Digestion – the complete process

fats

proteins

minerals

vitamins carbohydrates

water fibre

The 7 food groups represent large chemicals.

These chemicals are often chains of smaller, more useful chemicals, joined together.

Energy from food:

One example is carbohydrates.

Carbohydrates are made of long chains of identical small sugar molecules.

Energy molecules in food

carbohydrate

sugar molecule

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

Small sugar molecules

This is because we can’t release sugars from carbohydrates by physically breaking them up.

Problem One

Physical means like slicing and cleaving food does not break down the long chain molecules and release the sugars.

How can we release energy from food?

- releasing smaller sugars

The chain of sugars is held together by…

Chemical breakdown

Chemical bonds require a chemical technique if they are to be broken.

chemical bonds

- The food we start with is often large in size. Problem 2

Problem with food size

Being large, the food tends to be unable to dissolve.

We say it is large and insoluble.

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

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.

The digestive system:

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.

The only visible parts of the digestive system are the entry and exit points.

AnusMouth

External digestive system

What happens to the food in our bodies?

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.

What happens to the food in our body?

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.

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.

physical chemical

Chemical and physical digestion

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…

In we go!

Open wide

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!

anus

mouth

The digestive tract

In addition, the tube passes through organs on its route from the mouth to the anus.

But how does a 9m tube fit into a space, which is less than a metre long?

It is extremely folded!

Our guts

Digestive system diagram

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.

Physical digestion

Each is designed to do a different job.

premolar molar

canine

2nd premolar

latent incisor

1st molar

3rd molar

2nd molar

1st premolar

central incisor

Teeth:

incisorcanine

Diagram of a tooth

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.

Tooth size and shape

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.

Molars

Found behind the premolars and are used to grind hard food.

Premolars

Found behind the canines and are used to grind soft food.

Mammalian tooth types

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.

Action in the mouth

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.

Saliva

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.

What is an enzyme?

Enzyme properties:

Somewhere on the surface of the enzyme is an important region known as the active site.

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

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.

Specificity of enzymes

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.

The environment matters

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 10pH< 7

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.

The bulk of the food that enters the digestive system is from the three main food groups:

proteins carbohydrates fats

Food groups and enzymes

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.

The product of the chemical breakdown of carbohydrates is sugar. The sugar is known as glucose.

Enzymes that digests carbohydrates are known as carbohydrases.

Digestive enzymes:

sugar

sites of enzyme attack

The digestion of carbohydrates can be represented by the following equation.

SugarsCarbohydratescarbohydrase

Enzyme driven reaction

carbohydrase

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.

Proteins and amino acids

Imagine a bead necklace made up of over 20 different kinds of bead.

amino acids

sites of enzyme attack

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.

proteaseProtein Amino Acids

Fat in our food

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.

Fats are made up of a molecule of glycerol phosphate attached to three fatty acid molecules.

fatty acids

glycerol phosphate

Fat digestion can be represented by the following equation:

lipaseFat Fatty Acids + Glycerol Phosphate

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.

Enzymes for digesting fat

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.

Carbohydrase in saliva

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.

The food could now pass down either the trachea (windpipe) or the gullet/oesophagus.

sugars

chemically and physically digested

physically digested

physical digestion

chemical digestion

CarbohydratesFatsProteins

Digestive action of the mouth - summary

Contents

Digestion – Part Two

Lipid digestion

Absorption

The small intestine

Mouth to gullet

Gastric processes

Summary

Digestive action of the mouth – reminder

The food could now pass down either the trachea (windpipe) or the gullet/oesophagus.

sugars

chemically and physically digested

physically digested

physical digestion

chemical digestion

CarbohydratesFatsProteins

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.

Mouth to the gullet:

Protecting the windpipe

How it gets down the gullet

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.

Epiglottis

The swallowing process finishes with a muscle known as a sphincter relaxing and the food passes into the stomach.

It allows the stomach to shut off both entry and exit points and hold food inside it.

The sphincter muscle found at the entry and exits points of the stomach acts rather like a drawstring on a bag.

Keeping it down

The stomach is basically a muscular bag, filled with hydrochloric acid (HCl).

cross section of stomach

food enters from the gullet

glandular tissue makes:hydrochloric acid, mucus and protease enzyme

muscle tissue

The basics on the stomach

liquids mix with the fooddigested food leaves

…attacks any microbes (bacteria) that may have been swallowed accidentally when the food was eaten.

