2. Enzymes and the Digestive System

Question 1

Read pages 18 and 19!

Answer 1

Done!

Question 2

What are carbohydrates made up of?

Answer 2

Carbon combined with water

Question 3

Name some of the properties of carbon.

Answer 3

⋆ Carbon readily forms bonds with other carbon atoms to create long chains called backbones, which come in lots of different sizes
⋆ Carbon-containing molecules are known as organic molecules because life is based around carbon
⋆ Relatively few elements can attach to the carbon so life is made from a very small number of chemical elements

Question 4

Which are the four most common elements found in biological polymers?

Answer 4

Carbon, hydrogen oxygen and nitrogen

Question 5

What is the basic monomer in a carbohydrate?

Answer 5

A sugar (saccharide)

Question 6

Name the properties of monosaccharides.

Answer 6

⋆ Sweet-tasting
⋆ Soluble
⋆ (CH2O)n where n is between 3 and 7
⋆ Glucose (C6H12O6) can take a number of different forms and be arranged in many different ways

Question 7

Draw alpha glucose.

Answer 7

Check in your notes!

Question 8

Almost all monosaccharides and some disaccharides are reducing sugars. How do we test for reducing sugars?

Answer 8

⋆ Benedick’s solution is an alkaline solution of copper (II) sulphate
⋆ The test:
1. Add 2cm^3 of liquefied food sample to a test tube
2. Add an equal volume of Benedick’s reagent
3. Heat the mixture in a gently-boiling water bath for five minutes
⋆ Colour changes:
None (stays blue) , Very low (green), Low (yellow), Medium (orange), High (red)
⋆ The Benedick’s test is semi-quantitative because it an only be used to estimate the amount of reducing sugar

Question 9

Name three disaccharides and their constituent monosaccharides.

Answer 9

⋆ Glucose + glucose –> maltose
⋆ Glucose + fructose –> sucrose
⋆ Glucose + galactose –> lactose

Question 10

How do monosaccharides join together to form a disaccharide?

Answer 10

⋆ When monosaccharides join together a water molecule is removed
⋆ This is called a condensation reaction
⋆ The bond formed is called a glycosidic bond

Question 11

How can disaccharides be broken back down into monosaccharides?

Answer 11

Water can be added to disaccharides under the right conditions to break glycosidic bonds in a process known as hydrolysis, an addition of water that causes breakdown. (Make sure you know the diagram!)

Question 12

Look up for test for non-reducing sugars on page 23!

Answer 12

Done!

Question 13

Name some of the characteristics of polysaccharides.

Answer 13

⋆ Polymers – many monosaccharides joined together
⋆ Monosaccharides are held together by glycosidic bonds
⋆ Very large so insoluble and good for storage
⋆ When hydrolysed they break down into disaccharides and monosaccharides

Question 14

Name three polysaccharides and their characteristics.

Answer 14

⋆ Starch – many alpha glucose molecules that are only found in plants and form an insoluble, compact product for storage
⋆ Glycogen – animal equivalent of starch
⋆ Cellulose – made of monomers of beta glucose to support cell walls

Question 15

How do we test for the presence of starch?

Answer 15

Add iodine to a dimple tray and wait for it to turn blue-black

Question 16

Read about carbohydrate digestion and lactose intolerance in your textbook.

Answer 16

Done!

Question 17

How are proteins structured?

Answer 17

Monomer units (amino acids) which combine to make polymers called polypeptides

Question 18

How many amino acids are there?

Answer 18

100 amino acids have been identified, with 20 occurring naturally in proteins

Question 19

Learn the structure of an amino acid.

Answer 19

Done!

Question 20

Describe the primary structure of proteins.

Answer 20

⋆ Polymerisation through a series of condensation reactions forms amino acids into polypeptides
⋆ The primary structure is the sequence of amino acids in a polypeptides
⋆ Amino acids can be arranged in many different ways so there are lots of different types of primary protein structures
⋆ The sequence determines a protein’s properties and shape
⋆ A mutation will affect its shape and function

Question 21

Describe the secondary structure of proteins.

Answer 21

⋆ Amino acids that make up a linked polypeptide have –NH and -C=O groups
⋆ The H+ in the –NH group and the O2- in the –C=O group form weak hydrogen bonds
⋆ This causes the long polypeptide chain to twist into a three dimensional shape such as alpha helix of beta pleated sheet (see diagrams in textbook)

Question 22

Describe the tertiary structure of proteins.

Answer 22

⋆ The alpha helices and beta pleated sheet can be twisted and folded into a 3D structure
⋆ This is maintained by a number of bonds:
Strong disulphide bonds
Ionic bonds between carboxyl and amino groups not involved in peptide bonds which are easily broken by changed in pH
Lots of hydrogen bonds which are easily broken
⋆ The 3D shape makes each protein distinctive and allows it to be recognised by and interact with other molecules

Question 23

Describe the quaternary structure of proteins.

Answer 23

⋆ Large proteins consist of lots of polypeptide chains arranged in many ways
⋆ Prosthetic (non-protein) groups are also linked with molecules e.g. haem group in haemoglobin

Question 24

How do we test for the presence of proteins?

Answer 24

⋆ Biuret tests identify peptide bonds
⋆ The test:
Add an equal volume of protein solution and NaOH at room temperature
Add a few drops of dilute copper (II) sulphate solution and mix gently
A purple colouration indicates the presence of peptide bonds or else the solution will remain blue

Question 25

What are the properties of fibrous proteins?

Answer 25

⋆ Structural proteins e.g. collagen
⋆ Polypeptide strands tightly wrapped around one another
⋆ Chains linked by cross-bridges so form very stable molecules
⋆ Suitable as structural proteins because of their cross-linking, parallel layers that move over one another, external R-groups that are non-polar which makes them insoluble

Question 26

Collagen is a fibrous protein. What are its properties?

