Chapter 12: Digestive System
For a long time, people believed that the stomach worked like a grinding mill. Food went in, muscles crushed it, and the pieces were somehow taken up by the body. It was a sensible guess. You can feel your jaws crush a biscuit, so why should the stomach not do the same, only harder?
In the 1770s an Italian priest and naturalist, Lazzaro Spallanzani, decided to test the idea instead of arguing about it. He fed birds small metal tubes with holes in their walls. Meat was packed inside each tube. The walls protected the meat from being crushed by any grinding action. When the tubes came out again, the meat inside had partly disappeared.
Something was reaching the meat through those holes and dissolving it. Spallanzani went further. He swallowed small bags of food himself, and he collected samples of his own stomach fluid. He then placed meat in that fluid in a warm glass vial, far away from any muscle. The meat slowly dissolved there too.
That single observation changed the question. Digestion was not mainly a matter of crushing. It was a chemical change, carried out by a fluid. The rest of this chapter follows that fluid, and several others like it, from your lips to the end of your food canal.
Diagram
Fig. 12.0-1 — Spallanzani’s Test Of Digestion Two-part line drawing: on the left, a perforated metal tube containing a piece of meat, shown being swallowed by a bird, with the tube later recovered and the meat partly dissolved; on the right, a glass vial of collected stomach fluid holding a piece of meat, standing in a warm water bath. Label perforated tube, meat, recovered tube, stomach fluid, meat in vial, warm water bath.
12.1 Need For A Digestive System
Every cell in your body needs food. But the cells of your leg cannot reach the chapati you have just swallowed. Food has to be delivered to them, and blood is the delivery service.
That sets a hard condition. Whatever is to reach the cells must first be able to get into the blood.
12.1.1 Why Food Must Be Broken Down
Think about where swallowed food actually is. It lies inside a long tube that runs through your body, with an opening at each end. Until food crosses the wall of that tube, it has not truly entered the body at all.
To cross that wall, food must do two things. It must dissolve in water, because blood carries substances in dissolved form. And its particles must be small enough to pass through the cell membranes of the tube’s lining.
Most of what we eat fails both tests. A piece of boiled potato is neither dissolved nor small. So the body has to change it.
Some foods pass the first test but not the second. Sucrose, the sugar we get from sugarcane, dissolves in water quite easily. Even so, a sucrose molecule is too large to be carried through a cell membrane, and it must be split into smaller units first.
This whole process of change is called digestion — the breaking down of food into a form that can dissolve and pass through living membranes into the blood.
Diagram
Fig. 12.1-1 — Why Food Must Be Changed Before It Can Enter The Blood Simple sectional line drawing of a short length of the gut wall: the cavity of the gut on the left, a single layer of lining cells in the middle, a blood capillary on the right. Show a large insoluble food particle stopped at the lining, and small soluble molecules passing through the lining cells into the capillary. Label gut cavity, food particle (large, insoluble), lining cells, small soluble molecules, blood capillary.
12.1.2 Enzymes And Their Characteristics
Starch can be broken down into sugar in a test tube if you boil it long enough with acid. Your body cannot use that method. It works at about 37 °C (degrees Celsius), and it cannot afford to cook itself.
The body’s answer is a set of chemical helpers. An enzyme is a protein made by living cells that greatly speeds up one particular chemical reaction without being used up in it. Because it is not used up, the same enzyme molecule can do the same job over and over again.
This makes an enzyme a biological catalyst — a substance that changes the rate of a reaction while remaining unchanged itself at the end of it.
Each enzyme works on one kind of substance only, and that substance is called its substrate. An enzyme that splits starch will not touch a protein lying beside it, much as a key cut for one lock will not open another.
The way an enzyme acts on its substrate is shown in Fig. 12.1-2.
Diagram
Fig. 12.1-2 — An Enzyme Acting On Its Substrate Three-stage line drawing read left to right: (1) an enzyme molecule with a notch in it, and a matching substrate molecule approaching; (2) the substrate fitted into the notch; (3) two smaller product molecules moving away while the same enzyme molecule remains unchanged. Label enzyme, substrate, products, unchanged enzyme.
There are hundreds of enzymes in the body, and only some of them are concerned with digestion. All of them, however, share the same general characteristics.
Characteristics Of An Enzyme
- It is a protein, and it is therefore destroyed by heating.
- It acts on only one kind of substance, its substrate. That is, it is specific.
- It always forms the same end product or products from that substrate.
- It affects only the rate of the reaction, and it always speeds the reaction up.
- Like any catalyst, it can be used again and again.
- It works best at one particular pH, that is, at one particular degree of acidity or alkalinity.
- It works best within a narrow range of temperature, usually between 35 °C and 40 °C. This range is called its optimum temperature.
The first and the last of these are linked. Heat destroys an enzyme because heat damages proteins, in the same way that heat turns the clear white of an egg into a solid white lump. That change cannot be undone, and such an enzyme never works again.
12.2 The Plan Of The Digestive System
Before following food through the body, it helps to see the whole layout at once. The digestive system is built out of just two kinds of parts.
The first is the alimentary canal, a long muscular tube that begins at the mouth and ends at the anus. The second is the digestive glands, organs that pour digestive juices into that tube. A gland is simply a structure whose cells make a useful fluid and release it where it is needed.
The Canal Itself
The alimentary canal is about 9 metres long in an adult, far longer than the body that holds it. It fits inside the abdomen because it is tightly coiled, most of all in the small intestine.
The tube is not the same all the way along. Each region differs in structure from the region before it, and each difference matches a different job — chewing, storing, churning, absorbing. You will meet these regions one by one, in the order shown in FC 12.1.
Flowchart
FC 12.1 — Regions Of The Alimentary Canal Mouth → pharynx → oesophagus → stomach → small intestine (duodenum → jejunum → ileum) → large intestine (caecum → colon → rectum) → anus
The Glands That Serve It
Some digestive glands are small and lie in the lining of the canal itself. They are not separate organs at all; they are pockets of gland cells in the wall, and they release their juice directly into the space beside them.
Others are large organs lying outside the canal and connected to it by tubes called ducts. Two of these are the liver and the pancreas, both of which empty into the small intestine. Three pairs of salivary glands serve the mouth.
Fig. 12.2-1 shows the canal and these associated glands together.
Diagram
Fig. 12.2-1 — The Alimentary Canal And Its Associated Glands Full-length front view of the human digestive system in outline; label buccal cavity, parotid gland, submandibular gland, sublingual gland, oesophagus, cardiac sphincter, diaphragm, stomach, pyloric sphincter, liver, gall bladder, hepatic duct, common bile duct, pancreas, pancreatic duct, duodenum, jejunum, ileum, caecum, appendix, ascending colon, transverse colon, descending colon, rectum and anus. Line drawing with straight, non-crossing lead lines.
12.3 The Mouth
Digestion begins before food has travelled a single centimetre. The mouth, or oral cavity, is the space in which food is chewed and mixed with saliva. Its front boundary is formed by the upper and lower lips.
12.3.1 Lips And Tongue
The lips look like a simple opening, but they do four separate jobs.
- They close the mouth, so that food and liquid stay in while chewing.
- They make sucking and sipping possible.
- They shape many of the sounds of speech.
- They sense touch and heat, which is why you test a hot cup of tea with your lips before drinking.
Inside the mouth is the tongue, a muscular organ that can change shape in almost any direction. That freedom of movement is what makes its five jobs possible.
- It moves food about while you chew and mixes it with saliva.
- It pushes the food backwards during swallowing.
- It carries the taste buds, so it tastes the food.
- It cleans food particles off the teeth after eating.
- It shapes speech, along with the lips and teeth.
12.3.2 The Teeth — Dentition
The teeth cut and break food into smaller bits. That sounds like a small service, but it is what makes chemical digestion possible at a useful speed.
Think of one large lump of food. Enzymes can only reach the surface of it, because they work on whatever they can touch. Break that lump into many small bits and the total surface goes up sharply, while the amount of food stays the same. More surface means more places for enzymes to act at once, so digestion proceeds faster and more completely.