The food is then subjected to a coordinated attack.

Firstly the

the hydrochloric acid

When the food enters the stomach. The sphincter contracts behind it.

Gastric processes:

HCl

Food bolus

Microbes

These enzymes begin digesting the proteins in the swallowed food.

These proteins are broken down to release the amino acids.

proteaseprotein

Secondly, the hydrochloric acid provides the perfect conditions for protease enzyme.

Protease enzymes work best under acidic conditions (pH < 7).

Protein digestion

amino acids

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!

Mucus and muscles

presence of HCl

The hydrochloric acid, mucus, food and enzyme solution is given the name - gastric juice.

muscle and glandular tissue

layer of mucus

wall of stomach is protected

Making gastric juice

muscular tissue of stomach

protease enzyme and hydrochloric acid

sugars

amino acids

carbohydrates proteins

fats

fatprotein

carbohydrate

sugar

physical digestion

chemical digestion

The stomach’s digestive action - summary

and after the stomach…

It is also clear that the fat has yet to be chemically digested.

This happens in the next section of the digestive system.

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

Contents

Digestion – Part Two

Lipid digestion

Absorption

The small intestine

Mouth to gullet

Gastric processes

Summary

When the gastric juices leave the stomach, they pass into 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.

X-ray of the small intestine

The small intestine:

Luckily two substances are produced to stop this happening. The first is mucus and we have seen how this works. The second is bile.

A nasty problem!

The gastric juices are very acidic. There could be a danger of the lining of the small intestine being digested.

Bile is made in the liver. It is a yellow/green liquid that is naturally alkaline (pH > 7).

Dealing with the acid

pH >7 NeutralisationpH <7+

This is essential for lipase (the enzyme that digests fat) to function properly.

This means that when bile and the gastric juices meet, the result is neutralisation of the acid.

Why neutralise?

The active site of the lipase enzymes is only effective in conditions above pH 7.

Parts of the small intestine

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

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.

liver produces bile

bile is released from the gall bladder and passes down through the bile duct

A whole lot of bile

fat within the gastric juice

The pancreas and the small intestine now release enzymes.

Both organs produce all three enzyme types:

Influx of enzymes

small intestine

pancreascarbohydrases

proteases

lipases

fat

lipase

fatty acids

glycerol phosphate

If we just consider fat, the lipase begins to break its chemical bonds:

Lipid digestion:

bile emulsifies the fat muscular action of the small intestine

lipase enzymeprotease enzymecarbohydrase enzyme

sugar

amino acids fatty acidsglycerol phosphate

chemical digestion physical digestion

sugar fat

carbohydrates amino acids protein

Digestion in small intestine - 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

Digestive processes - summary

enzymes come into contact with food.

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.

Design of the small intestine

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.

What about the waste?

However, if we consider what it has to do, we will understand why this idea cannot be correct.

At first glance, the small intestine appears to have a flat surface.

Inside the small intestine

Contents

Digestion – Part Two

Lipid digestion

Absorption

The small intestine

Mouth to gullet

Gastric processes

Summary

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.

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.

Surface area and absorption

The small intestine

Have a look inside a human’s intestines

The result of this folding means that the surface area of the lining of the small intestine is enormous!

Maximising the surface area

This increases the number of places where small soluble food molecules can pass across and move into the blood.

Direction of Food

Epithelium of small intestine

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.

Where is the blood?

Remember that the small intestine lining is made up of thousands of villi.Here is a summary of the process of absorption.

sugars

amino acids

fatty acids and glycerol phosphates

A 

S 

B 

R 

O 

T 

P 

N 

O 

I 

BLOOD

Any indigestible food will leave the small intestine without having been absorbed.

Digestive products in the blood

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.

The large intestine

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.

Water regulation

The blood reabsorbs the excess water that is mixed with the waste food.

Getting back excess water

LARGE

INTESTINE

BLOOD

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.

Waste material then passes into a storage organ called the rectum.

rectumThis waste material mainly consists of indigestible food. It makes up the bulk of the faeces (solid excrement) that will be excreted.

Expelling the waste

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.

Through constant contraction and relaxation, the food is kept moving through the system, from the mouth to the anus.

2. Muscular tissue

This is responsible for the production of the digestive enzymes.

1. Glandular tissue

Important body tissues in digestion

Chew it all over

Chew it over