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Secondary 4 Pure Biology Preliminary Examination Paper 3

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TuitionGoWhere Practice Paper – Pure Biology Secondary 4

PRELIMINARY EXAMINATION – Version 3

TuitionGoWhere Secondary School (AI)

Subject: Pure Biology (6093)
Level: Secondary 4
Paper: Paper 2 (Structured and Free Response)
Duration: 1 hour 30 minutes
Total Marks: 75

Name: _________________________
Class: _________________________
Date: _________________________

INSTRUCTIONS TO CANDIDATES

This paper consists of three sections: Section A, Section B, and Section C.
Answer all questions in Section A and Section B.
In Section C, answer one question only.
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 55 minutes on Section A, 20 minutes on Section B, and 15 minutes on Section C.

SECTION A: Structured Questions (50 marks)

Answer all questions in this section. Write your answers in the spaces provided.

1. Fig. 1.1 shows an electron micrograph of an animal cell.

(a) Identify the organelles labelled P and Q in Fig. 1.1.

P: _________________________ [1]
Q: _________________________ [1]

(b) State one function of organelle P.

___________________________________________________________________________ [1]

___________________________________________________________________________ [2]

(d) A student observed that the cell in Fig. 1.1 contains many more organelle P compared to a typical skin cell. Suggest a reason for this observation.

___________________________________________________________________________ [2]

[Total: 7 marks]

2. A student investigated the effect of temperature on the activity of the enzyme amylase. Amylase breaks down starch into reducing sugars. The student set up five test tubes, each containing 5 cm³ of 1% starch solution and 1 cm³ of amylase solution, at different temperatures. After 10 minutes, the student tested each tube for the presence of starch using iodine solution. The results are shown in Table 2.1.

Table 2.1

Temperature (°C)	Colour with iodine solution after 10 minutes
10	Blue-black
20	Blue-black
30	Brown
40	Yellow-brown
50	Yellow-brown
60	Blue-black

(a) Explain why iodine solution is used to test for the presence of starch.

___________________________________________________________________________ [2]

(b) Describe the relationship between temperature and amylase activity shown by the results in Table 2.1.

___________________________________________________________________________ [3]

___________________________________________________________________________ [4]

(d) The student repeated the experiment but added 2 cm³ of amylase solution instead of 1 cm³ to each tube. Predict and explain how this would affect the time taken for the iodine test to show a yellow-brown colour at 40 °C.

___________________________________________________________________________ [3]

[Total: 12 marks]

3. Fig. 3.1 shows three red blood cells placed in solutions of different water potentials.

Cell A: In distilled water (high water potential)
Cell B: In 0.9% salt solution (same water potential as cytoplasm)
Cell C: In 5% salt solution (low water potential)

(a) State the term used to describe the appearance of Cell C.

___________________________________________________________________________ [1]

(b) Explain what has happened to Cell A to cause its appearance.

___________________________________________________________________________ [3]

(c) A patient was given an intravenous drip containing distilled water by mistake. Explain why this is dangerous, using your knowledge of osmosis.

___________________________________________________________________________ [4]

(d) Explain why a 0.9% salt solution is described as an isotonic solution for red blood cells.

___________________________________________________________________________ [2]

[Total: 10 marks]

4. Fig. 4.1 shows the structure of a cell membrane.

(a) Name the main type of molecule labelled X in Fig. 4.1.

___________________________________________________________________________ [1]

(b) State one function of the cell membrane.

___________________________________________________________________________ [1]

(c) Glucose molecules are too large to pass through the cell membrane by simple diffusion. Explain how glucose is absorbed into cells lining the small intestine.

___________________________________________________________________________ [4]

(d) Oxygen moves from the alveoli into the blood by diffusion. Explain two features of the alveoli that make this process efficient.

___________________________________________________________________________ [4]

[Total: 10 marks]

5. A student carried out food tests on four unknown solutions (W, X, Y, and Z). The results are shown in Table 5.1.

Table 5.1

Solution	Iodine test	Benedict's test (after heating)	Biuret test	Ethanol emulsion test
W	Blue-black	Blue	Blue	Colourless
X	Brown	Brick-red precipitate	Blue	Colourless
Y	Brown	Blue	Purple	Colourless
Z	Brown	Blue	Blue	White emulsion

(a) Identify the main type of biological molecule present in solution W. Give a reason for your answer.

