has a crown above the gum, a neck at the gum line and a root fixed in a socket of the jaw bone. The crown is covered by enamel, the hardest substance in the body, which protects the softer dentine beneath it. Inside is the pulp cavity, which holds blood vessels and nerves, and the root is covered by cement, which fixes it in the socket.

Experiment: the action of saliva on starch

1. Take 1 mL of 1 per cent starch solution in each of two test tubes, A and B.
2. Add 1 mL of saliva to test tube A and 1 mL of water to test tube B, and leave both undisturbed for 20 to 30 minutes.
3. Add a few drops of dilute iodine solution to both tubes.
4. Tube B turns blue-black, showing that starch is still present. Tube A shows little or no blue-black colour.
5. Conclusion: the salivary amylase in saliva has digested the starch into sugar.

How is food swallowed and moved down the oesophagus?

When the bolus is swallowed, it passes through the pharynx, the passage shared by food and air.

A flap of tissue called the epiglottis closes the opening of the windpipe (the glottis) during swallowing, so that food goes into the oesophagus and not into the air passage.

If we talk or laugh while eating, a particle may slip into the windpipe and cause choking and coughing.

The oesophagus is a narrow, muscular tube about 25 cm long that runs through the chest behind the windpipe and passes through the diaphragm to the stomach. No digestion takes place in it. Its job is to carry food down, which it does by peristalsis:

1. The circular muscles just behind the bolus contract, squeezing the tube and pushing the bolus forward.
2. The muscles just ahead of the bolus relax, widening the tube to receive it.
3. This wave of contraction and relaxation passes along the tube, moving the bolus steadily towards the stomach.
4. Mucus from glands in the lining lubricates the passage.

Because peristalsis pushes food actively, we can swallow even when lying down or upside down. The same movements carry food through the rest of the alimentary canal.

What happens to food in the stomach?

The stomach is a large, J-shaped, muscular organ that expands when food enters it. Its thick muscular walls churn the food and mix it thoroughly with the digestive juices. After some hours of churning, the food becomes a thick, creamy paste called chyme.

The gastric glands in the stomach wall secrete gastric juice, which contains:

• Hydrochloric acid (HCl): it makes the medium acidic, which the enzyme pepsin needs; it activates pepsin from its inactive form; and it kills many germs that enter with the food.
• Pepsin: a protein-digesting enzyme that breaks proteins into smaller molecules called peptones and peptides.
• Rennin: present mainly in infants (and in the young of many mammals), it curdles milk by acting on the milk protein casein, so that milk stays in the stomach long enough to be digested. In humans, pepsin and acid also curdle milk.
• Mucus: it protects the inner lining of the stomach from the action of the acid under normal conditions.

Too much acid can cause the burning feeling that people call "acidity". The exit of food from the stomach is controlled by a ring of muscle, the pyloric sphincter, which releases the chyme in small amounts into the small intestine.

What happens to food in the small intestine?

The small intestine is the longest part of the alimentary canal, about 6 to 7 m long in an adult, and fits into the abdomen because it is extensively coiled.

It is the site of the complete digestion of carbohydrates, proteins and fats, and of most absorption.

Its first part, the C-shaped duodenum, receives bile from the liver and pancreatic juice from the pancreas; the jejunum and the long ileum follow.

Bile from the liver

The liver, the largest gland in the body, secretes bile, a greenish-yellow alkaline fluid that is stored in the gall bladder and reaches the duodenum through the bile duct. Bile contains no enzymes, but it does two important jobs:

1. It makes the acidic chyme alkaline, so that the pancreatic enzymes can act.
2. Its bile salts break large fat globules into tiny droplets. This emulsification greatly increases the surface on which the fat-digesting enzyme lipase can act, much as soap breaks up grease.

Pancreatic juice

The pancreas, a leaf-shaped gland lying in the loop of the duodenum, secretes pancreatic juice. It contains trypsin, which digests proteins and peptones into peptides;

pancreatic amylase, which digests starch into maltose; and lipase, which breaks emulsified fats into fatty acids and glycerol.

