Biology Grade 10 · Unit 1 · Digestive System

Biology Grade 10 · Unit 1 · Digestive System · Athens Academy Islamabad

Think of your body as a house in a Rawalpindi street. Food arriving at the gate is like a delivery of wheat sacks — too big to fit through any door inside the house. Something must break those sacks into small handfuls before they can be carried room to room. That breaking-down job is exactly what your digestive system does, every single day, without you thinking about it.

This unit has seven ideas. Learn them in order and the whole chapter locks into place. I have marked every point that examiners come back to, year after year.

1The Five Steps: Ingestion to Egestion

Before you learn any organ, learn the five words that describe the whole journey. Every question in this chapter sits inside one of these five steps.

Picture a sugarcane juice stall. The cane goes in (ingestion). The crusher squeezes it into juice (digestion). The juice is poured into a glass and drunk (absorption). Your body uses that sugar for energy (assimilation). The crushed dry pulp is thrown out (egestion). Same five steps, same order.

Definition — examinable

Ingestion is the taking in of food.
Digestion is the breakdown of large food molecules into small soluble food molecules with the help of enzymes.
Absorption is the passing of the small digested molecules out of the digestive tube into the blood.
Assimilation is the taking of these molecules from blood into the cells, where they are used for various purposes.
Egestion is the removal of the undigested part of the food out of the body.

Our food is made of carbohydrates, proteins and fats. These are very large molecules. Every cell of the body needs them for proper functioning — but only in small pieces. That is the whole reason digestion exists.

One more point examiners like: the digestive system cannot work alone. It is helped by the transport system (the blood), which carries digested food from the digestive tube to all parts of the body.

Memory hook

"In Delhi All Aunties Eat" — Ingestion, Digestion, Absorption, Assimilation, Egestion.

The two A's are the tricky pair. Absorption = food enters the blood. Assimilation = food enters the cells. Blood first, cells second — always in that order.

⚠ Exam trap

Students swap absorption and assimilation almost every year. If the sentence mentions blood capillaries or the wall of the villus, it is absorption. If it mentions body cells using glucose, it is assimilation.

2The Human Alimentary Canal

The human digestive system consists of two sets of organs. Learn this split first — it is a common short question.

The two sets of organs in the digestive system

Set	What it does	Parts
Alimentary canal	Concerned with ingestion, digestion, absorption and egestion of undigested food	Mouth, oral cavity, pharynx, oesophagus, stomach, small intestine, large intestine
Digestive glands	Associated with the alimentary canal and help in digestion	Salivary glands, gastric glands, intestinal glands, liver, pancreas

Definition — examinable

The alimentary canal — also known as the gut, digestive tract or gastrointestinal tract — is a continuous tube about 9 metres long in an adult. It begins at the mouth and ends at the anus, and is lined internally by mucous membrane. It is specialised at various points along its length, each region designed to carry out a different function.

Figure 1. The alimentary canal — one continuous tube, about 9 metres long, from mouth to anus.

Regions of the canal, in order

Structure of the main regions — SLO B-10-R-02

Region	Key structure you must state
Mouth	The external opening leading into the oral cavity.
Oral cavity	Upper and lower jaws around the sides and front. The upper jaw is fixed, the lower is movable. Contains teeth and tongue.
Pharynx	Leads from the mouth and nasal cavity to the oesophagus and trachea. A common passage for food and air.
Oesophagus	Muscular tube about 25 cm long, pharynx to stomach. No digestion takes place here.
Stomach	Dilated, roughly J-shaped, placed transversely on the left of the abdomen just beneath the diaphragm. Two openings, each guarded by a valve.
Small intestine	Small in diameter, three parts: duodenum (20–25 cm), jejunum (2.5 m), ileum (4 m). Bile duct, hepatic duct and pancreatic duct all empty into the duodenum.
Large intestine	1.5 m long, larger in diameter than the small intestine. Three parts: colon, rectum, anus.