___________________________________________________________________________ [2]

(b) Identify the main type of biological molecule present in solution X. Give a reason for your answer.

___________________________________________________________________________ [2]

(d) State one role of the type of biological molecule present in solution Z in living organisms.

___________________________________________________________________________ [1]

(e) A student wanted to confirm that solution X contained a reducing sugar. Describe a control experiment the student should set up.

___________________________________________________________________________ [3]

[Total: 10 marks]

6. Fig. 6.1 shows the lock-and-key model of enzyme action.

(a) Label the following on Fig. 6.1:

(i) The active site [1]
(ii) The substrate [1]

(b) Explain what is meant by the term "enzyme specificity".

___________________________________________________________________________ [3]

(c) A protease enzyme breaks down protein in the stomach. The optimum pH for this enzyme is pH 2. Explain what would happen to the activity of this enzyme if it entered the small intestine, where the pH is approximately pH 8.

___________________________________________________________________________ [4]

(d) State two factors, other than pH, that affect the rate of enzyme activity.

___________________________________________________________________________ [2]

[Total: 11 marks]

SECTION B: Data-Based Question (15 marks)

Answer all parts of this question. Write your answers in the spaces provided.

7. A group of students investigated the effect of different concentrations of sucrose solution on the mass of potato cylinders. They cut six potato cylinders, each 5 cm long, and weighed them. Each cylinder was placed in a different concentration of sucrose solution for 30 minutes. After 30 minutes, the cylinders were removed, blotted dry, and reweighed. The percentage change in mass was calculated for each cylinder. The results are shown in Table 7.1.

Table 7.1

Concentration of sucrose solution (mol/dm³)	Percentage change in mass (%)
0.0	+12.5
0.2	+6.0
0.4	+1.5
0.6	-4.0
0.8	-9.5
1.0	-14.0

(a) (i) Plot a graph of the results on the grid provided in Fig. 7.1. Use a suitable scale and label both axes. Join the points with a straight line of best fit. [4]

(ii) Use your graph to estimate the concentration of sucrose solution that has the same water potential as the potato cells. Show on your graph how you obtained your answer.

Concentration: _________________________ mol/dm³ [2]

(b) Explain why the potato cylinder placed in 0.0 mol/dm³ sucrose solution gained mass.

___________________________________________________________________________ [3]

(d) The students blotted the potato cylinders dry before reweighing them. Suggest why this step was important.

___________________________________________________________________________ [2]

(e) State one variable that the students should have controlled in this investigation.

___________________________________________________________________________ [1]

[Total: 15 marks]

SECTION C: Free Response Question (10 marks)

Answer one question only from this section. Write your answer in the space provided. You are advised to spend no more than 15 minutes on this section.

EITHER

8. (a) Describe the structure of a DNA molecule. [4]

(b) Explain how the structure of DNA allows it to carry genetic information and replicate accurately. [6]

9. (a) Describe the process of protein synthesis, from the DNA in the nucleus to the formation of a polypeptide chain. [4]

(b) Explain the importance of enzymes in living organisms, using named examples. [6]

END OF PAPER

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Answers

TuitionGoWhere Practice Paper – Pure Biology Secondary 4

PRELIMINARY EXAMINATION – Version 3

ANSWER KEY AND MARKING SCHEME

TuitionGoWhere Secondary School (AI)

Subject: Pure Biology (6093)
Level: Secondary 4
Paper: Paper 2 (Structured and Free Response)
Total Marks: 75

SECTION A: Structured Questions (50 marks)

Question 1: Cell Structure [7 marks]

(a)
P: Mitochondrion / Mitochondria [1]
Q: Ribosome [1]

(b)
Site of aerobic respiration / Releases energy (ATP) from glucose / Site of energy production [1]
Accept: "Powerhouse of the cell" or equivalent.