The pancreas also has groups of cells, the islets of Langerhans, that produce the hormones insulin and glucagon, so it is both a digestive and an endocrine gland.

Intestinal juice

Glands in the wall of the small intestine secrete intestinal juice (succus entericus). Its enzymes complete digestion: they convert peptides to amino acids, the sugars maltose, sucrose and lactose to glucose and other simple sugars, and fats to fatty acids and glycerol.

What else do the liver and the pancreas do?

The liver and the pancreas are the two largest digestive glands, and both have important jobs beyond digestion.

Functions of the liver

The liver is a large, reddish-brown organ in the upper right part of the abdomen, just below the diaphragm. Besides making bile, it:

• Stores carbohydrate: it converts excess glucose into glycogen and stores it, changing it back to glucose when the blood sugar falls.
• Breaks down excess amino acids: in deamination, it removes the nitrogen-containing part and converts it into urea, which is later removed by the kidneys.
• Removes harmful substances: it breaks down or makes harmless many poisonous substances, including alcohol and some drugs (detoxification).
• Stores vitamins and iron: it stores vitamins such as A and D, and iron from worn-out red blood cells.
• Produces heat: its many chemical activities release a good deal of heat, which helps keep the body warm.
• Breaks down worn-out red blood cells and makes plasma proteins: the iron is stored and the rest of the haemoglobin is changed into bile pigments; the liver also makes plasma proteins, including those needed for blood clotting.

Functions of the pancreas

The pancreas is a mixed gland. Most of it is an exocrine gland, pouring pancreatic juice through a duct into the duodenum.

Scattered through it are the islets of Langerhans, an endocrine part that releases the hormones insulin and glucagon directly into the blood.

Insulin lowers the blood sugar level by helping cells take up glucose and the liver store it as glycogen. Glucagon does the opposite: it raises the blood sugar level by making the liver change stored glycogen back into glucose. A shortage of insulin causes diabetes mellitus.

Because the blood from the intestine, carrying the absorbed glucose and amino acids, reaches the liver first through the hepatic portal vein, the liver acts as a checkpoint between the gut and the rest of the body.

Which enzymes digest which foods?

The table brings together the main digestive juices, where they come from, their enzymes and what each enzyme does.

Note: Starch digestion begins in the mouth, protein digestion begins in the stomach, and fat digestion begins in the small intestine. All three are completed in the small intestine.

How is digested food absorbed and assimilated?

Absorption takes place mainly in the small intestine, chiefly in the jejunum and the ileum. Its inner lining has thousands of tiny finger-like projections called villi, which greatly increase the surface area for absorption. The villi are richly supplied with blood vessels.

1. Each villus has a thin wall, one cell thick, so digested food passes through easily.
2. Inside each villus is a network of blood capillaries, which absorb glucose, amino acids, minerals, water-soluble vitamins and water.
3. In the centre is a lymph vessel called a lacteal, which absorbs fatty acids and glycerol, which are then carried in the lymph.
4. Blood from the capillaries of the villi drains through veins into the hepatic portal vein, which carries the absorbed glucose and amino acids first to the liver, and from there the food is distributed to every cell of the body.

A finger-like projection of the intestinal lining, covered by a single layer of epithelial cells.

Inside, draw a branching network of blood capillaries, with an artery bringing blood in and a vein carrying it out, and a central lymph vessel, the lacteal, running up the middle.

Assimilation

Assimilation is the use of absorbed food by the body's cells, where it is used for obtaining energy, building new tissues and repairing old ones.

• Glucose is used by cells in respiration to release energy. Excess glucose is stored as glycogen in the liver and muscles.
• Amino acids are used to build proteins for growth and repair, enzymes and some hormones. Excess amino acids cannot be stored; they are broken down in the liver (deamination), and the nitrogen-containing part is converted to urea.
• Fatty acids and glycerol are used for energy, or recombined into fat and stored under the skin and around organs.

