Practical Skills: Endorsement Assessment (OCR A Level Biology): Flashcards

A fixed volume of distilled water is added to one side to create high humidity, and a fixed volume of drying agent is added to the other side to create low humidity.

They keep the woodlice at a safe distance from the water and drying agent, while still allowing them to move between the two sections.

• Divide each choice chamber into two sections, adding distilled water to one side and drying agent to the other to control humidity\n\n- Insert gauze platforms to keep the woodlice a safe distance from the water and drying agent\n\n- Divide the woodlice into two even-sized groups and drop them gently into the centre of chambers kept in the dark (group A) and in the light (group B)\n\n- Record the position of the woodlice after 15 minutes and repeat several times for all conditions

• A significant percentage of woodlice moved to the humid side, helping to prevent water loss from their respiratory surfaces\n\n- The woodlice responded to humidity more actively when in the light\n\n- The woodlice tended to stop moving in humid conditions, keeping them within favourable conditions that reduce water loss

It helps to prevent water loss from the respiratory surfaces of the woodlice, and stopping in humid conditions keeps them within favourable conditions that reduce dehydration.

False — such chemicals must not be administered to participants in a school setting.

True

A set of procedures used when culturing microorganisms that prevent the microbes being investigated from escaping or becoming contaminated with other unwanted, possibly pathogenic, microbes.

They ensure the microbes being investigated do not:

• escape from the culture

• become contaminated with another unwanted, possibly pathogenic, microbe

This is especially important in preventing the accidental culture of human pathogens.

Any four of, for example:

• Washing hands thoroughly to disinfect them

• Disinfecting work surfaces with disinfectant or alcohol

• Sterilising all apparatus, glassware and collecting loops before use

• Sterilising the culture medium (broth or agar plate)

• Using flamed loops or sterile swabs when transferring cultures

• Flaming culture bottlenecks to prevent contamination

• Only removing petri dish lids when necessary

To prevent contamination of the cultures and to reduce the risk of accidentally culturing or ingesting potentially pathogenic microbes, keeping both the investigation and the person safe.

To reduce the risk of culturing microorganisms that are pathogenic to humans, as human pathogens grow best at body temperature.

When transferring cultures, using flamed loops or sterile swabs avoids collecting unwanted microbes from the atmosphere.

A live bacterial cell that is able to divide and form a colony on an agar plate.

It is used to test for antibiotic resistance in bacteria.

• Pre-soak paper discs in the different antibiotic solutions

• Spread a sample of diluted bacterial broth onto the surface of a sterile agar plate

• Lightly press the paper discs onto the agar, evenly distributed and not touching each other or the edges

• Incubate the plate overnight at an optimum temperature for bacterial growth

• Examine the results with the petri dish lid on

So that the effects of the different antibiotics can be fairly compared (it acts as a control variable).

That the bacteria are vulnerable (susceptible) to that antibiotic.

There are no bacteria present in the clear area, because the antibiotic has diffused outwards and killed the bacteria where its concentration is high enough.

More effective antibiotics require a lower concentration to kill the bacteria, so they produce larger clear zones.

It allows multiple antibiotics to be tested at once.

That the bacteria are completely resistant to that antibiotic.

False — cultures should not be grown at body temperature, to reduce the risk of culturing microbes that are pathogenic to humans.

True

They allow the internal structures of organisms and organs to be examined, so that theories can be made about how they function.

The gills (the gas exchange surface in fish).

The spiracles, tracheae and tracheoles.

The gas exchange surface exposed by dissecting a fish is the gills.

• People worry about how the animals are raised and killed

• It goes against the religious beliefs of some individuals

• It should come from a reputable source

• It should be disposed of correctly

• If multiple specimens are used, they should be from individual organisms of the same species and roughly the same age

To avoid contamination with biological material, which could cause an allergic reaction.

• Cut away from your body

• Keep your fingers far from the blade to reduce the chance of cutting yourself

• Scissors: for cutting large sections of tissue, where cuts do not need to be precise

• Scalpel: for finer, more precise cutting (it must be sharp to ensure this)

To move the other sections of the specimen aside, leaving the desired structure exposed.

