Carbohydrates

Question 1

What elements make up carbohydrates?

Answer 1

C H O

Question 2

What are the tests for reducing and non-reducing sugars?

Answer 2

• benedict’s test

- reagent strips test

Question 3

Explain benedict’s test (reducing sugars)

Answer 3

• glucose solution in test tube, add a few drops of benedict’s solution, stand tube in 100 degrees C water bath
• positive result = blue to brick-red precipitate
• reducing sugar reduces copper (II) ions in Benedict’s solution to copper (I) oxide

Question 4

Explain the test for non-reducing sugars

Answer 4

• sucrose solution in test tube
  3 drops of dilute hydrochloric acid
  shake
  place in 100 degrees C water bath for 5 mins
  allow to cool
  3 drops of dilute sodium hydroxide + mix (neutralise)
  repeat reducing sugar test
• positive result = blue to brick-red precipitate
• acid hydrolyses sucrose into glucose + fructose (positive)

Question 5

Explain the test for starch

Answer 5

• drop of starch solution in depression on spotting tile
  add drop of iodine solution
• positive result = orange to blue-black precipitate
• coloured polyiodide complex formed in presence of starch

Question 6

Explain reagent strip test for reducing sugars

Answer 6

• quantitative measure of reducing sugars in a substance

- colour-coded chart used to approximate the concentration of sugars

Question 7

Define monosaccharide

Answer 7

small organic molecules used as building blocks for more complex substances (carbohydrates)

Question 8

What is the general formula for a monosaccharide?

Answer 8

(CH2O)n

Question 9

Define disaccharide

Answer 9

two monosaccharides joined together by a glycosidic bond

Question 10

How is a disaccharide synthesised?

Answer 10

CONDENSATION REACTION

• two monosaccharides bond together
• 2 alpha glucose next to each other = 2 hydroxyl groups react
• OH from 1 and OH from 4 join to produce water molecule
• leftover O atom bonds two monosaccharides together and this is the 1- 4 glycosidic bond (covalent)

Question 11

How is a disaccharide broken down?

Answer 11

HYDROLYSIS REACTION

• addition of water molecules
• breaks glycosidic bond and O joins with one of the H from water
• 2 individual monosaccharides created

Question 12

What is the difference between a hexose and a pentose monosaccharide?

Answer 12

• hexose contains 6 carbon atoms and pentose contains 5 carbon atoms
• hexose has more stereoisomers than pentose

Question 13

What is sucrose?

Answer 13

a disaccharide formed by 2 monosaccharides:

• glucose and fructose
• glycosidic bond at carbon 1 and carbon 2
• known as cane sugar or just sugar

Question 14

What is maltose?

Answer 14

a disaccharide formed by 2 monosaccharides:

• 2x alpha glucose
• glycosidic bond at carbon 1 and carbon 4
• wheat, cornmeal, barley and other ancient grains

Question 15

What is lactose?

Answer 15

a disaccharide formed by 2 monosaccharides:

• alpha glucose and beta galactose
• glycosidic bond at carbon 1 and carbon 4
• commonly found in milk and milk products

Question 16

What is ribose and deoxyribose?

Answer 16

pentose monosaccharides that are important components of biological molecules:

• ribose is present in RNA nucleotides
• deoxyribose is present in DNA nucleotides

Question 17

What is starch?

Answer 17

• found in plants
• alpha glucose joined by glycosidic bonds form two slightly different types of polysaccharide - collectively known as starch
• glucose = product of photosynthesis + stored as starch

Question 18

What is amylose?

Answer 18

• type of starch
• formed by alpha glucose molecules by 1,4 glycosidic bond only
• bond is angled; long chain forms helix, stabilised by by H bonds
• polysaccharides = more compact + less soluble than glucose molecules alone
• preferable use of energy storage in plants (grains in chloroplast + nucleus & lysosomes in chloroplasts)

Question 19

What is amylopectin?

Answer 19

• type of starch
• formed by alpha glucose molecules with 1,6 glycosidic bond or 1,4 glycosidic bonds
• branched-chain polymer
• branching takes place every 25 glucose subunits
• compact + insoluble
• energy store for grains in chloroplast + nucleus & lysosomes in chloroplasts

Question 20

What is glycogen?

Answer 20

• secondary long-term energy store in animals + fungi
• found in liver, muscle and kidney cells
• formed by condensation reactions of glucose molecules, glycosidic bond between carbons 1,4
• branching takes place approximately every 10 glucose subunits via 1,6 glycosidic linkages
• less branching than amylopectin = more compact = less space needed for storage (good for mobile animals)
• branching = lots of free ends for glucose molecules to be added / removed = speeds up storing / releasing glucose molecules required by the cell
• insoluble, compact and branched

Question 21

What is cellulose?

Answer 21

• formed by beta glucose molecules, which cannot join together as easily as alpha molecules can - alternate beta molecule is flipped upside down
• unable to branch / coil = straight-chain polymer
• H bonds with each other = microfibrils = fibres
• strong and insoluble
• used in cell walls
• part of our diet - hard to break down into monomers + forms ‘fibre’ / ‘roughage for a healthy digestive system