IGCSE Biology: Answer Key - Characteristics & Classification

Answers Answers 1 Characteristics and classification of living organisms 3 4 1.1 Characteristics of living organisms The base sequences of these two species would be the most similar/there would be fewer differences between them. [1] A species is a group of individuals that can reproduce to produce fertile offspring. An example is the whale, Orcinus orca. Recall and connect 1 Knowledge recall and Exam skills questions 1 The answer to this question will depend on the three organisms chosen by the student. 2 a movement, growth and respiration b c photosynthesis [1] The earthworms detected the presence of light [1] and moved to the dark region of the tray [1]. This demonstrates sensitivity [1]. [1] 1.2 The biological classification system There may be some confusion that arises in communication between scientists if the common names are used. For example, two of the three elephants include the word ‘African’ in their common name. In addition, the common names of the elephants may be different from language to language. 1.3 Keys Knowledge recall and Exam skills questions 1 The answer to this question will depend on the type of items chosen by the student. The basis of organisation tends to be physical characteristics. 2 a Knowledge recall and Exam skills questions 1 Similarities include all the seven characteristics of life: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition. 2 a It is an internationally agreed system [1]; in which the scientific name of an organism is made up of two parts showing the genus and species [1]. b Any three from: Hylobates [1], Varanus [1], Ramphastos [1], Orcinus [1], Carcharodon [1]. c Hylobates funereus and Orcinus orca (because they are both mammals). This means two different directions/options are provided for making a selection. [1] [1] © Cambridge University Press and Assessment 2024 1 CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE b There are several different keys that could be produced by the student. 1 mark for each point: Key includes all five species from Figure 1.2 Key includes question statements which are clearly opposite Key contains yes/no branches in appropriate location Boxes contain appropriate descriptors for each group Key is able to be read and used correctly One example is shown below: [5] Does the animal live in water? YES NO Does the animal have scales? YES Carcharodon carcharias Does the animal have feathers? NO Orcinus orca YES NO Ramphastos toco Does the animal give birth to live young? YES Hylobates funereus 3 NO Varanus komodoensis The answer to this question will depend on the two other organisms chosen by the student. 1.4 Kingdoms Knowledge recall and Exam skills questions 1 The answer to this question will depend on the organisms chosen by the student. Most, if not all, will be animals. 2 a 3 2 Plants can produce their own food through photosynthesis, while animals must consume other organisms to obtain their nutrients. [1] b a presence or absence of a nucleus animals, prokaryotes, protoctista [1] [1] b Any one from: Plants have chloroplasts/chlorophyll but fungi do not. Plants photosynthesise but fungi do not (they are saprophytic). Both have cell walls but only plants have cell walls containing cellulose. [1] [1] [1] © Cambridge University Press and Assessment 2024 Answers 4 Cell structure Nutrition Movement Reproduction Prokaryote unicelled prokaryotes absorptive or photosynthetic flagella sexual and asexual Protoctist unicelled eukaryotes absorptive or photosynthetic flagella, cilia or streaming sexual and asexual Fungi multi-celled eukaryotes absorptive, heterotrophic non-motile or saprobic Plant cellulose cell wall and plastids autotrophic non-motile Animal without cell wall and plastids heterotrophic highly motile sexual and asexual sexual and asexual sexual and asexual 1.5 Groups within the animal and plant kingdoms 1.6 Viruses Knowledge recall and Exam skills questions 1 movement, respiration, sensitivity, growth, reproduction, excretion and nutrition 2 Viruses are unable [1] to carry out all or most of the seven characteristics of life [1]. For example, they are unable to reproduce unless they infect a host cell. 3 (largest) human cell, bacterium, virus (smallest) 1 Knowledge recall and Exam skills questions Humans are more closely related to fish than to spiders. Fish and humans are both vertebrates – they have a backbone – whereas a spider is an arthropod. 2 kingdoms; backbone; mammals; birds; reptiles; amphibians; fish; arachnids; insects; spores; dicotyledons; monocotyledons; flowers; roots 3 a They have a backbone. b Any two from: scaly skin [1], rubbery shell [1], internal fertilisation [1] Any two from: external ears [1], mammary glands [1], diaphragm [1], bones in the middle ear [1], different types of teeth [1], sweat glands [1], uterus and placenta [1] Any one from: cold-blooded (X); warm-blooded (Y) c d heart with <4 chambers (X) heart with 4 chambers (Y) [1] 2 Cells 2.1 Animal and plant cells Knowledge recall and Exam skills questions 1 a b c [1] Recall and connect 2 Vertebrates are animals with a supporting rod (backbone) running along the length of the body. Arthropods are animals with jointed legs, but they have no backbone. (They are a very successful group, because they have a waterproof exoskeleton that allows them to live on dry land.) d cell membrane, cytoplasm, nucleus, mitochondria, ribosomes, small vacuole cell membrane, cytoplasm, nucleus, mitochondria, ribosomes, large vacuole, chloroplasts, cell wall If a cell has a cell wall (made from cellulose) it is a plant cell. The presence of chloroplasts also confirms a plant cell but the absence of chloroplasts does not confirm an animal cell. cell membrane (partially permeable barrier that controls what enters and leaves the cell); cell wall (tough outer layer surrounding cells in plants, fungi and bacteria for protection); cytoplasm (the site of most metabolic reactions); vacuole (fluidfilled space often used for storage or separation of solutions, e.g. cell sap in plants); nucleus (storage of genetic information); chloroplasts (site of photosynthesis); mitochondria (site of aerobic respiration); ribosomes (site of protein synthesis) © Cambridge University Press and Assessment 2024 3 CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE 2 a plant tissue b Chloroplasts [1] and a cell wall [1] are only found in plant cells. (Note: the vacuole is not visible in this image so will not give credit.) Chloroplasts (site of photosynthesis) [1]; cell wall (protects and supports the cell and stops it from bursting if it swells with water) [1]. c d [1] Function Name Label The site where the genetic material is stored. nucleus A Site of aerobic respiration. mitochondrion B A partially permeable barrier separating the cell from the outside environment. cell membrane C One mark for each correct row Recall and connect 1 Animal cells have a nucleus but no cell wall or chloroplasts, whereas plant cells have a nucleus but also have cell walls and some will have chloroplasts. Animals feed on organic substances made by other organisms, whereas plants will produce organic substances through photosynthesis from inorganic materials. 2.2 Bacterial cells Knowledge recall and Exam skills questions 1 a peptidoglycan (not cellulose) 2 b a cell membrane cell membrane, cytoplasm, ribosomes b 3 Component Animal Plant Bacteria Cell membrane present present present Cell wall absent present (cellulose) present (peptidoglycan) Nucleus present present absent Cytoplasm present present present Vacuole present (small) present (large, contains sap) absent Chloroplasts absent present (in some cells) absent Mitochondria present present absent Ribosomes present present present a This cell does not have a nucleus/the DNA stored is in the cytoplasm but animal cells do have a nucleus, which stores the DNA [1]. The cell has a cell wall and animal cells do not [1]. This cell does not have mitochondria but animal cells do [1]. b A plasmid is a small loop of DNA stored in the cytoplasm of bacterial cells. Recall and connect 2 All prokaryotes are unicellular, while protoctists can be unicellular or multicellular. Prokaryotic cells do not have a nucleus but protoctist cells do. Prokaryotic cells have a cell wall that is not made from cellulose, whereas not all protoctists have a cell wall. 4 © Cambridge University Press and Assessment 2024 [1] Answers 2.3 Specialised cells Knowledge recall and Exam skills questions 1 a b 2 a b 3 A unicellular organism has only one cell that must be able to carry out all functions, so can’t be specialised. Any four from: ciliated cell in the airways; neurone in the nervous system; red blood cell in the blood; sperm cell in the testes; egg cell in the ovaries; root hair cell in the roots of plants; palisade mesophyll cell in the leaf of a plant. Many cells of a particular cell type make up a tissue; multiple tissues make up an organ; multiple organs make up an organ system. 3 Movement into and out of cells 3.1 Diffusion Knowledge recall and Exam skills questions 1 2 Mitochondria are present in high numbers as aerobic respiration is carried out here. Aerobic respiration releases energy that will be used to power the contraction of the muscle cells. Specialised cell type Function red blood cell [1] transport oxygen palisade mesophyll cell photosynthesis [1] sperm cell [1] male gamete in animal reproduction root hair cell [1] absorption of water and minerals in plants neurone conduct electrical impulses [1] Knowledge recall and Exam skills questions 2 3 4 magnification = image size ÷ actual size image size = actual size × magnification actual size = image size ÷ magnification a 3.4 × 1000 = 3400 µm b (4.2 × 106) ÷ (1 × 106) = 4.20 m measured size in mm of image [1] ÷ 0.5 = magnification [1] image size measured in mm [1] both sizes converted to the same unit (either image size × 1000 or actual size ÷ 1000) [1] image size ÷ actual size = magnification [1] cell membrane b Because the substance would also need to move through a cell wall. A particle has kinetic energy when it is moving [1]. In order for the particle to move from a region of high concentration to low concentration, it must have kinetic energy [1]. a b 2.4 Sizes of specimens 1 a 3 Movement of oxygen from the lungs into the blood/movement of carbon dioxide from the blood into the lungs. [1] c i pH [1] ii Any one factor and explanation from: Temperature [1] – the higher the temperature, the more kinetic energy particles have, so the faster the rate of diffusion [1]. Surface area [1] – the greater the surface area, the faster the rate of diffusion [1]. Distance [1] – the greater the distance over which a particle must move, the lower the rate of diffusion [1]. Concentration gradient [1] – a higher concentration gradient will increase the rate of diffusion [1]. Mass of particles [1] – more massive particles move slower, reducing the rate of diffusion [1]. Osmosis involves the movement of water molecules in all directions – including from a region of low water potential to high. However, as a relatively higher number of water molecules move down the water potential gradient, there is a net flow in one direction. Recall and connect 1 Molecules absorbed by animal cells include glucose and oxygen; those excreted include carbon dioxide, water and urea. In plant cells that are undertaking photosynthesis, molecules absorbed are carbon dioxide and water, whereas those excreted include oxygen. © Cambridge University Press and Assessment 2024 5 CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE 3.2 Osmosis 5 Knowledge recall and Exam skills questions 1 The net movement of particles from a region of their higher concentration to a region of their lower concentration (i.e. down a concentration gradient), as a result of their random movement. Diagram showing the arrangement of dialysis tubing inside a beaker, with the solutions inside the dialysis tubing and the beaker having different water potentials. Recall and connect 2 In animal cells, the cytoplasm contains a lot of water. In plants, the vacuole also contains lots of water. 