The two stomach openings — a favourite short question

Components of gastric juice

Opening	Between	Guarded by
Cardiac opening	Oesophagus → stomach	Cardiac sphincter. The stomach region around it is the cardiac end.
Pyloric opening	Stomach → duodenum	Pyloric sphincter (muscles)

Memory hook

"Cardiac is near the heart, pyloric is near the pipe onward."

Cardiac = food coming in (top, near the heart). Pyloric = food going out to the duodenum.

Parts of the large intestine

The colon is itself divided into parts. Start at the caecum and follow the food.

Caecum — the first part of the colon. The ileum enters it from the side. It has a pouch-like end at its base to which the appendix is attached, about 10 cm long.
Ascending colon (right)
Transverse colon (across)
Descending colon (left)
Sigmoid colon, which connects to the rectum
Rectum — a straight, 8-inch chamber connecting the large intestine to the anus. It temporarily stores the faeces.
Anus — the last part of the digestive tract, a 2-inch canal surrounded by sphincter muscles that allow control of stool.

Memory hook

"Come And Take Delicious Sweets Right Away" — Caecum, Ascending, Transverse, Descending, Sigmoid, Rectum, Anus.

3Associated Organs

Six associated organs: teeth, tongue, salivary glands, liver, gallbladder and pancreas. These are not part of the tube, but the tube cannot work without them.

1. Teeth — four types

Enzyme · region · substrate · product — SLO B-10-R-04

Type	Function
Incisors	Bite into food and cut it into smaller pieces
Canines	Tear food
Premolars	Help to chew and grind up food
Molars	Chew food and grind it up

Memory hook

"Incisors cut, Canines tear, Premolars and Molars grind."

Think of eating a seekh kebab: front teeth cut it, the pointed ones tear the meat, the back teeth grind it into paste.

2. Tongue

The tongue is a muscular organ in the mouth cavity and is a digestive organ. It moves food around the mouth to help mastication (chewing) and swallowing. It also helps in chewing, speaking and breathing, and it carries taste buds. The five tastes are umami, sweet, salty, sour and bitter.

3. Salivary glands

The salivary glands produce saliva. There are three major pairs.

Common wrong answers vs. correct answers

Gland	Position
Sublingual	Below either side of the tongue, under the floor of the mouth
Submandibular	Below the jaw; its saliva enters the mouth from under the tongue
Parotid	Just in front of the ears; saliva enters through small ducts near the upper molars

Word help: sub- means under, lingual means tongue → under the tongue. Mandible is the lower jaw → under the jaw. The names tell you the position.

4. Liver

The liver is an organ and a gland. It is spongy, wedge-shaped and reddish-brown, located on the right side of the body, under the ribs. It secretes bile. It has two main parts: the larger right lobe and the smaller left lobe. The lobes contain many blood vessels and thousands of small lobes called lobules, which connect with many bile ducts that transport bile from the liver to the duodenum.

5. Gallbladder

The gallbladder is just below the liver. Bile is stored in the gallbladder and flows through the cystic duct and the common bile duct into the small intestine when food is being digested.

⚠ Exam trap

Bile is made in the liver but stored in the gallbladder. The MCQ "Bile is formed in ___" has the answer liver, not gallbladder. Made ≠ stored.

6. Pancreas

The pancreas lies behind the stomach and in front of the vertebral column. It is surrounded by the gallbladder, liver and spleen. Its head is on the right side of the body, tucked beside the curve of the duodenum; its tail extends to the left side, near the spleen.

4Swallowing and Peristalsis

The two main processes in the movement of food are swallowing and peristalsis. Swallowing gets food past the danger point; peristalsis carries it the rest of the way.

4.1 Swallowing — four actions

The tongue presses upwards and back against the roof of the mouth, forcing the bolus to the back of the mouth.
The soft palate closes the nasal cavity at the back.
The larynx cartilage around the top of the trachea is pulled upwards, so that the glottis (the opening of the trachea) lies under the back of the tongue.
The epiglottis, a flap of cartilage, helps prevent food from entering the glottis instead of the oesophagus.

Definition — examinable

The beginning of the swallowing action is voluntary, but once the food reaches the back of the mouth, swallowing becomes an automatic (involuntary) action. The food is then forced into and down the oesophagus by peristalsis.

