A-level Biology practicals: skills and methods

What you have to be able to do

• Focusing with the coarse then fine adjustment, low power first
• Calibrating an eyepiece graticule against a stage micrometer
• Measuring a structure in graticule divisions and converting to micrometres
• Recognising an artefact introduced by preparation rather than present in the living cell

The calculations involved

• Magnification as image size over actual size, rearranged either way
• Converting between millimetres, micrometres and nanometres
• Standard form for anything below a micrometre

Errors to avoid

• Calibration is specific to the objective in use. Change objective and the division is worth a different distance.
• A correct number written with the wrong prefix is a different quantity, so carry the unit through every line.
• Resolution and magnification are separate quantities. A microscope can magnify past its resolution and produce a larger blur.

Safety

• Slides and coverslips break into sharp fragments. Carry slides flat, and put broken glass in the broken-glass container, never the ordinary bin.
• Never angle a microscope mirror towards direct sunlight, which can damage the eye looking down the eyepiece.
• A microscope lamp that plugs into the mains is an electrical appliance. Keep water and stains away from the lamp and cable, and report a damaged lead rather than using it.
• A stained slide carries the hazards of its stain, so check the CLEAPSS Hazcard and your school's risk assessment for whichever stain is in use.

Methods in the lessons

• Calibrate a graticule against a stage micrometer and measure a cell
  In Microscopy, magnification and resolution
• Identify xylem, phloem and sclerenchyma in a stem section
  In Plan diagrams and tissue sections

Taught alongside: Cells, microscopy and biological organisation

What you have to be able to do

• Softening a root tip in warm hydrochloric acid so the middle lamellae break down and the cells separate, then squashing it to a single cell layer
• Choosing a stain for what it binds to rather than for its colour
• Cutting a section thin enough to transmit light
• Lowering a coverslip to exclude air bubbles

The calculations involved

• Counting a sample and expressing part of it as a proportion
• Mitotic index from a count of cells in division against the total
• Deciding how large a count has to be before the proportion means anything

Errors to avoid

• A mitotic index without the number of cells counted is not a measurement. State the denominator.
• Cells fixed at one instant are a snapshot, so the proportion in a stage reflects how long that stage lasts, not how important it is.
• Squashing too hard destroys the chromosomes you came to look at, and not enough leaves overlapping layers that cannot be scored.

Safety

• Hydrochloric acid used to soften root tips harms the eyes and skin, and its classification depends on its concentration, so check the CLEAPSS Hazcard and your school's risk assessment. Wear eye protection and warm it in a water bath, never over a flame.
• Stains stain skin and clothing, and some carry further hazards: acetic orcein, for example, is made up in ethanoic acid. Check the CLEAPSS Hazcard and your school's risk assessment for each stain, and wear eye protection.
• Cut sections with a scalpel or a single-edged blade on a tile, never a loose double-edged razor blade, cutting away from the fingers that hold the tissue. Blades go in the sharps container.
• Coverslips are thin enough to break under a thumb. Press a squash through folded filter paper, and dispose of broken glass in the broken-glass container.

Methods in the lessons

• Prepare a stained root tip squash and calculate a mitotic index
  In The cell cycle and mitosis
• Grow pollen tubes across a range of sucrose concentrations
  In Sexual reproduction in flowering plants

Taught alongside: Cells, microscopy and biological organisation · Cell cycles, reproduction and development

What you have to be able to do

• Benedict's test for reducing sugars, and the acid hydrolysis step that brings a non-reducing sugar into range, then neutralising with sodium hydrogencarbonate, since Benedict's test needs alkaline conditions
• Iodine in potassium iodide for starch
• Biuret for peptide bonds
• The emulsion test for lipids, and why the water goes in second

The calculations involved

• Reading a colour change as an ordered scale rather than a number
• Judging when a qualitative result needs a quantitative follow-up

Errors to avoid

• Biuret detects the peptide bond, so it responds to any polypeptide and says nothing about which protein is present.
• A brick-red Benedict's result is not a concentration. If a question asks how much, colorimetry is the answer, not a colour word.
• A negative test can mean absent, or below the threshold of the test. Those are different claims and only one is safe to write.