2 cell membrane 3 net [1]; higher [1]; potential [1]; lower [1]; potential [1]; partially [1] 3.3 Active transport 4 a Knowledge recall and Exam skills questions b c To ensure that a valid comparison of the results could be made. [1] [1] Boiling the carrot causes the partially permeable cell membranes to break [1]. So no osmosis would occur [1]. i The gradient of the line for carrot Q is increasing [1], suggesting that the rate of osmosis was increasing [1]. ii The solution containing carrot P [1], because water moves into the carrot [1] from a high to low water potential [1]. iii Diagram showing cell that is turgid (in carrot taken from solution P). [1] 1 Diffusion involves the movement of particles from a high concentration to a low concentration, so a gradient is required for this to happen. 2 The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient) [1] using energy from respiration [1]. 3 a b c Diagram showing a cell that is flaccid (in carrot taken from solution Q). [1] The graph increases before reaching a plateau. [1] As the concentration increases, a higher concentration gradient is achieved, which results in more particles moving through the protein carriers. However, at very high concentrations, when all protein carriers are involved in active transport, no further entry points are provided. [1] The concentration of mineral ions in the soil is lower than that inside the root hair cell cytoplasm. [1] Recall and connect 3 respiration 4 Biological molecules 4.1 Carbohydrates, fats and proteins Knowledge recall and Exam skills questions 1 iv 6 Cells in animal tissue do not have a cell wall that withstands the turgor pressure inside a cell when water enters by osmosis [1]. Animal cells would burst [1]. © Cambridge University Press and Assessment 2024 a carbon, hydrogen and oxygen b Starch: add iodine solution and if a blue-black colour change is seen then starch is present. Reducing sugars: add Benedict’s solution and heat. If the solution changes from blue to green/yellow/red a reducing sugar such as glucose is present. Answers 2 a b c 3 a b 4 A carbohydrate contains twice as many hydrogen atoms as oxygen atoms, but lipids have more hydrogen atoms per oxygen atom. A carbohydrate is a sugar molecule or is made up out of sugar molecules, whereas a lipid is made up of glycerol and fatty acid molecules. nitrogen Proteins: add biuret solution and if the blue solution changes to purple (lilac, mauve), protein is present. Lipids: add ethanol to food in a test tube. Pour the ethanol into a tube with distilled water. If a cloudy emulsion forms, lipids are present. Many glucose molecules [1] joined in a straight line [1]. Positive test result DNA nucleotide bases are complementary so if a G is on one strand, then the second strand has a C in the same place. The other base pairs are A-T, C-G, T-A. DNA is made up of nucleotide bases [1], forming two strands [1], twisted into a helix [1]. The strands are joined by complementary base pairs (A-T, G-C) [1]. a A is complementary with T so if there is 20% A then there is 20% T [1], 60% of bases are G and C [1] so G is 60 ÷ 2 = 30% [1]. 5.1 Biological catalysts Knowledge recall and Exam skills questions starch Benedict’s solution green/ yellow/red reducing sugars ethanol a white, cloudy emulsion is formed lipids biuret purple (lilac, mauve) colour proteins Proteins are made up of amino acids [1]. There are (20) different amino acids [1]. The order and type of amino acids in a protein determines its structure [1]. b 5 Enzymes blue-black b DNA stores genetic information and determines which proteins are made in a cell. In the nucleus. Tests for this group of biological molecules carbon (C), hydrogen (H), oxygen (O), nitrogen (N) [2 for all 4, 1 for 2–3, 0 for 0–1] a Recall and connect 1 iodine solution a 1 b 1 2 a proteins b Any sensible reaction such as respiration, and so on. A biological catalyst that acts to increase the rate of a chemical reaction [1] without being used up or changed [1]. a b One mark for each correct row 5 Knowledge recall and Exam skills questions 2 Add Benedict’s solution to both cereals in separate test tubes [1]. Heat in a water bath [1]. The cereal that goes most towards red has the highest glucose content [1]. Reagent name 4.2 The structure of DNA [2] Orlistat has a similar shape to lipids [1]. It is complementary to the shape of the active site of lipase and therefore it binds to it [1]. This prevents lipids from attaching to the active site and forming an enzyme–substrate complex [1]. Lipid digestion into fatty acids and glycerol will therefore be reduced/will not occur/these molecules will not be absorbed/lipids will be egested [1]. © Cambridge University Press and Assessment 2024 7 CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE 3 The shape of an enzyme is specific to its substrate. In particular, the shape of the active site is complementary to the substrate, so that the substrate can bind to it. Example diagram: enzyme substrate active site enzyme–substrate complex 3 presence of strong acid, which provides its optimum pH. This will cause more protein to be digested, meaning that a greater area of blue agar will be seen [1]. Well Q contains boiled protease, which is denatured. No protein will be digested around this well, meaning that no blue agar will be seen [1]. Well R contains protease in the presence of strong alkali, which is far from its optimum pH. This will cause less protein to be digested, meaning that a lower area of blue agar will be seen [1]. c This is a control. It allows a valid comparison between the other wells (it shows that protease is responsible for the digestion of protein, and the change in colour of the biuret solution). [1] optimum; kinetic; denatured; substrate; pH Recall and connect 2 products 4 In hot solutions, the kinetic energy of solute molecules is much greater than in a cold solution. Therefore, the solute particles in a cold solution will move much more slowly than those in a hot solution. 6 Plant nutrition Enzymes have a specific shape, the shape of the active site [1] is specific to its substrate [1]. The substrate binds to the enzyme at the active site forming an enzyme–substrate complex [1]. The enzyme changes the substrate into the products and the products are released [1]. 6.1 Making carbohydrates using light energy Knowledge recall and Exam skills questions Recall and connect 1 1 Use biuret solution. This will change in colour from blue to purple (lilac, mauve) if proteins are present. 5.2 Factors that affect enzymes a carbon dioxide + water glucose + oxygen (allow light energy + chlorophyll above the arrow) b The energy comes from light (not always sunlight e.g. in a greenhouse). Plants capture this using the molecule chlorophyll. starch [1] An insoluble molecule will not affect the water potential [1] and will not cause water to move into the cell [1]. Sucrose is soluble and starch is not [1]. Sucrose can dissolve in water and travel through the plant (in the phloem, by translocation) [1]. Knowledge recall and Exam skills questions 1 Approximately 37°C and pH 7–8. 2 a protein b 8 2 protease amino acids [1] The plate will be coloured purple where there is protein remaining, but blue where protein has been digested into amino acids by the protease [1]. Well P contains protease in the © Cambridge University Press and Assessment 2024 a b c Answers 3 a b 4 a b c 5 a b c Molecule nectar amino acids chlorophyll Use attract production capture pollinators of proteins light energy Ion needed none nitrate 6.2 Leaf structure Knowledge recall and Exam skills questions 1 magnesium ne mark per correct row O The different carbohydrates are used for different purposes in the cell. [1] Any three from: As the nitrate concentration increases the number of plants increases until 0.20 mg dm–3.[1] Between 0.20 and 0.30 mg dm–3 there is very little increase. [1] The plant’s leaves change from small and yellowish at 0.00 and 0.05 mg dm–3 to the normal appearance. [1] Use of data. [1] Any three from: Nitrate is required for the creation of amino acids. [1] Amino acids are required to make proteins. [1] Proteins are used for the plants to grow and reproduce. [1] As the plant has access to more nitrates it can grow at a faster rate as it can make more amino acids. [1] Any one from: lack of space [1] lack of a named other nutrient [1] lack of water [1] have not left enough time for reproduction [1] Tube A acts as a control/to compare the effect of having any organisms in the water. Tube B. Because the aquatic plant will photosynthesise and produce oxygen. Only the aquatic plant will respire. In tube D the plant still photosynthesises but both the snail and the plant respire. Tube C. Oxygen levels will drop as no photosynthesis occurs but respiration does (by the snail). a b c 2 a b c d Recall and connect 2 The factors that affect the rate of diffusion of a substance are concentration gradient, diffusion distance, temperature and the surface area across which diffusion can take place. 6.3 Factors affecting photosynthesis Knowledge recall and Exam skills questions 1 a b Recall and connect 1 The taking in of materials for energy, growth and development; (plants require light, carbon dioxide, water and ions; animals need organic compounds, ions and usually need water). allow diffusion of gases (named gas carbon dioxide/water/oxygen) to and from the cells of the mesophyll [1] Any two from: It is thin to reduce the diffusion distance of gases and ensure all cells can photosynthesise. [1] Palisade cells are packed at the top of the leaf to ensure they can catch as much light energy as possible. [1] Air spaces allow the diffusion of gases to and from the atmosphere. [1] Guard cells form stomata to allow gas exchange to occur. [1] Water vapour is less dense than air and will rise. [1] Having the stomata at the bottom prevents excess water loss. [1] A – upper epidermis F – guard cells B – palisade mesophyll D – vascular bundle c Use a plant with variegated leaves (leaves with some cells with chloroplasts and some without). Destarch the plant by leaving it in the dark for a day to ensure no starch is left in the plant. Expose the plant to light for a certain period to allow photosynthesis to occur. Take a leaf and test it for starch. The green parts will have formed starch, whereas the white part will not. A control is an experiment that has a known positive/negative outcome that can be used to compare the effect of your actions. By destarching the plant – leaving it in the dark for a certain amount of time – to use up its starch reserves. © Cambridge University Press and Assessment 2024 9 CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE 2 a b c 10 Any three from: The plants in the higher carbon dioxide concentration have a higher rate of photosynthesis, at all temperatures. Use of data (e.g. biggest difference at 35°C approx. 