Understanding tip

This is why you choke when you laugh while eating. Laughing pulls the epiglottis open at the wrong moment, and food heads for the glottis instead of the oesophagus. The epiglottis is a traffic policeman at a junction where two roads meet — food road and air road.

4.2 Peristalsis

Definition — examinable

Peristalsis is the rhythmic wave of muscular contraction and relaxation in the wall of the alimentary canal that causes food to move through the alimentary canal.

The alimentary canal has layers of muscle in its wall which push the ingested food forward. A contraction of the circular muscles just behind the bolus pushes the food forward into the next region, where the longitudinal muscles contract and the oesophagus becomes wide. This wave of contraction is called peristalsis.

Figure 2. Peristalsis — circular muscle contracts behind the bolus, longitudinal muscle contracts ahead of it.

Memory hook

"Squeeze behind, widen ahead."

⚠ Exam trap

"How does food move through your digestive tract?" The answer is by wavelike muscle contractions — not by gravity. You can swallow while lying down or standing on your head; gravity is not doing the work.

5Chemical Digestion — Region by Region

This is the heart of the chapter and the biggest source of marks. Learn it as a journey: what enzyme is released, what it acts on (substrate), what it produces (product). Note the -ase ending on enzyme names — amylase, lipase, maltase, lactase. Once you see -ase, you know you are looking at an enzyme.

5.1 Digestion in the oral cavity

Here food is tasted, smelled and felt. The teeth, cheeks and tongue chew the food by crushing and grinding, so food is physically broken into smaller pieces. Food in the oral cavity stimulates the salivary glands to secrete saliva. The water and mucous in saliva moisten, soften and lubricate the food.

Salivary amylase digests starch into maltose.
Lipase, produced by cells in the tongue, breaks down triglycerides mostly to diglyceride and free fatty acids.

The food is rolled by the tongue into a small, slippery, spherical mass called the bolus, which is swallowed and enters the oesophagus through the pharynx.

5.2 Digestion in the stomach

On the peristaltic reflex, the cardiac sphincter relaxes to allow food into the stomach. Food in the stomach stimulates the gastric glands to secrete gastric juice, which consists of mucous, HCl, pepsin and rennin.

Diarrhoea, constipation and ulcer — SLO B-10-R-07

Component	What it does
Mucous	Forms a coating over the stomach. Prevents the digestion of the stomach wall by the enzyme pepsin.
Hydrochloric acid (HCl)	(a) Stops the action of salivary amylase. (b) Converts the inactive enzyme pepsinogen to the active form pepsin, providing an acidic medium suitable for enzyme action. (c) Kills many microorganisms.
Pepsin	Secreted as inactive pepsinogen. HCl, or already activated pepsin, converts pepsinogen to pepsin. Pepsin digests proteins into polypeptides (chains of many amino acids) and peptides (chains of few amino acids).
Rennin	Secreted only in infants. Helps in the digestion of milk protein.

By the churning action of the stomach wall, food is mixed into a soupy mixture called chyme. When the chyme reaches a certain degree of acidity, the pyloric sphincter relaxes and a little food passes into the duodenum.

⚠ Exam trap

"Why is pepsin formed in the inactive form pepsinogen?" Because active pepsin would digest the protein of the stomach wall itself. The stomach protects itself twice — mucous coating, and secreting the enzyme in an inactive form. Both marks come from that pair.

5.3 Digestion in the small intestine

Starch, proteins and fats are all enzymatically broken down in the small intestine. When food enters the duodenum, the chyme stimulates three things:

the gall bladder to release bile;
the pancreas to secrete pancreatic juice;
the intestinal glands to secrete intestinal juice.

All these secretions are alkaline and neutralise the acidic chyme. The pancreatic and intestinal enzymes can work only in this alkaline medium.

Definition — examinable

Bile is a greenish-yellow liquid manufactured in the liver and stored in the gallbladder. It is released as soon as food enters the duodenum. Bile emulsifies fats, causing them to break down into small droplets called an emulsion. Emulsification provides a relatively large surface area of lipid for the action of lipase enzymes.

