1. Digestion – the complete process

2. Energy from food: The 7 food groups represent large chemicals. fats
proteins
minerals
vitamins
carbohydrates
water
fibre
These chemicals are often chains of smaller, more useful chemicals, joined together.

3. Energy molecules in food
One example is carbohydrates.
Carbohydrates are made of long chains of identical small sugar molecules.
carbohydrate
sugar molecule

4. Small sugar molecules The small sugar molecules are very useful.
The body has to break these large food molecules up into single or small chain sugar molecules. These are used to make…
ENERGY

5. How can we release energy from food?
Problem One
- releasing smaller sugars
Physical means like slicing and cleaving food does not break down the long chain molecules and release the sugars.
This is because we can’t release sugars from carbohydrates by physically breaking them up.

6. Chemical breakdown The chain of sugars is held together by…
chemical bonds
Chemical bonds require a chemical technique if they are to be broken.

7. Problem with food size Problem 2
- The food we start with is often large in size.
Being large, the food tends to be unable to dissolve.
We say it is large and insoluble.

8. Food solubility
The food needs to be soluble so that it can dissolve in the blood and thus, be transported around the body.
The smaller the food, the more likely they will dissolve.
So the digestive system has to cope with both these problems.
Blood vessel
Digestion
Soluble product

9. The digestive system:
The digestive system, being an organ system, is made of a group of organs all working together.
Each organ has a particular function and only by working together will they get the job done.

10. External digestive system
The only visible parts of the digestive system are the entry and exit points.
Anus
Mouth
The sound of a rumbling stomach and the fact that food looks very different when it leaves, compared to when it enters mean that the body must be doing something to the food during its journey.
What happens to the food in our bodies?

11. What happens to the food in our body?
It is digested.
This means it is broken down.
This digestion happens in 2 ways.
As we know all food has a physical shape and is made of chemicals. These chemicals are held together by chemical bonds.

12. Chemical and physical digestion
Our digestive system uses both:
chemical digestion
physical digestion
As we move through the digestive system, we will see one or both of these methods in action at any one time.
physical
chemical
The shape of the food must be physically changed so that it can fit through the small diameter of the digestive system.
This allows useful chemicals to be released and dissolve in the blood. To be broken down chemically, the bonds must be broken.

13. In we go!
Digestion is the chemical and physical breakdown of large insoluble molecules into small soluble molecules.
Let’s take a close look at how this happens…
Open wide

14. The digestive tract
All food enters our digestive system through the mouth and waste material leaves through the anus.
The digestive system is really one long tube with an opening at each end.
Stretched out it is a 9m tube!
mouth
anus

15. Our guts
But how does a 9m tube fit into a space, which is less than a metre long?
It is extremely folded!
In addition, the tube passes through organs on its route from the mouth to the anus.

16. Digestive system diagram

17. Physical digestion
The mouth is where digestion begins. Here we find both chemical and physical methods of digestion.
We will consider physical digestion first.
If you look in the mirror and smile, you immediately notice your teeth.
You will also realise that your teeth are different shapes.
You have 4 basic types of teeth; each type is designed for a different role.

18. Teeth: premolar molar canine incisor canine 2nd premolar
Each is designed to do a different job.
premolar
molar
canine
incisor
canine
2nd premolar
latent incisor
1st molar
3rd molar
2nd molar
1st premolar
central incisor

19. Diagram of a tooth

20. Tooth size and shape
The shape and size of each tooth is related to the function they have in digesting food.
If we look at the teeth of other animals many of them too have these 4 types of teeth.
However, the number of each type, their size and their shape differ between species.
This is because other organisms have different diets.

21. Mammalian tooth types Canine
Sharp pointed teeth, which are used to bite and tear food.
Incisors
Small rectangular shaped teeth, which are found between the canines. They are used for cutting food.
Premolars
Found behind the canines and are used to grind soft food.
Molars
Found behind the premolars and are used to grind hard food.

22. Action in the mouth
Together, these teeth can break up most foods that we put into our mouths. The mechanical act of chewing food is part of physical digestion.
Once the teeth have digested the food, it may be small enough to be swallowed. However, some food can be sharp and it would be uncomfortable to swallow. The food also needs chemically breaking down.
Therefore, the mouth produces a substance that solves both of these problems at the same time.

23. Saliva As mentioned, the saliva has two jobs.
These glands (a special type of tissue) produce saliva, a sticky liquid.
As mentioned, the saliva has two jobs.
Being a liquid, it softens the food and allows the digested food to be rolled into a ball just before it is swallowed.
It also contains a chemical known as an enzyme.

