OCR A-Level Biology

Designing an investigation

An investigation starts with a question that can be answered by measurement, and a prediction (a hypothesis) that follows from the biology. The independent variable is the one the investigator changes, the dependent variable is the one measured, and every other factor that could affect the dependent variable is a control variable that has to be kept constant or monitored. If a variable cannot be held constant, for example room temperature over several days, it is recorded so that its effect can be judged afterwards.

A plan that can be followed by someone else states:

• the range and intervals of the independent variable (at least five values, evenly spaced, unless the biology suggests otherwise);
• what is measured, with which apparatus, in which units and how often;
• how each control variable is kept constant, and how;
• how many repeats are made at each value, so that a mean can be calculated and anomalies spotted;
• the control experiment, which differs from the test only in the factor being tested, and shows that the result is caused by that factor. A tube with boiled enzyme, or with water in place of enzyme, is a typical control;
• the hazards and how the risk is reduced.

Implementing the plan

Using apparatus correctly means measuring in the right units, to the resolution the instrument allows, and recording observations and data in a table or a drawing as they are made.

Choosing apparatus

The apparatus must measure the dependent variable at a suitable resolution. A measuring cylinder reads to about 1 cm³, a graduated pipette or a syringe to 0.1 cm³ or better, a balance to 0.01 g or 0.001 g, a thermometer to 0.5 or 1 °C, and a stopwatch to 0.01 s although human reaction time limits timings to about 0.2 s. A larger quantity measured with the same instrument has a smaller percentage error, so the choice of volumes matters as well as the choice of apparatus.

Instruments that give a continuous record, such as a data logger with a temperature probe or a pH probe, remove the need to read a scale at intervals and allow rates to be found from smooth curves. A colorimeter measures how much light of a chosen colour a solution transmits or absorbs, so it gives a quantitative result for something that would otherwise be judged by eye. A potometer measures water uptake by a shoot, and a respirometer measures gas uptake or release. Other sections of these notes describe how each is used.

Serial dilutions

A serial dilution makes a series of solutions whose concentrations fall by the same factor each step. In a tenfold series, $1\ \mathrm{cm^3}$ of the stock solution is added to $9\ \mathrm{cm^3}$ of water to make a $10^{-1}$ solution. $1\ \mathrm{cm^3}$ of that is then added to a further $9\ \mathrm{cm^3}$ of water to make $10^{-2}$, and so on. A fresh pipette or syringe is used for each transfer so that solution is not carried over, and each tube is mixed before it is sampled. Serial dilutions give a wide range of concentrations from one stock, which is useful for a calibration curve, for the effect of a substrate or hormone at several concentrations, and for making bacterial cultures countable.

A dilution by a factor other than ten works the same way. Adding $5\ \mathrm{cm^3}$ of solution to $5\ \mathrm{cm^3}$ of water halves the concentration at each step, giving $100\%$, $50\%$, $25\%$, $12.5\%$ and so on.

Using a light microscope

Microscope work has a fixed order, so that the specimen is not damaged and the image is in focus.

1. Place the prepared slide on the stage and clip it in place, with the lowest-power objective lens in position.
2. Look from the side and use the coarse focus to bring the lens close to the slide, then look through the eyepiece and focus away from the slide, so the lens cannot be driven into the slide.
3. Sharpen the image with the fine focus, and adjust the light and diaphragm for contrast.
4. Move to a higher-power objective only once the specimen is centred and in focus, and then use the fine focus only.

Preparation of a slide is described in the note on microscopy, together with staining, the graticule and magnification.

Scientific drawings

A scientific drawing is made from observation, and it is not an artistic picture. It is large enough to show detail, uses sharp continuous lines with no shading or colour, and has a title saying what it shows and the magnification or a scale bar. Lines from labels are drawn with a ruler and do not cross, and they end on the structure they name. A low-power drawing shows the arrangement of tissues (a plan) and does not show individual cells, while a high-power drawing shows a small number of cells in detail. Proportions are copied accurately, so the drawing can be used to measure. Annotations give observations or functions, such as the thickness of a wall, and are written beside the labels.

Qualitative tests and separations

Qualitative reagents identify biological molecules by a colour change, and the tests are given in the note on biochemical tests. Colorimetry and chromatography give quantitative or separated results, and they are described there too. Gel electrophoresis separates fragments of DNA or proteins by size and charge, and is described in the note on sequencing and profiling.

Aseptic technique and microbiology

Microorganisms are everywhere, so any culture must be protected from contamination and the person must be protected from the culture. Aseptic technique includes:

• wiping the bench with disinfectant, and washing hands;
• sterilising inoculating loops in a flame until red hot, and cooling them in the air near the flame before touching the culture;
• flaming the neck of a bottle or tube each time it is opened and closed, and keeping it tilted so that airborne microbes are not drawn in;
• lifting the lid of a Petri dish only as far as is needed, and taping it shut afterwards with two or four pieces of tape, not all round the edge, so that air can still enter and anaerobic pathogens are not encouraged;
• incubating at a temperature of about 25 °C in a school laboratory, which is well below body temperature and so does not favour human pathogens;
• sterilising used cultures in an autoclave before disposal.

Cultures are grown on agar plates, which are solid, or in a nutrient broth, which is liquid. On a plate, a lawn of bacteria is spread over the whole surface, and a clear zone around a disc or well soaked in an antibiotic or disinfectant shows that growth has been stopped. The diameter of the clear zone is measured, and the area $\pi r^2$ is a better measure of the effect than the diameter. A control disc with no antimicrobial shows that the disc itself has no effect. In a broth the growth of the culture is followed by counting cells, or by measuring how much light the cloudy culture absorbs or scatters in a colorimeter.

Using organisms and dissection

Experiments on whole organisms, such as measuring the plant response to light or an animal's physiological function, are carried out safely and ethically. Ethical practice means causing the minimum of harm, stress or distress, not using more organisms than needed, choosing the least sentient organism that answers the question, and returning organisms to their habitat unharmed where appropriate. Human volunteers, for example in pulse or breathing measurements, take part only if they want to, are fit to do so, and can stop at any time.

Dissection of an animal or plant organ uses a sharp scalpel or scissors on a dissecting board, with the cut made away from the hand and fingers. The organ is identified, and then external and internal structures are observed, drawn and labelled. Specimens of organs such as the heart, kidney, gaseous exchange system of a fish or an insect, and plant stems are examined in this way. A stem is cut in transverse and longitudinal sections and stained so that the xylem vessels show up.

Fieldwork

Sampling in the field uses quadrats, sweep nets, pitfall traps and pooters, and the choice of method depends on the organism. The sampling methods, and what is calculated from the results, are covered in the note on biodiversity and sampling.

Information technology and research

Computer modelling, data loggers and software for processing and plotting data all help in collecting accurate results and analysing them. Sources are referenced when information is taken from books, journals or websites, and the reliability of a source is judged by who produced it, whether the evidence is primary or secondary, and whether the claims have been reviewed by other scientists.