3.3- Carbohydrates

Question 1

What are carbohydrates?

Answer 1

• molecules that only contain carbon, hydrogen and oxygen.
• general formula = Cn(H2O)n
• also known as saccharides.

Question 2

What are mono, di and poly saccharides (with examples)?

Answer 2

• Monosaccharides= single sugar unit. Eg. Glucose, fructose, ribose.
• Disaccharides = 2 monosaccharides linked together. Eg. Lactose, sucrose.
• Polysaccharides= more than 2 monosaccharides linked together. Eg. Glycogen, cellulose, starch.

Question 3

Glucose.

Answer 3

Chemical formula = C6H12O6
- composed of six carbons = hexose monosaccharide.

Two structural variations:

• alpha glucose (OH down)
• beta glucose (OH up)
• glucose molecules are polar and soluble in water, due to hydrogen bonds that form between the hydroxyl group and water molecules.
  Solubility in water is important as it means glucose is dissolved in the cytosol of cells.

Question 4

Explain the condensation reaction of two alpha glucose molecules.

Answer 4

When two alpha glucose molecules are side by side, two hydroxyl groups react. -Bonds are broken and are reformed in different places.

• two hydrogen atoms and an oxygen atom are removed from the glucose monomers and join to form a water molecule.
• a bond forms between carbons 1 and 4 on the glucose molecules = 1,4 glycosidic bond.
• maltose is formed (disaccharide)

Question 5

What makes up sucrose and lactose?

Answer 5

Fructose + glucose = sucrose (sugar)

Galactose + glucose = lactose (milk)

Fructose and galactose = hexose monosaccharides.

Question 6

What are the two main pentose monosaccharides?

Answer 6

1. Ribose (the sugar present in RNA nucleotides)

2. Deoxyribose ( the sugar present in DNA nucleotides)

Question 7

Amylose.

Answer 7

• one of the polysaccharides in starch.
• formed by alpha glucose molecules joined together by only 1,4 glycosidic bonds.
• this long chain of glucose forms a helix which is stabilised by hydrogen bonding. This makes the polysaccharide more compact and less soluble than the glucose molecules used to make it.

Question 8

Amylopectin.

Answer 8

• another type of starch polysaccharide.
• formed when glycosidic bonds form in condensation reactions between carbon 1 and 6 on two glucose molecules.
• contains both 1,4 and 1,6 glycosidic bonds.
• therefore has a branched structure:
  the 1,6 branching point occur approximately once every 25 glucose subunits.

Question 9

What is glycogen and its structure

Answer 9

• is the energy store molecule in animals and fungi.
• more branched than amylopectin so it is more compact and less space is needed for it to be stored.
• the branching means that there are many free ends where glucose molecules can be added/removed which speeds up the process of releasing/storing glucose molecules.

Question 10

How are glucose stores released?

Answer 10

(stored as starch by plants and glycogen by animals and fungi until needed).

To release glucose a hydrolysis reaction takes place. requires the addition of water and bonds are broken.
The reaction is catalysed by enzymes.

Question 11

Cellulose

Answer 11

• beta glucose molecules are unable to join together in the same way as alpha glucose molecules.
• due to this every second beta glucose monomer is flipped upside down.
• this forms 1,4 glycosidic bonds.
• when a polysaccharide is formed in this way it is unable to coil or form branches.

Instead cellulose is formed which is a straight chain molecule. (Unbranched)

Question 12

Explain the formation of cellulose fibres.

Answer 12

• cellulose molecules make hydrogen bonds with eachother forming microfibrils.
• these microfibrils join together forming macro-fibrils which then combine to form fibres
• these fibres are strong and insoluble and are used to make cell walls.
• cellulose is an important part of our diet as fibres are hard to break down and are necessary for a healthy digestive system