Safety

• Benedict's test and the hydrolysis step use near-boiling water, which scalds. Heat tubes in a water bath rather than over a flame, lift them with a holder, and point the open end away from people.
• Benedict's solution contains copper(II) sulfate, and biuret reagent contains sodium or potassium hydroxide. Check the CLEAPSS Hazcard and your school's risk assessment for the reagents in use, and wear eye protection throughout.
• Sodium hydrogencarbonate fizzes as it neutralises the acid. Add it a little at a time so the tube does not froth over.
• Iodine solution stains skin and clothing; check the CLEAPSS Hazcard and your school's risk assessment. Wipe up spills at once.
• The emulsion test uses ethanol, which is highly flammable. No naked flame on the bench while the ethanol is out, and keep its bottle closed.

Methods in the lessons

• Test for reducing sugars, non-reducing sugars and starch
  In Carbohydrate structure and function
• Test for lipids with ethanol and water
  In Lipids: triglycerides, phospholipids and ester bonds
• Test for protein with biuret reagent
  In Protein structure and function

Taught alongside: Biological molecules, water and inorganic ions · Nutrition, digestion and health evidence

What you have to be able to do

• Making a serial dilution and stating the concentration at each step
• Selecting a filter of the colour the solution absorbs
• Zeroing the instrument against the right blank
• Plotting the calibration curve and interpolating the unknown

The calculations involved

• Serial dilution arithmetic, including a factor applied repeatedly
• Plotting a curve of best fit and reading a value back from it
• Recognising that interpolation is safe and extrapolation is not

Errors to avoid

• The blank is the solvent and everything else except the substance being measured. A blank of pure water when the samples carry buffer measures the buffer as well.
• A calibration curve is valid only across the range you calibrated. An unknown darker than your strongest standard needs diluting and re-reading, not extending the line.
• The filter is the complementary colour to the solution: a blue solution is measured through a red filter.

Safety

• Use a pipette filler or a syringe for every volume. Nothing is pipetted by mouth.
• The chemical hazards are in the solutions being measured, so check the CLEAPSS Hazcard and your school's risk assessment for each one before making the dilution series.
• Keep liquids away from the colorimeter and its lead, do not overfill cuvettes, and wipe the outside before they go into the instrument.

Methods in the lessons

• Find a glucose concentration from a colorimeter calibration curve
  In Carbohydrate structure and function
• Titrate food and drink against DCPIP for vitamin C
  In Diet, energy and how to read a health claim

Taught alongside: Biological molecules, water and inorganic ions · Nutrition, digestion and health evidence

What you have to be able to do

• Choosing what to measure: product formed, substrate lost, or gas collected
• Holding temperature with a water bath and pH with a buffer
• Taking readings often enough early on to see the initial slope
• Recognising when the substrate has run down rather than the enzyme having stopped working

The calculations involved

• Rate as a gradient, and the initial rate as the gradient at t = 0
• Drawing a tangent to a curve and taking its gradient
• Rate as the reciprocal of a time to a fixed end point, with units

Errors to avoid

• The initial rate is the only rate that reflects the conditions you set. Any later gradient also reflects how much substrate is left.
• A buffer is a control, not a detail. Without one, an enzyme that releases an acidic product changes its own pH as it works.
• Using time-to-endpoint as a proxy for rate inverts the axis: a longer time is a slower reaction, so a graph of time slopes the opposite way to the graph of rate it stands in for.

Safety

• Many enzymes are respiratory sensitisers, particularly as powders, and repeated exposure can cause an allergic reaction. Avoid raising dust or splashing the solutions, wash skin that the enzyme touches, and check the CLEAPSS Hazcard and your school's risk assessment.
• Hydrogen peroxide, the substrate for catalase, harms the eyes and skin, and its hazard rises with concentration: check the CLEAPSS Hazcard and your school's risk assessment, and wear eye protection.
• A hot water bath scalds and stands near mains electricity. Lift tubes out with a holder, and keep cables and plugs clear of the water.

Methods in the lessons

• Measure the rate of an enzyme-controlled reaction
  In Factors affecting enzyme rate: temperature, pH and concentration
• Draw a tangent for initial rate and calculate Q10
  In Factors affecting enzyme rate: temperature, pH and concentration
• Measure the effect of gibberellin on amylase in germinating seeds
  In Plant responses: tropisms, auxins and the classic experiments

Taught alongside: Enzymes and metabolic control

What you have to be able to do

• Cutting tissue to a standard size so surface area is not a confounding variable
• Washing cut tissue to remove pigment released by the cutting itself
• Holding each sample in its treatment for the same length of time
• Measuring the leaked pigment by colorimetry rather than by eye