• It can be hard to see some of the smaller, finer structures within organs

• The specimens do not reflect how the tissue would look in a living organism

• If only a single specimen is dissected, anomalies within it may be ignored or glossed over

Sharp instruments give clean, precise cuts with as little damage as possible.

Blunt instruments are dangerous, as more force is required to cut, and they do not give precise cuts, making internal structures harder to distinguish.

Name the specific tools that should be used (e.g. scissors and forceps).

True — sharp instruments give clean, precise cuts and are safer, as less force is needed.

False — gills are found in fish; insects use spiracles, tracheae and tracheoles.

External openings on the surface of an insect through which air enters the gas exchange system.

The smallest tubes of the insect gas exchange system, where gas exchange with the body tissues takes place.

A line picture that shows specific features observed when a specimen is viewed under a microscope (or from a photomicrograph).

A small amount of blood spread onto a glass microscope slide, stained and covered with a coverslip, so that the different blood cells can be examined under a microscope.

• A title

• The magnification under which the observations were made

• Use a sharp HB pencil (and a good eraser)

• Draw on plain white paper

• Use clear, single lines with no shading

• Make the drawing as large as the space allows

• Only draw well-defined structures

• Use proper proportions

• They should not cross or have arrowheads

• They should connect directly to the part being labelled

• They should be kept to one side, parallel to the top of the page

• They should be drawn with a ruler

So that the drawing clearly shows the specific features and proper proportions of the structures observed, rather than obscuring them with thick shading.

Cell drawings are typically made when viewing cells at higher magnification.

Plan drawings are made of tissues viewed at lower magnification and never show individual cells.

They have no nucleus and a distinct biconcave shape.

White blood cells have irregular shapes and possess a nucleus, whereas red blood cells have a regular biconcave shape and no nucleus.

A distinctive lobed nucleus. Neutrophils make up roughly 70% of all white blood cells.

Lymphocytes can be identified by their very large nuclei, which nearly occupy the entire cell.

Neutrophils have distinctive lobed nuclei and make up roughly 70% of all white blood cells.

Only ever draw what you actually see, not what you think you see.

The eyepiece graticule.

True

False — red blood cells have no nucleus and a distinct biconcave shape.

False — label lines should have no arrowheads and should not cross.

A method of investigating the abundance and distribution of species and populations within a habitat.

• Random sampling: the positions of the sampling points are completely random / due to chance

• Non-random sampling: the positions of the sampling points are deliberately chosen by the person carrying out the sampling

Any three of:

• Sweeping nets

• Pitfall traps

• Pooters

• Tullgren funnel

• Mark up a grid on a map or to-scale drawing of the area and label the grid with coordinates

• Use a random number generator to choose a set of coordinates

• Place a quadrat at each generated coordinate

To avoid sampling bias, which could lead to over- or under-estimation (e.g. subconsciously choosing areas that are easier to access or that look like they contain more individuals).

To determine the number of quadrats required for a representative sample, a running mean is calculated and quadrats are added until there is no further significant change in its value.

Divide the whole area by the area of one quadrat, then multiply this value by the mean number of individuals per quadrat.

• Use a grid quadrat split into 100 smaller squares, where each square = 1% cover

• Count the number of squares in which the species occupies over half the square (e.g. 30 squares = 30% cover)

Usually used for plants.

Estimating percentage cover is subjective, so using the same person for all samples controls this variable and keeps estimates consistent.

The probability that a species will be found within any quadrat in the sample area. Calculated as: (number of quadrats the species was present in ÷ total number of quadrats) × 100.

It suggests the species lives in groups/clumps in preferred areas of the habitat, rather than being evenly distributed. Percentage cover and frequency used together give a good picture of a species' distribution.

They can only be used for sessile and immobile species (e.g. plants and slow-moving animals). Motile animals move and cannot be reliably counted this way, so a different method (mark-release-capture) is needed.

