3.2: Carbohydrates, lipids and proteins

Question 1

List 3 examples each of monosaccharides, disaccharides and polysaccharides:

Answer 1

Monosaccharide: Glucose, fructose and galactose.
Disaccharide: Maltose, lactose and sucrose.
Polysaccharide: Starch, glycogen, cellulose.

Question 2

State one function of glucose, lactose and glycogen in animals, and of fructose, sucrose and cellulose in plants.

Answer 2

In animals, glucose is used as an energy source for the body and lactose is the sugar found in milk which provides energy to new borns until they are weaned. Finally, glycogen is used as an energy source (short term only) and is stored in muscles and the liver.

In plants, fructose is what makes fruits taste sweet which attracts animals and these then eat the fruits and disperse the seeds found in the fruits. Sucrose is used as an energy source for the plant whereas cellulose fibres is what makes the plant cell wall strong.

Question 3

Outline the role of condensation and hydrolysis in the relationship between monosaccharides, disaccharides and polysaccharides:

Answer 3

• Condensation (dehydration) reactions occur when molecules are covalently joined together and water is formed as a by-product
• In carbohydrates, the bond that is formed is called a glycosidic linkage
• The opposite of a condensation reaction is a hydrolysis reaction, which requires a water molecule to break a covalent bond between two subunits
• Monosaccharides are single monomers that are joined to form disaccharides, while sugars containing multiple subunits (more than 10) are called polysaccharides.

Question 4

Define Carbohydrates, lipids and proteins:

Answer 4

Carbohydrates: Are organic compounds consisting of one or more simple sugars that as monomers follow the general basic formula of (CH20)n.

Lipids: Are a group of organic molecules that are insoluble in water but soluble in non-polar organic solvents. Common lipids include triglycerides (fats and oils), phospholipids and steroids.

Proteins: Are large organic compounds made out of amino acids.

Question 5

Outline the role of condensation and hydrolysis in the relationship between fatty acids, glycerol and triglycerides:

Answer 5

• A condensation reaction occurs between the three hydroxyl groups of glycerol and the carboxyl groups of three fatty acids
• This reaction forms a triglyceride (and three molecules of water)
• The bond between the glycerol and the fatty acids is an ester linkage
• When one of the fatty acids is replaced by a phosphate group and phospholipid is formed
• Hydrolysis reactions will, in the presence of water, break these molecules down into their constituent subunits.

Question 6

State three functions of Lipids:

Answer 6

1. Lipids can be used for energy storage in the form of fat in humans and oil in plants.
2. Lipids can be used as heat insulation as fat under the skin reduces heat loss.
3. Lipids allow buoyancy as they are less dense than water and so animals can float in water.

Question 7

Compare the use of carbohydrates and lipids in energy storage:

Answer 7

Carbohydrates and lipids can both be used as energy storage however carbohydrates are usually used for short term storage whereas lipids are used for long term storage. Carbohydrates are soluble in water unlike lipids. This makes carbohydrates easy to transport around the body (from and to the store). Also, carbohydrates are a lot easier and more rapidly digested so their energy is useful if the body requires energy fast. As for lipids, they are insoluble which makes them more difficult to transport however because they are insoluble, lipids do not have an effect on osmosis which prevents problems within the cells in the body. They also contain more energy per gram than carbohydrates which makes lipids a lighter store compared to a store of carbohydrates equivalent in energy.

Question 8

State the names of the four bases in DNA:

Answer 8

1. Adenine
2. Thymine
3. Guanine
4. Cytosine