Chapter 14: Human Digestive System (Summary Sheet)

Part A · Why the System Exists

1 Why a Digestive System Is Needed

Blood can carry only substances that are dissolved in it. Food arrives as large, insoluble pieces, so nothing eaten can reach a cell in the form it arrives in.
Digestion is the breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed into the body.
Food stays inside the lumen — the hollow centre of the tube — the whole way down. Only the digested molecules cross the wall into the body.
Two kinds of breakdown, one aim: physical breakdown makes the pieces smaller (teeth, churning) and changes only the size; chemical breakdown makes the molecules simpler (enzymes) and produces new substances.
Both speed digestion up, because enzymes can act only at the surface of the food: a larger exposed surface means faster chemical digestion.

2 The Five Processes, in Order

The order matters: each process depends on the one before it.

Process	What happens	Where
Ingestion	Food is taken into the mouth	Mouth
Digestion	Food is broken down, physically and then chemically by enzymes	Mouth, stomach and small intestine
Absorption	The products pass through the wall of the gut into the blood	Mainly the small intestine; water also in the large intestine
Assimilation	The absorbed substances are taken into the liver and other tissues and become part of the body	Liver and body tissues
Egestion	Undigested material is held and then expelled from the body as faeces	Rectum, then the anus

3 The Alimentary Canal and the Organs Around It

The alimentary canal (the gut) is one continuous muscular tube running from the mouth to the anus. Food travels inside it, in this order: mouth → gullet → stomach → small intestine → large intestine → rectum → anus.
Other organs lie alongside the canal and empty their secretions into it. Food never passes through them, but digestion depends on them.

Organ beside the canal	What it contributes
Salivary glands	Release saliva into the mouth, onto the food
Liver	Makes bile, which is needed for fat digestion
Gall bladder	Stores bile; the bile duct carries it into the small intestine
Pancreas	Makes pancreatic juice, delivered by a duct into the small intestine

Part B · The Parts and How They Work Together

4 The Mouth: Cutting, Wetting and Starting on Starch

Job	What happens	Why it matters
Physical digestion	The incisors cut and the molars crush and grind the food; the tongue turns the mouthful so every surface meets the teeth	Smaller pieces expose a far larger surface area, and enzymes work only at the surface, so the chemical digestion that follows is faster
Chemical digestion	Saliva from the salivary glands contains a carbohydrase, which starts breaking starch down into simpler sugars	Starch digestion begins in the mouth and carries on briefly in the gullet, because the enzyme is swallowed with the food
Other functions	Saliva moistens the food and the tongue shapes it into a rounded lump, the bolus	A soft, compact bolus slides down easily; dry, crumbly food is hard to swallow and can be inhaled

5 The Gullet: a Tube That Only Pushes

The gullet (oesophagus) is a narrow muscular tube from the back of the mouth to the stomach.
No digestion and no absorption take place in it — nothing is ground up there and no new enzyme is added. The only chemical digestion at this stage is the starch digestion already started in the mouth.
Its work is transport: the muscular wall pushes the bolus steadily down to the stomach.

Exam tip: a question about the oesophagus usually wants two points — no digestion and no absorption — and then the mechanism: the muscular wall pushes the bolus down to the stomach.

6 The Stomach: Churning, Acid and Protein

Job	What happens	Why it matters
Physical digestion	The muscular wall contracts and relaxes to churn the food, mixing it with gastric juice and breaking up the lumps into a semi-liquid, chyme	Churning exposes more surface to the enzyme, and a semi-liquid meal can flow on into the small intestine
Chemical digestion	Gastric juice contains a protease and hydrochloric acid	Proteins are broken down into shorter chains of amino acids; only proteins are digested to any great extent here
Other functions	The acid kills most bacteria taken in with the food, and mucus coats the lining	Mucus protects the stomach wall from being digested by its own enzyme and acid

A large meal stays in the stomach for a few hours — that time is what allows the churning and the protein digestion to be completed.