• Take a first large sample; catch, count and mark individuals in a way that won't affect survival

• Return marked individuals to the habitat and allow them to mix with the population

• After sufficient time, capture a second large sample

• Count the marked and unmarked individuals, and use the proportion of marked to unmarked to estimate the population size

N = (n₁ × n₂) ÷ m₂

• N = population estimate

• n₁ = number of marked individuals released

• n₂ = number of individuals in the second sample (marked and unmarked)

• m₂ = number of marked individuals in the second sample

• Marked individuals have sufficient time to disperse and mix back into the main population

• Marking does not affect the survival rate of individuals (e.g. does not increase predation)

• The marking remains visible and does not rub off

• The population size stays constant during the study (no significant births, deaths or migration)

False — quadrats and transects can only be used for sessile and immobile species; mobile animals require the mark-release-capture method.

True

A tool that allows for the quick and efficient gathering of data, either for simple data collection (e.g. breathing rate) or for more complex data collection using a number of probes monitoring different variables attached to a main computer.

The information can be formatted into a table, and the computer is then able to:\n\n- Calculate the mean\n\n- Perform statistical tests\n\n- Plot graphs using the data

Information from a data logger can be inputted into a computer and formatted into a table.

• Ecological succession / zonation\n\n- Infectious disease transmission / epidemiology\n\n- Predator-prey relationships\n\n- Natural selection\n\n- Genetic drift

Time can be sped up to predict the future outcome of populations and environments, allowing changes that would normally take very long time periods and many generations to be modelled over much shorter, more experimentally appropriate timescales.

The program usually starts off with a simple simulated population, with no particular selection pressures acting upon it. Natural selection can then be investigated by changing various factors and observing the effects on the simulated population.

• The presence or absence of different biotic and abiotic selection pressures (e.g. predators, disease, food availability)\n\n- Making new, specifically selected mutations appear in the population\n\n- Changing the likelihood that a new mutation will appear\n\n- Switching which alleles are dominant and which are recessive\n\n- Changing the environment the species is in\n\n- Changing one or more of the adaptations of the species

The effects of the changing factors are modelled, and the probabilities of different outcomes for the simulated population can be calculated.

A gradual change in allele frequencies in a small population due to chance and not natural selection. When a population is significantly small, chance can affect which alleles get passed onto the next generation, so over time some alleles can be lost or favoured purely by chance.

When a population is significantly small, chance can affect which alleles get passed onto the next generation, so over time some alleles can be lost or favoured purely by chance.

A gradual change in allele frequencies in a small population due to chance, and not natural selection, is called genetic drift.

Computer programs similar to those used to model natural selection can be used to model genetic drift.

True

False — genetic drift is caused by chance, not natural selection.

The surface area to volume ratio of the agar cubes.

• Cut coloured agar into cubes of different sizes (e.g. 0.5 cm, 1 cm and 2 cm cubes)\n\n- Place each cube into a boiling tube containing a diffusion solution (e.g. dilute hydrochloric acid)\n\n- Time the diffusion of the acid into the cubes (shown by the indicator changing colour)\n\n- Calculate the rate of diffusion and compare it with the surface area to volume ratio of each cube

The agar can be made up with Universal Indicator only, or with very dilute sodium hydroxide solution and Universal Indicator to create purple agar.

So that the acid's diffusion into the agar can be monitored by a change in colour of the indicator in the agar blocks.

• The time taken for the acid to completely change the colour of the indicator in the agar blocks (measured with a stopwatch)\n\n- The distance travelled into the block by the acid in a given time period, e.g. 5 minutes (measured with a microscope and stage micrometer)

A microscope and a stage micrometer (essentially a mini ruler).

Rate of diffusion = 1 ÷ time taken

A graph showing how the rate of diffusion (rate of colour change) changes with the surface area to volume ratio of the agar cubes.

As a cube increases in size, its volume increases faster than its surface area (volume is cubed, surface area is squared).\n\nThis gives a larger cube a smaller surface area to volume ratio, so diffusion into the centre takes longer and is less effective.

• Mass — a digital balance\n\n- Time — a digital stopwatch\n\n- Volume — a measuring cylinder

• Temperature — a digital thermometer (though water baths have one built in)\n\n- Length — a ruler

The greater the surface area to volume ratio of an agar cube, the faster the rate of diffusion.