Teeth also help in forming speech sounds, and they give the face its shape.
12.3.2.1 Types Of Teeth And Their Work
An adult human normally has 32 teeth. They are not all alike; each shape suits a different task, as Fig. 12.3-1 shows.
Incisors are the four front teeth in the centre of each jaw. Their cutting edges are broad and sharp like a chisel, and they are used for biting and cutting.
Canines stand one on either side of the incisors in each jaw. They are conical and sharply pointed, which suits holding and tearing food.
Premolars are two on each side in each jaw, next to the canines. Each has two hill-like projections, called cusps, on its surface, and is therefore described as bicuspid. Premolars grind and crush the food.
Molars are the last three teeth on each side in each jaw. Their biting surface is larger than that of the premolars, and they are the principal grinders and crushers. The last molar of each side in each jaw is called the wisdom tooth, so named because it appears last, at about 17 to 20 years of age, when the body is reaching maturity.
Diagram
Fig. 12.3-1 — Kinds Of Teeth In The Adult Upper Jaw Surface view of the upper jaw of a normal human adult showing the teeth in their natural position, with one half of the jaw marked off; label incisors (2), canine (1), premolars (2), molars (3) and wisdom tooth (last molar), and mark the total for one half of the jaw as 2 + 1 + 2 + 3 = 8. Below the jaw, four separate side-view sketches of a single incisor, canine, premolar and molar, each labelled.
Four shapes in one mouth is worth a name of its own. Teeth of different kinds in the same jaw are called heterodont teeth (hetero: different, dont: teeth). Human and other mammalian teeth are heterodont. Animals such as lizards and frogs have homodont teeth (homo: similar), where every tooth in the jaw is of the same kind and does the same work.
The four types are compared side by side in Table 12.1.
12.3.2.2 Two Sets Of Teeth And The Dental Formula
Mammalian teeth appear in two sets during life. A jaw that grows must be refitted, and this is how it is done.
The first set is the milk teeth, and it has 20 teeth — every kind except the premolars. They begin pushing through the gums when a child is about 7 to 8 months old, sometimes painfully, and the set is complete at about 2 years. These are the temporary teeth, also called the deciduous teeth.
They do not simply fall out. Their roots are gradually dissolved away inside the jaw, so the crown is left loosely held and comes away easily. Beneath each one, a permanent tooth is already waiting, as Fig. 12.3-2 shows. The replacement is complete by about 12 years of age.
Diagram
Fig. 12.3-2 — Temporary Teeth And The Permanent Teeth Below Them Cut-away side view of a child’s lower jaw showing the temporary teeth in place above and the developing permanent teeth lying within the jaw bone below them; label incisors, canine, premolars, molars, temporary teeth, developing permanent teeth and jaw bone.
The number of permanent teeth of a mammal is written in a short form called the dental formula. It gives the number of incisors (i), canines (c), premolars (pm) and molars (m), always in that order, and always for one half of each jaw only. The upper line is the upper jaw and the lower line the lower jaw.
Because the formula covers only one half of each jaw, the count it gives must be multiplied by two to get the whole mouth.
Human child, up to about 2 years: \[\frac{2,\ 1,\ 0,\ 2}{2,\ 1,\ 0,\ 2} = 20 \text{ (milk teeth)}\]
Human adolescent, up to about 17 to 20 years: \[\frac{2,\ 1,\ 2,\ 2}{2,\ 1,\ 2,\ 2} = 28 \text{ (permanent teeth)}\]
Human adult: \[\frac{2,\ 1,\ 2,\ 3}{2,\ 1,\ 2,\ 3} = 32 \text{ (permanent teeth, with 4 wisdom teeth added)}\]
Read the three formulae downwards and the story of a mouth appears in them. The child has no premolars at all. The adolescent has gained them but not the last molars. The adult has 32 only after the four wisdom teeth arrive.
12.3.2.3 Structure Of A Tooth
All types of teeth are built to the same general plan, whatever their shape. Each tooth has a crown, the part exposed above the gum, and a root, the part embedded in a cup-like socket of the jaw bone. Between the two is the neck, a slight constriction at gum level.
The number of roots varies with the work the tooth does. Incisors and canines have a single root, or fang. Premolars and the lower molars have two, and the upper molars have three. A tooth that must resist heavy grinding forces is anchored by more than one root.
Cut a tooth down its length, as in Fig. 12.3-3, and four materials appear.
Enamel is the material covering the crown. It is the hardest substance in the body, which is what allows the biting surface to survive a lifetime of crushing.
Dentine forms the bulk of the tooth. It is harder than bone but not as hard as enamel. Running through it are minute canals carrying strands of cytoplasm from cells that lie in the pulp cavity, so the dentine is a living material and not a dead block.
Cementum is a bone-like layer covering the root and fixing it in its socket.
Pulp is the soft connective tissue filling the central space of the tooth, called the pulp cavity. It contains blood capillaries, lymph vessels and nerve fibres, which pass out through an opening at the base of the root and join those of the rest of the body.
Now the pieces explain each other. The blood capillaries in the pulp feed the living cells of the tooth. The nerve fibres are the reason a deep cavity hurts, since pain is felt only once the damage passes the enamel and reaches this inner tissue.
Diagram
Fig. 12.3-3 — Internal Structure Of A Tooth Vertical section through two teeth shown one above the other: A, a canine with a single root, and B, a premolar with two roots. On A label crown, neck, root, enamel, dentine, gum, jaw bone and cementum. On B label enamel, dentine, pulp cavity, cementum and periodontal membrane. Straight, non-crossing lead lines.
12.3.3 The Salivary Glands And Saliva
Chewing alone would leave a dry crumbly mass in the mouth. What turns it into something swallowable is saliva, and saliva comes from three pairs of salivary glands, shown in Fig. 12.3-4.
- The parotid glands lie just in front of and below each ear.
- The submandibular glands lie close to the inner side of the lower jaw on each side.
- The sublingual glands lie below the tongue.
Each gland sends its secretion into the mouth through a duct. A small quantity of saliva is being made at all times, and the flow rises sharply while you eat. Even the sight or smell of food, or just the thought of a dish you like, can start it — the effect everyone knows as the mouth watering. Over a whole day, an adult produces roughly 1000 to 1500 mL of saliva.
Diagram
Fig. 12.3-4 — The Three Pairs Of Salivary Glands Side view of the human head and neck in outline showing the oral cavity, tongue and jaw; label parotid gland and its duct, submandibular gland and its duct, sublingual gland and its duct, tongue and opening of each duct into the mouth.
Saliva is a very slightly acidic fluid, of pH 6.8. It is about 99 per cent water, and the rest is salts, mucus and an enzyme, salivary amylase, also called ptyalin.
What Saliva Does
- It moistens and lubricates the lining of the oral cavity and the surface of the tongue, which makes speaking and swallowing possible.
- It moistens and lubricates the food itself, which again helps swallowing.
- It acts as a solvent. Substances in food dissolve in it, and only dissolved substances can stimulate the taste buds of the tongue.
- It makes food particles stick together into a bolus, a soft rounded ball that can be swallowed as one mass.
- It begins the digestion of starch. Its enzyme ptyalin converts starch into maltose.
- It cleans the mouth and tends to destroy germs, which helps prevent tooth decay.
- A dry mouth, caused by too little water in the saliva, produces the feeling of thirst. Saliva therefore takes part in keeping the body’s water balance.
The starch-digesting step is written like this.
\[\text{Starch} \xrightarrow{\text{ptyalin (salivary amylase)}} \text{Maltose (a disaccharide)}\]
This one reaction can be tasted. Starch itself has no sweet taste, but maltose does.
12.4 Swallowing And Peristalsis
Swallowing feels like a single action, but it is several actions carried out at the same instant. They have to be, because at the back of the mouth the food passage and the air passage cross each other.
The pharynx is the space at the back of the mouth where these two routes meet. Air arrives there from the nasal passage above and must go down into the windpipe. Food arrives from the mouth in front and must go down into the oesophagus behind. Getting food into the right tube means shutting the other two openings for a moment.