Answer 26

⋆ The structure of collagen:
PS: unbranched polypeptide chain
SS: tightly wound polypeptide chain
TS: chain twisted into a second helix
QS: three polypeptide chains twisted together as individual fibres then wound together into a rope shape
⋆ Found in tendon which link muscles to bones
⋆ The point where one collagen molecule ends and the next begins is spread out so they are not all in the same position, which means breakage is not so much of a problem

Question 27

What are the properties of globular proteins?

Answer 27

⋆ Enzymes
⋆ Carry out metabolic functions
⋆ Circular shapes, soluble in water

Question 28

What are the functions of proteins?

Answer 28

⋆ Carrier proteins
⋆ Antibodies
⋆ Structural proteins
⋆ Hormones
⋆ Enzymes 
⋆ Transport proteins
⋆ Contractile proteins

Question 29

What is the function of an enzyme?

Answer 29

⋆ Globular proteins that act as catalysts
⋆ Alter the rate of a reaction by providing an alternative reaction pathway with a lower activation energy without being used up themselves
⋆ Effective in small amounts and can be reused

Question 30

How is an enzyme structured?

Answer 30

⋆ Only a small region of the large protein is functional
⋆ This is known as the active site and is made up of a small number of amino acids
⋆ The active site forms a small, hollow depression within the large enzyme molecule

Question 31

How is an enzyme-substrate complex formed?

Answer 31

⋆ The molecule acting on the enzyme is called the substrate
⋆ The substrate is held in place within the active site by bonds that temporarily form between certain amino acids of the active site and groups on the substrate molecule
⋆ The enzyme-substrate complex is created
⋆ Products of the reaction are released from the surface of the enzyme

Question 32

What is the lock and key model of enzyme action?

Answer 32

⋆ Like a lock and key, a substrate will only fit the active site of a particular enzyme
⋆ Enzymes are specific to the reactions they catalyse
⋆ One limitation of this model is that the enzyme, like lock, is considered a rigid structure
⋆ Scientists observed that other molecules could bind to enzymes at sites other than the active site
⋆ This altered the activity of the enzyme
⋆ The other binding molecules therefore alter the enzyme’s shape
⋆ It has a flexible, not rigid, structure

Question 33

What is the induced fit model of enzyme action?

Answer 33

⋆ The enzyme has a certain general shape, but it alters in the presence of the substrate
⋆ As it changes its shape, the enzyme puts a strain on the substrate molecule which distorts a particular bond and consequently lowers the activation energy needed to break the bond

Question 34

Read page 33, about the factors affecting enzyme action.

Answer 34

Done!

Question 35

What is the effect of temperature on enzyme action?

Answer 35

⋆ Rise in temperature increases the kinetic energy of molecules
⋆ Molecules move around more rapidly and collide with each other more often
⋆ This means that the enzyme and substrate molecules come together more often in a given time, so the rate of reaction increases
⋆ After temperatures of around 45 degrees in humans, the hydrogen and other bonds in the molecule begin to break
⋆ This results in the shape of the active site changing as well as the site of the overall protein
⋆ This slows down the rate of reaction as the substrate is less able to bind to the active site
⋆ Around 60 degrees, the enzyme stops working all together
⋆ Denaturation is a permanent change and the enzyme will not function again
⋆ The optimum working temperature varies from enzyme to enzyme, but most in our bodies work well at round 40 degrees

Question 36

Why does our body temperature have to be kept at 37 degrees?

Answer 36

⋆ Although higher temperatures would slightly increase metabolic rate, the advantages are offset by the additional energy (food) that would be required to maintain a higher temperature
⋆ Other proteins apart from enzymes may be denatured
⋆ A further rise in temperature e.g. during illness might denature the enzymes

Question 37

What is the effect of pH on enzyme action?

Answer 37

⋆ Alters the charges on the amino acids that make up the active site so the substrate can no longer attach to it and a complex cannot be formed
⋆ Causes the bond maintaining the tertiary structure to break, making the enzyme change shape so the substrate no longer fits with the active site and the enzyme is denatured
⋆ pH fluctuations inside organisms are usually small, so they are more likely to reduce an enzyme’s activity than to denature it

Question 38

What is the effect of substrate concentration on enzyme action?

Answer 38

⋆ If we start off at low substrate concentrations and gradually increase the concentration whilst the enzyme concentration remains constant, the rate of reaction will increase because the active sites are not working to their full capacity
⋆ As more and more substrate is added, the active sites become gradually occupied
⋆ Eventually, all active sites are occupied so the rate of reaction levels off
⋆ In other words, there is an excess of substrate

Question 39

How do competitive inhibitors work?

Answer 39

⋆ Molecular shape similar to that of the substrate which allows them to occupy the enzyme’s active site
⋆ Competes with the substrate for active sites
⋆ The difference in concentration between the inhibitor and the substrate determines the extent to which the inhibitor will reduce the rate of reaction
⋆ Sooner or later, the substrate molecules will all occupy an active site, but this will take longer with more competitive inhibitors
⋆ An example of competitive inhibition occurs when with an important respiration enzyme that acts on succinic acid
⋆ Malonic acid can inhibit the enzyme because it has a similar molecular site

Question 40

How do non-competitive inhibitors work?

Answer 40

⋆ Attach themselves to the enzyme at a site other than the active site
⋆ This alters the shape of the enzyme and therefore the active site
⋆ The enzyme therefore cannot function
⋆ The difference in concentration between the substrate and the inhibitor is not significant, since the enzyme is denatured either way

Question 41

Read about controlling metabolic pathways on page 37.

Answer 41

Done!