Secondary 4 Pure Biology Preliminary Examination Paper 5

TuitionGoWhere Practice Paper – Pure Biology Secondary 4

PRELIMINARY EXAMINATION – Version 5 – ANSWERS

SECTION A (60 marks)

Question 1: Cell Structure and Function

(a)
P: Mitochondrion / Mitochondria
Q: Rough endoplasmic reticulum / RER
R: Ribosome / Ribosomes

(b)
Synthesises / transports proteins;
(accept: folds/modifies proteins, transports proteins to Golgi body)

(c)
Muscle cells require a lot of energy / ATP for contraction;
Mitochondria are the site of aerobic respiration / release energy;

(d)
Membrane system: Rough endoplasmic reticulum;
Role: Synthesises / transports proteins;

Question 2: Movement of Substances

(a)
Working for 0.0 mol/dm³:
Percentage change = (change in mass / initial mass) × 100
= (+0.25 / 2.50) × 100 = +10%

(b)
Water potential of solution is higher than water potential of potato cells / potato cells have lower water potential;
Water enters cells by osmosis;
Cells become turgid / gain mass;

(c)
Water potential of potato tissue is equivalent to 0.4 mol/dm³ sucrose solution;
Because there is no net movement of water / no change in mass;
Indicating water potentials are equal;

Question 3: Biological Molecules and Enzymes

(a)
As temperature increases from 10 °C to 40 °C, the rate of reaction increases;
(accept: rate increases proportionally / steadily)

(b)
As temperature increases, kinetic energy of molecules increases;
Molecules move faster, increasing frequency of collisions between enzyme and substrate;
More enzyme–substrate complexes formed per unit time;

(c)
Above 40 °C, the enzyme is denatured;
The active site loses its specific shape;
Substrate can no longer bind / enzyme–substrate complex cannot form;

(d)
Disagree;
Enzyme from hot spring bacteria would have a higher optimum temperature / would still be active above 40 °C;
The graph shows the enzyme denatures above 40 °C, indicating it is not from a hot spring bacterium;

Question 4: Cell Specialisation

(a) (i) Red blood cell:
Adaptation: Contains haemoglobin;
Explanation: Binds to oxygen for transport around the body;
(accept: biconcave shape – increases surface area for oxygen diffusion; no nucleus – more space for haemoglobin)

(ii) Root hair cell:
Adaptation: Long / elongated projection / root hair;
Explanation: Increases surface area for absorption of water and mineral ions;

(iii) Muscle cell:
Adaptation: Contains many mitochondria;
Explanation: Provides energy / ATP for contraction;
(accept: contains protein fibres – can contract/shorten)

Question 5: Diffusion and Active Transport

(a)
Set A: 60 arbitrary units
Set B: 5 arbitrary units

(b)
Potassium ions are absorbed by active transport;
Active transport requires energy from respiration;
Warm, aerated conditions allow respiration to occur;

(c)
In Set B, low temperature reduces rate of respiration / enzyme activity;
Lack of aeration means less oxygen available for respiration;
Less energy / ATP produced, so less active transport;

(d)
Aeration provides oxygen;
Oxygen is needed for aerobic respiration;
Respiration releases energy / ATP required for active transport;

Question 6: Osmosis in Plant and Animal Cells

(a)
Osmosis is the net movement of water molecules;
From a region of higher water potential to a region of lower water potential;
Through a partially permeable membrane;

(b)
Water moves out of the cell by osmosis;
Cell shrinks / shrivels / becomes crenated;
Because water moves from higher water potential (inside cell) to lower water potential (outside solution);

(c)
Water moves out of the cell by osmosis;
Cell becomes flaccid / plasmolysed (cell membrane pulls away from cell wall);
Plant cell has a cell wall which prevents bursting / maintains shape;
Red blood cell has no cell wall, so it shrinks/crenates;

Question 7: Food Tests

(a)
Protein is present;
Biuret test turned purple, which is a positive result for protein;
Other tests were negative;

