Secondary 4 Combined Science Biology Preliminary Examination Paper 3

TuitionGoWhere Practice Paper — Combined Science Biology Secondary 4

Answer Key — Version 3 of 5

Section A: Multiple Choice and Short Answer Questions [20 marks]

Question 1 [1 mark]

Answer: C — Mitochondrion

Marking note: Award 1 mark for C only. The mitochondrion is the organelle where aerobic respiration occurs (Krebs cycle and electron transport chain on the inner membrane/cristae).

Question 2 [1 mark]

Answer: Diffusion

Marking note: Award 1 mark for "diffusion" or "diffusion down a concentration gradient." Oxygen moves passively from a high concentration in the alveoli to a lower concentration in the red blood cell. Do not accept "active transport" or "osmosis."

Question 3 [1 mark]

Answer: Glucose (carbohydrate / sugar)

Marking note: Award 1 mark for "glucose" or "carbohydrate" or "sugar." Glucose is the primary substrate for cellular respiration to release energy (ATP).

Question 4 [2 marks]

(a) [1] Answer: Mitochondrion

Marking note: Award 1 mark for correct identification. Structure B is described as a double-membraned organelle with cristae — this is the mitochondrion.

(b) [1] Answer: Aerobic respiration / releases energy (ATP) / produces ATP / site of respiration

Marking note: Award 1 mark for any correct function. Accept "site of aerobic respiration" or "produces energy/ATP." Do not accept vague answers like "makes energy."

Question 5 [2 marks]

(a) [1] Answer: Sample R. The iodine test turned blue-black, which indicates the presence of starch.

Marking note: Award 1 mark for correctly identifying Sample R AND linking the blue-black colour change of iodine to the presence of starch.

(b) [1] Answer: Sample Q

Marking note: Award 1 mark for Sample Q only. Benedict's test produced a brick-red precipitate, indicating the presence of reducing sugar.

Question 6 [2 marks]

(a) [1] Answer: Cell X has chloroplasts but Cell Y does not / Cell Y has a long extension (root hair) but Cell X does not / Cell X is rectangular whereas Cell Y is elongated.

Marking note: Award 1 mark for any valid visible structural difference. The key visible difference is the presence of chloroplasts in Cell X and their absence in Cell Y.

(b) [1] Answer: The long extension (root hair) increases the surface area for absorption of water and mineral ions.

Marking note: Award 1 mark for linking the elongated/large surface area of the root hair cell to increased absorption. Do not accept "it absorbs water" without reference to surface area.

Question 7 [2 marks]

Definition: An enzyme is a biological catalyst (protein) that speeds up the rate of a chemical reaction without being used up / without being changed.

Marking note (definition): Award 1 mark for stating that an enzyme is a biological catalyst / protein that speeds up reactions. Must include the idea of catalysis and that it is not consumed.

Factor: Temperature / pH

Marking note (factor): Award 1 mark for either "temperature" or "pH." These are the two main factors affecting enzyme activity.

Question 8 [2 marks]

(a) [1] Answer: 37°C

Marking note: Award 1 mark for 37°C. This is the temperature at which the rate of reaction is highest (the peak of the curve).

(b) [1] Answer: Above 37°C, the enzyme is denatured. The active site changes shape so the substrate can no longer fit / bind to it.

Marking note: Award 1 mark for stating that the enzyme is denatured AND explaining that the active site shape is altered so the substrate cannot bind. Both ideas needed for the mark.

Question 9 [3 marks]

(a) [1] Answer: Muscle cell

Marking note: Award 1 mark for muscle cell. It has 1 200 mitochondria per cell, the highest number in the table.

(b) [2] Answer: Muscle cells require a large amount of energy (ATP) for contraction. Since mitochondria are the site of aerobic respiration where ATP is produced, muscle cells need more mitochondria to meet their high energy demand.

Marking note: Award 2 marks as follows:

• 1 mark for stating that muscle cells need a lot of energy / ATP for contraction.
• 1 mark for linking mitochondria to ATP production / aerobic respiration.

Award only 1 mark if the answer states energy need but does not link to mitochondria, or vice versa.

Question 10 [2 marks]

Answer (any two of the following):

1. Diffusion involves the movement of any molecules/particles, whereas osmosis involves the movement of water molecules only.
2. Diffusion can occur in gases, liquids, and solids, whereas osmosis occurs across a partially permeable membrane.
3. Diffusion does not require a membrane, whereas osmosis requires a partially permeable membrane.
4. Diffusion occurs down a concentration gradient of the solute/gas, whereas osmosis occurs down a water potential gradient.

