AHS 2021 S4 6093 Prelim P2 Ans

1 AHS 2021 Bio Paper 2 Prelim Exam Answers 1 (a) (i) (ii) B E 1 1 (b) Water/ urea/ glucose/ amino acids/ named ion e.g. Na+ 1 (c) H; Cells are constantly carrying out aerobic respiration thus the concentration of oxygen is low and oxygen will diffuse into the cells. 1 1 (d) Blood flows through a partially permeable dialysis tubing bathed in dialysis fluid; The dialysis fluid contains no urea, to ensure that urea will diffuse out of the blood into the fluid; Fluid contains useful substances like glucose/ amino acids/ salts (any two excluding water, which cannot be described using “concentration”) at concentrations similar to those of a healthy person, to prevent these from diffusing out of the blood; While excess salts and water move out by diffusion and osmosis respectively; Molecules like proteins, platelets and blood cells (any two) are too large to diffuse through the dialysis tubing; (any 4, but must address both parts of the qtn) 1 1 1 1 1 2 (a) (i) letter on Fig. 2.1 name of blood vessel oxygenated or deoxygenated blood D pulmonary vein oxygenated E aorta oxygenated 1,1 1,1 (ii) Has valves; To prevent backflow such that blood flows in one direction back to the heart; OR Has a large lumen relative to the diameter of the entire vessel; To reduce resistance to blood flow; OR Less elastic/muscular walls (than arteries); Enables vein to be squeezed by (surrounding skeletal) muscles to push the blood along 1 1 1 1 1 1 (b) (i) Percentage change = (135 – 181) / 181 x 100% = - 25.4% (3 s.f.) 1 1 (ii) Blood glucose conc. above the norm stimulated the islets of Langerhans in the pancreas; 1

2 Which released more insulin into the bloodstream; Stimulating the conversion of excess glucose into glycogen in the liver 1 1 3 (a) (i) Guard cells 1 (ii) Carbon dioxide diffuses through the stomata; into the intercellular air spaces; Dissolves in the thin film of moisture on the (spongy/palisade) mesophyll cells; Diffuses into chloroplast of mesophyll cell; 1 1 1 1 (b) (i) Carbon dioxide + Water Glucose + Oxygen 1 (ii) Accept between 0600 to 0624 1 (iii) Carbon dioxide concentration/ temperature/ concentration of chlorophyll within the plant 1 (iv) Rate of aerobic respiration was equal to the rate of photosynthesis; OR Any carbon dioxide produced during aerobic respiration was taken up by the plant for photosynthesis 1 (v) So that rate of photosynthesis is greater than rate of respiration/ glucose produced by the plant is not all used up in respiration; This ensures that there is surplus glucose/ food to be used to build new protoplasm/ some specific fate of glucose contributing to increase in mass described 1 1 4 (a) (i) Movement of sucrose and amino acids through the phloem from leaves to all parts of the plant. 1 (ii) Phloem 1 (b) (i) Curve 2 1 (ii) Smooth curve continuing from day 5 to day 6 i.e.: 1 Light Chlorophyll

3 Correct calculation made based on student’s graph (e.g. 37 – 35 = 2 mm) 1 (iii) Explain transpiration: Water constantly evaporates from the mesophyll cells in the leaf into the intercellular air spaces; before diffusing out of the leaf as water vapour; Explain transpiration pull; The decrease in water potential in the mesophyll cells causes water to be drawn out of the xylem by osmosis; Creating a suction force that pulls water up as a continuous column 1 1 1 1 5 (a) X: stigma Y: ovule 1 1 (b) Bee lands on the petal on flower S and moves in to collect nectar; in the process, pollen on anthers are brushed onto hairy back of insect; as bee moves into flower T, its back brushes against the sticky stigma, causing pollen to be transferred first point described at least once, for either flower; clear reference to pollen from S being transferred to stigma on T 1 1 1 (c) Parental phenotype: Dominant x Dominant Parental genotype: Rr x Rr Gametes: R r x R r Random fertilization 1 1

4 F1 genotype: RR Rr Rr rr F1 phenotype: Dominant Dominant Dominant Recessive F1 phenotypic ratio: 3 dominant : 1 recessive 1 1 (d) (i) Z, W, Y, X 1 6 (a) (i) (ii) All the substrate has been converted to products. 1 (iii) Graph with steeper gradient; levels off at same height as first graph; labelled clearly 1 (iv) At higher temperature, enzyme and substrate have higher kinetic energy; This increases the frequency of collisions between enzyme and substrate to form enzyme-substrate complexes more quickly, hence the rate of reaction is higher. 1 1 (b) Genotype refers to the genetic makeup of the organism while phenotype refers to the observable trait; An individual with two copies of the recessive allele/ is homozygous recessive with respect to the LDHA gene (genotype) will exhibit fatigue, muscle pain and cramps during exercise (phenotype) 1 1

5 7 (a) At the placenta, nutrients (e.g. glucose and amino acids) and oxygen diffuse from mother’s blood to foetus’ blood (at least two useful substances mentioned); Nutrients used for growth; Oxygen to enable aerobic respiration in fetus to release energy for growth; These are transported by the umbilical cord to the foetus; Antibodies are also transported across the placenta to protect foetus against diseases; Amniotic fluid is incompressible and therefore helps to protect foetus against physical injury/ acts as a shock absorber Also accept, if link to “nourished and protected” is explained clearly: metabolic wastes diffuse across placenta from foetus’ blood to mother’s blood to be excreted, preventing the build-up of toxic waste products in the foetus; Placenta produces progesterone to maintain the uterine lining to ensure foetus remains connected to mother to continue to receive nutrients for growth 1 1 1 1 1 1 1 Any 5 (b) Transmission of hormones in the blood is slower than the transmission of nerve impulses through neurones; hence nervous control allows a faster/immediate response to the source of danger/ provides effective protection 1 1 (c) Is a depressant which slows down brain functions, resulting in Slower reaction time/ increases reaction time; Poor muscle coordination/ slurred speech/ blurred vision; May lead to reduced self-control; Any 2 8E (a) Gland cells in the trachea (A) produce mucus to trap dust and bacteria particles; Cilia on the epithelial cells then sweep the mucus with trapped particles upwards away from the lungs; C-shaped cartilage keep the trachea open by providing structural support, to allow air to pass through; (MAX 2 for trachea) Thin film of moisture (B) dissolves the oxygen gas so that it can diffuse into the blood plasma; Wall of alveolus (C) is one-cell thick, to shorten the diffusion distance for oxygen to travel to the blood capillary; Red blood cell (D) contains haemoglobin to bind to the oxygen to transport it to body cells. 1 1 1 1 1 1

6 (b) Feature of comparison Gaseous exchange Aerobic respiration Location Occurs between the alveoli and blood capillaries Occurs in (the mitochondria of) body cells Type of process