30% increase). The peak of photosynthetic rate occurs at a higher temperature with more carbon dioxide (25 to 37°C). Any four from: Between 10 and 37°C the rate increases because: kinetic energy of particles increases this increases the chance of collision between the reactants and the enzymes more enzyme: substrate complexes are formed the rate of photosynthesis increases as a result. Above 37°C the rate decreases because: the enzymes start to denature the increased kinetic energy breaks the bond within the enzymes the active site loses its shape and the substrates no longer fit reaction not catalysed so lower rate of reaction. Any five from: Plant growth likely to have increased. Higher rate of photosynthesis. More glucose is produced. More starch is produced. More cellulose is produced. More amino acids can be produced. More energy for respiration (so can carry out more work). More cellulose to make new cell walls. More proteins to make new cells. Increase in CO2 links to enhanced greenhouse effect. Likely to increase average temperature. Increases rate of photosynthesis further. Any other valid point (e.g. projecting own graph). 3 a b [1] [1] [1] [1] [1] [1] [1] [1] c [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] oxygen [1] Description (max 2): Negative relationship. [1] As the distance increases the rate of bubble production decreases. [1] No significant change between 5 and 15 cm. [1] Use of data (e.g. 46 bubbles at 5 cm to 5 bubbles at 45 cm). [1] Explanation (max 2): Increasing the distance of the bulb to the plant decreases the light intensity. [1] Less light energy hits the plant/chlorophyll/ leaves. [1] Light energy is required to carry out photosynthesis. [1] Bubbles of oxygen represent the rate of photosynthesis. [1] Less light energy means less photosynthesis, so fewer bubbles. [1] Any two pairs (suggestion and explanation) from: Use a gas syringe/other valid method of measuring volume, [1] because bubbles can vary in size/observer can miscount bubbles. [1] Use a heatshield/beaker of water, [1] to control temperature as temperature influences the rate of photosynthesis. [1] Add a known mass of sodium hydrogen carbonate, [1] to control the level of carbon dioxide in the water as it influences the rate of photosynthesis. [1] 7 Human nutrition 7.1 Diet Knowledge recall and Exam skills questions 1 A diet consisting of a variety of different types of food and providing adequate amounts of the nutrients necessary for good health. 2 Nutrient deficiency disorders include scurvy (insufficient vitamin C) and rickets (insufficient vitamin D). These vitamin deficiencies tend to occur when people obtain insufficient fresh fruit [1] © Cambridge University Press and Assessment 2024 Answers (vitamin C) or animal products such as dairy products and eggs (vitamin D). Most components of a balanced diet, including carbohydrates, protein and fats, are rarely lacking. 3 Some (molecules in) food will move through intestines/egested/will not be absorbed. [1] Recall and connect 2 From smallest to largest: cell, tissue, organ, organ system. protein [1] – from nuts [1] vitamin D [1] – sunlight [1] OR vitamin D supplements [1] 7.3 Digestion iron [1] – spinach [1] Knowledge recall and Exam skills questions Recall and connect 1 Starch – turns brown iodine solution blue/black. Protein – turns blue biuret solution purple (lilac/mauve). Reducing sugars – turns blue Benedict’s reagent brick red (when heated). Lipids – produces a cloudy emulsion when ethanol is added (OWTTE) 1 hydrochloric; protease; amino acids; optimum; lipase/amylase 2 a chemical [1]; amino [1] b 7.2 The human digestive system Starch consists of the same residues (sugars), rather than different types. [1] c Bile emulsifies fats [1]. It breaks up large droplets/globules into smaller ones with greater surface area [1]. This makes the action of enzymes more efficient [1]. A – enamel, B – dentine, C – capillary, D – cement Knowledge recall and Exam skills questions 3 1 ingestion, digestion, absorption, assimilation Recall and connect 3 Similarities Differences Suggested diagram both are sites of absorption digestion occurs in the small intestine, but not the large intestine 2 enzyme substrate water absorbed products of digestion are from both sites absorbed from the small intestine only active site small intestine is the site of entry of secretions (e.g. bile, pancreatic juice) large intestine contacts the external environment (via anus) a b c P – oesophagus [1]; Q – small intestine/ duodenum [1] i P contains (more) protein; Q contains more amino acids. ii In the stomach, proteins are digested into amino acids by protease. iii Contain similar quantities of starch/lipid. (All the ingested) food will not enter the stomach. Stomach will not (fully) digest (some) food/proteins. enzyme–substrate complex [1] [1] [1] products [1] [1] © Cambridge University Press and Assessment 2024 11 CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE 7.4 Absorption and assimilation 4 Knowledge recall and Exam skills questions 1 By osmosis into the bloodstream. 2 Assimilation is the process by which small molecules are used to produce larger molecules. This takes place in the tissues of the human body. To reach these tissues, small molecules must travel in the bloodstream. They enter the blood from the intestines by the process of absorption. 3 a b These structures are villi [1]. They have a large surface area [1], which increases the rate of absorption across their surface [1]. i ii lacteal by diffusion [1] into lymph [1] [1] a b xylem [1] Max two from: Xylem vessels are long, continuous tubes [1], made from dead cells [1], strengthened by lignin [1]. Max one from: To allow a continuous flow of water from the roots to the shoots [1]. To strengthen and maintain the shape of the xylem vessels [1]. Recall and connect 1 Photosynthesis produces glucose in plants. It consists of a set of enzyme-catalysed reactions using carbon dioxide and water and producing glucose as well as oxygen. Recall and connect 4 8.2 Transport of water Diffusion and active transport can involve most types of substance, whereas osmosis is restricted to the movement of water. Whereas diffusion and osmosis do not require chemical energy to occur, active transport does. Osmosis and active transport require a partially permeable membrane; diffusion does not. Knowledge recall and Exam skills questions 1 a b 8 Transport in plants 8.1 Xylem and phloem Knowledge recall and Exam skills questions 1 2 3 12 a Xylem vessels carry water and mineral ions. 2 Phloem vessels carry sucrose and amino acids (dissolved in water). b Xylem vessels are made up of many hollow, cells joined end to end. The end walls have disappeared allowing water to flow through without obstruction. Xylem vessels are strengthened by lignin to keep the plants upright and withstand the pressure differences caused by transpiration. a In the centre of the root. b A suitable drawing showing the outer edge of the root and the central vascular bundle, with the xylem vessels in the centre and the phloem between the xylem. Xylem transports water and mineral ions, and provides support to the plant [1]. Phloem transports sucrose and amino acids [1]. © Cambridge University Press and Assessment 2024 c a b c Water moves from the soil into the root hair cortex cells by osmosis. Water will move between the root cortex cells towards the xylem vessels by osmosis. As water moves out of the leaf cells and out of the leaf it is replaced by water from the xylem by osmosis. This reduces the pressure at the top of the xylem vessel causing a flow of water from the higher pressure roots to the low pressure leaves. a potometer Any three from: peak between 10 and 12 h [1], slow increase between 4 and 9 h [1], rapid increase between 4 and 6 h [1], relatively constant between 10 and 12 h [1], steady decrease from 15 to 20 h [1], use of data [1] Work out the area below the graph. [1] Method for working out area (e.g. counting squares). [1] (Approx 6 dm3) Any four from: different size [1], different environmental condition (named) [1], different number of leaves/stomata [1], different species [1], different rates of evaporation [1], different thickness of xylem [1], different ages [1], different surface area of roots [1], any other valid point [1]. Answers Recall and connect 2 Water is used in transpiration, chemical reactions and to give cell structure. Nitrate ions are used in the production of amino acids (as well as nucleic acids). Magnesium ions are used in the production of chlorophyll. 8.3 Translocation of sucrose and amino acids 9 Transport in animals 9.1 Circulatory systems Knowledge recall and Exam skills questions 1 A system of blood vessels with a pump and valves to ensure one-way flow of blood. 2 a A single circulation is a system in which blood passes through the heart once during one complete circuit of the body. [1] b The heart of an organism with a single circulatory system carries only one type of blood (deoxygenated). The septum in the heart of an animal with a double circulatory system is to prevent oxygenated and deoxygenated blood from mixing together. [2] Quicker transport of blood around the body. [1] Knowledge recall and Exam skills questions 1 2 a sucrose and amino acids b c translocation A source has high concentration of the substance transported and a sink has a low concentration of the substance transported. The substance (e.g. sucrose) will move from the source to the sink. Sources and sinks can change during the year. A suitable drawing without feathering and showing the outline of the tissues. [1] a b c 3 a b The xylem (on the inside of the bundle) and phloem (on the outside of the bundle) are labelled. [1] amino acids [1] Any three from: Sucrose moves from the leaf to the roots when the plant is photosynthesising [1] because the concentration of sucrose is higher in the leaf than the roots [1]. When the leaf is not photosynthesising it does not produce sucrose [1]. The concentration is lower in the leaf than the root and sucrose moves to the leaf [1]. Any valid point (e.g. reference to growth and development) [1]. The bulb is the source and the growing leaf is the sink. The amino acids will be used for growth of the stem and leaves and sucrose will provide the energy for this growth through respiration. The leaf is the source and the roots, growing bulb and flower are the sinks. c Recall and connect 1 Absorption allows for the intake of the products of digestion into the bloodstream. This happens in the small intestine and allows the human body to gain the nutrients by the processes of diffusion and active transport. 