Biological Molecules

Question 1

What chemical elements are present in carbohydrates?

Answer 1

carbon, hydrogen and oxygen

Question 2

What chemical elements are present in proteins?

Answer 2

carbon, hydrogen, oxygen, nitrogen (sometimes sulfur)

Question 3

What chemical elements are present in lipids (fats and oils)?

Answer 3

carbon, hydrogen, oxygen

Question 4

Describe the structure of carbohydrates as large molecules made up from smaller basic units

Answer 4

• Simple sugars formed from carbon hydrogen and oxygen atoms, sometimes arranged in a ring shaped molecule.
• One example of a simple sugar is glucose (monosaccharide)
• Simple sugar molecules can link together to form larger molecules (disaccharides eg sucrose, polysaccharides eg starch and glycogen, long chains of glucose molecules)

Question 5

Describe the structure of proteins as large molecules made up from smaller basic units

Answer 5

• Long chains of amino acids linked together
• Over 20 different types of amino acid
• They can be joined together in different orders to make hundreds of thousands of different types of protein

Question 6

Describe the structure of lipids as large molecules made up from smaller basic units

Answer 6

• fatty acids and glycerol
• most common type of lipid in body is called a ‘triglyceride’
• made of two different sub units called 1x glycerol and 3x fatty acids

Question 7

What are food tests for carbohydrates detecting?

Answer 7

Presence of starch and glucose

Question 8

Testing for starch solutions and results:

Answer 8

• Iodine
• Negative = yellow/ brown
• Positive = blue/ black

Question 9

Testing for glucose solutions and results:

Answer 9

• Benedicts solution
• needs to be heated above 80 degrees
• negative = stays blue
• slightly positive = green or yellow
• positive = orange or brick red

Question 10

Testing for protein solutions and results:

Answer 10

• Biuret solution
• negative = blue
• positive = lilac/ purple

Question 11

Testing for fat solutions and results:

Answer 11

• ethanol
• negative = colourless
• positive = white emulsion

Question 12

Define reagent:

Answer 12

A chemical that indicates the presence of a substance, usually by changing colour.

Question 13

What may be necessary when adding a reagent to a food sample which changes colour depending on what biological molecules are present?

Answer 13

Sometimes it may be necessary to crush the food or add water to the food before adding the reagent.

Question 14

How to carry out test for starch?

Answer 14

• place food on dropping tile

- drop iodine solution onto food

Question 15

How to carry out test for glucose?

Answer 15

• add the food to a test tube
• add Benedict solution
• put water bath at 85 degrees then leave for 5-10 mins

Question 16

How to carry out test for protein?

Answer 16

• add the food to a test tube

- add a few drops of Biuret’s solution

Question 17

How to carry out test for fats?

Answer 17

• add food to a test tube
• add a few drops of ethanol, put bung + shake
• pour solution into test tube containing water

Question 18

Why does a cloudy layer form for fats?

Answer 18

Fats don’t dissolve in water but do in ethanol

Question 19

Which two chemicals are found in Biuret reagent?

Answer 19

sodium hydroxide (NaOH) and hydrated copper(II) sulfate

Question 20

Practical to show how enzymes work

Answer 20

• Visking tubing acts like the walls of the intestine.
• It is a ‘partially permeable membrane’, so only small molecules can fit through its small holes
• The enzyme amylase can break down long starch molecules into small glucose molecules
• They can then pass through the intestine wall and into the blood to go to all body cells and be used for respiration
• Pour starch and amylase solution into risking tubing in a boiling tube
• After 25 minutes the water in the test tube can be tested for glucose using iodine
• Should be positive as amylase breaks down starch into glucose

Question 21

What are enzymes?

Answer 21

• biological catalysts

- speed up chemical reactions in cells

Question 22

Synthetic reactions vs Breakdown reactions

Answer 22

• synthetic = large molecules built up from small
• eg making glycogen from glucose
• breakdown = large molecules to smaller ones
• eg making glucose from glycogen

Question 23

Enzymes are proteins?

Answer 23

Yes, they are made up of long chains of amino acids which are folded into specific shapes

Question 24

The lock and key hypothesis

see diagram

Answer 24

the substrate fits into the enzyme molecule

the products of the reaction are released at the end, leaving the enzyme free to be used again

Question 25

Properties of enzymes

Answer 25

• work very rapidly
• only needed in small amounts
• they are specific, will only catalyse one reaction

Question 26

Why are enzymes specific?

Answer 26

Due to specific active site, meaning they can only lock together with a substrate that has a specific shape and matches

Question 27

Why is the reaction low at low temperatures?

