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Secondary 4 Pure Biology Practice Paper 4

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TuitionGoWhere Practice Paper - Pure Biology Secondary 4

TuitionGoWhere Practice Paper (AI)

Subject: Pure Biology (6093) Level: Secondary 4 Paper: Practice Paper Version 4 Duration: 1 hour 45 minutes Total Marks: 80

Name: _________________________ Class: _________________________ Date: _________________________

Instructions to Candidates

This paper consists of three sections: Section A, Section B, and Section C.
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 no more than 30 minutes on Section A, 40 minutes on Section B, and 35 minutes on Section C.
You may use a calculator.

Section A: Multiple Choice Questions

Answer all questions. Circle the letter of the correct answer.
[10 marks]

1. Which organelle is responsible for synthesising proteins in a cell?

A. Mitochondrion B. Ribosome C. Golgi body D. Chloroplast

[1 mark]

2. A student placed a drop of blood into distilled water and observed it under a microscope. The red blood cells burst. Which process caused this?

A. Diffusion of water out of the cells B. Active transport of water into the cells C. Osmosis of water into the cells D. Diffusion of solutes out of the cells

[1 mark]

3. Which of the following correctly pairs a food test with its positive result?

Test	Positive Result
A. Benedict's test	Purple colour
B. Iodine test	Brick-red precipitate
C. Biuret test	Blue-black colour
D. Ethanol emulsion test	Cloudy white emulsion

[1 mark]

4. The diagram below shows the molecular structure of an enzyme and its substrate.

Which statement best explains why the enzyme can only catalyse one specific reaction?

A. Enzymes are proteins made of amino acids. B. The active site has a specific shape complementary to the substrate. C. Enzymes are denatured at high temperatures. D. Enzymes lower the activation energy of reactions.

[1 mark]

5. A plant cell was placed in a concentrated salt solution. After 30 minutes, the cell membrane pulled away from the cell wall. What is this process called?

A. Turgor B. Crenation C. Lysis D. Plasmolysis

[1 mark]

6. Which of the following elements is found in proteins but NOT in carbohydrates?

A. Carbon B. Hydrogen C. Oxygen D. Nitrogen

[1 mark]

7. The graph below shows the effect of pH on the activity of two enzymes, P and Q.

At which pH does enzyme P work best?

A. pH 2 B. pH 4 C. pH 7 D. pH 9

[1 mark]

8. Which statement about active transport is correct?

A. It moves substances down a concentration gradient. B. It does not require energy from respiration. C. It requires carrier proteins in the cell membrane. D. It is the same process as diffusion.

[1 mark]

9. A student investigated the effect of temperature on the rate of diffusion of potassium permanganate crystals in water. Which variable must be kept constant?

A. The temperature of the water B. The mass of potassium permanganate used C. The time taken for the crystals to dissolve D. The colour of the solution

[1 mark]

10. Large biological molecules are made from smaller repeating units. Which row correctly matches the large molecule to its smaller unit?

Large Molecule	Smaller Unit
A. Starch	Amino acid
B. Protein	Glucose
C. Fat	Glycerol and fatty acids
D. Glycogen	Nucleotide

[1 mark]

Section B: Structured Questions

Answer all questions in the spaces provided.
[40 marks]

11. Fig. 11.1 shows an electron micrograph of an animal cell.

(a) Identify the organelles labelled X and Y. [2 marks]

X: _________________________

Y: _________________________

(b) Organelle X is described as the "powerhouse" of the cell. Explain why. [2 marks]

(c) Explain why organelle Y is often found attached to the endoplasmic reticulum. [2 marks]

[Total: 6 marks]

12. A student carried out an investigation to determine the effect of sucrose concentration on the mass of potato strips. Five potato strips of equal mass were placed in different concentrations of sucrose solution for 30 minutes. The results are shown in Table 12.1.