7 The Small Intestine: Where Digestion Finishes

This is where digestion is completed, and where the products are taken into the body.
No physical digestion happens here — food arrives already semi-liquid and is not ground up further. All the remaining work is chemical, done by enzymes.
Enzymes come from two places at once: the cells lining the small intestine release their own, and the pancreas adds more by a duct. The liver adds bile, which is not an enzyme.
By the end of the small intestine the food has been reduced to small, soluble molecules that are ready to be absorbed.

What is digested	Enzyme class	What it becomes
Carbohydrates	Carbohydrase	Simple sugars, such as glucose
Proteins	Protease	Amino acids
Fats	Lipase	Fatty acids and glycerol, after bile has broken the fat into tiny droplets

Bile and lipase: two different jobs on the same fat

	Is it an enzyme?	What it does to fat
Bile	No — it contains no enzymes	Acts physically: it breaks the large globules into a huge number of tiny droplets (emulsification), to increase the exposed surface area of fat. It is also alkaline, which helps neutralise the acid from the stomach
Lipase	Yes — a fat-digesting enzyme	Acts chemically: it breaks the fat molecules down into fatty acids and glycerol. The droplets bile made give it the surface it needs

8 The Large Intestine, Rectum and Anus

The large intestine follows the small intestine. It is shorter but wider — the width is where its name comes from.
Almost no chemical digestion happens in this part of the canal. Its main job is to absorb water and mineral salts from undigested food, through the wall.
The material it holds is what the small intestine could not break down — plant fibre, together with bacteria, shed cells, mucus and salts.
The undigested waste is held for a time in the rectum and then leaves the body through the anus as faeces: this is egestion.

Part C · Digestion by Enzymes

9 Enzymes: One Substrate Each

An enzyme speeds up the breakdown of a particular food substance. It is not used up, so a small quantity can digest a much larger quantity of food.
Each enzyme acts on one kind of substrate and no other: a carbohydrase will not touch protein, and a lipase will not touch starch.

Class of enzyme	Substrate	End product	Where it is found
Carbohydrase	Starch and other carbohydrates	Simple sugars, such as glucose	Saliva in the mouth, and the small intestine
Protease	Proteins	Amino acids	The stomach, and the small intestine
Lipase	Fats	Fatty acids and glycerol	The small intestine

Every enzyme works fastest at its own optimum temperature and pH. Too far outside them it works more slowly, and if the temperature is raised too far or the pH is very wrong the enzyme is denatured: it loses the shape it needs and stops working altogether.
Denaturing by heat is permanent — cooling it back does not restore the enzyme's activity.
The three classes are what is required: carbohydrases, proteases and lipases. Specific names of enzymes are not needed.

10 Optimum pH: Why Each Region Has Its Own

The gut is not one uniform chemical environment, so each region's secretions have been matched to the pH there, and the enzymes present have their optimum close to it.

Where it acts	Secretion and its source	What it contains	Optimum pH there
Mouth	Saliva, from the salivary glands	A carbohydrase, which digests starch	About 7, close to neutral
Stomach	Gastric juice, from glands in the stomach wall	A protease, hydrochloric acid, mucus	About 2, strongly acidic
Small intestine	Pancreatic juice, from the pancreas, and intestinal juice from the lining	A carbohydrase, a protease and a lipase	About 8, alkaline

The change of pH is needed because the acid from the stomach would slow the intestinal enzymes down; bile helps neutralise it as the food enters the small intestine.

Part D · Absorption and What the Products Are Used For

11 The End Products and Their Uses

The end products of digestion are the small, soluble molecules left at the end: glucose from carbohydrates, amino acids from proteins, and fatty acids and glycerol from fats.

End product	How the body uses it
Glucose	Broken down in cellular respiration, releasing the energy every cell needs; any not needed at once is stored as glycogen in the liver and muscles
Amino acids	The raw material for growth and for the repair of damaged or worn-out tissue
Fatty acids and glycerol	Used to release energy, and to build stored fat, which also insulates the body and protects the organs

Absorbed products are taken into the body's cells and used for cellular processes such as respiration, growth and tissue repair.

