Biological Molecules

Question 1

What are the five properties of water and what are their biological importance?

Answer 1

1. High specific heat capacity - Allows water to act as a buffer against sudden temperature changes.
2. Large latent heat of vaporization - Provides a cooling effect with little loss of water through evaporation
3. Strong cohesion and surface tension in water - Allows water to be pulled up xylem tissue in continuous columns in plants.
4. Solvent - Metabolic reactions can take place faster in solution. Substances can be easily transported around an organism.
5. Metabolite - Involved in many metabolic reaction, including condensation and hydrolysis reactions. Used in photosynthesis.

Question 2

What is a polymer?

Answer 2

A molecule made of many monomers (repeating units) joined together.

Question 3

What reaction joins monomers together?

Answer 3

Condensation reaction.

Question 4

What reaction breaks polymers down into monomers?

Answer 4

Hydrolysis.

Question 5

What is the difference between Α glucose and β glucose?

Answer 5

On carbon-1, the hydroxyl (-OH) group points upwards/above ring in β glucose. It points downwards in A glucose.

Question 6

What is maltose?

Answer 6

A disaccharide formed by condensation of two glucose molecules.

Question 7

What is sucrose?

Answer 7

A disaccharide formed by condensation of glucose and fructose.

Question 8

What is lactose?

Answer 8

A disaccharide formed by condensation of glucose and galactose.

Question 9

How is a disaccharide formed?

Answer 9

Two monosaccharides link together to form a disaccharide in a condensation reaction. A glycosidic bond is formed and water is released.

Question 10

What is the difference between condensation and hydrolysis reactions?

Answer 10

Condensation reactions produce water and involve making bonds.

Hydrolysis reactions require water and break bonds.

Question 11

What makes starch and glycogen good for storage?

Answer 11

• Coiled so compact so store lots of glucose in a small space.
• Large and insoluble so cannot diffuse out of the cells in which they are stored in.
• Insoluble and have no osmotic effects.
• Branched so quickly hydrolysed to provide glucose for respiration. The branching means many ends are available for enzymes.

Question 12

What is the structure of cellulose and how does it make it suited for its function?

Answer 12

• Made up of straight unbranched chains that run parallel to each other and many hydrogen bonds form cross-linkages between chains.
• Many hydrogen bonds are collectively strong, so provide a high tensile strength.
• Makes the cell wall rigid and prevents osmotic lysis.
• Cellulose molecules are grouped to form microfibrils.

Question 13

What is the test for starch?

Answer 13

Add iodine solution. If starch is present, the colour changes from brown/orange to blue-black.

Question 14

What is the test for reducing sugars?

Answer 14

1. Add Benedict’s solution.
2. Heat.
3. If a reducing sugar is present, the colour of the sample will change to red.

Question 15

What is the test for non-reducing sugars?

Answer 15

1. Heat the sample with Benedict’s solution. If there is no colour change, a reducing sugar is not present, but a non-reducing sugar might be.
2. Boil a fresh sample with dilute acid such as hydrochloric acid.
3. Neutralise such as with an alkali.
4. Re-heat the sample with Benedict’s solution.

Question 16

What are some issues with the Benedict’s test?

Answer 16

• Non-specific: a positive result just tells us that a reducing sugar is present, but it doesn’t tell us which one. However, a biosensor can be used to test for specific sugars.
• Test is qualitative: colour change is used to determine the result, so we cannot obtain a value for the concentration of reducing sugar. However, it is only semi-qualitative as we can estimate how much reducing sugar is present by the colour change.
• Subjective: colorimeter can be used to quantify results. A serial dilution of a glucose solution can be carried out, producing a set of solutions of known concentrations. Benedict’s solution can be performed on each of these and colour intensity can be measured using a colorimeter. A calibration curve can be produced from the results.

Question 17

What is the role of inorganic ions in the body?

Answer 17

Hydrogen ions: Affects pH levels.

Iron ions: Component of haemoglobin in red blood cells.

Phosphate ions: Component of DNA, ATP and phospholipids.

Sodium ions: Involved in the absorption of glucose and amino acids in the small intestine by cotransport.

Question 18

What are all amino acids made of?

Answer 18

• An amine group.
• A carboxyl group.
• A side chain (variable group).

Question 19

How is a peptide bond formed between two amino acids to form a dipeptide?

Answer 19

Condensation reaction with the loss of a molecule of water. Bond forms between the amine group of one amino acid and the carboxyl group of another amino acid.

Question 20

Describe the structure of each level of protein structure.

Answer 20

Primary structure:
- The sequence of amino acids in a polypeptide chain.
- Joined by peptide bonds.

