COMMAND WORD · BIOLOGY
Combine two or more relationships to produce a new equation, rather than putting one number into one formula.
Relationship 1 with relationship 2 gives a new equation.
Original question
Derive an equation for cardiac output in terms of heart rate, end diastolic volume and end systolic volume.
Annotated answer
Relationship 1Stroke volume = end diastolic volume − end systolic volume. Relationship 2Cardiac output = heart rate × stroke volume. New equationCardiac output = heart rate × (end diastolic volume − end systolic volume).
The labels match the frame. Read them in order. This is one accurate route, not the only one.
The last line should stand alone, with the quantities named.
Defining cardiac output in words and never writing an equation.
Original practice
Mark points shown after marking
Magnification = image size ÷ object size. On a micrograph, a scale bar shows that 20 mm on the image represents 5 µm on the object.
Derive an expression for the actual length of a structure that measures L mm on the same image.
(2)1 mm on the image represents µm, so the actual length = µm.
Total for this question: 2 marks
5 µm corresponds to 20 mm, so 1 mm represents 0.25 µm. Actual length = 0.25L µm when L is in millimetres. That expression is the derivation. Measuring one structure would be a calculation.