9.2 The heart Knowledge recall and Exam skills questions 1 Too much fat in the diet causes fat to be deposited in the walls of the coronary arteries. This causes a blockage, which restricts blood flow. This means that less blood reaches the heart muscle, which is provided with less glucose and oxygen. To treat coronary heart disease, drugs such as aspirin can be taken to reduce blood clotting. Surgical operations such as a coronary bypass are also used, which involve attaching a piece of blood vessel to carry blood around the blocked artery. Angioplasty is another surgical approach, which requires the insertion of a tube or balloon into the artery, which is then inflated to widen the artery. A stent can be inserted, which acts as a tube to hold the arteries open and helps restore blood supply to the heart muscle. © Cambridge University Press and Assessment 2024 13 CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE 2 a P – left atrium [1]; Q – right ventricle [1] aorta b Any three from: The size/volume of the chambers is reduced [1]; less (volume of) blood pumped out of the heart [1]; less oxygen/ glucose delivered to muscles [1]; muscles require oxygen/glucose for (aerobic) respiration [1]. coronary vein hepatic vein hepatic portal vein renal vein ECG/pulse rate/listening to sounds of valves closing [1] pulmonary vein vena cava c 3 arteries; veins; valves; septum N/A Recall and connect 2 2 Mitochondria are the site of aerobic respiration, where energy is released. Muscle cells, such as those in the heart, need lots of energy for contraction. Knowledge recall and Exam skills questions a P: hepatic vein [1]; Q: hepatic artery [1]; R: aorta (branch of) [1]; S: hepatic portal vein [1] b 9.3 Blood vessels 1 a Suggested table: Any one from: have a lumen [1]; have a wall with elastic/ muscular tissue [1]. c Any two from: lumen is wider [1]; wall is thinner [1]; valves are present [1]. d Any one from: blood contains less oxygen [1]; blood is under lower pressure [1]; blood contains a different concentration of dissolved nutrients [1]. It has a lumen that is very narrow, which allows red blood cells to squeeze through slowly, allowing enough time for diffusion to occur [1]. Its wall is only one cell thick, which reduces the diffusion distance for dissolved nutrients, oxygen and glucose [1]. Type of blood vessel Width of lumen Structure of wall artery relatively narrow; it varies with heart beat, as it can stretch and recoil thick and strong; containing muscle and elastic tissue 3 vein wide; contains valves quite thin; containing far less muscle and elastic tissue than arteries Recall and connect 3 capillary very narrow; just very thin, only one wide enough cell thick for a red blood cell to pass through Diffusion is the movement of particles from high to low concentration without the use of energy. Active transport is the movement of particles from low to high concentration with the use of energy. 9.4 Blood Knowledge recall and Exam skills questions 1 Clotting factors in the blood are activated by platelets upon injury. These cause the conversion of the soluble plasma protein fibrinogen to insoluble fibrin to form a mesh, which traps red blood cells to form a clot. 2 a b Examples in the human body coronary artery hepatic artery renal artery pulmonary artery 14 © Cambridge University Press and Assessment 2024 It is a (group of) cells with similar structures [1]. These work together to perform a shared function [1]. Answers b c i(percentage change in rate = (final rate – initial rate) ÷ initial rate × 100) (one mark for working) = (450 – 950) ÷ 950 × 100 [1] = –53 (%) [1] ii Skeletal muscles – any two from: Muscle cells need glucose/oxygen [1] for aerobic respiration [1] to contract (during vigorous exercise) [1]. Skin – any two from: Vasodilation occurs [1]. Arterioles supplying capillaries in the skin dilate [1] to increase rate of heat loss [1]. Any two from: Blood clotting to prevent blood loss. [1] Defence against infectious disease. [1] Transport of dissolved nutrients/mineral ions/vitamins/products of digestion. [1] Heat transfer. [1] 3 blood cells, ions, nutrients, urea, hormones, carbon dioxide 4 Phagocytes surround/engulf pathogens [1] and digest them through phagocytosis [1]. 2 3 Recall and connect 4 For making haemoglobin, the red pigment in blood, which carries oxygen. 10 Diseases and immunity Recall and connect 1 10.1 Transmission of pathogens Knowledge recall and Exam skills questions 1 a b c aAny four from: Keeping your own bacteria and viruses away from your food by washing your hands before handling food and not coughing or sneezing over your food. Keeping animals away from foods by covering the food. Keeping food refrigerated to slow bacterial growth. Keeping raw meat away from other food. Cooking food to the correct temperature to kill the internal bacteria. b Sewage is the waste liquid that comes from houses including urine and faeces. Untreated or raw sewage contains many bacteria, some of which are pathogens (causing diseases such as poliomyelitis and cholera). Treating the sewage kills the bacteria making the water safe to drink and preventing transmission of the pathogens. A is the correct answer [1]. A transmissible disease is a disease caused by a pathogen that can spread from person to person in a population. Only the common cold is caused by a pathogen. The mutation causing sickle-cell anaemia (B) is passed from parent to offspring but is not caused by a pathogen. The remaining answers are examples of diseases caused by environmental factors (C and D) and can’t be passed to another person. Direct contact (e.g. skin-to-skin, skin-tosurface, blood-to-blood), indirect contact (e.g. airborne, animal-borne, food, water). Pathogens can cause disease symptoms by: Producing toxins that interfere with the body’s cells. Infecting cells and destroying them as part of their lifecycle. Triggering the immune response. The body has a number of defences that prevent pathogens from getting into the blood. These include: the skin, mucus in the airways, stomach acid, hairs in the nose. Bacteria are made up out of cells and viruses are not. Viruses are surrounded by a protein capsid and contain some other proteins and genetic material. Bacteria are surrounded by a plasma membrane and cell wall and contain cytoplasm and ribosomes, as well as their genetic material. 10.2 The immune response Knowledge recall and Exam skills questions 1 a b Lymphocytes will produce antibodies in response to a specific antigen on the pathogen. These antibodies will bind to the antigen on the pathogen and cause their destruction. Memory lymphocytes are formed as part of the immune response. When exposed to the pathogen again they will divide quickly, making large numbers of antibodies, preventing the infection from developing. © Cambridge University Press and Assessment 2024 15 CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE c d 2 3 Passive immunity is acquired from an external source (e.g. through breast milk or an injection). Active immunity is acquired by the action of the immune system in response to a pathogen or injected antigen. Vaccination prevents transmission of a pathogen from person to person. If enough people are vaccinated the pathogen cannot find another host, preventing its spread. This is called herd immunity. a Any three from: In 2010–2011 the prevalence is 8%. [1] It decreases to 3% in 2012 [1] and continues to fall to 1.5% by 2016. [1] Valid comparison of data from two time points. [1] b Any four from: 12- to 13-year-olds were vaccinated from 2008, reaching the 16–18 age bracket from 2012 [1]. Vaccinated individuals were exposed to an antigen from HPV [1]. This generated an immune response [1], including the production of memory lymphocytes [1]. This developed immunity to HPV [1], prevented development of infection [1], prevented spread of infection [1]. c Suggest: The number of cervical cancer cases is going to decrease [1]. Explain: As the vaccinated population reaches 30 years old in 2025 [1], lower HPV prevalence will mean lower number of cervical cancer cases caused by HPV [1]. The action of the lymphocytes will generally start working a few days after the initial infection. C is the correct answer [1]. Recall and connect 2 Phagocytes target pathogens by carrying out phagocytosis. Phagocytosis is when the cell engulfs pathogens and digests them inside of the cell. Lymphocytes produce antibodies in response to exposure to a specific antigen. 16 11 Respiration and gas exchange 11.1 Respiration Knowledge recall and Exam skills questions 1 a b 2 c a Any two from: active transport; growth/ division; muscle contraction. Any one from: (digestion of) starch/ carbohydrates; breakdown of glycogen; ingestion of food containing glucose or carbohydrates. in the bloodstream Species Type of Reactant(s) Product(s) respiration humans aerobic glucose + carbon oxygen [1] dioxide + water [1] anaerobic glucose [1] lactic acid [1] yeast b 3 a b anaerobic glucose [1] ethanol + carbon dioxide [1] Any one from: Aerobic respiration releases more energy. [1] Anaerobic respiration builds up an oxygen debt.[1] The build-up of lactic acid in the muscles [1] during intense physical exercise [1]. Lactic acid is produced in the muscles by anaerobic respiration during intense physical exercise [1]. Lactic acid is broken down by aerobic respiration to carbon dioxide and water in the liver [1]. Recall and connect 1 pH and temperature 11.2 Gas exchange in humans Knowledge recall and Exam skills questions 1 © Cambridge University Press and Assessment 2024 The heart contains two sides, left and right. The right side of the heart contains deoxygenated blood, which is sent to the lungs via the pulmonary artery. The left side contains oxygenated blood, which it receives from the lungs via the pulmonary vein. Answers 2 a i 12 Coordination and response The volume of each breath increases [1] but reaches a plateau at 6% carbon dioxide [1]. ii 3 The breathing rate remains constant until a concentration of 3% [1] but then increases [1]. b Any four from: If there is more carbon dioxide in inspired air, the diffusion gradient for this gas between the alveoli and the capillaries is less [1]. This means that the rate and depth of breathing must increase to eliminate the carbon dioxide produced during respiration from the bloodstream [1]. The carbon dioxide also dissolves in blood plasma to make it more acidic (reduces the pH of blood) [1]. This causes breathing rate and depth to increase, by stimulating receptors in the brain [1]. Finally, the increase in carbon dioxide in inspired air may be associated with a reduction in the proportion of that air consisting of oxygen, meaning that more air must be inspired each time to maintain normal oxygen intake into the bloodstream [1]. c Inspired air and expired air have the same concentration of nitrogen and noble gases. There is a higher proportion of oxygen in inspired air [1]. However, expired air has a higher proportion of carbon dioxide and water vapour. Oxygen diffuses from the air in the alveoli into the surrounding capillaries, while carbon dioxide diffuses in the opposite direction. Water evaporates from the lining of the lungs and forms vapour, which explains its increase in expired air [1]. contracts [1]; external [1]; volume [1]; domed [1]; contract [1]; pressure [1] Recall and connect 2 Small cells have a greater surface are to volume ratio. This allows more space for the diffusion of gases. For example, the alveoli walls in the lungs are made up of numerous small cells increasing the ability to exchange gases. 