Marking note: Award 1 mark per valid difference, up to 2 marks. Each difference must clearly distinguish between the two processes. Vague answers such as "they are different" are not accepted.

Section B: Structured Response Questions [25 marks]

Question 11 [3 marks]

(a) [1] Marking note: Award 1 mark if all three labels are correctly placed on the diagram:

• Phospholipid bilayer — the double layer of phospholipids forming the main structure.
• Channel protein — a protein spanning the bilayer with a pore/hole through which molecules can pass.
• Cholesterol — a small molecule embedded within the phospholipid tails.

(b) [2] Answer: The membrane is described as "fluid" because the phospholipids and proteins can move laterally (sideways) within the membrane. It is described as a "mosaic" because the proteins are scattered throughout the phospholipid bilayer in an irregular pattern, like tiles in a mosaic.

Marking note: Award 2 marks as follows:

• 1 mark for explaining "fluid" — phospholipids/proteins can move laterally within the membrane.
• 1 mark for explaining "mosaic" — proteins are scattered/embedded in an irregular pattern within the bilayer.

Award 1 mark if only one term is correctly explained.

Question 12 [4 marks]

(a) [2] Answer: pH 7. At pH 7, the iodine solution remained brown, which means no starch was present — the amylase had completely broken down the starch. At all other pH values, the iodine turned blue-black, meaning starch was still present and the enzyme was less active.

Marking note: Award 2 marks as follows:

• 1 mark for identifying pH 7.
• 1 mark for explaining that the brown colour of iodine indicates starch has been broken down / no starch remains.

(b) [2] Answer: If the enzyme was boiled, the amylase would be denatured. The active site would change shape and the substrate (starch) would no longer be able to fit into it. The starch would not be broken down, and the iodine test would remain blue-black.

Marking note: Award 2 marks as follows:

• 1 mark for stating the enzyme would be denatured.
• 1 mark for explaining that the active site shape changes so the substrate cannot bind / fit.

Question 13 [4 marks]

(a) [3] Answer: The Visking tubing contains 10% sucrose solution (low water potential / high solute concentration). The surrounding distilled water has a high water potential (low solute concentration). Water molecules move by osmosis from the distilled water (high water potential) through the partially permeable membrane into the sucrose solution (low water potential). This increases the volume of liquid inside the tubing, causing the liquid level in the capillary tube to rise.

Marking note: Award 3 marks as follows:

• 1 mark for stating that water moves from high water potential (distilled water) to low water potential (sucrose solution).
• 1 mark for identifying osmosis as the process / water moving through a partially permeable membrane.
• 1 mark for linking the influx of water to the rise in liquid level in the capillary tube.

(b) [1] Answer: A partially permeable membrane / a difference in water potential (concentration gradient) across the membrane.

Marking note: Award 1 mark for either answer. Both conditions are necessary for osmosis.

Question 14 [3 marks]

(a) [1] Ribosome: Site of protein synthesis / where amino acids are joined together to form proteins / where mRNA is translated into polypeptides.

Marking note: Award 1 mark for stating that ribosomes synthesise proteins. Do not accept "makes proteins" without reference to synthesis.

(b) [1] Golgi apparatus: Modifies, packages, and sorts proteins / packages proteins into vesicles for transport / processes proteins received from the endoplasmic reticulum.

Marking note: Award 1 mark for any correct function related to modifying, packaging, or sorting proteins.

(c) [1] Mitochondrion: Produces ATP (energy) through aerobic respiration, which provides the energy needed for protein synthesis and transport.

Marking note: Award 1 mark for stating that mitochondria produce ATP/energy for cellular processes including protein secretion.

Question 15 [4 marks]

(a) [2] Answer: Juice M contains a higher concentration of vitamin C. It required only 8 drops to decolourise the DCPIP, compared to 15 drops for Juice N. This means each drop of Juice M contains more vitamin C, so a smaller volume is needed to reduce the same amount of DCPIP.

Marking note: Award 2 marks as follows:

• 1 mark for identifying Juice M.
• 1 mark for explaining that fewer drops means a higher concentration of vitamin C per drop.

(b) [2] Answer (any two of the following):

1. Volume of DCPIP solution used (1 cm³).
2. Concentration of DCPIP solution.
3. Temperature of the experiment.
4. Same method of adding drops (dropwise, same dropper).

Marking note: Award 1 mark per valid controlled variable, up to 2 marks. Do not accept "same fruit juice" or "same volume of juice" as these are the independent variable being tested.

Question 16 [4 marks]

(a) [2]

• Diagram 1: Isotonic solution
• Diagram 2: Hypotonic solution
• Diagram 3: Hypertonic solution

Marking note: Award 2 marks (1 mark for each correct pair, all three must be correct for full marks). Award 1 mark if two out of three are correct.

