BIOLOGY SPECIFICATION NOTES
Edexcel Biology Salters-Nuffield A AS/A-Level
Key:
Definitions are in turquoise
Core practicals are in orange
Equations are in green
Topic 6: Infection, immunity and forensics (IIF)
6.1 Understand how to determine the time of death of a mammal by examining the extent of
decomposition, stage of succession, forensic entomology, body temperature and degree of
muscle contraction. p.79
As soon as a person dies, a series of physical and chemical changes start to take place.
These occur in a known order due to the timing of a death which can be used to estimate
the time of death.
Body temperature
- Lasts from 30-60 minutes, but is useful within first 24 hours
- Normal body temperature is between 36.2 - 37.6 C
- Core body temperature measured through rectum or
abdominal stab
- Long thermometer or electronic temperature probe used
- Cooling of the body follows sigmoid curve
- Not very accurate as other factors must be taken into
account. These include: body size and fat (insulation), body
position, clothing, air movement, humidity, and temperature of
surroundings, which encompasses the environment.
- A body immersed in water cools faster than in air, water better conductor of heat
Rigor mortis
- Complete is seen at 6-9 hours after death
- Rigor mortis is the stiffening of muscles. Rigor mortis eventually passes and muscles
are relaxed again when the muscle tissue breaks down. This is because:
- After death, muscles are starved of oxygen and oxygen-dependent reactions stop
- Anaerobic respiration in cells, lactic acid produced
- pH of cells falls, inhibits enzymes and stops respiration
- ATP for muscle contraction no longer produced, bonds between muscle proteins are
fixed
- Proteins can’t move to shorten the muscle, so muscles and joints are fixed
- The stages of rigor mortis can be divided up into four as well.
- < 3 hours: Warm, not stiff
- 3-8 hours: Warm, stiff
- 8-36 hours: Cold, stiff
- 36-48 hours: Cold, not stiff
, Decomposition
- Between 36-72 hours after death
- First is autolysis, where the body’s enzymes (from digestive tract and lysosomes)
break down cells
- Bacteria from gut and lungs invade tissues, releasing enzymes that aid
decomposition
- Loss of oxygen favours growth of anaerobic bacteria
- First sign of decomposition is putrefaction, green discoloration of skin in abdomen
- Spreads across body, darkening
- After about a week, gas or liquid blisters appear
- Body smells and becomes bloated due to methane, CO2, ammonia and hydrogen
forming
- Fluid from putrefaction drains away, soft tissues shrink and decay rate is reduced
- Temperature and injuries slow or speed up this process. A high temperature can
actually denature enzymes.
Forensic entomology and stage of succession can also be used to help detect the time since
death.
Forensic entomology involves the study of insects to help determine when the body died.
The temperature, air, ground, and maggot mass are measured, and as the stages of maggot
development are clearly defined, can be used to determine the death. Some maggots may
be killed, to retain the age they are when they are found, and some are left to grow, so their
development is completed and the species can be identified. It generally takes insects one
day to lay their eggs. If the length of time for an egg to form and pupate is subtracted from
the date of pupation, the date the eggs were laid can be worked out.
The different species found on the body can also be used. There is a predictable sequence
of organisms that feed on the body. For example:
1. Bluebottles, house fly
2. Flesh flies
3. Beetle larvae, moths
4. Cheese skipper
6.2 Know the role of microorganisms in the decomposition of organic matter and the
recycling of carbon. p.85
- There are other microorganisms involved in the decomposition of the body
- Bacteria from the gut invade the tissues after death
- Other bacteria and fungi from surrounding colonate
- These microorganisms are called decomposers
- The body is a great source of energy
- Organic carbohydrates, proteins, fats and nucleic acids are used as a food
source
- Energy is released through aerobic and anaerobic respiration
- Energy allows decomposers to grow more rapidly
