Enzymes and the digestive system

2.1 Enzymes and digestion

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Settler Activity: Can you find today's 16 key words?

O L E S A R D Y H O B R A C EE E N I T S E T N I L L A M SS T N O I T S E G E Q U A R AO A S S I X B T A E M Y Z N EP F N A K T B F Z D U N T S TH N U E H C A M O T S S F L OA G E R S D I L A Y F S E I RG A E C E B D P I O K N B P PU I V N R E C T U M N U R A ES D N A L G Y R A V I L A S TN H O P H Y D R O L A S E E NE W E C B H Y D R O L Y S I ST E N I T S E T N I E G R A LA A B S O R P T I O N I O N T

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Learning Objectives

All students should be able to:• Define digestion.• Give the structure and function of the major parts

of the digestive system.• Understand how the digestive system breaks down

food both physically and chemically.• Understand the role of villi and microvilli.• Outline the role of enzymes in digestion.Specification reference: 3.1.2

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Success Criteria

• I can rearrange fragments of a sentence to give the correct definition of digestion.

• I can label the digestive system.• I can name the structures of the digestive system when

given their function and vice versa.• I can draw a labelled diagram of the lining of the small

intestine and answer questions related to its key features. • I can answer exam questions on digestion and digestive

enzymes.

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Starter Activity: Can you rearrange these fragments of a sentence to give the correct definition of digestion?

Digestion is the process in which large molecules are hydrolysed by enzymes

into small molecules which can be absorbed and assimilated.

Digestion

is the process in

which large

molecules are

hydrolysed by

enzymes

into small

molecules which

can be absorbed

and assimilated.

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Activity 1: The human digestive system• The human digestive system is made up of a long

muscular tube and its associated glands.• The glands produce enzymes that break down large

molecules into small ones ready for absorption. • The digestive system therefore provides an interface

with the environment because food substances enter the body through it.

Missing words: small interface glands enter enzymes muscular large absorption

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Activity 2: Can you label the human digestive system?

A

BC

D

EFG

HIJ

K

LMNOP

Q

RST

U

VWX

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Activity 4: Question 1• Which parts of the digestive system

does the food pass through?

• Mouth, oesophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (caecum, colon, rectum), anus.

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!!!• The contents of the intestines are

NOT inside the body! Molecules and ions only truly enter the body when they cross the cells and cell-surface membranes of the epithelial lining of the intestines.

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Activity 5: How well do you know the major parts of the

digestive system?

???

???

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This part is the final section of the intestines. The faeces are

stored here before periodically being removed

via the anus in a process called egestion.

RECTUM

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These are situated near the mouth. They pass their

secretions via a duct into the mouth. These secretions

contain the enzyme amylase, which breaks down starch

into maltose.

SALIVARY GLANDS

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This part is a muscular sac with an inner layer that produces enzymes. Its role is to store and digest food, especially proteins. It has glands

that produce enzymes which digest protein. Other glands in its wall

produce mucus. The mucus prevents this organ being digested

by its own enzymes.

STOMACH

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This is a large gland situated below the stomach. It

produces a secretion. This secretion contains proteases to digest proteins, lipase to digest

lipids and amylase to digest starch.

PANCREAS

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This part absorbs water. Most of the water that is reabsorbed

comes from the secretions of the many digestive glands. The food

within this part therefore becomes drier and thicker in consistency and

forms faeces.

LARGE INTESTINE

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This is a long muscular tube. Food is further digested here by enzymes that are

produced in its walls and by glands that pour their secretions into it. The inner walls of this organ are folded into villi,

which gives them a large surface area. The surface area of these villi is further

increased by millions of tiny projections, called microvilli, on the epithelial cells of each villus. This adapts this organ for its

purpose of absorbing the products of digestion into the bloodstream.

SMALL INTESTINE

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The lining of the small intestine

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This part carries food from the mouth to the stomach. It is therefore adapted for transport rather than for

digestion or absorption. It is made up of a thick muscular

wall.

OESOPHAGUS

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Activity 6: Question 1• Can you distinguish between the terms

‘absorption’ and ‘assimilation’?

• Taking soluble molecules into the body.

• Incorporating absorbed molecules into body tissues.

(Absorption)

(Assimilation)

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!!!• All organisms are made up of the same

biological molecules and therefore our food consists almost entirely of other organisms or parts of them. We must first break them down into molecules that are small enough to pass across cell surface membranes.

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Activity 6: Question 2 • Unscramble the anagrams to reveal

the two stages of human digestion.

• Hysipalc robewkand• Halcmice tindogies

(Physical breakdown)

(Chemical digestion)

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Activity 7: Physical Digestion• If the food is large, it is broken down into

smaller pieces by means of structures such as the teeth. This not only makes it possible to ingest the food but also provides a large surface area for chemical digestion. Food is churned up by the muscles in the stomach wall and this also physically breaks it up.

Missing words: surface area physically stomach teeth chemical ingest large

muscles

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Activity 8: Chemical Digestion• This breaks down large, insoluble molecules into smaller, soluble

ones. It is carried out by enzymes.

• All digestive enzymes function by hydrolysis (the splitting up of molecules by adding water to the chemical bonds that hold them together). The general term for such enzymes is hydrolases.

• Enzymes are specific and so it follows that more than one enzyme is needed to break down a large molecule. Usually one enzyme splits a large molecule into sections and these sections are then hydrolysed into smaller molecules by one or more additional enzymes.

Missing words: specific enzymes hydrolysed hydrolysis smaller

large additional insoluble soluble hydrolases splits

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Activity 9: Question 1 • Can you name the three different

types of digestive enzyme?

1. These breakdown carbohydrates, ultimately to monosaccharides.

2. These breakdown lipids (fats and oils) into glycerol and fatty acids.

3. These breakdown proteins, ultimately to amino acids.

Carbohydrases

Lipases

Proteases

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Chemical digestion continued...• Once the large food molecules have been hydrolysed into

monosaccharides, glycerol, fatty acids and amino acids, they are absorbed by various means from the small intestine into the blood.

• They are carried to different parts of the body and are often built up again into very large molecules, although these are not necessarily of the same type as the molecules from which they were derived.

• These molecules are incorporated into body tissues and/or used in processes within the body. This is called assimilation.

Missing words: fatty acids, tissues, large, monosaccharides, glycerol, amino acids, assimilation, derived, blood,

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Plenary Activity: Answer these 4 questions under test conditions!

1. State one way in which the stomach is adapted:a) To churn foodb) To prevent the enzymes it produces from

digesting the surface of the stomach.2. What is hydrolysis?3. Which two structures produce amylase?4. Suggest a reason why the stomach does not

have villi or microvilli.

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Answers1.a. By having a muscular wall.b. By secreting mucus.2. Hydrolysis is the breakdown of molecules by the

addition of water to the bonds that hold these molecules together.

3. Salivary glands, pancreas4. Villi and microvilli increase surface area to speed up

the absorption of soluble molecules. As the food in the stomach has not yet been broken down into soluble molecules they cannot be absorbed and so villi and microvilli are unnecessary.

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Learning Objectives

All students should be able to:• Define digestion.• Give the structure and function of the major parts

of the digestive system.• Understand how the digestive system breaks down

food both physically and chemically.• Understand the role of villi and microvilli.• Outline the role of enzymes in digestion.Specification reference: 3.1.2