Cambridge International A Level Biology Revision Guide

Chapter P1: Practical skills for AS
The practical skills that you will develop during your
AS level course are tested by a practical examination.
In this examination, you will work in a laboratory,
answering questions on an examination paper. The
questions will test a range of skills, which can be classified
into three groups:
■■
■■
■■
manipulation, measurement and observation
presentation of data and observations
analysis, conclusions and evaluations.
During your AS level course, each time you carry out an
experiment, you will use a variety of these skills. In this
chapter, we will look at the different components of the
skills in detail, and consider what you must be able to do
to work to the best of your abilities.
The practical examination usually includes two
questions, sometimes three. One of these questions will
probably be a ‘wet practical’ – an experiment that will
involve you in manipulating apparatus, perhaps making
up and measuring out solutions, making measurements
and observations, recording them and drawing
conclusions from a set of results. One of the questions will
involve observing a biological structure, perhaps using a
microscope, and recording your observations in the form
of a diagram.
Experiments
Many of the experiments that you will do during your
course involve investigating how one thing affects another.
For example:
■■
■■
■■
■■
investigating how enzyme concentration affects the
rate of activity of rennin
investigating how temperature affects the rate of
activity of catalase
investigating how surface area affects the rate of
diffusion
investigating how the concentration of a solution
affects the percentage of onion cells that become
plasmolysed.
We will concentrate on the first of these experiments –
the effect of enzyme concentration on the rate of
activity of rennin – to illustrate how you should approach
an experiment, and how you should answer questions
about it.
Rennin is an enzyme that clots milk. It is found in
the stomachs of young mammals, which are fed on milk.
Rennin, also known as chymosin, is used commercially in
cheese-making. Its substrate is a protein called casein. In
fresh milk, the casein molecules are dispersed in the milk
as little micelles (groups of molecules organised rather
like a cell membrane), which spread evenly through the
milk to form a homogeneous emulsion. Rennin splits the
casein molecules into smaller molecules, which breaks up
the micelles and causes the protein to clump together into
small lumps, a process called clotting (Figure P1.1). These
lumps separate out from the liquid milk, producing the
curd that can be made into cheese.
micelles of casein
molecules dispersed in
the milk
rennin enzyme breaks
down the casein into
smaller molecules
the smaller molecules
stick together – the
casein coagulates
Figure P1.1 The effect of rennin on milk.
Variables and making
measurements
micelle
casein
molecule
rennin
In this experiment, you would be investigating the effect of
the concentration of rennin on the rate at which it causes
milk to clot.
The concentration of rennin is the independent
variable. This is the factor whose values you decide on, and
which you change.
The rate at which the rennin causes the milk to clot is
the dependent variable. This is the variable which is not
under your control; you do not know what its values will
be until you collect your results.
247