What is the role of the large intestine, and how is waste removed?

The unabsorbed food, mostly water, fibre (cellulose) and undigested matter, passes from the ileum into the large intestine. It is wider but much shorter than the small intestine, about 1.5 m long, and has three parts:

• The caecum, a small pouch at the junction with the small intestine, which bears the vermiform appendix. The appendix has no digestive role in humans; if it becomes inflamed (appendicitis), it may have to be removed.
• The colon, which runs up, across and down the abdomen. Its wall absorbs most of the remaining water and some minerals from the waste, making it semi-solid.
• The rectum, where the semi-solid waste, now called faeces, is stored for a time.

The faeces are removed from the body through the anus; this is egestion. The exit is regulated by the anal sphincter, a ring of muscle. Bacteria living in the large intestine help to make certain vitamins, such as vitamin K.

Cellulose, or roughage, is not digested by humans, but it is important in the diet. It adds bulk to the food, which helps the muscles of the intestine grip and move it, and so prevents constipation.

If too little water is absorbed, the result is loose, watery stools (diarrhoea), which can cause dangerous loss of water and salts.

A side view of the head showing the parotid gland in front of and below the ear with its duct opening into the mouth near the upper teeth, the submandibular gland below the lower jaw, and the sublingual gland beneath the tongue.

Glossary

• Digestion — The breakdown of complex food into simple, soluble substances that can be absorbed.
• Enzyme — A biological catalyst, such as amylase or pepsin, that speeds up the breakdown of food.
• Peristalsis — Wave-like contractions of the muscles of the alimentary canal that push food forward.
• Bolus — The soft ball of chewed food mixed with saliva that is swallowed.
• Chyme — The thick, creamy paste formed when food is churned with gastric juice in the stomach.
• Epiglottis — A flap that closes the windpipe during swallowing so food enters the oesophagus.
• Bile — An alkaline fluid made by the liver and stored in the gall bladder that emulsifies fats; it has no enzymes.
• Emulsification — The breaking of large fat globules into tiny droplets by bile salts.
• Villi — Finger-like projections of the small intestine lining that increase the surface for absorption.
• Lacteal — The lymph vessel in a villus that absorbs fatty acids and glycerol.
• Assimilation — The use of absorbed food by body cells for energy, growth and repair.
• Egestion — The removal of undigested waste as faeces through the anus.
• Dental formula — The number of each type of tooth in one half of each jaw; for an adult, 2123 over 2123.

Common errors and misconceptions

• Misconception: Bile is an enzyme that digests fat. Correct: Bile has no enzymes. It emulsifies fats and makes the medium alkaline; lipase digests the fat.
• Misconception: Bile is made by the gall bladder. Correct: Bile is made by the liver and only stored in the gall bladder.
• Misconception: Most digestion takes place in the stomach. Correct: The stomach begins protein digestion; the complete digestion of all foods takes place in the small intestine.
• Misconception: Food is absorbed in the large intestine. Correct: Digested food is absorbed mainly in the small intestine (chiefly the jejunum and ileum); the large intestine absorbs mostly water and some minerals.
• Misconception: The oesophagus digests food. Correct: No digestion takes place in the oesophagus; it only carries food by peristalsis.
• Misconception: Fats are absorbed into the blood capillaries of the villi. Correct: Fatty acids and glycerol are absorbed into the lacteals, the lymph vessels of the villi.
• Misconception: The hydrochloric acid in the stomach digests food. Correct: The acid provides the acidic medium for pepsin and kills germs; pepsin digests the proteins.

Exam-style questions with model answers

Q1. Name the enzyme that acts on starch in the mouth, and its product. [1 mark]

1. Salivary amylase (ptyalin) in saliva acts on starch in the mouth and converts it into the sugar maltose.

Q2. State two functions of hydrochloric acid in the stomach. [2 marks]

1. It provides the acidic medium needed for the action of pepsin, and activates pepsin from its inactive form.
2. It kills many of the germs that enter the stomach with the food.