(c)

Ribosomes are the site where amino acids are joined together / linked [1]
To form polypeptide chains / proteins [1]
Marking note: Must link ribosome location to the process of protein assembly. Do not accept "ribosomes make protein" without explanation of the process.

(d)

The cell requires more energy / ATP [1]
Because it is likely a metabolically active cell (e.g., muscle cell, liver cell, sperm cell) / carries out more aerobic respiration [1]
Marking note: Award [1] for identifying higher energy demand and [1] for linking to cell function. Accept any reasonable suggestion of a cell type with high energy requirements.

Total: 7 marks

Question 2: Enzyme Activity [12 marks]

(a)

Iodine solution changes from yellow-brown to blue-black in the presence of starch [1]
A blue-black colour indicates starch is present; a yellow-brown colour indicates starch is absent / has been broken down [1]
Marking note: Must describe the colour change and what each colour indicates about starch presence.

(b)

As temperature increases from 10 °C to 40 °C, amylase activity increases / more starch is broken down [1]
At 40–50 °C, amylase activity is at its maximum / optimum (all starch broken down) [1]
Above 50 °C (at 60 °C), amylase activity decreases / enzyme is denatured (starch remains) [1]
Marking note: Award marks for describing the three phases of the relationship. Must reference the data (colours/temperatures).

(c)

At 40 °C, the enzyme is at or near its optimum temperature [1]
Enzyme and substrate molecules have high kinetic energy / more frequent successful collisions [1]
All starch is broken down (no blue-black colour with iodine) [1]
At 60 °C, the high temperature has denatured the enzyme / changed the shape of the active site [1]
The substrate can no longer fit into the active site / enzyme-substrate complex cannot form [1]
Starch is not broken down (blue-black colour with iodine) [1]
Marking note: Maximum 4 marks. Award marks for explaining both temperatures. Must include denaturation explanation for 60 °C.

(d)

The time taken would decrease / starch would be broken down faster [1]
Because there are more enzyme molecules / active sites available [1]
More enzyme-substrate complexes can form per unit time / increased rate of reaction [1]
Marking note: Must predict the direction of change and explain using enzyme concentration reasoning.

Total: 12 marks

Question 3: Osmosis and Red Blood Cells [10 marks]

(a)
Crenated / Crenation [1]

(b)

Distilled water has a higher water potential than the cytoplasm of the red blood cell [1]
Water enters the cell by osmosis / down the water potential gradient [1]
The cell swells and bursts / undergoes haemolysis (because it has no cell wall to resist expansion) [1]
Marking note: Must include direction of water movement and the consequence (bursting/haemolysis).

(c)

Distilled water has a much higher water potential than blood plasma / red blood cells [1]
Water would enter the red blood cells by osmosis / down the water potential gradient [1]
Red blood cells would swell and burst / undergo haemolysis [1]
This would reduce the oxygen-carrying capacity of the blood / lead to anaemia / could be fatal [1]
Marking note: Award marks for water potential comparison, direction of water movement, consequence to cells, and physiological danger. Accept any reasonable medical consequence.

(d)

The 0.9% salt solution has the same water potential as the cytoplasm of red blood cells [1]
There is no net movement of water into or out of the cells / cells neither swell nor shrink [1]
Marking note: Must explain "same water potential" and "no net movement".

Total: 10 marks

Question 4: Cell Membrane and Transport [10 marks]

(a)
Phospholipid / Phospholipid molecule [1]

(b)

Controls movement of substances into and out of the cell / selectively permeable / partially permeable [1]
Accept any one valid function: compartmentalisation, cell recognition, etc.

(c)

Glucose is absorbed by active transport [1]
Active transport requires energy / ATP [1]
Carrier proteins / protein pumps in the cell membrane transport glucose [1]
Glucose moves against its concentration gradient / from low to high concentration [1]
Marking note: Must identify active transport, energy requirement, carrier proteins, and movement against gradient.

