AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
(HT only) Students should be able to express the answer in standard form.
Full specification wording
Culturing microorganisms (biology only): (HT only) Students should be able to express the answer in standard form.
In culturing work you often calculate numbers of bacteria after growth, or areas of zones of inhibition, and the values can be very large or very small.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 8 of 11
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In culturing work you often calculate numbers of bacteria after growth, or areas of zones of inhibition, and the values can be very large or very small. Standard form writes a number as A × 10ⁿ, where A is at least 1 and less than 10, and n is an integer. For example, 4 500 000 bacteria becomes 4.5 × 10⁶, and 0.000 32 mm becomes 3.2 × 10⁻⁴ mm. To convert, move the decimal point until one non-zero digit remains on its left, then count the moves: left gives a positive index, right gives a negative index. To multiply, multiply the A values and add the indices; to divide, divide the A values and subtract the indices. Always check the final A value is between 1 and 10, and keep units with the answer.
What earns the marks
- Writes a number in the form A × 10ⁿ with A at least 1 and less than 10 and n an integer.
- Converts correctly between ordinary numbers and standard form by counting decimal place moves.
- Uses a positive index for numbers greater than or equal to 10 and a negative index for numbers less than 1.
- Applies standard form to a culturing calculation, such as a bacterial population or a zone of inhibition area, and states the unit.
- Checks that the coefficient A is not less than 1 and not 10 or more, adjusting the index if needed.
- Multiplies or divides values in standard form by handling coefficients and indices separately.
Common mistakes and how to put them right
Writing a coefficient that is 10 or more, such as 45 × 10⁵.
Put it right: Rewrite it as 4.5 × 10⁶.
Using the wrong sign for the index when the original number is less than 1.
Put it right: Remember that small numbers take a negative index.
Dropping the unit after converting to standard form.
Put it right: Carry the unit through, for example 3.2 × 10⁻⁴ mm.
Exam tips
- Decide the index by counting how many places the decimal point moves, not by guessing.
- After converting, read the coefficient back and confirm it lies between 1 and 10.
- Show the conversion step in your working so an arithmetic slip does not hide a correct method.
Try the lesson
The start of the Culturing microorganisms (biology only): (HT only) Students should be able to express the answer in standard form. lesson, made from checked teaching notes for this specification point.
Have a go first
A bacterial culture contains 8.4 × 10⁵ cells per cm³. A student takes a 5.0 cm³ sample. How many cells are in the sample, expressed in standard form?
Listen
In culturing work you often calculate numbers of bacteria after growth, or areas of zones of inhibition, and the values can be very large or very small. Standard form writes a number as A × 10ⁿ, where A is at least 1 and less than 10, and n is an integer. For example, 4 500 000 bacteria becomes 4.5 × 10⁶, and 0.000 32 mm becomes 3.2 × 10⁻⁴ mm. To convert, move the decimal point until one non-zero digit remains on its left, then count the moves: left gives a positive index, right gives a negative index. To multiply, multiply the A values and add the indices; to divide, divide the A values and subtract the indices. Always check the final A value is between 1 and 10, and keep units with the answer. A strong answer earns marks for these points. Writes a number in the form A × 10ⁿ with A at least 1 and less than 10 and n an integer. Converts correctly between ordinary numbers and standard form by counting decimal place moves. Uses a positive index for numbers greater than or equal to 10 and a negative index for numbers less than 1. Applies standard form to a culturing calculation, such as a bacterial population or a zone of inhibition area, and states the unit. A common slip: Writing a coefficient that is 10 or more, such as 45 × 10⁵. Instead, rewrite it as 4.5 × 10⁶. Exam tip: Decide the index by counting how many places the decimal point moves, not by guessing.
Key words
Say what each word means before you turn the card.
Standard form
Try answering aloud or in your head before turning the card.
Index
Try answering aloud or in your head before turning the card.
Coefficient
Try answering aloud or in your head before turning the card.
Converting
Try answering aloud or in your head before turning the card.
See every definition
- Standard form
- A way of writing a number as A × 10ⁿ, where A is at least 1 and less than 10, and n is an integer.
- Index
- The integer n in standard form; it is positive for numbers greater than or equal to 10 and negative for numbers less than 1.
- Coefficient
- The value A in standard form, which must be at least 1 and less than 10, and is adjusted with the index if needed.
- Converting
- Moving the decimal point until one non-zero digit remains on its left, then counting the moves: left gives a positive index, right gives a negative index.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Culturing microorganisms (biology only): (HT only) Students should be able to express the answer in standard form.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge