Biology AS: Enzymes & the Digestive System

Contains: Enzymes & Digestion

Carbohydrates - monosaccharides, disaccharides, polysaccharides

Carbohydrate Digestion


Enzyme Action

Factors Affecting Enzyme Action

Enzyme Inhibition

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Enzymes & Digestion-Digestive System

The major parts of the digestive system are:

1. Oesophagus: Carries food from the mouth to the stomach.

Adapted for transport-it is made up of a thick muscular wall.

2. Stomach: A muscular sac with an inner layer that produces enzymes.

It stores and digests foods (esp. proteins).

It has glands that produce enzymes to digest proteins.

Other glands in the stomach wall produce mucus, which prevents the stomach from being digested by its own enzymes.

3. Small Intestine: A long muscular tube where food is further digested.

Intestinal walls and glands produce the enzymes needed.

Inner walls have villi and microvilli which increase surface area.  

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Enzymes & Digestion-Digestive System

The major parts of the digestive system are:

 4. Large Intestine: This absorbs water; comes from digestive secretions.

The digested food becomes drier and thicker --> faeces.

5. Rectum: Faeces are stored here before being removed by the anus.

This process is called egestion.

**Salivary Glands: Situated near the mouth, their secretions enter through a duct.

These contain amylase, which breaks down starch-->maltose.

**Pancreas: A large gland situated below the stomach.

It secretes pancreatic juices, which contain proteases, lipase, amylase.

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Enzymes & Digestion

The two stages of digestion:

1. Physical breakdown: If food is large, it is broken down by the teeth.

This makes it possible to ingest food but also provides a large surface area for chemical digestion.

Food is also churned up by the stomach muscles.

2. Chemical digestion: Large, insoluble molecules into small, soluble ones.

Carried out by digestive enzymes using hydrolysis.

Hydrolysis-Splitting molecules by adding water to chemical bonds. These enzymes are called hydrolases.

The three main types of digestive enzyme are carbohydrases (carbs->monosaccharides), lipases (lipids->glycerol and fatty acids) and proteases (proteins->amino acids).

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Carbon readily forms bonds with other carbon atoms.

This allows sequences of carbon to form, which become the 'backbone' for other atoms to attach to.

Carbon-containing molecules are known as organic molecules.

Individual organic molecules are known as monomers, which can join together to form polymers.

Proteins and carbohydrates are usually polymers made up of the four base elements: carbon, hydrogen, oxygen and nitrogen.

In carbohydrates, the monomer unit is a saccharide (sugar)/ monosaccharide.

A pair of saccharides is call a disaccharide, larger numbers are polysaccharides.

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Examples of disaccharides:

i. Glucose linked to glucose to form maltose.

ii. Glucose linked to fructose to form sucrose.

iii. Glucose linked to galactose to form lactose.

When monosaccharides join, a molecule of water is removed - condensation reaction.

The bond formed is called a glycosidic bond.

When water is added to a disaccharide under suitable condittions, it breaks the glycosidic bond, releasing the constituent monosaccharides.

This is called hydrolysis.

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Test For Non-Reducing Sugars

Non-reducing sugars do not change the colour of Benedict's reagent when heated with it.

In order to detect a non-reduing sugar it must first be broken down into its monosaccharide components by hydrolysis.

1. Add 2cm*3 of the sample (in liquid form) to an equal volume of B.r.

2. Place in a gently boiling water bath for 5 minutes. If the solution remains blue a reducing sugar is not present.

3. Add another 2cm*3 of the sample to an equal volume of dilute HCl and place in a gently boiling water bath for 5 minutes. This will hydrolyse any dissachride present.

4. Slowly add sodium hydrogencarbonate to neutralise the HCl as B.r. will not work in acidic conditions.

5. Retest the resulting solution with B.r. If a non-reducing sugar was present in the original sample, B.r. will turn orangey-brown.

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Polysaccharides are polymers, formed by combining many monosaccharide molecules.

The monosaccharides are joined by glycosidic bonds, formed by condensation reactions.

As polysaccharides are large molecules, they are insoluble. This feature makes them suitable for storage.

When they are hydrolysed, polysaccharides break down into disaccharides or monosaccharides.

Some polysaccharides, such as cellulose, are used for structural support rather than storage.

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Test For Starch

Starch is a polysaccharide that is found in many parts of plants in the form of small granules or grains.

It is formed by the linking of between 200 and 100000 alpha glucose molecules by glycosidic bonds in a series of condensation reactions.

Starch is easily detected by its ability to change the colour of iodine in potassium iodide from yellow to blue-black.

1. Place 2cm*3 of the sample into a test tube or spotting tile.

2. Add 2 drops of iodine solution and shake or stir.

3. The presence of starch is indicated by a blue-black colouration.

** This test is carried out at room temperature.

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Carbohydrate Digestion: Starch

Enzymes are specific, therefore it usually takes more than one to break down a large molecule.

Typically, one enzyme will break the molecule down into smaller sections then other enzymes will break these sections down into their monomers.

These enzymes are usually produced in different parts of the digestive system. This is because each enzyme works fastest at a different pH.

1. Amylase: Produced in the mouth and the pancreas.

Amylase hydrolyses alternate glycosidic bonds to produce the disaccharide maltose.

2. Maltase: Produced by the lining of the intestine.

Maltase hydrolyses maltose into alpha glucose (monosaccharide).  

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Starch Digestion in Humans

1. Food enters the mouth and subjected to physical breakdown (chewing). This increases the surface area of the food.

2. Saliva containing salivary amylase enters the mouth from salivary glands.

3. Amylase starts hydrolysing starch into maltose. This also contains mineral salts which maintain the pH at neutral (optimum pH).

4. Food is swallowed and enters the stomach, where conditions are acidic. Acid denatures the amylase and prevents further hydrolysis of starch.