24. Enzyme properties: What is an enzyme?
Enzymes are chemicals, which act to speed up chemical reactions. They are produced from glandular tissue, which is found all over the body.
In order to understand how an enzyme works, you have to think of it as having a particular shape.
Somewhere on the surface of the enzyme is an important region known as the active site.

25. What’s so special about enzymes?
We will use the shape below to represent on particular enzyme.
enzyme
In order for an enzyme to be able to speed up or catalyse a reaction, it must attach to the chemicals that are reacting. It does so using its active site.
Active site

26. Specificity of enzymes
The red areas on these two reacting chemicals represents the areas where the active site of the enzyme will attach.
The enzyme will attach to both at the same time. +
Enzymes are very specific.
Enzymes can only speed up certain reactions.
If the shape of the reacting chemicals does not match the shape of the active site, the enzyme will not be able to work.

27. The environment matters
Therefore, enzymes are specific to certain reactions.
Enzymes are also very particular about the environment that they work in.
To understand this, think of how you do homework.
You probably have a certain place to work, or you work at a certain time, you may like listening to music whilst you work or else you can only work if it is completely silent.

28. Enzymes and pH Different enzymes work best in different conditions.
If the condition is wrong, their active site can change shape. Say one particular enzyme works best in acidic conditions (pH less than 7). If the pH rises and the conditions become alkaline, the enzyme changes shape and stops working. It can no longer fit with the reacting particles of the chemical reaction.
pH 10
pH< 7

29. Food groups and enzymes
The bulk of the food that enters the digestive system is from the three main food groups:
proteins
carbohydrates
fats
Therefore, it is not surprising that the digestive system has enzyme-producing glands that relate to these three types of food.
Remember that the shape of the chemicals within the different food groups will be different. Therefore the shape of the enzymes that digest these chemicals will also be different.

30. Digestive enzymes: sites of enzyme attack sugar
Carbohydrates are chains of identical sugar molecules. The enzyme that digests carbohydrates must be able to break the chemical bonds between the individual sugar molecules.
sugar
sites of enzyme attack
The product of the chemical breakdown of carbohydrates is sugar. The sugar is known as glucose.
Enzymes that digests carbohydrates are known as carbohydrases.

31. Enzyme driven reaction
The digestion of carbohydrates can be represented
by the following equation.
carbohydrase
Carbohydrates
Sugars
carbohydrase

32. Proteins and amino acids
As with carbohydrates, proteins are made of chains of chemicals. However, instead of the chain containing identical molecules, in protein these molecules are different.
Protein is made up of chains of amino acids. There are over 20 different kinds of amino acid.
Imagine a bead necklace made up of over 20 different kinds of bead.
amino acids
sites of enzyme attack

33. Enzymes for digesting proteins
sites of enzyme attack
The enzymes that digest proteins must be able to break the chemical bonds between the different amino acids.
Enzymes that digest protein are known as
proteases.
amino acids
The digestion of proteins can be represented by the following equation.
protease
Protein Amino Acids

34. Fat in our food fatty acids site of enzyme attack glycerol phosphate
Fats are made up of a molecule of glycerol phosphate attached to three fatty acid molecules.
fatty acids
glycerol phosphate
site of enzyme attack
The enzymes that digest fats must be able to break the chemical bonds between the glycerol phosphates and the fatty acids.
Fats are also known as lipids.

35. Enzymes for digesting fat
The enzymes that digest fats must be able to break the chemical bonds between the glycerol phosphates and the fatty acids.
Fats are also known as lipids.
Enzymes that digest fat (lipid) are known as
lipases.
Fat digestion can be represented by the following equation:
lipase
Fat Fatty Acids + Glycerol Phosphate

36. Carbohydrase in saliva
Of these three enzymes, the only one that is released within the mouth is carbohydrase. This is partly because the conditions within the mouth are suitable for carbohydrase action.
It works best within an alkaline (pH > 7) environment.
The carbohydrase in saliva in combination with other digestive carbohydrases added later from the pancreas and the small intestine complete carbohydrate digestion.

37. Digestive action of the mouth - summary
Carbohydrates
Fats
Proteins
physical digestion
chemical digestion
The food could now pass down either the trachea (windpipe) or the gullet/oesophagus.
chemically and physically digested
physically digested
sugars

38. Digestion – Part Two Mouth to gullet Gastric processes
Contents
Digestion – Part Two
Mouth to gullet
Gastric processes
The small intestine
Lipid digestion
Absorption
Summary

39. Digestive action of the mouth – reminder
Carbohydrates
Fats
Proteins
physical digestion
chemical digestion
The food could now pass down either the trachea (windpipe) or the gullet/oesophagus.
chemically and physically digested
physically digested
sugars

40. Mouth to the gullet:
The semi-digested food has been formed into a ball by the tongue and is now swallowed.
The ball firstly moves to the pharynx, the region at the back of the mouth where there is a junction between two pipes.
It needs to pass down through the gullet. To ensure that it does not fall into the trachea and thus block our breathing, a small flap moves to cover the tube.

