The trouble with living material

There are eight required practicals on this course, and a practical question can draw on any of them. It is tempting to revise them as eight methods to remember. They are not really eight methods. They share one problem that chemistry does not have, and once you see it every practical question looks the same. ⭐⭐ The material is alive, and living things vary. Two seeds, two leaves or two people differ for reasons nobody set up and nobody can fully remove. So a single measurement of a single individual tells you almost nothing, because you cannot know whether that individual was typical or unusual. In chemistry, two careful runs of the same reaction should land in almost the same place, and if they do not, something went wrong. In biology, two runs land in different places because the two living things were different, and that is not a fault. It is the thing you are measuring. ⭐ So the craft of a biology practical is turning that variation from a nuisance into the measurement. You do it in two moves. You measure MANY individuals, not one, so their natural differences average out. And you set them against a COMPARISON GROUP that did not get the change, so you can say the change itself was responsible. ⚠️ That gives you the two questions to carry through everything that follows: HOW MANY, and COMPARED WITH WHAT?

Three ideas that tame the variation

Living things differ, and these three ideas are how a biologist stops that difference from wrecking the result. Almost every decision in a biology method is one of the three.

Tap the two choices that fight variation

A student writes up a biology method. Tap the TWO decisions that exist because living things vary from one another, rather than because a single reading needs to be taken more carefully.

I measured many plants rather than one, and worked out their average height.
I used a ruler with finer markings so I could read each height more exactly.
I grew a second group that was given no feed, kept the same in every other way, to compare against.
I read the scale at eye level each time to avoid a parallax error.

The odd plant that might be real

One plant in a set gives a value well away from the others. Why is it harder in biology than in chemistry to call that value a mistake and cross it out?

Because living things genuinely differ, so an unusual value may be a real individual rather than an error, and only more measurements can tell the two apart
Because in biology every unusual value must always be kept whatever its cause
Because biological instruments cannot be read finely enough to judge
Because biology never produces mistakes, only true results

Six words a living experiment needs

Six terms that a biology practical answer turns on. Learn each against the job it does.

Match each choice to what it guards

Measuring a large number of individuals rather than one or two
Growing a group that receives no treatment alongside the treated one
Keeping temperature and light the same for every group
Adding the values from all individuals and dividing to get one figure
Giving every individual the same length of time to respond before measuring

Lets the natural differences between individuals cancel out, so the result does not hang on one unusual one
Shows what would have happened without the change, so any difference can be put down to the treatment
Stops a second factor changing alongside the one being tested, which would otherwise share the credit
Turns a spread of genuinely different values into a single figure that two groups can be compared on
Makes sure a slow responder and a fast one are judged on equal terms rather than on how long each was left

Why one is never enough

Three of these explain why a single organism cannot settle a biology question. Choose the three.

Two individuals of the same kind differ for reasons no one set up, so one might mislead you
A single individual might be unusual purely by chance, and with one result you cannot tell
Only by measuring many can the natural differences even out into a figure worth trusting
A single organism measured with a finer instrument would settle the question
One organism always reads a fixed amount below the true value

How many, and compared with what

You may be asked about a biology experiment you have never done. You are not stuck, because the same two questions work on all of them. ⭐ First, HOW MANY? Were enough individuals measured for their natural differences to average out, or does the result rest on one or two that might be unusual? ⭐ Second, COMPARED WITH WHAT? Is there a group that did not get the change, so any difference can be pinned on the change rather than on how the individuals happened to be? Then say what would go wrong without each. That second half is where the marks are, because it shows you know what the decision was protecting. ⚠️ Keep the words straight. A larger sample makes a result more reliable, because chance differences even out. It does not make one reading more exact; that is a different idea and it is not what biology is usually short of. And where an experiment would use animals or human participants, it must be done with their consent and their care come first. That is a matter of ethics, not of method, and it never depends on the result.

Tall plants prove nothing alone

A study reports that plants given a new feed grew tall. Why is that finding worth little on its own, and what single addition would rescue it?

On its own it cannot show the feed caused the height, since the plants might have grown tall anyway; a group grown without the feed but treated the same in every other way gives the comparison that settles it
It is weak only because too few plants were used, and growing many more fed plants would fix it
It needs the heights measured with a finer ruler
It only needs the plants grown for a longer time

Order a living investigation

Put the stages of planning a biology investigation into the order they should happen.

Decide exactly what is being investigated
Choose the single factor that will be changed
List the other factors that could affect living things and decide how to keep each the same
Decide how many individuals to measure, so natural differences average out
Set up a comparison group that does not receive the change
Measure every individual and summarise each group by its average

Build the rescue for one plant

A single fed plant grew tall, and someone claims the feed works. Complete the advice.

The living-method run

Five quick decisions about living experiments. Three lives.

Judging a school by one pupil

Leave biology aside for a screen. Imagine judging a whole school by the exam result of a single pupil picked at random. If that one pupil did brilliantly, is the school excellent? If badly, is it failing? Plainly neither. Pupils differ enormously from one another, for all sorts of reasons no headteacher arranged, so one pupil tells you almost nothing about the school. ⭐ And the repair is not a more careful look at that pupil. You could check the marking of their paper to the last point and learn nothing new about the school, because the question was never really about that pupil. ⭐⭐ You fix it by looking at many pupils across the year, so that a brilliant one and a struggling one sit side by side and the typical picture appears. The more pupils you include, the more the individual differences cancel out and the safer your judgement becomes. And to say the school itself made the difference, rather than the pupils who happened to enrol, you would set its results beside another school taught differently but otherwise alike. Hold that picture. A living experiment works the same way: one individual settles nothing, many individuals reveal the typical picture, and a fair side-by-side comparison shows what the change itself was responsible for.

Complete the living-method facts

Living things differ from one another for reasons no one arranged, which is called natural _____. Because of this a single individual settles nothing, so a biologist measures a _____ of many individuals and summarises their differing values by taking the _____. To show that the factor being tested is really what caused any difference, the treated individuals are compared against a _____ group that was left untreated. Measuring more individuals makes the result more _____, because chance differences average out; and changing only one factor while holding the others the same keeps the test _____.

variation sample mean control reliable valid

Judge a living method you have not met

You are shown a method for a biology experiment you have never carried out, and asked to improve it. Work through your approach.

What do you check first?
The sample looks large enough. What is the next thing worth examining?
You have one improvement left to suggest. What earns most?

Explain what a sample and a control protect

Explain how a biology practical copes with the fact that living things vary, using the ideas of sample size, taking a mean and a control group. You may refer to any experiment you have carried out.

Explain why a single organism is rarely enough to answer a biology question
Explain how measuring a larger sample and taking a mean help, and what they protect against
Explain what a control group is and what it lets you say that the treated group alone cannot
Give one control variable and say what would go wrong if it were allowed to change
Explain why an unusual value in biology should be treated with more caution than in chemistry

Biology PAG Review

Get the method right under pressure

What you'll cover