Biological molecules Exam questions Flashcards
Describe the chemical reactions involved in the conversion of polymers to
monomers and monomers to polymers.
Give two named examples of polymers and their associated monomers to
illustrate your answer. (5)
- 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;
What is a monomer? (1)
(a monomer is a smaller / repeating) unit / molecule from which larger
molecules / polymers are made;
A biochemical test for reducing sugar produces a negative result with
raffinose solution.
Describe a biochemical test to show that raffinose solution contains a
non-reducing sugar. (3)
- Heat with acid and neutralise;
Accept boil/water bath for heat
Accept named alkali for neutralise
Accept named examples, eg HCl, NaHCO3 - Heat with Benedict’s (solution);
- Red precipitate/colour;
Lactulose is a disaccharide formed from one molecule of galactose and
one molecule of fructose.
Other than both being disaccharides, give one similarity and one difference
between the structures of lactulose and lactose. (2)
Similarity
1. Both contain galactose / a glycosidic bond;
Ignore references to hydrolysis and / or
condensation
Difference
2. Lactulose contains fructose, whereas lactose contains glucose;
Starch is a carbohydrate often stored in plant cells.
Describe and explain two features of starch that make it a good storage
molecule. (2)
Any two from:
1. Insoluble (in water), so doesn’t affect water potential;
2. Branched / coiled / (α-)helix, so makes molecule compact;
OR
Branched / coiled / (α-)helix so can fit many (molecules) in
small area;
3. Polymer of (α-)glucose so provides glucose for respiration;
4. Branched / more ends for fast breakdown / enzyme action;
5. Large (molecule), so can’t cross the cell membrane
Describe the structure of glycogen. (2)
- Polysaccharide of α-glucose;
OR
polymer of α-glucose; - (Joined by) glycosidic bonds
OR
Branched structure;
Suggest how glycogen acts as a source of energy.
Do not include transport across membranes in your answer. (2)
- Hydrolysed (to glucose);
- Glucose used in respiration;
Starch molecules and cellulose molecules have different functions in plant
cells. Each molecule is adapted for its function.
Explain one way in which starch molecules are adapted for their function in
plant cells. (2)
- Insoluble;
- Don’t affect water potential;
OR - Helical;
Accept form spirals - Compact;
Explain how cellulose molecules are adapted for their function in plant
cells. (3)
Long and straight chains;
2. Become linked together by many hydrogen bonds to form
fibrils;
3. Provide strength (to cell wall).
Describe how you would test for the presence of a lipid in a liquid sample of
food. (2)
- Add ethanol/alcohol then add water and shake/mix
- White/milky (emulsion)
Describe the induced-fit model of enzyme action and how an enzyme acts
as a catalyst.
(3)
- Substrate binds to the active site/enzyme
OR
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; - Reduces activation energy;
A competitive inhibitor decreases the rate of an enzyme-controlled
reaction.
Explain how. (3)
- Inhibitor similar shape to substrate;
Reject same shape
Accept ‘complementary to active site’ - Fits/binds to active site;
- Prevents/reduces enzyme-substrate complex forming;
Describe how the structure of a protein depends on the amino acids it
contains. (5)
) 1. Structure is determined by (relative) position of amino acid/R
group/interactions;
Accept for ‘interactions’, hydrogen bonds / disulfide
bridges / ionic bonds / hydrophobichydrophilic
interactions
2. Primary structure is sequence/order of amino acids;
3. Secondary structure formed by hydrogen bonding (between amino
acids);
Accept alpha helix/β-pleated sheet for ‘secondary
structure’
4. Tertiary structure formed by interactions (between R groups);
Accept for ‘interactions’, hydrogen bonds / disulfide
bridges / ionic bonds / hydrophobichydrophilic
interactions
5. Creates active site in enzymes
OR
Creates complementary/specific shapes in antibodies/carrier
proteins/receptor (molecules);
6. Quaternary structure contains >1 polypeptide chain
Explain how the active site of an enzyme causes a high rate of reaction. (3)
- Lowers activation energy;
- Induced fit causes active site (of enzyme) to change shape;
- (So) enzyme-substrate complex causes bonds to form/break;
Accept: description, of induced fit
Accept: enzyme-substrate complex causes stress/strain
on bonds.