Answer 27

Molecules have low kinetic energy and so move slowly, this means the enzyme and substrate are less likely to collide
Less successful collisions = less enzyme substrate complex will form and no product will be made

Question 28

What is the temperature at which the rate of an enzyme-controlled reaction is the fastest?

Answer 28

The ‘optimum temperature’

Question 29

Why does rate of reaction increase with temperature?

Answer 29

• As the temperatures increase the substrate and enzyme molecules gain kinetic energy and move more.
• This means the enzyme and substrate are more likely to collide.
• More collisions means more enzyme-substrate complexes, and more product formed.
• Therefore there is a higher rate of reaction.

Question 30

What happens to the enzymes at high temperatures? How does this affect the rate of reaction?

Answer 30

• At very high temperatures the enzyme is damaged and the active site change shape, this is called denaturing.
• When the enzyme is denatured the substrate no longer fits in the active site and no product is formed.
• Denaturing is irreversible, so once the enzyme is boiled it will never work again even if it is cooled down.

Question 31

Effect of pH on enzymes

Answer 31

• Each enzyme requires a certain pH to work efficiently. This is called the optimum pH. Some enzymes work best in acidic conditions, others in alkaline conditions.
• Changes in pH also alter the shape of an enzyme’s active site
  Enzymes are also sensitive to pH. Changing the pH of its surroundings will also change the shape of the active site of an enzyme.

Many amino acids in an enzyme molecule carry a charge. Within the enzyme molecule, positively and negatively charged amino acids will attract. This contributes to the folding of the enzyme molecule, its shape, and the shape of the active site.

Changing the pH will affect the charges on the amino acid molecules. Amino acids that attracted each other may no longer. Again, the shape of the enzyme, along with its active site, will change.

Extremes of pH also denature enzymes. The changes are usually permanent.

Enzymes work inside and outside cells, for instance in the digestive system where cell pH is kept at 7.0 to 7.4. Cellular enzymes will work best within this pH range. Different parts of the digestive system produce different enzymes. These have different optimum pHs. The optimum pH in the stomach is produced by the secretion of hydrochloric acid.

The optimum pH in the duodenum is produced by the secretion of sodium hydrogencarbonate.

The following table gives examples of how some of the enzymes in the digestive system have different optimum pHs:

Question 32

Practical showing effect of temperature on enzymes

Answer 32

(see sheets too)
Set up water baths at various temperatures (e.g. 0°C, 20°C, 40°C, 60°C and 80°C).
Add starch solution to 5 test tubes.
Add amylase solution to another 5 test tubes.
Place one starch and one amylase test tube into each water bath for 5 minutes - to allow the enzyme and substrate to reach the desired temperature.
Place 1 drop of iodine into each dimple on a spotting tile.
Add the amylase to the starch in the 0°C water bath.
Start the timer.
Every minute remove a sample of the starch-amylase solution and add it to a drop of iodine on the spotting tile.
Repeat step 8 until the iodine no longer changes colour - meaning that there is no starch present, in other words the amylase has broken all starch down.
Repeat steps 6-9 for each of the temperatures.
Record results.
Draw a graph to show the time taken for starch to be digested at different temperatures.

Results
At the optimum temperature the amylase will break down starch very quickly.
At low temperatures the amylase will break starch down slowly due to reduced kinetic energy.
At high temperatures the amylase will break starch down slowly or not at all due to denaturation of the enzyme’s active site.

Controlled variables
pH
Same volume and concentration of starch.
Same volume and concentration of amylase.

Question 33

Practical

Answer 33

Set up a Bunsen burner, heatproof mat, tripod and gauze.
Place a beaker of water on the gauze and adjust the flame to keep the water at about 35°C.
Now put two drops of iodine solution into each spot of a spotting tile.
Add 2 cm3 of amylase enzyme solution to a test tube.
Place 2 cm3 of starch solution into the same tube.
Finally add 1 cm3 of pH solution to the tube. This will keep the pH constant.
Mix the solution in the test tube and place it into the beaker of water on the Bunsen burner.
Use a pipette to remove a few drops of solution every 20 seconds from the test tube and put them into a different well of the spotting tile.
Repeat until the iodine solution stops turning black.
Record the time this takes.
Repeat with different pH solutions.

Conclusions
The enzyme amylase breaks down starch into glucose. If the enzyme is working effectively, this will happen quickly. At pH 7 it took the shortest time before the iodine no longer changed colour. This shows that the starch was broken down more quickly at this pH. The optimum pH for amylase is therefore pH 7

Risks
Iodine solution is an irritant. If it touches skin it should be washed off.
Goggles should be worn at all times.

Question 34

see diagrams and notes

Answer 34

see diagrams and notes