Secondary 4 Pure Biology Practice Paper 1

Answer Key - Pure Biology Secondary 4 Practice Paper (Version 1)

Section A: Cellular and Molecular Basis of Life

Question 1 (a) Ribosomes [1] (b)

• Biconcave shape increases surface area to volume ratio for faster diffusion of oxygen [1].
• Absence of nucleus provides more space for haemoglobin to bind oxygen [1].
• Contains haemoglobin which has a high affinity for oxygen [1]. (c)
• The 0.6 mol dm⁻³ sucrose solution has a lower water potential than the cell sap of the potato cells [1].
• Water moves out of the potato cells into the solution [1].
• This movement occurs by osmosis [1].
• Across a partially permeable membrane [1].

Question 2 (a) The movement of substances from a region of lower concentration to a region of higher concentration [1] using energy from respiration (ATP) [1]. (b)

• Mineral ion concentration in the soil is often lower than inside the root hair cell [1].
• Ions must move against a concentration gradient [1].
• Energy is required to power the carrier proteins in the cell membrane to transport these ions into the cell [1].

Question 3 (a) Carbon, Hydrogen, Oxygen, Nitrogen [1] (b)

• The enzyme has a specific 3D shape with an active site [1].
• The substrate has a complementary shape to the active site [1].
• The substrate fits into the active site to form an enzyme-substrate complex [1].
• The reaction is catalyzed, and products are released, leaving the enzyme unchanged [1]. (c)
• Pepsin is denatured at pH 8.0 [1].
• The strong alkaline conditions break the bonds maintaining the enzyme's tertiary structure [1].
• The shape of the active site is altered [1].
• The substrate is no longer complementary to the active site, so no enzyme-substrate complexes can form [1].

Question 4 (a) Benedict's Test [1] (b)

• Add Benedict's solution to the sample and heat in a water bath [1].
• Positive result: Color change from blue to green/yellow/orange/brick-red precipitate [2]. (c) Monosaccharides are single sugar units (e.g., glucose) [1], while polysaccharides are long chains of monosaccharides linked by glycosidic bonds (e.g., starch) [1].

Section B: Human Physiology

Question 5 (a) It transports nutrient-rich blood from the small intestine/villi directly to the liver [1] so that nutrients can be processed or stored before entering general circulation [1]. (b)

• One-cell thick epithelium/wall provides a short diffusion distance [1].
• Large surface area due to numerous villi and microvilli [1].
• Rich network of blood capillaries to maintain a steep concentration gradient [1].
• Presence of lacteals to absorb fats/fatty acids [1].

Question 6 (a)

• Arteries have thicker muscular/elastic walls; veins have thinner walls [1].
• Veins have valves to prevent backflow; arteries do not [1]. (b)
• The left ventricle pumps blood to the entire body (systemic circulation) [1].
• This requires much higher pressure to overcome resistance in distant capillaries [1].
• The thicker muscle allows for more powerful contractions to generate this pressure [1].

Question 7 (a)

• Definition: The breakdown of glucose in the presence of oxygen to release energy [1].
• Equation: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$ [2]. (b)
• During intense exercise, oxygen supply to muscles is insufficient [1].
• Muscles switch to anaerobic respiration, producing lactic acid [1].
• Lactic acid accumulates in the muscle tissues [1].
• After exercise, extra oxygen is required to oxidize the lactic acid back into pyruvate/glucose in the liver [1].

Question 8 (a)

• High blood pressure in the glomerulus [1].
• Forces small molecules (water, glucose, urea, salts) out of the blood [1].
• Through the walls of the glomerulus and Bowman's capsule into the nephron [1]. (b)
• High blood glucose is detected by the pancreas [1].
• Pancreas secretes insulin into the blood [1].
• Insulin stimulates liver and muscle cells to convert glucose into glycogen for storage [1].
• This removes glucose from the blood [1].
• Blood glucose levels return to normal, which inhibits further insulin secretion (negative feedback) [1].

Section C: Plants, Ecology and Genetics

Question 9 (a) $6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light, chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$ [2] (b)

• Wind removes the layer of water vapour from the leaf surface [1].
• This maintains a steeper water vapour potential gradient between the inside and outside of the leaf [1].
• This increases the rate of diffusion of water vapour out of the stomata [1]. (c)
• Xylem: Transports water and dissolved mineral ions from roots to leaves [2].
• Phloem: Transports sucrose and amino acids (organic solutes) from leaves to other parts of the plant (translocation) [2].

Question 10 (a)

• DDT is non-biodegradable and cannot be excreted [1].
• It accumulates in the fatty tissues of the organism [1].
• Predators eat many prey organisms, ingesting all the DDT stored in them [1].
• Consequently, the concentration of DDT increases at each successive trophic level [1]. (b)
• Restriction enzymes act as "molecular scissors" to cut the insulin gene from human DNA and cut the bacterial plasmid at specific sites [2].
• DNA ligase acts as "molecular glue" to join the insulin gene and the plasmid together, forming recombinant DNA [2].