Table 12.1

Sucrose concentration (mol/dm³)	Initial mass (g)	Final mass (g)	Change in mass (g)	Percentage change in mass (%)
0.0	2.50	2.75	+0.25	+10.0
0.2	2.50	2.60	+0.10	+4.0
0.4	2.50	2.50	0.00	0.0
0.6	2.50	2.35	-0.15	-6.0
0.8	2.50	2.20	-0.30	-12.0

(a) Calculate the percentage change in mass for the potato strip placed in 0.2 mol/dm³ sucrose solution. Show your working. [2 marks]

(b) Explain why the potato strip placed in 0.0 mol/dm³ sucrose solution increased in mass. [3 marks]

(c) Using the data in Table 12.1, estimate the sucrose concentration that is isotonic to the potato cell sap. Explain your answer. [2 marks]

(d) State one variable that must be kept constant in this investigation to ensure valid results. [1 mark]

[Total: 8 marks]

13. Fig. 13.1 shows the effect of temperature on the rate of an enzyme-catalysed reaction.

(a) Describe the trend shown in the graph between 10°C and 40°C. [2 marks]

(b) Explain why the rate of reaction increases between 10°C and 40°C. [2 marks]

(c) Explain why the rate of reaction decreases sharply above 50°C. [3 marks]

(d) The enzyme in this investigation was obtained from a bacterium living in a hot spring at 70°C. Sketch on Fig. 13.1 the expected curve for this enzyme. [1 mark]

[Total: 8 marks]

14. Fig. 14.1 shows two red blood cells placed in different solutions.

(a) Describe what has happened to the red blood cell in Solution A. [2 marks]

(b) Explain why the red blood cell in Solution A changed in this way. [3 marks]

(c) Solution B is an isotonic solution. Explain what is meant by an isotonic solution and state why the red blood cell in Solution B did not change shape. [2 marks]

(d) A patient in hospital requires an intravenous drip. Explain why the drip solution must be isotonic to blood plasma. [2 marks]

[Total: 9 marks]

15. A student tested four unknown food samples (W, X, Y, and Z) using Benedict's test, iodine test, Biuret test, and the ethanol emulsion test. The results are shown in Table 15.1.

Table 15.1

Food Sample	Benedict's Test	Iodine Test	Biuret Test	Ethanol Emulsion Test
W	Blue	Brown	Blue	Clear
X	Brick-red	Brown	Blue	Clear
Y	Blue	Blue-black	Blue	Clear
Z	Blue	Brown	Purple	Cloudy white

(a) Identify the food substances present in sample X. [1 mark]

(b) Identify the food substances present in sample Z. [2 marks]

(c) Explain why the Benedict's test must be heated in a water bath. [2 marks]

(d) A student claimed that sample W contains only water. Evaluate this claim. [2 marks]

(e) Describe how you would carry out the ethanol emulsion test for fats. [2 marks]

[Total: 9 marks]

Section C: Data-Based and Extended Response Questions

Answer all questions in the spaces provided.
[30 marks]

16. Fig. 16.1 shows the fluid mosaic model of the cell membrane.

(a) Identify the structures labelled P and Q. [2 marks]

P: _________________________

Q: _________________________

(b) Explain why the model is described as "fluid mosaic." [2 marks]

(c) Structure Q is involved in facilitated diffusion. Explain how facilitated diffusion differs from simple diffusion. [3 marks]

(d) Structure Q is also involved in active transport. Explain why active transport requires energy from respiration. [2 marks]

[Total: 9 marks]

17. A group of students investigated the effect of pH on the activity of catalase, an enzyme found in potato tissue that breaks down hydrogen peroxide into water and oxygen. They measured the volume of oxygen produced in 2 minutes at different pH values. The results are shown in Table 17.1.

Table 17.1

pH	Volume of oxygen produced in 2 minutes (cm³)
3	2
5	8
7	18
9	12
11	1

(a) Plot a graph of the results on the grid provided. Join the points with a smooth curve. [4 marks]

[Graph grid with axes: x-axis labelled "pH", y-axis labelled "Volume of oxygen produced in 2 minutes (cm³)"]

(b) Using your graph, determine the optimum pH for catalase. [1 mark]

(c) Explain why the volume of oxygen produced is low at pH 3 and pH 11. [3 marks]

(d) State two variables, other than pH, that must be kept constant in this investigation. [2 marks]

(e) Suggest how the students could modify the investigation to determine if the enzyme had been permanently denatured at pH 11. [2 marks]

[Total: 12 marks]

18. Fig. 18.1 shows the structure of a DNA molecule.

(a) Identify the components labelled R, S, and T. [3 marks]

R: _________________________

S: _________________________

T: _________________________

(b) State the complementary base-pairing rule in DNA. [1 mark]

(c) Explain how the structure of DNA allows it to store genetic information. [2 marks]

(d) A section of one DNA strand has the base sequence: A T G C C G T A. Write the base sequence of the complementary strand. [1 mark]

(e) Explain the relationship between DNA, genes, and chromosomes. [2 marks]

