Measuring Variables in Biology (SL IB Biology)

• When measuring a variable in an experiment, it is important to use the most appropriate instrument and measuring technique to achieve the most accurate value for that measurement to an appropriate level of precise

Measuring mass

• Mass is measured using a digital balance which normally gives readings to two decimal places
• Balances must be tared (set to zero) before use, this indicates that the container or balance is empty before taking the measurement
• The standard unit of mass is kilograms (kg) but in chemistry, grams (g) are most often used
  • 1 kilogram = 1000 grams

Measuring volume of liquids

• The most common units of measurement for volumes are cm³, dm³ or ml (millilitres) or l (litres)
• The volume of a liquid can be determined using several types of apparatus, depending on the level of accuracy needed
• For approximate volumes where high accuracy is not an important factor, measuring (or graduated) cylinders are used
  • These are graduated (have a scale so can be used to measure) and are available typically in a range of sizes from 10 cm³ to 1 litre (1 dm³)
• Volumetric pipettes are the most accurate way of measuring a fixed volume of liquid, usually 10 cm³ or 25 cm³
  • They have a scratch mark on the neck which is matched to the bottom of the meniscus to make the measurement
• Burettes are the most accurate way of measuring a variable volume of liquid between 0 cm³ and 50 cm³ (e.g. in a titration)
  • The tricky thing with burettes is to remember to read the scale from top to bottom as 0.00 cm³ is at the top of the column
• Whichever apparatus you use, you may see markings in ml (millilitre) which is the same as a cm³

Measuring the volume of gases

• The volume of a gas sometimes needs to be measured and is done by collecting it in a graduated measuring apparatus
• A gas syringe is usually the apparatus used
• A graduated measuring cylinder or burette inverted in water may also be used, provided the gas is not water-soluble
• If the gas happens to be heavier than air and is coloured, the cylinder can be used upright

Measuring time

• Time can be measured using a stopwatch or stop-clock which are usually accurate to one or two decimal places
• The units of time normally used are seconds or minutes although other units may be used for extremely slow reactions (e.g. rusting)
  • 1 minute = 60 seconds
• An important factor when measuring time intervals is human reaction time
  • This can have a significant impact on measurements when the measurements involved are very short (less than a second)

Measuring temperature

• Temperature is measured with a thermometer or digital probe
• Laboratory thermometers usually have a precision of a half or one degree
• Digital temperature probes are available which are more precise than traditional thermometers and can often read to 0.1 ^oC
• Traditional thermometers rely upon the uniform expansion and contraction of a liquid substance with temperature; digital temperature probes can be just as, if not, more accurate than traditional thermometers
• The units of temperature are degrees Celsius (ºC)

Measuring length

• Rulers can be used to measure small distances of a few centimetres (cm).
  • They are able to measure to the nearest millimetre (mm)
• The standard unit of length is metres (m)
• Larger distances can be measured using a tape measure
• Many distances in chemistry are on a much smaller scale, for example, a typical atomic radius is around 1 x 10^-10 m, so cannot be measured in this way
• When measuring length it is important to take note of the units that are being measured
  • 1 cm is 10 mm
  • 100 cm is 1 m

A ruler can measure distances to the nearest mm

Making counts

• This often involves taking counts of cells, by counting the cells in a known volume of a culture, the concentration can be assessed
  • Direct counting methods of cells include microscopic counts using a hemocytometer or a counting chamber
    • A hemocytometer works by creating a volumetric grid divided into differently sized cubes for accurately counting the number of particles in a cube and calculating the concentration of the entire sample
  • Counting the number of cells in a culture can also be carried by a method known as a streak plate, this involves plating a known volume of the cell culture onto a petri dish with a growth medium
    • Direct counting methods do not require highly specialised equipment sp are easy to perform but they can be quite time consuming
• Much of ecology involves counting organisms
  • Counting here can be difficult as many of organisms involved ma move, or are inconspicuous
  • Accurate counts are rare and so population sampling is carried out
  • Most counts are associated with errors due to the sizes of the populations or mistakes being made in detecting individuals or direct errors in counting such as large groups miscounted or individuals misidentified

Drawing annotated diagrams from observation

• To record the observations seen under the microscope (or from photomicrographs taken) a labelled biological drawing is often made
• Biological drawings are line pictures that show specific features that have been observed when the specimen was viewed
• There are a number of rules/conventions that are followed when making a biological drawing
• You can read in more detail the about drawing annotated diagrams from observations here

Making appropriate qualitative observations

• Classifying organisms is an example of making qualitative observations
• In qualitative classification, data can be classified based on attributes such as sex, colour of fur, number of limbs
• The attribute being observed cannot be measured so it is classed as qualitative data