12.1 The human nervous system Knowledge recall and Exam skills questions 1 a The central nervous system (CNS) and peripheral nervous system (PNS). b A diagram showing a cell body (containing a nucleus), with short dendrons and a single, long axon. The axon should terminate into axon terminals and be surrounded by a myelin sheath. motor neurone cell body c 2 a b c 3 a b c impulses The myelin sheath speeds up the rate of nervous impulses and insulates the axon. synapse A neurotransmitter is released into the synapse by the axon terminal once a nervous impulse reaches it. The neurotransmitter diffuses across the synaptic cleft and binds to the dendron of the next neurone, triggering a new impulse. The neurotransmitter needs to bind to a receptor in the membrane of the dendron. To do this, the shapes need to match. sensory neurone [1], relay neurone [1], motor neurone [1] (in that order) The receptor is found in the skin of the foot [1]. The effector is a muscle in the leg that will cause the foot to withdraw [1]. Any two features from: Automatic [1]: you don’t have to think about reacting, it happens involuntarily [1]. Rapid [1]: it happens very quickly [1]. Protective [1]: it prevents prolonged contact with harmful stimuli such as heat and bright light [1]. Recall and connect 1 ciliated cell, red blood cell, sperm cell, egg cell, root hair cell, palisade mesophyll cell © Cambridge University Press and Assessment 2024 17 CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE 12.2 Sense organs b Knowledge recall and Exam skills questions 1 2 a rod cells [1] and cone cells [1]; found in the retina [1] b c the pupil will constrict Any four from: The pupil/iris reflex occurs [1], which is controlled by circular and radial muscles in the iris [1]. To constrict the pupil, the circular muscles in the iris contract [1]. To dilate the pupil, the radial muscles contract [1]. These are antagonistic muscles [1]. [1] 2 a 3 b c a a retina cornea pupil optic nerve iris b lens b 3 a b 18 With any two from: The endocrine system: hormones travel by blood [1]; slow action by hormones [1]; long-lasting effect [1]. Reflex actions need to happen rapidly to protect the body. The nervous system acts more quickly than the endocrine system. [1] [1] Knowledge recall and Exam skills questions 1 Any four from: The suspensory ligaments hold the lens in position. The suspensory ligament is controlled by the ciliary muscle [1]. When the ciliary muscle contracts, the lens becomes flatter [1]. When the ciliary muscle relaxes, the lens becomes more rounded [1]. A flat lens refracts light less, focusing on distance [1]. A rounder lens refracts light more, focusing on close-up vision [1]. Knowledge recall and Exam skills questions a Insulin reduces blood glucose levels. glucagon Any two from: The nervous system: impulses travel via nerve cells [1]; rapid action by nervous impulses [1]; short-acting effect [1]. 12.4 Coordination in plants Light is refracted by the cornea and by the lens. the fovea [1]; accommodation [1] 12.3 Hormones 1 Any three from: Adrenaline causes the fight or flight response.[1] The heartbeat increases to carry more oxygen and glucose. [1] Breathing rate increases to allow more oxygen to enter the blood. [1] The pupils widen. [1] Blood glucose levels increase. [1] through the blood i adrenaline [1] ii oestrogen or progesterone [1] © Cambridge University Press and Assessment 2024 a Max 6 marks across all sections: Set up three (or more) plants: one in the dark [1] one lit from above [1] one lit from the side [1] Other valid suggestions such as changing the light angle by 15 degrees for the experiment. [1] Control variables (max 2): same type of plant [1] same size of plant [1] exclude external light (e.g. by using a box with a window) [1] same temperature [1] same volume of water added [1] leave for the same length of time [1] Measure the angle of growth of the shoot tip. [1] Repeats of each condition and calculate a mean.[1] Answers b 2 a b 3 a b c A control experiment is an experiment that is used as a sample to compare the effect of changing a variable. [1] The plant in darkness is a control experiment. As there is no light it will not influence growth. [1] OR The plant with overhead light will be a control. [1] As the light does not come from a specific direction no growth response will be seen. [1] Shoots have a positive phototropic response and roots have a negative phototropic response. Roots have a positive gravitropic response and shoots have a negative gravitropic response. Roots absorb water and minerals from the soil and anchor the plant. Growing down into the soil will increase the chance of this being successful. Shoots need light for photosynthesis and growing towards light and away from the soil will maximise the amount of light the leaves can catch. auxin [1] In the tip of the shoot [1]. Auxin elongates cells in the shoot [1]. Light causes auxin to move away from it to the shaded side of the shoot [1]. The shaded side will elongate more than the lit side [1] and the shoot will bend towards the light as a result [1]. Recall and connect 2 Diffusion is a passive process that causes the net movement of substances from an area of higher concentration to an area of lower concentration. Active transport is an active process using energy, that only occurs in living organisms and can move substances against a concentration gradient. 13 Excretion and homeostasis 13.1 Excretion Knowledge recall and Exam skills questions 1 All organisms produce unwanted or waste products because of metabolic reactions. Excretion is the removal of these products from the body. 2 a b 3 3: renal artery [1], 4: renal vein [1], 5: ureter [1] Any two from: more urea (in vessel 3) [1], more salt (in vessel 3) [1], higher pressure (in vessel 3) [1], more oxygenated (vessel 3) [1] c i bladder ii water [1], salt/named salt [1], urea [1] iii Any one from: urea – it is a toxic product of metabolism [1], salt – due to excessive intake in food/drink [1], water – due to low temperature/reduced sweating/excessive intake in food/drink [1] An appropriate sketch. One mark for both renal artery and renal vein; one mark for the structure of the nephron and the capillaries with glomerulus labelled. blood capillary glomerulus branch of renal artery branch of renal vein to ureter © Cambridge University Press and Assessment 2024 19 CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE 13.2 Homeostasis b Knowledge recall and Exam skills questions 1 2 3 starch; glucose; blood; respiration a negative feedback [1] b i Sugary meal → (Q) increase in blood glucose concentration [1] → (R) pancreas releases insulin [1] → (S) insulin stimulates the conversion of glucose to glycogen [1] → reduction in blood glucose concentration [1]. ii Cold shower → (Q) decrease in core body temperature [1] → (R) brain stimulates arterioles in skin to undergo vasoconstriction (sphincter muscle contraction) [1] → (S) less blood flows to the surface of the skin [1] → increase in core body temperature due to reduction in the rate of heat loss [1]. c i A chemical substance released from a gland, which travels into the blood to change the activity of a target organ. [1] ii Any two from: increase blood glucose concentration [1], increased breathing/heart rate [1], dilation of pupils [1]. c 2 a b 3 c a b One mark per correct column Speed of action Duration of effect hormonal control slower longer nervous control faster shorter Recall and connect 2 Used in respiration (to release energy) and converted into glycogen (for storage). c budding (yeast), binary fission (bacteria), fragmentation (Hydra), tubers (plants), runners (plants) An advantage is that asexual reproduction is rapid and does not require a mate. In a stable environment, offspring are adapted to the conditions. A disadvantage is that the lack of genetic variation increases the risk of pathogens destroying a population and it gives evolution less variation to act on. A gamete is a sex cell in a sexually reproducing organism (e.g. pollen, ovule, sperm, egg). A gamete is haploid, it has one copy of each chromosome, whereas other body cells with a nucleus have two copies of each chromosome (diploid). The number of chromosomes in a sexually reproducing organism would double every generation if gametes did not halve their chromosome number. meiosis A process involving the fusion of nuclei of two gametes to form a zygote and the production of offspring that are genetically different from each other. [1] A crop plant has certain characteristics that make it valuable (e.g. large fruit, quick-growing) [1]. Asexual reproduction results in genetically identical offspring [1]. This ensures the offspring have the same characteristics as the parents [1]. 28 [1] The pollen cell will fertilise the ovule [1], which also contains 28 chromosomes, restoring the original 56 chromosomes [1]. Recall and connect 1 the nucleus 14 Reproduction in plants 14.2 Sexual reproduction in flowering plants 14.1 Asexual and sexual reproduction Knowledge recall and Exam skills questions Knowledge recall and Exam skills questions 1 20 a 1 Asexual reproduction is the production of a new generation using only one parent. This is different from sexual reproduction as it requires two parents. Asexual reproduction leads to genetically identical offspring and sexual reproduction to genetically varied offspring. © Cambridge University Press and Assessment 2024 a A: petal [1]; B: sepal [1]; C: stamen/ filament [1] b The carpel consists of multiple parts. The stigma is where pollen will land [1], the style through which the pollen tube will grow towards the ovary/the style [1], which raises stigma for increased likelihood of Answers c 2 a b 3 a b c pollination [1] and the ovary, which contains the ovule/female gametes [1]. The petals are bright to attract pollinators [1]. The nectary produces nectar to reward pollinators [1]. Anthers are raised to ensure pollinators brushes past it [1]. The stigma is raised and sticky to remove pollen from pollinator [1]. Pollen from a wind-pollinated plant is smaller and smoother than that of an insect-pollinated flower. Pollen from an insect-pollinated plant is larger and often has barbs to allow it to attach itself to the insect. The anthers of a wind-pollinated plant are long and flexible to ensure they release their pollen. The stigma of the wind-pollinated plant is often feathery to increase its surface, increasing their chance of catching pollen in the air. self-pollination [1] The stamen and carpel can mature at different times [1]. When the stamen is fertile and producing pollen, the stigma would not be receptive and cannot be pollinated [1]. When the stigma is fertile, the anther is no longer producing pollen so self-pollination cannot occur [1]. The pollen grain grows a pollen tube from the stigma, [1], through the style, entering the ovary [1] and growing into an ovule [1]. 3 b Knowledge recall and Exam skills questions 1 Any one from: The sperm cell must swim to the site of the egg cell; the egg cell must support the growth of the embryo. 2 a faster [1]; requires less energy [1]; no need to find a mate/attract pollinators; [1]; less risk of reduced fitness in offspring [1] b When no other plants of the same species are around (e.g. deserted island or in a stable environment). [1] Self-pollination is when pollen from an anther fertilises an ovule within that same plant. Cross-pollination is when pollen from a plant fertilises the ovule of another plant. b Organ A is the prostate gland [1], which produces seminal fluid [1]. b a a The gametes contain half the chromosomes [1]. Chromosomes will be split at random during meiosis [1]. Gametes will fuse at random [1]. Different combination arise in all offspring [1]. Any two from: Asexually reproducing plants have little or no genetic variation. [1] Sexually reproducing plants have variation. [1] The greater the variation in the population, the more likely that an individual is resistant to the novel pathogen. [1] 15.1 The human reproductive systems Knowledge recall and Exam skills questions 2 Any three from: 15 Reproduction in humans 14.3 Advantages and disadvantages of different methods of reproduction 1 a Self-pollination requires the production of gametes, whereas asexual reproduction does not. Self-pollination produces some variety in the offspring produced, whereas asexual reproduction does not. 3 4 Organ B is a testis [1], which also produces testosterone [1]. c The zygote/cell C goes through mitosis/ cell division [1] to form an embryo, which moves down the oviduct [1]. It uses stored nutrients in the cytoplasm for this cell division [1]. It will implant in the uterus lining [1]. d Any line drawn to any place on the inside or wall of uterus. [1] The placenta produces hormones during pregnancy (e.g. progesterone and oestrogen) [1]. The placenta is the organ that exchanges substance between the maternal blood and fetal blood: oxygen, glucose and other nutrients (as well as antibodies) are transferred from the maternal blood to the fetal blood [1], but urea, carbon dioxide and other waste products are transferred in the other direction [1]. Any one from (for a max of two marks): Oestrogen [1] promotes the growth of the uterus lining/progesterone maintains it [1]. © Cambridge University Press and Assessment 2024 21 CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE Oestrogen [1] is secreted in higher concentrations before ovulation/progesterone is secreted in higher concentrations after ovulation [1]. Oestrogen [1] is secreted by the follicle/progesterone is secreted by the corpus luteum [1]. 