(b) [2] Answer: The solution surrounding the red blood cell in Diagram 2 has a higher water potential than the cell contents. Water molecules move by osmosis from the solution (high water potential) into the cell (lower water potential), causing the cell to swell.

Marking note: Award 2 marks as follows:

• 1 mark for stating the external solution has a higher water potential than the cell.
• 1 mark for stating water moves into the cell by osmosis, causing it to swell.

Question 17 [3 marks]

Answer: The concentrated salt solution has a lower water potential (higher solute concentration) than the cell cytoplasm. The cell cytoplasm has a higher water potential. Water molecules move by osmosis from the cell (higher water potential) across the partially permeable cell membrane into the concentrated salt solution (lower water potential). As water leaves the cell, the cell shrinks / decreases in volume.

Marking note: Award 3 marks as follows:

• 1 mark for correctly comparing water potentials (concentrated salt solution has lower water potential than the cell).
• 1 mark for identifying osmosis as the process.
• 1 mark for stating that water moves OUT of the cell, causing it to shrink.

Section C: Free Response / Extended Questions [15 marks]

Question 18 [5 marks]

(a) [3] Answer: Solution A is most likely cytoplasm. Reasons: (1) It has the highest water content (85%), which is characteristic of cytoplasm. (2) It contains a significant amount of protein (10%), which is consistent with the enzymes and structural proteins found in cytoplasm.

Marking note: Award 3 marks as follows:

• 1 mark for identifying Solution A.
• 1 mark for citing the high water content as a reason.
• 1 mark for citing the protein content (or other valid component) as a reason.

(b) [2] Answer: Solution C is most likely blood plasma. It has the highest water content (90%) and a relatively low protein content (2%), which is consistent with blood plasma composition. Blood plasma is mostly water with dissolved nutrients, mineral salts, and a small amount of protein.

Marking note: Award 2 marks as follows:

• 1 mark for identifying Solution C.
• 1 mark for a valid reason (high water content / appropriate composition matching plasma).

Question 19 [5 marks]

(a) [2] Answer: As temperature increases from 10°C to 70°C, the colorimeter reading (absorbance) increases, indicating that more red pigment (betalain) leaks out of the beetroot cells. The increase is gradual from 10°C to 40°C, then becomes much more rapid from 40°C to 60°C, and levels off between 60°C and 70°C.

Marking note: Award 2 marks as follows:

• 1 mark for describing the overall trend (absorbance increases with temperature).
• 1 mark for noting the change in rate (gradual at lower temperatures, rapid increase above 40°C, or levelling off at high temperatures).

(b) [2] Answer: At 60°C and 70°C, the high temperature denatures the proteins in the cell membrane and tonoplast (vacuole membrane). This damages the membrane structure, making it fully permeable. The red pigment betalain, which is normally contained within the vacuole, leaks out freely into the surrounding water, resulting in a high colorimeter reading.

Marking note: Award 2 marks as follows:

• 1 mark for stating that high temperature denatures membrane proteins / damages the membrane.
• 1 mark for explaining that this allows the pigment to leak out / the membrane becomes fully permeable.

(c) [1] Answer: To wash off any red pigment that may have already leaked out from the cut surfaces of the beetroot cylinders during preparation, so that it does not contaminate the distilled water at the start of the experiment.

Marking note: Award 1 mark for stating that washing removes surface pigment / prevents contamination of the distilled water at the start.

Question 20 [5 marks]

(a) [3] Answer (any three of the following):

1. Cell membrane
2. Nucleus
3. Cytoplasm
4. Mitochondrion
5. Endoplasmic reticulum
6. Ribosome

Marking note: Award 1 mark per correct structure, up to 3 marks. Both are eukaryotic cells, so they share typical eukaryotic organelles. Do not accept "cell wall," "chloroplast," or "large permanent vacuole" as these are plant-specific.

(b) [2] Answer: Chloroplast. The mesophyll cell is a leaf cell that carries out photosynthesis. Chloroplasts contain chlorophyll, which absorbs light energy for photosynthesis. The cheek epithelial cell is an animal cell and does not carry out photosynthesis, so it does not have chloroplasts.

Marking note: Award 2 marks as follows:

• 1 mark for identifying chloroplast (or large permanent vacuole / cell wall — accept any one plant-specific structure).
• 1 mark for explaining its function (photosynthesis / contains chlorophyll / absorbs light energy).

END OF ANSWER KEY

Total: 60 marks

© TuitionGoWhere Secondary School (AI) — Practice Paper Version 3 of 5