Biological Calculations and Data

Convert matching quantities before combining them. There are 1000 μm in 1 mm, but volume conversions involve cubed length factors: 1 cm³ is 1000 mm³. Magnification is a dimensionless length ratio, whereas a volume-per-time rate needs both a volume and time unit.

State the reference quantity in any percentage: percentage change uses the initial value, while percentage of a total uses that total. A change from 40% to 25% is a fall of 15 percentage points, not automatically a 15% relative decrease. Sense-check signs and approximate magnitudes before reporting precision.

For a stated additive-volume dilution, C1V1=C2V2. Several dilution steps multiply their factors. For ideal constant doubling, N=N0×2^(t/g), where g is generation time; lag, resource limits and death can make real cultures depart from this simple model.

For a progress curve, distinguish a mean rate over an interval from an instantaneous tangent gradient. Keep enough intermediate precision and round the final answer appropriately. A fitted straight line is useful only if the biological model and data support it; not every relationship should be forced through zero.

Use clear table headings with quantities and units. Bar charts suit separated categories, histograms suit grouped continuous data with appropriate frequency-density handling when class widths differ, and scatter or line-based plots can show relationships between quantitative variables. A logarithmic axis can make multiplicative growth patterns easier to interpret, but equal visual steps then represent ratios rather than equal additions.

A mean describes central tendency and SD describes dispersion. Label error bars by their actual meaning. Larger samples can improve estimation but cannot automatically remove biased selection or calibration error. AQA requires interpretation of standard deviation; manual SD calculation is not required in written examinations. Standard error and confidence intervals can be useful extensions, but are not substitutes for identifying what a plotted bar represents.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.