[Total: 9 marks]

END OF PAPER

Answers

TuitionGoWhere Practice Paper - Pure Biology Secondary 4

Answer Key and Marking Scheme

Paper: Practice Paper Version 4 Total Marks: 80

Section A: Multiple Choice Questions

[10 marks — 1 mark each]

Question	Answer	Explanation
1	B	Ribosomes are the sites of protein synthesis. Mitochondria produce ATP, Golgi body modifies and packages proteins, chloroplasts carry out photosynthesis.
2	C	Distilled water has a higher water potential than the cell contents. Water enters the red blood cells by osmosis, causing them to swell and burst (haemolysis).
3	D	Ethanol emulsion test gives a cloudy white emulsion with fats. Benedict's gives brick-red with reducing sugars, iodine gives blue-black with starch, Biuret gives purple with proteins.
4	B	The lock-and-key model states that the active site has a specific shape that is complementary to the substrate, allowing only specific substrates to bind.
5	D	Plasmolysis occurs when a plant cell loses water by osmosis in a hypertonic solution, causing the cell membrane to pull away from the cell wall.
6	D	Proteins contain carbon, hydrogen, oxygen, and nitrogen (and sometimes sulfur). Carbohydrates contain only carbon, hydrogen, and oxygen.
7	A	[Accept answer based on graph provided in question — enzyme P peaks at pH 2]
8	C	Active transport uses carrier proteins and requires energy (ATP) to move substances against the concentration gradient.
9	B	The mass of potassium permanganate must be kept constant. Temperature is the independent variable being changed.
10	C	Fats are made from glycerol and fatty acids. Starch is made from glucose, proteins from amino acids, glycogen from glucose.

Section B: Structured Questions

[40 marks]

Question 11 [6 marks]

(a) [2 marks]

X: Mitochondrion / Mitochondria [1 mark]
Y: Ribosome / Ribosomes [1 mark]

(b) [2 marks]

Mitochondria are the sites of aerobic respiration [1 mark]
where glucose is broken down to release energy in the form of ATP, which powers cellular activities [1 mark]

(c) [2 marks]

Ribosomes are the sites of protein synthesis [1 mark]
Proteins synthesised by ribosomes on the rough endoplasmic reticulum are transported through the ER for modification and transport to the Golgi body [1 mark]

Question 12 [8 marks]

(a) [2 marks]

Percentage change = (Change in mass ÷ Initial mass) × 100
= (+0.10 ÷ 2.50) × 100 = +4.0% [1 mark for correct working, 1 mark for correct answer]

(b) [3 marks]

The 0.0 mol/dm³ solution (distilled water) has a higher water potential than the potato cell sap [1 mark]
Water enters the potato cells by osmosis, down the water potential gradient [1 mark]
The cells become turgid, increasing the mass of the potato strip [1 mark]

(c) [2 marks]

The isotonic concentration is approximately 0.4 mol/dm³ [1 mark]
At this concentration, there is no net movement of water into or out of the cells, so there is no change in mass (0.0% change) [1 mark]

(d) [1 mark]

Any one of: temperature, volume of sucrose solution, time of immersion, surface area of potato strips, same potato variety [1 mark]

Question 13 [8 marks]

(a) [2 marks]

The rate of reaction increases as temperature increases from 10°C to 40°C [1 mark]
The rate reaches a maximum at approximately 40°C (the optimum temperature) [1 mark]

(b) [2 marks]

As temperature increases, the kinetic energy of enzyme and substrate molecules increases [1 mark]
This increases the frequency of effective collisions between enzyme and substrate, leading to more enzyme-substrate complexes formed per unit time [1 mark]

(c) [3 marks]

Above 50°C, the enzyme denatures [1 mark]
High temperatures break the hydrogen bonds that maintain the enzyme's specific three-dimensional (tertiary) structure [1 mark]
The active site loses its specific shape, so the substrate can no longer bind, and enzyme-substrate complexes cannot form [1 mark]

(d) [1 mark]

Curve should be shifted to the right, with optimum at approximately 70°C [1 mark for correct positioning]

Question 14 [9 marks]

(a) [2 marks]

The red blood cell has shrunk / become crenated [1 mark]
It has a spiky, shrivelled appearance [1 mark]

(b) [3 marks]

Solution A has a lower water potential than the cell contents (it is hypertonic) [1 mark]
Water moves out of the red blood cell by osmosis, down the water potential gradient [1 mark]
The cell loses water, shrinks, and becomes crenated [1 mark]

