IGCSE - Donuts

ScienceComplete Science forCambridge IGCSE2nd EditionRon Pickering, RoseMarie Gallagher andPaul Ingram, Stephen PopleEndorsed byUniversity of CambridgeInternational ExaminationsApplication boxes use relatableexamples to apply learning tothe real worldClear, straightforward Englishguarantees clarity for nonnative English speakersHighly visual layout caters todifferent learning styles andensures accessibility for all studentsO R G A N I S AT I O N A N D M A I N T E N A N C E O F O R G A N I S M S2.27O R G A N I S AT I O N A N D M A I N T E N A N C E O F O R G A N I S M SUptake of water and mineralsby rootsOsmosis: a reminderOBJECTIVES To understand that water may enter and leavecells by osmosisTo understand that dissolved substances may enter andleave cells by diffusion and active transportPlants need water and mineralsPlants need to obtain certain raw materials fromtheir environment. The roots of the plant areadapted to absorb both minerals and water fromthe soil. Water is essential to support the plant,as a reagent in many biochemical reactions andalso as a transport medium (see page 260). Thediagram below shows how water enters the plantthrough root hair cells.Minerals have a number of individual functionsand together have a great effect on the waterpotential of the plant tissues. Minerals fromthe soil are absorbed in the form of ions, forexample, magnesium enters the root as Mg2 ions and nitrogen enters as nitrate NO3 ions. Ifthe soil solution contains higher concentrationsof these ions than the root hair cell cytoplasm,the ions can enter by diffusion (see page 28).However, plants can continue to take up ionseven if the concentration gradient is in thewrong direction, that is, if the concentration ofthe ions is higher inside the cell than in the soilsolution.Leaves have a large surface area for photosynthesis. When the stomata areopen, water is lost by evaporation from spongy mesophyll cells (see page 94).Roots have an enormous surface area andpenetrate between the particles of soil.1 Water is drawn up the xylem to replacewater lost at the leaves. This upward flowof water is the transpiration stream (seepage 92). the cells can select which ions enter from thesoil solutionany factor that affects respiration, for examplelack of oxygen or low temperature, can reducethe uptake of ions. The diagram opposite showssome results that support these observations.2 Water (with any substances dissolved in it) isforced to cross the membrane and cytoplasm ofcells of the endodermis. The cell walls in theendodermis contain a waxy material whichmakes them impermeable to water. This allowsthe membranes of the endodermis cells to selectwhich substances can enter the xylem and bedistributed through the body of the plant.883 Water crosses the living cells ofthe cortex by (a) osmosis throughthe cells and (b) ‘suction’ throughthe freely permeable cellulose cellwalls. Almost all of the water movesacross the cortex by route (b).4 Water enters root hair cells byosmosis, from the thin film of watersurrounding the particles of soil. Thesoil water has a higher waterpotential than the cytoplasm of theroot hair cell, so that water movesdown a water potential gradient(see page 29).To increase crop yields, farmers may drain fields thatare liable to flooding. If the soil is not waterlogged,more oxygen in soil air spaces is available to the plants,so the rate of aerobic respiration in root cells is faster.This provides more energy for active transport, so thatthe growing plants will more quickly absorb mineralions present in the soil. Farmers may also cover theirfields with black polythene. This absorbs heat and helpsto raise the soil temperature, so that seed germinationand ion uptake by young roots will be faster.Ion uptake by active transport –energy is supplied by respirationso ions are taken up more quicklywhen more oxygen is available.Ion uptake by diffusion – noenergy is required so processis not affected by the lowconcentration of oxygenthat limits respiration.Oxygen concentration in soil Ion uptake depends on respirationQThe explanation of these observations is thatthe root hair cells use active transport tocarry out the selective uptake of ions againsta concentration gradient, using energy fromrespiration (see page 31).Active transport: applicationNote: Water movement through the plant occurs in thesequence 1–2–3–4: it begins with loss from the leaves,and is completed with water absorption from the soilsolution.‘Plateau’ suggests that activetransport of ions is limited bysome other factor that limitsrespiration, such astemperature.Experiments on the uptake of ions also show that: Root hair cells have anenormous surface area.This helps them absorbwater and minerals fromthe soil. The photographshows root hair cellsmagnified 300.The root hairs are verydelicate and easilydamaged.When plants are transplanted they recover much morequickly if the r oots are kept in a ball of soil or compostso that the root hairs are not disturbed.iA cell’s membrane controls the entry and exit of materialsto and from the cell (see page 29). A typical plant cellsuch as that found in the mesophyll layer of the leafhas a high concentration of solutes. As a result waterwill enter a plant cell by osmosis from an environmentwith a high water potential, until the water inside thecell forces the cell membrane up against the cellulosecell wall.When a plant cell contains plenty of water, the internalpressure of the cell contents against the cell wallsupports the cell. The cell is said to be turgid, andturgidity helps support the plant. If the plant does nothave a good supply of water, the cells lose their turgidityand slowly collapse. The cells are said to be flaccidand the plant is wilted.Rate of ion uptakeNEW EDITIONFully updated to match the latest Cambridge IGCSEsyllabuses, these trusted Science courses are now evenmore comprehensive. Plus, a new student CD-ROM ispacked with interactive exercises and exam-stylequestions to help your students maximise their exampotential. Now available with companion Teacher’sResource Kits.i1 Does a solution containing many molecules ofdissolved sugar and amino acids have a high or a lowwater potential? Explain your answer.2 Define osmosis in terms of water potential.3 How does the strength of the cellulose cell wall helppplants to support themselves?4 A scientist investigated the uptake of magnesiumiions by the roots of young cereal plants. He madethe following observations:a The rate of uptake was increased by raising thetemperature, so long as it did not exceed 40 C.b Uptake stopped if the roots were treated withcyanide, an ion that prevents respiration.c Ions were taken up even if they were present at alower concentration in the solution around theroots than in the root cells themselves.d If ion uptake continued for some time, theconcentration of sugars in the root cellsdecreased.What conclusions can be drawn from each of theseobservations?89 Uptake of water by root hair cellsComplete Biology for Cambridge IGCSE Students’ BookStep-by-step approachsimplifies complex ideas,enhancing comprehensionMore on the web.Visitus online to learn more abboouthe new studenuttt CD-ROMs atwww.oxfordsecondary.co.uk/iggccssee6Question panels highlight key concepts and confirmpupils’ understanding, drawing attention to anyweak areas that may need attention