(d)
Any two from:

Alveolar wall is one cell thick / thin wall → short diffusion distance [1]
Alveoli have a large surface area → more space for gas exchange [1]
Alveoli are surrounded by a dense network of capillaries → maintains concentration gradient / good blood supply [1]
Alveolar surface is moist → allows gases to dissolve before diffusing [1]
Marking note: Award [2] for each feature with explanation. Maximum 4 marks. Must state the feature AND explain how it increases efficiency.

Total: 10 marks

Question 5: Biological Molecules and Food Tests [10 marks]

(a)

Starch [1]
Iodine test gave a blue-black colour, which indicates the presence of starch [1]
Marking note: Must name the molecule and reference the test result.

(b)

Reducing sugar / Glucose [1]
Benedict's test gave a brick-red precipitate, which indicates the presence of reducing sugar [1]
Marking note: Accept "reducing sugar" or "glucose". Must reference the test result.

(c)

Biuret reagent detects peptide bonds [1]
Peptide bonds are present in proteins / polypeptides [1]
Marking note: Must link the colour change to the presence of peptide bonds in proteins.

(d)
Any one from:

Long-term energy storage / energy reserve [1]
Insulation / thermal insulation [1]
Protection of organs [1]
Component of cell membranes [1]
Source of metabolic water [1]
Marking note: Solution Z contains fats/lipids (positive ethanol emulsion test). Accept any valid role of lipids.

(e)

Use water / distilled water instead of the food sample [1]
Add the same volume of Benedict's solution [1]
Heat in the same way / same temperature and time [1]
The control should remain blue / show no colour change [1]
Marking note: Maximum 3 marks. Must describe a valid control with water, same procedure, and expected result.

Total: 10 marks

Question 6: Enzyme Action [11 marks]

(a)
(i) Active site: The indentation / groove on the enzyme where the substrate fits [1]
(ii) Substrate: The molecule that fits into the active site [1]
Marking note: Award marks for correct labelling on the diagram.

(b)

The active site of an enzyme has a specific shape [1]
Only a substrate with a complementary shape can fit into the active site [1]
Therefore, each enzyme can only catalyse one specific reaction / act on one specific substrate [1]
Marking note: Must explain the lock-and-key concept: specific active site shape, complementary substrate, and consequence of specificity.

(c)

The enzyme has an optimum pH of 2 / is adapted to work in acidic conditions [1]
At pH 8 (alkaline conditions), the enzyme would be denatured [1]
The shape of the active site would change / be altered [1]
The substrate would no longer fit into the active site / enzyme-substrate complex cannot form [1]
Enzyme activity would decrease significantly / stop [1]
Marking note: Maximum 4 marks. Must explain denaturation at non-optimum pH, change in active site shape, and consequence for activity.

(d)
Any two from:

Temperature [1]
Enzyme concentration [1]
Substrate concentration [1]
Marking note: Accept any two valid factors. Do not accept pH (already mentioned in question).

Total: 11 marks

SECTION B: Data-Based Question (15 marks)

Question 7: Osmosis Investigation [15 marks]

(a)(i)
Graph plotting [4 marks]:

Correct axes: x-axis = Concentration of sucrose solution (mol/dm³), y-axis = Percentage change in mass (%) [1]
Suitable linear scales using more than half the grid [1]
All six points plotted correctly (± half a small square) [1]
Straight line of best fit drawn (not dot-to-dot) [1]
Marking note: Deduct [1] if axes are not labelled with units. Deduct [1] if scale is inappropriate (too small, uneven intervals).

(a)(ii)

Concentration where the line crosses 0% change in mass [1]
Correct reading from graph: approximately 0.45–0.50 mol/dm³ [1]
Marking note: Accept answers in the range 0.45–0.50 mol/dm³. Must show working on the graph (horizontal line from 0% to the line of best fit, then vertical line down to x-axis).

(b)

The 0.0 mol/dm³ solution (distilled water) has a higher water potential than the potato cells [1]
Water enters the potato cells by osmosis / down the water potential gradient [1]
The cells gain water, increasing the mass of the cylinder [1]
Marking note: Must explain water potential difference, direction of water movement, and consequence for mass.

