B Biological Molecules

Question 1

How are amino acids bonded together?

Answer 1

• By a condensation reaction between the amine and carboxyl groups of two amino acids, forming a peptide bond.
• Results in a dipeptide, then a polypeptide.

Question 2

What is the primary structure of a protein?

Answer 2

The sequence of amino acids in a polypeptide.

Question 3

What is the secondary structure of a protein?

Answer 3

Folding of the polypeptide chain, due to forming hydrogen bonds between groups of peptide bonds, which forms either an alpha helix or a beta pleated sheet.

Question 4

What is the tertiary structure of a protein?

Answer 4

• The 3D folding of the polypeptide chain, caused by hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions and hydrophilic interactions between side (R) groups.
• Hydrophobic side groups position themselves to the inside of the protein, while hydrophilic side groups position themselves to the outside of the protein.

Question 5

What is the quaternary structure of a protein?

Answer 5

When 2 or more polypeptide chains are joined together.

Question 6

What are globular proteins (and examples)?

Answer 6

• Globular proteins have a spherical shape.
• They contain hydrophilic side groups on their outsides, meaning they are soluble in water.
• Eg 1. insulin: because it is soluble in water, insulin can diffuse through the cell/blood vessels and bind to it’s specific receptor to regulate blood glucose.
• Eg 2. protease: soluble in water so is able to diffuse through the cell and bind to it’s specific substrate to breakdown the peptide bond.

Question 7

What are globular conjugated proteins (and examples)?

Answer 7

• Globular proteins that have a prosthetic group (non-amino acid part of a proteins structure).
• Eg 1. haemoglobin: has a quaternary structure made of 4 polypeptide chains, each containing a haem group (which contains iron).
• Eg 2. Glycoproteins and lipoproteins have proteins and lipids as prosthetic groups, so are also conjugated proteins.

Question 8

What are fibrous proteins?

Answer 8

• Rod-like shaped proteins made up of long and strong fibres.
• Contain lots of hydrophobic side groups so are insoluble in water.
• Eg 1. collagen: found in bones, tendons and walls of arteries to strengthen them.
• Eg 2. elastin: found in cartilage, alveoli and walls of arteries to make them more flexible.
• Eg 3. keratin: found in skin, hairs and nails, strengthening them.

Question 9

What is the chemical test for proteins?

Answer 9

• Add the sample to a test tube, then add an equal volume of NaOH (to create alkaline conditions).
• Add a few drops of copper sulfate solution and mix gently.
• If proteins are present, the sample will turn purple (a negative test and it will remain blue).

Question 10

What are fatty acids composed of?

Answer 10

A hydrocarbon chain with a carboxyl group at the end.

Question 11

What is the structure of a triglyceride?

Answer 11

• 3 fatty acids, joined to an additional unit (glycerol).
• Glycerol has 3 OH groups, which react in condensation reactions with the OH from the carboxyl groups of 3 fatty acids to form 3 ester bonds and a triglyceride.
• Triglycerides can be broken down by water in hydrolysis reactions.
• The 3 fatty acids can be any length, and can be saturated or unsaturated.

Question 12

Compare the properties of triglycerides with saturated and unsaturated fatty acids

Answer 12

• Fats (eg. butter) are made of triglycerides with saturated fatty acids (straight, parallel hydrocarbon chains), meaning they don’t take up a lot of space and can be packed tightly together, forming a solid black at cool temperatures.
• Oils are made of triglycerides with unsaturated fatty acids, which have bends in their hydrocarbon chain. Therefore they take up a lot of space and cannot be tightly packed together, forming a liquid at cool temperatures.

Question 13

What are the properties of fatty acids (and therefore triglycerides)?

Answer 13

• Due to the hydrocarbon chain, the ratio of hydrogens and carbons to oxygens is much higher than in carbohydrates.
• They are large, non-polar molecules.
• Therefore triglycerides are insoluble in water, meaning they are hydrophobic.

Question 14

What are the functions of triglycerides?

Answer 14

1. They enable the release of large amounts of energy via respiration - due to the fatty acids having a high C+H : O ratio.
2. They are a good energy storage molecule - since triglycerides have a low mass:energy ratio, they are large so cannot cross the cell surface membrane, and are insoluble, so do not affect the water potentials of cells.
3. Provide insulation - triglycerides beneath our skin form a layer that provides thermal insulation. And triglycerides wrapped in layers around nerve cells (Schwann cells) provide electrical insulation.

Question 15

What is the structure of phospholipids?

Answer 15

A glycerol molecule, with two fatty acids attached with ester bonds, and a phosphate group.

Question 16

What are the properties of phospholipids?

Answer 16

• Contains a polar head, and non-polar fatty acid tails. Therefore the head of the phospholipid is hydrophilic, while the tails are hydrophobic.
• In water, phospholipids can form oil droplets (with hydrophilic heads facing outwards and hydrophobic heads facing inwards) called a micelle, a bilayer (tails facing inwards and heads facing outwards) or a monolayer (on the surface of the water).
• Therefore phospholipids are insoluble in water.

Question 17

What are functions of phospholipids?

Answer 17

• Cell-surface membranes and membranes around organelles are made of phospholipid bilayers.
• They separate the environments inside and outside, therefore they control the movement of molecules taken in and out of cells/organelles.
• Phospholipids make cell-surface membranes flexible: since they contain a mixture of saturated and unsaturated fatty acids, and unsaturated fatty acids cannot be tightly packed together, so the cell-surface membrane is a bendy flexible surface, therefore meaning small, non-polar molecules can pass through the gaps created by unsaturated fatty acids.
• Phospholipids can combine with a carbohydrate to form glycolipids, which stick out of the membrane and mark cells as self. it these were not present the body may assume it is a foreign cell and try to kill it (therefore phospholipids enable cell recognition).

Question 18

Are lipids soluble in alchohol?

Answer 18

Triglycerides and phospholipids are soluble in alcohol since alcohol also have a polar and non-polar region, so alcohols can mix with polar and non-polar substances.

Question 19

What is the chemical test for lipids?

Answer 19

Emulsion test:
1. Add 2cm3 of food sample to a dry, grease-free test tube.
2. Add 5cm3 of alcohol to the food sample and shake.
3. Lipids are soluble in alcohol, therefore use a syringe to remove the clear solution (lipids and alcohol), and add this to 5cm3 water in a new test tube and shake gently.
4. If a milky emulsion forms, it is a positive test for lipids.