Carbohydrates

Question 1

Test for reducing sugars

Answer 1

• Add food sample to test tube (if not liquid, grind in water).
• Add equal volume of Benedict’s reagent.
• Heat in gently boiling water for 5 minutes
  -positive test shows brick red solution

Question 2

Test for reducing sugars

Answer 2

• Food sample (in liquid form) added to equal part Benedict’s reagent. Placed in boiling water bath for 5 minutes. If no colour change then no reducing sugar.
• Add another sample to dilute HCl, placed in boiling water bath for 5 mins to acid hydrolyse any poly or disaccharides into monomers.
• Slowly add sodium hydrogen carbonate solution to the test tube to neutralise acid and test with litmus paper.
• Redo test for reducing solution. Brick red colour shows original sample had non reducing sugars.

Question 3

Test for starch

Answer 3

• Add iodine/potassium iodide solution to sample.
• Positive test shows blue black colour.

Question 4

Carbohydrates

Answer 4

Molecules consisting of only C, H and O.
Saccharides joined by glycosidic bonds in a condensation reaction.

Question 5

Disaccharides

Answer 5

Maltose - two alpha glucose molecules
Sucrose - alpha glucose + fructose
Lactose - alpha glucose + galactose
Formed by condensation reaction forming 1-4 glycosidic bonds.

Question 6

Glycogen

Answer 6

-Main energy storage molecule in animals.
- Polysaccharide of alpha glucose joined by 1-4 and 1-6 glycosidic bonds.
- Highly branched so enzymes can act on multiple branches simultaneously.
- Very compact maximising energy storage in small space.
- Insoluble so doesn’t affect water potential of cells and cannot diffuse out.
- Stored as small granules mainly in muscles and the liver
- More highly branched than starch as animals have a higher metabolic rate requiring higher release of glucose monomers for respiration.

Question 7

Starch

Answer 7

• Energy storage in plants
• Mixture of amylose and amylopectin
• Large and insoluble so doesn’t affect water potential and cannot diffuse in or out of cells.
• When hydrolysed, alpha glucose is easily transported and readily used in respiration.

Question 8

Amylose

Answer 8

• Unbranched chain of alpha glucose joined by 1-4 glycosidic bonds.
• Helical structure therefore very compact.

Question 9

Amylopectin

Answer 9

• Branched molecule comprised of alpha glucose joined by 1-4 and 1-6 glycosidic bonds.
• Branches structure allows enzymes to act on multiple ends simultaneously.

Question 10

Cellulose

Answer 10

• Component of cell walls in plants.
• Long, unbranched chains of beta glucose joined by glycosidic bonds.
• Adjacent monomers are flipped 180 degrees allowing hydrogen bonds to form between hydroxyl groups on adjacent parallel chains.
• Many hydrogen bonds contribute to overall structural strength.
• Cellulose molecules are grouped to form micro-fibrils which are arranged to form fibres to provide more strength.
• Stops plant cells bursting due to osmotic pressure by exerting inward pressure stopping influx of water, maintaining turgidity and rigidity of cell, maximising SA for photosynthesis.