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A Level H1 Biology Practice Paper 4

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Questions

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

Subject: Biology H1
Level: A-Level
Paper: Practice Paper (Version 4 of 5)
Duration: 1 hour 30 minutes
Total Marks: 60
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 are advised to spend approximately 1.5 minutes per mark.

Section A: Structured Questions

Answer all questions in this section.

1. Fig. 1.1 shows a simplified diagram of the fluid mosaic model of a cell membrane.

(Note: In the actual exam, a diagram would be provided showing a phospholipid bilayer with embedded proteins, cholesterol, and glycoproteins. Label A points to the hydrophilic head, Label B to the hydrophobic tail, and Label C to a channel protein.)

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

(b) Explain why substance X, which is a small, non-polar molecule, can cross the membrane at region B without the assistance of protein C, whereas substance Y, a large polar molecule, cannot. [3]

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 starch breakdown / arbitrary units
10	0.5
20	1.2
30	2.8
40	3.5
50	1.1
60	0.0

(a) Explain the increase in the rate of reaction between 10°C and 40°C. [3]

(b) Explain why the rate of reaction is zero at 60°C. [2]

3. Fig. 3.1 shows an electron micrograph of a liver cell.

(Note: Diagram shows a cell with prominent mitochondria, rough endoplasmic reticulum (RER), and Golgi apparatus.)

(a) Identify the organelle labelled M and state its function in this cell. [2]

(b) Liver cells are involved in the synthesis of plasma proteins. Explain the role of the Rough Endoplasmic Reticulum (RER) and the Golgi apparatus in this process. [3]

4. Isolated mitochondria were incubated in a solution containing either glucose or pyruvate. Carbon dioxide production was measured.

(a) Explain why carbon dioxide is produced when mitochondria are incubated with pyruvate but not when incubated with glucose. [3]

(b) State the specific stage of cellular respiration where this carbon dioxide is produced. [1]

5. Water is essential for life.

(a) Describe two properties of water that make it an effective solvent for biological reactions. [2]

(b) Explain how the polarity of water molecules contributes to its high specific heat capacity. [2]

Section B: Data and Diagram Interpretation

Answer all questions in this section.

6. Fig. 6.1 shows the change in DNA content of a cell during the cell cycle.

(Note: Graph shows DNA content doubling during S phase, staying constant during G2, halving during Meiosis I, and halving again during Meiosis II.)

(a) Identify the phase of the cell cycle labelled S. [1]

(b) Explain the biological process occurring during phase S that results in the change in DNA content. [2]

(c) With reference to Fig. 6.1, explain why the DNA content halves between stage X (end of Meiosis I) and stage Y (end of Meiosis II). [2]

7. Fig. 7.1 shows the structure of a triglyceride molecule.

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

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

(b) Triglycerides are used for energy storage in animals. Explain two structural features of triglycerides that make them suitable for this function. [4]

8. A student performed an experiment to determine the water potential of potato cells. Potato cylinders were placed in sucrose solutions of varying concentrations. The percentage change in mass was recorded after 2 hours.

Table 8.1

Sucrose Concentration / mol dm⁻³	Percentage Change in Mass / %
0.0	+15.0
0.2	+5.0
0.4	-2.0
0.6	-10.0
0.8	-18.0

(a) Explain why the potato cylinders gained mass in the 0.0 mol dm⁻³ sucrose solution. [3]

(b) Using the data in Table 8.1, estimate the water potential of the potato cells. Explain your reasoning. [2]

9. Fig. 9.1 shows the structure of an amino acid.

(Note: Diagram shows a central carbon, an amino group, a carboxyl group, a hydrogen atom, and an R-group.)

(a) Draw the structure of a dipeptide formed by the condensation of two amino acids. [2]

(b) Explain how the sequence of amino acids in a protein determines its three-dimensional structure. [3]

10. Fig. 10.1 shows the results of an experiment investigating the effect of pH on the activity of enzyme Z.

(Note: Graph shows a bell-shaped curve with optimum pH at 7. Activity drops sharply at pH 2 and pH 12.)

(a) State the optimum pH for enzyme Z. [1]

(b) Explain the shape of the curve at pH 2. [3]

Section C: Extended Response

Answer the question in this section.

11. Discuss the significance of the movement of substances across membranes to the process of photosynthesis in plant cells. [6]

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Answers

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

Answer Key and Marking Scheme Paper: Practice Paper (Version 4 of 5) Topic: Cells & Biomolecules

Section A: Structured Questions

1. (a) Phospholipid Arrangement:

Phospholipids form a bilayer [1].
Hydrophilic (polar) heads face outward towards the aqueous environment, and hydrophobic (non-polar) tails face inward, away from water [1].

(b) Transport Mechanisms:

Substance X is non-polar/lipid-soluble, so it can dissolve in and diffuse through the hydrophobic fatty acid tail region (B) of the membrane [1].
Substance Y is polar/large, so it is repelled by the hydrophobic interior of the membrane [1].
Therefore, Substance Y requires a transport protein (like C) to cross, whereas X does not [1].

2. (a) Temperature Increase (10-40°C):

As temperature increases, kinetic energy of enzyme and substrate molecules increases [1].
This leads to more frequent collisions between enzyme and substrate [1].
More enzyme-substrate complexes are formed per unit time, increasing the rate of reaction [1].

