A Level H2 Biology Cells Biomolecules Quiz

A-Level Biology H2 Quiz - Cells Biomolecules - Answer Key

1. A
[1]

2. A
[1]

3. C
[1]

4. B
[1]

5. Peptide bond
[1]

6.
(a) Triglycerides have three fatty acid tails, whereas phospholipids have two fatty acid tails and a phosphate group.
[1]
(b) The phosphate head is hydrophilic (attracted to water) and the fatty acid tails are hydrophobic (repelled by water). In an aqueous environment, the heads face outward towards the water, and the tails face inward, away from the water, forming a bilayer.
[2] (1 for hydrophilic/hydrophobic distinction, 1 for orientation)

7.
(a) As substrate concentration increases, there are more substrate molecules available to collide with enzyme active sites. This increases the frequency of successful collisions and the formation of enzyme-substrate complexes, thus increasing the rate of reaction.
[2]
(b) At point B, all enzyme active sites are saturated with substrate. The enzyme is working at its maximum velocity ($V_{max}$). Adding more substrate cannot increase the rate further because there are no free active sites available.
[2] (1 for saturation, 1 for $V_{max}$/no free sites)

8.
(a) Krebs cycle (or Link Reaction)
[1]
(b) The inner membrane contains the electron transport chain and ATP synthase. It is impermeable to protons, allowing a proton gradient to be established. Protons flow back into the matrix through ATP synthase, driving the synthesis of ATP from ADP and Pi (chemiosmosis).
[3] (1 for ETC/ATP synthase, 1 for proton gradient, 1 for chemiosmosis)

9.
(a)
Sugar: Deoxyribose (DNA) / Ribose (RNA)
Bases: A, U, C, G (RNA)
Structure: Single stranded (RNA)
[3] (1 per correct pair)
(b) Complementary base pairing ensures that each strand serves as a template for the synthesis of a new complementary strand. This results in two identical DNA molecules, ensuring genetic information is accurately passed on.
[2] (1 for template, 1 for accuracy/identity)

10.
(a) The rate of reaction increases as temperature increases.
[1]
(b) High temperatures break the hydrogen bonds and ionic bonds holding the tertiary structure of the enzyme together. The enzyme loses its specific shape, and the active site is no longer complementary to the substrate. The enzyme is denatured.
[3] (1 for bond breaking, 1 for shape change/active site, 1 for denaturation)

11.
(a) Selectively permeable means the membrane allows certain substances to pass through while restricting others.
[1]
(b) Facilitated diffusion moves substances down their concentration gradient and does not require energy. Active transport moves substances against their concentration gradient and requires energy (ATP).
[2] (1 for gradient direction, 1 for energy requirement)

12.
(a) To transport oxygen.
[1]
(b) Haemoglobin has four subunits (quaternary structure), each containing a haem group. This allows it to bind up to four oxygen molecules. The cooperative binding (change in shape upon oxygen binding) increases its affinity for oxygen in the lungs and releases it in tissues.
[2] (1 for 4 subunits/haem groups, 1 for cooperative binding/capacity)

13.
(a) Water molecules are held together by strong hydrogen bonds. A large amount of energy is required to break these bonds to raise the temperature of water.
[2] (1 for hydrogen bonds, 1 for energy requirement)
(b) It helps organisms maintain a stable internal body temperature (homeostasis) despite fluctuations in external temperature.
[1]

14.
(a) Peptide bond
[1]
(b) Hydrolysis
[1]

15.
(a) Protein synthesis (translation).
[1]
(b) Prokaryotic ribosomes are smaller (70S) than eukaryotic ribosomes (80S).
[1]

16.
(a) DNA fragments are separated based on their size (length/molecular mass). Smaller fragments move faster and further through the gel matrix than larger fragments because they encounter less resistance.
[3] (1 for size/mass, 1 for speed/distance, 1 for gel resistance)
(b) DNA is negatively charged due to the phosphate groups in its backbone. Therefore, it is attracted to the positive electrode (anode).
[1]

17.
(a)
A: Adenine (base)
B: Ribose (sugar)
C: Phosphate groups (or Triphosphate)
[3]
(b) ATP releases a small, manageable amount of energy when the terminal phosphate bond is broken. This energy is sufficient for most cellular processes without wasting energy as heat. It can be rapidly regenerated from ADP and Pi.
[3] (1 for small/manageable packet, 1 for immediate use, 1 for regeneration)

18.
(a) The induced-fit hypothesis suggests that the active site is not perfectly complementary to the substrate initially. When the substrate binds, the enzyme changes shape slightly to fit the substrate more closely, forming an enzyme-substrate complex.
[3] (1 for not perfect fit initially, 1 for shape change, 1 for tighter fit)
(b) Only the specific substrate can induce the correct conformational change in the enzyme to form the enzyme-substrate complex. Other molecules cannot fit or induce the change, ensuring specificity.
[2] (1 for specific substrate, 1 for conformational change requirement)

19.
(a) The sodium-potassium pump actively transports 3 sodium ions ($Na^+$) out of the neuron and 2 potassium ions ($K^+$) into the neuron against their concentration gradients. This creates an electrochemical gradient with the inside of the cell being negative relative to the outside, establishing the resting potential.
[4] (1 for 3 Na+ out, 1 for 2 K+ in, 1 for active transport/against gradient, 1 for negative inside/resting potential)
(b) Active transport moves substances against their concentration gradient (from low to high concentration). This requires energy to overcome the natural tendency of diffusion. ATP provides this energy.
[2] (1 for against gradient, 1 for energy source)

20.
(a)
Primary: Sequence of amino acids linked by peptide bonds.
Secondary: Folding into alpha-helices or beta-pleated sheets due to hydrogen bonds.
Tertiary: 3D folding due to interactions between R-groups (hydrogen, ionic, disulfide bonds).
Quaternary: Association of two or more polypeptide chains.
[4] (1 per level)
(b) Changes in pH affect the charge on the R-groups of amino acids. This disrupts the ionic and hydrogen bonds that maintain the tertiary structure. The enzyme denatures, and the active site changes shape, preventing substrate binding.
[3] (1 for charge on R-groups, 1 for bond disruption, 1 for denaturation/active site change)