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© Boardworks Ltd 2004 1 of 55 Digestion - Part Two KS4 Biology

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Digestion - Part Two

KS4 Biology

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Contents

Digestion – Part Two

Lipid digestion

Absorption

The small intestine

Mouth to gullet

Gastric processes

Summary

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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

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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:

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Protecting the windpipe

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How it gets down the gullet

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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

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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

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

cross section of stomachfood 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

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Contents

Digestion – Part Two

Lipid digestion

Absorption

The small intestine

Gastric processes

Summary

Mouth to gullet

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…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

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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

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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

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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

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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

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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

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Contents

Digestion – Part Two

Lipid digestion

Absorption

The small intestine

Mouth to gullet

Gastric processes

Summary

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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:

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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

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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.

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Parts of the small intestine

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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

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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

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The pancreas and the small intestine now release enzymes.

Both organs produce all three enzyme types:

Influx of enzymes

small intestine

pancreas

carbohydrases

proteases

lipases

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Contents

Digestion – Part Two

Lipid digestion

Absorption

The small intestine

Mouth to gullet

Gastric processes

Summary

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fat

lipase

fatty acids

glycerol phosphate

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

Lipid digestion:

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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

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 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

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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

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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?

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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

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Contents

Digestion – Part Two

Lipid digestion

Absorption

The small intestine

Mouth to gullet

Gastric processes

Summary

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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:

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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

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The small intestine

Have a look inside a human’s intestines

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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

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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?

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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

BLOOD

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

Digestive products in the blood

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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

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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

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

Getting back excess water

LARGE

INTESTINE

BLOOD

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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

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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

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Chew it all over

Chew it over

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“Digestion involves __________ types of process. These are firstly ____________ digestion and secondly _________ digestion. Chemical digestion is __________ out by ___________ which are chemical produced from ___________ tissue. Specific enzymes break down __________ types of food. For example ____________ enzyme breaks down ___________ . Mechanical digestion involves teeth and the tongue but mostly _________ contraction e.g. the squeezing of the muscular lining of the ________.”

chemical

mechanical two

stomachmuscle

carbohydrase

specific

carbohydrates glandularenzymes carried

chemicalmechanicaltwo

stomach

muscle

carbohydrasespecificcarbohydrates

glandularenzymes

carried

Find the best words to fill the gaps

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Use the words below to create 3 equations to show what happens when we digest:

1. Carbohydrates 2. Fats 3. Protein.

Digestion equations

carbohydrates fats amino acids

double sugar lipase glucose protein

single sugar 3 fatty acids protease

glycerol phosphate maltose carbohydrase

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True or false?

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Do you know your guts?

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Down the hatch! Part 1

What type of acid is found in the stomach? (12)The ___________ (10) links the stomach to the pharynx.The __________ (7) juices are found within the stomach.What happens to the water in the large intestine? (10)Where does the process of digestion actually begin? (5)This organ stores waste material before it is discharged from the body? (6)The _________ (6) moves the food around in the mouth, preparing it for its journey.

**

**

Questions

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Down the hatch! Part 2

___________ (5) transports the dissolved nutrients around the body.The first section of the digestive system is known as the l________. (5)The chemicals that help to break down food are collectively called __________. (7)We eat or ________ food. (6)This is the name for the soft fleshy part of the throat that stops food going down the trachea. E___________. (10)At the rear of the mouth, this region receives the bolus of food from the tongue. (7)The main part of the small intestine is known as the D____________. (8)

**

**

**

**

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Down the hatch! Part 3

These provide the mechanical breakdown of food in the mouth? (5)

This organ can be SMALL or LARGE? (9)

The collective name for the useful chemicals that are absorbed across the lining of the small intestine? (9)

The product of the previous question? (5)

The name for the muscular bag that contains acid? (7)

Food is __________ (7) down in the digestive system using both chemical and mechanical methods.

**

**

**

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You will notice that 9 of the questions have a small green asterisk beside them.

Take the first letter of each of the answers to these questions. Put these letters in the box below.

Now rearrange them to find the mystery word!

Down the hatch! Final Part

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Contents

Digestion – Part Two

Lipid digestion

Absorption

The small intestine

Mouth to gullet

Gastric processes

Summary

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20 questions

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H V C N D G B L O O D Z V U M R E Y L U R W B J X B A U C P Y T D P D M N U T R I E N T O N H V N S I R T G D I O S M B O D T A O H G G O M S K S K U C E K E U R F K O L C H U Y I E L S L E N Z Y M E S O H G M T L N O A T Y U I N G E S T L F H I C P B H Z O M Q X J W F T O Z O V H E Y R R S M T E S F B I R B G A S T R I C D I A G Q W G S I A G C F B Q L H P D M C X I N Q C U O L I N T E S T I N E H X J O S R E A B S O R B E D J T Y P X L O S T R P G C N J U L O N Z J P P P B T O N G U E N H V M I A Q K R

Digestion wordsearch