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A Level H1 Biology Practice Paper 2

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Questions

TuitionGoWhere Exam Practice (AI) - Biology H1 A-Level

Subject: Biology
Level: H1
Paper: Practice Paper (Version 2 of 5)
Duration: 1 hour 15 minutes
Total Marks: 40
Name: __________________________
Class: __________________________
Date: __________________________

Instructions to Candidates:

Answer all questions.
Write your answers in the spaces provided.
The number of marks is given in brackets [ ] at the end of each question or part question.
You may use a calculator.

Section A: Structured Questions

Answer all questions in this section.

1. Fig. 1.1 shows a diagram of a cell membrane.

(Note: Imagine Fig 1.1 shows a phospholipid bilayer with embedded proteins, cholesterol, and glycoproteins. Label A points to the hydrophilic head, Label B points to the hydrophobic tail, and Label C points to a channel protein.)

(a) With reference to Fig. 1.1, describe the arrangement of the phospholipids in the membrane. [2]

(b) Explain how the structure of the phospholipid molecule contributes to the formation of this arrangement in an aqueous environment. [2]

2. A student investigated the effect of temperature on the activity of the enzyme amylase. The results are shown in Table 2.1.

Table 2.1

Temperature / °C	Rate of reaction / arbitrary units
10	12
20	25
30	48
40	55
50	30
60	5

(a) With reference to Table 2.1, explain why the rate of reaction decreases significantly between 40°C and 60°C. [3]

(b) Suggest why the rate of reaction at 10°C is lower than at 40°C, even though the enzyme is not denatured. [2]

3. Fig. 3.1 shows a simplified diagram of the fluid mosaic model of a cell membrane.

(Note: Imagine Fig 3.1 shows a membrane with a carrier protein undergoing a conformational change to move glucose from outside to inside the cell.)

(a) Name the type of transport shown in Fig. 3.1. [1]

(b) With reference to Fig. 3.1, describe how glucose moves across the membrane. [2]

4. Mitochondria were isolated from liver cells and incubated in a solution containing pyruvate. Oxygen consumption was measured. In a second experiment, mitochondria were incubated in a solution containing glucose.

(a) Explain why carbon dioxide is produced when mitochondria are incubated with pyruvate. [2]

(b) Explain why no carbon dioxide is produced when mitochondria are incubated with glucose. [2]

5. Table 5.1 shows the percentage composition of nitrogenous bases in the DNA of three different organisms.

Table 5.1

Organism	Adenine (%)	Guanine (%)	Cytosine (%)	Thymine (%)
Human	30	20	20	30
Wheat	27	23	23	27
Bacterium	15	35	35	15

(a) State the relationship between the percentage of adenine and thymine in these organisms. [1]

(b) Explain this relationship with reference to the structure of DNA. [2]

(c) Calculate the percentage of guanine in the DNA of a virus that has single-stranded DNA and contains 20% adenine, 30% cytosine, and 10% thymine. [1]

6. Fig. 6.1 shows the structure of a triglyceride molecule.

(Note: Imagine Fig 6.1 shows one glycerol molecule bonded to three fatty acid chains via ester bonds.)

(a) Name the type of reaction that joins the fatty acids to the glycerol. [1]

(b) Explain why triglycerides are suitable for energy storage in animals. [2]

7. A patient suffers from a genetic disorder that affects the production of a specific membrane protein responsible for chloride ion transport. This leads to the accumulation of thick, sticky mucus in the lungs.

(a) Explain how the defect in the membrane protein leads to the accumulation of thick mucus. [3]

(b) Suggest why this condition makes the patient more susceptible to bacterial infections. [2]

8. Fig. 8.1 shows the results of an experiment where red blood cells were placed in solutions of different water potentials.

(Note: Imagine Fig 8.1 shows three cells: one swollen/burst, one normal, one shriveled.)

(a) Identify which cell was placed in a solution with a higher water potential than the cell cytoplasm. [1]

(b) Explain the change in appearance of the cell identified in (a). [2]

9. Collagen is a fibrous protein found in connective tissues.

(a) Describe the structural features of collagen that give it high tensile strength. [3]

(b) State one function of collagen in the human body. [1]

10. Fig. 10.1 shows the structure of an amino acid.

(Note: Imagine Fig 10.1 shows a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and an R-group.)

(a) Name the bond formed between two amino acids. [1]

(b) Explain how the sequence of amino acids determines the three-dimensional structure of a protein. [2]

Section B: Data Interpretation and Extended Response

Answer all questions in this section.