In the agar practical, the acid completely changing the colour of the indicator shows that diffusion has taken place.

The total surface area of an object divided by its volume; as an object gets larger this ratio decreases.

The net movement of particles from a region of higher concentration to a region of lower concentration.

False — a larger cube has a smaller surface area to volume ratio.

True

To investigate the effects of immersing plant tissue in solutions of different water potentials, and to use the results to estimate the water potential of the plant tissue itself.

• Cut potato cylinders to the same length (one or more per solution)

• Blot dry, then measure and record the initial mass of each

• Place each cylinder into a sucrose solution of a different concentration (at least 5 concentrations), leaving them for a set time (e.g. 30 minutes)

• Remove, dry to remove excess liquid, then measure and record the final length and mass

Percentage change in mass = (change in mass ÷ initial mass) × 100

A positive percentage change in mass indicates the solution had a higher water potential than the potato.

The potato has gained water by osmosis (net movement of water from the solution into the potato), so the solution had a higher water potential than the potato. The cells become turgid.

There is the greatest concentration gradient between the potato cells (higher water potential) and the sucrose solution (lower water potential), so the most water molecules move out of the cells by osmosis. The cells become flaccid, and may become plasmolysed.

The solution had the same water potential as the solution in the cytoplasm of the potato cells, so there was no concentration gradient and no net movement of water into or out of the cells.

Plot percentage change in mass against sucrose concentration. The point where the line of best fit crosses the x-axis (zero percentage change) gives the sucrose concentration inside the potato cylinders.

A plant cell that has taken in water by osmosis so that the water exerts turgor (hydrostatic) pressure on the cell wall, making the cell firm.

When a plant cell loses so much water by osmosis that the protoplast (cell membrane and its contents) shrinks and pulls away from the cell wall.

• Water leaves the cell through the partially permeable cell surface membrane by osmosis

• As water leaves the vacuole, the volume of the cell decreases

• The protoplast shrinks and stops exerting pressure on the cell wall

• The protoplast pulls away from the cell wall — the cell is plasmolysed

• Prepare epidermal strips (e.g. from red onion, which has coloured sap to make observations easier)

• Place the strips in a range of sucrose or sodium chloride solutions of gradually decreasing water potential

• View under a light microscope and count the number or percentage of cells that have undergone plasmolysis (may take several minutes)

Coloured-sap plants such as red onion are used because they make observations of plasmolysis easier under the microscope.

False — a negative change means the solution had a lower water potential, so water left the potato by osmosis and the cells became flaccid.

True

• Temperature

• Solvent concentration

Beetroot cells contain a dark purple-red pigment.

The higher the membrane permeability, the more of this pigment leaks out of the cell into the surrounding water.

A machine that passes light through a coloured liquid sample and measures how much light is absorbed, indicating how much colour (pigment) is present in the solution.

The small cuboid container that holds the liquid sample to be measured in a colorimeter.

It ensures the correct wavelength of light is used to measure the optical density of the specific pigment (e.g. the beetroot pigment betalain).

Using a cuvette containing distilled water, before each sample is inserted.

So they all have equal surface areas, volumes and mass.

Otherwise these factors could affect the rate at which pigment leaks out, making the comparison unfair.

To remove any pigment released during cutting, which would otherwise affect the results.

• Cut five equal-sized cubes of beetroot and rinse them

• Add each to a test tube containing the same volume of water

• Place each test tube in a water bath at a different temperature for the same length of time (e.g. ~30 minutes)

• Remove the beetroot pieces, leaving just the coloured liquid

• Use a colorimeter to measure the absorbance of each sample

The higher the absorbance, the more pigment has been released, due to a greater membrane permeability.

As temperature increases, the phospholipids in the membrane gain more energy and move more, so they become less tightly packed, increasing the permeability of the membrane.

• Phospholipids gain more energy and become less tightly packed

• At high temperatures the phospholipid bilayer may melt and break down

• Water inside the cells expands, putting pressure on the membrane and deforming channel and carrier proteins

• High temperatures break intermolecular forces in proteins, changing their conformation and function

• Channel or carrier proteins can deform at these low temperatures

• Ice crystals that form can pierce the cell membrane, making it highly permeable

Cuvettes may vary slightly in thickness, or be scratched, so they absorb slightly more light.