41. Protecting the windpipe

42. How it gets down the gullet

43. Epiglottis The flap is known as the epiglottis.
If ever food accidentally got into the trachea, we would choke and try to cough it back out.
How do we swallow?
Once the ball of food has passed through the top of the gullet, it is forced downwards by waves of muscular contraction.

44. Keeping it down
The swallowing process finishes with a muscle known as a sphincter relaxing and the food passes into the stomach.
The sphincter muscle found at the entry
and exits points of the stomach acts
rather like a drawstring on a bag.
It allows the stomach to shut off both entry and exit points and hold food inside it.

45. The basics on the stomach
The stomach is basically a muscular bag, filled with hydrochloric acid (HCl).
cross section of stomach
food enters from the gullet
muscle tissue
glandular tissue makes:
hydrochloric acid, mucus and protease enzyme
digested food leaves
liquids mix with the food

46. Gastric processes: Firstly the
When the food enters the stomach. The sphincter contracts behind it.
The food is then subjected to a coordinated attack.
Firstly the
the hydrochloric acid
…attacks any microbes (bacteria) that may have been swallowed accidentally when the food was eaten.
HCl
Microbes
Food bolus

47. Protein digestion protein amino acids
Secondly, the hydrochloric acid provides the perfect conditions for protease enzyme.
These enzymes begin digesting the proteins in the swallowed food.
These proteins are broken down to release the amino acids.
protein
protease
amino acids
Protease enzymes work best under acidic conditions (pH < 7).

48. Mucus and muscles
Thirdly, the muscular tissue of the stomach has the ability to contract and relax and in doing so, physically grinds the food inside it.
Mucus is produced to protect the lining of the stomach from the acid. If the mucus were not present, the hydrochloric acid would actually digest the tissue that had made it!

49. Making gastric juice muscle and glandular tissue presence of HCl
layer of mucus
wall of stomach is protected
The hydrochloric acid, mucus, food and enzyme solution is given the name - gastric juice.

50. The stomach’s digestive action - summary
protein
fat
sugar
carbohydrate
physical digestion
chemical digestion
muscular tissue of stomach
protease enzyme and hydrochloric acid
sugars
fats
carbohydrates
amino acids
proteins

51. Small soluble molecules
and after the stomach…
By this stage many large insoluble molecules are slowly being digested to produce the small soluble molecules that can easily be absorbed into the blood system.
Small soluble molecules
Glucose Amino Acids Glycerol phosphate
It is also clear that the fat has yet to be chemically digested.
This happens in the next section of the digestive system.

52. Digestion – Part Two Mouth to gullet Gastric processes
Contents
Digestion – Part Two
Mouth to gullet
Gastric processes
The small intestine
Lipid digestion
Absorption
Summary

53. The small intestine:
The name of this organ is deceiving. Although it is called small, it can stretch up to 6 metres in length. In order to fit into the body, it is heavily folded.
When the gastric juices leave the stomach, they pass into the small intestine.
X-ray of the small intestine

54. Dealing with the acid A nasty problem!
The gastric juices are very acidic. There could be a danger of the lining of the small intestine being digested.
Luckily two substances are produced to stop this happening. The first is mucus and we have seen how this works. The second is bile.
Bile is made in the liver. It is a yellow/green liquid that is naturally alkaline (pH > 7).

55. Why neutralise?
This means that when bile and the gastric juices meet, the result is neutralisation of the acid.
pH >7
Neutralisation
pH <7 +
This is essential for lipase (the enzyme that digests fat) to function properly.
The active site of the lipase enzymes is only effective in conditions above pH 7.

56. Parts of the small intestine

57. How is the fat digested? Let’s consider the process of fat digestion.
This section of the digestive system exemplifies the true idea of an organ system as a group of organs working together for a common function.
The influential organs in the digestion of fat are the:
Each organ plays its part in turning fat into fatty acids and glycerol phosphates.
liver
gall bladder
pancreas
small intestine

58. fat within the gastric juice
A whole lot of bile
liver produces bile
fat within the gastric juice
bile is released from the gall bladder and passes down through the bile duct
The bile and the fat meet within the small intestine.
The bile emulsifies the fat. This basically means the fat is physically broken into smaller pieces.
emulsification by bile
Notice that the fat has not been chemically digested, only physically. The result is a greater surface area over which the enzyme, lipase, can attack the fat.

