O Level Biology - Gcecompilation
O Level BiologyTeacher’s GuideMary Jones
ContentsUnit 1: Cell Structure .1Unit 2: Diffusion, Osmosis and Active Transport .7Unit 3: Enzymes .11Unit 4: Photosynthesis .16Unit 5: Animal Nutrition-Diet .23Unit 6: Animal Nutrition-Digestion .31Unit 7: Transport in Flowering Plants .35Unit 8: Transport in Humans .38Unit 9: Respiration .43Unit 10: Excretion .47Unit 11: Homeostasis .49Unit 12: Coordination .52Unit 13: Support, Movement and Locomotion .57Unit 14: The Use and Abuse of Drugs .59Unit 15: Micro-organisms and Biotechnology .61
Unit 16: Organisms and Environment .64Unit 17: Human Effects on Ecosystems .69Unit 18: Reproduction in Plants .73Unit 19: Reproduction in Humans .79Unit 20: Inheritance .84
Unit 1: Cell StructureThe Structure of Animal and Plant CellsThis covers the syllabus sections 1(a), (b), (c), (d), (e) and (f).It is suggested that students first see diagrams of cells, and learn about theirstructure. This will help them to understand what they see when they later lookat cells using a microscope, or when they see photomicrographs of cells. Theparts of the cells that are described are those that are required by the syllabus.There is no need to introduce other structures (for example, mitochondria) asthese may confuse students at this stage.The very small size of cells is emphasised here. Ask students to look at a rulerthat is marked off in millimetres, and imagine 100 animal cells fitting into onemillimetre.Investigation 1.1 Looking at animal cellsThis may be the first time that students have used a microscope. You may needto modify the instructions given so that they relate to the type of microscopesthat you have available. In any case, you should demonstrate the use of amicroscope to your students, before asking them to carry out this practical.Safety pointsLiver cells may carry bacteria. Dispose of any leftover suspension quickly. Donot pour it down the sink, as the rotting liver will smell badly.Students should wash their hands thoroughly after carrying out this experiment,to remove any trace of liver cells.Materials requiredEach group of students will need the following: a microscope and light source (which can be a lamp or a window)a clean microscope slideone or two clean cover slipsa long pin, or a mounted needle, to help with lowering the coverslipa pipettea small amount of methylene blue solution, with its own pipettea piece of filter paper or blotting paper1
a small amount of liver cell suspension. Prepare this by grinding orliquidising some fresh liver with a cold isotonic solution. The suspensionshould be prepared as close as possible to the time when it will be used, andshould be stored in a refrigerator.Methylene blue is suggested as a stain, because it is taken up by living cells. Itis a good idea for you to experiment with different concentrations of methyleneblue, to find out the best concentration for staining the liver cells, before thestudents carry out the practical. The stain should colour the cytoplasm lightblue, and the nucleus darker blue.Students are asked to draw one or two of the cells that they can see. It is veryimportant that they draw what they can see, and not what they think they oughtto be able to see. Diagrams that look exactly like Fig. 1.1a are useless!Investigation 1.2 Looking at plant cellsSafety pointsThere are no special safety issues relating to this investigation.Materials requiredEach group of students will need the following: a microscope and light sourcea clean microscope slideone or two clean coverslipsa long pin, or a mounted needle, to help with lowering the coverslipa pipettea small amount of iodine in potassium iodide solution, with its ownpipettea piece of filter paper or blotting papera piece of onion bulb or other source of plant cells—for example, a leaf fromwhich the lower epidermis can be peeledYou will need to show students how to peel a small piece of epidermis from theinside of one of the layers in an onion bulb, or from the lower surface of a leaf.Cut the piece of bulb to size first, rather than peeling off a very large piece ofepidermis and then trying to cut it into smaller pieces. The epidermis needs tobe put straight into water on the slide, before it begins to dry and curl. You maybe able to find a source of material that has coloured cytoplasm or cell sap, whichcan help students to see the cells more clearly.2
