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A-level Biology Unit 1 > Unit 1.1 (Part Two) Carbohydrates > Flashcards

Unit 1.1 (Part Two) Carbohydrates Flashcards

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1
Q

What type of compounds are carbohydrates?

A

Carbohydrates are organic compounds.

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2
Q

What chemical elements do carbohydrates contain?

A

carbon, hydrogen and oxygen

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3
Q

What are monosaccharides?

A

The monomer units or building blocks of carbohydrates are called monosaccharides.

Monosaccharides are small organic molecules and provide the building blocks for larger carbohydrates.

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4
Q

How are disaccharides made?

A

Two of these basic units (monosaccharides) can be joined to form a disaccharide or double sugar.

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5
Q

How are polysaccharides made?

A

Polysaccharides are polymers formed from many hundreds of monosaccharides.

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6
Q

What are the properties of monosaccharides?

A

They are all water soluble and taste sweet.

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7
Q

What is the general formula of monosaccharides?

A

They have the general formula (CH2O)n.

Their names are determined by the number of carbon atoms in the molecule (n).

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8
Q

What is the name for monosaccharides with 3 carbons?

What are they important in?

A

trioses.

They have the formula of C3H6O3.

They are important in metabolism and respiration.

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9
Q

What is the name for monosaccharides with 5 carbons?

What are they important in?

A

pentoses

They have the formula of C5H10O5.

They are important in the formation of nucleic acids, e.g. ribose, deoxyribose.

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10
Q

What is the name for monosaccharides with 6 carbons?

A

hexoses

They have the formula of C6H12O6

(e.g. glucose, fructose, galactose)

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11
Q

How are the structures of glucose, fructose and galactose different?

A

They are all isomers of each other, meaning that they have a different structural formula.

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12
Q

Draw the structure of pentose.

A
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13
Q

How do the carbon atoms of monosaccharides exist as?

A

The carbon atoms of monosaccharides usually exist as a ring structure when the sugar is dissolved in water.

They can alter their binding to make straight chains, with the rings and chains in equilibrium.

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14
Q

How does glucose exist as?

A

Two structural isomers

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15
Q

Draw an alpha glucose molecule.

A
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16
Q

Draw a beta glucose molecule.

A
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17
Q

What is the difference between an alpha glucose molecule and a beta glucose molecule?

A

On carbon atom 1 in the alpha glucose, the hydrogen is above, the OH is below, whereas, in beta glucose its the other way around.

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18
Q

What do the difference in forms of alpha and beta glucose result in?

A

These different forms result in biological differences when they form polymers such as starch and cellulose.

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19
Q

List the functions of monosaccharides.

A

Monosaccharides have several functions and can act as:

  1. A source of energy in respiration. C-H and C-C bonds are broken to release energy (exergonic), which is transferred to make ATP.
  2. Building blocks for larger molecules. Glucose, is used to make the polysaccharides starch, glycogen and cellulose.
  3. Intermediates in reactions, e.g., trioses, are intermediates in the reactions of respiration and photosynthesis.
  4. Constituents of nucleotides, e.g. deoxyribose in SNA, ribose in RNA, ATP, and ADP.
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20
Q

List the properties of disaccharides

A

Disaccharides are water-soluble and taste sweet.

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21
Q

Tell me the structure of disaccharides.

A

They consist of two monosaccharide units bonded together by the formation of a glycosidic bond and the elimination of water.

This is called a condensation reaction.

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22
Q

Draw a diagram of the condensation reaction that takes place in the process of producing a maltose molecule.

A
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23
Q

How can a disaccharide molecule be reverted back to their original monomer units?

A

By a hydrolysis reaction where a molecule of water is chemically added, breaking the glycosidic bonds.

24
Q

How is maltose formed?

A

From the monosaccharides glucose and glucose by a condensation reaction.

25
Q

What is the biological role of maltose?

A

Stored in seeds.

26
Q

How is lactose formed?

A

From the monosaccharides glucose and galactose by a condensation reactions.

27
Q

What is the biological role of lactose.

A

In mammalian milk.

28
Q

How is sucrose formed?

A

From the monosaccharides glucose and fructose by a condensation reaction.

29
Q

What is the biological role of sucrose?

A

transported in the phloem.

30
Q

Is sucrose a reducing sugar?

A

No. sucrose is a non-reducing sugar, meaning that it remains blue when the Benedict’s test is performed.

31
Q

List the monosaccharides that are reducing sugars.

A
  1. Glucose
  2. Fructose
  3. Galactose
  4. Maltose
32
Q

How can we test for the presence of reducing sugars?

A

Heating the reducing sugar with Benedict’s reagent.

If a reducing sugar, such as glucose, is present, the solution will turn from blue through green, yellow, and orange and finally a brick-red precipitate forms.

33
Q

What does the colour of the precipitate depend upon?

A

The colour of the precipitate depends upon the concentration of the reducing sugar.

34
Q

What type of test is the Benedict’s reagent?

A

A qualitative test because it can only be used to indicate the concentration of reducing sugar present in a solution.

35
Q

How do we test for sucrose?

A
  1. Heating with hydrochloric acid. (hydrolyses the glycosidic bond)
  2. Neutralise by adding sodium hydrogen carbonate until it stops fizzing (This is because Benedict’s reagent needs alkaline conditions to work.)
  3. Boil with Benedict’s reagent and if the solution turns red then sucrose was initially present.