What Happens In One Swallow
The tongue presses upward and back against the roof of the mouth, the palate, and this forces the bolus into the pharynx.
At the same moment the soft palate, the soft back part of the roof of the mouth, is lifted. It closes the opening between the throat and the nasal passage, so food cannot be pushed up into the nose.
The larynx, or voice box, sits at the entrance of the windpipe. It is pulled upward, close to the back of the tongue, and a flap called the epiglottis swings down over its opening, the glottis. The windpipe is now covered.
With the nasal passage sealed above and the windpipe covered in front, only one route is left open to the bolus — the oesophagus, or gullet. FC 12.2 sets the sequence out in order, and Fig. 12.4-1 shows the difference between the two positions.
Flowchart
FC 12.2 — Actions During One Swallow Bolus formed in mouth → tongue presses up and back against palate → bolus pushed into pharynx → soft palate rises and closes the nasal passage → larynx pulled upward and epiglottis covers the glottis → only the oesophagus remains open → bolus enters the oesophagus
Diagram
Fig. 12.4-1 — Position Of The Larynx During Breathing And During Swallowing Two side views of the head and neck in vertical section, one above the other. A — during breathing: the larynx is lowered and the glottis is open; label nasal cavity, buccal cavity, tongue, salivary gland, false palate, epiglottis, glottis, larynx (voice box), trachea (windpipe), pharynx, oesophagus and vertebra, with an arrow marked air passing into the trachea. B — during swallowing: the larynx is raised and the glottis is closed by the epiglottis; label food mass, glottis (closed), larynx, windpipe and oesophagus, with an arrow marked food passing into the oesophagus.
How Food Moves Onward
Once the bolus is inside the oesophagus, swallowing is over and something else takes charge. Food is not dropped down the tube by gravity. It is pushed.
The wall of the gut contains circular muscles that ring the tube. Just behind the bolus, these muscles contract and narrow the tube; just ahead of it, they relax and widen. The narrowing point then travels forward, and the food is squeezed along in front of it, rather as toothpaste is squeezed along a tube.
This travelling wave is called peristalsis.
Peristalsis is not limited to the oesophagus. It occurs through all regions of the gut, and it is the reason food keeps moving steadily along a canal about 9 metres long. To make that easier, every region of the gut secretes mucus, a slimy fluid that lubricates the food and the lining it slides against. Fig. 12.4-2 shows a single wave in progress.
Diagram
Fig. 12.4-2 — Peristaltic Movement In The Oesophagus Two successive line drawings of a length of the oesophagus in longitudinal section with a bolus inside; in the first, the wall behind the bolus is constricted and the wall ahead is relaxed, in the second the constriction has moved forward and the bolus has advanced. Label wave of constriction, wave of relaxation, bolus, circular muscle, and mark the direction of movement with arrows.
12.5 The Oesophagus
The oesophagus, or gullet, is the tube that conducts food from the throat to the stomach. It runs down the chest close to the backbone and passes through the diaphragm, the sheet of muscle that separates the chest from the abdomen, before opening into the stomach just below it.
Its job is transport and nothing more. The oesophagus produces no digestive enzyme, so no food is chemically changed on the way down. The only digestion still going on inside the bolus at this stage is the work of the ptyalin swallowed with it.
Food does not linger here. Peristaltic waves carry the bolus along, and the mucus of the lining keeps the passage smooth.
12.6 The Stomach
The stomach is an elastic bag lying just below the diaphragm, on the left side of the upper abdomen. In an average adult it can hold 2 to 3 litres of food.
Being elastic matters. It is what allows you to eat a full meal in twenty minutes and then release it into the intestine slowly, over hours. The stomach is as much a store as it is a digesting organ.
Its walls are highly muscular. They churn the food thoroughly, so that every part of it is mixed with the gastric juice secreted by the inner lining. Churning is mechanical work; the juice does the chemical work.
The Two Gateways
A bag that stores food under churning needs a closed door at each end, and it has one. A sphincter is a ring of muscle around an opening, which keeps that opening shut until it is told to relax, in the way a valve does.
The opening of the stomach into the intestine is called the pylorus (Greek pylon: gate, pylorus: gate-keeper). Its ring of muscle holds the exit closed so that food cannot leave until it has been properly churned. It also prevents material already in the duodenum from flowing back into the stomach.
At the front end, where the oesophagus enters, a similar ring called the cardiac sphincter prevents the churned, acidic contents from being pushed back up into the oesophagus. Fig. 12.6-1 shows both.
Diagram
Fig. 12.6-1 — The Stomach And Its Sphincters Longitudinal section of the human stomach in outline, with the lower end of the oesophagus above and the duodenum below; label oesophagus, cardiac sphincter, muscular wall, inner lining with gastric glands, pylorus, pyloric sphincter and duodenum, and mark the direction of food movement with arrows.
Gastric Juice
Gastric juice is a colourless, highly acidic liquid. It contains water, some salts, hydrochloric acid, and an enzyme called pepsin.
The acid is the striking part. Hydrochloric acid strong enough to attack food would seem an odd thing for the body to make, but it does two necessary jobs. It kills germs that have entered along with the food, and it converts the inactive form of the enzyme into its working form.
That second job needs explaining. The gland cells do not release pepsin itself. They release an inactive substance called pepsinogen, which the hydrochloric acid then changes into active pepsin inside the stomach cavity.
The reason for that arrangement is simple. Pepsin digests protein, and the cells that make it are themselves built of protein. By making it in an inactive form and switching it on only after it has left the cell, the gland avoids digesting itself. The lining is further protected by a thick layer of mucus that keeps the acid off the living cells beneath.
\[\text{Pepsinogen (inactive)} \xrightarrow{\text{HCl}} \text{Pepsin (active)}\]
\[\text{Proteins} \xrightarrow{\text{pepsin}} \text{Peptides}\]
Pepsin acts on proteins, breaking them into shorter chains called peptides. It does not finish the job — only about 20 per cent of the protein in a meal is digested here. The rest is dealt with further along.
The stomach of a baby produces a second enzyme, rennin, which is not found in adult humans. It too is released in an inactive form, prorennin. Rennin acts on casein, the soluble protein of milk, and converts it into insoluble paracasein — in everyday terms, it curdles milk. Curdled milk is a solid and stays in the stomach longer, giving pepsin time to work on it.
\[\text{Prorennin (inactive)} \xrightarrow{\text{HCl}} \text{Rennin (active)}\]
\[\text{Casein (soluble)} \xrightarrow{\text{rennin}} \text{Paracasein (insoluble)}\]
Leaving The Stomach
Food stays in the stomach for about 3 hours. By the end of that time, churning and gastric juice have reduced it to a pulp-like, semi-liquid form called chyme.
The pylorus then opens intermittently, letting the chyme pass into the intestine little by little. Releasing it in small instalments gives the intestine time to neutralise the acid and to digest each portion properly, which a sudden flood of chyme would not allow.
12.7 The Small Intestine
Most of what we call digestion happens here, and so does almost all absorption. The small intestine is a tube about 7 metres long and only 2.5 cm wide, coiled and folded to fit into the abdomen.
It is called small because of that narrow width, not because of its length. It is by far the longest part of the alimentary canal.
The small intestine does two jobs at once. It completes the digestion of every kind of food, and it absorbs the products. To do the first it receives two juices from outside — the bile and the pancreatic juice, both delivered into its upper part — while its own walls secrete a third, the intestinal juice.
12.7.1 The Three Regions
The tube is divided into three regions, one after the other.
The duodenum is the short upper part, next to the stomach. Its name means twelve, from duo: two and deni: ten, because its length is about twelve finger widths. The common bile duct opens into this part, and so does the pancreatic duct.
The jejunum is the next region, about 2 metres long. Its name means empty, because it is nearly always found empty in dissections and post-mortem examinations.
The ileum is the last and longest region, about 4 metres. Its name refers to twisting or rolling, which describes the movements of this part.
12.7.2 Villi And The Surface For Absorption
Absorption happens at a surface. Every molecule of digested food must cross the lining of the intestine to reach the blood, so the more lining there is, the more absorption can go on at the same time.
