Secondary 4 Combined Science Biology Cells Biomolecules Quiz

Secondary 4 Combined Science Biology Quiz - Cells Biomolecules

Answer Key

Section A: Multiple Choice & Short Answer (Questions 1–10)

1. C — Mitochondrion
[1 mark]
Common mistake: Students may select ribosome (protein synthesis) or Golgi apparatus (packaging/secretion). The mitochondrion is the site of aerobic respiration where ATP is produced.

2. C — Muscle cell
[1 mark]
Muscle cells require large amounts of energy (ATP) for contraction, so they contain many mitochondria. Red blood cells (in mammals) have no mitochondria. Fat storage cells and xylem vessels have relatively few mitochondria.

3. D — Carbohydrates
[1 mark]
Carbohydrates (especially glucose) are the primary immediate energy source. Lipids are for long-term energy storage. Proteins are for growth and repair. Nucleic acids carry genetic information.

4. C — Diffusion
[1 mark]
Oxygen moves from a high concentration in the alveoli to a lower concentration in the blood capillaries by diffusion, down the concentration gradient. No energy is required.

5. B — Iodine solution
[1 mark]
Iodine solution turns blue-black in the presence of starch. Benedict's solution tests for reducing sugars. Biuret reagent tests for proteins. Ethanol is used in the emulsion test for lipids.

6. The cell membrane controls/regulates the movement of substances into and out of the cell (partially/selectively permeable).
[1 mark]
Award 1 mark for stating that it controls/regulates entry and exit of substances. Accept "partially permeable" or "selectively permeable" as part of the answer.

7. Ribosome
[1 mark]
Ribosomes are the site of protein synthesis. Accept "rough endoplasmic reticulum" only if the answer specifies that ribosomes on its surface carry out protein synthesis.

8. Amylase breaks down starch into maltose (sugar).
[1 mark]
Award 1 mark for stating that amylase digests/breaks down starch into maltose. Accept "breaks down starch into sugar" as a simplified answer.

9. Diffusion
[1 mark]
Carbon dioxide moves from the muscle cell (high concentration) to the blood capillary (low concentration) by diffusion, down the concentration gradient. Accept "diffusion down a concentration gradient" for full credit.

10. Lipid (fat/oil)
[1 mark]
Lipids are insoluble in water and serve as a long-term energy store. Accept "fat" or "oil". Do not accept "protein" — proteins are not primarily energy stores and some are water-soluble.

Section B: Structured Response (Questions 11–17)

11.
(a) X = Mitochondrion
[1 mark]

(b) The mitochondrion is the site of aerobic respiration, where energy (ATP) is released from glucose.
[1 mark]
Accept: "site of respiration" or "produces energy/ATP".

(c) The highly folded inner membrane (cristae) increases the surface area for the attachment of enzymes involved in aerobic respiration, allowing more ATP to be produced.
[2 marks]
*Mark breakdown:

• 1 mark for "increases surface area"
• 1 mark for linking increased surface area to more enzymes / more respiration / more ATP production*

12.
(a) Sample P contains starch. The iodine test turned blue-black, which is the positive result for starch.
[1 mark]

(b) Sample Q contains reducing sugar. The Benedict's test produced a brick-red precipitate, which is the positive result for reducing sugar.
[1 mark]

(c) Sample R contains protein. The Biuret test turned purple, which is the positive result for protein.
[1 mark]

13.
(a) Muscle cells require more energy (ATP) than skin cells because they contract frequently. Since mitochondria are the site of aerobic respiration where ATP is produced, muscle cells need more mitochondria to meet their higher energy demand.
[2 marks]
*Mark breakdown:

• 1 mark for stating that muscle cells need more energy for contraction
• 1 mark for linking higher energy demand to more mitochondria*

(b) Muscle cells carry out continuous, intense contraction which requires a very large and sustained supply of ATP. Liver cells, while requiring energy for detoxification, do not need as much ATP at any one time because detoxification reactions are less energy-intensive than muscle contraction. Therefore, liver cells require fewer mitochondria.
[2 marks]
*Mark breakdown:

• 1 mark for comparing the energy demands (muscle cells need more ATP than liver cells)
• 1 mark for linking this to the number of mitochondria*

14.
(a) As temperature increases from 10 °C to 40 °C, the enzyme activity increases (the volume of oxygen collected increases from 2 cm³ to 30 cm³).
[1 mark]
Award 1 mark for describing the increase in activity. Do not award if the student only quotes figures without describing the trend.

(b) At 50 °C and 60 °C, the enzyme (catalase) is denatured. The high temperature causes the active site of the enzyme to change shape, so the substrate (hydrogen peroxide) can no longer fit into the active site, and the reaction slows down or stops.
[2 marks]
*Mark breakdown:

• 1 mark for stating that the enzyme is denatured
• 1 mark for explaining that the active site changes shape and the substrate can no longer bind*

(c) The optimum temperature is 40 °C.
[1 mark]
This is the temperature at which the highest volume of oxygen (30 cm³) was collected, indicating maximum enzyme activity.

15.
(a) Any two of the following:

• Cell wall
• Chloroplast
• Large central vacuole
  [2 marks — 1 mark each]
  Do not accept "cytoplasm", "nucleus", "cell membrane", or "mitochondrion" as these are found in both plant and animal cells.

