2023 SASS Bio Prelims Ans

Prelim Examination 2023/4Exp ANSWERS Paper 1 – MCQ 1 A 11 B 21 B 31 D 2 B 12 B 22 C 32 C 3 A 13 B 23 A 33 B 4 D 14 A 24 D 34 D 5 C 15 C 25 A 35 D 6 D 16 C 26 B 36 B 7 B 17 B 27 C 37 B 8 C 18 B 28 A 38 A 9 A 19 A 29 C 39 C 10 D 20 A 30 C 40 D Paper 2 – Section A (50 marks) Qn. Answer Marks Comments 1a - An individual with anemia have a lower red blood cell count than a healthy individual; 1 R: less blood cells (not specific) Generally well done. 1b - Red blood cells contain haemoglobin that binds reversibly with oxygen - This allows red blood cells to transport oxygen from the lungs to the rest of the body - A lower red blood cell count results in a decrease in oxygen transported to the brain 1 1 1 Poorly done. Many students did not state the function of haemoglobin. Many also did not specify the brain despite the question stating ‘light-headness’.

Prelim Examination 2023/4Exp ANSWERS 1c - Tachycardia / faster heart rate / heart failure / heart cells die / heart to be overworked / heart to be strained - to compensate for its reduced ability of blood to carry oxygen 1 1 R: high blood pressure (condition is not specific to the heart) A: weaker contractions as heart muscle tissue does not receive sufficient oxygen 2ai - A consists of layers of long cylindrical, closely packed cells while B consists of irregularly shaped cells with numerous large intercellular air spaces between them. - A contains more chloroplasts than B. - B contains the transport tissues / xylem and phloem while A does not. - Cells in B are covered with a thin film of moisture while cells in A does not. Any 2 2 R: functional differences (a handful of students did not read the question properly) Poorly done question, while most students can identify the 4 structures, many are not familiar with the structural features. Some students are not making comparisons but rather simply listing structural features. 2aii - C is made up of dead cells while D is made up of living cells. - C has a continuous lumen / no cross walls while D contains cross-walls / sieve plates between cells. - C contains lignin deposits while D does not. Any 2 2 2b 3 1 mark for accurate drawing a pair of guard cells and stoma 1 mark for any three correctly labelled structures 1 mark for drawing criterias (solid lines without shading, appropriate size, neat labels) Poorly done. Many are not clear about structures in guard cells, shaded the nucleus, had label lines cutting across excessively and labelled stomata (plural).

Prelim Examination 2023/4Exp ANSWERS 2c - A moist environment has higher humidity; - which means higher accumulation of water vapour outside the stomata; - This makes the water vapour concentration gradient between the leaf and the atmosphere less steep / more gentle; - This results in a lower rate of transpiration; Any 2 for 1m 2 A significant number of students were confused between water potential and concentration gradient. 3a - Structure C is the diaphragm, - which contracts and flattens down - Structure A is the intercostal muscles - external intercostal muscles contract, while the internal intercostal muscles relax - Structure B is the ribs, - which raises upwards and outwards - This causes the thoracic volume to increase. - Atmospheric pressure is now higher than pressure in the lungs, air is inhaled. Any 2 for 1m 4 Most students were familiar with the process of inhalation but majority of students were not able to identify A and B correctly. 3b - Carbonic anhydrase catalyses the reaction between carbon dioxide and water to form carbonic acid - which is then converted into hydrogencarbonate ions, allowing carbon dioxide to be transported in blood plasma. - With lower levels of carbonic anhydrase, less carbon dioxide is excreted/exhaled at the lungs / slower rate of gaseous exchange. - This could result in an increase in blood acidity / the accumulation of carbon dioxide can be toxic to the individual / rapid breathing to expel carbon dioxide Any 3, each 1m 3 Poorly done question. Many students skipped this question. Some were unsure which reaction was catalysed by carbonic anhydrase. Some students associated it to carbon monoxide and carboxyhemoglobin. A: catalyzes conversion of carbon dioxide to carbonic acid A: lower efficiency of removal of carbon dioxide from body

Prelim Examination 2023/4Exp ANSWERS 3c - sticky mucus paralyses cilia / reduces the sweeping action of cilia; - mucus and dust particles cannot be removed, air passages become blocked; - breathing becomes difficult / laboured; - resulting in persistent coughing to clear air passages; - this increases risk of infection / inflammation of the airways / may lead to emphysema Any 2 for 1m 2 Generally well done. 4a A – Graafian follicle B – Corpus luteum 1 1 A number of students skipped this question. Many had spelling errors (max 1m) 4bi - During the fertile periods from day 11 to 16. - Sperms can live for approximately three/four days in the female reproductive system - An egg can survive up to about one/two days after its release from an ovary. 1 1 1 A: +/- 1 day for fertile period Explanation must match suggested fertile period. Generally well done. 4bii - The method assumes that the woman’s menstrual cycle is highly regular / follows a 28- day cycle / ovulation always occurs on day 14th - However, there are various factors such as emotional stress, illness and malnutrition which may affect the menstrual cycle. 1 1 Generally well done. 5a - Order of bases in DNA determines order of bases in mRNA formed during transcription. - This in turn determines order of amino acids in protein formed during translation. - Hence, an individual with a mutation in the gene produces aspartoacylase enzymes that are of the wrong 3D conformation / with an altered active site / non- functional. 1 1 1 Poorly done, most students are not familiar with processes of transcription and translation. 5b - Myelin sheath insulates the axon, - allowing nerve impulses to jump from node to node. - Malformation of myelin shealth will thus slow down the transmission of impulses along the nerve fibre, - resulting in the individual having a slower reaction time. Any 2 for 1m 2 Many were incorrect about what was being insulated, some stated “neurone”, “dendron”, “node of ranvier”. 5ci - The disease is only expressed in homozygous recessive condition / The individual is diseased only if he inherit two recessive alleles for the gene. 1 Many students are unclear about what this term meant.

Prelim Examination 2023/4Exp ANSWERS 5cii Mother Father phenotype of parents normal normal genotype of parents Nn Nn gametes N n N n genotype of offspring NN Nn Nn nn phenotype of offspring normal normal normal diseased - 1 1 1 1 Generally well done. Rejected phenotypes: “infected” - as this disease is inherited and not due to infection “do not” - as it is simply not a phenotype. 6a - Q → P → S → R 1 Generally well done. 6b - 4 1 A handful did not read the question properly - “this cell” 6c - Stage S, - During anaphase, centromere splits and the spindle fibres shorten , pulling sister chromatids apart to opposite poles of the cell. 1 1 Generally well done. 6d Similarities • Disintegration of nuclear envelope and nucleolus • Chromatin thread condense, coil and shorten into chromosomes • Both are forms of nuclear division Differences Mitosis Meiosis One nuclear division Two nuclear divisions Daughter cells have equal number of chromosomes as parent cell. Daughter cells have half the number of chromosomes as pa