(c)

The 1.0 mol/dm³ sucrose solution has a lower water potential than the potato cells [1]
Water leaves the potato cells by osmosis / down the water potential gradient [1]
The cells lose water, decreasing the mass of the cylinder / cells become flaccid/plasmolysed [1]
Marking note: Must explain water potential difference, direction of water movement, and consequence for mass.

(d)

To remove excess water / sucrose solution from the surface of the cylinders [1]
So that the mass measured is only the mass of the potato tissue / to avoid inaccurate (higher) mass readings [1]
Marking note: Must explain the purpose (removing surface liquid) and the consequence of not doing so (inaccurate mass).

(e)
Any one from:

Length / size / surface area of potato cylinders [1]
Volume of sucrose solution used [1]
Time of immersion (30 minutes) [1]
Temperature [1]
Variety / type of potato [1]
Marking note: Accept any valid controlled variable. Must be a variable that could affect the results if not controlled.

Total: 15 marks

SECTION C: Free Response Question (10 marks)

Question 8: DNA Structure and Function [10 marks]

(a) Describe the structure of a DNA molecule. [4 marks]

DNA is a double helix / two strands twisted around each other [1]
Each strand is made up of nucleotides [1]
Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base [1]
The two strands are held together by hydrogen bonds between complementary base pairs: adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G) [1]
Marking note: Award marks for double helix, nucleotide composition, and complementary base pairing. Accept a labelled diagram with annotations.

(b) Explain how the structure of DNA allows it to carry genetic information and replicate accurately. [6 marks]

Carrying genetic information:

The sequence of bases along the DNA strand forms the genetic code [1]
Each gene (section of DNA) contains the code for one specific polypeptide / protein [1]
The genetic code is a triplet code: three bases code for one amino acid [1]

Accurate replication:

During replication, the two strands of DNA separate / unzip (hydrogen bonds break) [1]
Each strand acts as a template for the formation of a new complementary strand [1]
Free nucleotides pair with exposed bases according to complementary base pairing rules (A-T, C-G) [1]
This ensures that each new DNA molecule is identical to the original / semi-conservative replication [1]
Marking note: Maximum 6 marks. Must cover both carrying information and accurate replication. Award marks for clear explanation of the template mechanism and complementary base pairing ensuring accuracy.

Total: 10 marks

Question 9: Protein Synthesis and Enzymes [10 marks]

(a) Describe the process of protein synthesis, from the DNA in the nucleus to the formation of a polypeptide chain. [4 marks]

The DNA unwinds and one strand acts as a template [1]
mRNA is synthesised / transcribed (copying the DNA code) in the nucleus [1]
mRNA moves out of the nucleus to the ribosome in the cytoplasm [1]
At the ribosome, tRNA molecules bring specific amino acids; the ribosome reads the mRNA code (translation) and amino acids are joined to form a polypeptide chain [1]
Marking note: Award marks for transcription (DNA → mRNA in nucleus), mRNA movement, and translation (mRNA → polypeptide at ribosome). Accept a clear flow diagram.

(b) Explain the importance of enzymes in living organisms, using named examples. [6 marks]

Enzymes are biological catalysts that speed up chemical reactions without being used up [1]
They lower the activation energy required for reactions to occur [1]
Without enzymes, metabolic reactions would be too slow to sustain life [1]

Named examples (any two, with explanation):

Amylase: Breaks down starch into maltose/sugars in the mouth and small intestine; essential for digestion of carbohydrates [1]
Catalase: Breaks down hydrogen peroxide (toxic by-product of metabolism) into water and oxygen in liver cells; prevents cell damage [1]
DNA polymerase: Catalyses the synthesis of new DNA strands during DNA replication; essential for cell division and growth [1]
Protease (e.g., pepsin): Breaks down proteins into amino acids in the stomach; essential for protein digestion and absorption [1]
Marking note: Maximum 6 marks. Award up to [3] for general importance of enzymes (catalysts, activation energy, essential for life). Award up to [3] for named examples with clear explanation of their roles. Must name at least two enzymes with functions.

Total: 10 marks

END OF ANSWER KEY