OCR A Level Biology:复习笔记1.1.4 Using Units

Using Appropriate Units

  • It essential that the correct scientific measurements are used when discussing biological experiments
  • Ensure that the correct symbols are used in conjunction with the unit of measurement
    • E.g. m3 for cubic metres


Units of Measurement Table

  • cm3 is the same as millilitre (ml)
  • dm3 is the same as litre (l)

Exam Tip

Be careful when using the word "amount" in your answers. "Amount" has a very specific meaning in science - "mole". Instead refer to the mass, volume or concentration of a substance!

Significant Figures

  • Significant figures must be used when dealing with quantitative data
  • Significant figures are the digits in a number that are reliable and absolutely necessary to indicate the quantity of that number
  • There are some important rules to remember for significant figures
    • All non-zero digits are significant
    • Zeros between non-zero digits are significant
      • 4107 (4.s.f.)
      • 29.009 (5.s.f)


    • Zeros that come before all non-zero digits are not significant
      • 0.00079 (2.s.f.)
      • 0.48 (2.s.f.)


    • Zeros after non-zero digits within a number without decimals are not significant
      • 57,000 (2.s.f)
      • 640 (2.s.f)


    • Zeros after non-zero digits within a number with decimals are significant
      • 689.0023 (7.s.f)



  • When rounding to a certain number of significant figures:
    • Identify the significant figures within the number using the rules above
    • Count from the first significant figure to the specified number
    • Use the next number as the ‘rounder decider’
    • If the decider is 5 or greater, increase the previous value by 1


Worked Example

Write 1.0478 to 3 significant figures.

Step 1: Identify the significant figures

They are all significant figures

Step 2: Count to the specified number (3rd s.f.)


Step 3: Round up or down


Exam Tip

An exam question may sometimes specify how many significant figures the answer should be, make sure you keep an eye out for this!