5 Both the ovaries and testes are the site of gamete production. They are also the sites of hormone production and secretion. However, the type of gamete produced by each of these organs is different (sperm or eggs) and the type of hormones produced are different (testosterone, or oestrogen and progesterone). Recall and connect 1 Recall and connect 2 A disease that is caused by a pathogen and that can be transmitted from an infected person to an uninfected person. 16 Chromosomes, genes and proteins 16.1 Chromosomes and cell division Knowledge recall and Exam skills questions 1 Sperm have: a streamlined shape; flagellum (plural: flagella): a long, whip-like ‘tail’ structure found on sperm cells, used for swimming; acrosome: a structure containing digestive enzymes, in the head of a sperm cell. Eggs have: a jelly coat change and become impenetrable, so no more sperm can get through; cytoplasm containing yolk – an energy store. a b c 15.2 Sexually transmitted infections Knowledge recall and Exam skills questions 1 sexually transmitted infection; sexual; human; virus; AIDS 2 a 2 The number of HIV particles rises steeply and then falls steeply [1]. It reaches a peak of 90 000 HIV particles per mm3 of blood [1]. a b b 3 4 22 More than one year after infection the number of HIV particles increases [1]. During the same time, the number of white blood cells decreases [1]. The increasing virus numbers may be causing the death/reduced cell division of the white blood cells [1]. c Any one from: Some people have more white blood cells/more resistance [1]; some people might be taking medicines to reduce the effect of HIV [1]. Various outcomes to this task are possible, but students should include sexual intercourse, sharing needles and blood transfusions. To control the spread of STIs, condoms, sterile medical equipment and screening of fluids for transfusion should be used. 3 a Genetic material is stored in the form of chromosomes; long linear strands of DNA nucleotides. DNA is the chemical that contains genetic information. A gene is a section of DNA that codes for a particular protein. An allele is a version of a gene that has a different DNA sequence and therefore codes for a different form of the same protein. Different humans will have different combinations of alleles (genotype), which will result in different characteristics (phenotype). The environment will also act on this to cause further variation. The egg cell is haploid and the skin cell is diploid. If the diploid number is 38 then the haploid number is 19. Meiosis is called reduction division as there is a reduction in the number of chromosomes in the cell from the diploid parent cell to the haploid gametes that are formed. mitosis [1] b 4 A sexually transmitted infection (STI) is an infection that can be transmitted through sexual contact. © Cambridge University Press and Assessment 2024 Any two from: The cell needs to grow [1]. DNA needs to be replicated [1]. Cellular components need to be replicated [1]. Any three from: Mitosis produces genetically identical cells/meiosis produces genetically distinct cells [1]. Mitosis produces two cells/meiosis produces four cells [1]. Mitosis produces diploid cells/meiosis produces haploid cells [1]. Mitosis produces body cells/meiosis only produces gametes [1]. Answers Recall and connect 1 c To halve the number of chromosomes so that when two gametes fuse during fertilisation the diploid number is restored. XB X 16.2 Inheriting genes Knowledge recall and Exam skills questions 1 a AA [1] and Aa [1] b 50% of the offspring are likely to have polydactyly (Aa). This is a 1:1 ratio with the chance of not having polydactyly [1]. A a c 2 a b 1A:1B (simplified) Io Io IA IAIo IAIo I II IBIo c a b B o XX B XAY XB XBXB XBY A 16.3 Genes and protein synthesis [1] [1] [1] a Aa aa Y A The expected phenotypic ratio will be 1 calico female: 1 black female; 1 orange male; 1 black male. You would check their offspring for polydactyly by counting their fingers [1]. If any have five fingers the person must have been heterozygous as that is the only way two recessive alleles could have combined [1]. If all have polydactyly you can’t guarantee that they are homozygous as this may have occurred by random mutation [1]. co-dominance B 3 a Aa aa Both alleles are expressed [1] and produce different proteins in different hair cells [1], resulting in a calico coat [1]. Knowledge recall and Exam skills questions 1 a Different genes are active in different cells [1]. Different proteins are synthesised as a result [1], resulting in a different structure and function [1]. b A gene codes for a protein [1]. A gene is made up of a stretch of DNA with a particular sequence [1]. A mRNA copy of a gene is produced in transcription. This mRNA leaves the nucleus and binds to a ribosome [1]. The sequence of bases in the mRNA will code for the sequence of the amino acids in a protein. The ribosome joins the amino acids together to make a new protein [1]. Ricin will stop protein synthesis in cells. [1] Cells will no longer be able to replace damaged proteins or respond to new stimuli. [1] Cells will get too damaged and will die within a few days. [1] stem cells Blood stem cell (haematopoietic stem cell) which can turn into red blood cells, lymphocytes or platelets. Different genes are expressed in different cells, resulting in different specialised cells. amino acids The sequence of bases in the mRNA that binds to the ribosome making the protein. To transport the sequence of bases in the mRNA stored in the gene out of the nucleus and to the ribosome where it will act as a template to produce a protein. c Genes on other chromosomes are spread equally between male and female individuals – each child has an equal chance of having a recessive allele expressed in the phenotype. Sex-linked disorders are found on the X chromosome. Males have only one X chromosome so a recessive allele will be seen in the phenotype, whereas females will need two copies of this allele. XBY [1] A B X and X are co-dominant. [1] This means they are both seen in the phenotype. [1] 2 a b c 3 a b c © Cambridge University Press and Assessment 2024 23 CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE Recall and connect 2 2 a An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment [1]. The different colours of the moths will help them survive predation by enhancing their camouflage [1]. b i ii DNA consists of two strands, twisted into a double helix. The two strands are held together by hydrogen bonds between the A and T nucleotides and between the G and C nucleotides. 17 Variation and selection 17.1 Variation Knowledge recall and Exam skills questions 1 The nucleus of a human cell contains the chemical substance DNA. 2 a iGene mutation is a random change in the base sequence of DNA. [1] ii 3 Either ionising radiation, or some type of (named) chemical. [1] b i continuous [1] OR discontinuous [1] ii If continuous, this is because the red blood cells have different sizes [1]. If discontinuous, this is because the red blood cells are either biconcave or sickle in shape (there are no intermediate shapes of red blood cell) [1]. c Any one from: meiosis [1], random mating [1], random fertilisation [1] Some mutations may not harm the organism and may instead provide it with an adaptive feature that enhances its survival and ability to reproduce. [1] Recall and connect 1 Discontinuous variation is usually caused by genes only and continuous variation is caused by both genes and the environment. 3 Any year between 100 and 150 years. [1] There is competition (for food, space, mates) between the two types of moth [1]. When air pollution is higher, the white moths are more likely to be seen by predators [1], which means that they will be eaten and cannot reproduce to pass on the alleles genes for that colour [1]. c Any two from: caused by the environment/surroundings [1]; competition between individuals for (named) resource(s) [1]; features are adaptations to the environment [1]; maintains variation/less variation in selective breeding [1]; random mating [1]; reference to named selective agent(s) [1]; natural selection is slower [1]. mutation; resistant; kills; survive Recall and connect 2 Red blood cells have a biconcave shape, which maximises their surface area to absorb oxygen. 18 Organisms and their environment 18.1 Energy flow and ecosystems Knowledge recall and Exam skills questions 1 a 17.2 Selection Knowledge recall and Exam skills questions 1 24 Natural selection is the process by which populations become better adapted to their environment over time. Individuals that are better adapted to their environment and can compete successfully against other individuals for resources have a greater chance of reproduction. These individuals pass on their alleles to the next generation. © Cambridge University Press and Assessment 2024 b c A population is the number of individuals of a specific species within an ecosystem. In contrast, a community is the collection of all the populations of all the different species in an ecosystem. Energy enters ecosystems in the form of sunlight, which is converted into biological molecules by plants during photosynthesis. A secondary consumer will eat a primary consumer (a herbivore), which gets the energy in its biomass from producers. Answers 2 a b c 3 a b c The rest is eventually transferred into thermal energy, through decomposition and respiration. Energy is also stored in undigested materials. The length of the chain is limited as there is less energy available at each trophic level. Eventually there is so little energy that no further trophic level can be supported. Top predators are generally low in numbers as there is only a little energy available to them. A pyramid of energy shows the available energy in each trophic level, whereas a pyramid of numbers only shows the number of individuals and does not represent the energy stored within them. A pyramid of biomass does not show the energy that is available to the next trophic level. grass → sika deer → Amur leopard [1 mark for correct order; 1 mark for arrows in the right direction] grass: first trophic level sika deer: second trophic level Amur leopard: third trophic level [all 3 correct 2 marks, 2 correct 1 mark, 1 or 0 correct no mark] A single Amur leopard will need a large area to live. [1] A single Amur leopard needs to eat multiple sika deer. [1] As a result, their number is limited. [1] c 2 a b c 3 a b c Recall and connect 1 Kingdoms: prokaryotes, fungi, plants, animals, protoctists Any three from: mammals, birds, reptiles, fish, amphibians. Some of the nitrogen-containing compounds will go into the soil [1], replenishing those used by the plants without root nodules previously [1]. Denitrifying bacteria use ammonia and nitrates as a food source [1]. They turn these into atmospheric nitrogen [1]. Any three from: carbohydrates/sugars, proteins/amino acids, DNA/RNA/nucleotides, lipids. Nitrogen-containing – any two from: DNA/ RNA/nucleotides, proteins/amino acids. Decomposers break down carbon-containing compounds in dead materials and waste and release carbon dioxide, which can be used in photosynthesis. i photosynthesis ii respiration Decomposers break down dead material and waste [1], releasing inorganic compounds (e.g. carbon dioxide, ammonium ions, mineral ions), into the ecosystem to be reused by other organisms [1]. Without decomposers, material cycles will break down [1]. No new matter is formed on Earth so needs to be recycled. Energy in ecosystems is produced by the Sun [1]. All energy is eventually converted to waste thermal energy [1]. Plants can become carnivorous [1]. The plants have no source of nitrogen in the soil [1], so can gain it from breaking down animals such as insects instead (from their amino acids) [1]. Recall and connect 2 18.2 Nutrient cycles Elements in protein: carbon, hydrogen, oxygen, nitrogen. Knowledge recall and Exam skills questions Nitrogen is not found in carbohydrates. 