6.5.4Unit 6: Practical Skills in Biology II
Types of Data Provided by Forensic Analysis — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 6 parts Lesson 20 of 48 Unit quiz
Section 1 of 6Decomposition and Succession
Rate depends on conditionsdecay is slower in cold or anaerobic surroundings and faster when warm.
Decomposers
- Bacteria and fungi secrete enzymes that break down the tissues, and the changes follow a sequence.
Early signs
- Within days the skin may appear green, and gases such as methane cause bloating.
Later stages
- Soft tissue liquefies over weeks, then months or years pass until only the skeleton remains.
Succession
- Different organisms arrive in order
- bacteria, then flies, then beetles;
- unlike in ecology, earlier arrivals mostly remain.
- bacteria, then flies, then beetles;
Section 2 of 6Forensic Entomology
1
Flies are the first insects to colonise a body, often within hours of death.
2
Flies lay eggs, and blowfly eggs hatch after roughly a day into larvae.
3
The presence of larvae therefore suggests that death occurred more than about 24 hours earlier.
4
As tissue dries, flies leave, but beetles can feed on dry tissue and remain.
5
Temperature, humidity, oxygen and drugs in the body can all alter the speed of insect development.
Section 3 of 6Body Temperature: Algor Mortis
Body Temperature: Algor Mortis
Cause
- After death, respiration stops, so no more heat is produced and the body cools towards the temperature of its surroundings.
Rate
- The fall is gradual and roughly 1.5–2 °C each hour in the early period.
Variables
- Air temperature, clothing, body fat and surface area to volume ratio all change the rate of cooling.
Use
- The difference between normal body temperature (about 37 °C) and measured temperature gives an estimate of time since death.
Section 4 of 6Rigor Mortis
Cause
- Without oxygen, muscle cells respire anaerobically and make lactic acid, which lowers pH and denatures the enzymes that make ATP.
Locked muscles
- Without ATP, myosin heads cannot detach from actin, so the muscles stay contracted and stiff.
Timing
- Stiffness begins a few hours after death, spreads through the body, and then passes off after about a day or more.
Variation
- Higher temperatures speed rigor mortis, and the level of muscle development also influences it.
Section 5 of 6Worked Example
Question
- A body is found with a temperature of 28 °C.
- Assume it was 37 °C at death and cooled by 1.5 to 2.0 °C per hour.
- Estimate the time since death.
- Temperature fall = 37 − 28 = 9 °C.
- At 2.0 °C per hour: 9 ÷ 2.0 = 4.5 hours.
- At 1.5 °C per hour: 9 ÷ 1.5 = 6.0 hours.
Answer Death occurred about 4.5 to 6 hours earlier, as an estimate only.
Section 6 of 6Exam Checklist & Tips
6.5.4 – Key Points for the Exam
- Forensic entomology The study of insects on a body to help estimate time of death.
- Algor mortis The cooling of the body after death.
- Rigor mortis The stiffening of muscles after death.
- Succession The change in the organisms present in a habitat over time.
Common Mistakes
- Giving an exact time of death, when all methods only provide estimates.
- Saying rigor mortis happens because ATP is present;
- it happens because ATP is lacking, so myosin heads cannot detach from actin.
- Using body temperature alone without considering air temperature, clothing or body size.
- Stating that all decomposition proceeds at the same rate whatever the conditions.
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