The inner lining of the ileum is therefore not smooth. It is thrown into a vast number of tiny finger-like projections called villi (singular, villus), shown in Fig. 12.7-1. Packed side by side, they raise the inner surface of the intestine to many times the whole outer surface of the body, and it is this enormous surface that makes efficient absorption possible.
Between the bases of the villi are small openings. Through these, the intestinal juice made by the gland cells of the wall is poured into the cavity of the intestine.
Each villus is covered by an epithelium, a lining layer only one cell thick. A single cell is all that separates the food in the intestine from the transport vessels inside the villus, which is about as short a crossing as living tissue can offer.
Inside each villus lie an artery, a vein and the blood capillaries connecting them, together with a lymph vessel called a lacteal. Between them these vessels carry away everything the villus absorbs.
Diagram
Fig. 12.7-1 — Microscopic Structure Of An Intestinal Villus Longitudinal section of a single finger-like villus, enlarged; label epithelium (one cell thick), blood capillaries, artery, vein, lacteal, lymphatic duct and the vessel leading to the hepatic portal vein. Show the artery and vein in the core of the villus with the capillary network between them and the lacteal in the centre.
Four Adaptations Of The Ileum
- It is very long, so a great deal of surface is provided.
- Its lining carries a large number of villi, which increase that surface further.
- Its epithelium is only one cell thick, so absorbed substances have the shortest possible distance to travel.
- It is narrow, which slows the movement of food and leaves time for absorption to be completed.
12.7.3 Bile
Bile is a yellowish-green, watery fluid produced in the liver. It leaves the liver through the hepatic duct, which is joined by the cystic duct from the gall bladder to form the common bile duct. Bile may flow straight into the duodenum, but more usually it is stored for a while in the gall bladder and released when food arrives. Fig. 12.7-2 shows the arrangement of these ducts.
Its colour comes from the pigments biliverdin and bilirubin, which are produced when dead and worn-out red blood cells are broken down. These pigments are waste products; bile is thus a route by which the body disposes of them as well as a digestive fluid.
Bile contains no enzyme at all. It works in two other ways.
First, it contains a large amount of sodium bicarbonate. The chyme arriving from the stomach is strongly acidic, and the enzymes of the pancreatic and intestinal juices cannot work in acid. The bicarbonate neutralises that acid and makes the contents alkaline, so those enzymes can act.
\[\text{Acidic chyme} \xrightarrow[\text{NaHCO}_3]{\text{bile}} \text{Alkaline chyme}\]
Second, bile salts act on fats. Fat does not mix with the watery contents of the gut; it gathers into large blobs, and an enzyme can only reach the outside of a blob. Bile salts lower the surface tension of the fat and break it into a great many tiny droplets. This is called emulsification, and it hugely increases the fat surface exposed to enzymes.
\[\text{Fat} \xrightarrow{\text{bile}} \text{Emulsified fat (tiny droplets)}\]
Notice that this is the same principle you met with the teeth. Bile does to fat, chemically, what chewing does to a mouthful of food.
Diagram
Fig. 12.7-2 — The Bile Duct And Pancreatic Duct Opening Into The Duodenum Line diagram showing the underside of the liver, the gall bladder, the pancreas and the first part of the small intestine; label from liver, hepatic duct, gall bladder, cystic duct, common bile duct, pancreas, pancreatic duct, common opening into the duodenum, and duodenum.
12.7.4 Pancreatic Juice
The pancreas is a whitish gland lying behind the stomach. Its juice reaches the duodenum through the pancreatic duct, which opens by an aperture common with that of the bile duct.
Pancreatic juice contains three enzymes, and between them they attack all three main kinds of food.
Amylopsin, also called pancreatic amylase, digests the starch left over from the mouth into maltose.
Trypsin acts on the proteins and polypeptides that pepsin did not finish, producing smaller peptides and some amino acids. Like pepsin, it is dangerous to the cells that make it, so it too is secreted in an inactive form, trypsinogen. It is switched on inside the duodenum by an enzyme called enterokinase, also called enteropeptidase, which the lining of the duodenum secretes.
Steapsin, or pancreatic lipase, acts on emulsified fats and splits them into fatty acids and glycerol. This is why bile must arrive first: steapsin works on droplets, and bile is what makes the droplets.
\[\text{Trypsinogen (inactive)} \xrightarrow{\text{enterokinase}} \text{Trypsin (active)}\]
\[\text{Proteins and polypeptides} \xrightarrow{\text{trypsin}} \text{Smaller peptides + amino acids}\]
\[\text{Leftover starch} \xrightarrow{\text{amylopsin}} \text{Maltose}\]
\[\text{Emulsified fat} \xrightarrow{\text{steapsin}} \text{Fatty acids + glycerol}\]
12.7.5 Intestinal Juice
The intestinal juice is secreted by the gland cells in the wall of the intestine itself. It finishes what the earlier juices began, and its enzymes deal with molecules that are already quite small.
Erepsin, a mixture of peptidases, converts the remaining peptides into amino acids. Maltase splits maltose into glucose. Sucrase, also called invertase, splits sucrose into glucose and fructose. Lactase splits lactose, the sugar of milk, into glucose and galactose. Traces of lipase digest any remaining fat into fatty acids and glycerol.
\[\text{Peptides} \xrightarrow{\text{erepsin (peptidases)}} \text{Amino acids}\]
\[\text{Maltose} \xrightarrow{\text{maltase}} \text{Glucose}\]
\[\text{Sucrose} \xrightarrow{\text{sucrase}} \text{Glucose + fructose}\]
\[\text{Lactose} \xrightarrow{\text{lactase}} \text{Glucose + galactose}\]
\[\text{Emulsified fat} \xrightarrow{\text{lipase}} \text{Fatty acids + glycerol}\]
The three juices are set side by side in Table 12.2.
12.7.6 Absorption Of Digested Food
The final products of digestion are mainly absorbed in the small intestine itself, and the route a product takes depends on what it is.
Amino acids and the simple sugars — glucose, fructose and galactose — have relatively small molecules. They pass through the thin epithelium of the villi into the blood capillaries inside, and so enter the blood circulation. That blood does not go straight round the body. It travels first to the liver, through the hepatic portal vein.
Fatty acids and glycerol take a different road. They are absorbed into the lymph vessels, that is, into the lacteals of the villi. From there they travel through the lymphatic system, a network spreading over the whole body, which finally empties its contents into the blood stream.
Absorption is not hurried. Food passes very slowly through the small intestine, taking about four hours to reach the large intestine — time enough for digestion to be completed and for most of the products to be taken up.
FC 12.3 and the trace that follows put the whole journey of one starch molecule together, from the first bite to the blood.
Flowchart
FC 12.3 — Digestion Of Starch From Mouth To Blood Starch in food → ptyalin in mouth → maltose → (no change in oesophagus or stomach) → amylopsin in duodenum acts on leftover starch → maltose → maltase in ileum → glucose → absorbed by villus epithelium → blood capillary → hepatic portal vein → liver
12.8 The Large Intestine
By the time the contents leave the small intestine, almost everything useful has been taken out of them. What is left is mostly material the body cannot digest, mixed with a great deal of water. The large intestine deals with both.
It is about 1.5 metres long — much shorter than the small intestine, but far wider, which is where its name comes from. It has three parts: the caecum, the colon and the rectum, shown in Fig. 12.8-1.
Diagram
Fig. 12.8-1 — The Large Intestine Front view of the large intestine in outline with the end of the ileum entering it; label ileum, caecum, vermiform appendix, ascending colon, transverse colon, descending colon, rectum and anus, and mark the direction of movement of the contents with arrows.
Caecum
The caecum is a small blind pouch at the junction of the small and large intestines. Blind means closed at one end: material enters and leaves by the same side, and nothing passes through the far end.
From its closed end projects a narrow, worm-shaped tube, the vermiform appendix. In human beings it is a vestigial organ, that is, a structure that has been reduced during the course of evolution and no longer performs its original function. In many plant-eating mammals the corresponding region is large and houses bacteria that break down cellulose; in humans it does no such work.
