AQA AS Level Biology Paper 1, November 2020: Question 7
10 marks · Medium difficulty · Practical Techniques & Data Analysis
Describe binary fission in bacteria, interpret a growth rate graph for Bacillus subtilis, convert units of mass, and suggest environmental variables to increase cell growth rate.
Practise this questionQuestion
Question text
07.1 Describe binary fission in bacteria.
[3 marks]
The cell growth rate of the bacterium Bacillus subtilis is proportional to its mass
immediately after binary fission.
Figure 7 shows this relationship.
Figure 7
07.2 The mass of the bacterial cells was measured in femtograms (fg).
1 fg (femtogram) = 1 × 10−15 g
*15* Place a tick ( ) in the box next to the number that is equal to 680 fg
[1 mark]
0.000 000 000 006 8 g
6.8 × 10−13 g
6.8 × 10−15 g
6.8 × 10−17 g 17
A scientist determined the growth rate of a B. subtilis cell by measuring its mass for
5 minutes.
In those 5 minutes, the cell’s mass increased by 90 fg
*0167.*3 Use this information and Figure 7 to determine the mass of this cell immediately after
binary fission.
Show your working.
[2 marks]
Answer fg
07.4 Suggest and explain how two environmental variables could be changed to increase
the growth rate of these cells.
[4 marks]
Suggestion 1
Explanation
Suggestion 2
Explanation
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
07.1 1. Replication of (circular) DNA; 3 1. Accept nucleoid
1. Reject
2. Replication of plasmids;
chromosome
3. Division of cytoplasm (to produce daughter 1. Reject mitosis
cells);
3. Ignore genetically
identical
07.2 6.8 × 10−13; 1
07.3 Correct answer of 660 = 2 marks;; 2
Accept for 1 mark,
18 (fg minute‾1)
OR
0.30 (fg s‾1)
OR
Correct use of interpolation lines
07.4 Principle of marking pairs: 4 max
Named environmental variable;
Correct effect on growth rate;
Examples
1. Increased (concentration of) glucose;
2. Increased respiration;
3. Increased (concentration of) oxygen;
4. Increased respiration;
5. Increased temperature;
6. Increased enzyme activity;
7. Increased (concentration of) phosphate;
8. Increased ATP/DNA/RNA;
9. Increased (concentration of) nucleotides;
10. Increased DNA synthesis;
TOTAL 10
How to answer it
Bacterial Growth and Binary Fission Study Guide
What this question tests
This question assesses your understanding of prokaryotic cell division (binary fission), standard form conversions involving femtograms (fg), interpreting graphical data with rates, and applying biochemical and physiological knowledge to explain how environmental variables alter metabolic and growth rates in bacteria.
Describe binary fission in bacteria.
✅ Correct Answer Requirements
- Replication of circular DNA (accept nucleoid).
- Replication of plasmids.
- Division of cytoplasm to produce daughter cells.
❌ Common Errors & Examiner Traps
- The "Mitosis" Trap: Examiners strictly reject references to "mitosis", "spindle fibres", or "chromosomes" since prokaryotes lack a true nucleus and linear chromosomes.
- Incomplete descriptions that omit plasmid replication or cytoplasmic division.
Standard Form Unit Conversion
The mass of the bacterial cells was measured in femtograms (fg). Given that 1 fg = 1 × 10⁻¹⁵ g , identify the number equal to 680 fg .
✅ Correct Answer
6.8 × 10⁻¹³ g
🧠 Exam Technique & Working
Convert 680 fg to grams by multiplying by 1 × 10⁻¹⁵ :
680 × 10⁻¹⁵ = 6.8 × 10² × 10⁻¹⁵ = 6.8 × 10⁻¹³ g
Determining Cell Mass Using Graphs and Rates
A scientist determined the growth rate of a B. subtilis cell by measuring its mass for 5 minutes. In those 5 minutes, the cell's mass increased by 90 fg. Use this information and Figure 7 to determine the mass of this cell immediately after binary fission.
✅ Correct Answer
660 fg (2 marks)
📐 Step-by-Step Calculation
- Calculate growth rate per minute:
90 fg ÷ 5 minutes = 18 fg min⁻¹ - Convert growth rate to seconds (matching Figure 7 units of fg s⁻¹):
18 ÷ 60 = 0.30 fg s⁻¹ - Use Figure 7: Find 0.30 fg s⁻¹ on the y-axis, follow across to the line, and read down to the x-axis to find the mass immediately after binary fission.
❌ Common Errors
Forgetting to convert minutes to seconds is the most common way students lose the final accuracy mark. If you calculated 18 fg min⁻¹ or 0.30 fg s⁻¹ correctly but failed to read the graph accurately, 1 mark was awarded.
Suggest and Explain Environmental Variables
Suggest and explain how two environmental variables could be changed to increase the growth rate of these cells.
💡 Marking Pair Principle
Marks are awarded in marking pairs: 1 mark for naming/suggesting a valid environmental variable with an upward change, and 1 mark for explaining its correct biochemical effect on growth rate.
✅ Example Acceptable Answers
- Suggestion 1: Increase glucose/nutrient concentration.
Explanation: Increases rate of respiration, providing more ATP for synthesis reactions. - Suggestion 2: Increase temperature (up to optimum).
Explanation: Increases kinetic energy and enzyme-substrate complex formation, increasing metabolic rate. - Alternative 3: Increase oxygen concentration.
Explanation: Increases aerobic respiration and ATP production.
Topics
Biology · 3.2 Cells
Question and mark scheme from the AQA AS Level Biology examination, Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.