TKGS Prelim paper 2 ANS

TKGS SEC 4 BIO PRELIM P2 2025 MARKING SCHEME Paper 2 (50 m) Q Ans Mark Remarks 1 (a) Temperature of solutions; Particles move faster at higher temperature / cells or cell membrane are destroyed at higher temperature; Duration of experiment; Longer duration allows more time for substances to diffuse; R – size of tissue / type of tissue / freshness of tissue / concentration of solution /volume of solution / container size / humidity / temperature of room / location 2 This was not answered well with poor reference to the question context. Answers were vague and not specific. Explanations were also not specific. For example, temperature of solutions is correct, but students did not explain in relation to kinetic energy of molecules in solution. (b) Active transport; As concentration of oxygen increases from 0 to 20 arbitrary units, rate of potassium ion uptake increases from 7 to 100 arbitrary units; Increased rate of aerobic respiration to release more energy; R – no units in description of data 3 Many answers did not include reference to data. Some data lacked units. Common mistakes include interpreting the main method as diffusion. (c) Discs from the same root tissue will have minimal differences in concentration of K+ ions; To improve reliability of data; R – improve accuracy / make the experiment fair 2 It is incorrect to write that all discs from the same root tissue have the same concentration / water potential. Reference should be made that the concentration / water potential difference is at a minimum compared to using discs from different root tissues. (d) Drawing of a plasmolysed cell; Correct labelling of nucleus and cytoplasm; 2 Many answers did not include vacuoles. Those that did, the size of

the vacuoles were too big. A plasmolysed cell’s vacuole should be smaller in size. 2 (a) Urea; 1 This was well answered. (b) Glomerular filtrate has higher water potential than urine; Water molecules are selectively reabsorbed along the tubules; Into the blood; By osmosis; Max 3 Most answers stated the correct comparison of water potential between glomerular filtrate and urine. Many answers failed to include “reabsorbed”, into the blood and by osmosis. (c) (i) Glomerulus membrane (basement membrane) becomes fully permeable; No longer prevent large molecules e.g. proteins from moving out of the blood; Into Bowman’s capsule; Max 2 Many answers were not specific to link the damaged glomeruli to its partially permeability membrane. Many answers did not interpret that proteins are large molecules. Instead, they explained incorrectly that the proteins cannot be selectively reabsorbed. Many answers also failed to mention that the glomerulus filtrate was first formed in the Bowman’s capsule. (ii) Carbon, hydrogen, oxygen, nitrogen; Amino acids; 2 This was well answered. 3 (a) Proteins in faeces are broken down/decomposed; By bacteria/fungi; Amino acids formed are absorbed into the plant tissues; Amino acids are used to form new protoplasm / any function of proteins; Max 3 This was not well answered. Majority of answers did not specify the decomposition of protein in faeces into amino acids, so that the amino nucleus cytoplasm

R – proteins are absorbed into the plant and broken down into amino acids. acids are able to be absorbed into the plant tissues. Proteins are inferred as large molecules, so they are unable to be absorbed into the plant directly. Hence the sequence of writing has to be done right. Majority of answers were correct in the paraphrasing of the word “growth” in the question into the functions of proteins. (b) Nutrient-poor; 1 The reason why the plant benefits from the relationship is due to nutrient-poor soils. (c) Nectar provides glucose; Oxygen in the chamber / enters from the atmosphere via the openings; R – oxygen from photosynthesis 2 This was well-answered with correct reference to the context. 4 (a) (i) Stomach; (label + name correctly) 1 This was well answered. Some students labelled without naming, losing all marks from this section. (ii) Small intestine; (label + name correctly) 1 Majority of students made the mistake of answering large intestine. Large intestine function is to absorb water but small intestine function is to absorb nutrients, including water. As small intestine is longer and before large intestine, most of the water is already absorbed in small intestine before reaching large intestine

(iii) Rectum; (label + name correctly) 1 Quite a number answered anus or large intestine instead of rectum. Anus is a muscle controlling the release of waste from rectum, large intestine function is to absorb water and mineral salts, not store faeces. (b) (i) Liver; (correct position + name correctly) 1 Common mistakes: 1. Too small, liver is same size if not larger than stomach 2. Liver is beside and behind the stomach, many students placed the liver too low or too high. Stomach rests directly against the diaphragm, so nothing in abdomen can be higher than the stomach. 3. Pancreas is not related to bile at all. (ii) - breaks up/emulsify large globules of fats into smaller droplets; - increases SA:vol of fats; - faster fat digestion + lipase; Max 2 Common mistakes: 1. Break down is not the same as break up, break down is for enzyme action and chemical digestion, break up is for physical digestion. 2. Fail to mention faster/higher rate of digestion 3. Fail to mention lipase If this question had more marks, students are also expected to state the products of fat digestion by lipase (glycerol and 3 fatty acids) (c) (i) Villus; 1 Quite a number of students answered microvilli. Although there is microvilli at the end of that label, what is being labelled is the entire structure, not a

small part of a cell. Microvilli would be the answer if the diagram was a single epithelial cell instead of wall of small intestine. (ii) - cannot maintain steep conc gradient of digested food substances between lumen and blood; - rate of absorption of digested flood substances will be reduced; R – less food absorbed 2 Many students failed to distinguish between (i) and (ii), linking the 2 answers together. Another common mistake was talking about surface area to volume ratio instead of concentration gradient. 5 (a) (i) - more black moths are eaten by predators; - black moths are not able to camouflage/blend in with the bark/surrounding; - less black moths survive to reproduce; 3 R: pale moth able to hide/escape from predator (these are behaviours) R: they mutate to adapt to the environment (mutation is random and does not start occurring because of the environment colour) (ii) Increase; 1 (b) (i) Gg + Gg; 1 (ii) (i) Parental phenotype pale speckled moth pale speckled moth Parental genotype Gg Gg Gametes G g G g Random fertilisation 4 R: white R: Punnet’s square

Offspring genotype GG Gg Gg gg Offspring phenotype pale speckled pale speckled pale speckled black Proportion of black moth: 25% Headings; F1 genotype; F1 phenotype; %; (iii) - a form of gene that exerts its effect/expresses itself in the presence of other alleles / - gives the same phenotype in a homozygous or heterozygous condition; 1 R: any ref to likely / chance R: dominant allele is the gene... R: dominant allele is the trait... Note to candidates: Many candidates have the misconception that the most common allele in a population is the dominant one. This is not true. An example