Colon
The colon is much broader than the ileum and is a little more than a metre long. It takes a three-sided path around the abdomen. It passes upward on the right side as the ascending colon, crosses to the left just below the stomach as the transverse colon, and runs down the left side as the descending colon.
Rectum
The rectum is the last part, about 15 cm long, and it opens at the anus. The anus is surrounded by circular muscles, that is, sphincters, which keep it closed except when the bowels are emptied.
What The Large Intestine Does
The large intestine secretes no enzyme. No digestion takes place in it, and very little digested food is absorbed from it.
What it does absorb, in large quantity, is water. That is not a small matter. Several litres of fluid are poured into the alimentary canal every day as saliva, gastric juice, bile, pancreatic juice and intestinal juice, and if that water were simply lost the body would rapidly dry out. The colon takes most of it back.
As the water is removed, the contents become semi-solid. This material is called faeces. It passes into the rectum, where it is held until it is expelled at intervals. The expulsion of the undigested remains of food from the alimentary canal is called defaecation.
Faeces are made up of roughly:
- 75 per cent water;
- 25 per cent solid matter, which itself consists of about 30 per cent dead bacteria, 10 to 20 per cent fat, 2 to 3 per cent protein and about 30 per cent roughage — the indigestible plant fibre that passes through the canal unchanged.
The composition varies from person to person and with the kind of food eaten, so these figures describe a general pattern rather than a fixed recipe. The characteristic odour of faeces is produced by the action of bacteria on them.
12.9 Digestion Region By Region
You have now followed food along the whole canal, region by region. It is worth seeing the same journey the other way round — as a single view of which juice acts where, on what, and with what result.
Two things stand out when the whole canal is set out at once. First, no single region digests everything; each takes the food a stage further and passes it on. Second, digestion is finished in the ileum, and every end product listed there is small enough to be absorbed. Table 12.3 gives the complete picture.
Read down the last column and the three end points of digestion appear plainly. Carbohydrates end as simple sugars, proteins end as amino acids, and fats end as fatty acids and glycerol. Nothing smaller is needed, because these are exactly the forms the villi can absorb.
12.10 Assimilation Of Food
Digestion breaks food down and absorption carries it into the blood. Neither of these has yet made the food part of you. Glucose floating in the blood is still just glucose. The last step is to put it to use — to build it into tissue, store it, or burn it for energy.
That step is called assimilation.
The absorbed food leaves the gut by two routes, as you saw in the villus. Simple sugars, amino acids, vitamins and minerals go by the hepatic portal system, which delivers them to the liver. Fatty acids and glycerol go by the lymphatic system.
What Happens To Glucose
Blood from the intestine reaches the liver first, through the hepatic portal vein, and the liver decides what happens to the sugar in it.
If there is more glucose than the body needs at that moment, the liver converts the excess into glycogen, an insoluble carbohydrate that can be stored. This conversion is called glycogenesis. Storing it in an insoluble form matters, because a large amount of dissolved sugar sitting in the blood would upset the body’s chemistry.
When glucose is needed again — during exercise, or hours after a meal — the liver reconverts the stored glycogen into glucose and returns it to the blood. This reverse conversion is called glycogenolysis.
The glucose is needed in the tissues for two things: as the fuel of cellular respiration, which releases the energy every cell runs on, and as raw material for building certain other compounds. FC 12.4 shows both directions.
Flowchart
FC 12.4 — The Fate Of Absorbed Glucose Glucose absorbed by villi → hepatic portal vein → liver → (if in excess) glycogenesis → glycogen stored in liver → (when needed) glycogenolysis → glucose → blood → body tissues → used in cellular respiration or in building compounds
What Happens To Amino Acids
Amino acids circulate in the body and serve as the building blocks of protein. Cells take them up and join them into the proteins that make muscle, enzymes and much else.
Amino acids cannot be stored. There is no protein equivalent of glycogen, so any amino acids beyond what the body can use at the time must be dealt with at once. They are broken down in the liver by a process called deamination.
In deamination, the nitrogen-containing amino group is removed from the amino acid. That part is converted into urea, a soluble waste which the body excretes. The remaining part of the molecule forms glucose, which can be used like any other glucose.
\[\text{Excess amino acid} \xrightarrow{\text{deamination in liver}} \text{Urea (excreted)} + \text{Glucose (used)}\]
What Happens To Fats
The fatty acids and glycerol absorbed by the villi travel mainly through the lymphatic system. Its largest vessel, the thoracic duct, empties into the large veins carrying blood to the heart, so these products join the blood only after this detour.
Some of this fat is used in the body cells to build certain compounds. The excess is deposited as a store, chiefly beneath the skin as subcutaneous fat, and in smaller quantity around certain internal organs.
12.11 The Liver
The liver is the largest gland in the body, weighing about 1.5 kg on average in an adult. It is a reddish-brown organ lying in the upper right side of the abdomen, just below the diaphragm.
You have already met it twice. It makes the bile that neutralises acid and emulsifies fat, and it receives, through the hepatic portal vein, everything the villi absorb into the blood. Its position explains the second of these: blood coming from the intestine reaches the liver before it reaches any other organ, so nothing absorbed from a meal enters general circulation until the liver has dealt with it.
Other Functions Of The Liver
Bile is only one item on a long list. The liver takes part in so many processes that it is sometimes described as the body’s chemical workshop.
- Control of blood sugar levels. It retains excess glucose received from the intestine, stores it as glycogen, and releases it again as glucose when needed.
- Control of amino acid levels. Excess amino acids are deaminated here, producing urea, which is excreted, and sugar, which is used.
- Synthesis of foetal red blood cells. The liver produces red blood cells in the embryo. In adults, red blood cells are produced in the bone marrow instead.
- Storage. It stores iron, copper and several vitamins, including vitamins A and D.
- Detoxification. It renders harmless a range of substances, including drugs and alcohol.
- Production of fibrinogen and prothrombin, two substances used in the clotting of blood.
- Production of heparin, a substance that prevents blood from clotting inside the vessels.
- Production of heat, released by the chemical activity going on in its cells.
- Regulation of blood volume, by acting as a temporary store for excess water.
- Excretion of toxic and metallic poisons.
- Destruction of dead red blood cells, whose breakdown yields the pigments that colour the bile.
Read items 6 and 7 together and an apparent contradiction appears: the same organ makes substances that start clotting and a substance that prevents it. There is no contradiction. Blood must clot at a wound and must not clot inside an intact vessel, and the liver supplies both halves of that balance.
12.12 Certain Experiments On Digestion
Everything said so far about enzymes can be shown on a laboratory bench. The three experiments below test three separate claims: that saliva digests starch, that pepsin needs acid, and that pepsin is destroyed by heat.
1. The Action Of Saliva On Starch
Prepare a starch suspension by heating 1 gram of starch in 100 mL of water in a beaker.
Put some of this suspension into two test tubes. Test the sample in the first for starch, using iodine solution, and the sample in the second for sugar. The first gives a positive starch test; the second shows no sugar. This tells you what you began with.
Now take a little of the starch suspension in a third test tube and add an equal amount of saliva, which you can collect beforehand by chewing some gum. Hold this tube in your hand for 15 minutes to keep it warm, or stand it in a beaker of water at about \(38^\circ\)C.
Then test samples from this third tube for starch and for sugar. The starch test is no longer positive, and sugar is now present.
The starch has not vanished; it has been changed. Ptyalin in the saliva has converted it into maltose, which is a sugar. Keeping the tube at about \(38^\circ\)C matters, because that is close to the optimum temperature of the enzyme.
Flowchart
FC 12.5 — Procedure Of The Saliva And Starch Experiment Heat 1 g starch in 100 mL water → divide suspension between three test tubes → test tube 1 with iodine (starch present) → test tube 2 for sugar (sugar absent) → add equal volume of saliva to test tube 3 → keep at about 38°C for 15 minutes → test tube 3 for starch (now absent) and for sugar (now present)
2. The Action Of Pepsin On Egg White
Egg white is almost pure protein, which makes it a convenient test substance.
