Lesson 5 of 5 / Movement of substances
Describe a trend and calculate a percentage change
Why does "the mass decreases" score almost nothing?
I can describe a trend with quoted figures and units, calculate percentage change to 3 significant figures with its sign, and deal with an anomalous result.
I can define osmosis and investigate and describe its effects on plant and animal tissues.Syllabus
Syllabus K327 / K328, Topic 2(b). Define osmosis and investigate and describe its effects on plant and animal tissues.
About 7 min
Make a guess. You are not marked.
A graph shows mass falling as sugar concentration rises. A student writes "As the concentration increases, the mass decreases." How many marks out of 3 is this likely to score?
Show the answer
About 1, because it quotes no figures.
Mark schemes give marks for quoted values with units. A direction alone usually earns one mark at most.
The key idea
A full description gives the direction, quotes the first and last values with units, and quotes the value at any turning point or level-off. Percentage change uses the original value as the denominator and keeps its sign.
A trend is the overall pattern in how one variable changes as another variable changes. To quote a value means to copy the actual number from the table or graph, together with its unit.
Explore the idea
What does a full-mark description of this graph say?
Illustrative results: invented for this activity
Move 1: DirectionStart with the overall direction: as sucrose concentration increases, the percentage mass change decreases.
Units appear in every quoted value. A "describe" answer stops here; a "describe and explain" answer then gives the osmosis reason for each range.
Explanation
Examiners give marks for data, not for general words. "As the concentration increases, the mass decreases" shows the direction but quotes nothing, so it usually earns at most one mark. Mark schemes for every Biology paper reject figures written without units. Writing "-12.0" is not enough; write "-12.0%" or "-12.0% mass change".
Use the same four moves every time. First, state the overall direction: increases, decreases or stays the same. Second, quote the first and last values, each with its unit and the matching value of the other variable. Third, find any change in the pattern, such as a peak, a turning point or a level-off, and quote where it happens. Fourth, describe each range separately if the pattern changes. A turning point is where a rising pattern starts to fall, or a falling pattern starts to rise. A level-off (plateau) is where the values stop changing much.
In a "describe" question, do not explain. Explaining means giving the biological reason, such as water leaving cells by osmosis. If the question says "describe and explain", describe each range with figures first, then explain each range in turn.
Percentage change compares a change with the starting value. Percentage change = (new value - original value) / original value x 100. The original value is always the denominator, so a carrot cylinder that falls from 2.45 g to 2.12 g has changed by (2.12 - 2.45) / 2.45 x 100 = -13.5%. Dividing by the final mass instead gives 15.6%, which is wrong. Keep the minus sign: it is the only thing that tells the reader the mass went down.
Significant figures (s.f.) count digits from the first non-zero digit. Many papers ask for answers to 3 significant figures. -13.469% to 3 s.f. is -13.5%, and -10% to 3 s.f. is written -10.0%, because the final zero counts. Round only at the end of a calculation, not at each step.
An anomalous result is a reading that does not fit the pattern shown by the other results. Repeats show anomalies clearly. If three repeats at one concentration are -1.0%, -1.4% and +4.6%, the +4.6% reading does not fit, so leave it out and give the mean as -1.2%. Then repeat that reading. Say that a result is anomalous only when you can point to the pattern it breaks.
Step by step
- 1
State the direction
Say whether the dependent variable increases, decreases or stays the same.
- 2
Quote the ends
Give the first and last values with units, each paired with its independent-variable value.
- 3
Quote the change in pattern
Give the value and position of any peak, turning point or level-off, then describe each range.
- 4
Calculate with care
Use the original value as the denominator, keep the sign and round to 3 s.f. only at the end.
Worked example
Carrot cylinders in sucrose solution
Invented mean results: 0.0 mol/dm3, +14.0%; 0.2 mol/dm3, +6.4%; 0.4 mol/dm3, -1.2%; 0.6 mol/dm3, -8.8%; 0.8 mol/dm3, -12.0%; 1.0 mol/dm3, -12.4%. Describe the trend.
One way to explain it
As the sucrose concentration increases from 0.0 mol/dm3 to 0.6 mol/dm3, the percentage mass change decreases steadily from +14.0% to -8.8%. It passes 0% between 0.2 mol/dm3 and 0.4 mol/dm3. From 0.8 mol/dm3 to 1.0 mol/dm3 it levels off, changing only from -12.0% to -12.4%.
Why this answer works
- The direction comes first.
- Every value carries its unit: mol/dm3 for concentration and % for mass change.
- The level-off is a change in pattern, so it gets its own sentence with figures.
- No explanation is given, because the question only says "describe".
Watch out for this
A student says: "Quoting the numbers is enough; the units are obvious from the table." What is wrong with this?
Show the answer
Mark schemes reject figures without units. Write the unit every time you quote a value, for example "-8.8% at 0.6 mol/dm3".
Check your understanding
Using the carrot results above, which answer would score full marks for "Describe the trend"?
Show the answer
Mass change falls from +14.0% at 0.0 mol/dm3 to -8.8% at 0.6 mol/dm3, then levels off at -12.0% to -12.4% from 0.8 to 1.0 mol/dm3.
It gives the direction, quotes values with units at both ends and describes the level-off with its figures.
Quick recall
Why does "the mass decreases" score almost nothing?