(b) The large central vacuole is filled with cell sap (a solution of sugars, salts, and water). When the vacuole is full of water, it pushes the cytoplasm against the cell wall, keeping the cell turgid and providing structural support to the plant.
[1 mark]
Accept any answer that links the vacuole to maintaining turgidity / rigidity / structural support.

16.
(a) The water outside the tubing will remain brown-yellow (negative result) when tested with iodine solution. Starch molecules are too large to pass through the pores of the dialysis tubing, so no starch enters the surrounding water.
[2 marks]
*Mark breakdown:

• 1 mark for correct prediction (no colour change / remains brown-yellow)
• 1 mark for correct explanation (starch molecules are too large to pass through the membrane)*

(b) The water outside the tubing will turn brick-red (positive result) when tested with Benedict's solution after heating. Glucose molecules are small enough to diffuse out of the dialysis tubing into the surrounding water. Glucose is a reducing sugar, so it gives a positive Benedict's test.
[2 marks]
*Mark breakdown:

• 1 mark for correct prediction (brick-red precipitate / positive result)
• 1 mark for correct explanation (glucose is small enough to diffuse through the membrane)*

17. Enzymes are biological catalysts because they speed up (increase the rate of) chemical reactions in living organisms without being used up or permanently changed in the process. They lower the activation energy required for the reaction to occur. After the reaction, the enzyme remains unchanged and can be reused for further reactions.
[3 marks]
*Mark breakdown:

• 1 mark for stating that enzymes speed up/increase the rate of reactions
• 1 mark for stating that enzymes are not used up / remain unchanged / can be reused
• 1 mark for stating that they are biological (found in living organisms) OR that they lower activation energy*

Section C: Data Interpretation & Extended Response (Questions 18–20)

18.
(a) Molecule A has the smallest molecular size. This is because molecule A diffuses across the membrane at the fastest rate (reaches the highest rate of diffusion in the shortest time), and smaller molecules diffuse more quickly through a partially permeable membrane than larger ones.
[2 marks]
*Mark breakdown:

• 1 mark for identifying molecule A
• 1 mark for linking faster diffusion rate to smaller molecular size*

(b) Molecule A reaches a plateau before molecule B because molecule A is smaller and diffuses more rapidly, so the concentration gradient across the membrane is established and reaches equilibrium sooner. Molecule B, being larger, diffuses more slowly and therefore takes longer to reach equilibrium.
[2 marks]
*Mark breakdown:

• 1 mark for explaining that molecule A reaches equilibrium faster because it diffuses more quickly
• 1 mark for linking this to the concentration gradient / equilibrium being established*

(c) Any one of the following:

• Temperature
• Concentration gradient (difference in concentration on either side of the membrane)
• Surface area of the membrane
• Thickness of the membrane
  [1 mark]

19.
(a) As pH decreases (becomes more acidic), the activity of pepsin increases (digestion time decreases). Pepsin works best at low pH (acidic conditions). At pH 1–2, digestion is fastest (4–5 minutes). As pH increases towards 5, activity decreases sharply, and at pH 5, no digestion occurs.
[2 marks]
*Mark breakdown:

• 1 mark for describing the trend (activity increases as pH decreases / pepsin works best in acidic conditions)
• 1 mark for referencing data from the table to support the trend*

(b) At pH 5, the enzyme pepsin is denatured. The active site of pepsin changes shape because the pH is too far from its optimum (pH 1–2). The substrate (protein/egg white) can no longer bind to the active site, so no digestion occurs.
[2 marks]
*Mark breakdown:

• 1 mark for stating that pepsin is denatured at pH 5
• 1 mark for explaining that the active site changes shape and the substrate cannot bind*

(c) Hydrochloric acid in the stomach creates an acidic environment (low pH) which is the optimum pH for pepsin to function. Without this acidic pH, pepsin would be less active or denatured, and protein digestion would be very slow or not occur. The acid also helps to kill harmful bacteria in food.
[2 marks]
*Mark breakdown:

• 1 mark for stating that HCl provides the optimum acidic pH for pepsin
• 1 mark for explaining that this allows efficient protein digestion (or for mentioning the antibacterial role)*

20.
(a) Root hair cells are found underground in the soil where there is no light. Chloroplasts are the organelles responsible for photosynthesis, which requires light. Since root hair cells do not carry out photosynthesis, they do not need chloroplasts.
[2 marks]
*Mark breakdown:

• 1 mark for stating that root hair cells are in the dark / underground / no light reaches them
• 1 mark for linking this to the fact that photosynthesis requires light, so chloroplasts are not needed*

(b) The long, thin extension of the root hair cell greatly increases the surface area to volume ratio of the cell. This allows the cell to absorb water and mineral ions from the soil more efficiently by diffusion and active transport, because a larger surface area means more contact with the soil solution.
[2 marks]
*Mark breakdown:

• 1 mark for stating that the extension increases the surface area (to volume ratio)
• 1 mark for linking increased surface area to more efficient absorption of water and mineral ions*

(c) The mitochondrion is responsible for providing energy for active transport. The mitochondrion carries out aerobic respiration, breaking down glucose in the presence of oxygen to produce ATP (energy). This ATP is used to power the active transport of mineral ions into the root hair cell against the concentration gradient.
[2 marks]
*Mark breakdown:

• 1 mark for naming the mitochondrion
• 1 mark for explaining that it produces ATP through aerobic respiration, which provides energy for active transport*

END OF ANSWER KEY