Understanding tip

Bile contains no enzyme, yet it speeds up fat digestion. Think of washing an oily pot: the dishwashing liquid does not eat the oil, it just breaks the big oily layer into tiny drops so the water can reach every part. Bile is that dishwashing liquid.

Pancreatic juice contains amylase, lipase and trypsinogen.

Pancreatic amylase is the starch-digesting enzyme. It converts polysaccharides to maltose and even to glucose.
Pancreatic lipase converts neutral fats (triglycerides) into glycerol and fatty acids.
Trypsin converts proteins into polypeptides.

Digestion in the jejunum and ileum. The lining of the jejunum and ileum secretes several enzymes:

Erepsin converts peptides into amino acids.
Enterokinase activates pancreatic trypsinogen into its active form, trypsin.
Lactase converts lactose to glucose.
Maltase converts maltose to glucose.

In the ileum the digestion process is complete.

Figure 3. The three food groups and the enzymes that finish them off in the small intestine.

Master enzyme table — learn this one by heart

Enzyme	Where	Substrate	Product
Salivary amylase	Oral cavity	Starch	Maltose
Lipase (tongue)	Oral cavity	Triglycerides	Mostly diglyceride + free fatty acids
Pepsin	Stomach	Proteins	Polypeptides and peptides
Rennin	Stomach (infants only)	Milk protein	Digested milk protein
Pancreatic amylase	Small intestine	Polysaccharides	Maltose, even glucose
Pancreatic lipase	Small intestine	Neutral fats	Glycerol + fatty acids
Trypsin	Small intestine	Proteins	Polypeptides
Erepsin	Jejunum / ileum	Peptides	Amino acids
Enterokinase	Jejunum / ileum	Trypsinogen	Trypsin (activation)
Lactase	Jejunum / ileum	Lactose	Glucose
Maltase	Jejunum / ileum	Maltose	Glucose

Students often write	The correct statement
Bile is an enzyme that digests fat	Bile contains no enzyme; it emulsifies fat so lipase can work
Pepsin works in the small intestine	Pepsin needs an acidic medium; the small intestine is alkaline, so pepsin cannot act there
Digestion of protein begins in the mouth	Protein digestion begins in the stomach with pepsin
Trypsin is secreted in active form	It is secreted as trypsinogen and activated by enterokinase
Rennin is present in all adults	Rennin is secreted only in infants

6Absorption, Assimilation and Defecation

6.1 Absorption in the small intestine

The internal lining of the small intestine is modified into tiny finger-like projections called villi.

Simple sugars and amino acids pass through the wall of the villi into the blood capillaries. Mineral salts and vitamins also pass into the blood capillaries.
Absorption occurs by a combination of diffusion and active transport.
Glycerol and fatty acids are absorbed by the lacteal, and ultimately poured into the blood.

The absorbed food in the blood capillaries of the villi passes into the blood stream. It is carried away in the capillaries, which join to form veins. These veins unite to form one large vein called the hepatic portal vein. This vein carries all the blood from the intestine to the liver, which may store or alter any of the digestion products.

Memory hook

"Sugar and amino go by blood; fat goes by lacteal."

6.2 Absorption in the large intestine

From the large intestine, water and some vitamins are absorbed into the blood. That is the whole job of the large intestine in absorption — short, and often asked.

6.3 Assimilation

The products of digestion are carried round the body by the blood and absorbed by the body cells. The body cells use glucose, amino acids and fats. This uptake and use of food is known as assimilation. All cells use glucose during cellular respiration to produce ATP.

6.4 Defecation

The semi-solid waste — the faeces or stool — is passed into the rectum by peristalsis and expelled at intervals through the anus. The removal of undigested matter from the body is called egestion or defecation.

7Role of the Liver

The liver has many functions. Eight of the most vital roles are listed below — an extensive-answer question sits on exactly this list.

The liver cells manufacture bile.
Excess glucose is stored as glycogen.
The liver converts glycerol and amino acids to glucose molecules.
The liver removes the amino group from amino acids — this is called deamination.
Ammonia is converted into urea in the liver.
The majority of the plasma proteins are synthesised in the liver.
It destroys old red blood cells.
It stores fat-soluble vitamins (A, D, E and K) and mineral ions such as iron.