Secondary structure:
- Folding of the polypeptide chain due to the formation of many weak hydrogen bonds.
- Produces secondary structures such as alpha helixes and beta-pleated sheets.

Tertiary structure:
- Further folding of the secondary structure where the whole chain folds into a 3D specific shape.
- Stabilised by ionic bonds, hydrogen bonds and disulphide bonds.

Quaternary structure:
- Protein containing more than one polypeptide chain.

Question 21

What are fibrous proteins?

Answer 21

Form long chains running parallel to each other with cross-bridges between chains. This prices very stable molecules such as collagen. They tend to have a structural role in organisms.

Question 22

What are globular proteins?

Answer 22

Carry out metabolic functions.

Question 23

What is the role of collagen in humans and what are other examples of fibrous proteins?

Answer 23

• Collagen gives strength and structure in tendons, skin and blood vessels.
• Other fibrous proteins are keratin (hair) and elastin (skin, blood vessels and lungs).

Question 24

What is the test for proteins?

Answer 24

• Add Biuret solution to a sample of solution.
• If protein is present, the sample will turn lilac.

Question 25

What is the equation to work out the Rf value?

Answer 25

Distance from origin to solute ———————————————————— Distance from origin to solvent front

Question 26

Why are Rf values used?

Answer 26

The distance moved by the spot depends on the distance moved by the solvent. The Rf value gives a ratio, allowing comparison.

Question 27

Why is it important to draw the origin line in pencil?

Answer 27

So the pencil line does not dissolve in the solvent and won’t move up the chromatogram and interfere with the results.

Question 28

Why is it important that the solvent line is above the line drawn on the chromatography paper?

Answer 28

The line would not the separate the pigments in the solution.

Question 29

What is meant by metabolism?

Answer 29

All the chemical reactions in the cells.

Question 30

What is the difference between intracellular enzymes and extracellular enzymes?

Answer 30

Intracellular are enzymes used within a cell and extracellular are released from cells.

Question 31

What is the mechanism of an enzyme action?

Answer 31

- Enzyme has a specific tertiary structure and shape, which is complementary to the substrate. - Substrate binds into active site. - Forms an enzyme-substrate complex.

Question 32

What is the induced fit model?

Answer 32

- A substrate and an enzyme’s active site are not fully complementary to begin with. - When a substrate binds with an enzyme, it induces changes in the enzyme’s tertiary structure. - The active site changes shape to become complementary to the substrate. - This puts strain on the substrate and distorts the bonds, making them weaker, reducing the activation energy.

Question 33

What is the effect of temperature on enzyme activity?

Answer 33

- As temperature increases, enzyme and substrate molecules gain more kinetic energy. - They collide more frequently. - So a great number of enzyme-substrate complexes are formed. - This leads to an increased rate of reaction. - However, the increased kinetic energy causes hydrogen bonds to break. - The active site is no longer complementary to the substrate so less ES complexes are formed. - Enzyme’s active site denatures.

Question 34

How does change in pH affect enzyme activity?

Answer 34

- A change in pH causes some of the hydrogen and ionic bonds that maintain the enzyme’s structure to break. - The shape of the active site changes. - No enzyme - substrate complexes can be formed. - The enzyme has been denatured.

Question 35

How does change in substrate concentration affect enzyme activity?

Answer 35

- As substrate concentration increases, enzyme activity increases. - With more substrates in the solution, there are more successful enzyme and substrate collision so more enzyme-substrate complexes form. - However, enzyme activity levels of at a maximum as there is another limiting factor. - The reaction remains constant as substrate concentration increases. - This is because all the enzyme active sites are full occupied. The enzyme concentration is the limiting factor as it is limiting the rate of reaction.

Question 36

How does enzyme concentration affect enzyme activity?

Answer 36

- As enzyme concentration increases, enzyme activity increases until a certain point where increase in enzyme concentration has no further increase in enzyme activity. - With more enzymes in the solution, there are more successful enzyme and substrate collisions, so enzyme-substrate complexes form. - If substrate concentration becomes limiting, adding additional enzymes will not increase the rate.

Question 37

How do competitive inhibitors affect enzyme activity?

Answer 37

- They have a similar shape to that of the enzyme’s substrate. - It binds to the active site of the enzyme. - The active site is occupied and blocked. - Fewer enzyme-substrate complexes are formed and the rate of reaction is reduced.

Question 38

How do non-competitive enzymes affect enzyme activity?

Answer 38

- Non-competitive inhibitors bind to some other region of the enzyme (the allosteric site). - This changes the tertiary structure so changing the shape of the active site. - The substrate cannot bind. - Fewer enzyme-substrate complexes are formed and so reaction rate decreases.