Once again, it is very important that students are encouraged to draw what theycan genuinely see. It is unlikely that they will be able to see a clear line separatingthe vacuole from the cytoplasm, for example. They should not be surprised thatthere are no chloroplasts, if they know that onion bulbs grow underground.Answers to questions1.1StructureAnimal cellsPlant cellscell membrane cell wall cytoplasm nucleus permanent vacuole containing cell sap chloroplasts 1.2StructureFunctioncell membranecontrols what enters and leaves the cellcell wallholds the cell in shape and stops it from burstingcytoplasmcontains enzymes and other substances; manymetabolic reactions take place herenucleuscontains chromosomes made of DNA, that determinewhat proteins the cell will makepermanentvacuolecontainingcell sapcontains cell sap, which is made up of sugars andother substances dissolved in waterchloroplastscontains chlorophyll and carries out photosynthesis3
Special Types of CellsThis covers syllabus sections 1(g) and (h).Both the red blood cell and the root hair cell are modified to give them a largesurface area to volume ratio, which speeds up the rate at which they can absorboxygen and water respectively. You could demonstrate this concept to yourstudents using a set of square wooden building blocks.Demonstration: Shape, Size and Surface Area to Volume RatiosYou will need lots of small square blocks—preferably with sides of 1 cm, as thismakes the calculations easier.(1) First show the students one block, and ask them to tell you: its volumeits surface area—they will need to work out the surface area of eachface, and then count the number of faces(2) Write these values down on the board, in a table like this:number of bricksin the stackvolume/cm3surfacearea/cm2116surface area tovolume ratio(3) Now add 7 more blocks to the first one to make a cube, and ask the studentsagain for the volume and surface area. Add these values to the table.SizesCube made of 8 bricks . . . . . . and so on . . .4
(4) Carry on adding bricks to make larger cubes, writing the volume and surfacearea in the table each time.(5) Now draw a second table on the board, and repeat the whole thing, usingthe same number of bricks each time but now arranging them in a long flatshape rather than a cube.(6) When you have two complete sets of data, show the students how tocalculate the surface area to volume ratio. They can then fill in the valuesfor each stack of bricks in the final column.Discuss results with the studentsAsk them what patterns they can see. They should notice that: as the volume of the stack increases, the surface area to volume ratiodecreasesfor a particular volume, the surface area to volume ratio is always greaterfor a long, flat shape rather than a cubic one.Discuss with them what this means. They should be able to see that : a small cell has a larger surface area to volume ratio than a large cella flattened cell has a larger surface area to volume ratio than a spherical cellYou can now relate this to the shape of a red blood cell. Its size and shape giveit a large surface area to volume ratio. This means that the cell has a lot of surfacethrough which oxygen can get in and out. So more oxygen can pass into or outof the cell at any one moment in time than if the cell was large and spherical—thus speeding up the rate at which oxygen moves in and out.5
Answers to questions1.3 abRed blood cells transport oxygen from lungs to other parts of thebody.Haemoglobin combines with oxygen in the lungs, and then releases itwhere it is needed.The lack of a nucleus makes more space for haemoglobin.The small size, and the biconcave shape, gives the red blood cell alarge surface area to volume ratio. This speeds up the movement ofoxygen into and out of the cell. Their small size helps them to squeezethrough the tiniest capillary, and get close to the body cells whichneed oxygen.1.4 abRoot hair cells absorb water and inorganic ions from the soil.The shape of the root hair cell gives it a large surface area to volumeratio. This speeds up the rate at which water and inorganic ions canmove into the cell.1.5 aXylem vessel elements transport water and inorganic ions around theplant. They also help to support the plant.Having no living contents makes it easy for water to move through thexylem vessels.Having lignin in their walls makes the xylem vessel elements verystrong, so they help to hold the plant upright.Having no end walls means that water can easily flow from one xylemvessel element to the next.bHow cells are organizedThis covers section 1 (i) of the syllabus.Demonstration: different types of cells and tissuesIf possible, use a projector and transparencies to show students some differenttypes of plant and animal cells, and the tissues of which they are part. (If youcannot do this, then show the students photomicrographs instead.) You candiscuss the function of each tissue that they see, and also consider how the cellsthat make up the tissue are adapted for their functions. Some suggestions are: TS leaf, showing epidermal, palisade mesophyll and spongy mesophylltissuesTS stem or root, showing epidermis, cortex, xylem, phloemThe wall of part of the alimentary canal of a mammal—you may be able to6