Memory hook

"Bile, Glycogen, Glucose, Deamination, Urea, Plasma, RBC, Vitamins" — Big Green Gates Do Usually Protect Rich Villages.

⚠ Exam trap

"Which part of the digestive system is not in contact with food?" — the liver. Food never enters it; only blood and bile do. Students answer "caecum" here and lose the mark.

8Structure of the Villus

Definition — examinable

A villus (plural: villi) is a finger-like projection found in the small intestine. Each villus is covered by simple columnar epithelium and contains a blood capillary network and a lymph capillary called a lacteal.

Crypts. Crypt means small cavity. Interstitial crypts are small tubular glands formed in the small intestine between the bases of the villi. Crypt cells of the small intestine provide stem cells for renewal of the intestinal epithelium.

Goblet cells. A villus also has goblet cells. Their primary function is to secrete mucin and create a protective mucus layer.

Why villi matter: the villi increase surface area for absorption. This one line earns marks in almost every paper.

Figure 4. Structure of a villus — label all six parts when this is asked as a drawing question.

Drawing tip

When the paper says "draw and label the longitudinal section of villus", six labels earn the marks: simple columnar epithelium, lacteal, capillary network, goblet cells, intestinal crypt, and the villus itself. Draw large, label with straight lines, no arrowheads needed.

9Disorders of the Gut

Disorder	Symptoms	Causes	Treatment / prevention
Diarrhoea	Stools become watery; increased frequency of defecation	Infection of the lower GI tract and nervous stimulation. In infection such as food poisoning from contaminated food, the intestinal wall becomes irritated and peristalsis increases	May result in dehydration, so ORS (oral rehydration solution), or a mixture of salt and sugar solution, is given to the patient
Constipation	Decrease in frequency of daily stools, or difficulty in defecation, which may result in abdominal pain	Faeces move too slowly along the large intestine, so more water than usual is absorbed and faeces become hard. Causes: medications, lack of exercise, not enough liquids, not enough fibre (fruit, vegetables, cereals), a change in routine or eating habits	Treatment: a laxative to soften the faeces for easy discharge. Prevention: plenty of water or fluids, and fibrous food — fruits, vegetables
Ulcer	Main symptom is stomach pain	Acidity, smoking and bacterial infection	Prevention: (a) stop smoking, (b) avoid spicy food and food containing acids, (c) avoid stress

Definition — examinable

An ulcer is an open sore in the wall of the gut caused by the gradual disintegration of the tissue. It may be gastric (stomach) ulcer or peptic (duodenal) ulcer.

The textbook notes that constipation is called the mother of all diseases. Examiners have used this line directly.

⚠ Exam trap

"The diet component which prevents constipation is ___" — the answer is fibre, not protein and not vitamin. Fibre holds water in the faeces and keeps them soft.

10Worked Scenarios

Worked scenario 1 — Pancreatic juice fails to reach the small intestine

GivenA patient's pancreatic duct is blocked, so pancreatic juice cannot reach the duodenum. Bile and intestinal juice still arrive normally.

RequiredPredict which foods will remain undigested and explain why.

ApproachList what pancreatic juice contains, then remove each enzyme's action from the digestive picture.

Steps 1. Pancreatic juice contains pancreatic amylase, pancreatic lipase and trypsinogen.
2. Without pancreatic amylase, polysaccharides are not converted to maltose in the small intestine, so starch digestion is badly reduced.
3. Without pancreatic lipase, neutral fats are not converted to glycerol and fatty acids. Bile still emulsifies the fat, but emulsification only increases surface area — it does not digest. So fat stays undigested.
4. Without trypsinogen, no trypsin is formed even though enterokinase is present, so proteins are not converted to polypeptides in the small intestine.
5. Some digestion still happens: pepsin in the stomach has already acted on protein, and erepsin, maltase and lactase in the jejunum and ileum still work on whatever smaller molecules exist.

