3.1.2 Carbohydrates

Question 1

What is a monosaccharide?

Answer 1

A single monomer of a carbohydrate

Question 2

What is a disaccharide?

Answer 2

Sugar formed when two monosaccahrides are joined by glycosidic linkage (Pair of monosaccharides)

Question 3

What is a polysaccharide?

Answer 3

A chain of monosaccharides joined together by glycosidic bonds

Question 4

What is the general formula for monosaccharides?

Answer 4

(CH^2O)n where n is ≥ 3 and counts for the number of carbons

Question 5

What can monosaccharides be classified by?

Answer 5

the number x of carbons present: 3(triose) 4 (tetrose) 5(pentose) 6(hexose) 7(heptose) etc

Question 6

What are some features of monosaccharides?

Answer 6

They are sweet tasting and water soluble. Their atoms usually form a ring and are hydrophilic.

Question 7

What is meant by the term hydrophilic?

Answer 7

A substance that is attracted to water and diffuses in water

Question 8

What enables their solubility?

Answer 8

They have a large number of OH groups (hydroxyls) and can form hydrogen bonds with H^2O molceules

Question 9

Give an example of some monosaccharides

Answer 9

Glucose, fructose, galactose

Question 10

What are the four types of glycosidic bonds?

Answer 10

1-2 beta, 1-4 beta, 1-4 alpha 1-6 alpha

Question 11

What are the two forms of glucose and what is the difference?

Answer 11

The alpha and beta isomers (isomers have same molecular formula, different arrangement of atoms). In alpha glucose, the hydroxyl group points below the ring while in beta glucose, it points above the ring.

Question 12

What does the energy glucose releases during cellular respiration make?

Answer 12

ATP (Adenosine triphosphate) which is the energy currency of the cell, it provides energy for reactions in living organisms

Question 13

What disaccharide is formed by glucose + glucose

Answer 13

Maltose

Question 14

What disaccharide is formed by glucose + fructose?

Answer 14

Sucrose

Question 15

What disaccharide is formed by glucose + galactose?

Answer 15

Lactose

Question 16

How are disaccharides formed?

Answer 16

By the condensation reaction between monosaccharides with water as a byproduct

Question 17

What is a glycosidic bond?

Answer 17

A type of covalent bond that joins a carbohydrate molecule to another group, which may or may not be another carbohydrate

Question 18

How are disaccharides broken down?

Answer 18

Through hydrolysis

Question 19

How are polysaccharides formed?

Answer 19

When three or more monosaccharides are joined through condensation reactions

Question 20

What monosaccharides make up starch?

Answer 20

(Alpha) glucose

Question 21

What makes starch good for storage?

Answer 21

It’s helical shape (amylopectin)- which makes it compact and enables it to fold so lots can be stored in a small space.

Question 22

What are the two types of glucose polymer in starch and what is the difference?

Answer 22

Amylose and amylopectin. Amylose linked by 1,4-glycosidic bonds and is long unbranched, perfect for energy release. Amylopectin linked by α-1,4-glycosidic bonds and α-1,6-glycosidic bonds and is long and branched and has a helical shape for storage.

Question 23

What makes starch suitable for its role?

Answer 23

Insoluble- won’t draw out water easily by osmosis, changing water potential. Large size-doesn’t easily diffuse out of cell. Hydrolysed into glucose- easily transported and regularly used in respiration, Compact- lots stored in small space.

Question 24

What type of cells are starch and cellulose only found in?

Answer 24

Plant cells

Question 25

What is the chemical formula of monosaccharides?

Answer 25

C6H12O6

Question 26

What is the chemical formula of disaccharides?

Answer 26

C12H22011

Question 27

What is the shape of cellulose molecules?

Answer 27

Cellulose is a straight chain polymer. Unlike starch, no coiling or branching occurs and the molecule adopts an extended and rather stiff rod-like conformation

Question 28

What are microfibrils and what’s its purpose?

Answer 28

Long, straight cellulose chains joined by hydrogen bonds that provide rigidity for cell

Question 29

(Exam Question) Why is the rise of glucose in the blood slower if the carb is starch rather than sucrose

Answer 29

The digestion of starch is more complex than the digestion of sucrose. Sucrose is digested by a one step process involving the enzyme sucrase which breaks it down to form glucose and fructose via hydrolysis. Starch breakdown has broadly two steps. The first involves the enzymes amylase which breaks it down to maltose. The second step results in the production of the final product, glucose, which is formed using maltase which breaks down the maltose to give glucose.

Question 30

Describe the function and structure of cellulose

Answer 30

A polymer of beta glucose which gives rigidity to plant cell walls (prevents bursting under turgor pressure, holds up stem). 1)1,4 glycosdic bonds 2) Long unbranched, straight chain molecule 3)Alternate glucose molecules rotated 180degrees 4)H-bond crosslinks between parallel strands form microfibrils- high tensile strength

Question 31

What happens when a polysaccharide is hydrolysed?

Answer 31

It is broken down into monosaccharides and disaccharides

Question 32

Describe the test for reducing sugars?

Answer 32

Benedict’s Test 1) Add 2cm^3 of the food sample to a boiling tube 2)Add an equal volume of Benedict’s regenant 3)Heat the solution in a water bath for 3-5 minutes 4) Observe for a colour change

Question 33

What do the different colours indicate?

Answer 33

Blue- no reducing sugars Green-Low conc of reducing sugars Orange- Moderate Red- High conc.

Question 34

How does the colour change occur?

Answer 34

Reducing property of simple carbohydrates. The copper (II) ions in the Benedict’s solution are reduced to Copper (I) ions, which causes the colour change. The blue copper sulphate which is reduced to form an insoluble red precipitate of copper oxide

Question 35

What is Benedict’s solution?

Answer 35

A clear blue liquid that changes colour and gives a precipitate depending on how much is reduced

Question 36

What is cellulose? What must be done before condensation reactions can occur to form cellulose?How is the structure of cellulose different to starch? What is structurally different that gives them different physical properties? Why is it impossible for humans to digest food containing cellulose? Cellulose is a major component of…
What does cellulose provide for and how ?

Answer 36

Cellulose is formed by beta 1-4 glycosidic linkages and crosslinks, making it rigid. Starch has alpha 1-4 and 1-6 glycosidic linkages without the tight crosslinks of cellulose.

Human digestive enzymes cannot break down the beta 1-4 glycosidic bonds in cellulose as the enzyme that can is absent in humans

Question 37

What is glycogen? Features, why suitable, where made etc

Answer 37

Glycogen is found in animals and bacteria but never in plant cells.
Glycogen is very similar in structure to starch but has shorter chains and is more highly branched.
It is sometimes called ‘animal starch’ because it is the major carbohydrate storage product of animals.
Major (but small) stores in liver and muscle cells
Most alpha gluc joined by 1,4 glycosidic bonds, branch point 1,6

• it is insoluble and therefore does not tend to draw water into the
cells by osmosis
• being insoluble, it does not diffuse out of cells
• it is compact, so a lot of it can be stored in a small space
• It is more highly branched than starch and so has more ends that can be acted on simultaneously by enzymes. It is therefore more rapidly broken down to form glucose monomers, which are used in respiration. This is important to animals which have a higher metabolic rate and therefore respiratory rate than plants because
they are more active.