The calculations involved

• Plotting a response against a treatment level
• Identifying a threshold from a curve that is flat and then steep
• Percentage transmission and absorbance, related by absorbance = log₁₀(100 ÷ %T), so one falls as the other rises

Errors to avoid

• The pigment on the outside of a freshly cut piece came from the knife, not from the treatment. Washing is the control that makes the reading mean something.
• At high temperatures, typically well above the physiological range, the damage is to the protein and the lipid together, so an answer that mentions only denatured protein is half the story; the exact temperature at which leakage climbs varies with the tissue.
• Deeper colour means more leakage means a more permeable membrane. State that chain explicitly; a lone absorbance value is not an explanation.

Safety

• Cut tissue on a tile with the blade moving away from you. Push a cork borer into tissue resting on the tile with a twisting motion, never into tissue held in the hand.
• Water baths at the top of the temperature range scald. Lift tubes with a holder or tongs, and let them cool before handling.
• Ethanol, if alcohol concentration is the variable, is highly flammable: no naked flames while it is out. For any other alcohol, check the CLEAPSS Hazcard and your school's risk assessment.

Methods in the lessons

• Measure membrane permeability with beetroot and a colorimeter
  In Membrane structure and the fluid mosaic model

Taught alongside: Membranes and transport across cells

What you have to be able to do

• Preparing a graded series of solutions of known water potential
• Blotting and weighing tissue consistently before and after
• Setting up a potometer without trapping air, and cutting the shoot under water
• Changing one of light, temperature, humidity and air movement deliberately while holding the other three steady

The calculations involved

• Percentage change in mass, and why percentage rather than raw grams
• Reading the intercept where the change is zero
• Rate of water uptake from distance moved and the bore of the tube

Errors to avoid

• The intercept is the answer. The point where mass change crosses zero is the solution whose water potential matches the tissue.
• A potometer measures uptake, not transpiration. Most water taken up is transpired, but some is used and retained, so uptake is the honest word for what was measured.
• Water potential is zero for pure water at atmospheric pressure and negative for every solution at atmospheric pressure, so a more concentrated solution has a lower value. The sign catches people out more often than the concept does.

Safety

• Cut tissue and shoots on a tile, or under water, with the blade moving away from the hand. Blades go in the sharps container.
• Glass capillary tubing snaps when it is forced into a bung, and the broken end cuts deeply. Use apparatus already assembled where possible; otherwise wet the tube, hold it in a cloth close to the bung and push with a gentle twist.
• A lamp or fan used to change conditions stands near water. Keep cables and plugs dry, and let a filament lamp cool before moving it.
• Some plants have irritant sap. Use a species your school's risk assessment approves, and wash your hands afterwards.

Methods in the lessons

• Find the water potential of potato tissue from a dilution series
  In Diffusion and osmosis
• Measure the rate of water uptake with a potometer
  In Xylem and the transpiration stream

Taught alongside: Membranes and transport across cells · Plant transport and mineral nutrition

What you have to be able to do

• Handling instruments and material safely, and disposing of both correctly
• Working from the outside inwards, cutting away from yourself
• Drawing in clean single lines with no shading and no sketching
• Labelling with straight ruled lines that do not cross

The calculations involved

• Drawing to a stated scale and calculating the magnification of the drawing
• Measuring a real structure and expressing it in sensible units

Errors to avoid

• A biological drawing carries a scale bar or a magnification. Without one it records shape and nothing else.
• Draw what is in front of you, not what the textbook diagram shows. The specimen often differs from the diagram, and those differences are what the drawing is meant to record.
• Naming a structure is not describing it. Questions usually want the feature and the function it serves in the same sentence.

Safety

• Scalpels and dissecting scissors are sharp. Cut on a board, away from yourself and others, pass instruments handle-first, and fit or remove scalpel blades only with the method your school uses. Blades go in the sharps container.
• Animal organs come from a food supplier or a biological supplier, never from an animal of unknown origin. Disinfect the bench afterwards, wash your hands, and dispose of the material as the risk assessment directs.
• Insects for dissection come from a supplier, are killed humanely by the method your school approves, and are used only in the numbers the work needs.
• Some people are allergic to insects such as locusts, or to the material being dissected. Say so before the practical rather than during it.