11. Fig. 11.1 shows the effect of substrate concentration on the rate of an enzyme-catalyzed reaction.

(Note: Imagine Fig 11.1 is a graph where the rate increases linearly at low substrate concentrations and then plateaus at high concentrations.)

(a) With reference to Fig. 11.1, explain why the rate of reaction increases as substrate concentration increases from point A to point B. [2]

(b) Explain why the rate of reaction remains constant from point B to point C, even though substrate concentration continues to increase. [2]

Graph space

12. Water is essential for life.

(a) Explain how the polarity of water molecules contributes to its role as a solvent in biological systems. [2]

(b) Explain the significance of the high specific heat capacity of water for living organisms. [2]

13. Fig. 13.1 shows a diagram of a prokaryotic cell.

(Note: Imagine Fig 13.1 shows a bacterium with a cell wall, plasma membrane, cytoplasm, nucleoid region with circular DNA, ribosomes, and plasmids.)

(a) Identify two structures present in this prokaryotic cell that are also found in eukaryotic cells. [2]

(b) State one structural difference between the DNA in this prokaryotic cell and the DNA in a eukaryotic nucleus. [1]

14. Glycogen is a polysaccharide found in animals.

(a) Describe the structure of glycogen. [2]

(b) Explain why glycogen is a more suitable storage molecule than glucose for animals. [2]

15. Fig. 15.1 shows the results of an experiment investigating the effect of pH on the activity of pepsin, an enzyme found in the stomach.

(Note: Imagine Fig 15.1 is a bell-shaped curve with peak activity at pH 2.)

(a) State the optimum pH for pepsin activity based on Fig. 15.1. [1]

(b) Explain why pepsin activity decreases at pH 7. [2]

16. Discuss the significance of the movement of substances across membranes to the function of a cell. [6]

17. Fig. 17.1 shows a diagram of a nucleotide.

(Note: Imagine Fig 17.1 shows a phosphate group, a pentose sugar, and a nitrogenous base.)

(a) Name the components labeled X, Y, and Z. [3]

X: __________________________ Y: __________________________ Z: __________________________

(b) Describe how nucleotides are joined together to form a polynucleotide chain. [2]

18. Compare and contrast the structures of starch and cellulose. [4]

19. Explain how the fluidity of the cell membrane is maintained and why this is important for cell function. [3]

20. A student observed a plant cell under a microscope after placing it in a concentrated salt solution. The cytoplasm shrank away from the cell wall.

(a) Name this process. [1]

(b) Explain why this process occurred. [2]

[END OF PAPER]

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Answers

TuitionGoWhere Exam Practice (AI) - Biology H1 A-Level

Answer Key and Marking Scheme (Version 2)

Total Marks: 40

Section A: Structured Questions

1. (a) Phospholipids form a bilayer [1]; with hydrophilic heads facing outward towards the aqueous environment and hydrophobic tails facing inward [1]. (b) The phosphate head is polar/hydrophilic and interacts with water [1]; the fatty acid tails are non-polar/hydrophobic and avoid water, clustering together in the center [1].

2. (a) High temperature breaks hydrogen bonds/ionic bonds holding the tertiary structure [1]; the enzyme loses its specific shape/active site changes shape [1]; substrate can no longer bind to the active site (enzyme is denatured) [1]. (b) At lower temperatures, molecules have less kinetic energy [1]; fewer successful collisions between enzyme and substrate per unit time [1].

3. (a) Facilitated diffusion [1]. (b) Glucose binds to the carrier protein [1]; the protein changes shape/conformation to release glucose on the other side [1]. (c) Glucose is polar/large [1]; it cannot pass through the hydrophobic core of the phospholipid bilayer [1].

4. (a) Pyruvate enters the mitochondria and is converted to Acetyl-CoA, which enters the Krebs cycle [1]; CO2 is produced as a waste product of the Krebs cycle/link reaction [1]. (b) Glycolysis occurs in the cytoplasm, not in the mitochondria [1]; isolated mitochondria lack the enzymes for glycolysis, so glucose cannot be broken down into pyruvate [1].

5. (a) The percentage of adenine is equal to the percentage of thymine [1]. (b) Adenine forms complementary base pairs with thymine via two hydrogen bonds [1]; therefore, they must be present in equal amounts in double-stranded DNA [1]. (c) 40% [1]. (100 - 20 - 30 - 10 = 40).

6. (a) Condensation [1]. (b) Triglycerides have a high ratio of energy-storing C-H bonds to carbon atoms [1]; they are insoluble in water, so they do not affect the water potential of cells [1].

