Biological Molecules Questions

Question 1

Describe the chemical reactions involved in the conversion of polymers to monomers and monomers to polymers.
Give two named examples

Answer 1

A condensation reaction joins monomers together and forms a
(chemical) bond and releases water;

A hydrolysis reaction breaks a (chemical) bond between
monomers and uses water;

A suitable example of polymers and the monomers from which
they are made;

A second suitable example of polymers and the monomers
from which they are made;

Question 2

What is a monomer?

Answer 2

(a monomer is a smaller / repeating) unit / molecule from which larger
molecules / polymers are made;

Question 3

Describe a biochemical test to show that raffinose solution contains a non reducing sugar

Answer 3

Heat with acid and nuetralise

Heat with benedicts solution

Red precipitate colour

Question 4

Suggest method to measure quanitity of a reducing sugar

Answer 4

Filter and dry precipitate

Find mass/weight

Question 5

Give one similarity and one difference between structyures of lactulose (galactose and one molecule of fructose)
And lactose

Answer 5

Both contain galactose / a glycocidic bond

Lactulose contains fructose whereas lactose contains glucose

Question 6

Describe two differences between the structure of a cellulose molecule and a glycogen molecule

Answer 6

Cellulose is made up of β-glucose (monomers) and glycogen is
made up of α-glucose (monomers);
Cellulose molecule has straight chain and glycogen is
branched;
Cellulose molecule has straight chain and glycogen is coiled;
glycogen has 1,4- and 1,6- glycosidic bonds and cellulose has
only 1,4- glycosidic bonds

Question 7

Describe and explain two features of starch that make it a good storage
molecule.

Answer 7

Insoluble (in water), so doesn’t affect water potential;
Branched / coiled / (α-)helix, so makes molecule compact;
OR
Branched / coiled / (α-)helix so can fit many (molecules) in
small area;
Polymer of (α-)glucose so provides glucose for respiration;
Branched / more ends for fast breakdown / enzyme action;
Large (molecule), so can’t cross the cell membrane

Question 8

Describe structure of glycogen

Answer 8

Polysaccharide of α-glucose;
OR
polymer of α-glucose;
2. (Joined by) glycosidic bonds
OR
Branched structure;

Question 9

Describe how glycogen acts as a source of energy

Answer 9

Hydrolysed (to glucose);
2. Glucose used in respiration;

Question 10

Explain difference in sturcture of starch molecule and celullose molecule shown in diagram

Answer 10

Starch formed from α-glucose but cellulose formed from β-glucose;
Position of hydrogen and hydroxyl groups on carbon atom 1
inverted.

Question 11

Explain one way in which starch molecules are adapted for their function in plant cells.

Answer 11

Insoluble
Does not affect water potential

Helical;
4. Compact;
OR
5. Large molecule;
6. Cannot leave cell.

Question 12

Explain how cellulose molecules are adapted for their function in plant
cells.

Answer 12

Long and straight chains;
2. Become linked together by many hydrogen bonds to form
fibrils;
3. Provide strength (to cell wall).

Question 13

Describe how you would text for the presence of a lipid in a liquid sample of food.

Answer 13

Shake with) ethanol / alcohol;
1. Accept named alcohol
2. Then add (to) water;
2. Order must be correct
3. White / milky / cloudy (layer indicates oil).

Question 14

Describe how a triglyceride molecule is formed.

Answer 14

One glycerol and three fatty acids;
2. Condensation (reactions) and removal of three molecules of water;
3. Ester bond(s) (formed)

Question 15

Describe how an ester bond is formed in a phospholipid molecule.

Answer 15

Condensation (reaction)
2. OR
Loss of water;
Between of glycerol and fatty acid;

Question 16

Describe the induced-fit model of enzyme action and how an enzyme acts
as a catalyst.

Answer 16

Substrate binds to the active site/enzyme
OR
2. Enzyme-substrate complex forms;
Accept for ‘binds’, fits
Active site changes shape (slightly) so it is complementary to
substrate
OR
Active site changes shape (slightly) so
distorting/breaking/forming bonds in the substrate;
3. Reduces activation energy;

Question 17

A competitive inhibitor decreases the rate of an enzyme-controlled
reaction.

Answer 17

Inhibitor similar shape to substrate;
Reject same shape
Accept ‘complementary to active site’
2. Fits/binds to active site;
3. Prevents/reduces enzyme-substrate complex forming

Question 18

Describe how the structure of a protein depends on the amino acids it
contains.

Answer 18

Structure is determined by (relative) position of amino acid/R group/interactions;

Primary structure is sequence/order of amino acids;

Secondary structure formed by hydrogen bonding (between amino
acids);

Tertiary structure formed by interactions (between R groups);

Creates active site in enzymes

Question 19

Explain how the active site of an enzyme causes a high rate of reaction.

Answer 19

Lowers activation energy;
2. 3. Induced fit causes active site (of enzyme) to change shape;
(So) enzyme-substrate complex causes bonds to form/break

Question 20

Describe two other ways in which all dipeptides are similar and one way in
which they might differ.