Methods in the lessons

• Dissect an insect's tracheal system and a fish gill, and draw them
  In Gas exchange in insects, fish and plants

Taught alongside: Exchange surfaces and gas exchange · Animal transport and cardiovascular biology

What you have to be able to do

• Flaming a loop and a bottle neck, and working close to the flame
• Pouring and spreading a plate without lifting the lid clear
• Placing discs at a spacing that keeps their zones separate
• Taping the lid on with two or four short strips rather than sealing it all round, which would favour anaerobes, then inverting and incubating well below body temperature, at the temperature the school's risk assessment sets, and never opening a plate once grown

The calculations involved

• Measuring the diameter of a zone of inhibition, across the disc and in two directions, and halving it to get the radius
• Calculating the area of a zone from that radius, and reading the question to see which of the two it wants
• Population growth as a doubling process, and mean generation time

Errors to avoid

• Diameter and area are both in use, and the question decides which. Some published methods measure the diameter and then calculate the area from it; others compare diameters directly and are marked that way. Measure the diameter either way, since the area cannot be found without it, and then give whichever the question asked for.
• The two rank drugs in the same order but not by the same margins. Area goes as the square of the radius, so a zone twice as wide has four times the area, and a table of areas will spread the same discs further apart than a table of diameters. Say which measure a comparison rests on before drawing a conclusion from it.
• The plate is incubated below body temperature deliberately, to avoid selecting for organisms that grow well in a human.
• A clear zone means growth was inhibited. Whether the organisms were killed or merely stopped is a separate question the plate does not answer.

Safety

• Treat every culture as a possible pathogen. Use only the cultures your school's risk assessment approves, from a recognised supplier, and do not culture from your own body or the surroundings unless the risk assessment covers it.
• Disinfect the bench before and after, wash your hands, and keep hands away from your mouth and eyes. Report any spill at once; it is covered with disinfectant and left before it is cleared.
• Ethanol for flaming a spreader is highly flammable. Keep its beaker covered and away from the Bunsen burner, tie long hair back, and let a flamed loop or spreader cool before it touches the culture.
• Some people are allergic to antibiotics such as penicillin. Handle antibiotic discs with forceps, not fingers.
• Once incubated, a plate stays taped and is not opened. Used plates and cultures are made safe by autoclaving or pressure-cooking before disposal, by the teacher or technician.

Methods in the lessons

• Test plant extracts against bacteria on agar, aseptically
  In Industrial biotechnology and fermentation
• Compare antibiotics by their zones of inhibition
  In Evidence for evolution, resistance and genetic drift
• Follow the growth of a bacterial culture by turbidity or counts
  In Industrial biotechnology and fermentation
• Isolate single colonies by streak plating
  In Industrial biotechnology and fermentation

Taught alongside: Pathogens, disease and immunity

What you have to be able to do

• Concentrating the spot by applying and drying it repeatedly
• Drawing the origin in pencil, since ink would itself run, and keeping it above the solvent surface
• Running the tank sealed so the atmosphere stays saturated
• Marking the solvent front the moment the run is stopped

The calculations involved

• Rf as distance moved by the pigment over distance moved by the solvent
• Recognising Rf as a ratio, so it has no units and never exceeds one
• Comparing an unknown Rf against reference values run in the same solvent

Errors to avoid

• If the origin sits below the solvent, the pigment dissolves into the tank instead of running up the paper.
• Rf values depend on the solvent, so a value compared against a table from a different solvent proves nothing.
• Measure to the centre of each spot, consistently, and say that you did. Spots spread, and the edge is not a defined point.

Safety

• The usual running and extraction solvents are flammable and give off vapour. Use them in a well-ventilated room or a fume cupboard, with no naked flames, keep the tank lidded, and check the CLEAPSS Hazcard and your school's risk assessment for the solvent you use.
• Grinding leaves with sand in a mortar can flick grit and solvent. Wear eye protection while grinding.
• Glass capillaries used for spotting break easily. Dispose of them in the broken-glass container.