1 a b mutualistic/both sides benefit Bacteria will produce nitrogen-containing compounds for the plant. Plants will produce sugars and other organic molecules for the bacteria. The plants with root nodules will have nitrogen-fixing bacteria. [1] [1] 18.3 Populations Knowledge recall and Exam skills questions 1 [1] [1] a There is a slow increase from 2.0 to 2.5 million in the first 6 hours. [1] The population increases from 2.5 to 20.0 million between 6 and 24 hours. The population decreases from 20.0 to 15.0 million between 24 and 48 hours. © Cambridge University Press and Assessment 2024 25 [1] [1] CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE b c d 2 a b c 3 a b c 26 Calculation of change in population: 6.5 – 2.5 = 4 million [1] Calculation of rate per hour: 4 million ÷ 6 hours = 0.67 million hour-1 [1] Any two from: The bacteria need time to adapt to their environment [1]. They need to make the enzymes required to break down their food [1]. There is a limited number of bacteria so growth will be slower [1]. Any one from: The food in the liquid medium has been used up [1], so bacteria no longer have the resources to survive and reproduce [1]. OR Waste materials have built up [1], killing the bacteria [1]. The populations have been there for a long time and have reached equilibrium. The same number of each species die as are born over time. Another factor would become limiting, such as food supply. The reintroduction of the stoats would cause a reduction in the population of rabbits initially. The stoat population would increase, further reducing the rabbit population. Eventually, there would not be enough rabbits for the stoat population and their population would reduce. This would allow the rabbit population to recover and eventually a balanced population would be reached. The increase in food productivity due to the increased use of machinery, fertilisers and artificial selection has increased food yields. The improvement of healthcare, in particular vaccines and antibiotics, has greatly improved the chance of children surviving to adulthood. It would be pyramidal in shape with a broad base and a narrower top. It would not have a broad base. There would be a narrower base than upper levels, there would be a bulge in the population at higher age when the population is greater. 19 Human influences on ecosystems 19.1 Human pressures on ecosystems Knowledge recall and Exam skills questions 1 Suggestions include: larger human populations/ greater biodiversity. 2 a b 3 a Fertilisers, herbicides, insecticides, large-scale monocultures and selective breeding. Fertilisers can result in eutrophication due to the leaching of substances applied to the soil into nearby water bodies. A similar effect can happen if the waste materials produced by animals subject to intensive livestock production enters nearby water bodies. The number of bacteria is highest at site 2 (closest to the sewage centre) and there is a decrease from site 2 to site 5 (furthest away from the sewage centre) [1]. There is an increase from site 2 to site 5 [1]. b 4 Bacteria respire aerobically [1] as they act to decompose the algae that grew [1] in response to the mineral ions/sewage released near at site 2. Aerobic respiration requires oxygen, which is therefore reduced in concentration [1]. c Any one from: nitrate [1], ammonium [1], phosphate [1] Reasons include: climate change, habitat destruction, hunting, overharvesting, pollution and introduced species. Recall and connect 1 The element nitrogen, usually in the form of nitrate or ammonium ions, is required to make proteins. If a plant fails to obtain enough, it develops weak growth and yellow leaves. The element magnesium is required to make chlorophyll. Deficiency results in a yellowing between the veins of leaves. © Cambridge University Press and Assessment 2024 Answers 20 Biotechnology and genetic modification 19.2 Conservation Knowledge recall and Exam skills questions 1 2 3 4 Population sizes of organisms at the top of food webs already tend to be small because energy has been lost along food chains. a 20.1 Biotechnology Knowledge recall and Exam skills questions Correct percentage decrease calculation: ((540 – 490) ÷ 540) × 100 [1] 1 9.25 % [1] or = 9.3 % to one decimal place [1] b Any three from: climate change [1], habitat destruction [1], hunting/poaching [1], pollution and introduced species [1] c Any three from: providing habitats or protected areas [1]; introducing laws to prevent poaching [1]; education [1]; breeding programmes [1]; providing veterinary care [1] This can result in inbreeding [1], which reduces genetic diversity. A loss of genetic variation can increase the risk of genetic diseases and bring about a reduced ability to survive changes in the environment [1], resulting in extinction [1]. Sustainable management is the process by which resources are produced as rapidly as they are removed from the environment so that they do not run out. Two examples are forests and fish stocks. 2 a Sucrose is extracted from sugar cane. Yeast is added. Yeast uses anaerobic respiration, producing ethanol as a waste product. The ethanol produced is purified and used to dilute petrol, producing biofuel. b Advantages: Bioethanol is carbon neutral, unlike fossil fuels. Sugar cane is sustainable as we can keep growing more, unlike fossil fuels. This does not increase carbon dioxide levels in the atmosphere. Disadvantages: It uses land that could be used for food production or natural habitats. It encourages monoculture farming, reducing biodiversity. Ethanol does not release as much energy per gram as fossil fuel. Biofuel still requires petrol so still increases carbon dioxide levels. a a protease (e.g. pepsin or trypsin) b A fermenter contains nutrient broth that provides the nutrients required for the bacteria. Air can be provided for aerobic respiration if required. A paddle can keep the mixture suspended increasing the rate of reaction. Temperature and pH levels can be measured and maintained. The product can be harvested (either periodically or at the end of a growth cycle). To avoid contamination so that the correct bacteria grow. Forests: using education, protected areas, quotas and replanting. Fish stocks: using education, closed seasons, protected areas, controlled net types and mesh size, quotas and monitoring. Recall and connect 2 Percentage change is equal to the difference between two values, divided by the initial value, then multiplied by one hundred. c 3 a b c glucose → ethanol + carbon dioxide [1] Making bread [1]: The carbon dioxide produced is trapped in bubbles in the dough causing it to rise [1]. Making alcohol [1]: The ethanol produced is used in alcoholic beverages such as beer and wine [1]. Pectinase breaks down pectin, a component of cell walls in plants. [1] Any one from: This makes it easier to extract juice from fruit [1]. It can be used to clarify juices such as apple juice that are high in pectin [1]. © Cambridge University Press and Assessment 2024 27 CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE GM crops are often sterile and more expensive [1]; loss of biodiversity [1]. Recall and connect 1 Bacterial cell: no nucleus, mitochondria, often have a flagellum, has a cell wall made from peptidoglycan, contains circular DNA and plasmids. Yeast cell: has a nucleus, has mitochondria, has a cell wall made out of chitin, contains linear chromosomes. 20.2 Genetic modification Recall and connect 2 The DNA sequence in a gene will be copied into an mRNA strand. This leaves the nucleus and attaches to a ribosome. The sequence of the nucleotides on the mRNA strand will determine the sequence of amino acids in the protein. This is called protein synthesis. Knowledge recall and Exam skills questions 1 a b 2 a b c 3 a b 28 Any five from: The gene for hGH is identified in the human genome [1]. Restriction enzymes are used to cut out the human gene from DNA [1]. This produces a fragment with sticky ends [1]. A bacterial plasmid is cut with the same restriction enzymes [1], producing complementary sticky ends which are joined using DNA ligase [1]. This produces a recombinant plasmid [1], which is inserted into bacteria [1]. The recombinant bacteria can then be grown in a fermenter to produce large quantities of hGH [1]. Any two from: ensures a constant supply [1]; easy to make hGH on demand [1]; reduces risk of human diseases spreading [1]; does not require cadavers [1]. It allows the production of crops that supplement deficiencies (e.g. Golden Rice). It increases the yield of food. It can reduce the need for pesticides (e.g. Bt crops). It might encourage the overuse of herbicides/ pesticides. It might cause the development of superweeds. It might cause allergic reactions. Public opinion is against it. The bacteria are less likely to escape the lab; less visible; established for longer. Practical skills for IGCSE Knowledge recall and Exam skills questions 1 aThey should be accurate (e.g. correct size for measuring volume) and precise enough to allow a reliable and repeatable result to be generated. bFirst you would need to identify those areas where a risk is present (e.g. chemicals, glassware, scalpels etc.). You would then need to state the risk caused by these factors and then provide a way of mitigating/minimising the risk. cIncreasing reliability is achieved by increasing the number of repeats carried out. This allows identification of anomalous results as well as the calculation of a mean. 2 a seconds (s) b mol dm–3 (or possibly g dm–3) c µm3 d m2 3 Any two from: pesticide resistance [1]; pesticide production (e.g. Bt) [1]; addition of a novel gene (e.g. Golden Rice) [1] Answer must contain at least one point from both positive and negative aspects for a max of three marks: Positive aspects: increased crop yield [1]; less herbicide needed as only weeds will be affected [1]; less run-off into water system [1]. Negative aspects: the herbicide might kill other plants outside of the field if it is used more [1]; © Cambridge University Press and Assessment 2024 a Benedict’s test for reducing sugars [1] b Any five from: Independent variable: type of drink. [1] Dependent variable: colour change of Benedict’s solution (from blue to yellow to brick red). [1] At least one control variable identified: volume of drink, volume of Benedict’s solution, temperature of water bath, time left in water bath. [1] Control experiment identified (one from water with no glucose or a glucose solution of a known concentration). [1] A method that controls for the above variables. [1] Mention of repeats. [1] Answers cThe Beta drink contained the most glucose as the colour change was greatest [1]. The Alpha drink contained the least glucose as there was no colour change [1]. as the enzyme denatures and no collisions can occur [1]. eCarry out the same experiment with temperatures between 40 and 50°C [1]. dThe Alpha drink contained sweetener/nonreducing sugar [1], which would taste sweet but not cause a reaction with the Benedict’s solution [1]. 