Q3. Why is bile important in digestion, though it contains no enzymes? [3 marks]

1. Bile is alkaline, so it neutralises the acidic chyme coming from the stomach and provides the alkaline medium the pancreatic enzymes need.
2. Its bile salts emulsify fats, breaking large fat globules into tiny droplets.
3. This greatly increases the surface area on which lipase can act, so fats are digested much more efficiently.

Q4. Name the three enzymes of pancreatic juice and state the action of each. [3 marks]

1. Trypsin digests proteins and peptones into peptides.
2. Pancreatic amylase digests starch into maltose.
3. Lipase digests emulsified fats into fatty acids and glycerol.

Q5. How is the small intestine adapted for the absorption of digested food? [4 marks]

1. It is very long, about 6 to 7 m, and coiled, which gives food a long time and a large area for absorption.
2. Its inner lining bears thousands of finger-like villi, which greatly increase the surface area.
3. Each villus has a wall only one cell thick and a rich network of blood capillaries, which absorb glucose, amino acids, minerals and water.
4. Each villus also has a central lacteal, which absorbs fatty acids and glycerol into the lymph.

Q6. Describe the digestion of a meal containing starch, protein and fat as it passes through the alimentary canal. [5 marks]

1. In the mouth, the teeth grind the food and salivary amylase begins to digest starch into maltose.
2. In the stomach, the food is churned into chyme; in the acidic medium provided by hydrochloric acid, pepsin digests proteins into peptones and peptides.
3. In the duodenum, bile from the liver makes the chyme alkaline and emulsifies the fat; pancreatic juice then acts, with trypsin digesting proteins to peptides, amylase digesting starch to maltose, and lipase digesting fats to fatty acids and glycerol.
4. Intestinal juice completes digestion, converting peptides to amino acids and maltose and other sugars to glucose.
5. The end products are absorbed through the villi of the small intestine, sugars and amino acids into the blood and fatty acids and glycerol into the lacteals; the large intestine then absorbs water, and the waste is egested through the anus.

Key takeaways

• Digestion breaks complex starch, proteins and fats into glucose, amino acids, and fatty acids and glycerol that can be absorbed.
• The alimentary canal runs from the mouth through the pharynx, oesophagus, stomach, small and large intestines to the anus, and food is moved by peristalsis.
• Saliva from three pairs of salivary glands contains salivary amylase, which begins starch digestion in the mouth.
• Gastric juice contains hydrochloric acid, pepsin for proteins, rennin in infants, and mucus that protects the stomach lining.
• Bile from the liver has no enzymes; it makes chyme alkaline and emulsifies fats for lipase.
• Pancreatic juice (trypsin, amylase, lipase) and intestinal juice complete the digestion of all foods in the small intestine.
• Villi of the small intestine absorb sugars and amino acids into blood capillaries and fatty acids and glycerol into lacteals.
• The large intestine absorbs water and stores faeces in the rectum until egestion; cellulose adds bulk and prevents constipation.

Test yourself

Name the three pairs of salivary glands.

The parotid glands, the submandibular (sub-maxillary) glands and the sublingual glands.

What is the dental formula of an adult human?

I 2/2, C 1/1, PM 2/2, M 3/3, giving 32 teeth in all.

What prevents food from entering the windpipe during swallowing?

The epiglottis, a flap that closes the opening of the windpipe while food passes into the oesophagus.

What is the function of mucus in the stomach?

Mucus protects the inner lining of the stomach from the action of hydrochloric acid under normal conditions.

Where is bile stored, and how does it reach the intestine?

Bile is stored in the gall bladder and reaches the duodenum through the bile duct.

What is the role of rennin?

Rennin, found mainly in infants, curdles milk by acting on the milk protein casein.

Which vein carries absorbed food from the intestine to the liver?

The hepatic portal vein carries blood with absorbed food from the intestine to the liver.

Why is roughage important even though we cannot digest it?

It adds bulk to food, helping the intestinal muscles move it along, and so prevents constipation.