Secondary 4 Combined Science Biology Preliminary Examination Paper 4

Answer Key - Combined Science Biology Secondary 4 (Prelim V4)

Question 1 (a) Cell wall / Chloroplast / Large central vacuole. (Any one) [1] (b) Nucleus [1] (c) Onion cells have a rigid cell wall made of cellulose [1] which provides structural support and maintains a fixed shape, whereas cheek cells lack a cell wall [1].

Question 2 (a) Cardiac muscle cells have a much higher metabolic rate/energy demand for constant contraction [1], thus they require more mitochondria to produce more ATP via aerobic respiration [1]. (b) Red blood cells lack mitochondria to provide more space for haemoglobin [1] to maximize oxygen transport capacity [1].

Question 3 (a) Osmosis [1] (b) Water moves from the potato cells (higher water potential) [1] to the sucrose solution (lower water potential) [1] across a selectively permeable membrane. (c) It allows certain small molecules (like water) to pass through [1] while preventing larger molecules (like sucrose) from passing [1].

Question 4 (a) Active transport [1] (b) Active transport moves substances against a concentration gradient (from low to high) [1], whereas diffusion moves substances down a gradient (high to low) [1]. Active transport requires energy in the form of ATP [1].

Question 5 (a) Diffusion [1] (b) Oxygen enters the alveoli $\rightarrow$ diffuses across the alveolar wall into the pulmonary capillaries [1] $\rightarrow$ binds to haemoglobin in red blood cells [1] $\rightarrow$ transported via pulmonary vein to left atrium $\rightarrow$ left ventricle $\rightarrow$ aorta [1] $\rightarrow$ systemic arteries to liver [1] $\rightarrow$ diffuses from liver capillaries into tissue fluid [1] $\rightarrow$ diffuses across the liver cell membrane into the mitochondrion [1].

Question 6 (a) Enzyme [1] (b) High temperatures cause the enzyme to denature [1]. The heat breaks bonds in the enzyme's protein structure, changing the shape of the active site [1], so the substrate can no longer fit (lock-and-key mechanism fails) [1].

Question 7 (a) Carbon dioxide + water $\rightarrow$ glucose + oxygen (in presence of light and chlorophyll) [1] (b) Palisade mesophyll has a higher density of chloroplasts [1] because it is located at the top of the leaf to maximize light absorption [1]. Spongy mesophyll has fewer chloroplasts [1] as it is loosely packed to facilitate gas exchange.

Question 8 (a) Bile salts emulsify fats [1], breaking large fat droplets into smaller droplets to increase the surface area for lipase action [1]. (b) Presence of villi/microvilli to increase surface area for absorption [1]; one-cell thick epithelium to shorten diffusion distance [1]; rich network of blood capillaries to maintain concentration gradient [1]; presence of lacteals for fat absorption [1].

Question 9 (a) Oestrogen [1] (b) From day 14-28, the corpus luteum secretes progesterone [1]. Progesterone maintains the thickness of the uterine lining [1] and makes it more vascular/secretory [1] to prepare for the implantation of a fertilized egg [1].

Question 10

• High Glucose: Pancreatic $\beta$-cells detect high glucose $\rightarrow$ release insulin [1]. Insulin increases glucose uptake by muscle/fat cells [1] and stimulates the liver to convert glucose to glycogen [1], lowering blood glucose [1].
• Low Glucose: Pancreatic $\alpha$-cells detect low glucose $\rightarrow$ release glucagon [1]. Glucagon stimulates the liver to break down glycogen into glucose [1], raising blood glucose [1].
• Feedback: This is a negative feedback loop as the response opposes the original stimulus to maintain homeostasis [1].