Stir the white of an egg into about 500 mL of water. Boil the mixture and filter it through glass wool. The filtrate is a cloudy liquid, a suspension of tiny particles of protein. If the protein is digested into soluble peptides, the cloudiness will disappear, so the clearing of the liquid is your signal.
Take four test tubes, marked A, B, C and D, and put about 3 mL of the suspension into each. Add a few drops of 1 per cent pepsin solution and of hydrochloric acid as follows. Acids must be handled carefully.
- Test tube A — only pepsin.
- Test tube B — only hydrochloric acid.
- Test tube C — pepsin and hydrochloric acid.
- Test tube D — hydrochloric acid and boiled pepsin solution.
Stand all four in a beaker of water at about \(35^\circ\)C for nearly 10 minutes.
Only test tube C changes. Its cloudy suspension turns into a clear solution. The other three stay cloudy, as Fig. 12.12-1 shows.
Diagram
Fig. 12.12-1 — Testing The Conditions Pepsin Needs Row of five test tubes standing in a beaker of water at 35°C, each labelled with its contents and its result: A, egg-white suspension + pepsin, still cloudy; B, suspension + hydrochloric acid, still cloudy; C, suspension + pepsin + hydrochloric acid, now clear; D, suspension + hydrochloric acid + boiled pepsin, still cloudy; E, suspension + pepsin + sodium bicarbonate, still cloudy. Label beaker of water at 35°C and mark each tube A to E.
Setting up four tubes instead of one is the point of the experiment, not extra work. Each of A, B and D removes exactly one thing from the recipe in C, so when only C works, you can say which missing ingredient mattered. Comparing a test with such deliberately incomplete versions is what makes a result trustworthy.
Three conclusions follow.
- Pepsin can digest protein only when the medium is acidic, because pepsin alone in A did nothing.
- Acid alone has no effect on the protein, because B did nothing.
- Boiled pepsin fails to digest protein, because D did nothing even though acid was present. The enzyme has been destroyed by heat.
3. Effect Of Acidity And Alkalinity On An Enzyme Reaction
Conclusion 1 above says pepsin needs acid. A fifth tube tests the opposite condition directly.
In the same experiment, take a fifth test tube, E. Put the egg white suspension into it, add a similar quantity of pepsin, and then add a few drops of mild sodium bicarbonate solution to make the contents alkaline. Keep it at \(35^\circ\)C alongside the others.
No change is noticed in the cloudiness of the suspension. The protein has not been digested.
Pepsin, then, does not act in an alkaline medium. Taken with tube C, this shows that the enzyme is not simply switched on or off by having acid present; it works only within a particular range of pH, exactly as the sixth property of enzymes states.
12.13 Practical Work On Food Tests
Knowing what foods contain is one thing; showing it is another. Each of the tests below produces a colour change or another visible sign that a particular class of food is present. Two of them involve heating and strong chemicals, so work carefully and keep the mouth of a test tube pointed away from yourself and from others.
1. Test For Glucose
Add a little glucose to a blue-green chemical called Fehling’s solution in a test tube. Tilt the tube away from you and heat it over a flame until bubbles begin to appear, without letting it boil over.
The colour changes from blue-green to a deep brick red, and a precipitate appears. A small piece of apple or grape gives the same result, because both contain glucose.
Sucrose does not respond to this test as it is. It gives a positive result only after it has been boiled with dilute hydrochloric acid and then neutralised with sodium bicarbonate. The boiling with acid splits the sucrose into glucose and fructose, and it is those products that react.
2. Test For Starch
Shake a little starch powder in a test tube with some water and boil it to make a solution. Allow the solution to cool, then add a few drops of iodine solution.
The solution turns blue-black, which indicates the presence of starch. Foods such as potato and rice give a positive result.
Iodine solution is prepared by dissolving 1 g of iodine and 1 g of potassium iodide in 100 mL of distilled water. It should be diluted before use.
3. Test For Protein
Place a piece of hard-boiled egg white in a test tube. Add enough drops of concentrated nitric acid to immerse the sample. Concentrated acids burn skin and clothing, so this step needs great care.
Heat the tube gently, then cool it by holding it under a tap, and add ammonium hydroxide.
The colour changes in two stages: first a white precipitate turns yellow on heating, and then the yellow turns orange when the ammonium hydroxide is added.
4. Test For Fats And Oils
Rub a piece of groundnut, a walnut or a little butter on a plain sheet of paper.
The spot rubbed becomes translucent, that is, it lets light through without being clear. Hold the paper against the light to see it. Unlike a water mark, this spot does not dry out and disappear.
Two Further Tests
Food contains water and minerals as well, and both can be shown simply.
For water, take a piece of cobalt chloride paper, which is blue when dry and turns pink when it meets moisture. Touch the dry paper directly to the cut surface of a food such as potato or banana, and watch the colour change.
For minerals, place a piece of banana or potato in a crucible and heat it until it burns completely. Ash is left behind that will not burn any further. That ash is the mineral matter of the food.
All six tests are collected in Table 12.4.
12.14 Summary
- Food cannot be used as swallowed. It must be made soluble, and its large molecules must be cut into units small enough to cross the cell membranes of the gut wall.
- Digestion is the breakdown of naturally occurring foodstuffs into a diffusible form that can be absorbed through living membranes.
- Digestion is brought about by enzymes. An enzyme is a protein, is destroyed by heating, is specific to one substrate, always gives the same end product, only speeds up a reaction, can be used again and again, and works best at a particular pH and at an optimum temperature of about 35 to 40 degrees Celsius.
- The digestive system consists of the alimentary canal, a muscular tube about 9 metres long running from mouth to anus, and the digestive glands that pour juices into it.
- The lips close the mouth, allow sucking and sipping, help in speech and sense touch and heat. The tongue moves food, aids swallowing, carries the taste buds, cleans the teeth and helps in speech.
- The teeth break food into small pieces, increasing its surface area for enzymes to act on.
- Human teeth are heterodont, of four kinds: incisors for biting and cutting, canines for holding and tearing, premolars for grinding and crushing, and molars as the principal grinders. The last molar on each side of each jaw is the wisdom tooth.
- Teeth appear in two sets. The 20 temporary or deciduous teeth are complete by about 2 years and are replaced by the 32 permanent teeth by about 12 years, apart from the wisdom teeth.
- The dental formula gives the number of incisors, canines, premolars and molars for one half of each jaw. It is 2,1,0,2 over 2,1,0,2 for the milk set and 2,1,2,3 over 2,1,2,3 for the adult set.
- A tooth has a crown, a neck and a root. Enamel covers the crown and is the hardest substance in the body; dentine forms the bulk of the tooth; cementum fixes the root in its socket; pulp fills the central cavity and contains blood capillaries, lymph vessels and nerve fibres.
- Saliva is secreted by three pairs of glands — parotid, submandibular and sublingual. It lubricates, dissolves food for tasting, forms the bolus, cleans the mouth and contains ptyalin, which converts starch into maltose.
- During swallowing the soft palate closes the nasal passage and the epiglottis covers the glottis, leaving the oesophagus as the only open route.
- Peristalsis is the wave of constrictions produced by the circular muscles of the gut, which pushes food along. It occurs through all regions of the gut.
- The oesophagus conducts food from the throat to the stomach and secretes no enzyme.
- The stomach stores and churns food. Its cardiac and pyloric sphincters guard its two openings.
- Gastric juice contains hydrochloric acid, which kills germs and converts pepsinogen into pepsin, and pepsin, which digests proteins into peptides. Rennin, found in babies only, curdles the milk protein casein into paracasein.
- Food remains in the stomach for about 3 hours and becomes chyme, which the pylorus releases into the duodenum a little at a time.
- The small intestine, about 7 metres long, has three regions — duodenum, jejunum and ileum — and carries out both digestion and absorption.
- The lining of the ileum bears villi. Each villus has a one-cell-thick epithelium, blood capillaries and a lacteal. Great length, many villi, a thin epithelium and a narrow bore adapt the ileum for absorption.