(c) [2 marks]

An isotonic solution has the same water potential as the cell contents [1 mark]
There is no net movement of water into or out of the cell, so the cell retains its normal shape [1 mark]

(d) [2 marks]

If the drip solution is not isotonic, water will move into or out of the red blood cells by osmosis [1 mark]
This could cause red blood cells to burst (in a hypotonic solution) or shrink/crenate (in a hypertonic solution), which would be harmful to the patient [1 mark]

Question 15 [9 marks]

(a) [1 mark]

Reducing sugar (only) [1 mark]

(b) [2 marks]

Protein [1 mark]
Fats / Lipids [1 mark]

(c) [2 marks]

Heating provides activation energy for the reaction [1 mark]
This allows reducing sugars to react with copper(II) sulfate in Benedict's solution, producing a colour change from blue to brick-red precipitate [1 mark]

(d) [2 marks]

The claim may not be entirely correct [1 mark]
Sample W gave negative results for reducing sugar, starch, protein, and fats, but it could contain non-reducing sugars (e.g., sucrose) or other dissolved substances not tested for [1 mark]

(e) [2 marks]

Add about 2 cm³ of the food sample to a test tube [1 mark]
Add about 2 cm³ of ethanol and shake vigorously; then add an equal volume of water and shake again. A cloudy white emulsion indicates the presence of fats [1 mark]

Section C: Data-Based and Extended Response Questions

[30 marks]

Question 16 [9 marks]

(a) [2 marks]

P: Phospholipid / Phospholipid bilayer [1 mark]
Q: Protein / Carrier protein / Channel protein [1 mark]

(b) [2 marks]

"Fluid" because the phospholipid molecules can move laterally within the membrane [1 mark]
"Mosaic" because proteins are scattered throughout the phospholipid bilayer, forming a mosaic pattern [1 mark]

(c) [3 marks]

Facilitated diffusion involves carrier proteins or channel proteins in the membrane, while simple diffusion does not [1 mark]
Both processes move substances down the concentration gradient and do not require energy [1 mark]
Facilitated diffusion allows larger or charged molecules (e.g., glucose, ions) to cross the membrane that cannot pass through the phospholipid bilayer by simple diffusion [1 mark]

(d) [2 marks]

Active transport moves substances against the concentration gradient (from lower to higher concentration) [1 mark]
Energy (ATP) is needed to change the shape of the carrier protein to move the substance across the membrane [1 mark]

Question 17 [12 marks]

(a) [4 marks]

Correct axes labelled with units [1 mark]
All points plotted correctly [1 mark]
Appropriate scales chosen [1 mark]
Smooth curve drawn through points [1 mark]

(b) [1 mark]

pH 7 [1 mark]

(c) [3 marks]

At pH 3 and pH 11, the enzyme is denatured [1 mark]
Extreme pH values disrupt the ionic and hydrogen bonds that maintain the enzyme's specific three-dimensional shape [1 mark]
The active site loses its specific shape, so the substrate (hydrogen peroxide) can no longer bind, and few enzyme-substrate complexes are formed [1 mark]

(d) [2 marks]

Any two of: temperature, concentration/volume of hydrogen peroxide, mass/surface area of potato tissue, time of reaction [1 mark each]

(e) [2 marks]

Return the enzyme from pH 11 to pH 7 (the optimum pH) [1 mark]
Measure the volume of oxygen produced. If the enzyme was permanently denatured, activity will not recover. If the change was reversible, activity will increase [1 mark]

Question 18 [9 marks]

(a) [3 marks]

R: Phosphate group [1 mark]
S: Deoxyribose sugar [1 mark]
T: Nitrogenous base / Base [1 mark]

(b) [1 mark]

Adenine (A) pairs with Thymine (T); Cytosine (C) pairs with Guanine (G) [1 mark]

(c) [2 marks]

The sequence of nitrogenous bases along the DNA strand forms the genetic code [1 mark]
Different sequences of bases code for different proteins, allowing DNA to store vast amounts of genetic information [1 mark]

(d) [1 mark]

T A C G G C A T [1 mark]

(e) [2 marks]

DNA is a long molecule that carries the genetic code [1 mark]
A gene is a specific sequence of DNA bases that codes for one polypeptide/protein. Chromosomes are structures made of tightly coiled DNA, containing many genes [1 mark]

END OF ANSWER KEY