Topic 1---A: Biological molecules- 3. Polysaccharides

Question 1

What is a polysaccharide?

Answer 1

• They are carbohydrates
• Made from large numbers of their monomers (monosaccharides)
• There formed when two or more monosaccharides join together by condensation reactions.

Question 2

What are examples of polysaccharides?

Answer 2

• Starch
• Glycogen
• Cellulose

Question 3

How can polysaccharides be broken down into their constituent monosaccharides?

Answer 3

By hydrolysis reactions

Question 4

Starch

Answer 4

• Cells get energy from glucose
• Plants store excess glucose as starch so when a plant needs more glucose for energy, it breaks down starch to release the glucose.
• Most commonly found in leaves and storage organs.
• It is compacted into amyloplasts.
• It’s a mixture of two polysaccharides of alpha-glucose- amylose and amylopectin.

Question 5

Amylose

Answer 5

• Long unbranched chain of a-glucose
  -Has a coiled structure (helix) due to the angles of the glycosidic bond.
• This makes it compact so it’s really good for storage as you can fit more in a small space.
• 1-4 glycosidic bond
• 2 accessible ends

Question 6

Amylopectin

Answer 6

• Long, branched chain of a-glucose
• Branched structure increases surface area for rapid hydrolysis back to glucose.
• 1-4 and 1-6 glycosidic bond

Question 7

Properties of starch

Answer 7

• Insoluble in water so it doesn’t affect water potential so it doesn’t cause water to enter by osmosis which would make them swell.
• Large so it can’t escape
• Compact so can store lots of energy
• When hydrolysed it forms a-glucose which is used to meet energy requirements.

Question 8

Glycogen

Answer 8

• Animal cells get energy from glucose too.
• Animals store excess glucose as glycogen.
• Another polysaccharide of a-glucose.
• Located mainly in muscle and liver cells.
• 1-4 and 1-6 glycosidic bonds
• Structure is very similar to amylopectin but it has more side branches meaning enzymes that break amylopectin down can get to the glycosidic bonds easily meaning stored glucose can be released quickly.

Question 9

Properties of glycogen

Answer 9

• Compact molecule so it’s good for storage.
• Branched structure increases surface area for rapid hydrolysis
• Insobule so it won’t affect water potential

Question 10

Cellulose

Answer 10

• Made of long, unbranched chains of beta glucose
• When beta glucose molecules bond they form straight cellulose chains which are linked together by hydrogen bonds to form microfibrils.
• Strong fibres means glucose provides structural support for cells.
  -1-4 glycosidic bonds

Question 11

Cellulose structure

Answer 11

• Many hydrogen bonds provide collective strength
• Insoluble so won’t affect water potential

Question 12

Iodine test (starch)

Answer 12

• Add iodine dissolved in potassium iodide solution to the test sample.
• If there is starch present the sample changes from browny-orange to a dark blue-black colour