- Bile from the liver contains no enzyme. It neutralises the acid chyme with sodium bicarbonate and emulsifies fats into tiny droplets.
- Pancreatic juice contains amylopsin, which digests starch to maltose; trypsin, activated from trypsinogen by enterokinase, which digests proteins and polypeptides; and steapsin, which digests emulsified fats.
- Intestinal juice contains erepsin, maltase, sucrase, lactase and lipase, and completes digestion.
- Amino acids and simple sugars are absorbed into the blood capillaries of the villi and reach the liver by the hepatic portal vein. Fatty acids and glycerol are absorbed into the lacteals and travel by the lymphatic system.
- The large intestine, about 1.5 metres long, consists of caecum with the vestigial vermiform appendix, colon and rectum. It secretes no enzyme, absorbs water, and the semi-solid faeces formed are expelled by defaecation.
- Assimilation is the conversion of absorbed digested food into body material. The liver stores excess glucose as glycogen by glycogenesis and releases it by glycogenolysis; excess amino acids are deaminated into urea and glucose; excess fat is stored below the skin.
- The liver is the largest gland in the body. Besides making bile, it controls blood sugar and amino acid levels, stores iron, copper and vitamins A and D, detoxifies substances, makes fibrinogen, prothrombin and heparin, produces heat and destroys worn-out red blood cells.
- Experiments show that saliva converts starch into sugar, that pepsin digests protein only in an acidic medium and not in an alkaline one, and that boiled pepsin does not work because heat destroys the enzyme.
- Food constituents are identified by tests: Fehling’s solution for glucose, iodine solution for starch, nitric acid followed by ammonium hydroxide for protein, a translucent spot on paper for fats, cobalt chloride paper for water and the ash left on burning for minerals.
Flowchart
FC 12.6 — The Whole Journey Of Food Mouth: chewing, saliva, starch → maltose → oesophagus: peristalsis only → stomach: churning, HCl and pepsin, proteins → peptides, chyme formed → duodenum: bile neutralises and emulsifies, pancreatic juice acts on starch, protein and fat → ileum: intestinal juice completes digestion → villi absorb: sugars and amino acids → hepatic portal vein → liver; fatty acids and glycerol → lacteals → lymphatic system → large intestine: water absorbed, faeces formed → rectum → defaecation
12.15 Key Terms
- Alimentary canal — the muscular tube running from the mouth to the anus along which food travels.
- Amylopsin — the pancreatic enzyme that digests starch into maltose.
- Assimilation — the conversion of absorbed digested food into body material.
- Bile — the yellowish-green fluid made by the liver, which neutralises acid chyme and emulsifies fats.
- Bilirubin — a bile pigment formed from the breakdown of worn-out red blood cells.
- Biliverdin — a green bile pigment formed from the breakdown of worn-out red blood cells.
- Bolus — the soft rounded ball of chewed food mixed with saliva that is swallowed as one mass.
- Caecum — the small blind pouch at the junction of the small and large intestines.
- Canines — the conical, pointed teeth beside the incisors, used for holding and tearing food.
- Cardiac sphincter — the ring of muscle at the entrance of the stomach that prevents backflow into the oesophagus.
- Catalyst — a substance that changes the rate of a reaction without being used up in it.
- Cementum — the bone-like layer covering the root of a tooth and fixing it in its socket.
- Chyme — the pulp-like semi-liquid form into which food is reduced in the stomach.
- Colon — the broad main part of the large intestine, where most water is absorbed.
- Common bile duct — the duct formed by the joining of the hepatic and cystic ducts, which carries bile into the duodenum.
- Crown — the part of a tooth exposed above the gum.
- Deamination — the removal of the nitrogen-containing amino group from excess amino acids in the liver, forming urea and glucose.
- Deciduous teeth — the first set of 20 teeth, shed and replaced by the permanent set; also called temporary teeth.
- Defaecation — the expulsion of the undigested remains of food from the alimentary canal.
- Dental formula — a short form giving the number of incisors, canines, premolars and molars in one half of each jaw.
- Dentine — the material forming the bulk of a tooth, harder than bone but softer than enamel.
- Digestion — the breakdown of foodstuffs into a soluble, diffusible form that can be absorbed through living membranes.
- Digestive glands — the glands that secrete digestive juices into the alimentary canal.
- Duodenum — the short first region of the small intestine, which receives bile and pancreatic juice.
- Emulsification — the breaking of fat into tiny droplets by bile salts, which increases the surface available to enzymes.
- Enamel — the hardest substance in the body, covering the crown of a tooth.
- Enterokinase — the enzyme from the duodenal lining that converts trypsinogen into active trypsin.
- Enzyme — a protein made by living cells that speeds up a chemical reaction without being used up.
- Epiglottis — the flap that covers the glottis during swallowing.
- Epithelium — the lining layer of cells, one cell thick, covering a villus.
- Erepsin — the mixture of peptidases in intestinal juice that converts peptides into amino acids.
- Faeces — the semi-solid waste formed in the large intestine after water has been absorbed.
- Fehling’s solution — the blue-green reagent that turns brick red on heating with glucose.
- Gastric juice — the acidic juice of the stomach, containing hydrochloric acid and pepsin.
- Gland — a structure whose cells make a useful fluid and release it where it is needed.
- Glottis — the opening at the top of the windpipe.
- Glycogen — the insoluble carbohydrate in which the liver stores excess glucose.
- Glycogenesis — the conversion of excess glucose into glycogen in the liver.
- Glycogenolysis — the reconversion of stored glycogen into glucose.
- Hepatic portal vein — the vein carrying absorbed sugars and amino acids from the intestine to the liver.
- Heterodont — having teeth of different kinds in the same jaw, as in mammals.
- Homodont — having teeth all of the same kind, as in lizards and frogs.
- Ileum — the last and longest region of the small intestine, where absorption mainly occurs.
- Incisors — the broad, chisel-edged front teeth used for biting and cutting.
- Intestinal juice — the juice secreted by glands in the wall of the intestine, which completes digestion.
- Iodine solution — the reagent that turns blue-black in the presence of starch.
- Jejunum — the middle region of the small intestine, about 2 metres long.
- Lactase — the intestinal enzyme that splits lactose into glucose and galactose.
- Lacteal — the lymph vessel inside a villus, which absorbs fatty acids and glycerol.
- Lipase — an enzyme that digests fats into fatty acids and glycerol.
- Maltase — the intestinal enzyme that splits maltose into glucose.
- Molars — the last three teeth on each side of each jaw, the principal grinders and crushers.
- Mucus — the slimy fluid secreted by the gut lining, which lubricates food and protects the wall.
- Neck — the slight constriction of a tooth between the crown and the root.
- Oesophagus — the tube that conducts food from the throat to the stomach.
- Optimum temperature — the narrow temperature range, usually 35 to 40 degrees Celsius, at which an enzyme works best.
- Oral cavity — the mouth, where food is chewed and mixed with saliva.
- Pancreas — the whitish gland behind the stomach that secretes pancreatic juice into the duodenum.
- Pepsin — the gastric enzyme that digests proteins into peptides in an acidic medium.
- Pepsinogen — the inactive form of pepsin, converted to pepsin by hydrochloric acid.
- Peristalsis — the wave of constrictions of the circular muscles of the gut that pushes food along.
- Permanent teeth — the second and final set of 32 teeth that replaces the milk set.
- Pharynx — the space at the back of the mouth where the food and air passages meet.
- Premolars — the two bicuspid teeth on each side of each jaw, used for grinding and crushing.
- Ptyalin — another name for salivary amylase, the enzyme of saliva that converts starch into maltose.
- Pulp — the soft connective tissue in the central cavity of a tooth, containing capillaries, lymph vessels and nerves.
- Pylorus — the guarded opening of the stomach into the duodenum.
- Rectum — the last part of the large intestine, which stores faeces before defaecation.
- Rennin — the gastric enzyme of babies that curdles the milk protein casein into paracasein.
- Root — the part of a tooth embedded in a socket of the jaw bone.