Methods in the lessons

• Separate photosynthetic pigments by chromatography and find Rf values
  In Chloroplast structure and the light-dependent reactions

Taught alongside: Photosynthesis and primary productivity

What you have to be able to do

• Setting up a respirometer with an alkali to absorb carbon dioxide
• Running a thermobarometer alongside, so a pressure change from the room is subtracted rather than measured
• Using a redox indicator such as DCPIP to follow light-driven electron transfer in chloroplast extract (the Hill reaction, also described as dehydrogenase activity in chloroplast extracts); in respiring cells the same indicator class reports dehydrogenase activity
• Equilibrating apparatus before the first reading is taken
• Measuring the oxygen an aquatic plant such as Elodea or Cabomba gives off, as a gas volume in a photosynthometer rather than a bubble count, with hydrogencarbonate supplying the carbon dioxide

The calculations involved

• Volume of gas from the distance a meniscus moves and the tube's bore
• Rate per unit mass of tissue per unit time, and why all three parts are needed
• Reading a rate from the steepest straight part of a curve

Errors to avoid

• Without a control tube, a respirometer measures the weather. Room temperature and atmospheric pressure move the meniscus as readily as respiration does.
• In a respirometer with alkali present, the movement measures oxygen uptake. Remove the alkali and it measures the difference between two gases, which is a different quantity.
• A boiled or killed sample is the control that shows the change was biological. Stating that it was included earns the mark; assuming it does not.

Safety

• Soda lime and potassium hydroxide, used to absorb carbon dioxide, are corrosive or irritant to the eyes and skin: check the CLEAPSS Hazcard and your school's risk assessment. Wear eye protection, handle them with a spatula, and keep them behind gauze so no organism touches them.
• Living organisms in a respirometer, such as woodlice or maggots, are kept in the apparatus no longer than the readings need, handled gently, and returned to suitable conditions afterwards.
• A lamp near a water bath or a beaker of pondweed brings heat and mains electricity close to water. Keep cables dry, and do not touch a filament or halogen lamp until it has cooled.
• DCPIP stains skin and clothing; check the CLEAPSS Hazcard and your school's risk assessment. A blender and a centrifuge, if chloroplasts are extracted, are used only as the risk assessment directs.

Methods in the lessons

• Isolate chloroplasts and time the reduction of DCPIP
  In Chloroplast structure and the light-dependent reactions
• Measure the rate of photosynthesis in pondweed
  In Limiting factors and primary productivity
• Measure oxygen uptake with a respirometer
  In Anaerobic respiration and the respiratory quotient
• Measure the rate of respiration in yeast with methylene blue
  In Anaerobic respiration and the respiratory quotient

Worked in full

• The Hill reaction: DCPIP and isolated chloroplasts
  Reduce a dye with illuminated chloroplasts, time the colour change against a standard, and read a rate against the light intensity it was measured at.

Taught alongside: Respiration and cellular energy · Photosynthesis and primary productivity

What you have to be able to do

• Generating random coordinates and placing quadrats on them
• Running a belt or line transect where the question is about a gradient
• Mark, release and recapture for a mobile population, with a mark that neither harms nor advertises the animal
• Using a choice chamber or maze with the stimulus as the only variable that differs

The calculations involved

• Mean, and a measure of spread, from a set of quadrat counts
• Percentage cover against frequency, and when each is appropriate
• The Lincoln index for a population estimate from a recapture
• A statistical test chosen for the question asked, with the null hypothesis written before the data are seen

Errors to avoid

• Random means generated coordinates, not a walk that felt unplanned. Questions ask how randomness was achieved and reject answers that only assert it.
• The recapture estimate assumes marked animals mix back in, the mark stays on, and nothing joins or leaves in between. Quote the assumption the question is probing rather than all three by rote.
• A transect answers a question about a gradient. Using one where there is no gradient produces a tidy graph of nothing.

Safety

• Fieldwork is planned against the site: weather, uneven ground, and above all water, whether a river, a pond or a tide. Work in groups, under supervision, and stay within the agreed area.
• Cover cuts and wash your hands before eating, particularly after working with soil or in or near fresh water. Do not touch plants you cannot identify, since some sting, irritate the skin or are poisonous.
• Animals that are sampled, marked or placed in a choice chamber are handled gently, kept for as short a time as possible, and returned to where they were found. Marks must not harm them.
• Drying agents used in a choice chamber can be corrosive or irritant: check the CLEAPSS Hazcard and your school's risk assessment, and keep them beneath the gauze, out of the animals' reach.