Answer 20

mine/NH2 (group at end);
Accept amino/NH3
+
2. Carboxyl/COOH (group at end);
Accept carboxylic / COO−
3. Two R groups;
4. All contain C and H and N and O;
Accept examples of different R groups
Difference
5. Variable/different R group(s);
3
(c) 1. Moved to negative (electrode) because positive(ly charged);
2. 3. (Spots move) different distances/rates because (amino acids)
different charge/mass;
Accept size for ma

Question 21

Describe how a non-competitive inhibitor can reduce the rate of an
enzyme-controlled reaction.

Answer 21

Attaches to the enzyme at a site other than the active site;
Accept ‘attaches to allosteric/inhibitor site’
Changes (shape of) the active site
OR
Changes tertiary structure (of enzyme);
(So active site and substrate) no longer complementary so
less/no substrate can fit/bind;

Question 22

Describe how a peptide bond is formed between two amino acids to form a
dipeptide.

Answer 22

Condensation (reaction) / loss of water;
Accept each marking point if shown clearly in
diagram.
2. Between amine / NH2 and carboxyl / COOH;

Question 23

The secondary structure of a polypeptide is produced by bonds between
amino acids.
Describe how

Answer 23

Hydrogen bonds;
Accept as a diagram
Reject N - - - C / ionic / disulfide bridge / peptide
bond
2. Between NH (group of one amino acid) and C=O (group);
OR
Forming β pleated sheets / α helix;

Question 24

Two proteins have the same number and type of amino acids but different
tertiary structures.
Explain why

Answer 24

Different sequence of amino acids
OR
Different primary structure;
If candidate assumes proteins are the same, accept
effect of different pH/ temperature
2. Forms ionic / hydrogen / disulfide bonds in different places

Question 25

Formation of an enzyme-substrate complex increases the rate of reaction. Explain how.

Answer 25

Reduces activation energy; Accept ‘reduces Ea’. 2. Due to bending bonds OR Without enzyme, very few substrates have sufficient energy for reaction;

Question 26

Describe the structure of DNA.

Answer 26

Polymer of nucleotides; Accept ‘Polynucleotide’ Accept for ‘phosphate’. phosphoric acid 2. Each nucleotide formed from deoxyribose, a phosphate (group) and an organic/nitrogenous base; 3. Phosphodiester bonds (between nucleotides); 4. Double helix/2 strands held by hydrogen bonds; 5. (Hydrogen bonds/pairing) between adenine, thymine and cytosine, guanine;

Question 27

Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule.

Answer 27

Condensation (reaction)/loss of water; 2. (Between) phosphate and deoxyribose; 3. (Catalysed by) DNA polymerase;

Question 28

Describe how the separation of strands occurs.

Answer 28

DNA helicase; 2. Breaks hydrogen bonds between base pairs/ AT and GC/complementary bases OR Breaks hydrogen bonds between polynucleotide strands;

Question 29

Describe the role of DNA polymerase in the semi-conservative replication of DNA

Answer 29

Joins (adjacent DNA) nucleotides; Reject suggestions that it forms hydrogen bonds or joins complementary bases. Reject ‘nucleotide bases’. 2. (Catalyses) condensation (reactions); 3. (Catalyses formation of) phosphodiester bonds (between adjacent nucleotides);

Question 30

Use your knowledge of semi-conservative replication of DNA to suggest: 1. the role of the single-stranded DNA fragments 2. The role of the DNA nucleotides.

Answer 30

Role of single-stranded DNA fragments 1. Template; 2. Determines order of nucleotides/bases; Role of DNA nucleotides 3. Forms complementary pairs / A – T, G - C OR Forms complementary (DNA) strand;

Question 31

Give two features of DNA and explain how each one is important in the semi-conservative replication of DNA.

Answer 31

W eak / easily broken hydrogen bonds between bases allow two strands to separate / unzip; Two strands, so both can act as templates; may appear in the same feature Complementary base pairing allows accurate replication;

Question 32

Describe the role of two named enzymes in the process of semi- conservative replication of DNA.

Answer 32

DNA) helicase causes breaking of hydrogen/H bonds (between DNA strands); Reject ‘helicase hydrolyses hydrogen bonds’. DNA polymerase joins the (DNA) nucleotides; Forming phosphodiester bonds

Question 33

Describe the function of each of these enzymes. DNA helicase_ DNA polymerase ___

Answer 33

DNA helicase – (unwinding DNA and) breaking hydrogen bonds / bonds between chains / bases / strands; 2. DNA polymerase – joins (adjacent) nucleotides OR forms phosphodiester bond / sugar-phosphate backbone;

Question 34

Adenosine triphosphate (ATP) is a nucleotide derivative. Contrast the structures of ATP and a nucleotide found in DNA to give two differences.

Answer 34

ATP has ribose and DNA nucleotide has deoxyribose; 2. ATP has 3 phosphate (groups) and DNA nucleotide has 1 phosphate (group); 3. ATP – base always adenine and in DNA nucleotide base can be different / varies;