Describe a biochemical test to confirm the presence of protein in a
solution. (2)
- Add biuret (reagent);
- (Positive result) purple/lilac/violet /mauve;
Describe how a non-competitive inhibitor can reduce the rate of an
enzyme-controlled reaction. (3)
- 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;
Accept ‘no longer complementary so less/no enzyme substrate complexes form’
Describe the structure of DNA. (5)
) 1. 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;
Describe how a phosphodiester bond is formed between two nucleotides
within a DNA molecule. (2)
- Condensation (reaction)/loss of water;
- (Between) phosphate and deoxyribose;
- (Catalysed by) DNA polymerase;
In the process of semi-conservative DNA replication, the two strands within
a DNA molecule are separated. Each then acts as a template for the
formation of a new complementary strand.
Describe how the separation of strands occurs. (2)
- DNA helicase;
- Breaks hydrogen bonds between base pairs/ AT and
GC/complementary bases
Describe the role of DNA polymerase in the semi-conservative replication
of DNA. (2)
- Joins (adjacent DNA) nucleotides;
Reject suggestions that it forms hydrogen bonds or
joins complementary bases.
Reject ‘nucleotide bases’. - (Catalyses) condensation (reactions);
- (Catalyses formation of) phosphodiester bonds (between adjacent
nucleotides);
Name the two scientists who proposed models of the chemical structure of
DNA and of DNA replication. (1)
Watson and Crick
Give two features of DNA and explain how each one is important in the
semi-conservative replication of DNA. (2)
- Weak / 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;
Describe the role of two named enzymes in the process of semiconservative replication of DNA. (3)
- (DNA) helicase causes breaking of hydrogen/H bonds (between DNA
strands);
Reject ‘helicase hydrolyses hydrogen bonds’. - DNA polymerase joins the (DNA) nucleotides;
Reject if suggestion that DNA polymerase joins the
complementary nucleotides or forms H bonds.
Reject if joining RNA nucleotides or forming RNA. - Forming phosphodiester bonds;
Describe how an ATP molecule is formed from its component molecules. (4)
- and 2. Accept for 2 marks correct names of three components adenine,
ribose/pentose, three phosphates;;
Accept for 1 mark, correct name of two components
Accept for 1 mark, ADP and phosphate/Pi
Ignore adenosine
Accept suitably labelled diagram - Condensation (reaction);
Ignore phosphodiester - ATP synthase;
ATP is an energy source used in many cell processes. Give two ways in
which ATP is a suitable energy source for cells to use. (2)
- Releases relatively small amount of energy / little energy lost as
heat;
Key concept is that little danger of thermal death of
cells - Releases energy instantaneously;
Key concept is that energy is readily available - Phosphorylates other compounds, making them more reactive;
- Can be rapidly re-synthesised;
- Is not lost from / does not leave cells
Give two ways in which the hydrolysis of ATP is used in cells (2)
- To provide energy for other reactions/named process;
Reject ‘produce’ energy - To add phosphate to other substances and make them more
reactive/change their shape;
Describe how ATP is resynthesised in cells. (2)
- From ADP and phosphate;
Accept
Reject P/Phosphorus
Reject use of water in the reaction - By ATP synthase;
- During respiration/photosynthesis;
Explain five properties that make water important for organisms (5)
) 1. A metabolite in condensation/hydrolysis/ photosynthesis/respiration;
2. A solvent so (metabolic) reactions can occur
OR
A solvent so allowing transport of substances;
3. High (specific) heat capacity so buffers changes in temperature;
For ‘buffer’ accept ‘resist’.
4. Large latent heat of vaporisation so provides a cooling effect (through
evaporation);
Reject latent heat of evaporation
5. Cohesion (between water molecules) so supports columns of water
(in plants);
6. Cohesion (between water molecules) so produces surface tension
supporting (small) organisms;
State and explain the property of water that can help to buffer changes in
temperature. (2)
- (water has a relatively) high (specific) heat capacity;
Ignore numbers relating to heat capacity - Can gain / lose a lot of heat / energy without changing temperature;
OR
Takes a lot of heat / energy to change temperature;
Give two properties of water that are important in the cytoplasm of cells.
For each property of water, explain its importance in the cytoplasm. (4)
- Polar molecule;
- Acts as a (universal) solvent;
- (Universal) solvent;
- (Metabolic) reactions occur faster in solution;
OR - Reactive;
- Takes place in hydrolysis / condensation / named reaction;