see the epithelium (which may contain goblet cells to secrete mucus), andalso smooth muscle cellsThe wall of the trachea or a bronchus of a mammal—you may be able tosee the ciliated cells and goblet cells in the epithelium, and also cartilageand smooth muscle cellsAnswers to questions1.6 eye – organnerve – organkidney – organleaf – organepithelium of a leaf – tissuexylem vessel element – cell1.7 artery – circulatory systemstomach – digestive systemskull – skeletal systemUnit 2: Diffusion, Osmosis and Active TransportDiffusionThis covers the syllabus section 2(a).Students need to know something about the kinetic theory before they canunderstand diffusion, so check that this has been covered in physics or chemistrylessons. Emphasize that the particles are just moving randomly, and notpurposefully from one place to another.You may be able to demonstrate the situation described in ‘How diffusionhappens’. Choose something with a strong scent, but that is not harmful. Abottle of strong perfume would be ideal.Investigation 2.1 DiffusionThere are many possible ways to demonstrate diffusion—for example, colourspreading from a copper sulphate crystal in a glass container of water, brominegas inside a gas jar moving into another held above it, ammonia graduallyturning successive pieces of red litmus paper blue as it diffuses down a tube. Thisexample has been chosen as it introduces the idea of membranes which let somethings through and not others, and also the starch-iodine test.7
Take care, however, to avoid the very common misconception that diffusiononly takes place across a membrane.Students will not understand this investigation unless they understand what asolution is. They need to be aware that the starch solution is a mixture of starchmolecules and water molecules, and that the iodine solution is a mixture ofiodine molecules and water molecules.Safety pointsNone of the materials that are used are toxic. However, iodine solution will stainclothes.Materials requiredEach group of students will need: a small beaker or other containera piece of Visking tubing, long enough to allow it to be tied into a knot atone enda length of strong threada small amount of iodine in potassium iodide solution (as used for starchtests)some starch ‘solution’ (make this using soluble starch)—the concentrationdoes not mattertwo dropper pipettesStudents are asked to explain their results. Encourage them to think about eachtype of molecule separately – the water molecules, starch molecules and iodinemolecules – rather than thinking about the solution moving.Further workAssessment Objectives C5 in ‘O’ Level Biology is to design or plan aninvestigation, and this investigation provides opportunity for students to developthis skill. When they have done Investigation 2.1, they could be asked how theycould modify this procedure to investigate the effect of temperature on the rateof diffusion. You could deal with this as a class discussion, and use it to introducethe idea of variables. In this instance, students should think about: what they would need to change (the temperature of the liquids)what they would need to keep the same (the volumes of liquids, the size ofthe Visking tubing, the concentrations of the starch solution)8
what they would measure (the time taken for the blue colour to first appearinside the tubing, or for a particular depth of colour to be reached) and howthey would measure it.OsmosisThis covers syllabus sections 2(b) and (c).Do take care that students understand that osmosis is just a special kind ofdiffusion. They often become very confused with the use of the term‘concentration’ to describe how much water is present, and this can lead toerroneous answers to examination questions such as ‘Osmosis is the opposite todiffusion, because water moves from a low to a high concentration rather thanfrom high to low concentration’. The use of the term ‘water potential’ can avoidthis confusion right from the start.Do not be tempted to do anything quantitative about water potential—even ALevel students get confused with this! Keep it very simple.Another common confusion is that the plant cell wall stops water from enteringthe cell, and this is why it does not burst. Make sure that students realize thatthe water does move freely through the cell wall, and that the reason it does notburst is because the wall resists the expansion of the cell.You might like to place some pieces of raw potato or other