3.3 : Carbohydrates

Question 1

What are carbohydrates? General formula?

Answer 1

Molecules that only contain the elements carbon, hydrogen and oxygen. The elements in carbohydrates usually appear in the ration Cx(H2O)y.

Question 2

Define the terms monosaccharide, disaccharide and polysaccharide. Give examples.

Answer 2

Monosaccharide - a simple sugar unit e.g. glucose, fructose and ribose

Disaccharide - double sugars, formed from two monosaccharides e.g. lactose and sucrose

Polysaccharide - polymers, formed from two or more
monosaccharides e.g. glycogen, cellulose and starch

Question 3

What kind of monosaccharide is glucose? Molecular structure?

Answer 3

It is a monosaccharide composed of 6 carbons and therefore it is a hexose monosaccharide. Formula : C6H12O6

Question 4

What is triose, pentose, hexose etc?

Answer 4

Triose = 3 carbon atoms 
Pentose = 5 carbon atoms 
Hexose = 6 carbon atoms

Question 5

Why are glucose molecules polar and soluble in water? Why is this important?

Answer 5

This is due to the hydrogen bonds that form between the hydroxyl groups and water molecules. This is important as it means that glucose is dissolved in the cytosol of the cell.

Question 6

What are the two types of glucose molecule?

Answer 6

The two structural variations are alpha and beta. The difference is that the OH group on carbon 1 is in opposite positions. These are called structural isomers.

Question 7

Give some examples of trioses, pentoses, and hexoses.

Answer 7

Triose : glyceraldehyde
Pentose : ribose, deoxyribose
Hexose : glucose, fructose

Question 8

What are the two main functions of carbohydrates?

Answer 8

source of energy

structural function e.g. Cellulose -> cell walls, ribose -> nucleic acids

Question 9

How do two alpha glucose molecules bond?

Answer 9

When they are side by side , the two hydroxyl groups interact. When this happens , two hydrogen atoms and an oxygen atom are removed and join to form water. A bond form between carbons 1 and 4 on the glucose and the molecules are now joined.

Question 10

What kind of bond do the two alpha glucose molecules form and what is the reaction called?

Answer 10

A covalent bond called a glycosidic bond is formed. The reaction is called a condensation reaction because a water molecule is formed. As carbon 1 of one molecule is joined to the carbon 4 of the other, this bond is specifically called a 1,4 glycosidic bond.

Question 11

What does the bonding of two alpha glucose molecules form?

Answer 11

Maltose - a disaccharide

Question 12

Give some information about the different sugars.

Answer 12

• fructose naturally occurs in fruit
• fructose and glucose make sucrose which known as cane sugar or just sugar
• galactose and glucose make lactose, which is commonly found in milk and milk products.
• fructose is sweeter than glucose and glucose is sweeter than galactose
• ribose ( a pentose ) is the sugar present in RNA nucleotides and deoxyribose is the sugar present in DNA nucleotides