biological molecules - carbohydrates

Question 1

name the 3 monosaccharides

Answer 1

1. glucose
2. fructose
3. galactose

Question 2

what are the constituent monomers of 1. lactose 2. maltose 3. sucrose

Answer 2

1. glucose + galactose
2. glucose + glucose
3. glucose + sucrose

Question 3

outline the process used to test for reducing sugars

Answer 3

• add 2cm3 of food sample to 2cm3 of benedict reagent in a test tube
• (if sample is not already in liquid form then grind it up in water
• heat the mixture by placing the test tube in a warm water bath for about 5 minutes
• if a reducing sugar is present the solution turns orange - brown colour.

Question 4

outline the process for non reducing sugars

Answer 4

• carry out steps for reducing sugars. if a reducing sugar is not present then the reagent stays a blue colour
• add 2cm3 of the food sample to 2cm3 of dilute HCl in a test tube , ( this hydrolyses the disaccharides into their constituent monosaccharides )
• slowly add sodium carbonate to the test tube to neutralise the acid ( benedict’s reagent does not work in acidic conditions)
• now heat the sample with 2cm3 of benedict’s reagent in a warm water bath, the reagent should turn an orange brown colour - due to the reducing sugars produced.

Question 5

give 3 example of polysaccharides

Answer 5

starch, glycogen and cellulose

Question 6

outline the test for starch

Answer 6

using iodine dissolved in potassium iodide

• place the food samples into white spotting tiles , about 2 drops.
• then add 2 drops of iodine dissolved in potassium iodide to the sample
• if starch is present then the it is indicated by a blue/black colour change.

Question 7

what makes the structure of starch suitable for it role

Answer 7

• insoluble - meaning water potential is not affected , so water is not drawn into the cell via osmosis ( could damage cell
• unbranched form is compact - meaning a lot can be stored in a small space
• when hydrolysed - ∝glucose is formed , readily used for processes such as respiration
• branched form has many ends meaning enzymes can work on it simultaneously - means that glucose monomers are released rapidly .

Question 8

what are the 2 polysaccharides that make starch

Answer 8

the 2 polysaccharides are - amylose and amylopectin.

• amylose - long un branched chains of ∝glucose
• amylopectin - long branched chains ∝glucose

Question 9

outline 2 similarities and 2 differences between starch and glycogen

Answer 9

similarities :
- they are both insoluble therefore water potential does not affect it ( no water enters the cells damaging them via osmosis)
- both are compact - are able to store more in a small space
differences:
-starch is only found in plants whereas glycogen is only found in animals and bacteria
-glycogen is more highly branched than starch although they have similar structures

Question 10

in animals how is glycogen usually stored

Answer 10

usually stored as small granules found in the muscles and liver

Question 11

how does cellulose differ from glycogen and starch

Answer 11

• made from B glucose instead of ∝glucose
  -straight unbranched chains (parallel allowing hydrogen bonds to be formed)
  -they are grouped together forming microfibrils
  which provides rigidity to the cells.