(b)
Add Benedict's solution to the sample;
Heat in a water bath / boil;
Observe colour change (blue to green/yellow/orange/brick-red if reducing sugar present);

(c)
Ethanol dissolves fats;
When water is added, the fat comes out of solution / forms a fine suspension;
This suspension scatters light, appearing cloudy white;

Question 8: Enzyme Specificity

(a)
Enzymes have an active site with a specific shape;
Only a substrate with a complementary shape can bind / fit into the active site;

(b)
Substrate molecule fits into the active site of the enzyme (like a key into a lock);
Forms an enzyme–substrate complex;
Reaction occurs / bonds in substrate are broken;
Products are released;
Enzyme remains unchanged and can be reused;

Question 9: Carbohydrates, Fats, and Proteins

(a)
Carbon, hydrogen, oxygen;

(b)
Carbon, hydrogen, oxygen, nitrogen;
(accept: sometimes sulfur/phosphorus)

(c)
Many glucose / simple sugar units join together;
By condensation reactions / forming glycosidic bonds;

(d)

1. Energy storage / source of energy;
2. Thermal insulation;
3. Protection of organs;
4. Component of cell membranes;
   (any two)

SECTION B (20 marks)

Question 10: Enzymes – Extended Response

(a)
Enzymes are proteins;
They have a specific three-dimensional shape;
Contain an active site;
Active site has a shape complementary to the substrate;
This allows the enzyme to bind specifically to its substrate;
Enzyme–substrate complex forms, lowering activation energy;

(b)
Use starch solution and amylase;
Set up a series of buffer solutions with different pH values (e.g., pH 4, 5, 6, 7, 8, 9);
Add equal volumes of amylase to each;
Add equal volumes of starch solution;
Start timing;
At regular intervals, remove samples and test with iodine solution;
Record time taken for iodine to remain yellow-brown (starch no longer present);
Rate of reaction = 1/time;
Repeat and calculate mean;
Control variables: temperature, enzyme concentration, substrate concentration;

(c)
Biological washing powders contain enzymes (e.g., proteases, lipases);
Enzymes break down stains (e.g., proteins, fats) into smaller, soluble molecules;
These molecules can be washed away more easily;
More effective at lower temperatures (save energy);
Limitation: Enzymes can be denatured at high temperatures;
(accept: may cause allergic reactions; may damage certain fabrics like wool/silk)

Question 11: Cells and Transport – Extended Response

(a)
Similarities: Both have cell membrane, cytoplasm, nucleus, mitochondria, ribosomes;
Differences: Plant cell has cell wall (animal cell does not);
Plant cell has chloroplasts (animal cell does not);
Plant cell has large central vacuole (animal cell has small/temporary vacuoles);
Plant cells often have a fixed/regular shape due to cell wall;
Cell wall provides structural support (important for plants which do not have skeletons);
Chloroplasts allow photosynthesis to produce food;
Large vacuole stores water/solutes, maintains turgor pressure;

(b)
Diffusion: Net movement of particles from a region of higher concentration to a region of lower concentration;
Example: Oxygen diffusing from alveoli into blood capillaries;
Importance: Allows gas exchange for respiration;

Osmosis: Net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane;
Example: Water absorption by root hair cells from soil;
Importance: Allows plants to obtain water for photosynthesis/support;

Active transport: Movement of molecules/ions from a region of lower concentration to a region of higher concentration, against the concentration gradient, using energy;
Example: Absorption of mineral ions by root hair cells;
Importance: Allows plants to obtain minerals needed for growth even when soil concentration is low;

(c)
Patient's red blood cells have a lower water potential than the distilled water;
Water enters the cells by osmosis;
Red blood cells have no cell wall;
Cells swell and burst / haemolyse;
This would reduce the number of red blood cells / cause harm;
Therefore, distilled water should not be used;
A saline solution with the same water potential as blood plasma should be used (isotonic);