Secondary 4 Pure Biology Practice Paper 3

Answer Key - Pure Biology Secondary 4 Practice Paper (Version 3)

Section A

Question 1 (a) Modifies, sorts, and packages proteins/lipids for secretion or delivery to organelles. [1] (b) Muscle cells require more energy for contraction [1]; mitochondria are the site of aerobic respiration to produce ATP. [1]

Question 2 (a) The net movement of water molecules from a region of higher water potential to a region of lower water potential [1] through a partially permeable membrane. [1] (b) The 0.6 mol dm⁻³ sucrose solution has a lower water potential than the cell sap of the potato [1]. Water moves out of the potato cells into the solution by osmosis [1]. The cells lose turgor/become plasmolysed, leading to a decrease in mass. [1]

Question 3 (a) Benedict's test. [1] (b) Solution changes from blue to green/yellow/orange/brick-red precipitate. [1] (c) Monomers: Amino acids [1]. Bond: Peptide bond. [1]

Question 4 (a) Pepsin. [1] (b) As pH increases away from the optimum (2.0), the chemical bonds maintaining the enzyme's tertiary structure are disrupted [1]. The active site changes shape and is no longer complementary to the substrate [1]. The enzyme is denatured, preventing enzyme-substrate complexes from forming [1].

Question 5 (a) Long extension/projection [1]. Increases surface area for absorption of water and mineral ions [1] from the soil. [1] (b) Diffusion: Passive (no energy), moves down concentration gradient [1]. Active Transport: Requires energy (ATP), moves against concentration gradient [1].

Question 6 (a) $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$ [2] (b) During intense exercise, oxygen supply is insufficient for aerobic respiration [1]. Cells switch to anaerobic respiration, producing lactic acid [1]. Oxygen debt is the amount of oxygen required after exercise to break down the accumulated lactic acid in the liver [1].

Question 7 (a) Insulin. [1] (b) High blood glucose is detected by the pancreas [1]. Pancreas secretes insulin into the blood [1]. Insulin stimulates the liver and muscle cells to convert glucose into glycogen for storage [1]. This lowers blood glucose levels back to the set point. [1]

Question 8 (a) Prevents the backflow of blood from the left ventricle into the left atrium. [1] (b) Left ventricle wall is thicker [1]. It must generate higher pressure to pump blood to the entire body (systemic circulation) [1], whereas the right ventricle only pumps blood to the lungs (pulmonary circulation) which is a shorter distance/lower resistance. [1]

Section B

Question 9 (a) High blood pressure in the glomerulus [1] forces small molecules (water, glucose, urea, salts) [1] through the basement membrane/podocytes [1] into the Bowman's capsule [1]. (b) Glucose is a useful substance [1]. It is completely reabsorbed [1] into the blood capillaries surrounding the proximal convoluted tubule by active transport. [1] (c) Blood is passed through a partially permeable membrane [1]. The dialysis fluid has a lower concentration of urea than the blood [1]. Urea moves from the blood into the fluid by diffusion [1] down the concentration gradient. [1]

Question 10 (a) Palisade mesophyll cells are columnar/closely packed [1]. They contain a high density of chloroplasts [1] to maximize light absorption [1]. They are located just below the upper epidermis to receive maximum sunlight [1]. The cells are arranged vertically to allow efficient gas exchange. [1] (b) $\text{CO}_2$ is a raw material for photosynthesis [1]. Increasing its concentration increases the rate of the Calvin cycle [1], thus increasing the overall rate of photosynthesis. [1] (c) Transpiration is the loss of water vapour from the aerial parts of a plant [1]. High humidity means the air outside the leaf has a higher water vapour concentration [1]. This reduces the water vapour potential gradient between the leaf interior and the air [1], slowing the rate of diffusion of water vapour out of the stomata. [1]

Question 11 (a) Double helix structure [1]. Composed of two strands of polynucleotides [1]. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base [1]. Bases pair complementarily (A-T, C-G) via hydrogen bonds. [1] (b) A gene is a specific sequence of DNA that codes for a specific polypeptide [1]. A polypeptide is a chain of amino acids [1]. One or more polypeptides fold into a specific 3D shape to form a functional protein. [1] (c) Restriction enzymes cut DNA at specific recognition sequences [1] to isolate a target gene or open a plasmid. DNA ligase joins the target gene and the plasmid [1] by forming phosphodiester bonds [1], creating a recombinant DNA molecule. [1]

Question 12 (a) Energy flows linearly from producers to consumers [1]. Only about 10% of energy is transferred to the next level [1]. Energy is lost as heat during respiration [1], through excretion/egestion of undigested matter [1], or because not all organisms at one level are consumed. [1] (b) Toxins are non-biodegradable and cannot be excreted [1]. They accumulate in the tissues of organisms [1]. Because consumers must eat many prey to survive, the toxin concentration increases at each successive trophic level [1]. Top predators ingest the highest accumulated dose. [1] (c) Burning fossil fuels releases $\text{CO}_2$ [1]. $\text{CO}_2$ is a greenhouse gas that traps infrared radiation/heat in the atmosphere [1]. This leads to an increase in the average global temperature (global warming). [1]