biological molecules and food tests 2.7, 2.8, 2.9

Question 1

what is the test for reducing sugars (glucose)?

Answer 1

benedicts solution

Question 2

what’s the method for reducing sugars test - benedicts solution?

Answer 2

1. add benedicts solution to a sample of food (5cm3)
2. place in a water bath at 80 degrees for 5 minutes
3. if glucose is present the colour will change from blue to brick red
   if it is yellow, green or orange it is a positive but lower concentration of sugar

Question 3

what’s the colour change for a positive sugar-reducing test - benedicts solution?

Answer 3

blue to brick red

Question 4

what’s the test for starch?

Answer 4

iodine

Question 5

what’s the method for testing for starch - iodine?

Answer 5

1. add a few drops of orange iodine solution to the food sample on a spotting tile
2. a blue-black colour indicates the presence of starch

Question 6

what’s the colour change for a positive starch test - iodine?

Answer 6

orange to blue-black

Question 7

what’s the test for protein?

Answer 7

biuret

Question 8

what’s the method for testing for proteins - biuret?

Answer 8

1. add 2cm3 water to a food sample and shake (if not a liquid)
2. add an equal volume of dilute sodium hydroxide and shake
3. add two drops of 1% copper sulfate solution
4. if protein is present a pale purple colour will appear

Question 9

what’s the colour change for a positive test for proteins - biuret?

Answer 9

blue to pale purple

Question 10

what’s the test for lipids?

Answer 10

emulsion

Question 11

what’s the method for testing for lipids - emulsion?

Answer 11

1. place the food sample in a tube
2. add a small amount of ethanol and shake to dissolve any lipid in the alcohol
3. add an equal amount of water
4. a cloudy white colour indicates the presence of lipids

Question 12

what’s the colour change for a positive lipids test - emulsion?

Answer 12

cloudy white

Question 13

why do leaves store glucose as starch?

Answer 13

glucose is not a good storage molecule as it is soluble, meaning it can draw water through osmosis causing plants to swell
starch is insoluble molecule

Question 14

what are carbohydrates made of?

Answer 14

carbon, hydrogen, oxygen
made up of simple sugars

Question 15

what is the use of carbohydrates?

Answer 15

carbohydrates are broken down in a chemical reaction, respiration, to release energy in the form of ATP
this energy is used for other processes

Question 16

what are the simple sugars of carbohydrates?

Answer 16

simple sugars - glucose and fructose - are the smallest units of carbohydrates

Question 17

examples of complex carbohydrates:

Answer 17

many sugar molecules (1000s) are joined together to make complex carbohydrates:
- starch, used to store glucose in plants
- glycogen, used to store glucose in animals and fungi
these are better storage molecules as they are less soluble than simple sugar and so have less effect on water movement i n and out of cells

Question 18

what are fats (lipids) made of?

Answer 18

carbon, hydrogen, oxygen
made of 3 fatty acids joined to glycerol molecules

Question 19

what is the function of fats (lipids)?

Answer 19

• energy storage (can used in respiration)
• part of cell membranes
• thermal insulation
• electrical insulation (around nerve cells)
• buoyancy (helps marine animals such as whales to float)

Question 20

what are proteins made of?

Answer 20

carbon, hydrogen, oxygen, nitrogen
made amino acids joined together
20 different amino acids which can be combined in any order
shape of a protein helps carry out its job

Question 21

what are the functions of proteins?

Answer 21

• structural molecules
• controlling chemical reactions - these are enzymes
• messenger molecules - hormones
• combating disease - antibodies
• transport

Question 22

what do you do before a food test (preparation)?

Answer 22

• break up food using a pestle and mortar
• transfer food into a beaker with distilled water
• stir mixture with a glass rod to dissolve some of the food
• filter solution with a lined filter paper funnel to get rid of any solid bits of food