ResultFat digestion is affected most severely, because pancreatic lipase is the only fat-digesting enzyme in the small intestine. Starch and protein digestion are seriously reduced but not fully stopped.

Answer: Fats, and most starch and protein, would remain undigested. Fat is worst affected because bile can emulsify it but cannot digest it without lipase.

Worked scenario 2 — Why pepsin cannot work in the small intestine

GivenSome pepsin passes with the chyme from the stomach into the duodenum.

RequiredExplain why this pepsin cannot carry on digesting protein there.

ApproachCompare the medium (pH condition) each enzyme needs with the medium actually present in the region.

Steps 1. Pepsin is activated by HCl and works in an acidic medium in the stomach.
2. When chyme enters the duodenum, bile, pancreatic juice and intestinal juice are released.
3. All three secretions are alkaline and neutralise the acidic chyme.
4. In this alkaline medium pepsin loses the condition it needs, so it cannot act.
5. Protein digestion is instead continued by trypsin, which is suited to the alkaline medium.

ResultThe medium in the small intestine is alkaline, which does not suit pepsin.

Answer: Pepsin requires an acidic medium. The small intestine is alkaline because of bile, pancreatic and intestinal secretions, so pepsin cannot perform its enzymatic activity there.

Worked scenario 3 — A child with severe diarrhoea

GivenA child has watery stools and an increased frequency of defecation after eating contaminated food.

RequiredIdentify the disorder, explain the mechanism, and state the immediate treatment.

ApproachMatch the symptoms to the disorder, then trace the effect on water absorption.

Steps 1. Watery stools with increased frequency identify the disorder as diarrhoea.
2. The contaminated food causes food poisoning, an infection of the lower GI tract.
3. The intestinal wall becomes irritated and peristalsis increases.
4. Because food moves faster, the large intestine has less time to absorb water, so the stool stays watery.
5. Loss of water and salts leads to dehydration, which is the real danger.

ResultIncreased peristalsis reduces water absorption in the large intestine, causing watery stool and dehydration.

Answer: The disorder is diarrhoea. The patient must be given ORS (oral rehydration solution), or a mixture of salt and sugar solution, to replace lost water and salts.

11Revision Checklist

Tick each box only when you can say the answer out loud without looking. This is your last-night revision list.

I can define ingestion, digestion, absorption, assimilation and egestion, in order.
I can name the two sets of organs and list the parts of each.
I can state the length of the alimentary canal (9 m), the oesophagus (25 cm) and the large intestine (1.5 m).
I can name the three parts of the small intestine with their lengths.
I can name both stomach openings and the sphincter guarding each.
I can list the parts of the colon in order, from caecum to anus.
I can state the function of all four types of teeth.
I can name the three pairs of salivary glands and their positions.
I can describe the four actions of swallowing, including the role of the epiglottis.
I can define peristalsis and say which muscle contracts behind the bolus.
I can list the four components of gastric juice and the three functions of HCl.
I can explain why pepsin is secreted as pepsinogen.
I can complete the enzyme–substrate–product table for every enzyme in this unit.
I can explain what bile does and state that it contains no enzyme.
I can say what is absorbed by the blood capillaries and what by the lacteal.
I can name the vein that carries blood from the intestine to the liver.
I can state what the large intestine absorbs.
I can list all eight roles of the liver.
I can draw and label the villus with six labels.
I can give symptoms, causes and treatment for diarrhoea, constipation and ulcer.

Final word from Athens Academy

This chapter looks long, but it is really just one journey followed from the mouth to the anus, with a few helpers standing at the side. If you can tell that journey as a story — food goes in, teeth cut it, saliva starts on the starch, the stomach churns it with acid, the small intestine finishes everything off, the villi soak it up, the large intestine takes the water, the rest leaves — then you already understand most of the marks in this paper.

Go through it three times: once reading, once covering the tables and testing yourself, once drawing the villus from memory. That is all it takes. You can do this.

Athens Academy Islamabad · Biology · Grade 10 · Unit 1 · Digestive System · FBISE · Exam 2027

🌱 Chapter 1 · Digestive System