O Level Biology Practice Paper 4

TuitionGoWhere Practice Paper - Biology O-Level (Version 4) - Answer Key

Section A: Cells and The Chemistry of Life

Question 1 (a) Cell wall; Chloroplasts; Large central vacuole (Any two). [2] (b) (i) Muscle cell / Sperm cell / Heart cell (Any cell requiring high energy). [1] (ii) Mitochondria are the sites of aerobic respiration [1]; they produce ATP/energy required for contraction/movement. [1] (c) Modifies proteins/lipids [1] and packages them into vesicles for secretion or transport to other organelles. [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) (i) The sucrose solution has a lower water potential than the cell sap [1]. Water moves out of the vacuole/cell by osmosis [1], causing the cell to become flaccid/plasmolysed and lose mass [1]. (ii) The cells would become turgid [1] as water enters the vacuole by osmosis. [1] (c) Mineral ions are in lower concentration in the soil than in the root hair cell [1]. They must be moved against the concentration gradient [1], which requires energy in the form of ATP produced by respiration. [1]

Question 3 (a) Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N). [2] (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], which lowers the activation energy for the reaction to occur [1]. (c) The change in pH disrupts the chemical bonds holding the enzyme's 3D structure [1]. The active site changes shape/is denatured [1]. The substrate can no longer fit into the active site [1], and no enzyme-substrate complex is formed [1].

Section B: The Human Body - Maintaining Life

Question 4 (a) Deoxygenated blood is pumped from the right ventricle to the lungs via the pulmonary artery [1]. Blood is oxygenated in the lungs [1] and returns to the left atrium via the pulmonary vein [2]. Oxygenated blood is then pumped from the left ventricle to the rest of the body via the aorta [1]. This ensures high-pressure delivery of oxygen to tissues [1]. [6] (b) Walls are one cell thick for a short diffusion distance [1]. Large surface area for maximum gas exchange [1]. Richly supplied with capillaries to maintain a steep concentration gradient [1]. Moist lining to allow gases to dissolve [1]. [4]

Question 5 (a) Stimulus (heat) is detected by receptors in the skin [1]. Impulse is transmitted by a sensory neurone to the spinal cord [1]. A relay neurone in the spinal cord passes the impulse to a motor neurone [1]. The motor neurone transmits the impulse to the effector (muscle) [1]. The muscle contracts [1], pulling the hand away from the heat [1]. [6] (b) Insulin is secreted by the pancreas when blood glucose is high [1]; it stimulates the liver to convert glucose to glycogen [1]. Glucagon is secreted when blood glucose is low [1]; it stimulates the liver to convert glycogen back to glucose [1]. [4]

Question 6 (a) Blood is filtered under high pressure in the glomerulus [1]. Small molecules (water, glucose, urea, salts) are forced out into the Bowman's capsule [1]. Large molecules (proteins) and blood cells remain in the blood [1] because they are too large to pass through the capillary walls [1]. [4] (b) Hypothalamus detects low water potential [1]. Pituitary gland releases more ADH (anti-diuretic hormone) [1]. ADH increases the permeability of the collecting ducts/distal convoluted tubule [1]. More water is reabsorbed into the blood by osmosis [1]. Less water is excreted in the urine [1], making the urine more concentrated [1]. [6]

Section C: Living Together and Continuity of Life

Question 7 (a) As light intensity increases, the rate of photosynthesis increases proportionally [1]. Eventually, the rate plateaus [1]. At this point, light is no longer the limiting factor [1]; the rate is limited by temperature or CO2 concentration [1]. [4] (b) Pollen grain lands on stigma [1] $\rightarrow$ pollen tube grows down through the style [1] $\rightarrow$ enters the ovule through the micropyle [1]. This is important to deliver the male gamete(s) directly to the female gamete for fertilisation [1]. [4]

Question 8 (a) (i) Parents: Tt x tt. Gametes: (T, t) and (t, t). Offspring: Tt, Tt, tt, tt. [4] (ii) 1 Tall : 1 Short. [1] (b) Mitosis produces 2 daughter cells [1] that are genetically identical to the parent cell [1]. Meiosis produces 4 daughter cells [1] that are genetically different from the parent and each other [1]. [4] (c) Down Syndrome (Trisomy 21). [1]