Solution: use the same cuvette for every reading, or repeat many times and find a mean.

So that only the pigment in the sample is measured, not the water or the cuvette.

False — a higher absorbance indicates more pigment has leaked, showing greater membrane permeability.

True

It can be used to carry out a semi-quantitative test, in which the colour change is used to estimate the concentration of reducing sugar present in the solution.

A test that gives an estimate of concentration (rather than an exact value), typically by comparing an observed colour change against a set of known standards.

So that there is more than enough copper(II) sulfate present to react with any reducing sugar in the sample, ensuring the colour change reflects the amount of sugar and not a shortage of Benedict's solution.

The intensity of the colour change relates to the concentration of reducing sugar present. A positive test is shown along a spectrum of colour:\n\n- Green = low concentration\n\n- Brick-red = high concentration

A positive semi-quantitative Benedict's test is shown along a spectrum of colour, from green (low concentration) to brick-red (high concentration of reducing sugar).

Set up standard solutions with known concentrations of a reducing sugar (such as glucose), produced by carrying out a serial dilution of an existing stock solution.

A series of dilutions made from a stock solution, in which the concentration decreases by the same quantity between each test tube.

• Doubling dilutions, where the concentration is halved between each test tube\n\n- A desired range of concentrations (e.g. 0, 2, 4, 6, 8, 10 mmol dm⁻³)

• Add the same volume of (excess) Benedict's solution to each sample\n\n- Heat all tubes in a boiled water bath, ideally at the same temperature each time\n\n- Leave for a set time (around 5 minutes) to allow the colour changes to occur\n\n- Record the colour change for each known concentration

The same procedure is carried out on the sample of unknown concentration, and its colour is compared to the standard solution colours to estimate the concentration of reducing sugar present.

An instrument that beams a specific wavelength (colour) of light through a sample and measures how much of that light is absorbed (in arbitrary units), providing a quantitative measurement.

A filter is chosen so that a contrasting colour of light is shone through the sample, as this will be absorbed rather than reflected (e.g. shine green light through a red sample).\n\nThis is because a sample looks red as that wavelength is being reflected, while the other wavelengths are absorbed.

• Place a blank into the colorimeter and take a reference reading\n\n- It should read 0 (no light being absorbed)\n\n- Repeat this step periodically while taking measurements to check the reading is still 0

• Measure the absorbance of the known concentrations and plot a calibration (standard) curve of absorbance against concentration\n\n- Measure the absorbance of the unknown sample\n\n- Read the unknown concentration off the calibration curve

True — this ensures there is more than enough copper(II) sulfate to react with all the reducing sugar present.

False — a colorimeter provides a quantitative measurement of how much light is absorbed by the sample.

A reagent that determines whether or not a substance is present in a sample, without indicating its quantity or concentration.

Ethanol (used in the emulsion test).

Biuret reagent.

Benedict's solution and iodine.

Lipids are nonpolar molecules, so they do not dissolve in water but will dissolve in organic solvents such as ethanol.

To determine, quickly and easily in a lab, whether a sample contains lipids.

• Add ethanol to the sample to be tested\n\n- Shake the mixture to mix\n\n- Add the mixture to a test tube of water

A milky emulsion forms (the solution appears 'cloudy').\n\nThe more lipid present, the more obvious the milky colour of the solution.

The solution remains clear (no milky emulsion forms).

In the emulsion test, a positive result is shown by the formation of a milky emulsion.

It is a qualitative test - it shows whether lipid is present but does not give a quantitative value for how much lipid is in the sample.

Lipids are nonpolar molecules, so they do not dissolve in water but do dissolve in ethanol.

False — it is a qualitative test that only shows whether lipid is present, not how much.

False — lipids are nonpolar, so they dissolve in ethanol but not in water.

A cloudy mixture of tiny lipid droplets suspended in water, giving the milky appearance seen in a positive lipid test.

The Biuret test.

