Edexcel International A Level Biology Microbiology, Immunity and Forensics Exam Questions

About Edexcel International A Level Biology Microbiology, Immunity and Forensics

This topic is part of Unit 4 (Energy, Environment, Microbiology and Immunity) of Edexcel International A Level Biology. It covers how bacteria are cultured using aseptic technique and how populations are measured, then how pathogens such as Mycobacterium tuberculosis and HIV cause disease. You study the non-specific and specific immune responses, active and passive immunity, the evolutionary race between pathogens and hosts, and how antibiotics work. The forensics section explains DNA profiling using PCR and gel electrophoresis, and how time of death is estimated. Practical questions on serial dilutions, growth curves and the effect of antibiotics on bacteria are common in this unit.

What you need to know

• Culturing bacteria Use aseptic technique to grow bacteria in broth or on agar. Measure population size using serial dilution and plate counts, a haemocytometer or turbidity.
• Growth curves Bacterial growth has lag, exponential, stationary and death phases. Exponential growth is shown on a log scale.
• Pathogens TB is caused by Mycobacterium tuberculosis, which survives in macrophages. HIV is a retrovirus that infects T helper cells, leading to AIDS and opportunistic infections.
• Immune response Non-specific responses include inflammation, lysozyme, interferon and phagocytosis. Specific responses involve antigen-presenting cells, T helper cells, T killer cells, B cells, plasma cells, antibodies and memory cells.
• Antibiotics Bactericidal antibiotics kill bacteria; bacteriostatic antibiotics prevent growth. Resistance develops by natural selection; hospital acquired infections are controlled by hygiene and careful prescribing.
• Forensics DNA profiling uses PCR to amplify short tandem repeats and gel electrophoresis to separate fragments. Time of death is estimated from body temperature, rigor mortis, decomposition and insect succession.

Common mistakes in Microbiology, Immunity and Forensics

• Forgetting the dilution factor when calculating bacterial numbers.
• Saying the exponential phase continues indefinitely; nutrients and waste eventually limit growth.
• Describing antibodies as cells; they are proteins secreted by plasma cells.
• Writing that HIV directly causes death; opportunistic infections cause death after T helper cells are destroyed.
• Confusing coding DNA with the short tandem repeats used in DNA profiling.

Try three Microbiology, Immunity and Forensics exam-style questions