Methods in the lessons

• Measure the effect of caffeine on the heart rate of Daphnia
  In Variables and controls: what an experiment can show
• Sample with quadrats and transects and relate distribution to an abiotic factor
  In Succession and environmental change
• Estimate a population of mobile animals by mark, release and recapture
  In Populations and their limits
• Investigate an animal's response to light or humidity in a choice chamber
  In Receptors: transduction and the reflex arc
• Measure habituation in snails to a repeated touch
  In Brain imaging, development and learning
• Measure the effect of temperature on seedling growth or hatch rate
  In The carbon cycle and climate change

Taught alongside: Ecology, populations and environmental change · Classification, biodiversity and conservation

What you have to be able to do

• Mashing tissue to break cell walls before any reagent is added
• Treating with a warm salt and detergent solution, and saying which reagent does which job
• Filtering the mixture to take out wall and organelle debris
• Adding a protease to hydrolyse the histones and the other protein in the extract
• Layering ice-cold ethanol down a tilted tube so it floats rather than mixes, and spooling the fibres from the boundary

The calculations involved

• Making up a solution of stated concentration by mass and volume
• Judging the volume of ethanol against the volume of extract, since too little leaves the DNA in solution
• Yield as mass of DNA recovered per gram of starting tissue, and why a visible tangle is not a measurement of it

Errors to avoid

• Each reagent answers a different structural question, and a question that asks for the job of one wants that one: detergent for the phospholipid of the membranes, salt to shield the negative charge on every phosphate of the backbone, protease for the histones, cold ethanol because DNA will not dissolve in it.
• The ethanol must be ice-cold and must be poured slowly down the side of a tilted tube. Warm ethanol keeps more of the DNA in solution, and shaking the layers together gives a cloudy suspension instead of fibres at a boundary.
• The white threads are DNA together with whatever protein survived the protease, so this is a preparation and not a pure sample. Purity is a separate claim needing a separate test.
• Mashing and heating both shear long molecules. The fibres are visible because DNA runs to millions of base pairs, so handling it roughly removes the very property being demonstrated.

Safety

• Ethanol is highly flammable. No naked flames in the room while it is out, and keep the bottle closed between uses.
• Proteases are respiratory sensitisers, particularly as powders such as biological washing powder. Avoid raising dust, wash off skin contact, and check the CLEAPSS Hazcard and your school's risk assessment.
• Detergent irritates the eyes, so wear eye protection. The warm water bath and any knife used to chop the tissue carry the usual scald and cut hazards.

Taught alongside: Nucleic acids, genomes and protein synthesis

What you have to be able to do

• Filling, knotting and sealing a length of dialysis tubing without leaks, and rinsing the outside before it goes into the beaker
• Setting the tubing up two ways: as a model gut, with starch and glucose inside and the surrounding water sampled at intervals, and as an osmometer, with sucrose inside and a capillary through the bung
• Sampling the outside liquid and testing it with Benedict's solution and with iodine, so that what crossed and what did not are both shown
• Cutting agar cubes of several edge lengths from one block, so the jelly, the indicator and the alkali are identical between them
• Timing the colour change to the centre of each cube against a stated endpoint

The calculations involved

• Surface area, volume and the ratio between them for a cube, and the same for a cylinder when the tubing is the shape in question
• Rate as height risen up a capillary per minute, or as the reciprocal of the time for a cube to lose its colour
• Plotting the ratio against the time to the centre, and reading the relationship the two quantities have

Errors to avoid

• Rinse the filled tubing before it is suspended, or the sugar spilled on the outside while filling gives a positive result within seconds and the experiment measures the washing-up.
• The cubes must be cut from one block and dropped in together. Cutting them from separate blocks changes the alkali concentration between them, and staggering the start makes the acid weaker for whichever cube goes in last.
• The endpoint is a judgement about a fading colour, so state how it was fixed and keep one person judging it. Cutting each cube in half at a set time and measuring the depth the colour has penetrated is the more defensible alternative.
• The models are stand-ins, and their limits are examinable. Dialysis tubing has pores and no proteins, so it models simple diffusion and osmosis and nothing else; agar is a jelly with no membrane at all, so it shows how far diffusion reaches rather than anything about selectivity.

Safety

• The acid and the alkali in the agar-cube method harm the eyes and skin, and their hazards depend on concentration. The indicator in the agar has hazards of its own. Check the CLEAPSS Hazcard and your school's risk assessment for each. Wear eye protection, and move the cubes with forceps or a spatula.
• Cut agar on a tile with the blade moving away from you.
• Testing the dialysis water uses Benedict's solution in near-boiling water and iodine solution, with the hazards and controls given for the biochemical tests.
• Glass capillary tubing through a bung snaps if it is forced. Use an osmometer already assembled, or wet the tube, hold it in a cloth and push with a gentle twist.

Taught alongside: Membranes and transport across cells · Exchange surfaces and gas exchange