7. (a) The defective protein prevents chloride ions from moving out of the cell [1]; this lowers the water potential inside the cell/creates a water potential gradient [1]; water moves out of the mucus into the cells by osmosis, making the mucus thick and sticky [1]. (b) Thick mucus traps bacteria but cannot be cleared effectively by cilia [1]; bacteria multiply in the stagnant mucus, leading to infection [1].

8. (a) The swollen/burst cell [1]. (b) The solution had a higher water potential than the cell cytoplasm [1]; water entered the cell by osmosis, causing it to swell and burst (haemolysis) [1].

9. (a) Collagen consists of three polypeptide chains wound together in a triple helix [1]; hydrogen bonds between the chains provide stability [1]; cross-links between adjacent collagen molecules provide tensile strength [1]. (b) Provides strength to tendons/ligaments/bones/skin [1].

10. (a) Peptide bond [1]. (b) The sequence of amino acids (primary structure) determines the interactions between R-groups [1]; these interactions (hydrogen bonds, ionic bonds, disulfide bridges) determine the folding into the tertiary structure [1].

Section B: Data Interpretation and Extended Response

11. (a) As substrate concentration increases, there are more substrate molecules available to collide with enzyme active sites [1]; more enzyme-substrate complexes are formed per unit time [1]. (b) All enzyme active sites are saturated/occupied [1]; adding more substrate cannot increase the rate because there are no free active sites [1]. (c) Line should start at the origin, rise more slowly than the original curve, but eventually reach the same maximum rate (Vmax) [1].

12. (a) Water molecules are polar, with a partial negative charge on oxygen and partial positive charge on hydrogen [1]; this allows water to surround and dissolve charged/polar ions and molecules [1]. (b) Water can absorb a large amount of heat energy with only a small change in temperature [1]; this helps organisms maintain a stable internal temperature/homeostasis [1].

13. (a) Ribosomes [1]; Plasma membrane/Cytoplasm [1]. (b) Prokaryotic DNA is circular/naked (not associated with histones) [1]; Eukaryotic DNA is linear/associated with histones.

14. (a) Glycogen is a polymer of alpha-glucose [1]; it is highly branched [1]. (b) Glycogen is insoluble, so it does not affect the water potential of the cell [1]; its branched structure allows for rapid release of glucose when needed [1].

15. (a) pH 2 [1]. (b) pH 7 is above the optimum pH [1]; changes in pH alter the charge on the amino acids in the active site, disrupting bonding and changing the shape of the active site [1].

16. Marking Guidance for Extended Response (6 marks):

Level 3 (5-6 marks): Detailed explanation of multiple transport mechanisms linked to specific cell functions. Clear scientific terminology.
Level 2 (3-4 marks): Description of transport mechanisms with some link to function.
Level 1 (1-2 marks): Basic description of transport or function without clear linkage.

Indicative Content:

Nutrient Uptake: Glucose/amino acids enter cells via facilitated diffusion or active transport (e.g., in intestinal epithelial cells) to provide substrates for respiration/protein synthesis [1-2].
Waste Removal: Urea/CO2 leaves cells by diffusion to prevent toxicity [1].
Ion Balance: Active transport of ions (e.g., Na+/K+ pump) maintains resting potential in nerve cells, essential for impulse transmission [1-2].
Osmoregulation: Water movement by osmosis is controlled by solute concentration; essential for maintaining cell turgor in plants or volume in animals [1].
Cell Signaling: Receptor proteins in the membrane allow detection of hormones/neurotransmitters, triggering cellular responses [1].

17. (a) X: Phosphate group [1]; Y: Pentose sugar (Deoxyribose/Ribose) [1]; Z: Nitrogenous base [1]. (b) Nucleotides are joined by condensation reactions [1]; forming phosphodiester bonds between the phosphate of one nucleotide and the sugar of the next [1].

18.

Similarities: Both are polysaccharides/polymers of glucose [1]; both contain glycosidic bonds [1].
Differences: Starch contains alpha-glucose, cellulose contains beta-glucose [1]; Starch is helical/branched (amylopectin), cellulose forms straight chains held by hydrogen bonds into microfibrils [1]; Starch is for storage, cellulose is for structural support [1]. (Award max 4 marks)

19.

Cholesterol molecules fit between phospholipid tails, restricting their movement at high temperatures and preventing packing at low temperatures [1];
Unsaturated fatty acid tails have kinks that prevent tight packing, increasing fluidity [1];
Fluidity allows for membrane flexibility, vesicle formation, and the movement of proteins within the membrane for transport/signaling [1].

20. (a) Plasmolysis [1]. (b) The salt solution had a lower water potential than the cell cytoplasm [1]; water left the vacuole/cytoplasm by osmosis, causing the protoplast to shrink away from the cell wall [1].