A reagent containing an alkali (e.g. sodium hydroxide) and copper (II) sulfate, which reacts in the presence of peptide bonds to test for protein.

• An alkali (e.g. sodium hydroxide)

• Copper (II) sulfate solution

Peptide bonds (the reagent reacts in the presence of peptide bonds).

• Add sodium hydroxide to the food solution to make it alkaline

• Add a few drops of copper (II) sulfate solution (which is blue) to the sample

• Repeat using a control solution (containing no protein)

• Compare the colours of the control and the food sample

A colour change from blue to lilac/mauve indicates protein is present.

In the Biuret test, if no colour change is observed and the solution remains blue, then no protein is present.

The reagent reacts with peptide bonds, so samples containing only amino acids or dipeptides (which lack two or more peptide bonds) give a negative result.

The colour change can be very subtle, so a white tile background makes it easier to detect the change from blue to lilac/mauve.

A control solution containing no protein (e.g. distilled water) provides a reference colour to compare against the food sample, making any colour change easier to identify.

It only indicates whether protein is present or absent (via a colour change); it does not give a numerical value for the amount of protein in the sample.

A test that shows whether a substance is present or absent but does not measure how much of it is present.

False — it is a qualitative test that only shows whether protein is present or absent.

False — at least two peptide bonds must be present, so amino acids and dipeptides give a negative result.

A technique used to separate a mixture into its individual components, relying on differences in solubility between the solutes in the mixture.

• The mobile phase

• The stationary phase

The components separate as the mobile phase travels over the stationary phase.

Differences in the solubility of each component affect how far it travels.

Components with higher solubility travel further, as they spend more time in the mobile phase.

• Mobile phase: the solvent (a liquid, e.g. water or ethanol) in which the sample molecules move

• Stationary phase: the chromatography paper

• Place a spot of the mixture on the chromatography paper and leave it to dry

• Suspend the paper in a solvent

• As the solvent travels up, the components move at different speeds (larger molecules move slower)

• The mixture separates into spots or bands, producing a chromatogram

The pattern of separated spots or bands produced on the chromatography paper after a mixture has been separated.

During paper chromatography, larger molecules move slower than smaller ones.

Colourless mixtures must be stained first so the spots can be seen.

Coloured mixtures such as ink or chlorophyll do not need staining.

Each amino acid is more or less soluble in the mobile phase than the others, so they separate depending on their:

• Charge

• Size

Dry the chromatography paper and then spray it with ninhydrin solution.

Ninhydrin reacts with amino acids to produce an easily visible blue-violet colour.

The line drawn across the chromatogram to show how far the solvent travelled. The distance from the origin line (where samples were placed) to the solvent front is the distance moved by the solvent.

Rf = distance moved by solute ÷ distance moved by solvent

The Rf value is a ratio, so it is always lower than one and has no units.

A smaller Rf value indicates the molecule is less soluble and larger in size (it has travelled a shorter distance during the mobile phase).

Each pigment has a unique Rf value, so pigments extracted from a leaf can be separated and identified by comparing their Rf values to known values.

Place known standard solutions beside the sample, then compare and match the spots.

If a sample spot is the same distance from the origin line as a standard, the mixture must contain that substance.

• Carotenoids have the highest Rf values (close to 1)

• Chlorophyll A has an intermediate Rf value

• Chlorophyll B has a much lower Rf value

True

False — the Rf value is a ratio, so it is always lower than one.

A series of dilutions of a stock solution in which the concentration decreases by the same quantity between each successive test tube.

By taking a series of dilutions of a stock solution, so that the concentration decreases by the same quantity between each test tube.

• Doubling dilutions – the concentration is halved between each test tube\n\n- A desired range of concentrations (e.g. 0, 2, 4, 6, 8, 10 mmol dm⁻³)

A type of serial dilution in which the concentration is halved between each successive test tube.

To create a standard (a known range of concentrations) against which unknown concentrations can be compared.

Serial dilutions are completed to create a standard to compare unknown concentrations against.

• Visual comparison\n\n- Measured using a calibration/standard curve\n\n- Measured using a colourimeter

• Counting bacteria or yeast populations\n\n- Determining unknown glucose, starch or protein concentrations

In a doubling dilution, the concentration is halved between each successive test tube.

