Rates per minute and 1000 ÷ time, gradients, reading an optimum and an intercept, and choosing a chart.
What you will learn
- Calculate a rate from a count or a time
- Read an optimum or an intercept from a graph
- Choose the right chart
Rates
- Count in a time: rate = amount ÷ time (bubbles per minute, cm³/min).
- Time to finish: rate = 1000 ÷ time (amylase, reaction times).
- From a straight-line graph: rate = gradient.
Read an optimum at the peak of a curve. In osmosis, the concentration where the best-fit line crosses 0% change in mass is the concentration of the cell contents.
On a curved graph the rate changes. The mean rate over a time is the total change ÷ the total time; the rate at the start (the initial rate) is found from the first part of the curve, where it is steepest. The rate at one instant is the gradient of a tangent: a straight line drawn touching the curve at that point. To predict a value on a straight-line graph, read it off the line, or use gradient × time.
Choosing a chart: a line graph when both variables are continuous (such as time and volume); a bar chart when one variable is categoric, in separate groups (such as types of food); a scatter graph to look for a correlation between two measurements (such as height and mass).
The rest of this lesson and its 9 practice questions are locked.
In this lesson
- Rates
9 practice questions, marked as you go; about 20 minutes in all.
Key terms
Optimum, Mean rate, Line graph, Bar chart, Scatter graph
In this unit: Required practicals and maths skills
The skills behind every biology practical: units and magnification, means and percentages, rates and graphs, sampling, ratios and planning.