Part E · Health, Food and Lifestyle Choices

12 Diabetes: When Blood Glucose Cannot Be Controlled

In diabetes, the body cannot control the concentration of glucose in the blood: it is unable to store or use the glucose present, so the concentration becomes abnormally high and the excess is lost in the urine.
Signs to recognise: feeling weak and tired; losing weight without trying; passing urine very often and feeling very thirsty; wounds that heal slowly, and catching infections more easily.

	Type 1	Type 2
Cause	The pancreas does not produce enough insulin	The body's cells respond poorly to the insulin that is produced
Who it affects	Often younger people	Often adults, particularly those who are overweight
Managed by	Insulin, with a controlled diet	A healthy diet and regular exercise, sometimes with medication

Because type 2 is closely linked to body weight and lifestyle, sensible food and lifestyle choices matter: regular meals, less sugar and fat, more fibre, and keeping active.

13 Components of a Healthy Diet

Food group	Examples	What it contributes
Rice and alternatives	Rice, noodles, bread, cereals	Energy; especially important when the diet is low in fat
Vegetables and fruits	Foods from plants	Vitamins and minerals, together with fibre, which gives the gut something to push against and helps prevent constipation
Meat and alternatives	Meat, dairy products, eggs, beans	Protein, which the body breaks into amino acids and uses to build its own proteins
Fats, oils, sugar and salt	Foods supplying calories and little else	Mainly energy. Some fat is needed, but only a small quantity, so this group is used sparingly

The healthy diet pyramid is the usual guide: foods at the base make up the largest part of what is eaten, and foods towards the top are eaten in steadily smaller amounts.

14 Gut Bacteria, Food Safety and Common Problems

Bacteria in the gut can be beneficial or harmful.

Condition	What is happening	What helps
Beneficial bacteria	Live in the large intestine and break down material the body's own enzymes cannot manage, helping digestion along	—
Harmful bacteria	The same bacteria cause illness if they reach places they do not belong, or if food carries bacteria from poor hygiene → food poisoning	Wash hands and food, store and cook food properly
Constipation	Waste moves too slowly, so too much water is absorbed and the waste becomes dry and hard	More water, more fibre, more exercise
Gastritis	Inflammation of the stomach lining, causing pain, indigestion, nausea and vomiting	Avoid too much alcohol, caffeine, oily food, certain painkillers and stress
Food poisoning	The bowel moves its contents along too quickly, so water is not absorbed in time	Replace lost water, and hygiene to prevent it

Part F · Exam Tips from the Syllabus Pointers

15 Exam Tips: What Each Syllabus Pointer Asks For

The MOE pointer	What earns the mark
Explain the importance of the digestive system	State the reason, not the parts: food is large and insoluble, blood carries only dissolved substances, so food must be broken down before it can be absorbed and reach the cells.
Explain how the main parts work together to perform a function	Name the part, then its job, then the link: the mouth starts starch digestion, the stomach churns and digests protein, the small intestine finishes digestion and absorbs, the large intestine absorbs water from undigested food, and the rectum and anus egest the waste.
Describe how the system helps in the digestion of food	Separate physical from chemical at each stage, and name the enzyme class and the end product. Say where each enzyme works and why that pH suits it.
State that the end products are used for cellular processes	Give the product and its use: glucose → cellular respiration for energy; amino acids → growth and tissue repair; fatty acids and glycerol → energy and stored fat.
Investigate the effect of enzymes in digestion	Name the class of enzyme that acts on each food substance, and the end product it makes. Only the classes of enzyme are required — carbohydrase, protease and lipase.
Appreciate the importance of food and lifestyle choices in diabetes	Link the choice to the condition: a diet high in sugar and fat with little exercise raises the risk of type 2 diabetes; regular meals, more fibre and staying active help control blood glucose.
Show awareness that bacteria can be beneficial or harmful	Give both sides with an example: gut bacteria help break down food the body cannot digest, but the same bacteria cause infection if they reach other parts of the body or contaminate food.