False — the concentration decreases by the same quantity between each test tube.

True

A piece of apparatus used to investigate the effect of environmental factors on the rate of transpiration (by measuring water uptake by a plant shoot).

• Air movement (airflow)

• Humidity

• Temperature

• Light intensity

To prevent air from entering the xylem.

If air enters the apparatus, the readings will be affected, so the results will be inaccurate.

Any moisture on the leaves will affect the rate of transpiration.

After setting up the environmental factor being investigated, the plant is left to adapt to the new environment for 5 minutes before readings are taken.

The starting and end locations of the bubble are recorded over a set time period.

The further the bubble travels in the same time, the faster transpiration is occurring, and vice versa.

• Cut a shoot underwater and place it in the tube

• Set up the apparatus, sealing any gaps with Vaseline to make it airtight

• Dry the leaves of the shoot

• Introduce a single air bubble into the capillary tube

• Set up the environmental factor and allow the plant to adapt for 5 minutes

• Record the starting location of the air bubble

• Leave for a set time, then record the end location of the bubble

Set up a fan or hairdryer directed at the plant to increase airflow.

Spray water inside a plastic bag and wrap it around the plant to increase humidity.

Change the distance of a light source from the plant.

Change the temperature of the room the plant is in (e.g. a cold or warm room).

Keep all other factors the same, changing only the one factor being investigated.

False — high humidity decreases the rate of transpiration.

True

Many biological structures are too small to be seen with the naked eye.

Optical microscopes allow tissues, cells and organelles to be seen and studied — for example, the movement of chromosomes during mitosis can be observed.

• Light is directed through a thin layer of biological material supported on a glass slide

• The light is focused through several lenses so that an image is visible through the eyepiece

• Magnifying power is increased by rotating a higher power objective lens into place

• The eyepiece lens

• The objective lenses

• The stage

• The light source

• The coarse and fine focus

• Add a few drops of the sample to the slide using a pipette

• Cover the liquid/smear with a coverslip and gently press down to remove air bubbles

• Wear gloves to ensure there is no cross-contamination of foreign cells

• Use scissors to cut a small sample of the tissue

• Peel away or cut a very thin layer of cells (using a scalpel or forceps) to place on the slide

• A stain may be needed to make the structures visible

• Gently place a coverslip on top and press down to remove air bubbles

• It is easier to find what you are looking for in the field of view

• It helps prevent damage to the lens or coverslip in case the stage has been raised too high

Thin layers of material placed on slides can dry up rapidly, damaging the cells through dehydration.

• The focus is off — switch to the lower power objective lens and use the coarse focus

• The specimen may not be thin enough for light to pass through

• There could be cross-contamination with foreign cells or bodies

An internal ruler within the eyepiece of a microscope, consisting of a series of evenly spaced vertical lines, used to measure the size of an object being viewed.

A slide with an accurate scale in micrometres (µm), used to calibrate the eyepiece graticule. The smallest subdivisions typically measure 0.01 mm (10 µm).

The eyepiece graticule scale measures differently depending on the objective lens magnification used, and there are small differences between microscopes.

Calibrating the graticule against a stage micrometer (a scale of known size) works out the actual length of each graticule division at a specific magnification, so the graticule can then be used to measure the object.

• Line up the eyepiece graticule with the stage micrometer scale

• Count how many graticule divisions correspond to one micrometer division of known size

• Divide to find the length of one graticule division at that magnification

Cell structures are 3D, but when tissue samples are cut at different planes they may appear inconsistent in size on a 2D slide.

• Structures are 3D but samples are cut at different planes, giving inconsistent sizes on a 2D slide

• Optical microscopes have lower magnifying power than other microscopes, so some structures cannot be seen

• Slide preparation could alter the structure of cells

• Replace the micrometer with the specimen slide at the same magnification

• Count the graticule divisions spanned by the object

• Multiply the number of divisions by the length of one division to give the object's actual size

False — always start with the low power objective lens, as it is easier to find the specimen and helps prevent damage to the lens or coverslip.

True