59. Influx of enzymes small intestine
The pancreas and the small intestine now release enzymes.
Both organs produce all three enzyme types:
pancreas
small intestine
carbohydrases
proteases
lipases

60. Lipid digestion: lipase
If we just consider fat, the lipase begins to break its chemical bonds:
fat
lipase
fatty acids
glycerol phosphate

61. Digestion in small intestine - summary
chemical digestion
physical digestion
sugar
fat
carbohydrates
amino acids
protein
bile emulsifies the fat muscular action of the small intestine
lipase enzyme
protease enzyme
carbohydrase enzyme
sugar
amino acids
fatty acids
glycerol phosphate

62. Digestive processes - summary
 The three processes of chemical digestion can be summarized as follows.
Large insoluble food
Enzyme
Small soluble food
Carbohydrates
carbohydrase
sugars
Proteins
protease
amino acids
Fats
lipase
glycerol phosphates
and fatty acids

63. Design of the small intestine
There is a further advantage for the small intestine in being a narrow tube.
With the enzymes being produced in the lining of the tube, it is essential that the food be forced to mix with them to ensure that there is efficient digestion before the food passes through the organ.
If the tube is tight, the food is forced against the sides of the tube and thus, mixes directly with the enzymes.
enzymes come into contact with food.

64. What about the waste?
Now that the large insoluble molecules have been digested down into the small soluble molecules, the body must separate them from the waste food that has not been digested.
No food is 100% useful and so there will always be some waste that needs excreting.
In order to understand how the body carries out this selection procedure, we need to take a closer look at the lining of the small intestine.

65. Inside the small intestine
At first glance, the small intestine appears to have a flat surface.
However, if we consider what it has to do, we will understand why this idea cannot be correct.

66. Digestion – Part Two Mouth to gullet Gastric processes
Contents
Digestion – Part Two
Mouth to gullet
Gastric processes
The small intestine
Lipid digestion
Absorption
Summary

67. Absorption:
The small intestine is the site of absorption of useful molecules of digested food.
These molecules must pass across the lining of the small intestine and enter the blood stream.
Their destinations are the cells of the body.
Our body cells constantly need these chemicals and therefore the absorption process must be very efficient to keep up with the high demand.

68. Surface area and absorption
If absorption were to occur across a flat lining, then not enough molecules would pass across the lining in the desired time.
The only way to improve this situation would be to create a larger surface area over which absorption could occur.
That is exactly what is present in the small intestine.
Its lining is in fact a highly folded lining, which creates an enormous surface area in a small space.

69. Have a look inside a human’s intestines
The small intestine

70. Maximising the surface area
The result of this folding means that the surface area of the lining of the small intestine is enormous!
Direction of Food
Epithelium of small intestine
This increases the number of places where small soluble food molecules can pass across and move into the blood.

71. Where is the blood?
The blood is found in minute small vessels known as capillaries. The capillaries are found protruding into the villi.
The blood approaches the villus, picks up the absorbed food molecules and then leaves.

72. Digestive products in the blood
Remember that the small intestine lining is made up of thousands of villi.
Here is a summary of the process of absorption. A B S
sugars B L O D O R
amino acids P
fatty acids and glycerol phosphates T I O
Any indigestible food will leave the small intestine without having been absorbed. N

73. The large intestine
Let us now move on to the next organ in the system, the large intestine or colon.
As the food enters this organ, all that is left is waste material and water. The body will want to leave the waste material within the digestive system but the water is valuable.

74. Water regulation
This water will need to be retained by the body in order to prevent dehydration.
Remember that all the liquids you drink provide the largest source of water for the body.
The blood reabsorbs the excess water that is mixed with the waste food.
Again, if something is going to be absorbed, it must have somewhere to go.

75. Getting back excess water
The blood reabsorbs the excess water that is mixed with the waste food.
LARGE
INTESTINE B L O D

76. Expelling the waste rectum
Waste material then passes into a storage organ called the rectum.
rectum
This waste material mainly consists of indigestible food. It makes up the bulk of the faeces (solid excrement) that will be excreted.
In order for the waste material to be removed, another sphincter muscle must relax. This opens the anus and the faeces can pass out of the system.

77. Important body tissues in digestion
We have now finished our journey through the digestive system. We have seen the chemical and physical digestion of large & insoluble into small & soluble food.
It is important to remember that the digestive system relies heavily on the presence of two important types of body tissue.
This is responsible for the production of the digestive enzymes.
1. Glandular tissue
Through constant contraction and relaxation, the food is kept moving through the system, from the mouth to the anus.
2. Muscular tissue

78. Chew it over
Chew it all over