(b) Temperature at 60°C:

The high temperature breaks the hydrogen bonds/ionic bonds holding the tertiary structure of the enzyme [1].
The active site changes shape (denaturation), so the substrate can no longer bind [1].

3. (a) Organelle M:

Name: Mitochondrion [1].
Function: Site of aerobic respiration / production of ATP [1].

(b) Protein Synthesis and Secretion:

RER: Ribosomes on the RER synthesize proteins which enter the lumen of the RER for folding/modification [1].
Transport vesicles carry proteins from RER to the Golgi apparatus [1].
Golgi: Modifies (e.g., glycosylation), sorts, and packages proteins into secretory vesicles for transport out of the cell [1].

4. (a) Glucose vs. Pyruvate:

Glucose cannot enter the Krebs cycle directly; it must first be broken down into pyruvate via glycolysis [1].
Glycolysis occurs in the cytoplasm, not in the mitochondria [1].
Isolated mitochondria lack the enzymes for glycolysis, so glucose cannot be metabolized to produce CO₂. Pyruvate can enter the mitochondrial matrix and undergo the Link Reaction/Krebs cycle, producing CO₂ [1].

(b) Stage:

Krebs Cycle (Citric Acid Cycle) / Link Reaction [1]. (Accept either, as both produce CO2 in mitochondria, but Krebs is the major source).

5. (a) Solvent Properties:

Water is polar [1].
It forms hydrogen bonds with charged/polar solutes, allowing them to dissolve [1].

(b) Specific Heat Capacity:

Hydrogen bonds between water molecules require a large amount of energy to break [1].
This allows water to absorb much heat with only a small rise in temperature, stabilizing thermal conditions [1].

Section B: Data and Diagram Interpretation

6. (a) Phase S:

Synthesis phase / S phase [1].

(b) Process in S Phase:

DNA replication occurs [1].
Each chromosome is replicated to form two sister chromatids, doubling the DNA content [1].

During Meiosis I, homologous chromosomes separate into different cells [1].
This reduces the DNA content by half in each daughter cell compared to the cell at the end of Meiosis I [1].

7. (a) Reaction Type:

Condensation / Esterification [1].

(b) Suitability for Storage:

Triglycerides are insoluble in water, so they do not affect the water potential of cells/osmotic balance [1].
They have a high energy-to-mass ratio (more C-H bonds than carbohydrates), providing more energy per gram [1].
They are compact/hydrophobic, allowing large amounts of energy to be stored in a small volume [1].
(Any 2 points, 2 marks each for explanation) -> Total 4 marks.

8. (a) Mass Gain at 0.0 M:

The sucrose solution has a higher water potential (is hypotonic) than the potato cells [1].
Water enters the potato cells by osmosis [1].
This causes the cells to gain mass [1].

(b) Estimation of Water Potential:

The point where there is no change in mass (0% change) represents isotonic conditions [1].
From the table, this occurs between 0.2 and 0.4 mol dm⁻³ (closer to 0.4). Estimate approx 0.3 - 0.35 mol dm⁻³ [1].

9. (a) Dipeptide Structure:

Correct linkage of two amino acids via a peptide bond (C-N) [1].
Removal of water molecule (H from amine, OH from carboxyl) shown or implied in structure [1].

(b) Sequence and Structure:

The sequence of amino acids (primary structure) determines the interactions between R-groups [1].
These interactions (hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions) cause the polypeptide to fold [1].
This folding results in the specific tertiary (3D) shape of the protein [1].

10. (a) Optimum pH:

pH 7 [1].

(b) Shape at pH 2:

Low pH (high H⁺ concentration) disrupts ionic and hydrogen bonds in the enzyme's tertiary structure [1].
The active site changes shape (denaturation) [1].
Substrate can no longer bind effectively, reducing/Stopping activity [1].

Section C: Extended Response

11. Significance of Membrane Transport in Photosynthesis:

CO₂ Uptake:
- CO₂ is a raw material for the Calvin Cycle (light-independent reaction).
- It enters leaf mesophyll cells via diffusion through stomata and cell membranes down a concentration gradient.
- Without efficient CO₂ entry, the rate of photosynthesis is limited.
Water Uptake:
- Water is a raw material for the light-dependent reaction (photolysis).
- Water enters root hair cells via osmosis and is transported to leaves.
- Water movement into guard cells (via active transport of ions followed by osmosis) regulates stomatal opening, controlling CO₂ entry.
Ion Transport:
- Minerals like Magnesium (for chlorophyll) and Nitrates (for enzymes/proteins) are absorbed by root cells via active transport against concentration gradients.
- These are essential for the synthesis of photosynthetic pigments and enzymes.
Product Export:
- Glucose produced in photosynthesis is converted to sucrose for transport.
- Sucrose is loaded into phloem sieve tubes via active transport (co-transport with H⁺).
- This removes products from the leaf, preventing feedback inhibition and maintaining the concentration gradient for continued photosynthesis.
Thylakoid Membrane Function:
- The thylakoid membrane contains the electron transport chain and ATP synthase.
- It maintains a proton gradient (H⁺) by pumping protons into the thylakoid lumen during the light-dependent reaction.
- This gradient drives chemiosmosis to produce ATP, which is essential for the Calvin Cycle.

(Marking Guide: 6 marks available. Award 1 mark for each distinct, well-explained point linking membrane transport to photosynthesis. Max 6 marks.)