Lesson 7 - Polysaccharides (carbohydrates)

Question 1

What are polysaccharides?

Answer 1

● Polymers formed from many monosaccharides joined together by glycosidic bonds (many condensation reactions)
● When hydrolysed, they break down into disaccharides/monosaccharides
● They generally perform one of two functions:
- Energy store
- Structural role

Question 2

Key info about starch

Answer 2

● Found in plant cells in the form of small granules or grains
● Starch’s main role = energy storage in plants
● Made of 𝛂 glucose monomers
● Structure = coiled & branched (due to the presence of 1-4 and 1-6 glycosidic bonds)

Question 3

What are the 2 types of starch?

Answer 3

Amylose
Amylopectin

Question 4

Amylose

Answer 4

● Formed from 𝛂 glucose monomers
● 1,4 glycosidic bonds between the monomers
● The chain forms a coiled shape

Question 5

Amylopectin

Answer 5

Formed from 𝛂 glucose monomers
𝛂 1,4 AND 𝛂 1,6 glycosidic bonding between the monomers
Branched shape due to the extra 𝛂 1,6 glycosidic bonding

Question 6

What’s the test for starch?

Answer 6

● Add iodine in potassium iodide to test solution
● Shake or stir
● If starch is present, the solution will change from yellow/orange to blue-black

Question 7

Glycogen - key info

Answer 7

● Found in animal cells
● Formed from the monosaccharide 𝛂 glucose
● Has a very similar structure to Amylopectin in Starch
● BUT there is less 𝛂 1,4 - glycosidic bonds and more 𝛂 1,6 glycosidic bonds which results in it having a very branched structure
● It’s main role - energy storage in animals
● Also stored in small granules like starch, found mainly in the muscles and liver (energy hungry organs)

Question 8

Cellulose

Answer 8

● Role - structural role (provides strength) found in plant cell walls
● Formed from the monosaccharide 𝛃 glucose
● This small change (OH group on C1) causes huge differences in the structure and function
Cellulose
● Every other glucose molecule is rotated (flipped) by 180° to allow the glycosidic bonds to form between the 𝛃 glucose molecules
● Straight, unbranched chains are formed

Question 9

Glycogen structure related to function

Answer 9

1) helix/coiled/branched so compact;
2) polymer of glucose so easily hydrolysed
3) branched, so more ends for faster hydrolysis
4) glucose (polymer) so provides respiratory substrate/used in respiration for energy (release)
5) insoluble so doesn’t affect water potential

Question 10

Starch structure related to function

Answer 10

1) helical/coil shape so compact
2) large (molecule)/insoluble so osmotically inactive
3) branched so glucose is easily hydrolysed/released for respiration
4) large (molecule) so can’t leave cell/cross cell-surface membrane

Question 11

Cellulose structure

Answer 11

1) long, straight/unbranched chains of B-glucose
2) joined by hydrogen bonding
3) to form (micro/macro) fibrils
4) provides rigidity/strength

Question 12

Function of cellulose in cell walls

Answer 12

• Provides strength in cell walls
• Prevents plant cells bursting when water enters by osmosis
• Maintains turgidity and rigidity in plant cells and therefore in the whole plant (stems and leaves)
• Maximises surface area for light absorption for photosynthesis