OR

Use sucrase, an enzyme that hydrolyses sucrose into glucose and fructose.
The Benedict’s test will then give a positive result.
(This only works for sucrose)

36
Q

How do we get an actual value to the concentration of sugar present?

A

Using a biosensor.

37
Q

What are polysaccharides made up of?

A

Polysaccharides are polymers made up of hundreds of monosaccharide units.

38
Q

What is a polymer?

A

A long chain of monomers.

39
Q

How are polysaccharides made?

A

A condensation reaction occurs between monosaccharide units which are held together by glycosidic bonds. The long molecules formed may be branched or unbranched.

40
Q

Tell me the properties of polysaccharides.

A
  1. They are insoluble.
  2. They are not sweet to taste
41
Q

What are the two groups of polysaccharides?

A
  1. Storage (e.g. starch, glycogen. These tend to be folded to give a compact molecule.)
  2. Structural (e.g. cellulose, chitin. These tend to be coiled or straight chained.)
42
Q

Why is glucose stored as a polysaccharide?

A
  1. Insoluble so are osmotically inert.
  2. Cannot diffuse out of the cell
  3. Is a compact molecule and can be stored in a small space.
  4. Carries a lot of energy in its
    C-H and C-C bonds.
  5. Glucose units can be easily added or removed easily by condensation or hydrolysis.
43
Q

What happens if glucose is not stored as polysaccharides?

A

It is soluble in water and so it would increase the concentration of the cell contents (lower the water potential), and consequently draw water in by osmosis.

44
Q

How is glucose stored in plant cells?

A

As Starch.

45
Q

Where are starch grains found in high concentrations?

A

Starch grains are found in high concentrations in seeds and storage organs such potato tubers.

46
Q

Tell me the structure of starch.

A

Starch is a mixture of two polysaccharides - amylose and amylopectin.
It is made of alpha glucose molecules bonded together in two different ways.

Amylose is a linear, unbranched molecule with alpha-1.4- glycosidic bonds forming between the C1 on one glucose monomer and the C4 on the adjacent one.
This is repeated, forming a chain, which coils into a alpha-helix, making it compact.

Amylopectin also has chains of glucose monomers joined with alpha-1,4-glycosidic bonds.
They are cross-linked with alpha-1,6-glycosidic bonds and fit inside the amylose.
When a glycosidic bond forms between the C1 atom on one glucose molecule and the C6 atom on another, a side branch is seen.
Alpha-1,4-glycosidic bonds continue on from the start of the branch.

Starch grains are made of successive layers of amylose and amylopectin that show up as growth rings.

47
Q

How do you test for the presence of starch?

A

Iodine turns from brown to blue-black in the presence of starch.

The depth blue-black colour gives an indication of relative concentration.

This is a qualitative test.

48
Q

Is the iodine test reliable?

A

As temperature increases, the colour intensity decreases so above about 35 degrees Celsius, this test is unreliable.

It is also unreliable at very low pH, as the starch is hydrolysed.

49
Q

How is glucose stored in humans?

A

As Glycogen.

50
Q

Where is glycogen stored?

Why?

A

In humans, it is stored in the liver and muscle cells.

During prolonged periods of exercise, blood glucose may fall, and glycogen is quickly converted to glucose to meet the body’s energy needs by insulin.

51
Q

Describe the structure of glycogen.

A

It is a long chain of alpha glucose, very similar in structure to amylopectin.

It has a alpha-1,4-glycosidic bond and alpha-1,6-glycosidic bonds.

The difference is that glycogen molecules have shorter alpha-1,4-linked chains and so are more branched than amylopectin.

Both starch and glycogen can be hydrolysed to a alpha-glucose, which is soluble and can be transported to where energy is needed.

52
Q

What is cellulose?

A

Cellulose is a structural polysaccharide found in plant cell walls.
It provides strength and rigidity.

53
Q

Tell me the structure of cellulose.

A

A polymer of beta glucose molecules with every other beta glucose rotated a 180 degrees.

It is a linear chain of beta-1,4-glycosidic bonds only

There are many chains lying parallel, cross-linked by hydrogen bonds between hydroxyl (OH) groups.

Between 60-70 cellulose molecules become tightly cross-linked to form microfibrils.
The microfibrils are in turn held in bundles called fibres.

A cell wall is made up of several layers of fibres, which run parallel to each other in a layer but at an angle to the adjacent layers.

This laminated structure also contributes to the strength of the cell-wall.

Cellulose fibres are freely permeable because there are spaces between the fibres.
Water and its solutes can penetrate through these spaces in the cell-wall, to the cell membrane.

54
Q

What is chitin?

A

A structural polysaccharide found in the exoskeleton of insects and in fungal cell walls.

55
Q

Tell me about the structure of chitin.

A

It resembles cellulose, with long chains of beta-1,4-linked monomers, but has groups derived from amino acids added replacing a hydroxyl group to form a heteropolysaccharide.

It is strong, waterproof and lightweight.

Like cellulose, the monomers are rotated through 180 degrees and the long parallel chains are cross-linked to each other by hydrogen bonds, forming microfibrils.

A-level Biology Unit 1 (3 decks)

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