2.1.2 Biological Molecules

Question 1

Many multicellular organisms need to be able to convert monosaccharides into polysaccharides and back again.
Mammals convert the monosaccharide glucose into a highly branched polysaccharide called glycogen, which gets stored in liver cells.
Explain why mammals store glycogen instead of glucose.

Answer 1

Glycogen is…
Insoluble, no has no effect on water potential (1)
Metabolically inactive (1)
Compact/lots can be stored in a small space (1)
Able to store large amounts of energy (1)
Highly branched so has lots of ends for adding/removing glucose when needed OR can be broken down quickly to release glucose (1)

Question 2

Fig. 20 shows the disaccharide lactose, which is found in milk. Another disaccharide is maltose. Maltose and lactose both contain the same number of atoms of each element,
C, H and O. State two other structural similarities between lactose and maltose.

Answer 2

Two, 6-membered rings/hexoses (1)
1-4 glycosidic bond (1)
Two CH2OH groups (1)
Rings contain one oxygen atom (1)

Question 3

Complete the table below to identify differences between the structures of lactose and maltose.

Answer 3

Lactose
Contains β-glucose (1)
β-glycosidic bond (1)
Sugars in opposing orientation/flipped (1)

Maltose
Contains α-glucose (1)
α-glycosidic bond (1)
Both monomers in same direction (1)

Question 4

Explain how the structure of galactose allows it to be used as a respiratory substrate.

Answer 4

Bonds contain energy (1)
Bonds can be broken by respiratory enzymes (1)
Soluble so can move within cell (1)
H/OH groups can form H bonds with water (1)
AVP (1)

Question 5

E. coli usually grows in conditions where the extracellular concentration of lactose is low. In such
conditions lactose does not easily cross the bacterial cell surface membrane.
Suggest and explain why lactose is unable to cross membranes.

Answer 5

Too big (1)
Unable to pass through phospholipids (1)

OR

No/small concentration gradient (1)
Needs carrier protein (1)

Question 6

Give three properties of cellulose that make it suitable as the basis of plant cell walls

Answer 6

Insoluble (1)
Unreactive/inert (1)
High tensile strength (1)
Flexible (1)
Can form hydrogen bonds with neighbouring chains (1)

Question 7

Explain how the structure of glycogen differs from that of amylopectin to make it better suited as an energy store in animals.

Answer 7

Glycogen is…
More branched (1)
More coiled (1)
More compact (1)
Branching gives many/more free ends (1)
Where glucose can be added or removed (1)
Speeds up hydrolysis (1)