- Roughage — indigestible plant fibre that passes through the alimentary canal unchanged.
- Saliva — the slightly acidic fluid of the mouth, containing water, salts, mucus and ptyalin.
- Salivary amylase — the starch-digesting enzyme of saliva.
- Salivary glands — the three pairs of glands, parotid, submandibular and sublingual, that secrete saliva.
- Soft palate — the soft back part of the roof of the mouth, which closes the nasal passage during swallowing.
- Sphincter — a ring of muscle around an opening, which keeps it closed until it relaxes.
- Steapsin — the pancreatic enzyme that splits emulsified fats into fatty acids and glycerol.
- Substrate — the particular substance on which an enzyme acts.
- Sucrase — the intestinal enzyme, also called invertase, that splits sucrose into glucose and fructose.
- Temporary teeth — the milk teeth of the first set, shed during childhood.
- Tongue — the muscular organ of the mouth that moves food, carries taste buds and aids swallowing and speech.
- Trypsin — the pancreatic enzyme that digests proteins and polypeptides into smaller peptides and amino acids.
- Urea — the soluble nitrogenous waste formed in the liver by deamination and removed by excretion.
- Vermiform appendix — the narrow worm-shaped tube projecting from the caecum.
- Vestigial — reduced during evolution and no longer performing its original function.
- Villus — a tiny finger-like projection of the intestinal lining that absorbs digested food; plural, villi.
- Wisdom tooth — the last molar of each side of each jaw, appearing at about 17 to 20 years of age.
12.16 Points To Ponder
Pepsin works only in acid and trypsin only in alkaline conditions, yet both digest protein. What advantage might a body gain from using two enzymes with opposite requirements instead of one that works everywhere?
The stomach is lined with living cells made of protein, and it fills itself with an enzyme that digests protein. Apart from mucus and the release of pepsin in an inactive form, what else would you want to know before you were satisfied that the lining is safe?
Suppose an animal ate only food that was already in the form of small soluble molecules. Which parts of the human alimentary canal would such an animal have little use for, and which would it still need?
The appendix is described as vestigial in humans, while the same region is large in many plant-eating mammals. What does that difference suggest about the diet of our distant ancestors, and how far can such a conclusion be trusted?
Blood leaving the intestine goes to the liver before it goes anywhere else. What might go wrong if it went straight to the brain and the muscles instead?
Chewing increases the surface of food, and bile increases the surface of fat. Where else in this chapter does the body solve a problem by increasing surface area, and what does that repetition tell you about how living systems are built?
12.17 Objective Questions
Multiple Choice Questions
- The enzyme that acts on starch in the mouth is: (1 mark)
- pepsin
- ptyalin
- steapsin
- erepsin
- The number of premolars in one half of an adult human lower jaw is: (1 mark)
- 1
- 2
- 3
- 4
- Bile helps in digestion although it contains no enzyme, because it: (1 mark)
- splits proteins into peptides
- converts maltose into glucose
- emulsifies fats and makes the contents alkaline
- absorbs fatty acids into the lacteals
- Fatty acids and glycerol absorbed by a villus are carried away by the: (1 mark)
- hepatic portal vein
- blood capillaries
- lacteal
- pancreatic duct
- The conversion of excess glucose into glycogen in the liver is called: (1 mark)
- deamination
- glycogenolysis
- assimilation
- glycogenesis
Assertion-Reason
- In each of the following, choose:
- both assertion and reason are true, and the reason is the correct explanation of the assertion
- both assertion and reason are true, but the reason is not the correct explanation of the assertion
- the assertion is true but the reason is false
- the assertion is false but the reason is true
- Assertion: Boiled saliva does not convert starch into maltose. Reason: An enzyme is a protein and is destroyed by heating. (1 mark)
- Assertion: The ileum is narrow and very long. Reason: A narrow, long tube slows the movement of food and provides a large surface for absorption. (1 mark)
Match The Following
- Match each item in Column A with the correct item in Column B. (5 marks)
| Column A | Column B |
|---|---|
| a. Enterokinase | i. Curdles milk protein |
| b. Rennin | ii. Splits sucrose |
| c. Sucrase | iii. Activates trypsinogen |
| d. Cementum | iv. Emulsifies fat |
| e. Bile salts | v. Fixes the root of a tooth in its socket |
Fill In The Blanks
The wave of constrictions that pushes food along the gut is called …………… . (1 mark)
Excess amino acids are broken down in the liver by a process called ……………, producing urea. (1 mark)
The opening of the stomach into the duodenum is called the …………… . (1 mark)
True Or False
The oesophagus secretes a digestive enzyme. (1 mark)
Pepsin acts best in an alkaline medium. (1 mark)
The large intestine absorbs water but secretes no enzyme. (1 mark)
Name The Following
The hardest substance in the human body. (1 mark)
The lymph vessel present inside an intestinal villus. (1 mark)
The pancreatic enzyme that acts on emulsified fats. (1 mark)
The flap that covers the glottis during swallowing. (1 mark)
The pouch from which the vermiform appendix arises. (1 mark)
Odd One Out
Maltase, sucrase, lactase, trypsin. Give a reason for your choice. (2 marks)
Parotid gland, sublingual gland, gall bladder, submandibular gland. Give a reason for your choice. (2 marks)
12.18 Check Your Concepts
Define digestion. (2 marks)
State the two changes food must undergo before it can be absorbed. (2 marks)
Name the three pairs of salivary glands and state where each is situated. (3 marks)
Give a reason for each of the following. (3 marks)
- Boiled rice begins to taste sweet if chewed for a while.
- Pepsin is secreted in an inactive form.
- The wisdom tooth is so named.
Differentiate between the following pairs, stating the basis of difference in each case. (3 marks)
- Temporary teeth and permanent teeth.
- Heterodont and homodont dentition.
- Glycogenesis and glycogenolysis.
Write the dental formula of an adult human being and explain what the numbers stand for. (3 marks)
Explain, with an example, what is meant by saying that an enzyme is specific. (3 marks)
Why does the pylorus open only intermittently instead of staying open? (3 marks)
Describe the changes that a mouthful of bread undergoes in the mouth and in the stomach. (4 marks)
List four ways in which the ileum is adapted for the absorption of digested food. (4 marks)
State the composition of gastric juice and give the function of each of its components. (4 marks)
What would happen if the bile duct of a person were blocked? Explain your answer with reference to the digestion of fats. (4 marks)
Describe the experiment that shows that pepsin can digest protein only in an acidic medium. State the conclusions drawn. (5 marks)
Trace the path of a molecule of starch from the mouth until its final product reaches the liver, naming the enzymes and the regions involved. (5 marks)
Name any five functions of the liver other than the production of bile, and state briefly what each achieves. (5 marks)
12.19 Figure-Based Questions
- Study Fig. 12.3-3, which shows a vertical section of a tooth. (4 marks)
- Name the part that covers the crown.
- Name the soft tissue in the central cavity and state one function of it.
- Which labelled part fixes the root in its socket?
- State one difference between the tooth marked A and the tooth marked B.
- Refer to Fig. 12.7-1, the structure of an intestinal villus. (4 marks)
- Name the vessel that carries away fatty acids and glycerol.
- How many cells thick is the outer layer, and why does this matter?
- Name the vein that finally receives the sugars absorbed here.
- Redraw the villus and label any four parts.
- Fig. 12.4-1 shows the head and neck during breathing and during swallowing. (3 marks)
- Name the structure that closes the glottis in B.
- Name the passage that the soft palate closes during swallowing.
- State what would happen if the structure named in (a) failed to act.
- Fig. 12.7-2 shows the bile duct and the pancreatic duct opening into the duodenum. (3 marks)
- Name the organ that produces bile and the organ that stores it.
- Name the duct formed by the union of the hepatic and cystic ducts.
- Name one enzyme carried by the pancreatic duct and state the food it acts on.
- Draw a labelled diagram of the human alimentary canal with its associated glands, labelling the oesophagus, stomach, liver, pancreas, small intestine, large intestine and anus. (5 marks)