ScienceComplete Science forCambridge IGCSE:Teacher’s Resource KitsRon Pickering, RoseMarie Gallagher andPaul Ingram, Stephen PopleThis ever-popular Science course now offers invaluableteacher support. Containing a wealth of lesson ideas,PowerPoints, and customisable worksheets on anaccompanying CD-ROM, these new guides will helpenhance your lessons while saving you time.PRACTICALI N V E S T I G AT I O NIntroductionYou need:plain paperName:Browning of apples and pHHandling experimental observations and data:estimating the size of a populationO Step-by-step instructionssustain focus and minimisepotential for confusionStraightforward proceduralguidelines encourage independentthought and planningName:NEWA wealth of practical investigations linklearning with real world examples,developing comprehension and enthusiasmEndorsed byUniversity of CambridgeInternational ExaminationsO pen or pencilO Freshly cut apples gradually turn brown when they are left in air.Cooks try to stop this by dipping the slices of apple in lemon juice.bag or small boxDesign and carry out an investigation to find out whether pH affectsthe rate of the ‘browning’ reaction.Method:You are provided with a bag with some pieces of paper in it. Thepieces of paper represent animals in a population, and the bag isthe environment in which they live. The investigation looks at a‘capture-recapture’ technique for estimating their population size.Start by thinking about the following:1 Remove between 15 and 20 ‘animals’ from the habitat (the exactnumber does not matter) and record this number in the table.2 Mark all the pieces of paper with a small number 1 and putthem back in the bag. Shake for 1 minute to mix up the‘animals’.3 Remove 15–20 ‘animals’ from the bag and write down thisnumber in the table.M How will you prepare solutions with a range of pH values?M What apparatus will you need and how will you use it?M How will you decide when the apple pieces have turned brown?M Will you need to set up any controls?M Write down any hypotheses you are going to test.M Are there any safety hazards?M What safety precautions must you take?Plan your investigation4 Count how many of this second sample have got a number 1written on them (remember to look on both sides).Let your teacher check your plans5 Estimate the size of the population using this formula:Carry out your investigationNo. in first sample no. in second sampleWrite up:No. in second sample marked with a 16 Repeat steps 1–5 a further 4 times but mark the captured‘animals’ in the second step with a 2 the second time, a 3 thethird time, a 4 the fourth time and a 5 the fifth time. Ignore anyother numbers from the earlier samples.7 Display all the readings in the form of a table. Work out a meanvalue for the estimate of the population using your five sets ofresults. Record the mean value.M What you did (including diagrams).M What you found (your results including any tables).M What your conclusions are.M Whether your hypothesis was supported (proved) or not.M Any scientific explanation you can offer for your conclusions.M How your investigation could be improved.8 Tip out all of the ‘animals’ and count the actual population size.Record this value.9 Present yo

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