solid plant tissueinto water, and into concentrated sugar solution, some time before the lesson,and then allow the students to find out what a tissue made of turgid cells, andanother containing flaccid cells, feels like. This will help them to understand howturgor can help to support the soft parts of a plant, and why plants wilt whenthey have lost too much water.Answers to questionsThese questions should help students to check their understanding of osmosis.It also helps them to use what they have learnt about osmosis in single cells, andapply this knowledge to work out what will happen in tissues.2.1 higher, down, osmosis, membrane, burst, out of, shrink2.2 tissue, osmosis, water, wall, membrane, turgid, wall, water, flaccid, softer9
Investigation 2.2 Osmosis and plant tissuesAny dried fruit can be used for this investigation. It is suggested that studentsuse groups of 10 raisins, but you may need more than this if the balancesavailable cannot mass these with any degree of accuracy.Safety pointsNone of the apparatus or materials are potentially harmful. However, make surethat students do not put the raisins into their mouths.Materials requiredEach group of students will need: at least 10 raisins or pieces of other dried fruittwo containers, one containing pure water and the other a sugar-solution(the concentration is not important)access to a top pan balanceblotting paper or filter paper to dry the raisinsThe raisins in both groups should gain mass, but those in the water will gainmore than those in the sugar solution. As there is bound to be considerableinaccuracy in the results obtained, it is a good idea for all the groups in the classto share their results, so that any patterns can be more clearly seen. Talk thesethrough with the whole group, and try to draw out from them what washappening. They will need to think about the cell membranes around the raisincells, in which direction there is a water potential gradient, what was happeningto the water molecules and what was happening to the sugar molecules. Theyshould also think about the steepness of the water potential gradient in the twocontainers, and how this has affected the results.Active TransportThis covers syllabus section 2(d).10
Answers to ecules move down aconcentration gradient Molecules move against aconcentration gradient The cell uses energy to make ithappen Can only happen across apartially permeable membrane Always involves the movementof water molecules abThe concentration of potassium ions is greater inside the cells thanoutside. Left alone, the potassium ions would diffuse out of the cell.The cell must have been taking them in against their concentrationgradient.The cell has to use energy to move the ions into the cell by activetransport. It gets this energy from the breakdown of glucose inrespiration. If the roots do not have enough oxygen, then they cannotrelease enough energy from glucose, so cannot carry out activetransport.Unit 3: EnzymesThe introductory paragraphs of this Unit cover syllabus section 3(a).Students are likely to have met catalysts already in Chemistry lessons. If youknow that this is so, you could begin by asking them to tell you what acatalyst is.A very common mistake that students make is to think that all catalysts areenzymes. They need to understand that enzymes are just a particular type ofcatalyst, found in living organisms and made of protein.11
How enzymes workThis section covers syllabus section 3(b).Maltase is used as an example here because it catalyses a simple reaction inwhich a molecule is split into two equal parts.In this section, students are being asked to visualise events taking place thatinvolve molecules. It is worth taking a little time to ensure that they dounderstand that a molecule is the smallest possible particle of a substance. It isquite impossible for any of us to visualise the infinitesimally small size of thesemolecules, but you could at least ask your students to try!A number of important new terms are introduced, and Question 3.2 is includedto help them to learn them. Similarly, Question 3.1 provides them with asummary table of enzymes, substrates and products. This looks ahead to Chapter5, in which many of these enzymes will be met again.Answers to questions3.1 catalyst – any substance that increases the rate of a chemical reactionwithout itself being changedenzyme – a protein that functions as a biological catalystactive site – the part of an enzyme molecule into which the substratemolecule fits
Assessment Objectives C5 in ‘O’ Level Biology is to design or plan an . Osmosis. 10 Active Transport. transport 23. plants. 35. 38. o .
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