4 a i D epends on size of printed version/ to be determined. ii As i. bclear, unbroken outline [1]; a suitable size (at least 7 cm top to bottom) [1]; antennae seen [1]; mouthparts seen [1] Carry out repeats [1] and calculate a mean [1]. Recall and connect 1 [1] [2] ccollect a number of beetles (more than 5) [1]; measure the length of these beetles [1]; calculate the mean length of the beetles [1]; repeat with further samples from this population [1] 5 Any three from: food tests, enzyme practicals, microscopy, transpiration, respiration, tropisms, etc. Exam Practice answers Please note that answers are not provided for past paper questions where examiner commentary is included, or for those that require an improved response or mark scheme to be written. athe volume of oxygen produced [1]; the time it has taken to produce this [1] Exam practice 1 b i 2 An anomalous result is a result that does not fit the pattern and/or is significantly different from other repeats. [1] Trial 3 is the anomalous result. ii 9 cm3 min–1 [1] [1] cAxes labelled with the units from the table (°C on the x-axis and cm3 min–1 on the y-axis). [1] large graph drawn (at least 10 by 10 cm) [1]; linear scales on the axes [1]; all five points plotted accurately [1]; suitable line drawn (dot-to-dot or a curve peaking between 40 and 50°C) [1] d Any four from: Enzyme reactions are affected by temperature [1]. As temperature increases from 20 to 50°C, the rate of reaction increases [1], as the particles have more kinetic energy and more collisions occur [1]. a Any two features from: many (body) segments [1]; head and, body (segments) [1]; many (pairs of) legs/elongated bodies [1]. b Any two groups from: crustaceans [1], arachnids [1], insects [1] Exam practice 2 5 Any three from: Carbon dioxide is required for photosynthesis. [1] ( more carbon dioxide =) faster/maximum/ optimum, rate of photosynthesis; [1] ( more carbon dioxide =) more glucose/starch/ organic molecules produced. [1] arbon dioxide (concentration) is a limiting C factor; or reverse argument. [1] revents concentration falling below that P of atmosphere/or alternative wording. [1] Reference to more, growth/yield/profit. [1] The rate of reaction decreases after 50°C [1], © Cambridge University Press and Assessment 2024 29 CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE Exam practice 3 4 One mark for each correct row: function letter on Fig. 4.1 name structure that separates oxygenated and deoxygenated blood F septum structure that prevents backflow of blood from ventricle to atrium D blood vessel that carries deoxygenated blood B bicuspid/mitral/ atrioventricular, valve 6 H pulmonary artery vena cava structure that prevent backflow of blood from pulmonary artery to right ventricle K semilunar valve chamber of the heart that contains oxygenated blood C E left atrium left ventricle chamber of the heart pumps deoxygenated blood J G right atrium right ventricle b i Any three from: pulse rate increases and remains constant immediate/sudden/steep/rapid/ alternative wording, increase in pulse rate increases from 44–48 bpm to 164–170 bpm maximum/164–170 bpm at 4 min(utes)/ 2 min(utes) after race starts ii Any four from: adrenaline stimulates increase in heart/pulse rate increase in blood carbon dioxide (concentration)/acidity detected nerves stimulate heart to beat faster 30 reference to muscle contraction/ alternative wording muscles require more energy/muscles are doing more work (rate of aerobic) respiration increases increase demand for oxygen/glucose reference to removal of carbon dioxide/lactic acid/heat more blood/carbon dioxide to lungs (per unit time) more blood/oxygen/glucose to muscles additional valid point; e.g. reference to ATP/vasodilation in muscles 7 [1] [1] [1] [1] [1] [1] [1] © Cambridge University Press and Assessment 2024 Any four from: produces a toxin bacteria/toxin, attach to the wall of the, small/large, intestine correct reference to chloride ions secretion/loss chloride ions into the small intestine causing a water potential gradient/water potential of the intestinal lumen is lowered causing osmotic movement of water into the gut/water flows from the cells/blood into the lumen/gut loss of salts from the blood causing diarrhoea/dehydration Any five from: [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] (named) mechanical (barriers) [1] (named) chemical barriers [1] reference to active immunity [1] white blood cells/lymphocytes/phagocytes [1] (phagocytes) engulf (named) microorganisms/ phagocytosis [1] lymphocytes produce antibodies [1] reference to specific antigens/pathogens [1] reference to long-term immunity/ memory cells [1] Answers Exam practice 4 2 a i, ii One mark for each column: intercostal muscles diaphragm b 6 internal external pressure change in the thorax breathing in contract relax contract decreases breathing out relax contract/relax relax increases c There is a very short distance for air to diffuse through [1]. It is well supplied by blood in the surrounding capillaries [1]. i A tissue is a group of cells with similar structures [1] that work together to perform a shared function [1]. ii Cartilage forms incomplete rings around the trachea and bronchi [1] to keep the airways open [1]. a One mark per correct row: b c function name letter on Figure 6.1 conducts impulses to central nervous system (CNS) sensory neurone V conducts impulses to an effector motor neurone Y conducts impulses only within the CNS relay neurone/intermediate neurone X detects a stimulus or a change in the environment (in this case a change in temperature) receptor U causes a movement to occur by contracting/ carries out a response effector, muscle, biceps Z Any four in the correct order from: The presynaptic neurone contains vesicles containing neurotransmitters. [1] The postsynaptic neurone has receptors that correspond to these neurotransmitters. [1] When an impulse reaches the end of the axon/axon terminal the vesicles in the cytoplasm fuse with the membrane. [1] This releases the neurotransmitters into the synaptic cleft. [1] The neurotransmitters diffuse across the cleft and bind to the receptors. [1] This triggers an impulse in the postsynaptic neurone. [1] Any one of the following: pupil reflex [1] accommodation[1] tear reflex [1] corneal reflex [1] other valid reflexes within the eye [1] © Cambridge University Press and Assessment 2024 31 CAMBRIDGE IGCSE BIOLOGY: EXAM PREPARATION AND PRACTICE 8 a i glomerulus b [1] ii b c Blood vessel Q, which is narrower than R, is an artery. This high pressure is required for ultrafiltration. [1] iii Microvilli increase the surface area of the small intestine [1], which enables faster reabsorption of substances such as glucose [1]. There would be no protein in the Bowman’s capsule (region 1) [1], because these particles are too large to leave the glomerulus [1]. However, smaller particles such as all the glucose, salts and urea are filtered out of the glomerulus and into the Bowman’s capsule [1]. By the time the filtrate enters the loop of Henle (region 3), there is (re)absorption of all the glucose and of some salts by active transport (using energy from respiration) [1]. Most water is reabsorbed by osmosis in the loop of Henle [1], which causes the urea concentration to be increased in the loop of Henle [1]. Therefore, the urine, which passes out of the kidney, is mostly excess salts and toxic urea. i homeostasis [1] ii Any three from: If no food is eaten, the concentration of glucose in the blood is low [1]. This stimulates glucagon release from the pancreas [1], which stimulates the breakdown of glycogen stored in the liver [1], and the release of glucose into the blood [1]. iii a 4 5 a b a b One mark per correct row: function 32 letter from Figure 4.1 name of the organ meiosis to produce pollen seeds C anther pollination D stigma development of seeds E ovary protection of flower in the bud A sepal ovule [1] A nucleus with only one copy of each chromosome. [1] To keep the same number of chromosomes from one generation to the next. [1] OR To prevent the number of chromosomes from doubling with each generation. [1] Exam practice 6 Exam practice 5 4 i ii © Cambridge University Press and Assessment 2024 c Bacteria carry out these processes. A is the decomposition of amino acids/ deamination into ammonium ions in the soil. A is also nitrification, the conversion of ammonium ions into nitrite and nitrate ions. B is nitrogen-fixation, by bacteria in the root nodules of plants, or lightning. C is denitrification. When nitrates are used as an energy source in anaerobic conditions. active transport decomposer bacterium → Paramecium → Didinium Any two from: Ciliates eat the bacteria. Ciliates also eat dead and decomposing matter which can also cause suspended material. Predatory ciliates like Didinium will eat other ciliates. Any three from: Nitrifying bacteria turn ammonium ions into nitrite ions. Ammonium/nitrite ions are turned turned into nitrate ions Plants can absorb nitrate ions in the soil. Nitrate ions are used to make amino acids and proteins (nucleic acids/DNA) [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] Answers 7 a b 8 a b c Any three from: can be used to treat diabetes the insulin produced is human insulin/same shape as human insulin no immune response; no risk of infection from donor animals fewer ethical/religious objections due to animal source can be produced quicker and cheaper can be scaled up quickly/bacteria reproduce rapidly i plasmid made from DNA/nucleotides ii restriction enzymes/restriction endonuclease Changing the genetic material of a species [1] by inserting a gene from one species into another or by removing an allele [1]. [1] [1] [1] [1] [1] [1] [1] [1] [1] One mark per correct row: (M was provided as completed in the question) Letter from Figure 8.1 Name Description M chromosomes threads of DNA found in the nucleus N gene or allele section of DNA removed from human cell Q plasmid a small loop of bacterial DNA used as a vector R bacterium type of cell that is genetically engineered O the protein (insulin) specific chain of amino acids coded by the section of DNA removed from the human cell P fermenter vessel in which bacteria can be grown Any three from: asexual reproduction leads to clones only one parent is required no gametes are required insulin can be produced on demand all insulin is identical genetically engineering only has to be carried out once © Cambridge University Press and Assessment 2024 33 [1] [1] [1] [1] [1] [1]

IGCSE Biology: Answer Key - Characteristics & Classification

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