5. Food is passed onto the small intestine where it is mixed with a pancreatic secretion called pancreatic juice.

6. Pancreatic juice contains pancreatic amylase which continues to hydrolyse any remaining starch. Alkaline salts are produced by the pancreas and intestinal wall to maintain a neutral pH.

7. Muscles in the intestinal wall push food along the small intestine. Epithelial lining produces maltase which hydrolyses maltose into alpha-glucose.

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Carbohydrate Digestion: Disaccharides


In natural foods sucrose is contained within cells and therefore needs to be physically broken down to release it.

Sucrose moves into the small intestine and the epithelial lining produces sucrase.

Sucrase hydrolyses the single glycosidic bond in the sucrose molecule to produce glucose and fructose (monosaccharides).


The sugar found in milk and therefore milk products.

This is digested in the small intestine, whose epitheial lining produces an enzyme called lactase.

Lactase hydrolyses the glycosidic bond that links glucose and galactose (monosaccharides) to release them.

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Amino acid structure: Amino acids are the basic monomer units which combine to make polymers called polypeptides, which can be combined to make proteins.

Every amino acid has a central carbon atom to which four different chemical groups are attached:

Amino group: A basic group: --NH2

Carboxyl group: An acidic group: --COOH

Hydrogen atom: --H

R group: A variety of different chemical groups. Each amino acid has a different R group.

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Formation of a Peptide Bond

Amino  acid monomers can combine to form a dipeptide.

This is done via a condensation reaction i.e. the removal of a water molecule.

The water is made by combining an --OH from the carboxyl group with an --H from the amino group of another amino acid.

The two amino acids are then linked by a peptide bond between the carbon atom of one and the nitrogen atom of another.

A peptide bond can also be broken down by hydrolysis.

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Primary Structure of Proteins

Through a series of condensation reaction, many amino acid monomers can be joined together in a process called polymerisation.

The resulting chain is called a polypeptide. The sequence of amino acids in a polypeptide chain form the primary structure of any protein.

The primary structure determines the ultimate shape of the protein and therefore its function.

A change in a single amino acid can lead to a change in the shape of the protein and possibly stop it carrying out its function.

A protein's shape is specific to its function: change its shape and it will function less well, if at all.

Typically, a protein is made up of a number of polypeptide chains.

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Secondary & Tertiary Structure of Proteins

Secondary Structure

The linked a.a.s of a polypeptide possess both --NH and --C=O groups on either side of the peptide bond.

The hydrogen (--NH) has an overall positive charge, whilst the O (--C=O) has an overall negative charge.

These two groups readily form weak bonds, called hydrogen bonds. This cause the chain to twist into a 3-D alpha-helix.

Tertiary Structure

Secondary structure can be twisted into a complex shape, maintained by:

a. Disulfide bonds: Fairly strong and therefore not easily broken.

b. Ionic bonds: (carboxyl/amino groups) weaker than a. and broken by pH changes.

c. Hydrogen bonds: Numerous but easily broken.

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Quaternary Structure & Test for Proteins

Large proteins often form complex molecules containing a number of individual polypeptide chains that are linked in various ways.

There may also be non-protein (prosthetic) groups associated with the molecules, such as the iron-containing haem group in haemoglobin.

Test For Proteins

The most reliable protein test is the Buiret test, which detects peptide bonds:

1. Place the sample in a test tube and add an equal volume of sodium hydroxide solution at room temperature.

2. Add a few drops of dilute (0.05%) copper (II) sulphate solution and mix.

3. A purple colouration indicates the presence of peptide bonds and hence a protein. If no protein is present, the solution remains blue.

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Enzyme Action

For reactions such as: sucrose + water ---> glucose + fructose to take place naturally, a number of conditions must be satisfied:

1. Sucrose and water must collide with sufficient energy to alter the arrangement of their atoms to form glucose and fructose.

2. The energy of the products must be less than that of the substrates.

3. An initial boost of energy is needed to kick start the reaction. The minimum amount of energy needed to activate the reaction is called activation energy.

There is an activation energy level which must be initially overcome before the reaction can proceed.

Enzymes work by lowering the activation energy. This allows reactions to take place at a lower temperature than normal.

This enables some metabolic processes to take place rapidly at the average human body temperature (37degrees Celsius).

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Enzyme Structure

Enzymes are globular proteins that have a specific 3-D shape as a result of their sequence of amino acids.

Only a small region of the enzyme is functional. This is called the active site and is made up of a relatively small number of amino acids.

The molecule on which an enzyme acts is called a substrate. This fits into the active site to form the enzyme-substrate complex.

The substrate is held within the active site by bonds that temporarily form between certain amino acids of the active site and groups on the substrate molecule.

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Models of Enzyme Action

Lock & Key Model

A substrate will only fit an active site of one particular enzyme.

This model is supported by the observation that enzymes are specific in the reactions they catalyse.

The shape of the substrate (key) exactly fits the active site of the enzyme (lock).

One limitation of this model is that the enzyme is considered to have a rigid structure, whereas scientists have observed that other molecules could bind to enzymes at sites other than the active site.

In doing so, they altered the activity of the enzyme. This suggests that the enzyme's shape was being altered by the binding molecule.

In other words, its structure is flexible.

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Models of Enzyme Action

Induced Fit Model

This proposes that enzymes actually changes its shape slightly to fit the profile of the substrate.

Therefore the enzyme is flexible and can mould itself around the substrate.

The enzyme has a certain general shape, but this alters in the presence of the substrate.

As it changes its shape, the enzyme puts a strain on the substrate molecule. This strain distorts a particular bond and consequently lowers the activation energy.

The Induced Fit Model explains:

a. How other molecules can affect enzyme activity.

b. How the activation energy is lowered.

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